Protection, Basic • Fundamentals Of Cathodic Protection, Designing And Its Application
•Arindam Samanta 1
Mechanism Of Corrosion Corrosion is loss of metal by electrochemical reaction of metal with its
environment.
Almost always electrochemical reactions occur in aqueous solutions. In all aqueous electrolytes such as soil, water, metal atoms go into solution as
metal ions.
For corrosion (electrochemical) reaction to proceed the following conditions
must be satisfied––
i.
Setting up a galvanic cell in the same metal or between two different metals resulting in a potential difference in the cell and current flow.
ii.
Existence of anode & cathode in the cell.
iii.
Anode & cathode should be in contact with electrolyte.
iv.
Electrolyte shall be conductive to ions.
2
Mechanism Of Corrosion (Cont.) At anode metal goes into solution as metal ions(oxidation reaction). At cathode metal deposition or reduction of gases or cations occur (reduction
reaction).
Anodic sites have a more negative potential than cathodic sites in the same
electrolyte.
When metal atoms go into solution as metal ions, electrons are released at
anode which migrate to cathode by metal path where it is consumed by reduction reaction. This flow of electrons gives rise to DC current. Thus the magnitude magnitude of o f current which is called corrosion current is directly related to mass of metal going into solution by Faraday’s Law.
Passage of electrons through external metal path from anode to cathode
means flow of conventional current from anode to cathode through electrolyte. electrolyte. Thus any corrosion phenomenon is associated with passage of DC current from anode to electrolyte.
3
Mechanism Of Corrosion (Cont.)
4
Corrosion Reactions AT ANODE
M
M +z + Ze
AT CATHODE
2 H + +2e
• (Acid)
O2+ 4H+ + 4e
• (Acidic solutions) •
(Neutral
H2
or Basic solutions)
O 2+ 2H2O+ 4e
2 H2O 4OH –
For Iron
• Fe
Fe+++2e
• Fe ++ + 2OH–
Fe (OH)2
• 4Fe (OH)2 + O2 + 2H2 O • Fe(OH)3
4Fe (OH)3
FeOOH+H2O (Hydrated Ferric oxide, Red Rust)
5
Corrosion Rate expressed as mm/yr mm/yr or mils/year mils/year (mpy) or Mg/dm2/day(mdd). Corrosion rate expressed
•
The above units represent average rate of metal penetration or weight loss of metal excluding any adherent or non adherent corrosion products.
•
For steel in sea water rate is = 0.13 mm/yr. mm/yr.
•
For steel in soil rate is = 0.021 mm/yr. mm/yr.
Above is derived from Faradays Law m = I t a / n F
where F= F= Faraday Faraday constant =96500 coulumbs coulumbs /equivalent /equivalent I =current passed (Amps) (1Amp = 1coulumb 1 coulumb for 1 sec ) m= mass reacted a= atomic weight n= number of equivalents exchanged( valency) t = time 6
Corrosion Rate (cont.) Corrosion rate = m/t A
where A= surface area
=Ita/nFtA =ia/nF
where i( current density)=I/A density)=I /A
The above relationship relationship gives proportionality between mass mass loss per unit area per unit time ( mg/dm2/day) and current density( μA/cm2).The μA/cm2).The constant includes factor a/nF and conversion factor for units Corrosion rate expressed in penetration per unit time–––
•
a) corrosion rate in in mils mils per year (mpy) = 0.129 x a i /n d where d= density of the metal ( gm/cm3) i = μa/cm2 μa/cm2 mm/yr = 0.00327 x a i/n d • b) corrosion rate in mm/yr For Fe→Fe++ + 2e ,
1μa/cm2= 0.0116mm/ 0.0116mm/yr 7
Classification On Basis Of Corrosion Rate
a)
< 0. 0.15 mm mm/yr (<5mpy) – Excellent co corrosion Re Resistance.
b)
0.15–0.5 mm/yr mm/yr (5–20 mpy) mpy) –Good
c)
0.5 – 1.0 mm/yr (20–50 mpy) – Fair
d)
> 1.0 mm/yr (>50 mpy)–Unacceptable.
