Corrosion Basics

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Saudi Aramco Corrosion Corrosion Basics - A Refresh Refresher er

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Inspection File Reference: COE10300

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Engineering Encyclopedia

Inspection Corrosion Basics - A Refresher

CONTENTS

PAGES

WHAT IS CORROSION? ...................................................................................... 1 FORMS OF ATTACK............................................................................................4 Sweet Corrosion .......................................................................................... 4 Sour Corrosion ............................................................................................ 7 Stress Corrosion Cracking ........................................................................... 7 Oxygen Corrosion ....................................................................................... 9 Concentration Cell Attack ......................................................................... 12 Galvanic Corrosion....................................................................................12 Bacterial Corrosion....................................................................................13 CORROSION MONITORING AND CONTROL METHODS ............................ 15 GLOSSARY ......................................................................................................... 16

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CORROSION BASICS -- A REFRESHER WHAT IS CORROSION? Corrosion is the term generally used to describe a detrimental change in the physical properties of a metal through chemical or electrochemical reactions. Corrosion occurs in all phases of oil and gas production, both offshore and onshore, and in refinery operations. Corrosion is a process through which a metal is returned to a more stable state resembling the ore from which it was produced. This action, similar to metallurgy in reverse, is illustrated in Figure 1. Most metals are found in nature as metallic oxides or salts, as in the case of iron, and have the same chemical composition as rust, Fe2O3. The energy that converts iron ore to iron is the same energy released when the iron converts to rust.

FIGURE 1. Metallurgy in Reverse The basic requirements for corrosion are

• • •

A corrosion cell consisting of an anode and a cathode An electrolyte to complete the circuit Flow of direct current


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Figure 2 shows all of these requirements. The anode is the point at which the metal dissolves by going into solution. At the anode, metal atoms lose electrons and the resulting positive ions go into solution. The electrons migrate through the metal to the cathode. At the cathode, various ion species in solution remove these electrons to complete the corrosion reaction. A small, but measurable, electric current is produced and flows from the anode to the cathode through the electrolyte and then passes through the metal from the cathode to the anode. The quantity of current that passes through the cell is directly proportional to the amount of metal that corrodes. A current flow of 1 ampere per year will corrode approximately 20 pounds of steel.

FIGURE 2. Corrosion Cell (Note that the flow of electrons is opposite to the flow of cu rrent.)


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Typical reactions at the anode and cathode on a piece of steel are defined in the following equations. Anode

Fe

→

Fe ++ + 2e -

(Oxidation)

H2

(Reduction)

Cathode

2H+ + 2e-

→

Anodes and cathodes can form on a single piece of metal because of either local differences in the metal or in the environment. If the hydrogen produced adheres to the cathode and forms an insulating blanket, polarization results. Polarization introduces a resistance and interferes with current flow so that corrosion is decreased or stopped. Oxygen, if present, combines with the hydrogen in this insulating blanket to form water and thus removes the hydrogen film. Current flows again and corrosion proceeds. This is called cathodic depolarization. The rate of the destructive attack as a function of time will depend on process or service conditions such as pressure, temperature, velocity and impingement, pH and corrodent concentration, and the presence of oxygen or other depolarizers.


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FORMS OF ATTACK There are many types of corrosion. The most common types include:

• • • • • • •

Sweet corrosion Sour corrosion Stress corrosion cracking Oxygen corrosion Concentration cell attack Galvanic corrosion Bacterial corrosion

Sweet Corrosion Sweet corrosion results from CO2 gas or low molecular weight organic acids dissolving in water. CO 2 + H2O Fe + H2CO3

→

H2CO 3

→

(Carbonic acid)

FeCO3 + H2

This type of attack is characterized by a general loss of metal over the entire surface or by shallow areas of localized attack that are free of scale. Important factors that affect the solubility of CO2 and, therefore, the severity of attack include:

• • • •

CO 2 concentration Pressure Temperature Water composition


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The concentration of CO2 is shown by its partial pressure. The partial pressure of CO2 is obtained by multiplying the total pressure by the percentage of CO2. Corrosivity of a sweet system is determined by the following criteria:

• • •

Partial pressure above 30 psi usually indicates attack. Partial pressure between 7 and 30 psi may indicate corrosion. Partial pressure less than 7 psi is considered noncorrosive.

