Environment-Assisted Cracking
The three-legged model
Environment-assisted cracking requires a susceptible material, a specific environment, and tensile stress, usually residual stress from welding; removal of any leg prevents cracking. EAC is threshold driven, so inspection is for cracks rather than thinning, and the crack morphology is usually branched and brittle with little general metal loss.
Families to know
Chloride SCC occurs in 300-series stainless steels above roughly 140 deg F in the presence of chlorides and water, classically under insulation or in stagnant low-flow areas. Caustic SCC affects carbon steel and stainless steel where caustic concentrates at elevated temperature; avoid steam-traced stagnant caustic lines and ensure dilution. Amine SCC attacks carbon steel in amine units, especially in concentrated and warm locations and at weldments. Ammonia SCC attacks copper alloys. Carbonate SCC affects carbon steel with carbon dioxide, water, and high pH. Polythionic acid SCC affects sensitized 300-series stainless in refinery sulfur services after shutdown, when sulfide scale meets moisture and oxygen; prevent it with a soda ash wash, nitrogen purge, and low-sulfur startup fuel. Sulfide stress cracking, hydrogen-induced cracking, and stress-oriented hydrogen-induced cracking occur in wet H2S service and are controlled by hardness, chemistry, and post-weld heat treatment. Hydrogen stress cracking occurs in high-strength steels and in HF alkylation; liquid metal embrittlement occurs when a susceptible metal contacts a liquid metal.
Traps and a worked example
Traps include selecting chloride SCC for carbon steel, forgetting that SCC needs stress, and confusing HIC (blistering and stepwise cracking without applied stress) with SSC. Worked example: an insulated 304 stainless line at 180 deg F shows branched, transgranular cracking from the outside surface. Material, temperature, chlorides from insulation, and residual stress confirm external chloride SCC; the fix is a coating and watertight cladding, not a process change.