types of corrosion.pdf

CHAPTER 2

TYPES OF CORROSION

2.1 INTRODUCTION

It is a great challenge to classify the types of corrosion in a uniform way. One can divide the corrosion on the basis of appearance of corrosion damage, mechanism of attack, industry section, and preventive methods. There are many types and causes of corrosion. In the oil and gas production industries, corrosion is usually classified into four different types. This classification is mostly based on the types of corrosion in the oil field (see Oxford and Foss, 1958):

1. Sweet corrosion 2. Sour corrosion 3. Oxygen corrosion 4. Electrochemical corrosion

One can also classify corrosion types as follows (see http://www.cor- rosiondoctors.org/Forms/fretting. htm):

1. Uniform attack 2. Pitting corrosion 3. Crevice corrosion 4. Galvanic corrosion 5. Erosional corrosion 6. Fretting corrosion 7 . Cavitation

35

36 Chapter 2 Types of Corrosion

8. Intergranular corrosion 9. Stress corrosion

10. Dealloying (selective leaching) 11. Environmental cracking 12. Fatigue 13. Exfoliation

According to Bertness et al. (1989, p. 559), destruction of metal is influenced by various physical and chemical factors which localize and increase corrosion damage.

The conditions which promote corrosion include:

1. Energy differences in the form of stress gradients or chemical reac- tivities across the metal surface in contact with corrosive solution.

2. Differences in concentration of salts or other corrodants in electrolytic solution.

3. Differences in the amount of deposits, either solid or liquid, on the metal surfaces, which are insoluble in the electrolytic solutions.

4. Temperature gradients over the surface of the metal in contact with corrosive solution.

5. Compositional differences in the metal surface.

In this chapter, the authors tried to cover all of the above conditions and types of corrosion, giving a brief introduction for each. One should realize, however, that in practice usually more than one type of corrosion takes place.

2.2 SWEET CORROSION

Sweet corrosion is a common type of corrosion and can be defined as the deterioration of metal due to contact with carbon dioxide, fatty acids, or other similar corrosive agents, but excluding hydrogen sulfide H2S. One can recognize the sweet corrosion by pitting on the steel. Carbon dioxide systems are one of the most common environments in the oilfield industry where corrosion occurs. Carbon dioxide reacts with the moisture in the environment and forms a weak carbonic acid (H2C03) in water, which then reacts with metal (however, this reaction occurs very slowly):

CO, + H,O ++ H,CO,

2.3 Sour Corrosion 37

Fe+ H2C03 + FeCO,+ H2? (2.3)

Corrosion rates in a COz system can reach very high levels (thou- sands of mils per year), but corrosion can be effectively inhibited. The reaction of CO, with water will decrease pH*, which will cause corrosion. With increasing solubility of carbon dioxide in water, corrosion will increase, The solubility is determined by the temperature, pressure, and composition. Pressure increases the solubility, whereas temperature decreases the solubility. Sweet corrosion always occurs in gas-condensate wells. Condensate wells produce water with pH below 7 at the wellhead and commonly as low as 4 at the bottom of some wells (see Oxford and Foss, 1958). This is due to:

1. High COz contents of gas; usually up to 3%. 2. High total pressure, ranging from 1,000 to 8,OOOpsi at the

3. Presence of organic acids, such as acetic acid (CH3COOH). wellhead.

Erosion by high-velocity gas stream aggravates the corrosion.

2.3 SOUR CORROSION

The deterioration of metal due to contact with hydrogen sulfide and moisture is called sour corrosion. Although hydrogen sulfide is not corrosive by itself, it becomes a severely corrosive agent in the presence of water. The general equation of sour corrosion can be expressed as follows:

H2S+ Fe+ H 2 0 + FeSx + 2H+ H 2 0 (2.4)

