A welded 304 tank developed knife-edge corrosion grooves on both sides of the weld after just one year — this is usually not a weld quality issue but sensitisation: 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 judgement
Thin plate welds cool quickly with low sensitisation 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 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 sensitised during welding;
- 304L: carbon ≤ 0.03%, too little carbon to form continuous chromium carbides, sensitisation risk greatly reduced;
- 321 / 347: titanium or niobium added for stabilisation — 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 still sensitise 304L.
How to judge whether your service conditions require an upgrade
- Plate thickness: ≥6 mm or multi-pass multi-run welding keeps the heat-affected zone in the sensitisation 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; for occasional short-term heating, 304L is sufficient;
- Medium: nitric acid, chloride-containing water, pulp, high-purity water (high-resistivity water more readily causes selective corrosion of chromium-depleted zones) → sensitive;
- Structure: where welds are in stress concentration zones and post-weld solution annealing is not possible, leave a material allowance upfront.
Four things you can do (from lowest to highest cost)
- Material upgrade: 304 → 304L, small cost increase, the most cost-effective step;
- Control heat input: low current, fast welding speed, interpass temperature control, avoid large weaving, shorten the sensitisation time window;
- Post-weld solution annealing: heat to about 1050–1100℃ then rapid cool to re-dissolve chromium carbides into the matrix. Effective but costly (distortion, requires whole-component treatment), only for critical equipment;
- Surface treatment: pickling and passivation of welds and heat-affected zones to remove the chromium-depleted surface layer — treats the symptom, but sufficient for most service conditions.
How to verify
When disputed, carry out an intergranular corrosion test according to standards (e.g. sulfuric acid-copper sulfate method, or electrochemical methods) rather than relying on visual inspection. If the project requires this at procurement, the test method and acceptance criteria should be written into the contract, otherwise there will be disputes after delivery.
Common pitfalls
- Drawings specify "304" but intergranular corrosion resistance is actually needed, and only at acceptance is it found there is no basis;
- Treating 304L as universal: thick plate with high heat input will also sensitise;
- Post-weld grinding only without grinding plus passivation: the chromium-depleted layer is ground away but the passive film is not rebuilt, and corrosion still occurs;
- Using 321 but selecting the wrong filler metal (a matching stabilised filler should be used), making the weld itself the weak link;
- Assuming "no cracks in the weld means qualified": sensitisation is a microstructural issue, invisible to the naked eye.
Tell us your plate thickness, welding method, service temperature and medium, and we will give you selection advice for 304 / 304L / 321 with corresponding quotations, and specify the applicable standards and test requirements in the contract — the cost of this type of problem is almost entirely after the fact.
