Environmentally Induced Cracking (EIC)

Environmentally Induced Cracking is a general term for brittle mechanical failures that occur when a susceptible alloy is under tensile stress in a corrosive environment. If any one of these conditions are eliminated, EIC will not occur.

Stress Corrosion Cracking

Differential stresses from a variety of sources can cause metal to corrode. In stress corrosion, the area of highest stress becomes anodic while the area of lower stress becomes cathodic. This can often be observed at couplings.

Cracks can form in the anodic section, and are usually clean, sharp, and branching. They may be transgranular or intergranular. Tool marks and similar scratches on the surface of pipes may also become anodic. Cathodic protection is one method of mitigating stress corrosion cracking.

Almost all metals and alloys are susceptible to SCC in some environment. Iron-chromium stainless steels (e.g., type 410) are susceptible to SCC in chloride containing environment, e.g., seawater. Addition of nickel to Fe-Cr alloys alters the susceptibility; 8% nickel alloys (300 series stainless steels) shows minimum SCC resistance. Type 304 SS fails in ZrBr2 completion fluids. Resistance to SCC increases with increasing nickel content, become immune at nickel content 40% and above (e.g., nickel-base alloys).

Carbon steels are susceptible to SCC in hot caustic solutions. This type of failure is also known as caustic embrittlement. In both stainless steels and carbon steels, increasing stress, temperature, and concentration of specific ions, reduce SCC resistance.

Sulfide Stress Cracking (SSC), which causes catastrophic failures in high-strength drill pipes, casing, and sucker rods in sour environments, is a special case of hydrogen embrittlement and will be discussed in another section.

Corrosion Fatigue Cracking

In this type of corrosion, cracks are created under cyclic tensile stress. The cracks are usually transgranular and nonbranching, blunt, contain corrosion products (crud) and have striations (beachmarks).

In the oilfield, with the exception of metallurgical defects, practically all breaks in the body of sucker rods are caused by corrosion fatigue. It should also be recognized that all metals would eventually fail when they reach their endurance limit. During fatigue tests in air below a nominal threshold stress or fatigue limit, in excess of 100 million (108) cycles are necessary before the endurance limit is reached and the failure occurs. This condition is recognized by the presence of many fatigue crack on the shiny metal surface.

In water, however, the threshold stress level would be much lower than air, and in a corrosive environment threshold stress or fatigue limit may be completely lost. Failure may occur after few thousand cycles even at much lower stress levels. Fatigue cracks would be observed initiating from the corrosion pits. Mechanical damage, i.e., nicks, dents, gouges, and hammer blows may also provide sites for crack initiation. (See figure) With proper corrosion control measures, at least 108 cycles should be possible, i.e., approximately 20 years at 10 strokes per minute pumping rate, before the fatigue failure occurs.