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Corrosion Prevention Method Coating––––
•
Coating by coal tar enamel or polyethylene is primary method of external corrosion protection.
•
Protection by coating alone is not recommended recommended due to rapid attack of metal at coating holidays (areas of coating defects) due to large anodic current density at defects.
•
Therefore coating is always supplemented supplemented with cathodic protection which essentially protects exposed metal at coating defects.
•
Application of coating drastically reduces cathodic protection current requirement since C.P. Current is required at defect areas (holidays) of coating.
•
Current reduction is in order of 2000–100 folds depending on type of coating and electrolyte. electrolyte.
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Corrosion Prevention Method (cont.)
Cathodic Protection– Protection–––– ––– Cathodic •
Cathodic protection being electrochemi electrochemical cal technique, technique, arrests all forms forms of corrosion (uniform attack, galvanic, pitting, crevice, stress corrosion cracking) excepting H2 damage.
•
Most effective effective method of corrosion control brings corrosion rate to zero.
•
The method consists of supplying electrons from external source to the corroding metal so as to convert all anodic sites of corroding corro ding metal to cathode where by only reduction reaction occurs by consuming the supplied electrons.
•
By comparison with with value of asset protected cost of cathodic protection (installed + operating) is low.
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Type ypess of Cath Cathodic odic Prot Protec ection tion methods of Cathodic Cathodic Protection––– There are basically two methods SACP (Sacrificial (Sacrificial Anode Cathodic Cathodic Protection) ICCP (Impressed Current Cathodic Protection) electrochemically "active" In SACP system a galvanic anode, a piece of a more electrochemically
metal (Anodic) than the metal to be b e protected is connected with the structure to be protected. Due Due to potential difference difference in galvanic series the required cathodic cathodic current is generated. This technique is used when current requirement is low and the electrolyte has low resistivity.
For larger structures, or where electrolyte resistivity is high, galvanic anodes
cannot economically deliver enough current to provide protection. In these cases, cases, impresse impressed d current current cathodic cathodic protection protection (ICCP) system systemss are used. These consist of anodes connected to a DC power source, often a transformer–rectifier connected to AC power.
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Types of Cathodic Cathodic Prote Protection ction (cont.)
Sacrificial Anode Cathodic Protection
Impressed Current Cathodic Protection
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Cathodic Cath odic Pro Protect tection ion Reactions Reactions a) At Cathode:
O2+H2+4e
4OH– (neutral (neutral / alkaline alkaline solution solution))
2H2O+2e H2 H2+20H– (At Potential more–vethan–1.15v(Cu/CuSO4) more–vethan–1.15v(Cu/CuSO4) in neutral / alkaline solution) 2H++2e→H2↑(acidsolution), O2+4H+4e→2H2O(oxygenated acid solution) System) b) i) At Anode (Impressed Current System) 2H2O → O2 ↑ + 4H+ +4e (in oxygenated oxygenated solution) 2Cl– → 2Cl– → Cl2 ↑ +2e (in chlorine rich solution) •
ii) Sacrificial anode— Mg→ Mg ++ + 2e
Mg ++ + 2OH – →Mg( – →Mg( OH )2 ( soluble)
Al → Al +++ + 3e
Al +++ + 3OH – → – → Al (OH)3( soluble )
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Protection Criteria Plain carbon steel and low alloy ferrous materials exposed to soil, fresh water
and sea water water are fully cathodically cathodically protected when when their interface potential potential to electrolyte when when measured with a copper–copper copper–copp er sulfate sulfate reference electrode is minimum minimum –0.850 – 0.850 volts for soil / freshwater and –0.800 volts w.r w.r.t. .t. Ag / Agcl in seawater.
