1.What is the core reason why we cannot weld?
Color coated roll side:
Coating burnout: The high temperature of the welding arc (far exceeding the tolerance limit of the coating) will instantly burn the color-coated organic coating (topcoat and primer) near the weld, and may destroy the "chemical conversion layer" on the surface of the substrate (usually a galvanized sheet). This leaves the metal substrate directly exposed.
Galvanized coil side:
Zinc layer evaporation/oxidation: The melting point of zinc is about 420°C, and the boiling point is about 907°C, which is much lower than steel. The high temperature of welding will evaporate and oxidize the zinc layer near the weld, forming porous and discontinuous zinc oxide (white ash), losing its original dense barrier protection.
Failure of sacrificial anode protection: The core principle of galvanized rust prevention is that zinc acts as a "sacrificial anode" to corrode preferentially to protect iron. Welding destroys the continuity of the zinc layer and the good electrical connection between zinc and iron, making this cathodic protection effect greatly reduced or even ineffective in the weld area.
The weld metal itself:
The material of the welding rod or welding wire is usually different from the base steel. The chemical composition and metallographic structure of the weld metal are also different from the base metal. Its corrosion resistance is average in the exposed state.
2.What are the consequences after welding?
Accelerated electrochemical corrosion: The exposed steel (cathode), remaining zinc layer (anode) and weld metal will form a "primary cell" in a humid environment, which will in turn accelerate the corrosion of the exposed steel (weld area).
Stress concentration: There are residual stresses and possible microscopic defects in the weld, which are more likely to cause and accelerate corrosion.
Rust begins at welds: Therefore, welded joints are often the first place in the entire structure to start rusting, and where corrosion develops fastest.
3.What is the correct process?
Post-weld cleaning:
Thoroughly remove welding slag, spatter, zinc dust and oxides from the weld and heat-affected zone (about 20-50mm wide on both sides). Tools such as wire brushes and angle grinders can be used.
Surface treatment:
Sand the cleaned area to remove loose coating and rust, creating a clean, rough surface for increased adhesion.
Apply special anti-rust primer:
This is the most critical step. An anti-rust primer with strong weather resistance and excellent adhesion must be used, such as:
Epoxy zinc-rich primer: Provides cathodic protection and is one of the best options.
Epoxy primer: extremely strong adhesion and resistant to chemical media.
The coating must completely cover all areas affected by heat damage and extend outward appropriately.
4.How to apply midcoat and topcoat?
According to environmental needs, apply an intermediate paint (such as epoxy mica iron intermediate paint to increase barrier properties and layer thickness) on the primer.
Finally, apply a weather-resistant topcoat (such as acrylic polyurethane topcoat, fluorocarbon topcoat) that matches the color and performance of the color-coated roll.
5.What needs to be paid special attention to in such a harsh corrosive environment?
Even with the above-mentioned post-processing, the quality, integrity, thickness and long-term resistance to chemical corrosion of hand-painted coatings are far from being comparable to the complete coating system integrated by the original factory.
The weld is always a weak point. Complex chemical media, condensate and temperature changes will penetrate quickly, causing the protective layer to fail, corroding the weld and possibly causing equipment leakage.
Professional advice:
The preferred solution: avoid using color-coated coils, use fiberglass reinforced plastic (FRP), stainless steel or plastic-lined carbon steel as the main material, and use corresponding flange bolt connections or professional welding (such as pickling and passivation after stainless steel welding).
Secondary option: If it must be used, mechanical connections (such as riveting, bolted connections and sealant) should be used to retain the integrity of the original coating to the greatest extent. The joints must be sealed with a chemical-resistant sealant.