Increased pressure results in the increased solubility of CO2 in water and, therefore, higher corrosion rates up to a limit. This action is shown in Figure 3. As temperature increases, the solubility of CO2 in water decreases. If the pressure is kept constant, an increase in temperature will cause CO2 to be released from the solution. The accompanying rise in pH lowers the corrosion rate as shown in Figure 4. Water composition is another factor in sweet corrosion. Many dissolved minerals tend to buffer or to prevent the reduction of pH in a water with dissolved CO2, thereby influencing the rate of corrosion.

FIGURE 3. Corrosion Rate Vs. CO2 Partial Pressure


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CO 2 corrosion is particularly devastating in a high-velocity/high-pressure system where the combination of corrosion and erosion can lead to extremely high corrosion rates.

FIGURE 4. pH Vs. CO 2 Partial Pressure With Varying Temperature


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Sour Corrosion Sour corrosion results from the reaction of hydrogen sulfide and steel in the presence of water. Fe + H2S → FeS + 2H in H2O

This type of attack produces black iron sulfide scale and numerous pits. A galvanic reaction occurs between the black iron sulfide scale and the steel. The steel acts as an anode and the iron sulfide scale as a cathode. Additional problems occur with the precipitated iron sulfide. This insoluble precipitate is preferentially oil-wet and is commonly carried over into storage tanks, heater treaters, free water knockouts, and other production equipment where scale problems can occur. Disposal water will then carry significant concentrations of oil-wet iron sulfide that will plug filters and lines. Iron sulfide changes to iron oxide with time and exposure to air. There are four major sources of hydrogen sulfide in the oil field.

• •

Inflow of “sour” formation fluids

Bacterial decomposition of sulfates in drilling fluid (gyp muds, anhydrite contamination, etc.)

•

Thermal degradation (above 350-375 °F) of certain sulfur containing drilling fluid additives (lignosulfonates)

•

Make-up water in mud system containing sulfur compounds

Stress Corrosion Cracking Stress corrosion cracking is caused by the combined forces of stress and corrosion on an alloy. If either stress or corrosion is absent, then no cracking will occur. The best example of stress corrosion cracking in the oil field is hydrogen embrittlement. The embrittling action is caused by the liberation of hydrogen from hydrogen sulfide and from the corrosion process. The liberated hydrogen, frequently referred to as atomic hydrogen, is absorbed on the steel surface and migrates into the grain boundaries of the metal. After going into the steel, the liberated hydrogen combines either with itself to become molecular hydrogen or with carbon compounds in the steel. These compounds and the molecular hydrogen have larger molecules than the liberated hydrogen (atomic hydrogen) and are trapped in the steel. These large, trapped molecules then cause excessive pressure within the steel. As a result, the steel splits and blisters and may crack. This action is shown in Figure 5.


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FIGURE 5. Hydrogen Blistering


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Oxygen Corrosion In oil production, the removal of corrosives is primarily directed at the exclusion of oxygen. Oxygen is normally absent in oil producing systems or, at most, present in trace amounts. However, oxygen can enter a supposedly “closed” system through pumps, nongas-blanketed storage tanks, systems operating in a vacuum, and so forth. For example, the three most likely points where oxygen enters oil field water injection systems are at the wells, at the tanks, and through pump seals. The annulus of a well must be kept sealed or blanketed to exclude O2. This is particularly troublesome in water source wells equipped with electrical submersible pumps. Maintaining an effective seal around electrical cables or rotating shafts is extremely difficult. Gas blanketing is usually the only effective method of excluding oxygen from wells of this type. In the case of tanks, gas blanketing with natural gas or nitrogen is best. Oil blankets are not effective. Pump seals are another point of oxygen entry. If water is leaking from a seal, O2 can enter this site by diffusion against pressure. The presence of trace amounts (1 ppm or less) of oxygen greatly increases the effects of other corrodents. Corrosion caused by trace amounts of oxygen typically results in extreme attack in crevices, behind obstructions in the fluid flow, and in other shielded areas. Accelerated corrosion of this type takes place where oxygen is either absent or in small amounts. The removal of hydrogen by oxygen increases the cathodic reaction. Therefore, locations lacking oxygen tend to become anodic. The more oxygen in the system, the greater the weight loss as shown in Figure 6.


