A welded 304 tank developed "knife-cut" corrosion grooves on both sides of the weld after just one year of service — this is usually not a weld quality issue, but rather 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 general structural components in ambient-temperature fresh water.
Differences Between the Three Grades
- 304: carbon ≤ 0.08%, most susceptible to sensitization 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℃) applications.
Note: 304L only "reduces" risk, it does not "immunize." Extremely thick plate, high heat input, and multiple repairs can still cause sensitization in 304L.
How to Determine If Your Application Requires an Upgrade
- Plate thickness: ≥6 mm or multi-pass welding keeps the heat-affected zone in the sensitization temperature range for extended periods — 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, allow material margin upfront.
Four Actions You Can Take (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 rapid cool, re-dissolving chromium carbides back 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, removing the chromium-depleted surface layer — treats the symptom, but sufficient for most applications.
How to Verify
When disputes arise, 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, the test method and acceptance criteria should be specified in the contract; otherwise, disagreements will arise upon delivery.
Common Pitfalls
- Drawings specify "304" when intergranular corrosion resistance is actually needed — discovered only at acceptance that there is no basis;
- Treating 304L as a universal solution: it can also sensitize under thick plate and high heat input;
- Grinding without passivation after welding: the chromium-depleted layer is removed but the passive film is not rebuilt, and 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 incurred after the fact.
