BASIC CONCEPTS OF SUCKER ROD CORROSION
Table of Contents INTRODUCTION ................................................................................ 1 CORROSION ...................................................................................... 2 Electrochemical Corrosion .............................................................................. 2 Chemical Corrosion ......................................................................................... 3 Oxidation ......................................................................................................... 4 Galvanic Corrosion .......................................................................................... 4 Erosion-Corrosion ........................................................................................... 5 Hydrogen Embrittlement ................................................................................ 5 Corrosion Fatigue ............................................................................................ 5
FIELD SERVICE PROBLEMS .......... ..................... ..................... ..................... ..................... ..................... ............. 6 Conditions that Cause Sucker Rod and Coupling Failures ........ ................. .................. .............. ..... 6 Types of Sucker Rod and Coupling Failures ........ ................ ................. .................. .................. ................. .......... 6
DESIGNING TO REDUCE FAILURES.......... .................... ..................... ..................... .................... .......... 16 Mill Defects ................................................................................................... 16 Manufacturing Problems .............................................................................. 16 Handling Problems ........................................................................................ 17 Mechanical Damage ...................................................................................... 17 Improper Joint Makeup ................................................................................ 17 Bent Rods ...................................................................................................... 18 Guided Sucker Rods ...................................................................................... 19 Poor Pumping Conditions ............................................................................. 19 Corrosion Problems ....................................................................................... 20 Oxygen Corrosion .......................................................................................... 21 Problems Caused by a Stuck Pump ................ ......................... ................. ................. .................. .................. ............ ... 22 Hammering the Surface of Couplings ........................ ................................. ................. ................. .................. ......... 23 Thread Galling ............................................................................................... 23 Collection of Background Data and Selection ......... .................. .................. .................. ................. ........... ... 25 25 Non -Destructive Testing (NDT) .................................................................... 25 Mechanical Testing ....................................................................................... 25 Selection and Preservation of Fracture Surfaces ............. ...................... .................. ................. ........... ... 26 Macroscopic Examination of Fracture Surfaces ........ ................. ................. ................. .................. ........... 26 Fracture Classifications ................................................................................. 27
REFERENCES .......... .................... ..................... ...................... ..................... ..................... ..................... .................... .......... 29
© 1993-2016 Weatherford. All rights reserved.
i
BASIC CONCEPTS OF SUCKER ROD CORROSION
Table of Contents INTRODUCTION ................................................................................ 1 CORROSION ...................................................................................... 2 Electrochemical Corrosion .............................................................................. 2 Chemical Corrosion ......................................................................................... 3 Oxidation ......................................................................................................... 4 Galvanic Corrosion .......................................................................................... 4 Erosion-Corrosion ........................................................................................... 5 Hydrogen Embrittlement ................................................................................ 5 Corrosion Fatigue ............................................................................................ 5
FIELD SERVICE PROBLEMS .......... ..................... ..................... ..................... ..................... ..................... ............. 6 Conditions that Cause Sucker Rod and Coupling Failures ........ ................. .................. .............. ..... 6 Types of Sucker Rod and Coupling Failures ........ ................ ................. .................. .................. ................. .......... 6
DESIGNING TO REDUCE FAILURES.......... .................... ..................... ..................... .................... .......... 16 Mill Defects ................................................................................................... 16 Manufacturing Problems .............................................................................. 16 Handling Problems ........................................................................................ 17 Mechanical Damage ...................................................................................... 17 Improper Joint Makeup ................................................................................ 17 Bent Rods ...................................................................................................... 18 Guided Sucker Rods ...................................................................................... 19 Poor Pumping Conditions ............................................................................. 19 Corrosion Problems ....................................................................................... 20 Oxygen Corrosion .......................................................................................... 21 Problems Caused by a Stuck Pump ................ ......................... ................. ................. .................. .................. ............ ... 22 Hammering the Surface of Couplings ........................ ................................. ................. ................. .................. ......... 23 Thread Galling ............................................................................................... 23 Collection of Background Data and Selection ......... .................. .................. .................. ................. ........... ... 25 25 Non -Destructive Testing (NDT) .................................................................... 25 Mechanical Testing ....................................................................................... 25 Selection and Preservation of Fracture Surfaces ............. ...................... .................. ................. ........... ... 26 Macroscopic Examination of Fracture Surfaces ........ ................. ................. ................. .................. ........... 26 Fracture Classifications ................................................................................. 27
REFERENCES .......... .................... ..................... ...................... ..................... ..................... ..................... .................... .......... 29
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i
BASIC CONCEPTS OF SUCKER ROD CORROSION
Introduction Historically, the first recorded incidence of corrosion was a problem encountered by British ships operating in the Mediterranean early in the nineteenth century. Worms living in those waters would enter the wooden hulls and eat the timbers until ships required dry-docking for replacement of these wooden structures. The British decided to cover the ship hulls with thin sheets of copper and the worm problem was solved, or so it appeared. Soon the copper sheets began falling off the hulls, and the worm problem returned. The steel nails holding the copper had disintegrated where the copper and steel were in contact, and no one could explain the reason. We now know that a galvanic action occurs between dissimilar metals in sea water. Using copper nails resolved the problem. Last century, scientists began to recognize the tremendous scope of corrosion and the cost associated with failed equipment. Of note was the realization that stray current from street car railways was damaging and even destroying underground metal structures and communication cables.
Corrosion is a natural phenomenon that is not necessarily limited to metals. The effects of corrosion can be observed every day, everywhere in the world; and the cost of its damage to metal objects amounts to billions of dollars each year. Consider the following costs to an operator if a single rod string fails as a result of corrosive action: 1.
Downtime: lost production because the string can no longer activate the pump
2.
Workover: cost of a workover rig and the labor to pull the tubing and rod string
3.
Replacement: Replacement: cost of a new string of rods and possibly the cost of replacing tubing and the downhole pump
4.
Work: labor of company personnel who could be doing other things
Multiply those costs by the number of corrosion failures in rod strings in one year and the total cost to operators becomes enormous. Today the mechanics of corrosion are understood and its behavior can be controlled. In some cases it can be completely eliminated by following proper procedures. This guide focuses on the types of corrosion related to sucker rods, the causes, and ways to reduce sucker rod failures.
© 1993-2016 Weatherford. All rights reserved.
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Corrosion Corrosion can be defined as the deterioration of a substance or its properties because of a reaction with its environment. The substance we will consider is steel. Steel is an iron compound with alloying elements such as carbon and manganese. The driving force that makes metals corrode is a natural phenomenon. Iron, from which steel is produced, is found in combination with other elements, and these are called ores. Any iron-based product in a usable form is in only a temporary condition and eventually, if let unhampered, will return to its original form (an ore).
Electrochemical Corrosion Most corrosion in the oil field is attributable to the presence of water in large or small amounts. Corrosion in the presence of water is an electrochemical process. Please keep this in mind as we discuss other types of corrosion. Figure 1 shows that steel is not homogeneous. It is a mixture of iron, carbon, and other alloying elements; but for purposes of illustration, let’s consider only the iron and carbon elements. Some of the carbon is dissolved in the iron, and the balance exists as iron carbide.
Left unprotected in an atmosphere, a metal will release energy as it combines with other elements and returns to its natural state as an ore. The release of energy and its attendant combination with other elements to form ore is corrosion. One of the most obvious examples is a steel object left exposed to weather. It will begin to rust and, if left undisturbed, will completely deteriorate. Rust is a combination of iron and oxygen, in which the iron gives up its energy and returns to its natural state. There are multiple types of corrosion: •
Electrochemical corrosion
•
Chemical corrosion – Sour – Sweet
•
Oxidation
•
Galvanic
•
Corrosion-erosion
•
Hydrogen embrittlement
•
Corrosion fatigue
Figure 1: Photomicrograph showing a pearlite – ferrite microstructure. Pearlite is a mixture of cementite (Fe3C); Ferrite is almost pure iron.
Iron carbide (Fe3C) has a lower tendency to corrode than pure iron (Fe), thus with an electrode and in the presence of an electrolyte (such as water or salt water), the two different compositions will complete an electrical circuit and current will flow (Figure 2).
In most instances more than one type of corrosion contributes to a failure.
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BASIC CONCEPTS OF SUCKER ROD CORROSION
The area corroded is always the anode. The cathode remains unaffected. The affinity of iron in solution for oxygen is greater than the affinity of hydrogen for oxygen. Hydrogen gives up one electron when it goes into its ionic state. Oxygen gains two electrons and becomes negatively charged. In a simplified form, the reaction is as follows: Fe Figure 2: Schematic showing current flow between iron and iron carbide grains with resulting corrosion of the iron
3. Electrolyte. The fluid that transmits the current between anode and cathode. When iron corrodes, the iron dissolves, goes into solution, and gives up two electrons. Since an atom of iron contains an equal number of protons (positively charged particles) and electrons (negatively charged particles), the iron in solution is called an ion. The iron ion is positively charged.
Iron Atom
2+
Fe
+
Iron Atom
2H+ Hydrogen Ions
+
–
+2e
Electrons
→
→
FeO
Hydrogen Gas
H2↑
+
Iron Oxide
Hydrogen Gas
Chemical Corrosion Sour Hydrogen sulfide is very soluble in water. When both hydrogen sulfide and water are present in well fluids, sulfuric acid (H2SO4,) is formed. Although other reactions may be involved, simply stated, the final reaction is the formation of iron sulfide (FeS). H2S
Electrons
Hydrogen Sulfide
Hydrogen Gas
+
HHO
H2↑
+
The positively charged iron ion and the negatively charged oxygen ion combine to form iron oxide. The two positively charged hydrogen ions then form a molecule and escape at the cathode as a gas.