Measurement taken by placing the electrode in the electrolyte and measuring
the potential between structure and electrode with a high resistance voltmeter (see figure).
electrolyte metal interface interface temperature temperature upto 40C. For Above potentials are at electrolyte
temp > 40C, protection potential shall be–0.950 Volt Volt (Cu / Cuso4) in soil / fresh water and –0.900Volt (Ag / Agcl) in sea water.
drawing high current, minimum 100 mv potential shift For large bare structure drawing is recommended recommended as protection criteria
•
Shif Shiftt = Inst Instan antt ‘of ‘off’ poten potenti tial al (Cu/ (Cu/Cu CuSO SO4) – Natu Natura rall Potent Potential ial (Cu/C (Cu/CuSO uSO4)
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Protection Criteria (cont.) Basis of protection criteria
•
For effective effective cathodic protection ,all anodic anodic process must be stopped i.e. all anodic current shall be zero and there would not be solvation of metal atom. To satisfy above condition potential of metal should be equal to reversible potential of that that metal metal electrode Fe →Fe++ + 2e in its own ions
•
Reversible potential for above system according to Nernst equation with activity of Fe ions 10–6moles 10–6moles /litre would be at 250c E = –0.440 + 0.0295 log ( conc Fe ++) = –0.617 volt ( w.r.t standard Hydrogen electrode )
where –0.440 volt is the standard electrode potential of Fe /Fe++ at 25 degree C having activity of Fe ++ = 1N •
Therefore potential w.r.t. Cu/CuSO4 = –0.617 –( +0.337) = –0.954 Volt at 25 degree C.
•
At 35 degree C the w.r.t. Cu/CuSO4 is –0.85 Volt 15
Cathodic Protecti Protection on Over Protectio Protection n
Excessive current leads to very high cathode potential which has following
detrimental effects-----
•
Damage to coating coating by sapponification sapponification (softening of coating) coating) at potential more negative than –2.0 VOLTS ( cu/cuso4).
•
High pH
•
Hydrogen generation generation at potential more negative than than -1.2V (OFF) Cu/Cuso4 Cu/Cuso4 leading to blistering and and dis-bondment dis-bondment of coating.
•
Wastage of energy.
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Cathodic Cath odic Prot Protecti ection on Design Design The following steps are involved in designing a cathodic protection system
using ceramic anodes:--
a. Collect data. 1) History--Information from occupants in the area can indicate the severity of
corrosion problems. Data on failures and failure rates of nearby structures can be invaluable and must be considered.
structure to be protected and the area where it is 2) Drawings--- Drawings of the structure
or will be installed are needed to provide the physical dimensions of the structure for determining surface area to be protected, and locations of other structures in the area that may cause interference, of insulating devices, and of power sources.
requirement test and potential survey test results are needed 3) Tests---Current requirement
for existing structures that will be protected. Electrolyte (soil or o r water) resistivity tests and evaluation of conditions that could support sulfate-reducing sulfate-reducing bacteria are needed for all cathodic protection designs. This information will indicate the size of the cathodic protection system that will be required as well as the probability of stray current problems. Soil resistivity's contribute to both design calculations and location of the anode ground bed. 17
Cathodic Protectio Protection n Design (cont.) 4) Life-- The user must determine the required number of years that the structure
needs to be protected, or the designer must assume a nominal life span. The structure will begin to deteriorate from corrosion at the end of the cathodic protection system's system's design life unless unless the system is rejuvenated. rejuvenated.
complements the protective coating system. system. A 5) Coatings-- Cathodic protection complements good coating system system substantially reduces reduces the amount amount of cathodic protection current required. The coating efficiency has to be assumed.
6) Short circuits--- All short circuits must be eliminated from both new and
existing structures structures for which which a cathodic protection system system is being designed. designed.
protec ted---- b. Calculate surface area to be protected-----
The overall current current requirement requirement of a cathodic protection system system is directly proportional to the the surface area to to be protected. This includes underground underground or submerged submerged pipes buried tanks, and wetted surfaces (up to high water(level) of watertan watertanks ks (includin (including g risers).
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Cathodic Protectio Protection n Design (cont.) requirem ent--- c. Determine current requirement----
•
For existing structures, a current requirement test will provide the actual current requirement at the time of the test. Allowance should be made in the design for future degradation of coatings or structure additions that will increase the current requirement.
•
Total current required is given by the following equation:-I = (A)(I' (A)(I')(l )(l.0 .0 - CE)
where I is the total current requirement, A is the total surface area to be protected, I' is the estimated estimated current density density,, and CE is the efficiency efficiency of the coating coating system. d. Select anode type---
•
Ceramic anodes are made in a variety of shapes, such as, wires, rods, tubes, strips, disks, and mesh.
