How does Stress corrosion cracking occur and how to control it

Stress corrosion cracking involves a conjoint corrosion and straining of a metal resulted by a residual or applied stresses.

Irrespective of the increased use of polymers in the modern times, metals are still an essential part of structures for their high strength, toughness and potential to withstand high temperatures and high pressures. But the metals are prone to corrosion. Corrosion may occur in various forms. Here we discuss the stress corrosion cracking, also referred as SCC.

SCC may include different mechanisms when cracking is commonly caused by hydrogen embrittlement, it is also considered to be a part of SCC.

Stress corrosion cracking is a vigorous type of corrosion, it causes a remarkable loss of mechanical strength  with nominal metal loss, the damage is not evident from the causal evaluation. SCC may cause catastrophic damage to application components and structures. It is found that various major damages involved stress corrosion cracking including high pressure gas transmission pipes, boiler explosion and damage of power units and oil refineries.

The occurrence of stress corrosion cracking depends on the simultaneous presence of three factors:

a sensitive material, condition that cause stress corrosion cracking of that material and adequate tensile stress to cause stress corrosion cracking.

Occurrence of stress corrosion cracking

SCC is not an inevitable process and for various metals in most conditions, it does not occur. Hence it is easy to identify specific combinations of metal and environment that are sensitive to problem. But with the passage of time, more and more combinations are identified, particularly engineers endeavour to use materials more efficiently by increasing working stress and using economical materials.

Austenitic stainless steels experience stress corrosion cracking in hot solutions comprising of chlorides. A high chloride concentration although nominal magnitudes of chlorides are adequate at hot surfaces where chloride concentration may occur or where chloride is concentrated by pitting or crevice corrosion and issues can be noticed in fresh water.

The temperature suitable for SCC is normally above 70oC however stress corrosion cracking may occur at lower temperatures in some conditions, usually more acid solutions. The cracking progresses at low stresses and occurs at a result of residual stresses from welding or fabrication. The cracking is usually transgranular, however it may turn to an intergranular path due to sensitization of steel.

Protection from stress corrosion cracking

The first step to defence in controlling stress corrosion cracking is be informed of the feasibility in the design and development stages. By choosing a material that is not sensitive to stress corrosion cracking in the service condition and by processing and fabricating it suitably, the resultant scc problems can be prevented.

Mechanical needs like high yield strength can be tough to resolve with scc resistance. Residual stress can be eased by stress-relief annealing and is commonly used for carbons steels. Additionally by using high corrosion resistant nickel based alloys such as Hastelloy C276- a SCC resistant alloy, it is possible to significantly the control the problems that occur due to Stress corrosion cracking in various industrial processes.