–
H2
FeO Iron Oxide
Negative Oxygen Positive Hydrogen
2e
Although the cathode remains unaffected, another chemical reaction takes place at the cathode. The electrons left behind at the anode when iron went into solution travel to the cathode through the solid metal. Hydrogen in water has an affinity for those electrons and consumes them.
+
+
Positive Iron
2. Cathode. The cathode is the opposite side of the cell that is unaffected by the current flow and therefore does not corrode.
→
Water
+
1. Anode. The anode is that portion of the metal surface that is corroded.
→
H2O
Iron Atom
Fe
Some definitions are necessary to describe the electrical circuit:
Fe
+
+
→
Fe Iron Atom
FeS Iron Sulfide
H2↑
+
Hydrogen Gas
Iron sulfide is a black scale that clings to the surface of the metal and is cathodic to the iron. It accelerates the corrosion in the locality of the scale and causes deep pitting.
Sweet The term sweet corrosion refers to corrosion caused by the presence of carbon dioxide (CO2) in a producing well. Carbon dioxide easily dissolves in water to form carbonic acid. CO2
+
Carbon Dioxide
H2O
→
Water
H2CO3 Carbonic Acid
An electrochemical reaction occurs and the iron replaces hydrogen to form iron carbonate. Fe Iron
Figure 3: Schematic showing basic current and electron flow in localized corrosion cell at the surface of ferrous metals immersed in an electrolyte
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+
H2CO3 Carbonic
→
Fe2CO3 Iron Carbonate
+
H2 Gas
The end result is pitting. The severity of the pitting is determined by many factors, such as pressure, temperature, and the amount of CO2 present. Both sour and sweet corrosion can occur in the same well.
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Oxidation Corrosion of metal by oxidation is the ordinary rust observed on any unprotected piece of steel. It can only occur in the presence of water, and then only if dissolved oxygen is present. However, it will also occur in sea water and salt solutions. We will limit our discussion to salt water, a fluid that is prevalent in pumping wells. Water solutions rapidly dissolve oxygen from the air, and this is the source for the required oxygen in the corrosion process. Normally oxygen is not found in well fluids; but if it is introduced by any method, rusting will continue as long as oxygen is present. The simplified chemical reaction is:
When the three conditions listed above are met, the anodic (most active) metal will deteriorate while the cathodic (less active) metal will be relatively unaffected. Figure 4 indicates the relative anodic and cathodic properties of some commercial metals and alloys in sea water. From their positions on the scale in Figure 4, it becomes apparent that steel nails will be destroyed by the more noble copper. The scale also explains why gold is usually not found in chemical combination with other elements. Active or Anodic
H2O
4Fe Iron
+
3O2
4Fe2O3
Oxygen
Iron Oxide
For an example of the effect of oxygen on steel, consider the photos taken of the ocean liner Titanic that lies at the bottom of the Atlantic Ocean. Why has the hull not corroded away? Because at those depths, no oxygen exists.
Galvanic Corrosion Galvanic corrosion is very rare in the oil field. Three conditions are necessary for galvanic corrosion to occur: •
Electrochemically dissimilar metals must be present.
•
The metals must be in electrical contact.
•
The metals must be exposed to an electrolyte.
Electrochemical dissimilarity refers to the amount of energy stored by the metal when it is removed from its natural condition. An example of metals in electrical contact is the steel nails that held copper sheeting on the hulls of British ships in salt water. A potential can be created between any two dissimilar metals and even between different types of steels. The third condition for galvanic corrosion relates to the strength and type of electrolyte and the presence or absence of oxygen. Noble or Cathodic
Magnesium Magnesium Alloys Zinc Galvanized Steel Aluminum 1100 Aluminum 2024 (4.5 Cu, 1.5 Mg, 0.6 Mn) Mild Steel Wrought Iron Cast Iron 13% Chromium Stainless Steel Type 410 (active) 18-8 Stainless Steel Type–Tin Solders Lead Tin Muntz Metal Manganese Bronze Naval Brass Nickel (active) 76 Ni, 16 Cr, 7 Fe Alloy (active) 60 Ni, 30 Mo, 6 Fe, 1 Mn Yellow Brass Admiralty Brass Red Brass Copper Silicon Brass 70:30 Cupro Nickel G-Bronze Silver Solder Nickel (passive) 76 Ni, 16 Cr, 7 Fe Alloy (passive) 13% Chromium Stainless Steel Type 410 (passive) Silver Graphite Gold Platinum
Figure 4: Galvanic series of some commercial metals and alloys in sea water
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Erosion-Corrosion
Corrosion Fatigue
This type of corrosion is caused by corrosive fluid, impinging flow, or turbulence of the fluid upon metal surfaces. Impingement can be caused by solid material or gas bubbles entrapped in the fluid being pumped. These materials, moving against a metal surface, tend to abrade or erode the steel. The erosion takes the form of elongated pitting or a deep groove. The rapid movement of corrosive fluid removes a protective scale and exposes the underlying metal, which accelerates corrosion.
Thus far we have primarily considered the mechanics of corrosion alone. However, corrosion and fatigue, acting in concert, are the cause of many sucker rod failures.
Turbulence is a major factor in this type of attack. When a liquid flows over a metal, there is usually a critical velocity below which impingement does not occur, but above which impingement rapidly increases. To illustrate this phenomenon, consider a curve in a lazy stream. As long as the stream contains little water and the flow is gentle, the banks remain unchanged. Increase the water volume and the stream becomes more turbulent, thereby eroding the bank. The break out of gases in low-pressure zones can impinge on the surface of a sucker rod and remove any protective coating.
Hydrogen Embrittlement
Cyclic stressing of sucker rods does not typically cause a failure of the sucker rod string if the stresses are limited to a level below the endurance limit of the steel. Environmental conditions are extremely important in corrosion fatigue. Salt water is a corrosive medium. When dissolved gases such as hydrogen sulfide (H2S), carbon dioxide (C02), and free oxygen are present in salt water, corrosivity increases and fatigue life decreases. Usually fatigue begins at the rod surface as a pit in the metal, a nonmetallic inclusion, or some other steel defect. However, it may begin with some kind of mechanical damage. Though the stresses may be uniform over the balance of the rod, the stresses induced at the bottom of a pit will be considerably higher; and as corrosion continues to attack, the cross section is reduced. As stresses across that area increase, a fatigue failure results. This will be discussed later in this guide.
We have discussed chemical and electrochemical reactions in which positively charged hydrogen ions in an electrolyte are expelled from the system as hydrogen gas. However, all hydrogen may not leave. It is possible, even most probable in a sour well environment, that some of the hydrogen will enter the steel as either atomic or molecular hydrogen and diffuse into its structure. Once absorbed, the hydrogen molecules build pressure at crystallographic vacancies or discontinuities, such as voids, which generate microscopic cracks. This results in a brittle failure of the steel at stress levels considerably below the yield strength of the steel. The more acidic the well fluid, the more susceptible the steel is to hydrogen embrittlement. Since hydrogen embrittlement is related to the chemical and electrochemical reactions we described in sour wells, the term sulfide stress cracking is also used to describe this phenomenon.
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5
BASIC CONCEPTS OF SUCKER ROD CORROSION
Field Service Problems In this section we will discuss field problems that you will encounter with sucker rods and sucker rod couplings. We will also show examples to illustrate each mode of failure. Failure prevention of any component starts with a knowledge of failure mechanisms. To prevent failures, it is necessary to understand the many ways a product can fail. Each failure can be considered a valuable specimen from which to extract as much information as possible. The information can be used to improve product quality and skills in the application of sucker rods. Understanding product failures and being able to extend field service life is important to you, the salesman or the field engineer, and to the user who will benefit by receiving longer service from the equipment and by paying less for lifting.
Conditions that Cause Sucker Rod and Coupling Failures Examination of sucker rod failures reveals several enemies to dependable rod string operation. S.M. Bucaran, et aI, presented the paper (No. 55) NACE/72 "Proper Selection, Handling, and Protection of Downhole Materials—A Practical and Economical Approach" in which they stated that rod and coupling failures can be classified simply as caused by wear, corrosion, mechanical causes, or mishandling; and plenty of 1 cases involve two or more of the basic causes . Here is another list of causes for sucker rod and coupling failures: •
Overload condition
•
Application issues – – – – –
•
Rods in compression Rod wear Pumping speed Stuck pump Incomplete pump fillage
Corrosion problems – Water floods – Sour gas – CO2 injection
•
– Improper makeup procedures – Bent rods – Mechanical damage •
Inhibitors – Lack of inhibition – Poor inhibition program
•
Manufacturing defects – Mill defects – Forging defects – Threading defects
Types of Sucker Rod and Coupling Failures Metallurgical investigations of failed sucker rods typically reveal a fatigue pattern. The fatigue crack originates at the point of highest stress, and the stress peaks occur at the surface. Therefore, the fatigue failures are surface failures and the damage, or intrusion, is a sharp notch or roughness and a stress-raiser that starts the crack. Breaks in sucker rod pins have a similar appearance to fatigue failure, but they usually occur if the joint is not made up properly, a condition referred to as loss of displacement. A loss of displacement occurs when a connection loosens downhole. This can be caused by improper makeup procedures, applications issues, or mechanical damage. Coupling breaks will occur as a result of wear, corrosion, or loss of displacement. Hydrogen sulfide embrittlement failures have occurred in couplings with hardness greater than 23 Rockwell C. We will define and discuss four modes of sucker rod and coupling failures: •
•
•
•
© 1993-2016 Weatherford. All rights reserved.