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Cathodic Protectio Protection n Design (cont.)
e. Calculate number of anodes (N) or length of bare anode wire (L B )----
•
The number of anodes or length of anode wire required is determined from the total current required and the manufacturer's manufacturer's published current rating for a given life.
•
1) For rod, strip, tube and disk anodes:---
Total current current required / Manufacturers Manufacturers rated rated Number of anodes required = Total current for specific size, environment and life.
•
2) For wire anodes:----anodes:-----
Total footage of anode = Total current requirement / Manufacturers rated current
capacity per foot of wire for a given environment and life.
•
The number calculated will determine the minimum number of anodes or anode wire length required.
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Cathodic Protectio Protection n Design (cont.) f. Calculate the total circuit resistance (RT )---
•
N) The total circuit resistance (R T) consists of the anode-to electrolyte resistance (R N plus the interconnecting interconnecting wire resistance resistance (R W) plus the structure-to-electrolyte structure-to-electrolyte resistance (R C).
R T = R N + R W + R C •
A criterion of 2-ohm maximum ground bed resistance is often used to limit the rectifier output voltage and the associated hazards of overprotection. When the total required current is low, a higher total resistance is often acceptable. As the required current increases, the total resistance should be reduced.
•
The total anode-to-electrolyte resistance (R N N) is calculated in different ways ways according to the type of anode installation.
•
The anode-to-electrolyte anode-to-electrolyte resistance for a single anode is given by R A. For a single vertical anode:
where R A is the anode-to-electrolyte resistance for a single anode, p is the electrolyte resistivity in ohm-cm, L is the length of the backfill column in feet, and d is the diameter of the backfill column in feet. 21
Cathodic Protectio Protection n Design (cont.) •
For a single horizontal anode:
where R A is the anode-to-electrolyte resistance for a single anode, p is the electrolyte resistivity in ohm-cm, L is the length of the backfill column in feet, d is the diameter of the backfill column in feet and h is the depth of the backfill cylinder in feet.
•
For a multiple vertical anode:
where R N is the anode-to-electrolyte resistance, N is the number of Anodes, and Cc is the center-to-center spacing of the anodes in feet. This equation assumes a linear configuration of the the groundbed anodes. 22
Cathodic Protectio Protection n Design (cont.) •
If the anode dimensions are different, another empirical expression may be used:
where R N is the anode-to-electrolyte resistance, R A is the anode-to-electrolyte anode-to-electrolyte resistance for a single anode, p is the soil resistivity in ohm-cm, ohm-cm, N is the number of anodes used, P F is a paralleling factor and Cc is the center-to-center spacing of anodes in feet. This equation assumes a linear configuration configuration of o f the groundbed anodes. •
For a circle of rod anodes (as in a water storage tank):
where R N is the anode-to-electrolyte resistance, p is the electrolyte resistivity in ohm-cm, LB is the length of each rod anode in feet, D is the tank diameter in feet, A R is the radius of the anode circle in feet, and D E is an equivalent diameter factor. 23
Cathodic Protectio Protection n Design (cont.) •
For wire anode circle or hoop (in a water storage tanks):
where R A is the anode-to-electrolyte resistance, p is the electrolyte resistivity, D R is the diameter of the anode circle in feet, D A is the diameter of the anode wire in feet, and H is the depth below the high water level in feet. •
Wire resistance (R W) is the sum of both the rectifier-to anode lead and the rectifierto-protected-structure to-protected-structure lead.
where LW is the length of wire in thousands of feet and R MFT is the resistance of the wire in ohms per 1000 ft. 24
Cathodic Protectio Protection n Design (cont.) •
The structure-to-electrolyte structure-to-electrolyte resistance (R C) is dependent d ependent primarily on the condition of the coating.
where R S is the coating resistance in ohm-square feet and A is the total surface area. If the structure surface is bare, b are, negligible resistance is assumed (R C = 0). g. Calculate required rectifier voltage and current.
