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Types of Corrosion

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

1. Uniform attack. The entire area of the metal corrodes uniformly resulting in thinning of the metal. This often occurs to drillpipe, but usually 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.

2. Crevice corrosion. This is an example of localized attack in the shielded areas of metal assemblies, such as pipes and collars, rod pins and boxes, tubing and drillpipe joints. Crevice corrosion is caused by concentration differences of a corrodant 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 a drillstring and is the common cause of washouts and destruction under rubber pipe protectors.

3. 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 sea water. low-pH aerated brines, or drilling fluids. Upon formation of a pit. corrosion continues as in a crevice but. usually, at an accelerated rate.

4. 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.

5. 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.

Ring worm corrosion is a selective attack which forms a groove around the pipe near the box or the external upset end. This type of selective attack is avoided by annealing the entire pipe after the upset is formed.

6. Selective leaching. 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.

7. Erosion-corrosion. The combination of erosion and corrosion results in se­vere localized attack of metal. Damage appears as a smooth groove or hole in the metal, such as in a washout of the drillpipe, 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 abrasive 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.

8. 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 and formation and collapse of the 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, examples 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.

9. Corrosion due to variation in fluid flow. Velocity differences and turbulence 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 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, bacteria and sediments, in pipeline and vessels.

10. Stress corrosion. The term stress corrosion includes 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 and unstressed metal is cathodic. The degree to which these stresses are induced in pipes varies with the metallurgical properties and the cold work caused by the weight of the pipe, effects of slips, notch effects at tool joints and the presence of H2S gas. In the oil fields, H2S-induced stress corrosion has been instrumental in bringing about sudden failure of pipes.

In the absence of sulphide, hydrogen collects in the presence of the pipe as a film of atomic hydrogen which quickly combines with itself to form molecular hydrogen gas (H2). The hydrogen gas molecules are too large to enter the steel and, therefore, usually bubble off harmlessly.

In the presence of sulphide, however, hydrogen gradient into the steel is greatly increased. The sulphide and higher concentrat ion of hydrogen atoms work together to maximize the number of hydrogen atoms that enter the steel. Once in the steel, atomic hydrogen tries to accumulate to form molec­ular hydrogen which results in high stress in the metal. This is known as 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 sulphide stress cracking appears to be small, possibly as low as 1 ppm, but sufficient hydrogen must be available to establish a differential gradient required to initiate and propagate a crack. Laboratory tests suggest that H2S concentrations as low as 1-3 ppm can produce cracking of highly-stressed and high-strength steels (Wilhelm and Kane, 1987).

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

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