Intergranular corrosion of SS 304 stainless steel

Intergranular corrosion of SS 304 stainless steel is a critical issue that many buyers and engineers encounter-especially after welding or high-temperature exposure. Although SS 304 (EN 1.4301 / X5CrNi18-10) is widely regarded as a corrosion-resistant material, improper processing or service conditions can significantly reduce its performance.

This article explains what intergranular corrosion is, why it occurs in SS 304, how it compares with AISI 316 stainless steel, what tests are used to detect it, and how buyers can reduce project risk through proper material selection.

 

What Is Intergranular Corrosion?

Intergranular corrosion (IGC) is a form of localized corrosion that occurs along the grain boundaries of stainless steel while leaving the grains themselves relatively intact.

In SS 304 stainless steel, this phenomenon is typically associated with:

Chromium carbide precipitation

Chromium depletion at grain boundaries

Reduced corrosion resistance in localized zones

 

Why does 304 stainless steel (EN 1.4301) experience intergranular corrosion?

304 stainless steel (EN 1.4301) contains ≤0.08% carbon. When exposed to sensitizing temperatures between 450–850℃, the carbon at the grain boundaries combines with chromium to form Cr₂₃C₆ precipitates. This reduces the chromium content near the grain boundaries from 18.0–20.0% in the base metal to below the critical corrosion-resistant concentration of 12%, causing a loss of passivation ability and subsequently leading to intergranular corrosion.

How to Detect Intergranular Corrosion in SS 304?

1. ASTM A262 – Intergranular Corrosion Test (Most Important)

Practice	Purpose
Practice A	Screening (oxalic acid etch)
Practice E	Strauss test (quantitative)
Practice C	Huey test (nitric acid)

Widely accepted for EN 1.4301 stainless steel plate, pipe, and tube

 

2. Metallographic Examination

Microscopic analysis of grain boundaries

Identifies carbide precipitation

Often used after ASTM A262 screening

 

3. Chemical Composition Analysis

Confirms carbon content

Verifies compliance with EN 10088

Important for distinguishing 1.4301 vs 1.4306

 

4. PMI Testing (Positive Material Identification)

Confirms grade identity

Prevents 304 / 316 mix-ups

XRF identifies Mo (316 vs 304)

 

5. Corrosion Simulation Tests (Project-Specific)

Acid immersion tests

Process-media exposure testing

 

How Buyers Can Prevent Intergranular Corrosion?

Solution	Description
Use 304L (1.4306)	Lower carbon reduces carbide formation
Use Stabilized Grades	321 (1.4541) or 316Ti (1.4571)
Proper Heat Treatment	Solution annealing + rapid cooling
Welding Control	Minimize heat input & dwell time
Specify IGC Testing	ASTM A262 required in PO