One can recognize the sour corrosion by the black deposit, i.e., iron sulfide deposit on the surface of steel. The scale tends to cause faster corrosion because the iron sulfide plays the role of cathode, whereas the steel acts as the anode where corrosion occurs. Sour corrosion results in deep pitting on the equipment. Also, hydrogen released in the


above reaction can embrittle the steel (cause a sulfide stress cracking [SSC]). The presence of C 0 2 and/or O2 aggravates sour corrosion. The incidents of sour corrosion in refineries vary according to the sulfur content of oil blends used. Amine units are the hot spots for sour corrosion. The following refinery units are the places where sour corrosion is considerable (http://www.ionscience.com/Pages/HydroApp. htm#B ac):

1. Amine columns 2. CDU lower reflux circuits 3. Catalytic crackers 4. Coker gas recovery systems 5. Diesel drier overhead accumulators 6. Fuel gas mixing drums and knockout drums 7. Flame knockout drums 8. Compressor suction drums 9. Gas compressors

10. Hot wells 11. Gadoil pre-saturators, strippers, and absorbers 12. Hydrocracker reactor effluent air coolers 13. High-pressure sour gas separators 14. Low-pressure sour gas separators 15. Off-gas knockout pots 16. On-site flare knockout drums 17. Recycle gas knockout drums 18. VBU flare knockout drums 19. Sour fuel gas knockout drums 20. Sour gas gathering lines, absorber towers, knockout drums, flash

21. Sour water strippers 22. Sour flare lines 23. Vacuum off-gas knockout drums

drums

2.4 CLASSES OF CORROSION

Numerous types of steel destruction can result from the corrosion process, which are listed under the following classes of corrosion:

Uniform attack. The entire area of the metal corrodes uniformly result- ing in thinning of the metal.”his often occurs to drill pipe, but usually

2.4 Classes of Corrosion 39

is the least damaging of different types of corrosive attacks. Uniform rusting of iron and tarnishing of silver are examples of this form of corrosion attack.

Crevice corrosion. Crevice corrosion is an example of localized attack in the shielded areas of metal assemblies such as pipes and collars, rod pins and boxes, tubing, and drill pipe joints. Crevice corrosion is caused by concentration differences of corrodants over a metal surface. Electrochemical potential differences result in selective crevice or pitting corrosion attack. Oxygen dissolved in drilling fluid promotes crevice and pitting attack of metal in the shielded areas of drill string and is the common cause of washouts and destruction under rubber pipe protectors.

Pitting corrosion. Pitting is often localized in a crevice but can also occur on clean metal surfaces in a corrosive environment. An example of this type of corrosion attack is the corrosion of steel in high- velocity seawater, low-pH aerated brines, or drilling fluids. Upon formation of a pit, corrosion continues as in a crevice but, usually, at an accelerated rate.

Intergranular corrosion. Metal is preferentially attacked along the grain boundaries. Improper heat treatment of alloys or high- temperature exposure may cause precipitation of materials or non- homogeneity of the metal structure at the grain boundaries, which results in preferential attack. Weld decay is a form of intergranular attack. The attack occurs in a narrow band on each side of the weld owing to sensitizing or changes in the grain structure due to welding. Appropriate heat-treating or metal selection can prevent the weld decay. Ringworm corrosion is a selective attack which forms groove around the pipe near the box or external upset end. This type of selective attack is avoided by annealing the entire pipe after the upset is formed.

Galvanic or two-metal corrosion. Galvanic corrosion may occur when two different metals are in contact in a corrosive environment. The attack is usually localized near the point of contact.

Selective leaching. Selective leaching occurs when one component of an alloy is removed by the corrosion process. An example of this type of corrosion is the selective corrosion of zinc in brass.

Cavitation corrosion. Cavitation damage results in a sponge-like appearance with deep pits in the metal surface. The destruction may be caused by purely mechanical effects in which pulsating pressures cause vaporization with formation and collapse of bubbles at the metal surface. The mechanical working of the metal surface causes


destruction, which is amplified in a corrosive environment. This type of corrosion attack, example of which are found in pumps, may be prevented by increasing the suction head on the pumping equipment. A net positive suction head should always be maintained not only to prevent cavitation damage, but also to prevent possible suction of air into the flow stream. The latter can aggravate corrosion in many environments.

