Intergranular Corrosion Testing: ASTM A262 and Sensitization

The problem: sensitization

When austenitic stainless steel spends time in the range of roughly 450–850 °C — most commonly in the heat-affected zone next to a weld — carbon migrates to the grain boundaries and precipitates as chromium carbide. The metal immediately beside each boundary loses chromium, and can fall below the level needed to maintain the passive film. The grains themselves remain sound, but the boundaries form a continuous, corrosion-prone network. In an aggressive medium the steel is attacked along the boundaries and can literally lose grains — intergranular corrosion.

Three defences exist by grade design: L grades (carbon limited to 0.03% maximum) starve the reaction, stabilized grades (321 with titanium, 347 with niobium) lock the carbon up in harmless carbides, and correct solution annealing dissolves carbides that have formed.

The ASTM A262 practices

ASTM A262 is not one test but a landscape of practices, each aimed at particular alloys and attack mechanisms:

Practice	Test medium and nature	Typically used for
A	Oxalic acid etch — rapid screening; classifies the etched microstructure	First filter: acceptable structures need no further test
B	Ferric sulfate–sulfuric acid — mass-loss (corrosion rate) result	Molybdenum-bearing grades such as 316/317, and stabilized grades
C	Nitric acid (Huey test) — five boiling periods, mass loss per period	High-chromium grades; also reveals sigma-phase effects in molybdenum grades
E	Copper–copper sulfate–sulfuric acid (Strauss test) — exposure, then a bend test: cracks reveal attack	Chromium-depletion detection in grades such as 304/304L and stabilized types
F	Copper–copper sulfate–50% sulfuric acid — mass loss	Cast austenitic alloys and high-molybdenum grades

Europe covers similar ground in EN ISO 3651-2, with its own methods and evaluation rules; a specification should name one system and one practice, not just "IGC test".

When buyers specify IGC testing

• Welded equipment for chemical, pharmaceutical and food service, where the weld zones are the exposed area.
• Nitric-acid and strongly oxidizing services, where Practice C results are traditionally quoted.
• Any non-L, non-stabilized austenitic grade that will be welded or see the sensitizing temperature range in fabrication.
• Qualification of material after suspect heat treatment, straightening or repair.

Interpreting the result honestly

An A262 result certifies the condition of the tested sample — a sensitized specimen fails even if its grade chemistry is correct, and a well-made specimen of a marginal grade can pass. Mass-loss figures are practice-specific comparative rates, not predictions of service life, and acceptance limits (where numerical) are set by the purchase specification, not by A262 itself. The test is therefore best used as a gate on material condition and fabrication practice — exactly the things a chemistry certificate cannot show.