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FIGURE 6. Influence of Dissolved Oxygen on Corrosion of Steel

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The effect of NaCl content on the solubility of oxygen in water at various temperatures is shown in Figure 6A.

FIGURE 6A. Solubility of Oxygen in Water




Concentration Cell Attack Concentration cell attack is an intense localized corrosion that occurs within crevices and other shielded areas. This attack is usually associated with small volumes of stagnant solution caused by holes, gasket surfaces, lap joints, surface deposits, and crevices under bolts and rivet heads. This type of attack is shown in Figure 7.

FIGURE 7. Concentration Cell Galvanic Corrosion Galvanic corrosion is produced when current flows between two different metals or between areas of the same metal having different characteristics. This corrosion occurs because different metals have different tendencies to corrode and because metals are rarely homogeneous. It really is not necessary to have two different metals for galvanic corrosion. Iron sulfide scale acts as a cathode when covering steel and leads to accelerated corrosion of the steel. Galvanic corrosion is a very rapid and localized attack.




Bacterial Corrosion There are three major groups of bacteria in the oil field.

• • •

Slime formers Sulfate-reducing bacteria Iron bacteria

Bacteria may cause plugging and corrosion problems.

•

Plugging

•

Bacteria cells and “slime”

•

Chemical precipitates • •

•

FeS (sulfate-reducing bacteria) Fe(OH)3 (iron bacteria)

Corrosion

•

Production of corrosive substances (H2S)

•

Deposits accelerate corrosion

•

Depolarization of cathode of corrosion cell by sulfate-reducing bacteria

Indicators of bacterial activity include:

• • • •

Pressure increase in injection wells Black water Slime accumulations on filters Increase in sulfide




Bacteria growth tends to concentrate in the following locations:

• • •

In films on pipe surfaces or walls of tanks Filter beds Stagnant areas in a system

• • •

Tank bottoms Gauge settings, bull plugs sample connections Wellbore below perforations




CORROSION MONITORING AND CONTROL METHODS Corrosion problems should be anticipated and the need for corrosion monitoring recognized before a pressure vessel is fabricated or a pipeline laid. This permits the best possible material selection and design features to be incorporated in new construction. Whether in the plant or in the field, corrosion monitoring and control efforts should include examination of

• • •

The area of corrosion Reports of maintenance men and operating personnel Materials of construction, process streams, and corrosion products

Awareness of corrosion as a specific problem has resulted in more care taken in obtaining corrosion monitoring information and in preserving valuable evidence. The prevention or control of corrosion commonly uses one of the following methods:

• • • • •

Change in the environmental conditions Corrosion-resistant materials Separation of the metal from the corrosive environment by the use of coatings Use of inhibitors Cathodic/anodic protection




GLOSSARY anode

Area where corrosion occurs and where current leaves the metal and enters the solution

cathode

Area where no corrosion occurs and where current enters the metal from solution

corrosion

Detrimental change in the physical properties of a metal through chemical or electrochemical reactions

depolarization

Removal of the hydrogen film at the cathode

oxidation

Removal of electrons from an atom

polarization

Result of hydrogen adhering to a metal and reducing or stopping the current flow

reduction

Gain of electrons by an atom

sour corrosion

Corrosion caused by H2S dissolved in water

sweet corrosion

Corrosion caused by CO2 and/or low molecular weight organic acids dissolved in water


Corrosion Basics

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