Intergranular Corrosion and Sensitisation in Stainless Steel Pipe Fittings: Cause, Testing, and Prevention

The Sensitisation Mechanism

In standard austenitic stainless steels (Type 304, 316) with carbon content above approximately 0.03%, exposure to temperatures in the range 450–850°C causes chromium to diffuse from the bulk grain to the grain boundary, where it reacts with carbon to form chromium carbides (primarily Cr₂₃C₆). The chromium carbides precipitate along the grain boundary within seconds to minutes at the peak sensitisation temperature (~700°C). The chromium depletion zone immediately adjacent to the grain boundary — where chromium has diffused away to feed the carbide — falls below the ~12% Cr threshold needed for passivity. This depleted zone is now susceptible to preferential corrosion in oxidising acid environments, particularly in nitric acid (the classic test medium), polythionic acid (a risk in refinery service during shutdown), and concentrated sulphuric acid. The corrosion attacks only the chromium-depleted zone at the grain boundary, leaving the grain interiors relatively intact — the result is a rapid loss of structural integrity as grain boundaries dissolve, even though the bulk alloy composition appears correct.

Weld Decay

The most common source of sensitisation in pipe fittings is the heat of welding — the region of the fitting HAZ that is exposed to the 450–850°C sensitisation temperature range during the weld thermal cycle. This sensitised HAZ region is called "weld decay" because it appears as a band of corrosion attack parallel to and slightly away from the weld bead (in the zone heated to ~600–700°C, not at the weld fusion line itself). Weld decay was a common failure mode in standard 304 and 316 stainless pipe fittings in acidic process service before the introduction of L-grade variants. It occurs even in a single-pass weld on a fitting that was previously solution-annealed and non-sensitised — the weld thermal cycle re-sensitises the HAZ regardless of the fitting's prior heat treatment history.

Prevention: L-Grades and Stabilised Grades

Two material strategies prevent sensitisation: Low-carbon grades (304L, 316L) — carbon content ≤ 0.03% maximum (vs 0.08% for standard 304/316). At this low carbon level, there is insufficient carbon to form a continuous chromium carbide network at grain boundaries even after extended exposure to the sensitisation temperature range. 304L and 316L are standard for virtually all welded stainless steel pipe fittings in corrosive service — the L-grade suffix is not "lower quality" but specifically denotes sensitisation resistance. Stabilised grades (321, 347) — contain titanium (321, Ti/C ratio ≥ 5) or niobium (347, Nb/C ratio ≥ 8) that preferentially combines with carbon to form stable titanium or niobium carbides, leaving insufficient carbon available to form chromium carbides at grain boundaries. Stabilised grades are used where the L-grade carbon limit cannot be guaranteed, or in service above approximately 400°C where L-grades may gradually sensitise over very long time periods (intergranular sensitisation is a function of both time and temperature).

Testing for Sensitisation: ASTM A262

ASTM A262 provides five standard practices (A through E) for testing stainless steel for susceptibility to intergranular corrosion. The most commonly specified for pipe fittings are: Practice B (Ferric Sulphate-Sulphuric Acid Test, the "Streicher test") — measures the corrosion rate of the sensitised material in a boiling 50% H₂SO₄ + Fe₂(SO₄)₃ solution; sensitised material shows corrosion rates typically 10–100× higher than solution-annealed material. Practice E (Copper-Copper Sulphate-Sulphuric Acid Test, the "Strauss test") — the test specimen is bent after exposure; sensitised grain boundaries crack while unsensitised material bends without cracking. Both tests are destructive — they require a test coupon from the production heat. For pipe fitting procurement in critical service (nitric acid, polythionic acid risk, or any service requiring post-sensitisation performance guarantee), specifying ASTM A262 Practice E on a heat-representative coupon in the purchase order is the appropriate verification. Without this requirement, the supplier delivers material that meets the chemistry specification but has not been tested for sensitisation resistance.

Polythionic Acid SCC in Refinery Shutdown

In refinery service, sensitised stainless steel is at risk of polythionic acid stress corrosion cracking (PTA-SCC) during shutdown. Polythionic acids (H₂SₓO₆, x = 3–5) form when iron sulphide scale on the internal fitting surface contacts moist air during shutdown — a combination unique to the refinery environment. PTA-SCC is intergranular, occurs at ambient temperature, and can cause cracking in the sensitised HAZ of previously-welded stainless fittings within hours of first air contact. Prevention: specify 321 or 347 stabilised stainless for refinery service where the process contains H₂S and there is a risk of air ingress during shutdown; or use NACE RP0170 neutralisation procedure (soda ash wash before opening) on 304L/316L equipment during shutdown to raise the pH above the range where polythionic acids are stable.