•
The required rectifier voltage (V REC and maximum current rating should include at least an extra 20 percent to allow for variations in calculations from actual conditions and for changes in the system over the system's life.
•
VREC = (I) (R T) (1.2) = (IREC ) (R T)
•
where I is the total current required and R T is the total circuit resistance as calculated above and I REC is the minimum current rating for a rectifier for this particular application. application.
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Cathodic Protection Components Following are the major components of Cathodic Protection system :
•
Power source (for impressed current system)
•
Anodes
•
Cables
•
Backfill
•
Reference electrodes (permanent & portable)
•
Junction boxes
•
Potential measurement measurement test points
•
Corrosion probes / coupons
•
Potential recorder
•
Zinc grounding cells
•
Insulating joints / flanges 26
DC Power Source
Automatic potential control or constant voltage / constant current transformer Automatic Rectifier unit (TRU) of following : 25A / 25V D.C. 50A / 50V D.C.
PSP Feedback ility TR Unit Drawi
75A / 75V D.C. 50A / 75V D.C. 100A / 12V D.C. For cross country pipelines or underwater underwater structures auto potential potential control units are are
recommended. constant voltage / constant current units are recommended. recommended. For plant structures constant
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Anodes Sacrificial Anodes:--1.
Zinc
2.
Aluminum Alloy
3.
Magnesium
Impressed current current anodes: -------
1. Graphite 2. Silicon-chromium-iron Silicon-chromium-iron alloy 3. Metal oxide coated titanium 4. Platinis Platinised ed titanium titanium 5. Platinis Platinised ed niobium
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Sacrificial Anodes Sacrificial anodes generate their own DC Charges and require no external DC
Pow Power sou source rce. onegative in E.M.F Sacrificial anodes are made of metals which are more electrone seri series es tha than prot proteected cted metal etal They have fixed driving voltage to protected metal which is in range of 0.6-
0.25 0.25 volt olts. They are not pur pure metals but alloys . Alloyin oying g elemen ementts have ave several functions ons The which are :i. Givi Giving ng a fine fine grai graine ned d stru struct ctur uree to give give unif unifor orm m corr corros osio ion. n. ii. Redu Reduce ce self self –cor –corro rosi sion on by loca locall cell cell acti action on.. iii. Pre Prevent passivation due due to formation of ins insoluble corr orrosion produ oducts on anode surface
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Composition of Sacrificial Sacri ficial Anodes a) High purity Magnesium (AZ 63 C as per ASTM B 843)----
•
Element
% wt
•
Aluminium
5.3-6.7
•
Zinc
2.5-3.5
•
Manganese
0.15-0.7
•
Silicon
0.3 Max
•
Copper
0.05 Max
• Nickel
0.003 Max
•
Iron
0.003 Max
•
Others ( total )
0.3 Max
•
Magnesium
Remainder
• Note -Heavy -Heavy metals( Cu, Ni, Ni, Fe, Pb ) should never exceed exceed specified values values since they they lower efficiency due to local cell formation. •
Al,Zn, Si, Cd are activators to prevent surface films. 30
Packed Magnesium Anode
31
Composition of Sacrificial Sacri ficial Anodes
b)Zinc ( ASTM B418)
Type I
Type Type II
Element
% wt
% wt
Iron
0.005 max
0.0014 max
Lead
0.006 max
0.003 max
Copper
0.005 max
0.002 max
Aluminium
0.1-0.5
0.005max
Cadmium
0.025-0.07
0.003 max
Others ( total )
0.1 max
----
Zinc
remainder
remainder
• Note: 1) Cu ,Fe ,Pb should not exceed specified specified limit limit as they passivate the anode •2) Al and Cd are for grain refinement refinement and prevent passivation 32
Composition of Sacrificial Sacri ficial Anodes
c) Aluminium alloy( Al-Zn-Si-In alloy)
Galvanum Galvanum III
Alanode Alanode
Elem Elemen entt
wt %
wt %
Iron
0.12 max
0.13 max
Silicon
0.08-0.2
0.10 max
Copper
0.006
max 0.01 max
Zinc
2.0-4.0
0.5-5.0
Indium
0.01-0.02
0.005-0.05
Others (each) 0.02 max
0.02 max
Aluminum remainder
remainder
• Note: Heavy metal metal (iron, copper and and others i.e. Nickel, Nickel, lead should should not exceed specified limit) 33
Properties of Sacrificial Anodes
Magnesium
Zinc
Aluminium
•a) Open circuit potential(Cu/CuSo4) potential(Cu/CuSo4) -1.55 v
-1.1 v
-1.1 v
• b) Closed circuit circuit potential(Cu/CuSo4) potential(Cu/CuSo4) -1.50 v
-1.05 v
-1.05 v
•c) Electrochemical Electrochemical Capacity( Ah/Kg) 1100-1200
780
2500-2700
•d) Current efficiency efficiency
95%
50%
95%
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Use of Sacrificial Sacri ficial Anodes a) Magnesium Magnesium: External surface of Underground pipelines ,vessels equipments having soil resistivity upto 10,000 ohm-cm.
