What three conditions does stress-corrosion cracking need?
Stress-corrosion cracking needs a susceptible alloy, a sustained tensile stress and a specific environment, all present at the same time. The three legs of stress-corrosion cracking are usually stated as alloy, stress and environment, and the word carrying the weight is “and”.
A susceptible alloy in a benign environment does not crack. The same alloy in an aggressive environment with no tensile stress does not crack. Each of the three conditions for stress-corrosion cracking is individually ordinary; the coincidence of all three is what is rare, which is why stress-corrosion cracking so often appears years into service rather than at commissioning.
Is an alloy susceptible to stress-corrosion cracking in general, or only in certain environments?
An alloy is susceptible to stress-corrosion cracking only in particular environments; there is no such thing as an alloy susceptible to stress-corrosion cracking in the abstract. Austenitic stainless steels resist a great many environments and crack readily in hot chlorides. Copper alloys are largely indifferent to chlorides and crack in ammonia. Carbon steel is unremarkable in most waters and cracks in concentrated caustic.
The pairing of alloy and environment is the mechanism of stress-corrosion cracking, and a specification that selected a corrosion-resistant material without naming the service environment has not addressed stress-corrosion cracking at all.
The condition of an alloy matters as much as its grade. An austenitic stainless steel that has been sensitized, with chromium carbides precipitated at grain boundaries during welding or slow cooling, becomes vulnerable along those grain boundaries to environments that leave the same alloy untouched when solution annealed. ASTM A262 sets out the practices used to detect the susceptibility that sensitization creates.
What kind of stress causes stress-corrosion cracking?
The stress that causes stress-corrosion cracking is tensile, sustained, and present at the surface where the environment is, and it acts as a threshold rather than as a load case. That stress need not be the design load, and frequently is not: residual stress from welding, forming or machining routinely reaches a substantial fraction of yield and is enough on its own to supply the stress for stress-corrosion cracking.
Cyclic loading moves the problem into corrosion fatigue, a different mechanism from stress-corrosion cracking with different fracture features. Stress-corrosion cracking is a static-stress phenomenon, which is why a vessel sitting idle at ambient pressure is not thereby safe from stress-corrosion cracking.
Does the bulk process chemistry or the local chemistry govern stress-corrosion cracking?
The chemistry that governs stress-corrosion cracking is the local chemistry at the crack tip, not the bulk chemistry in the process stream. Evaporation at a hot surface, wet-dry cycling, capillary action in a crevice, and concentration under a deposit or beneath disbonded coating can each produce a local environment orders of magnitude more aggressive than the bulk.
The environment is the leg of stress-corrosion cracking most often argued about, because bulk conditions are recorded in logs and local conditions at the metal surface are not.
How can stress-corrosion cracking be prevented?
Stress-corrosion cracking can be prevented by removing any one of its three conditions: the susceptibility of the alloy, the tensile stress or the environment. Because the three conditions are necessary rather than merely contributory, stress-corrosion cracking is more tractable than mechanisms driven by accumulated damage.
A different alloy removes susceptibility. Post-weld heat treatment or solution annealing removes the residual stress. Drying insulation, controlling chloride ingress, adjusting chemistry or eliminating an ammonia source removes the environment. Any one of these measures prevents stress-corrosion cracking, and the choice between them is ordinarily a matter of cost and practicality rather than technical possibility.
The fact that removing one condition prevents stress-corrosion cracking is also what makes stress-corrosion cracking a specification question. The decision determining whether a component would crack by stress-corrosion cracking was usually made in a materials selection document, a weld procedure or an insulation detail, years before anything went wrong.
How does the three-condition test help in a stress-corrosion cracking dispute?
In a stress-corrosion cracking dispute, the three-condition test frames the investigation as three separate factual questions (which alloy and in what condition, what stress and from where, what species and at what concentration), which separates issues that otherwise blur together.
Each of the three stress-corrosion cracking questions has its own evidence and often its own custodian: mill certificates and material verification for the alloy, fabrication and heat-treatment records for the stress, and process and inspection history for the environment.
What do ASTM stress-corrosion cracking tests establish?
ASTM stress-corrosion cracking tests establish that an alloy could crack in a given environment; they do not establish that it did. The ASTM G-series provides standardized ways to hold exemplar material at a known stress in a known environment in order to establish susceptibility to stress-corrosion cracking.
Within the ASTM G-series, G30 covers U-bend specimens, G38 C-rings, G39 bent-beam and G49 direct-tension loading, with G44 describing alternate-immersion exposure and G36 the boiling magnesium chloride test used to rank austenitic stainless steels. ASTM G123 covers testing of stainless alloys in boiling acidified sodium chloride.
Which standards name stress-corrosion cracking environments in advance?
Some environments are aggressive enough that industry has written the constraint down in advance, in documents including NACE/AMPP MR0175 with ISO 15156 and API RP 571.
NACE/AMPP MR0175 with ISO 15156 governs materials selection for hydrogen sulfide-bearing oil and gas service, setting limits on alloy, hardness and condition; the hydrogen-dominated end of that spectrum is a distinct mechanism from stress-corrosion cracking and is addressed separately. API RP 571 catalogs damage mechanisms in refining equipment, including the stress-corrosion cracking families, with the conditions under which each is expected.
Where such a document applied and the selected material sat outside it, the stress-corrosion cracking specification question largely answers itself.
Does the three-condition framing settle a stress-corrosion cracking case?
Not by itself. The three-condition framing establishes that stress-corrosion cracking was possible and identifies which condition could have been removed, but it does not, by itself, establish when cracking initiated, how fast it grew, or whether the local environment that developed was foreseeable.
When stress-corrosion cracking initiated, how fast it grew and whether the local environment was foreseeable remain matters of inspection history, operating records and the standard of care at the time, and they are where competent stress-corrosion cracking analyses most often disagree.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.