A welded 304 tank developed "knife-cut" corrosion grooves on both sides of the weld after one year — this is usually not a weld quality issue, but sensitization: the heat-affected zone stayed in the 450–850℃ range, chromium carbides precipitated at grain boundaries, consuming the chromium near the boundaries, and that ring became a "chromium-depleted zone."
One-Sentence Judgment
Thin plate welds cool quickly with low sensitization risk; thick plate, multi-pass welding, and slow cooling after welding carry high risk. For long-term service at 400–800℃, or contact with nitric acid/chloride-containing media, or use in high-purity water systems, 304L (or 321/347) must be used. Ordinary 304 is usually fine for ordinary structural parts in room-temperature fresh water.
Differences Among Three Grades
- 304: carbon ≤ 0.08%, most easily sensitized during welding;
- 304L: carbon ≤ 0.03%, too little carbon to form continuous chromium carbides, sensitization risk greatly reduced;
- 321 / 347: titanium or niobium added for stabilization — carbon preferentially combines with titanium/niobium rather than chromium. Used for high-temperature service (400–800℃).
Note: 304L only "reduces" risk, it is not "immune." Very thick plate, high heat input, and multiple repairs can also cause sensitization in 304L.
How to Judge Whether Your Service Conditions Require an Upgrade
- Plate thickness: ≥6 mm or multi-pass multi-layer welding, heat-affected zone stays in the sensitization temperature range for a long time, 304L is recommended directly;
- Service temperature: long-term 400–800℃ (e.g., heat exchangers, exhaust systems) → use 321/347 or 310S; only occasional short-term heating, 304L is sufficient;
- Medium: nitric acid, chloride-containing water, pulp, high-purity water (high-resistivity water more easily selectively corrodes chromium-depleted zones) → sensitive;
- Structure: where welds are in stress concentration zones and post-weld solution treatment is impossible, leave material allowance first.
Four Things You Can Do (From Low to High Cost)
- Material upgrade: 304 → 304L, minimal cost increase, the most cost-effective step;
- Control heat input: low current, fast welding speed, interpass temperature control, avoid large weaving, shorten the sensitization time window;
- Post-weld solution treatment: heat to about 1050–1100℃ then rapid cool, re-dissolve chromium carbides back into the matrix. Effective but costly (distortion, requires whole-part treatment), only for critical equipment;
- Surface treatment: pickling and passivation of welds and heat-affected zones, remove the chromium-depleted surface layer — treats the symptom, but sufficient under most service conditions.
How to Verify
When disputed, conduct intergranular corrosion tests per standards (e.g., sulfuric acid-copper sulfate method, or electrochemical methods), not visual inspection. If the project requires it at procurement, test method and acceptance criteria should be specified in the contract, otherwise each party will have its own interpretation upon delivery.
Common Pitfalls
- Drawings specify "304" but intergranular corrosion resistance is actually needed, only discovered at acceptance that there is no basis;
- Treating 304L as universal: it also sensitizes under thick plate and high heat input;
- Post-weld grinding only without grinding + passivation: chromium-depleted layer removed but passive film not rebuilt, corrosion still occurs;
- Using 321 but selecting wrong filler metal (should use matching stabilized filler), the weld itself becomes the weak link;
- Assuming "no cracks in weld means qualified": sensitization is a microstructural issue, invisible to the naked eye.
Tell us your plate thickness, welding method, service temperature and medium, and we will provide 304 / 304L / 321 selection recommendations and corresponding quotations, and specify the execution standards and test requirements in the contract — the cost of such problems is almost entirely after the fact.