•Internal of tank ,vessel, condensers containing fresh water . •Seldom used for sea water application • Not recommended for internal of vessels/equipments handling explosive / combustible liquids and alloys susceptible to HIC. b) Zinc: Undergound application ( External surface pipelines / vessels vessels / sheet piling, piles for soil resistivity <1000 <100 0 Ohm–cm (only Type Type II to be used)
•For internal surface surf ace of vessels/condensers/tankage/equipments vessels/conden sers/tankage/equipments carrying carrying sea water ( Type Type I ). •For underwater sea water application (jetty piles, wharfs, offshore platform jackets, submarine pipelines, ship hulls, ballast tanks)
• Not recommended in fresh water and soils with resistivity > 1000 ohm cm electrolyte temperature is >500C • Not recommended where electrolyte
35
Use of Sacrificial Sacrif icial Anodes (cont.) c) Aluminium Aluminium Alloy
•Best choice for marine marine application because of its large large Ah/Kg and light weight. weight. All offshore offshore structures, submarine submarine pipelines, jetty piles, wharfs, other marine under water structures. •
Internal protection of tanks /vessels /condensers exchangers carrying sea water or brine up to temperature temperature of 75C. Only Only anode for concentrated concentrated brine at elevated temperature temperature (heater treaters, dehydrators)
• Not suitable for for soil application and fresh water water . Can be used in brackish water containing minimum 500 ppm chloride ions. Chemical Backfill----
• Sacrificial anodes when used in soil are always always pre packed with chemical chemical bacfill of composition 70% gypsum + 20 % bentonite +5% sodium sulfate. sulfate. •Backfill are essentially chemicals having high ionic conductivity( low resistivity)
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Anode Weight Weight Calculation Anode Wight
W( Kgs)= Ip X L x 8760 8760 / C x 0.85 0.85 where C = Current capacity of anode material ( Ah/kg)
Ip = Protection current ( Amps ) = A x i, A = Surface Area, i = current density. L = Design life of anode /CP system ( yrs) 1 yr= 8760 hrs 0.85= Utilisation factor Anode current output-
•
Individual anode current output( Ia Amps)= (Ea-Ec)/Ra where Ea( v)= Closed Circuit potential of anode( Cu/CuSo4) Ec( v) = Protective potential of cathode ( Cu/CuSo4) Ra( ohms) = Resistance of individual anode to earth 37
Impressed Current Anode •
These are made of metals/alloys having very low wear rate (gms/a-yr.)
•
Can have life of 30 years at large current discharge and therefore often termed as permanent anodes. anodes.
•
They need not be of more negative electro-chemical potential than protected metal.
•
These anodes are used for impressed current C.P. System using external DC Power source.
