What is Stress Corrosion Cracking? (SCC)
Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature.
SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks when closely examined microscopiccaly. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.
The stresses can be the result of the crevice loads due to stress concentration, or can be caused by the type of assembly or residual stresses from fabrication (e.g. cold working); the residual stresses can be relieved by annealing or other surface treatments. Unexpected and premature failure of chemical process equipment, for example, due to stress corrosion cracking, constitutes a serious hazard in terms of safety of personnel, operating facilities and the environment.
Typically Stress Corrosion Cracking is observed in critical equipment for indsutries that generate or work with certain chemicals, including for example fertilizer companies, on- & off-shore processing, and other petrochemical industrial sites but also for aerospace and in gas turbines or in critical equipment such as piping.
What causes Stress Corrosion Cracking? (SCC)
The cause of Stress Corrosion Cracking is dependent on many factors, but primarily the environment and the material used for the application. Some Austenitic Stainless Steels and aluminium alloys can crack in the presence of chlorides, limiting the usefullness of austenitic stainless steel for containing water with higher than a few parts per million content of chlorids at temperatures above 50 degrees celsius.
In copper alloys, cracking is observed due to ammoniacal solutions being present affecting the copper and forming cuprammonium ions. [Cu(NH3)4]2+ The copper-alloy ultimately cracks because of the residual stresses in the material and this is also known as ‘season cracking’.
Typical High-Tensile Steels suchj as 4140 or 4340 have been known to crack unexpectedly, becoming very brittle especially in the presence of alakalides or chlorides
How can we prevent Stress Corrosion Cracking? (SCC)
Besides carefully considering the applied materials for application and environment, mitigating sharp transitions in regions of the components that experience regular loading and thus experience stresses is key to reducing impacts of SCC.
Literature also specifies the type of materials, applications and environments that should be closely monitored, and such leading expert knowledge-bases are the American Petroleum Institute and/or the AMPP – Association for Materials Protection and Performance. (Former NACE – National Association of Corrosion Engineers/ SCPP – The Society for Protective Coatings.)
Fracture Mechanics Analysis – Crack Inititation and Propagation.
Typically SCC can be investigated by performing a so-called Fracture Mechanics analysis, where various computational methods are used to simulate components in their applied environment and to observe the stresses they typically encounter in daily operation. Such studies look at crack initiation as a baseline to determine the origin of the corrosion, as well as the crack-propagation over time as a result of experiencing typical loads.
It is not uncommon to also perform a metallurgical analysis and corrosion testing, where test specimens are generated in a laboratory and purposefully corroded to recreate the conditions observed in the field. The findings are then used to fine-tune such Fracture Mechanics Analysis to accurately deteremine strain, stress, torsional modes etc. dependent on the component geometry as well as an estimated remaining lifetime assessment.
Are you experiencing Stress Corrosion Cracking? Challenge Alloy-Search to adress your problems in the field.