Erosion-corrosion. The combination of erosion and corrosion results in severe localized attack of metal. Damage appears as smooth groove or hole in the metal, such as in a washout of the drill pipe, casing, or tubing. The washout is initiated by pitting in a crevice which penetrates the steel. The erosion-corrosion process completes the metal destruction. The erosion process removes protective films from the metal and exposes clean metal surface to the corrosive environment. This accelerates the corrosion process. Impingement attack is a form of erosion-corrosion process, which occurs after the breakdown of protective films. High velocities and presence of abra- sive suspended material and the corrodants in drilling and produced fluids contribute to this destructive process. The combination of wear and corrosion may also remove protective surface films and accelerate localized attack by corrosion. This form of corrosion is often overlooked or recognized as being due to wear. The use of inhibitors can often control this form of metal destruction. For example, inhibitors are used extensively for protection of downhole pumping equipment in oil wells.

Corrosion due to variation in fluid flow. Velocity differences and tur- bulence of fluid flow over the metal surface cause localized corrosion. In addition to the combined effects of erosion and corrosion, variation in fluid flow can cause differences in concentrations of corrodants and depolarizers, which may result in a selective attack of metals. For example, selective attack of metal occurs under the areas which are shielded by deposits from corrosion, i.e., scale, wax, bac- teria, and sediments, in pipelines and vessels.

Stress corrosion. The stress corrosion is produced by the combined effects of stress and corrosion on the behavior of metals. An example of stress corrosion is that local action cells are developed due to the residual stresses induced in the metal and adjacent unstressed metal in the pipe. Stressed metal is anodic, whereas the unstressed metal is cathodic. The degree to which these stresses are induced in pipes varies with (1) the metallurgical properties, (2) cold work, (3) weight of the pipe, (4) effects of slips, notch effects at tool joints, and ( 5 )

2.4 Classes of Corrosion 41

I

presence of H2S gas. In the oil fields, H,S-induced stress corrosion has been instrumental in bringing about sudden failure of drill pipes.

2.4.1 Stress-Induced Corrosion

Because of extra energy supplied by the stress, the portion of the same metal exposed to a higher stress acts as anode. This can be demon- strated by placing a bent nail into seawater (Figure 2.1). The initial points of corrosion attack (formation of rust) will be the more stressed areas, i.e., the head, the point, and middle portion of the nail as shown in Figure 2.1, with discoloration of water with rust in the vicinity.

In general, the more stressed part of a metal structure acts as anode where corrosion occurs. Thus, “stress begets rust!”

Hydrogen collects on the pipe as a film of atomic hydrogen which quickly combines with itself to form molecular hydrogen gas (H,). The hydrogen gas molecules are too large to enter the steel and, therefore, usually bubble off harmlessly.

In the presence of sulfide, however, hydrogen gradient into the steel is greatly increased. The sulfide and higher concentration of hydrogen atoms work together to maximize the number of hydrogen atoms that enter the steel. Once in the steel, atomic hydrogen is converted to

SALT WATER

Figure 2.1 Bent nail in seawater showing the flow of electrons (and electrical current)-stress cell.


molecular hydrogen, which gives rise to a high stress in the metal (hydrogen-induced stress). Presence of atomic hydrogen in steel reduces the ductility of the steel and causes it to break in a brittle manner.

The amount of atomic hydrogen required to initiate sulfide stress cracking appears to be small, possibility as low as 1 ppm. Sufficient hydrogen must be available, however, to establish a differential gradient required to initiate and propagate a crack. The H2S concentration as low as 1-3 ppm can produce cracking of highly stressed and high- strength steels (Wilhelm and Kane, 1987).

According to Cron and Marsh (1983, p. 1035), very low concentrations of H,S (0.1 ppm) and low partial pressures (0.001 atm) can cause SSC. The time to failure is decreased with increasing concentration of H2S and increasing stress. Maintaining a high temperature prevents cracking, because SSC does not occur at temperatures above =180"F.

Stress-corrosion cracking can occur in most alloys, and the corrodants which promote stress cracking may differ (few in number for each alloy). Cracking can occur in both acidic and alkaline environments, usually in the presence of chloride ion and/or oxygen.

