What Is Intergranular Corrosion?

Intergranular corrosion (IGC) is a form of localized corrosion that attacks the grain boundaries of austenitic stainless steels and nickel alloys. The attack is selective: the bulk of the grain remains intact while the thin boundary zone dissolves, eventually causing the metal to lose structural integrity and disintegrate along grain boundaries. In piping, IGC most commonly occurs in the heat-affected zone (HAZ) adjacent to welds on 304 and 316 stainless steel components.

The Sensitization Mechanism

Stage	What Happens	Temperature Range
1. Heating	Welding or thermal processing heats the metal into the sensitization range	425-870°C (800-1600°F)
2. Chromium carbide precipitation	Carbon diffuses to grain boundaries and combines with chromium to form Cr23C6	Occurs within minutes at 650-700°C
3. Chromium depletion	A narrow zone (~0.1 mm) adjacent to the boundary loses chromium below the 12% minimum for passivation	Adjacent to precipitates
4. Corrosive attack	The depleted zone is no longer “stainless” and corrodes preferentially in service	In-service exposure to corrosive media

The term “sensitization” refers to the condition where chromium carbides have precipitated at grain boundaries, making the alloy sensitive to intergranular attack.

Susceptible Materials

• Standard austenitic grades: ASTM A182 F304, F316, A312 TP304, TP316 (carbon content up to 0.08%)
• High-carbon variants: 304H and 316H (0.04-0.10% C) are deliberately sensitized for high-temperature creep strength but must not be used in corrosive environments without post-weld solution annealing
• Nickel alloys: Alloy 600, Alloy 800 can sensitize; Alloy 625 and C-276 are highly resistant

Prevention Methods

Method	How It Works	Application
L-grade steels	304L (0.030% C max), 316L (0.030% C max); low carbon limits carbide formation	Standard for all corrosive service welded piping
Stabilized grades	321 (titanium-stabilized), 347 (niobium-stabilized); Ti/Nb preferentially binds carbon	High-temperature service where L-grades lose creep strength
Solution annealing	Heat to 1040-1120°C and water quench; dissolves existing carbides	Post-weld treatment for standard-grade components
Controlled welding	Low heat input, fast cooling, limit interpass temperature to 150°C max	Minimizes time in the sensitization range

Common Mistake

Specifying 304 or 316 (standard carbon) instead of 304L or 316L for welded piping in corrosive service is a common procurement error. The cost difference is negligible, but the corrosion performance difference is critical. Always specify L-grades for welded stainless steel in contact with corrosive media.

Testing for Sensitization

ASTM A262 defines five practice tests (A through E) for detecting susceptibility to intergranular attack:

Practice	Method	Typical Application
Practice A	Electrolytic oxalic acid etch (screening test)	Quick go/no-go screening
Practice B	Ferric sulfate-sulfuric acid (Streicher test)	Quantitative; weight loss measurement
Practice C	Nitric acid boiling (Huey test)	For strongly oxidizing media
Practice E	Copper sulfate-sulfuric acid (Strauss test)	Most common for austenitic stainless steels

For duplex stainless steels, ASTM A923 provides methods to detect intermetallic phases (sigma, chi) that cause a related but distinct form of intergranular degradation.

Read the full guide to steel corrosion