•
Anodes range range from consumable consumable to non- consumable anodes
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Impressed Current Anode (cont.) a)
Fe-Si-Cr Cylindrical( Cylindrical( solid ) ---
Cr, • 14 % Si, 1%C, 1% Mn, 5% Cr
• Density Density 7-7.2gms/c 7-7.2gms/cm3 m3
current Si converted to to Si02 which is electronic conductor and • On passage of current excellen excellentt resistance resistance to acid. Material is bulky bulky ,heavy ,heavy and extreme extremely ly brittle brittle and there therefo fore re handled handled with with care care • Material and difficulty difficulty.. Requires good foundry practice during casting. For these reasons replaced by metal oxide coated Ti anodes
• Maximum allowable current for intermittent period soil / fresh water @ 30A / m2 = 7 amps, in salt water @ 50A / m2 =12Amps rate:-150-200 gms gms / Ayr in coke back fill or fresh fresh water at 15A/m2, 15A/m2, • Wear rate:-150-200 500gms/Ayr 500gms/Ayr in salt water water at25A/m2
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Image of Ceramic Anode
40
Impressed Current Anode (cont.) Titanium Tubular Tubular b) i) Metal Oxide Coated Titanium
•
25mm dia x1000 mm long x 0.8 mm thick 350gms
•
16mm dia x1000 mm long x 0.8mm 0.8mm thick 230gms 230gms
•
Ti tubes tubes coated with with noble metal oxides (RuO, IrO)of thickness20-30microns. thickness20-30microns.
•
Wearrate:- 2mg/Ayr 2mg/Ayr in soil (coke backfill) backfill) / salt water water at maximum maximum density of 8mg/Ayr 8mg/Ayr in fresh water water or salinemud. salinemud.
•
Individual Anode end or center connected to cable 16/50mm2 copper of desired length or multiple anodes mounted mounted on a single cable (anode string)
Oxid e Coated Titanium Wire Anode ii) Metal Oxide
•
1.5mmor3mmdiaTitaniumsolidwirecoatedwithoxides
41
Image of Tubular Tubular Anode
42
Different Parts of Anode
43
Anode Cable Connection
44
Vertical Canistered Anode
45
Horizontal Canistered Anode
46
Image of Multiple Anode String
47
Image of Vertical Vertical Shallow Anode Ground Bed
48
Horizontal Continous Anode Ground Bed
49
Deep Bed Anode
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Cable for CP System •Catho Cathodic dic protectio protection n cables cables are are usual usually ly copper copper conduct conductor or to to ha have less volt voltag agee drop and durability. •Header Cables / Bonding Cables are 25-50mm 2 single core. •Measu Measurem rement ent Cab Cables les are 4 or 6mm 6mm2 single single core. core. •Anode Anode Cables Cables (cable (cabless fac factor tory y joined joined to anode anode ) are are 6-10m 6-10mm m 2 singl singlee core. core. •All cables cables excepting excepting anode cables are armouredwith PE or PVC insulat insulation ion & PVC sheath. sheath. •Anode Anode cables cables are unarmoured unarmoured with with PVDF (Polyviny (Polyvinylide lidene ne fluoride fluoride ) insula insulatio tion n with with HMWPE sheathing for acid resistance. resistance.
51
Backfill of Anode Anodes in soil application are always used in combination with backfill.
Anodes are usually supplied prepacked with backfill.
The function of the backfill are---
•
Provide low resistance resistance around anode (lower (lower anode to earth resistance)
•
Prevent anode to come into contact with harmful ions in soil and contaminants which can passivate passivate the anode (Phosphates ,carbonates, ,carbonates, nitrates).
