A welded 304 tank developed "knife-cut" corrosion grooves on both sides of the weld after just 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."
Quick Assessment
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 normal structural parts in ambient-temperature fresh water.
Differences Between the 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 — allowing carbon to preferentially combine with titanium/niobium rather than chromium. Used for high-temperature service (400–800℃).
Note: 304L only "reduces" risk, it does not "immunize." Very thick plate, high heat input, and multiple repairs can still cause sensitization in 304L.
How to Determine If Your Application Needs an Upgrade
- Plate thickness: ≥6 mm or multi-pass multi-run welding, heat-affected zone stays in the sensitization temperature range for long periods — directly use 304L;
- Service temperature: long-term 400–800℃ (e.g., heat exchangers, exhaust systems) → use 321/347 or 310S; for occasional short-term heating, 304L is sufficient;
- Medium: nitric acid, chloride-containing water, pulp, high-purity water (high-resistivity water more easily causes selective corrosion of chromium-depleted zones) → sensitive;
- Structure: where welds are in stress concentration zones and post-weld solution annealing is not possible, allow material margin upfront.
Four Things You Can Do (From Lowest to Highest 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 annealing: heat to approximately 1050–1100℃ then rapidly cool, re-dissolving 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, removing the chromium-depleted surface layer — treats the symptom, but sufficient for most applications.
How to Verify
When disputed, conduct intergranular corrosion testing per standards (e.g., sulfuric acid-copper sulfate method, or electrochemical methods) rather than relying on visual inspection. If the project requires this at procurement, test method and acceptance criteria should be specified in the contract, otherwise disputes arise upon delivery.
Common Pitfalls
- Drawings specify "304" but intergranular corrosion resistance is actually needed, only discovered at acceptance with no basis;
- Treating 304L as a universal solution: it also sensitizes under thick plate and high heat input;
- Post-weld grinding without grinding + passivation: grinding removes the chromium-depleted layer but does not rebuild the passive film, corrosion still occurs;
- Using 321 but selecting the wrong filler metal (should use matching stabilized filler), making the weld itself the weak link;
- Assuming "no cracks in the 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 selection recommendations for 304 / 304L / 321 with corresponding quotations, and specify the applicable standards and testing requirements in the contract — the cost of these problems is almost entirely after the fact.
