Why Does Stainless Steel Welded Pipe Rust After Welding

Stainless steel welded pipes are widely used in industries such as construction, chemical processing, and food production due to their corrosion resistance and durability. However, many users notice rust spots appearing after welding, which can be concerning. Why does this happen? Let’s explore the reasons and solutions.

 

1. Oxidation During Welding

When stainless steel is exposed to high temperatures during welding, chromium (the key element for corrosion resistance) reacts with oxygen, forming chromium oxide. If the welding environment lacks proper inert gas protection (such as argon), excessive oxidation occurs, reducing the material’s rust resistance.

 

2. Carbide Precipitation (Sensitization)

Stainless steel contains carbon, which can combine with chromium at high temperatures (450–850°C) to form chromium carbides. This depletes chromium in the surrounding areas, weakening the passive oxide layer and making the welded zone prone to rust.

 

3. Contamination from Iron Particles

During cutting, grinding, or handling, iron particles from tools or other metals can adhere to the stainless steel surface. These particles can rust when exposed to moisture, leading to surface corrosion.

 

4. Improper Post-Weld Treatment

After welding, slag, heat tint (discoloration), and oxide layers must be removed. If not properly cleaned via pickling (using nitric-hydrofluoric acid) or passivation (restoring the chromium oxide layer), rust may develop over time.

 

5. Low-Quality Material or Incorrect Grade Selection

Some stainless steel pipes contain insufficient chromium or nickel, reducing corrosion resistance. Additionally, using grades unsuitable for specific environments (e.g., 304 in high-chloride conditions) accelerates rusting.

 

How to Prevent Rust After Welding?

Use Argon Shielding Gas (TIG/MIG Welding): Prevents oxidation during welding.

Post-Weld Cleaning: Remove heat tints with pickling paste or mechanical brushing.

Passivation Treatment: Enhances the chromium oxide layer.

Select the Right Grade: For harsh environments, consider 316L (with molybdenum) or duplex stainless steel.

Avoid Cross-Contamination: Use dedicated tools for stainless steel.

 

By understanding these factors and taking preventive measures, you can maintain the corrosion resistance of stainless steel welded pipes and extend their service life.