Engineers should monitor the condition of tubular goods (casing, pipelines, etc.) in areas that are subsidence-prone due to fluid (oil, water) withdrawal (see Chilingarian et al., 1995). Faults and fractures form as a result of subsidence, which can cause appreciable stress in the casing for example. In turn, stressed metals are corrosion-prone.

2.5 TYPES OF CRACKING IN DRILLING AND PRODUCING ENVIRONMENTS

Hydrogen embrittlement (sulfide cracking) and corrosion fatigue are two forms of cracking that are associated with drilling and producing environments.

2.5.1 Hydrogen Embrittlement (Sulfide Cracking)

Hydrogen embrittlement occurs as a sudden cracking of metal, caused by the entrapment of hydrogen within the lattice structure. The cracking of the metal may proceed in a stepwise rupturing manner. The corrosion process in an acid environment produces atomic hydrogen:

Fe+2H+ + Fe2++2Ho (atoms) (2.5)

2.5 Types of Cracking in Drilling and Producing Environments 43

Fe - 2e- + Fez+ (anode) (2.6)

2H++2e- +2Ho (cathode) (2.7)

Some of the hydrogen atoms that are formed in the cathodic reaction penetrate the metal with subsequent formation of molecular hydrogen. The remaining hydrogen atoms combine to form molecules of hydrogen gas at the metal surface. The adsorption of hydrogen atoms by the metal causes a loss in ductility and cracking of high-strength steels.

Materials, which interfere with the pairing of atoms of hydrogen to form hydrogen gas at cathodic areas of metal, enhance the penetration of atomic hydrogen into the steel. Hydrogen sulfide in drilling fluids supplies sulfide ions, which prevent the pairing of hydrogen atoms to form hydrogen gas. Thus, the penetration of atomic hydrogen into steels is promoted by the presence of hydrogen sulfide. The time to failure by hydrogen embrittlement is shortened by increasing:

1. Concentration of hydrogen sulfide 2. Stress 3. Strength and hardness of steel

This behavior is illustrated by Hudgins et al. (1966) in Figures 2.2 and 2.3. The embrittlement (sulfide stress cracking [SSC]) occurs gener- ally in steels with yield strengths above 90,000 psi or above Rockwell C-20-22 hardness. Lower-strength steels are not subject to SSC; however, they are subject to hydrogen blistering.

2.5.2 Hydrogen Blistering

Hydrogen penetration of low-strength steel may cause blistering, which appears as bumps on the metal surface. The hydrogen atoms form hydrogen molecules at points of defect in the metal. The hydrogen gas cannot penetrate into nor escape from the crystalline structure. The pressure of hydrogen gas increases sufficiently to part the metal, create a void. and raise a blister on the steel surface.

2.5.3 Corrosion Fatigue

Cyclic stresses at high levels cause fatigue failure of metal. In a corrosive environment, the progress of fatigue is accelerated by


15

4 0

35

u 30 [L

In- c 5 25 I"

20

- Applied stresses Expressed as % of yield deformation

k Y Week Month I I I I I I

Figure 2.2 Relationship (approximate) between hardness and time to failure a t different applied stresses, expressed as percentage of yield deformation, for carbon steels in 5% NaCl solutions containing 3,000 ppm H2S (After Hudgins, 1969, p. 42, figure 1; courtesy of Materials Protection).

electrochemical corrosion. Corrosion fatigue is the combined dction of corrosion and fatigue (cyclic stressing), which results in early fracture of metal. Corrosion fatigue is the principal cause of drill pipe failures. By establishing limits to the stresses repeatedly applied to metal, failure by metal fatigue in a noncorrosive environment can be avoided. The endurance limit is the maximum cyclic stress level which can be applied to a metal without a fatigue failure. In an environment with continued corrosion, instead of exhibiting an endurance limit, metal will fail due to the growth of corrosion fatigue cracks. The time to failure or number of stress cycles necessary to cause failure is decreased with increasing severity of corrosive environment and level of stress as illustrated in Figure 2.4.