Handling problems
Fatigue: the effects of stress concentration and corrosion Corrosion: pitting versus uniform attack, and the damaging effects of hydrogen sulfide embrittlement Wear: its effects on service life Tensile failure: not a service failure but an overload condition caused by carelessness
6
BASIC CONCEPTS OF SUCKER ROD CORROSION
Fatigue The word fatigue may suggest that metals become tired from supporting a load for a long time. Certainly metals do not tire in a biological sense; nor do they deteriorate as a direct result of supporting a constant load. However, they do fracture in a brittle manner when subjected to cyclical loading that varies sufficiently in intensity, even though one such cycle produces no detectable effect.
The process of fatigue consists of three stages (Figure 6): 1. Initial fatigue damage leading to crack initiation 2. Crack propagation until the remaining uncracked cross-section of a part becomes too weak to carry the imposed loads 3. Final, sudden fracture of the remaining cross-section
Fatigue is the progressive, localized, permanent structural change that occurs in a material subjected to repeated or fluctuating stresses that have a maximum value less than the tensile strength of the material. Fatigue fractures are caused by the simultaneous action of cyclic stress, tensile stress, and plastic strain, all three of which must be present. Although these three conditions are sufficient to cause fatigue, a host of other variables—such as temperature, crystal system of the metal, grain size, environment (corrosive or otherwise), metallurgical structure, and stress system—alter the conditions for fatigue. Figure 5 illustrates the three general types of fluctuating stresses that can cause fatigue.
Figure 6: Relative lengths of the various stages of a sucker rod body failure
Consequently, fatigue cracks are classified as brittle cracks, as identified by little or no evidence of ductility in the area adjacent to the second stage cracking. There is no necking or shear lips on the fracture as shown in the final stage of failure. Fatigue failures are always normal (90°) to the applied stress. Chevron marks, or a herringbone pattern, point to the fracture origin. Fatigue beach marks, clamshell marks, or families of striations may be observed with the unaided eye (Figure 7). The familiar beach marks can be observed macroscopically with a light microscope or under the electron microscope (Figure 8).
Figure 5: Typical fatigue stress cycles
Case 1 represents an idealized situation wherein stress fluctuates in a sinusoidal fashion from tensile to compression and the net resultant stress is zero. This is the most common form used to study fatigue in a laboratory, but it is also approached in service by a rotating shaft operating at a constant speed without overload.
Figure 7: The three stages of fatigue of a sucker rod body failure
In Case 2, we have the sinusoidal stress form but the resultant, or mean, stress is not zero. Case 3 shows an irregular or random stress cycle.
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7
BASIC CONCEPTS OF SUCKER ROD CORROSION
concentrated loads at a surface discontinuity. Such concentrated stresses produce local plastic strain (stress exceeds yield strength) that is critical in cyclic loading.
Figure 8: Field failure with visible beach marks
Fatigue beach marks are clearly evident with little or no magnification for lower strength, more ductile materials and for a variety of strength ranges under conditions of low load, high-cycle stresses. The distinguishable concoidal markings usually represent points of variation in the load environment. For higher strength materials and for high-load fatigue, the use of an optical or electron microscope is often required. With the close examination of a fatigue fracture face, much or all of the following information regarding the crack can be determined: 1. Point(s) of crack nucleation 2. Direction of crack growth 3. Size of prior crack 4. Relative magnitude of stress 5. Direction of loading (axial, bending, reverse bending, etc.)
In practice, prediction of the fatigue life of a material is complicated because the fatigue life is very sensitive to small changes in loading conditions, local stresses, service environment, and local characteristics of the material. However, the endurance limit of any material under cyclic stress is lower than its strength under static load. As load decreases, the number of cycles to failure increases until at some load, the number of cycles to failure becomes so large that we need not fear failure. Normally the endurance limit for sucker rods is defined as the maximum stress that the string will withstand without failure after ten million or more cycles of stress. Generally, when loads are low, only one crack is generated. Conversely, multiple cracking is a sign of high loads. In addition, the ratio of prior crack area to total cross-sectional area gives information about the magnitude of stress at final rupture. With the use of an electron microscope, direct measurements of striation spacing offers good insight into the stress environment during crack growth. The preparation of a metallographic specimen containing a crack is often helpful in identifying the crack mechanism. Fatigue cracks are invariably transgranular or transcrystalline and sometimes are branched. Fatigue striations are not evident, however, on profile. Bending fatigue failures can be divided into three classifications: one-way, two-way, and rotary. The fatigue crack formations associated with the type of bending load are shown in Figure 9.
Fatigue failures occur at apparently safe values of predetermined, calculated average stress over the crosssection of a sucker rod or coupling, and failure is caused by
Case
No Stress Concentration Low Overstress A
High Overstress B
Stress Condition Mild Stress Concentration Low Overstress C
High Overstress D
High Stress Concentration Low Overstress E
High Overstress F
1 One-way bending load
2 Two-way bending load
3 Reversed-bending loadrotation load Figure 9: Fracture appearances of fatigue failures in bending by Dr. Charles Upson, " Why Machine Parts Fail," Penton, Cleveland 13, Ohio
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8
BASIC CONCEPTS OF SUCKER ROD CORROSION
140
API Bulletin RP11BR "Recommended Practice for Care and Handling of Sucker Rods" recommends using a modified Goodman diagram (Figure 10) for determining the allowable range of stress for a string of sucker rods in noncorrosive service. Minimum Tensile Strength
130 120
T n
T 2
T 3
+
I S P , s s e r I S t S l e P b a , w s o l s l = A t r e
S , i s p 0 0 0 , 1 , t i m i l e c n a r u d n E
T 1.75
S u m i m i n M =
S A T 4
Rare cases
50% ratio
110 100 90 80 70 60
Normal for polished specimens
50 40
N I
S M
45° 0
–
0 S A
=
S A
=
(4T + M S ) SF (0.25T + 0.5625 S ) SF
S A
=
S A – SMIN
S A
=
Maximum available stress, PSI
S A
=
Maximum allowable range of stress, PSI
MIN
MIN
M = SMIN = SF = T
10 40
3
=
S er vi ce fa ct or Minimum tensil e strength, PSI
Hardness, Bhn 120 160 200 240 280 320 360 400 440 480 520 1.0 Mirror-polished 0.9 Fine-ground or commercially polished 0.8
0.6 0.5 0.4
0.1 0 60
Figure 11: Generalized relation of ultimate tensile strength (Su ) and fatigue strength (Sn ) Figure 12: Stress concentration factors for various surface conditions. This graph is a derating factor for influence of surface conditions on fatigue.
Sn
= 0.5(Su)(C1)(C0)(CS)
Sn
= Endurance limit
Su
= Ultimate strength
Sn /Su
= 0.5
C 1
= 1.0, 0.9, or 0.58 depending on whether the load is by bending, axial, or torsion, respectively
C 0
= A size factor, usually taken at 1.0 for diameters less than 0.4 inches and at 0.9 for diameters between 0.4 and 3.0 inches
C S
= Surface factor, which varies for the surface conditions shown in Figure 12
H o t - r o l l e d
0.3 0.2
80 100 120 140 160 180 200 220 240 260 Tensile strength, 1,000 psi, Su
M ac hi ne d
0.7
60
Slope of SA curve = 0.5625 Minimum stress, PSI (calculated or measured)
Figure 11 shows a number of variables that influence the fatigue properties. Note that surface notches and corrosion reduce fatigue strength. Figure 12 shows stress concentration factors for the various surface conditions.
S
Corroding specimens
2
Figure 10: Modified Goodman diagram for allowable stress and range of stress for sucker rods in noncorrosive service
C r o t c a f e c a f r u S
20
1
When:
T 3
Severely notched specimens
30
Corroded in tap water
As f o rg e d
Corroded in salt water 80 100 120 140 160 180 200 220 240 260 Tensile strength Su, ksi
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9
BASIC CONCEPTS OF SUCKER ROD CORROSION
Figure 13 shows the importance of limiting the system to low and intermediate hardness and indicates the importance of residual stress in fatigue; that is, materials with a greater endurance limit are higher in carbon and thus have a higher temperability. 160
H-11 Austempered
150 140
H-11 Conventional
130 i s p 0 0 0 , 1 – t i m i L e c n a r u d n E
A 120 110
Sucker rods have a hot-rolled surface (note derating factor for hot-rolled materials in Figure 12). For a sucker rod with 100,000-psi tensile strength, the endurance limit for complete stress reversals would be approximately 50,000 psi for polished test specimens and 30,000 psi for test specimens with a hot-rolled surface. A salt water environment further reduces fatigue strength. The API modified Goodman diagram (Figure 10) has been adjusted to indicate a derating for a hot-rolled, shot-blasted surface and a substantial safety factor of 2. The adjustment or derating for corrosion (service factor) is selected by each user as his experience indicates. Figure 14 indicates typical service factors for various environments.