•
Retain moisture around the anode
a) Chemical Backfill for sacrificial anodes
fo r sacrificial anodes. anode s. • Never use coke or graphite as backfill for 52
Backfill of Anode (cont.) b) Calcined petroleum Coke Backfill for permanent anodes ( Impressed Current
anodes):
•Extremely low resistivity •Increases anode area( electronic conductor) •Lower anode to earth resistance •Extends life of anode ,majority ,majority of current discharged discharged electrolytically electrolytically at backfill to soil interface •Helps venting of gas because of high porosity po rosity •Provides Low resistance around anode when wet Properties of calcined petroleum coke backfill----
•
a) Chemical Chemical composition: Carbon 97.6%, Sulfur 0.7%,Ash 0.5%,Nitrogen 0.5%, Volatiles 0.4%, Moisture 0.3%
2.1gms/cm3, • b) Density 2.1gms/cm3, 53
Backfill of Anode (cont.) •c) Resistivity < 50 ohm-cm( loose) •d) Particle size Analysis >1 mm 2% 0.5-1mm 41% 0.12-0.5 42.6% 0.05-0.12 10.6% <0.05 3.7 % •e) Consumption Consumption 1 kg/A-yr kg/A-yr at max 10A/m2. 10A/m2. single anode anode is prepacked prepacked in a galvanis galvanised ed sheet sheet steel caniste canisterr Each single Anode is centrally located in canister around which the loose coke backfill is
poured and compacted compacted
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Reference Electrode •Stable non-polarizable electrodes with stable electrode potential •Consists of high purity metal metal element in the solution of its own salt. •Most commonly used electrodes are •Saturated Copper-copper Sulfate (Cu/CuSO4) •Silver-silver Chloride/Sea Water ( Ag / Agcl) •Silver-silver Silver-silver Chloride / Saturated KCl (Ag/AgCl/KCl) (Ag/AgCl/KCl) •High purity zinc
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Reference Electrode
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Test Station •
Test points are installed installed at regular intervals intervals or at desired locations to serve serve following functions :
-
Termination of measurement cables from structure and reference electrodes
-
Connection of sacrificial anodes
-
Bonding of different structures
-
Connection of grounding cells across
-
Connection of probes / coupons
57
Test Station (cont.)
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Corrosion Probes Coupons •
Coupons are installed to measure instant ‘off ‘off ’ potential measurement and are placed at critical locations locations as follows follows :
-
Areas of marginal protection
-
Location of large IR drop
-
Interference location
-
Highly corrosive / aggressive spots
•
With coupons, TR unit interruption interruption is not not required.
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Corrosion Probes Coupons (cont.)
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Cathodic Protection at TCL Babrala PCC/MCC/ASP/MLP
4Cx50 Sq.mm Al,P VC ( 400 MTRS )
OR
•
EXISTING POW ER SUP PLY S OURCE
PDB
WALL MTG TYPE 125A ( 6 O/G ) / 63A ( 3 O/G ) QTY - 10 Nos
3Cx25 Sq.mm Al TO T/R UNITS
T/R UNIT
+
-- --
--
INTPUT - 230V, 1PH A.C. OUTPUT - 75 V D.C / 75 A D.C. QTY - 35 Nos
•
CJB CJB
1Cx 95 Sq.mm Al, PV PVC ( 10 KMS)
1Cx 95 95 Sq. mm Al
QTY - 14 Nos CLASSIFIED / NON CLASSIFIE D
1Cx 50 50 S q. q. mm mm Cu (DRAINAGE CABLE)
F IRE HY DRA NT P IP IP E ( U/ G ) CW SUP PLY & RETURN PIPE ( U/ G ) CY & SY DRAIN PIPING ( U/G )
1Cx50 Sq.mm Cu,PVC Cu,PVC (DRAINAG (DRAINAGE CABLE)
1Cx 95 Sq. mm Al
AJB
AJB AJB
TANKS NAPHTHA BULK STORAGE NAPHTHA DAY TANK NPDU
1Cx50 Sq.mm Cu,PVC ( NEGATIVE HEADER CABLE ) ( 4 KMS ) FIRE HYDRANT PIPE ( U/G ) CW SUPPLY & RETURN PIPE ( U/G ) QTY - 38 Nos CY & SY DRAIN PIPING ( U/G ) CLASSIFIED / NON CLASSIFIED
1Cx10 Sq.mm Cu, XLPE ( 18 KMS) VERTICAL BED AT 4 MTR DEPTH
ANODE BED HORIZONTAL BED AT 1.5 MTR DEPTH
DEEP W ELL BED AT 16 MTR DEPTH
1000 x 20 x 3 MM - 69 Nos 2000 x 20 x 3 MM - 133 Nos MATERIAL - MMO (M IXED ME TAL OXIDE) COATED TITANIUM 2000x20x 3 MM CANNISTERED - 89 Nos MATERIAL - MMO (M IXED ME TAL OXIDE) COATED TITANIUM
16 MTR TUBULAR STRING ANODES - 6 Nos MATERIAL - MMO (M IXED ME TAL OXIDE) COATED TITANIUM
61
thanksYou Thank
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