Corrosion fatigue is enhanced by corrodants in a corrosive environment which cause pitting of steel, such as oxygen and acid gases (H2S and C02), which are often present in drilling and produced fluids. Increasing the strength of steels will not improve the resistance to corrosion fatigue. Instead, it may shorten the number of cycles to failure.

2.5 Types of Cracking in Drilling and Producing Environments 45

4 0

35

30

U 0 v; 2 5

I" 20

VI

C 0

l5 Stress level - 130% Yield detorm

10 0.5 1 5 10 50 100 500 1,000

Time to tailure, hours

Figure 2.3 Relationship (approximate) between hardness and time to failure for carbon steels in 5% NaCl solution containing various concentrations of H2S. Stress level = 130% of yield deformation (After Hudgins, 1969, p. 43, figure 2; courtesy of Materials Protection).

\ \ \

I I t I

lo4 lo5 106 10'

Cycles for failure ( N )

Figure 2.4 Relationship between endurance strength of steel stress and number of cycles needed for the occurrence of failure in corrosive and noncorrosive environments (After Bertness, 1957, p.131, figure 1; courtesy of Division of Production, American Petroleum Institute).


High-strength steels are more susceptible to pitting than the low- strength steels.

Notches, such as the ones caused by tongs or slips, and corrosion under protectors will accelerate corrosion fatigue failure. The relationship of fatigue to environment, tensile strength, and surface condition of steel is illustrated in Figure 2.5.

Figure 2.5 Relationship between endurance limit and tensile strength for polished, notched, and corroding specimens (applied to ordinary corrodible steels) (After Battelle Memorial Institute, 1949, p. 78, figure 93; courtesy of John Wiley and Sons, Inc.).

References and Bibliography 47

Inhibitors, which reduce the corrosion and the entry of corrosion- generated hydrogen into the rods, can reduce the frequency of corrosion fatigue failures provided that stress is within a reasonable range. Examples of field test of inhibitors for control of corrosion fatigue are discussed by Martin (1980,1983) (also see Chapter 4).

REFERENCES AND BIBLIOGRAPHY

Annand, R. R., 1981. Corrosion Characteristics and Control in Deep, Hot Gas Wells. Southwestern Petroleum Short Course.

Battelle Memorial Institute, 1949. Prevention of the Failure of Metals Under Repeated Stress. New York, NY. Wiley, 295 pp.

Bertness, T. A., 1957. Reduction of failures caused by corrosion in pumping wells. API Dril. Prod. Pract., 37: 129-135.

Bertness, T. A., Chilingar, G. V., and Al-Bassam, M., 1989. Corrosion in drilling and producing operations. In: G. V. Chilingar, J. Robertson, and S . Kumar (Editors), Surface Operations in Petroleum Production, IZ, Amsterdam: Elsevier, pp. 283-317.

Chilingar, G. V. and Beeson, C. M., 1969. Surface Operations in Petroleum Production. New York, NY American Elsevier, 397 pp.

Cron,C. J. and Marsh, G.A., 1983. Overview of economics and engineering aspects of corrosion in oil and gas production. J. Pet. Technol., June: 1033-1041.

Davis, J. B., 1967. Petroleum Microbiology. Amsterdam: Elsevier, 604 pp. Dean, H. J., 1977. Avoiding drilling and completion corrosion. Pet. Eng., lO(9):

Deming, H. G., 1940. Fundamental Chemistry. New York and London: John Wiley and Sons, Inc., 756 pp.

Doig, K. and Wachter, A. P., 1951. Bacterial casing corrosion in the Ventura Field. Corrosion, 7 : 221-224.

Fontana, M. G. and Greene, N. D., 1967. Corrosion Engineering. New York, NY McGraw-Hill, 391 pp.

Gatzke, L. K. and Hausler, R. H., 1983. Gas well corrosion inhibition with KP 223/KP 250. NACE Annual Con$, April 18-22, Anaheim, CA.

Hackerman, N. and Snavely, E. S., 1971. Fundamentals of inhibitors. In: NACE Basic Corrosion Course. NACE, Houston, TX, (9): 1-25.