A
B C
100 E, F
90
D
80
A = SAE 4063 B = SAE 5150 C = SAE 4052 D = SAE 4140 E = SAE 4340 F = SAE 2340
70 60 50
20
30
40
50
60
Rockwell “C” Hardness
Figure 13: Relation of hardness and fatigue strength for several steels
Corrosiveness/Derated Service Factor Environment
None 1.0
Mild 0.9
Moderate 0.8
Severe 0.7
H2S
0%
X < 10 ppm (0.001%)
10 ppm < X < 100 ppm (0.001% - 0.01%)
100 ppm < X (0.01% and greater)
CO2
0%
X < 250 ppm (0.025%)
250 ppm < 1500 ppm (0.025% - 0.15%)
1500 ppm < X (0.15% and greater)
Figure 14: Table of derating factors (NACE Standard MR0176-2000)
© 1993-2016 Weatherford. All rights reserved.
10
BASIC CONCEPTS OF SUCKER ROD CORROSION
The API modified Goodman diagram cuts off at neutral (zero) rather than at reversed stress because the rod string is susceptible to column buckling and should not be put into compression. To avoid exceeding the yield strength (plastic range), the upper boundary of the shaded portion in Figure 10 represents approximately 58% of the ultimate tensile strength, which is less than the yield strength. The API version of the Goodman diagram helps in visualizing the failure strength of a sucker rod string with a variety of loads. The relationship is shown in the shaded area of Figure 10: The top line on the shaded area represents the maximum stress and the bottom line represents the minimum stress with which the sucker rod can be loaded without fatigue failure in a noncorrosive environment. As the stress range (maximum stress minus the minimum stress) is reduced, the maximum stress and minimum stress can be increased. When the stress range is reduced to zero, the load is static. To operate in the safe range, it is necessary to determine the maximum allowable loading on the basis of calculated or measured minimum allowable loading.
Exercise #1 Assume a string of API Grade C sucker rods with a minimum tensile strength of 90,000 psi (T) is being used at a minimum downstroke stress of 10,000 psi (S min). At what peak polished rod stress (SA) can we operate this string in noncorrosive service (SF = 1)?
� =
4
+
SA = (90,000/4 + 10,000 x 0.5625) 1 SA = (22,500 + 5625) 1 SA = 28,125 PSI in noncorrosive service Converting to load for different size top rods: L = Load SA = Maximum available stress, psi Arod = Cross-sectional area of sucker rod body L = SA (Arod ) 2
5/8 in. — 28,125 psi × .307 in = 8,634 lb 2
Example
3/4 in. — 28,125 psi × .442 in = 12,431 lb
(Refer to notes covering calculations and Goodman diagram in Figure 10).
7/8 in. — 28,125 psi × .601 in = 16,903 lb
2
1 in.
2
— 28,125 psi × .785 in = 22,078 Ib
Example Allowable Sucker Rod Stress Determination Using Range of Stress
Exercise #2
Calculating maximum allowable stress:
What is the peak polished rod stress at which we can operate in H2S environments (remember SF for H2S is 0.80)?
� � =
=
4
4
�
+
+ 0.5625
Where:
=
4
+
= (90,000/4 + 10,000 × 0.5625) 0.80 = (22,500 + 5625) 0.80
SA = Maximum available stress, psi
SA = 22,500 lb stress in H2S service
M = Slope of SA curve = 0.5625
Exercise # 3
T = Minimum tensile strength, psi
For API Grade C, given a peak polished rod stress of 35,000 psi, calculate minimum allowable stress (Smin).
Per API 11B T = :
�−
Grade K = 90,000 psi
=
Grade C = 90,000 psi
4
÷ 0.5625
= (35,000 – 0.25 × 90,000) ÷ 0.5625
Grade D = 115,000 psi
= (35,000 – 22,500) ÷ 0.5625
Smin
= Minimum stress, psi (calculated or measured)
SF
= Service factor (noncorrosive = 1, H2S = 0.80)
= 22,222 psi in noncorrosive service
Calculating minimum allowable stress Smin:
− 4 =
÷ 0.5625
© 1993-2016 Weatherford. All rights reserved.
11
BASIC CONCEPTS OF SUCKER ROD CORROSION
To convert for different size top rods in corrosive service: 2
5/8 in. — 28,125 psi × 0.80 × 0.307 in. = 6,908 lb 2
3/4 in. — 28,125 psi × 0.80 × 0.442 in. = 9,945 lb 2
7/8 in. — 28,125 psi × 0.80 × 0.601 in. = 13,523 Ib 2
1 in. — 28,125 psi × 0.80 × 0.785 in. = 17,663 lb
cracking, but small pits with sharp roots are logical locations for crack initiation. There are several types of downhole corrosion and they affect sucker rods differently. However, all pumping wells produce fluids that are corrosive to some degree. Figures 15 and 16 show fatigue cracks in corrosion pits, one from a sour well (H2S) and the other from a sweet well (CO2).
Corrosion-Fatigue We have defined and discussed metal fatigue. We know that metals under cyclic loading have a limited useful strength in a noncorrosive environment and that the endurance limit (fatigue strength) is primarily dependent on tensile strength. Sucker rods that operate below the endurance limit in a noncorrosive environment will withstand an infinite number of pump strokes (stress reversals) before failure. These factors affect sucker rod fatigue: •
The range between minimum and maximum tensile stress: the wider the range, the lower the number of cycles to produce a fatigue failure
•
Pitting, stress cracking, and severe uniform corrosion
•
The environment in which the sucker rod is used
Figure 15: Start of fatigue cracks in corrosion pits in a sour well (H2S)
The environment has a very pronounced effect on fatigue strength. In laboratory tests, samples tested in air and in tap and salt water showed progressively lower fatigue strength. The more corrosive the environment, the lower the fatigue strength becomes. Refer to Figure 11 for derating factors for various surface conditions and environments. Corrosion combined with cyclic stress is more damaging than either corrosion or fatigue alone. The part played by corrosion in this type of degradation is extremely important. Without corrosion, fatigue failures would be greatly reduced, or very likely there would be no fatigue cracking except in the few cases in which fatigue failures were caused by mechanical notches. However, the presence of corrosion lowers the endurance limit of steel. There is actually no real endurance limit in a corrosive environment; whether the endurance will be higher or lower is dependent on the salt content of the water and the presence of oxygen, carbon dioxide, and hydrogen sulfide gases.
Figure 16: Start of fatigue cracks in corrosion pits of a well containing CO2 (sweet well)
Corrosion-fatigue failures can be considered similar to notch fatigue failures. The difference is that the corrosion-fatigue crack starts at the root of the corrosion pit, but notch fatigue starts at the root of mechanical defect. Corrosion-fatigue can be considered as a type of notch fatigue in which a point of stress concentration has been formed by a corrosion pit, and the fatigue progress is accelerated by the action of corrosion. Not all corrosion pits produce cracks at the root. General-type pitting typically does not produce sharp notches, but rather flat or roundedbottom pits. This type of pitting is less likely to produce
© 1993-2016 Weatherford. All rights reserved.
12
BASIC CONCEPTS OF SUCKER ROD CORROSION
Development of a Modified Goodman Diagram for Sucker Rods A fatigue test consists of placing a specimen in the fatigue machine, subjecting the specimen to a defined load (typically reverse bending by rotation), and running the machine until the specimen fails or until it has run in excess of ten million revolutions. The process for developing a modified Goodman diagram for sucker rods includes running a series of fatigue tests. When the test of the first specimen is complete and the number of stress reversals required to cause failure has been determined, an identical specimen is placed in the machine and tested to failure under a different load. This procedure is repeated several times using identical specimens and a different load each time. The results of these tests are plotted in terms of stress and number of cycles to fracture. A curve (called an S-N curve) similar to the one in Figure 17 is then drawn. 120
+
Smax s s e r t S
SR
0
Smin
– +
S A
s s e r t S
Smax
SR SM Smin
0
Figure 18: Variations in completely reversed cyclic stress Fluctuating stress about a mean stress is shown:
100
i s k , s s e r t s m u m i x a M
Control (no decarburization)
80
60 Decarburized
S A
= Stress Amplitude
SR
= Stress Range
SE
= Fatigue Strength
SM
= Mean Stress
Smax
= Maximum Stress
Smin
= Minimum Stress
SU
= Tensile Strength
40
20
104
105
106
107
100
Fatigue life, cycles
Figure 17: Effect of decarburization on fatigue strength of rotating beam specimens of SAE 4140 steel, tempered for normal hardness of Ro-48 [8].
The maximum stress that will not produce failure in the 7 material after ten million (10 ) cycles is referred to as the endurance limit . It has been determined that when steel is tested under normal reverse bending stress conditions, its fatigue strength (endurance limit) is approximately 50% of the tensile strength. The fatigue strength of steel with rough surface, as in hot-rolled sucker rods with no corrosion effects, is approximately 1/3 of the tensile strength when tested under reverse bending.
S m a x
50 A i s p 3 0 1 × E
S
B
SU SR
n e a M
S A SU
S
S
n i m
0 50 SM × 103
100
Tensile strength = 100,000 psi
The fatigue strengths under these various test conditions can be related by a Goodman diagram to show the maximum usable stress for sucker rod materials. It is important to note that Goodman diagrams are based on a linear relationship with the tensile strength, not on yield strength.
