Stress-corrosion cracking is easy to miss and reasonably easy to recognise once someone is looking for it. The damage does not announce itself the way corrosion does: there may be no measurable wall loss, no rust staining, nothing a thickness survey would flag. What there is instead is a network of fine, often branching cracks running through a section that is otherwise intact, and a surface chemistry that identifies which species drove them. Both are fragile forms of evidence, and both are routinely destroyed in the first hours after discovery.
Cracking without section loss
The defining feature is the mismatch between how little metal is missing and how compromised the component is. General or localised corrosion consumes material and can be quantified by thickness measurement. SCC propagates as tight cracks that may be all but invisible from outside, sometimes under nothing more than discoloration. Equipment can pass a wall-thickness inspection while carrying a crack network through most of the wall, which is why inspection aimed at finding cracks is not interchangeable with inspection aimed at measuring loss.
Branching, and what it indicates
Cracks that divide and re-divide as they run are characteristic. The pattern arises because propagation follows whichever local path the environment and microstructure favour rather than a single plane of maximum stress. That distinguishes SCC from fatigue, which tends to advance on one front and leaves beach marks or striations, and from overload, which leaves a single fast fracture. Multiple parallel cracks across an area, rather than one crack at a stress riser, points the same way.
Intergranular or transgranular
Which path the cracks take through the microstructure narrows the mechanism considerably. Chloride SCC of austenitic stainless steel is typically transgranular and heavily branched. Caustic cracking of carbon steel, and polythionic acid cracking of sensitised stainless during shutdowns, are typically intergranular, following grain boundaries whose chemistry has been altered. Metallography of a cross-section and scanning electron fractography of an opened crack answer this directly, and ASTM A262 provides the practices for establishing whether a stainless steel was sensitised.
The neighbouring mechanisms
Corrosion fatigue requires cyclic loading and shows fatigue features modified by corrosion; the loading history usually separates it. Hydrogen-driven cracking is a distinct mechanism, treated separately. Mechanical overload, creep and weld defects each leave their own signatures. These are not difficult to tell apart in the laboratory. They are easy to conflate in a description written at the site before anyone has looked at a section.
Chloride cracking under insulation
Austenitic stainless steel under thermal insulation is the classic setting. Insulation absorbs and holds water — rain, wash-down, condensation, leaking steam — against a hot surface, chlorides arrive in that water or leach from the insulation itself, and wet-dry cycling concentrates them. The cracking is hidden by the jacketing, so by the time insulation comes off during a turnaround the damage has had years to develop. Temperature matters: the mechanism is conventionally associated with surfaces above roughly 60 degrees Celsius, though that is a rule of thumb rather than a boundary.
Why a harmless concentration does not stay harmless
The most common objection to a chloride SCC finding is that the water chemistry logs show concentrations far below anything considered aggressive. The objection misunderstands where the reaction happens. Bulk concentration is not crack-tip concentration. Evaporation at a heated surface, boiling in a crevice, drying and rewetting under insulation or a deposit, and capillary retention in a tight gap can each raise local concentration by orders of magnitude. A few parts per million in the stream is entirely compatible with a near-saturated brine at the metal surface.
Caustic, amine and ammonia environments
Concentrated sodium hydroxide cracks stressed carbon and low-alloy steel — historically called boiler embrittlement — again with evaporation or steam blanketing doing the concentrating, and again at welds and bends. Amine solutions used for gas treating crack carbon steel in comparable fashion, which is why stress relief of amine service welds is standard practice. Ammonia cracks stressed brass and other copper alloys, the season cracking recognised long before the mechanism was understood, and trace exposure suffices. API RP 571 catalogues these families with the service conditions in which each is expected.
Preserving the evidence that identifies the species
Crack-face deposits and surface residues are often the only physical record of which species drove the cracking. Energy-dispersive spectroscopy on the fracture surface and ion chromatography of deposits and any retained liquid can identify chloride, sulfide, hydroxide or ammonia directly. Washing, pressure-cleaning, wire-brushing, sandblasting and grinding remove that record permanently, and every one of them is an ordinary reflex when a crack is found. Insulation, coating fragments, scale and residues should be bagged as recovered rather than swept up.
How the environment question is contested
Predictably, on chemistry: that logged concentrations were low, that the identified species has an alternative source, that the deposits were contaminated after the fact, or that the sample came from a location that does not represent the crack. Analyses documenting where each sample was taken, what the surface looked like before it was touched, and how the local concentrating mechanism is supposed to have worked hold up better than a single laboratory result offered on its own.
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.