Sensitization and Weld Decay in Stainless Steel

Sensitization and Weld Decay in Stainless Steel is a form of grain-boundary degradation in which austenitic stainless steel, held or slowly cooled through roughly 425 to 815 degrees Celsius, precipitates chromium-rich carbides at its grain boundaries, depletes the adjacent metal of chromium, and becomes locally susceptible to intergranular corrosion. When this happens in a band of the heat-affected zone (HAZ) beside a weld, the resulting attack is called weld decay.

The metallurgy: chromium carbide precipitation

Austenitic grades such as 304 and 316 resist corrosion because chromium in solid solution forms a passive oxide film. That protection depends on keeping roughly 10.5 to 12 percent chromium in solution at every point. Carbon has very low solubility in austenite at intermediate temperatures, so when the steel dwells in the sensitizing range, carbon and chromium combine as M23C6 carbide (predominantly Cr23C6) on grain boundaries. Chromium diffuses far more slowly than carbon, so a narrow zone flanking each boundary is drained of chromium, sometimes below the passivating threshold. The bulk grain interior stays protected; the depleted boundary does not.

Why welds are the classic case

A single weld pass imposes a full temperature gradient on the surrounding plate. Directly at the fusion line the metal is molten or above the carbide solvus, so no carbides survive. A short distance away the peak temperature falls squarely into the 425 to 815 degrees Celsius window and stays there long enough during cooling to precipitate carbides. That is why weld decay appears as one or two lines running parallel to the weld, offset a few millimetres to each side, rather than in the weld metal itself.

The role of carbon content and time

The amount and rate of carbide precipitation scale with carbon content. Standard 304 (up to 0.08 percent carbon) sensitizes readily; the effect is far weaker below about 0.03 percent. Exposure time matters as well: carbide precipitation is most rapid near 650 degrees Celsius, where sensitization can begin within minutes, while at the extremes of the range it may take hours. Time-temperature-sensitization (TTS) curves for each grade capture this C-shaped behaviour.

Prevention methods and how they work

Three routes prevent sensitization: remove the carbon, tie it up with a stronger carbide former, or dissolve the carbides after the fact and cool fast enough to keep them dissolved.

Knife-line attack in stabilised grades

Stabilised grades are not immune. In multi-pass welds, metal immediately next to the fusion line can be heated high enough (above roughly 1200 degrees Celsius) for the stabilising carbides (TiC, NbC) to dissolve and release carbon back into solution. If a later pass or service exposure then reheats that same narrow band into the sensitizing range, chromium carbides precipitate after all. The result is a very thin, sharply defined line of intergranular attack right at the weld edge, known as knife-line attack. A post-weld stabilising anneal near 900 degrees Celsius reprecipitates the titanium or niobium carbides and restores resistance.

Detection, testing and consequences

Sensitization is confirmed by standardised intergranular corrosion tests: ASTM A262 (practices A to F, including the oxalic-acid etch screen and the copper-copper-sulphate Strauss test) and EN ISO 3651. Electrochemical potentiokinetic reactivation (EPR) gives a quantitative, near-nondestructive measure. Beyond visible decay, sensitized boundaries also raise susceptibility to chloride stress corrosion cracking and can act as initiation sites for pitting corrosion. Because the failure is composition driven, verifying that installed material matches the specified L or stabilised grade through positive material identification guards against mixed-heat errors.

Managing the risk in operating plant

Sensitization is a records problem as much as a metallurgical one: knowing the weld procedures, heat treatments, and material certificates for every susceptible circuit. Fabrico lets you attach material grades, weld maps, and inspection results to each asset so recurring HAZ inspections stay scheduled and traceable. Book a Fabrico demo to see how that history is kept.

Frequently Asked Questions

Does weld decay occur in the weld metal or the base metal?

Almost always in the base metal, within the heat-affected zone a few millimetres from the fusion line, where the peak temperature falls into the 425 to 815 degrees Celsius sensitizing range. The weld metal and the fusion line itself reach too high a temperature to retain carbides.

Are ferritic and duplex stainless steels affected?

Ferritic grades can sensitize, but at higher temperatures and much shorter times because chromium diffuses faster, so low interstitial content is the main defence. Duplex grades resist classic weld decay better but have their own precipitation concerns, such as sigma phase at elevated temperatures.

Can a sensitized component be recovered?

Yes, if geometry and equipment allow. A full solution anneal near 1040 to 1120 degrees Celsius followed by rapid cooling dissolves the chromium carbides and restores corrosion resistance. This is often impractical for large fabricated assemblies, which is why grade selection is preferred over remediation.

Is an L grade always better than a stabilised grade?

Not always. L grades give the simplest protection for welded service up to moderate temperatures. Stabilised grades such as 321 and 347 keep carbon tied up during prolonged high-temperature service where even low-carbon steel could eventually sensitize, so the choice depends on operating temperature.