50
Endurance limit in reverse bending = 50,000 psi
Figure 19: The Goodman diagram, based on polished samples and reverse bending stress, helps to visualize the change of fatigue strength of a material under a variety of loads.
In Figure 19, the line AB represents the maximum stress with which a material can be loaded without fatigue failure. This stress level is at a minimum when the stress range, SR, is
© 1993-2016 Weatherford. All rights reserved.
13
BASIC CONCEPTS OF SUCKER ROD CORROSION
maximum (and is a complete reversal from tension to compression; the average stress is zero). As the stress range is reduced (left to right on diagram), the maximum stress can be increased and the minimum stress can be increased proportionately. When the stress range is reduced to zero, the maximum stress is increased to equal the tensile strength, SU, and this is a static load. The modified Goodman diagram in Figure 20 is based on as-produced sucker rods, and the endurance limit in reverse bending is T/3. Because sucker rods do not operate in compression, the diagram is shifted to the right. Point T/4 was selected as a safety factor because it is not practical to operate at T/2, which is a statistical value, and scatter in test results is to be expected. Point T/1.75 is an arbitrary value that represents about 57% of the tensile strength and is always below the yield strength.
T
T
questionable, the replacement of a worn part may be, particularly in the absence of established standards. In general, wear may be defined as damage to a solid surface caused by the removal or displacement of material by the mechanical action of a contacting solid, liquid, or gas. When a failure is caused by one type of wear, analysis may be relatively simple. However, many wear failures are caused by combined modes of wear. Sucker rods and couplings exhibit wear by one or a combination of the following modes: •
•
•
i ) ( p s s s t r e l e S b a l l o w A =
T 2 +
T 3
T 4
T 1.75
S A
•
i n
S m
0
–
T 3
•
Abrasion: Displacement of material from a surface by contact with hard projections on a mating surface (metalto-metal contact) or by hard particles, such as sand and corrosion products, trapped between two sliding surfaces (Figure 21) Adhesion wear: Wear occurs when two metallic surfaces slide against each other under pressure (also described as scoring, galling, seizing and scuffing) Erosive wear: Abrasive wear involving loss of surface material by contact with a fluid that contains foreign matter or particles Corrosive wear: A mode of wear in which chemical or electrochemical reaction contributes to the wear rate (Figure 22); for example, the pitting caused by CO2 corrosion. Erosion-Corrosion: A type of wear in which there is relative movement between a surface and a corrosive fluid (Figure 23). The fluid may or may not contain abrasive particles. In cavitation erosion, the repeated formation and collapse of vapor bubbles at the surface imposes contact stresses that may cause pitting or spalling.
T = SU = Tensile
Figure 20: Modified Goodman diagram based on as-produced sucker rods for which the endurance limit in reversed bending is T/3
Wear The Handbook on Failure Analysis and Prevention from the American Society for Metal describes wear as a surface phenomenon that occurs by displacement and detachment of material. Because wear usually implies a progressive loss of weight and alteration of dimensions over a period of time, wear problems generally differ from those entailing 3 outright breakage. Although worn parts may break, it is more likely that a worn part will be removed from service because it no longer performs satisfactorily or because its performance is marginal. Although the replacement of a broken part is not
© 1993-2016 Weatherford. All rights reserved.
Figure 21: Extreme abrasion wear on a coupling
14
BASIC CONCEPTS OF SUCKER ROD CORROSION
Figure 22: Corrosive wear of a rod body. The rod has rubbed against the tubing, exposed clean metal, and initiated localized corrosion attack and subsequent pitting and grooving.
Figure 23: A rod and guide exhibiting erosion-corrosion of the rod from well fluids passing through the gap in the guide.
Tensile Failures Typically, tensile failure of a sucker rod is not a service-oriented failure. Such failures typically have one of these causes: •
•
•
Tensile stress overload of the string while trying to free a stuck pump Pulling the pin off the rod upset while over-tightening the joint with uncalibrated power tongs Torsional overload in a PCP application
When load exceeds the tensile strength of the rod or pin, the failure is identifiable by the necked-down area (shown in Figure 24) and cup and cone (Figures 25 and 26) with the 45° shear lip.
Figure 24: A photo of a sucker rod pin overtightened using power tongs. Note the necking down of the pin undercut.
© 1993-2016 Weatherford. All rights reserved.
Figure 25: Tensile failure showing the cone breakface sheared at 45°. This was a lab failure created during routine testing.
Figure 26: Sucker rod pulled in two when a stuck sucker rod string was overpulled.
15
BASIC CONCEPTS OF SUCKER ROD CORROSION
Designing to Reduce Failures We have discussed sucker rod and sucker rod coupling failure mechanisms and presented examples of each failure mode. Understanding of the factors contributing to service failures is necessary to control them. Corrective action can improve the service life of the equipment. This chapter will describe various conditions that cause sucker rod and coupling failures and steps that are available for preventing future failures. In most cases, illustrations will be shown that represent the primary cause of each failure mechanism.
Mill Defects When a sucker rod fails prematurely, you will often hear the customer say "bad steel," "faulty material," or "that string was in service only a few months, and the string it replaced lasted 5 to 6 years—must be a bad heat of steel." These opinions are rarely justified because failures caused by faulty material almost never happen. However, if a customer does experience a failure related to a mill defect, the cause will most likely be a surface defect such as a scab or sliver. Figure 27 shows a scab (sliver), a loose or torn segment of material or debris rolled into the surface of the bar. One end of these particles of metal is metallurgically bonded to the body of the rod. The remaining section is rolled into the bar surface but only attached physically. If the particle is dislodged, a deep surface pit remains, normally with a sharp root. The scabs and slivers act as notches in the surface of the metal, reducing the fatigue-endurance limit by a ratio of perhaps two or three to one. The condition then develops into a notch fatigue failure.
Figure 27: Surface damage caused by a sliver
For mill defects, the obvious corrective measure is to inspect the affected string, preferably by flux-leakage magnetic equipment, and to discard rods with surface defects. Inspection service companies provide this service. Also most manufacturers of sucker rods have inspection equipment in house for inspection of bars before processing.
Manufacturing Problems In rare cases, sucker rods are shipped with manufacturing defects. The most common defects are forging laps in the bead, undersize pin threads, oversize coupling threads, forged-in scale pits in the rod body adjacent to the bead, forging laps on the square, and deep steel stamp marks in the flats of the upset square. Please bear in mind that when we talk about manufacturing defects, we are including all sucker rod manufacturers. No manufacturer is exempt. You will find, however, that failures caused by manufacturing defects are very uncommon. A deep stamp mark in the steel can result in a fatigue crack that progresses to ultimate failure. The stamp mark is a sharp notch that raises local stress and increases surface stresses in the notch to the point of exceeding the endurance limit. An example can be seen in Figure 28. This type of problem can be controlled by using steel stamps with less sharpness to reduce penetration in the square and reduce root sharpness at the bottom of the stamp mark. Sometimes early failures in surface notches in the upset are caused by a combination of high loading and the notch effect. In such cases, redesigning the string to a lower
© 1993-2016 Weatherford. All rights reserved.
16
BASIC CONCEPTS OF SUCKER ROD CORROSION
operating stress level will eliminate the failure in the square and improve service life.
Figure 29: Deformation in the surface of a rod and the resulting fatigue crack that led to tensile failure.
Improper Joint Makeup For proper makeup, the API sucker rod joint is designed so that the pin is in tension. The important factor is that the joint must be tightened sufficiently to induce a preload in the pin—a preload high enough to prevent the contact faces from separating when the string is under its maximum tensile load. If the joint has insufficient torque, the first full thread root will not only be subjected to a high range of stress, but will also be exposed to bending, as seen in Figure 30. Early failure occurs if the rod string carries any appreciable load. Figure 28: Classic example of a notch fatigue failure
Handling Problems Mishandling is a factor in rod and coupling failures. Examples include mechanical damage to the rod surface, improper joint makeup, bending or kinking the rods, and hammering the surface of couplings. In most cases, damage by mishandling could have been avoided or the damaged part could have been discarded to eliminate the possibility of an early failure. Training of field personnel will reduce handling-related problems. Field personnel can support users by keeping them informed of any handling-related failures. Field personnel might also suggest a course of action that could be effective in correcting handling problems.
Mechanical Damage Usually, mechanical damage is the result of a permanent deformation in the surface of the rod. Figure 29 shows an example in which fatigue started at the root of the damage and progressed to a depth at which the rod cross-section could not support the operating load. Figure 30: Results of improper makeup of coupling to rod. The photo on the top shows damage to the first full thread root; the photo on the bottom shows damage to the coupling. Both failures were caused by under-torquing.
© 1993-2016 Weatherford. All rights reserved.