Hilliard, H. M., 1980. Corrosion control in Cotton Valley production. SOC. Pet. Eng. Cotton Valley Symp., SPE 9062,Tyler, TX, May 21: 4 pp.

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Hudgins, C. M., McGlasson, R. L., Mehdizadeh, F'., and Rosborough, W. M., 1966. Hydrogen sulfide cracking of carbon and alloy steels. Corrosion, 22(8):

23-28.

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Ironite Products Co., 1979. Hydrogen Suljide Control. 41 pp. May, P. D., 1978. Hydrogen sulfide control. Drilling-DCW, April.


Martin, R. L., 1979. Potentiodynamic polarization studies in the field. Muter. Perform., 18(3): 41-50.

Martin, R. L., 1980. Inhibition of corrosion fatigue of oil well sucker rod strings. Muter. Perform., 19(6): 20-23.

Meyer, F. H., Riggs, 0. L., McGlasson, R. L., and Sudbury, J. D. 1958. Corrosion products of mild steel in hydrogen sulfide environments. Corrosion, 14(2):

National Association of Corrosion Engineers, 1979. Corrosion Control in Petroleum Production, NACE TPC Publ. No. 5: 101 pp.

NGAA (Natural Gasoline Association of America), 1953. Condensate Well Corrosion. Tulsa, OK: N.G.A.A., 203 pp.

Oxford, W. F. Jr. and Foss, R. E., 1958. Corrosion of Oil- and Gas-Well Equip- ment. Dallas, TX: Division of Production, American Petroleum Institute,

Rahman, S. S. and Chilingarian, G.V., 1995. Casing Design: Theory and Practice. Elsevier, Amsterdam, The Netherlands, 373 pp.

Ray, J. D., Randall, B. V., and Parker, J. C., 1978. Use of Reactive Iron Oxide to Remove H2S from Drilling Fluid. 53rd Annu. Fall Tech. Conf. SOC. Pet. Eng. AIME, Oct. 1-3, Houston, TX: 4 pp.

Rhodes, F. H. and Clark, J. M., 1936. Corrosion of metals by water and carbon dioxide under pressure. Znd. Eng. Chem., 28(9): 1078-1079.

Simpson, J. P., 1979. A new approach to oil-base muds for lower-cost drilling. J. Pet. Technol., 31(5): 643-650.

Snavely, E. S., 1971. Chemical removal of oxygen from natural waters. J. Pet. Technol., 23(4): 443-446.

Snavely, E. S. and Blount, F. E., 1969. Rates of reaction of dissolved oxygen with scavengers in sweet and sour brines. Corrosion, 25(10): 397-404.

Staehle, R. W., 1978. $70 billion plus or minus $21 billion (editorial). Corrosion,

Uhlig, H. H. (Editor), 1948. The Corrosion Handbook. New York, N Y Wiley,

Uhlig, H. H., 1965. Corrosion and Corrosion Control, 3rd ed. New York, N Y Wiley, 371 pp.

Watkins, J. W. and Wright, J., 1953. Corrosive action on steel by gases dissolved in water. Pet. Eng., 25(12, Nov.): B50-B57.

Wendt, R. P., 1979. The kinetics of Ironite Sponge H2S reactions. Pet. Div. Am. Soc. Mech. Eng., Energy Technol. Con$, Houston, TX, Nov. 5-9,1978:7 pp.

Wendt, R. P., 1979. Control of hydrogen sulfide by alkalinity may be dangerous to your health. Pet. Eng. Int., 51(6, May): 66-74.

Wendt, R. P., 1979. Alkalinity control of H2S in muds is not always safe. World Oil, 188 (2, Aug.): 60-61.

Whitman, W., Russell, R., and Altieri, V., 1924. Effect of Hydrogen-Ion Con- centration on the Submerged Corrosion of Steel. Znd. Eng. Chem., 16: 665.

Wilhelm, S. M. and Kane, R. D., 1987. Status Report: Corrosion Resistant Alloys, Petroleum Engineering International, March: 3641.

109t-115 t.

87 PP.

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1188 pp.

types of corrosion.pdf

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