Why can a component with stress-corrosion cracking pass a wall-thickness inspection?
A component with stress-corrosion cracking can pass a wall-thickness inspection because stress-corrosion cracking propagates as tight cracks rather than by consuming material, so there may be no measurable wall loss for a thickness survey to flag. The defining feature of stress-corrosion cracking is the mismatch between how little metal is missing and how compromised the component is.
General or localized corrosion consumes material and can be quantified by thickness measurement. Stress-corrosion cracks may be all but invisible from outside, sometimes under nothing more than discoloration. Equipment can pass a wall-thickness inspection while carrying a stress-corrosion crack network through most of the wall, which is why inspection aimed at finding cracks is not interchangeable with inspection aimed at measuring loss.
What does branched cracking indicate?
Branched cracking — cracks that divide and re-divide as they run — is characteristic of stress-corrosion cracking. The branching pattern arises because stress-corrosion cracks propagate along whichever local path the environment and microstructure favor rather than along a single plane of maximum stress.
Branching distinguishes stress-corrosion cracking 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, point the same way, toward stress-corrosion cracking.
Why does it matter whether stress-corrosion cracks are intergranular or transgranular?
Whether stress-corrosion cracks are intergranular or transgranular matters because the path the cracks take through the microstructure narrows the cracking mechanism considerably. Chloride stress-corrosion cracking of austenitic stainless steel is typically transgranular and heavily branched. Caustic cracking of carbon steel, and polythionic acid cracking of sensitized stainless steel 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 the intergranular-or-transgranular question directly, and ASTM A262 provides the practices for establishing whether a stainless steel was sensitized.
Is stress-corrosion cracking hard to tell apart from other failure mechanisms?
Stress-corrosion cracking is not difficult to tell apart from its neighboring mechanisms in the laboratory, but it is easy to conflate with them in a description written at the site before anyone has looked at a section. The neighboring mechanisms are corrosion fatigue, hydrogen-driven cracking, mechanical overload, creep and weld defects.
Corrosion fatigue requires cyclic loading and shows fatigue features modified by corrosion; the loading history usually separates corrosion fatigue from stress-corrosion cracking. Hydrogen-driven cracking is a distinct mechanism from stress-corrosion cracking and is treated separately. Mechanical overload, creep and weld defects each leave their own signatures.
Why is stainless steel under insulation the classic setting for chloride stress-corrosion cracking?
Austenitic stainless steel under thermal insulation is the classic setting for chloride stress-corrosion cracking because the 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.
Chloride stress-corrosion cracking under insulation is hidden by the jacketing, so by the time the insulation comes off during a turnaround the damage has had years to develop. Temperature matters: chloride stress-corrosion cracking of austenitic stainless steel is conventionally associated with surfaces above roughly 60 degrees Celsius, though that is a rule of thumb rather than a boundary.
Can a chloride concentration that looks harmless in the water chemistry logs still cause stress-corrosion cracking?
Yes. Water chemistry logs showing chloride concentrations far below anything considered aggressive do not rule out chloride stress-corrosion cracking, because bulk concentration is not crack-tip concentration. That low-concentration argument is the most common objection to a chloride stress-corrosion cracking finding, and it misunderstands where the reaction happens.
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 chloride concentration by orders of magnitude. A few parts per million of chloride in the stream is entirely compatible with a near-saturated brine at the metal surface.
Which metals do caustic, amine and ammonia environments crack?
Concentrated sodium hydroxide cracks stressed carbon and low-alloy steel, amine solutions used for gas treating crack carbon steel in comparable fashion, and ammonia cracks stressed brass and other copper alloys. Caustic cracking of carbon and low-alloy steel was historically called boiler embrittlement; in caustic cracking, evaporation or steam blanketing does the concentrating, and the cracking occurs at welds and bends.
Because amine solutions crack carbon steel, stress relief of amine service welds is standard practice. Ammonia cracking of stressed brass and other copper alloys is the season cracking recognized long before the mechanism was understood, and trace ammonia exposure suffices.
API RP 571 catalogs the caustic, amine and ammonia cracking families with the service conditions in which each is expected.
How is the evidence of which species drove stress-corrosion cracking preserved?
The evidence that identifies which species drove stress-corrosion cracking is preserved by leaving crack surfaces uncleaned and bagging insulation, coating fragments, scale and residues as recovered rather than sweeping them up, because 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 chemical record permanently, and every one of them is an ordinary reflex when a crack is found.
How are findings about a stress-corrosion cracking environment challenged?
Findings about the environment behind stress-corrosion cracking are predictably challenged 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.
Stress-corrosion cracking 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.