17
BASIC CONCEPTS OF SUCKER ROD CORROSION
Applying the proper circumferential displacement to the joint during makeup is highly recommended. When power tongs are out of calibration, too much torque can be applied during makeup, and the pin can break under tensile overload. Loose joints not only cause pin fatigue failures, but the separation of the pin-coupling faces also allows corrosive fluids to enter the coupling and initiate corrosion fatigue failures of the coupling or pin. Refer to API Publication RPIIBR, "Recommended Practice for Care and Handling of Sucker Rods," for proper methods to determine correct joint makeup and to control connection failures. Separation of joint faces has also been attributed to unscrewing of the joint. Without the drag of friction on the mating surfaces and with the smooth finish or rolled threads, each stroke permits a little rotation until the rod string separates with no apparent damage to either pin or coupling.
measured by dial indicator riding on the machined pin shoulder, the maximum allowance is 0.130 TIR. Most bent rods are caused by rough handling in shipment, by improper handling while running the string in and out of the hole, or by dropping the string (Figure 32). Bending can also be caused by fluid pound, gas pound, or tagging bottom. Failure caused by bending is identified by these features: •
All fatigue cracks are on one side of the bar.
•
Corrosion pits may or may not be present.
•
•
The bar is visibly bent and failure started on the concave side. The break face is not perpendicular to the rod body.
Bent Rods Straightness is important on heavily loaded rods. A bent rod (Figure 31) will produce a high order of cyclic stress variations and cause an early failure. Under load, any degree of bend imposes higher tensile stresses on the inside, or concave side, of the bend compared to the same load on a straight rod. A string operating at its maximum loading will be over stressed at the concave side of the bend. When the endurance limit is exceeded, the string will fail from fatigue that started on the inside of the bend.
Figure 32: Bent sucker rods most likely damaged by dropping the rod string
Rods that are bent to the maximum API recommended straightness, when under load, are stressed about 10% more on the concave side (Figure 33).
Figure 31: A load of rods, some bent
API Specification 11B Twenty-seventh Edition, November 1, 2011, for sucker rods, Page 24, Section A.6.1, “Straightness and Surface Finishes” specifies body and end straightness for 5/8-in. to 1 1/8-in.-diameter rods: •
Body straightness: Within any 12 inches, the maximum allowable bend is 0.065 inch (0.130 TIR)*.
*Total indicator run out (TIR) is the total dial gauge deflection measured at the rod surface as the rod is rotated 360°. The bend of TIR valves is twice the amount measured by straight edge. •
End straightness: As measured by supporting the rod body at a distance of 6 inches from the rod pin shoulder and
© 1993-2016 Weatherford. All rights reserved.
18
BASIC CONCEPTS OF SUCKER ROD CORROSION
P
L = Length in inches P = Load in pounds E = Mod. of elas. 29×10 6 ∆ = Out-of-straignt (in inches) d = Diameter (inches) of sucker rod A = Square area of sucker rod
d L
[Stress at Concave Area = ] (1)
∆
Stress =
4∆Ed L2
+
P A
As the rod guide contacts the tubing surface, it can remove corrosion inhibitor or any protective scale that has formed. This will increase the corrosion rate at the contact location. When this occurs, it typically appears as a groove the width of the rod guide vane. It can also appear as though the rod guide has worn through the tubing. Rod guides will add weight to the rod string and increase the contact friction on the tubing. This should be taken into consideration when using guides. Proper rod guide design and placement are critical. An improperly designed rod and guide system will shorten the run life of the sucker rod string.
Poor Pumping Conditions
X [Stress Concentration Factor = ] (2)
y P
SCF = 1 +
1.414 • 10 8 • ∆ • d 3 PL2 + 1.767 • 10 7 (d4)
Sucker-rod body wear and coupling wear can be indications of poor pumping conditions: •
Figure 33: Calculation of stress in concave area
Guided Sucker Rods Sucker rod guides have evolved from simple metal scrapers to a highly engineered thermoplastic product. With the large increase in directional and horizontal wells, rod guides are used not only to remove paraffin from tubing and sucker rods, but also to protect and stabilize the sucker rod string. Rod guides can significantly increase the life of a sucker rod string by eliminating rod and tubing wear. However, there are some disadvantages to using rod guides.
•
•
•
Application Issues During the pump cycle, all rod guides disrupt the fluid flow, some much worse than others. This causes an increase in fluid velocity and a low pressure zone. Because of this disruption, a low pressure zone is created on the upper side (closest to the surface) of the rod guide, as seen in Figure 34. The increase in fluid velocity can cause fluid erosion, and the low-pressure zone can cause erosion-corrosion and CO2 breakout.
•
Poor pumping speeds can induce compression loads on the pump downstroke and cause buckling and rod wear (Figures 35 and 36). This condition can be corrected by proper design of the rod string and reducing the pumping speed. Deviated wells cause coupling and rod wear (Figure 37). Wear is also an indication of cork-screwed tubing from improper tension on the anchor or packer. Fluid and gas pounding can cause rod buckling. Reduce pump size or reduce pumping speed to correct fluid pound. Gas interface can be reduced with a pressure regulator or application of a specific downhole pump and gas separator. Coupling wear can occur in a rod string with a rod rotator installed but no rod guide protection (Figure 38).
Figure 35: An example of rod wear.
Figure 34: Rod guides can increase the corrosion rates on production tubing.
© 1993-2016 Weatherford. All rights reserved.
Figure 36: Cracking along one side of the rod (flexing) caused by improper pumping conditions
19
BASIC CONCEPTS OF SUCKER ROD CORROSION
Carbon Dioxide Corrosion
Figure 37: Extreme coupling and rod wear that most likely was caused by running through a deviated well without rod guide protection. A spray metal coupling can be used when wear or corrosion is a problem. Caution should be taken when using spray metal couplings.
Pits created by carbon dioxide corrosion are normally deep with sharp edges and round bottoms as in Figure 39. The scale is iron carbonate, which is hard and grey to black in color. Pits may connect or channel in high fluidflow environments.
Figure 39: Corrosion damage to sucker rods by a carbon d ioxide (sweet corrosion) environment
Hydrogen Sulfide Corrosion Figure 38: Coupling wear in a rod string with a rod rotator installed but no rod guide protection
Corrosion Problems As discussed earlier, the word corrosion denotes destruction of metal by chemical or electrochemical action. Chemical corrosion, although starting rapidly, often slows as soon as an obstructive layer of corrosion products forms upon the metal surface. If, however, this corrosion product is continuously being cracked by bending or being removed by rubbing or other mechanical action, corrosion will continue unchecked at its original rapid rate. Familiar examples of this conjoint action on sucker rods are corrosion fatigue (in which cyclic stress ruptures the corrosion by-product layer), down-hole wear, and impingement attack by gas and fluid.
The iron sulfide produced by the action of hydrogen sulfide and water on steel typically adheres to the steel surface as a black powder or scale. The scale tends to cause a local acceleration of corrosion because the iron sulfide is cathodic to the steel. The pits are usually scattered on the metal surface and are saucer-shaped with round edges (Figure 40). Cracks will form in the root of the pit. When placed in dilute hydrochloric acid, the corrosion by-product (iron sulfide) will release an odor like that of rotten eggs. The hydrogen released in the reaction enters the steel to cause embrittlement or to form molecular hydrogen, which leads to blisters and cracks.
The major corrosives encountered in oil wells are carbon dioxide, hydrogen sulfide, and oxygen dissolved in water. Practically all well fluids produced by sub-surface pumps and rods are corrosive to some degree. The corrosivity varies not only from field to field, but also from well to well in the same 4 field. It also varies with time in any well. The different types of corrosion are generally characterized by pit shape and scale formation.
Figure 40: Corrosion damage by a hydrogen sulfide environment (sour corrosion)
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20
BASIC CONCEPTS OF SUCKER ROD CORROSION
MIC (Microbiologically Influenced Corrosion) MIC is a form of corrosion that originates from a large growth of bacteria that can live in oxygen-rich (aerobic) or oxygen-free (anaerobic) environments. Downhole environments tend to be oxygen free, so most MIC found in the wells are anaerobic. MIC tends to be localized, so large isolated pits can form on sucker rods. Two types of MIC are typically found downhole: sulfate reducing bacteria (SRB) and acid producing bacteria (APB). Both forms can cause significant damage if left untreated. SRB generate hydrogen sulfide and contribute to localized corrosion by their ability to grow in the absence of oxygen. The hydrogen sulfide reacts with iron in solution to form iron sulfide (FeS) precipitate and scale. The FeS scale is cathodic to steel (Figure 41), SRB corrosion pits tend to be isolated and to have shallow bottoms with soft edges. Many times SRB corrosion creates worm-like pits surrounding the larger pit.
Figure 43: Example of APB corrosion
Oxygen Corrosion Subsurface equipment in oil wells is subject to oxygen corrosion only if oxygen from the air is introduced into the well. The presence of carbon dioxide or hydrogen sulfide increases the rate of oxygen corrosion. Corrosion of downhole equipment in the oxygen environment usually shows a general form of attack, sometimes producing large, shallow, flat-bottom pits (Figure 44).
Figure 41: The FeS scale is cathodic to steel, resulting in corrosion of scale-free areas which, in turn, adds more iron to the solution.
Acid-producing bacteria produce organic acids as a byproduct that reduces the pH, which can dissolve the sucker rod. APB corrosion pits tend to interconnect with sharpedged, flat-bottomed pits. Figures 42 and 43 show examples of both types of MIC corrosion.
Figure 42: Example of SRB corrosion
Figure 44: A typical form of oxygen corrosion. Localized attack may result in deep pitting, and t he corrosion by-product is ferric oxide.
