How Stress corrosion cracking occurs in chloride environments

Austenitic stainless steels receive stress corrosion cracking in hot solutions comprising of chloride. A solution containing concentrated chloride should be present, however comparatively nominal magnitudes of chloride are adequate in heated surfaces where chloride content can occur or where chloride is concentrated by pitting or crevice attack and troubles may occur in tap water.

The factors resulting in SCC are different for those causing hydrogen embrittlement. The temperature often requires to be more than 70oC however stress corrosion cracking may occur at lower temperatures in some conditions, significantly higher acid solutions.  The cracking occurs at low stresses and usually due to residual stresses caused welding or fabrication. The cracking is usually transgranular, however it may switch to an intragranular path due to steel sensitization.

Carbon and low alloy steels can experience stress corrosion cracking in diverse environments that develop a security passivating layer of oxide or other materials. Cracking doesn’t occur even at noticeable corrosion rate which is not in case of hydrogen embrittlement. Diverse cases have been noticed to cause stress corrosion cracking such as strong caustic solutions, phosphates, nitrates, carbonates and hot water. The issues are considerable for economic and safety reasons. Caustic cracking of steam production boilers was a severe problem in the past century and boiler blasts took several lives.

Recently gas transmissions pipelines damaged due to carbonate solutions production in security layers. In such case the crack runs along the pipe length and may propagate for long distances by quick fracture. If a gas cloud that is released ignites, the resultant fireball is damaging.

Hydrogen embirttlement processes are often not firmly affected by the environment and that is needed is conditions that permit hydrogen to be developed by cathodic corrosion reaction and to penetrate the steel.

Other SCC mechanisms are much more specific and particular conditions are required for cracking to occur. It is because cracking is based on the feasibility of specific corrosion reactions at the crack edge with other reactions occurring on the crack walls and the sample surface. With just nominal variations in the environment one or other these needs may not be met and cracking will not occur.

While the need for a particular condition is advantageous in that it refers that stress corrosion cracking is relatively uncommon, it also makes life complicated for the materials engineer as it makes the occurrence of cracking instead unpredictable with slight differences in service environments causing remarkable changes in behavior.

The specific alloy chemistry, microstructure and heat processing of Hastelloy C22 bar can have a remarkable effect on stress corrosion cracking performance.

There are some general rules leading the effect of material strength on SCC sensitivity. For hydrogen embrittlement processes a strength usually increases the susceptibility, additionally higher strength materials have low K(ic) and hence damage by quick fracture with a nominal SCC crack. Processes that depend on plastic strain at the crack edge will be easier for lower strength materials. So various SCC systems like caustic cracking of carbon steels will become more vulnerable with reduction in strength.