Rust on stainless steel after welding is a common headache for many engineers and constructors. Looking at the yellow rust around the weld on the originally smooth surface, people's first reaction is often "is the material bought fake?" In fact, in most cases, the material is genuine, and the problem lies in the necessary process of welding itself, which breaks the delicate balance of stainless steel.
1. How does the stainless steel amulet work?
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Core barrier: chromium oxide film
The chromium content in stainless steel usually exceeds 10.5%. When exposed to air, chromium will react rapidly with oxygen, forming a very thin (about 1-3 nanometers) but extremely dense chromium oxide (Cr₂O₃) passivation film on the surface. This film is invisible to the naked eye, but it can be firmly attached like armor, effectively isolating the contact between steel matrix and corrosive media.
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A vivid metaphor:
This passivation film is like human skin. Good skin can resist external bacteria; Welding is equivalent to a "deep burn" in the local area, and the skin is destroyed. If it is not handled correctly in time, the wound will be infected and inflamed.
2. Analysis of the influence of welding thermal field on the microstructure and properties of steel
(1) The phenomenon of "chromium deficiency" in heat affected zone (HAZ)-the most fundamental inducement.
This is the core and most professional reason for joint rust. When the welding temperature reaches the sensitive range of 450°C to 850°C (especially during slow cooling), chromium in stainless steel will combine with carbon and precipitate chromium carbide (CRC) along the grain boundary.
- Serious consequences: chromium is "locked" in carbide and cannot participate in the formation of passive film. This leads to a sharp decrease in the chromium content in the grain boundary area around carbide (that is, "chromium-poor"), and sometimes even lower than the critical value of 10.5% to maintain rust resistance.
- Intuitive performance: in corrosive environment (even in humid air), chromium-poor grain boundaries will become the preferential corrosion path. Corrosion develops inward along the grain boundary, forming a visible rust line on both sides of the weld, which is professionally called "intergranular corrosion". In severe cases, the material may even break along the grain boundary.
(2) "Physical Change" of Weld Body
- Mix-up in materials: If you're using the wrong filler metal ((like welding 304 steel with 308 wire)or your settings are off, the weld might not have the same stuff in it as the base metal, which can make it rust faster.
- Grain size issues: The middle of the weld melts and reforms, making bigger crystals than the original metal. Big crystals usually mean weaker rust protection.
- Impurity enrichment and defects: impurities, porosity and slag inclusion may be introduced during welding, which become the starting point of corrosion.
(3) Destruction of surface state
- Oxidation and tempering color: at high temperature, a thick oxide scale (blue, purple, yellow and other tempering colors) will be formed in the weld and its vicinity. This oxide scale is loose and porous, and the metal below it is still in a chromium-poor state, and the color itself is the beginning of corrosion.
- Increased roughness: welding slag and spatter make the surface rough, which makes it easier to absorb moisture and corrosive ions, and it is not easy to form a complete passivation film.
3. common operations that aggravate problems in actual combat
- Failure to use suitable welding materials: If ordinary carbon steel covered electrode is misused when welding 316L stainless steel, the corrosion resistance of the weld will be completely destroyed.
- Lack of shielding gas or insufficient protection: In inert gas shielded welding, the gas flow and purity are not enough or the cover dragging time is short, which will lead to the air intrusion of the weld at high temperature and serious oxidation.
- Improper welding parameters: excessive line energy (high current and voltage, slow speed) will make the heat affected zone wider and the chromium deficiency more serious.
- No treatment after welding: it is considered that "welding is enough", and the weld and surrounding areas are not cleaned, pickled or passivated, which is equivalent to directly exposing the "burned" wound.
4. Professional solutions: from prevention to repair
Prevention stage:
- Material matching: choose low-carbon or stainless steel containing stabilizing elements (such as 304L and 316L, or 321 and 347 containing titanium/niobium). They can reduce the precipitation of chromium carbide.
- Process control: adopt low current and fast welding speed to reduce heat input; Ensure that inert gas protection is sufficient and effective; If necessary, preheat the thick plate or control the interlayer temperature to avoid staying in the sensitization temperature range.
- Welding material selection: choose welding material with matching chemical composition or even better (such as welding 304 with 316 welding material to increase molybdenum content).
Post-welding treatment:
(1) Mechanical cleaning: Use special stainless steel grinding wheel or wire brush (which must be special for stainless steel to avoid carbon steel pollution) to remove welding slag and splash.
(2) Acid pickling passivation: this is the core step to restore corrosion resistance.
- Pickling: use nitric acid-hydrofluoric acid mixed solution to remove the oxide scale and chromium-poor layer in weld and heat affected zone.
- Passivation: Treat with passivation solution such as nitric acid or citric acid to promote the surface to form a complete and dense chromium oxide film again.
(3) Electrolytic polishing: for high-demand products, the surface can be leveled by electrolytic polishing, and an excellent passivation film can be obtained at the same time.