Remedial measures Improving corrosion-fatigue life of sucker rods requires inhibiting corrosive wells, which also gives added protection to the tubing string and the well casing. Because corrosion is a surface reaction, any modification of the steel-corrosive media interface will affect the rate of corrosion. When added to a corrosive system, specific chemicals, called inhibitors, modify the interface to reduce the corrosion rate. All of the major inhibitor suppliers can furnish effective inhibitors and proper application for reducing corrosion in most fields. Even under the best conditions, however, inhibitors will not be 100% effective. Protective coatings, such as epoxy, have been used with some success on sucker rods. The difficulty is achieving a coating free from pin holes and handling damage. Spray metal couplings have been used successfully for many years to reduce corrosion rates. Oxygen corrosion in oil wells is best controlled by the exclusion of oxygen. The casing valve should always be closed to the atmosphere. If production is reduced by closing the casing, then a small check valve or ball and seat should be installed on the casing. This will allow gas to vent to the
© 1993-2016 Weatherford. All rights reserved.
21
BASIC CONCEPTS OF SUCKER ROD CORROSION
atmosphere by holding only an ounce or two of pressure to exclude oxygen from the annulus. The beneficial effects of lowering maximum stress levels in a corrosive environment have been discussed previously. Using alloy rods in place of carbon or carbon manganese rods has been successful as a means of improving rod life in a corrosive environment, but no sucker rod is impervious to corrosion. The most reliable way to avoid sulfide stress cracking is to use non-susceptible materials. If high-strength rods are necessary, then an effective inhibition program should be used.
Load Rod Type
MD
D
Problems Caused by a Stuck Pump Pulling on the rod string to unseat a stuck pump can cause accidental overload, excessive rod stretch, or tensile breakage. Excessive stretch occurs when the load exceeds the yield strength. Breakage occurs as the load increases from beyond the yield strength to the tensile strength. If sucker rods are permanently stretched by overload, there will most likely be localized, external, and perhaps internal damage that may lead to early failures. Consequently, the affected rods should be removed from service.
KD
Grade HD T66/XD
Calculating the maximum allowable pull can prevent stretching and tensile failure (Table 1). S67 67D
S87
S88
EL® rod
Size (in.)
(lb)
(DaN)
5/8
23,400
10,400
3/4
33,800
15,000
7/8
45,900
20,400
1
60,000
26,600
5/8
27,600
12,200
3/4
39,700
17,600
7/8
54,100
24,000
1
70,600
31,400
1-1/8
89,400
39,700
3/4
37,700
16,800
7/8
51,400
22,800
1
67,100
29,800
1-1/8
84,900
37,700
3/4
45,700
20,300
7/8
62,200
27,600
1
81,200
36,100
1-1/8
102,800
45,700
3/4
43,700
19,400
7/8
59,500
26,400
1
77,700
34,500
1-1/8
98,400
43,700
3/4
45,700
20,300
7/8
62,200
27,600
1
81,200
36,100
3/4
51,600
22,900
7/8
70,300
31,200
1
91,800
40,800
1-1/8
116,200
51,700
5/8
35,900
15,900
3/4
51,600
22,900
7/8
70,300
31,200
1
91,800
40,800
1-1/8
116,200
51,700
Table 1: Maximum weight indicator pull (load) that can be applied to a stuck sucker-rod string
© 1993-2016 Weatherford. All rights reserved.
22
BASIC CONCEPTS OF SUCKER ROD CORROSION
before rerunning. If the pump cannot be unseated, the tubing should be pulled and rods backed off.
Weight Size (in.)
(lb/ft)
(kg/m)
5/8
1.114
1.657
3/4
1.634
2.432
7/8
2.224
3.310
1
2.904
4.322
1-1/8
3.676
5.471
Table 2: Weight of sucker rods per foot Note: The ratings are based on 90% of the minimum yield strength for a sucker-rod string in “like new” condition. The maximum pull should be reached with a steady pull and not a shock load. For a tapered string, calculate the weight of the sucker rod above the smallest and lowest section, and add the calculated weight to the value tabulated here for the type and size of the lower section. For a single-taper sucker-rod string, the values tabulated here are the maximum pull.
Hammering the Surface of Couplings When pulling a well or when a coupling needs to be removed after installation, many rig crews “warm up” the coupling by hammering. Hammer blows cause mechanical damage to the coupling and can induce cracking. The cracks can be stress raisers and sites for corrosion fatigue. Damage from hammering can become points where localized corrosion will start its attack. Hammering the faces of any coupling may result in an improper joint makeup that can cause pin fatigue. Hammering on spray metal couplings causes surface cracks in the hard surfacing (Figure 45) and will cause localized corrosion and fatigue. Any coupling that has been hammered on should be discarded.
Calculations SF – safety factor Sa – allowable stress (psi) Sy – yield strength of sucker rod (psi) 2
A – cross-sectional area of sucker rod (in ) L – load in lbs Sa = Sy x SF L = Sa x A weight of rods = weight per foot (see Table 2) x length of rod section Sy – sucker rod maximum yield strength (psi) Type of Rod
API C
Minimum yield strength
60,000 PSI
Smallest rod
3/4 in.
500 feet of 7/8-in. rods above the top 3/4-in. rod 1. 60,000 psi × 0.90 = 54,000 psi 2. 54,000 psi × 0.442 sq. in. = 23,868 lb 3. Weight of 7/8-in. rods above the top 3/4-in. rod 500 ft × 2.224 lb/ft = 1,112 lb 4. Maximum allowable pull in pounds 25,194 + 1,112 = 24,980 lb
Figure 45: Spray metal coupling with damage caused by ha mmer blows to the surface
Thread Galling Thread galling can occur because of damaged or dirty threads (Figure 46). Joints seldom cross-thread because the pin must be aligned in the coupling recess before the first threads engage. Cross-threading, however, may be possible when power tongs are used in field assembly and the threads are damaged during stubbing. Coupling and pin threads must be clean and lubricated before assembly; if power tongs are used, the API RP11BR recommendation for sucker rod joint makeup should be followed.
Rods that have been in service for a length of time may be damaged by corrosion or corrosion-fatigue to a degree that breakage will occur when the maximum calculated loads are applied. Under these conditions, the string should be thoroughly inspected, and affected rods should be discarded
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Failure Analysis In discussing failure analysis work, you must consider failure prevention. Engineering teamwork is required to create the kind of reliability that will give a product a superior advantage over its competitors. However, a vital ingredient in attaining such reliability is an adequate method for analyzing the inevitable failures that occur during engineering tests or during service. You must also divide your products into two categories: (1) downhole equipment with no threat of injury or life and (2) surface equipment for which failure could be a threat to life or could cause injury.
Figure 46: Thread galling during makeup. This rod has been overtightened, which stripped the threads and deformed the makeup face.
All parts have a finite life. Acceptable limits are mostly established by the user, economic situation, environment, and competition. Remember that when a part fails, there is a reason for failure and there are corrective measures. This chapter is concerned primarily with general procedures, techniques, and precautions used in the investigation and analysis of metallurgical failures that occur in service. Figure 47 is a schematic of the stages of failure analysis.
Figure 47: Stages of a failure analysis
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Collection of Background Data and Selection
Non -Destructive Testing (NDT)
Failure investigation should be directed first at gaining a good understanding of the conditions under which the part was operating. The investigator must know as much as possible about the manufacturing processes and service histories of the failed component and must reconstruct insofar as possible the sequence of events leading to the failure. Unfortunately, in the majority of instances, the investigator will receive a failed part with little information about its history and operating conditions. In such cases, the physical evidence must be the sole basis for the analysis.
NDT Method
Service history depends on how detailed and thorough the recordkeeping was before the failure. Service history should include environmental details, such as normal or abnormal loading, accidental overloads, cyclic loads, temperature, temperature gradients, and corrosive environment. When service data is sparse, the analyst must deduce service conditions, and much depends on his skill and judgment because misleading deductions can be more harmful than the absence of information.
•
Radiography •
•
Ultrasonic •
Dye penetrate
Study of the Fracture Where fractures are involved, the next step in preliminary examination should be general photography of the entire fractured part, including broken pieces, to record their size and condition and to show how the fracture is related to the components. Next should be a careful examination of the fracture faces to determine the areas of prime interest and what magnification is needed to bring out fine details. Photograph the fracture face at the magnification determined to give the most revealing details.
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Inclusions, porosity, cracks Uses high-frequency sonar to find surface and subsurface defects Inclusions, porosity, thickness of material, position of defects Uses a die to penetrate open defects
•
Surface cracks and porosity
•
Eddy current
Measures differences in radiation absorption
•
Magnetic particle
Preliminary Examination The failed sample must be visually examined and documented photographically to create a permanent record of the evidence for later reference in light of new information that may become available. The failed parts should be examined before any surface cleaning to document the status of the surface condition; for example, rust and scale that indicate the type of environment in which the part was operating. The visual examination typically will allow the investigator to identify the mode of fracture (brittle, ductile, fatigue, etc.), points of initiation, and direction of propagation. Visual examination can be aided by low magnification tools such as a stereomicroscope. Photographic documentation should place particular importance on fracture surfaces and surface defects. The use of normal shadows can give depth to a surface, which makes it easier to photograph and to direct attention to important details.
