What Is Weld Decay? Sensitization

Weld decay is a form of intergranular corrosion that occurs in the heat-affected zone (HAZ) of austenitic stainless steel welds. During welding, the HAZ is heated to 450-850 deg C (the sensitization temperature range), causing chromium to combine with carbon and precipitate as chromium carbides (Cr23C6) along the grain boundaries. This depletes the adjacent grain boundary region of chromium below the 10.5% minimum needed for corrosion resistance, leaving a narrow zone vulnerable to preferential attack in corrosive environments.

How Weld Decay Develops

The process occurs in three steps:

1. Heating: Welding heats the HAZ into the sensitization range (450-850 deg C)
2. Carbide precipitation: Carbon diffuses to grain boundaries and combines with chromium to form Cr23C6
3. Chromium depletion: The zone immediately surrounding each grain boundary loses chromium below the passivation threshold

In service, a corrosive medium (acids, chlorides, or even aerated water) attacks the chromium-depleted grain boundaries preferentially, causing the grains to detach. The corrosion pattern follows a band parallel to the weld, typically 2-5 mm from the fusion line, corresponding to the zone that experienced the critical temperature range.

Susceptibility by Grade

Grade	Carbon Content	Sensitization Susceptibility
304	0.08% max	High (standard carbon grade)
304L	0.030% max	Low (insufficient carbon for significant carbide precipitation)
316	0.08% max	High
316L	0.030% max	Low
321	0.08% max (Ti-stabilized)	Very low (Ti preferentially binds carbon)
347	0.08% max (Nb-stabilized)	Very low (Nb preferentially binds carbon)
304H/316H	0.04-0.10%	Very high (elevated carbon for high-temperature strength)

Prevention Methods

Method	Mechanism
Use L-grades (304L, 316L)	Low carbon (max 0.030%) provides insufficient carbon for significant Cr23C6 precipitation
Use stabilized grades (321, 347)	Titanium or niobium combines with carbon before chromium can, forming stable TiC or NbC
Solution annealing after welding	Heating to 1050-1100 deg C dissolves carbides and restores chromium distribution; water quench prevents reprecipitation
Minimize heat input	Shorter time in the sensitization range reduces carbide formation
Avoid prolonged service at 450-850 deg C	Operational exposure to the sensitization range over time can cause in-service sensitization

Common Mistake

Do not assume that L-grade stainless steels are completely immune to sensitization. At 0.030% max carbon, L-grades have greatly reduced susceptibility but can still sensitize if held in the critical temperature range for extended periods (hours). For continuous high-temperature service in corrosive environments, stabilized grades (321, 347) are the safer choice.

Detection

Weld decay is detected in the field by visual inspection (surface pitting or grooving along the HAZ) and confirmed by metallographic examination. The standard laboratory test for sensitization susceptibility is ASTM A262, which includes five practices (A through E) using different acid solutions to evaluate intergranular corrosion resistance. Practice E (Strauss test, boiling copper sulfate-sulfuric acid) is the most common acceptance test for stainless steel piping materials.

In-service weld decay requires replacement of the affected section, as the grain boundary damage cannot be reversed in the field. Prevention through proper material selection is far more economical than repair.

Read the full guide to fillet welds