Capabilities
Uses a magnetic field and iron powder to locate surface and near-surface defects
•
Surface cracks and defects
•
Based on magnetic induction
•
Measures conductivity, magnetic permeability, physical dimensions, cracks, porosity, and inclusions
Table 3: Nondestructive Tests
Mechanical Testing Hardness testing, the simplest of the mechanical tests, is often the most versatile tool available to the failure analyst: •
Assists in evaluating heat treatment (hardness requirements)
•
Enables an approximation of the tensile strength of steel
•
Detects work hardening
•
Detects softening or hardening caused by overheating, by decarburization, or by carbon or nitrogen pickup.
Two other useful mechanical tests are tensile and impact tests and ductile-to-brittle transition tests. Remember that the effects of size in fatigue stress-corrosion and hydrogen embrittlement testing are not well understood. However, on the basis of the limited evidence available, it appears that resistance to these failure processes decreases as specimen size increases.
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Selection and Preservation of Fracture Surfaces Preservation of fracture surfaces is important to prevent important evidence from being destroyed. Usually protection during shipment will include wrapping samples in cloth, cotton covering, or other suitable soft material. Avoid contact of the fracture with chemicals, including water, that could result in corrosion damage.
Sectioning
shown in Figure 48. The direction of crack growth is almost always away from the tips of the chevrons. Chevron marks occur because nearly all cracks are stopped at an early stage in their development; as the crack front expands, the traces of the steps form chevron marks. The photograph in Figure 48 shows the fracture surface of a 5-in.-diameter, seal stem, tubular section. As indicated by the arrows, the fracture reveals multiple crack propagation paths depicted by the chevron marks.
When sectioning specimens, use these techniques to protect the fracture area: •
Flame cut far away from fracture
•
Dry saw
•
Carbide cutoff with liquid coolant
Secondary Cracks When a primary fracture has been damaged or corroded such that most of the evidence is obliterated, it is desirable to open any secondary cracks to expose their fracture surfaces for examination. Secondary cracks may provide more information than the primary fracture.
Macroscopic Examination of Fracture Surfaces Examination of fracture surfaces at 1× to 100× diameter can be done with the unaided eye, a hand lens, or a low-power stereoscopic microscope. Occasionally it may be advantageous to use a scanning electron microscope at low magnifications. Extensive information can be obtained from examining a fracture surface at low-power magnification. Fracture surfaces may give an indication of the stress system that produced failure. Failure in monotonic tension produces a flat (square) fracture normal to the maximum tensile stress under plane-strain conditions and to a slant (shear) fracture at about 45° if plane-stress conditions prevail. Because pure plane-strain and pure plane-stress conditions are ideal situations that seldom occur in service, many fractures are flat at the center but surrounded by a picture frame of slant fractures. The slant fracturing occurs because conditions approximating plane strain operate at the center of the specimen but relax toward plane-stress near the surfaces. An example of this behavior is the familiar cup-and-cone tensile fracture. In thin sheets or small-diameter rods, full slant fractures may occur because axial stresses are relaxed by plastic deformation that impedes plain-strain from developing, making the crack propagation easier in a slant direction.
Figure 48: Chevron patterns point to the multiple origins of the fracture.
Where fracture surfaces show both flat and slant structures, it may be generally concluded that the flat fracture occurred first. Conversely, if a fracture has begun at a free surface, the fracture-origin area is usually characterized by a total absence of slant fracture or shear lip. Low-power examination of fracture surfaces often reveals regions that have a different texture from the region of final failure. Fatigue, stress corrosion, and hydrogen embrittlement fractures may all show these differences. Figure 49, which shows the fracture surface at the sucker rod body, is an excellent example of the type of information that can be obtained by macroscopic examination. The chevron marks clearly indicate that the fracture origin is at the point marked by the arrow. This region, unlike the rest of the fracture, has no shear lip. The flat surface suggests that the stress causing the failure was tension parallel to the length of the rod. Note the difference in texture.
Macroscopic examination can usually determine the direction of crack growth and hence the origin of failure. With brittle, flat fractures, determination depends largely on the fracture surface showing "chevron marks" of the type
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Mode of Fracture
Typical Fracture Surface Characteristics Cup and cone
Ductile
Dimples Dull surface Inclusion at the bottom of the dimple
Brittle intergranular
Shiny Grain boundary cracking Shiny
Brittle transgranular
Cleavage fractures
Figure 49: Fracture surface of a sucker rod at approximately actual size, showing point of initiation (at arrow) Chevron marks, and development of shear lips.
Flat
Metallographic Examination
Striations (SEM)
Metallographic examination of polished and etched sections by optical microscopy is a vital part of failure investigation to determine: •
Class of material and structure
•
Whether abnormalities are present
•
Heat treatment
•
Corrosion, oxidation, work hardening of surfaces
•
•
Characteristics of any cracks and their mode of propagation. Location of sample in respect to fracture origin.
Fracture Classifications Although a satisfactory classification of failures involving fractures does not exist, fractures will be classified in terms of their growth mechanism. Crack initiation will not be considered. Table 4 lists different modes of fractures and characteristics of each.
Benchmarks
Fatigue
Initiation sites Propagation area Zone of final fracture
Table 4: Modes of fractures and their characteristics
Ductile Fracture Ductile fractures are characterized by tearing of metal and gross plastic deformation. Ductile tensile fractures in most materials have a gray, fibrous appearance and are classified on a macroscopic scale as flat-face (square) or shearface (slant) fractures. Flat-face tensile fractures in ductile materials are produced under plane-strain conditions (that is, in thick sections) with necking. These fractures typically occur normal to the direction of loading, and some shear lip is formed at the junction of the fracture surface and the part surface. Microscopic examination at >100× of flat-face tensile fractures in ductile materials will reveal equiaxed dimples in the flat-face region.
Brittle fractures There are two types of brittle fractures: transgranular clearage and intergranular. The transgranular facets observed on brittle fractures are produced by clearage along numerous parallel crystallographic planes, thus creating a terraced fracture surface. The intergranular fractures are grain surfaces that have been exposed by crack propagation along grain boundaries (a rock candy fracture surface).
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BASIC CONCEPTS OF SUCKER ROD CORROSION
Fatigue Fracture Fatigue fractures result from repeated or cyclic stresses, each of which may be substantially below the nominal yield strength. The most noticeable macroscopic features of classic fatigue-fracture surfaces are the progression marks (also known as bench marks, clamshell marks, or tide marks). Little macroscopic ductility is associated with fatigue fracture. Microscopically, surfaces of fatigue fractures are characterized by presence of striations, each of which is produced by a single cycle of stress; however, every cycle does not produce a striation.
Stress-Corrosion Cracking Stress-corrosion cracking is a mechanical-environmental failure process in which mechanical stress and chemical attack combine in the initiation and propagation of fracture in a metal part. Stress-corrosion cracks may be intergranular, transgranular, or a combination of both. Transgranular fractures that show branching are typical of stress-corrosion cracking of austenitic stainless steels of an 18cr-8 ni type. Figures 50 and 51 show examples of stresscorrosion cracking.
Figure 51: Transcrystalline stress corrosion cracking in a Type 316L stainless steel gas lift valve assembly at 50X magnification; solution etched using Fry’s reagent
Embrittlement by Liquid Metals Embrittlement can occur as a result of liquid metal around grain boundaries, that is, penetration of copper alloys by mercury and penetration of steel by molten tine and cadmium. Polished samples or polished and etched sections can be examined by microscope. Positive identification is possible by electron-microprobe analysis.
Hydrogen embrittlement (sulfide cracking) Positive identification of hydrogen embrittlement is often difficult, and it is frequently impossible to differentiate between hydrogen-induced, delayed fracture and stresscorrosion-cracking fracture. Analysis should include the following:
Figure 50: Intercrystalline stress corrosion cracks in precipitation hardened stainless steel UNS S17400 alloy at 300X magnification; solution etched using Vilella’s reagent
•
Check hardness
•
Check for H2S
•
Run metallographic examination
Chemical Analysis Perform chemical analysis to confirm analysis of materials. Unreported gaseous elements (or interstitial) have profound effects on mechanical properties of some metals. In steel the effects of oxygen, nitrogen, and hydrogen are of major importance. Oxygen and nitrogen may cause strain aging and quench aging. Hydrogen may induce embrittlement.
Failures Resulting From Corrosion Damage Various forms of corrosion attack render parts inoperable. These modes were discussed earlier in this document.
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BASIC CONCEPTS OF SUCKER ROD CORROSION
References 1.
Bucaram, S.M., Byors, H.G., and Kaplan, M. “Proper Selection, Handling, and Protection of Downhole Materials – A Practical and Economical Approach.” Paper No. 55. Corrosion/72
2.
Madayag. 1969. “Metal Fatigue, Theory and Design.” Wiley. New York. 2─126.
3.
“Failure Analysis and Prevention.” Metals Handbook. Vol. 10. American Society for Metals. Metals Park, Ohio.
4.
Corrosion of Oil-and-Gas-Well Equipment. Book 2 of the Vocational Training Series. API.
5.
Dvoracek, L. M. "Corrosion Fatigue Testing of Oil Well Sucker Rods Steels." Union Oil Company of California. Union Research Center, Brea, California 92621.
6.
Mehdizadeh, P., McGlasson, R. L., and Landers, J. E. 1963. "Corrosion Fatigue Performance of a Carbon Steel in Production Environments." Paper presented to the South Central Region Conference of NACE. New Orleans, Louisiana. October 18─22.
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