Post-weld cleaning for stainless steel often includes passivation to restore corrosion resistance. Which acids are commonly used for passivation?

Post-weld cleaning for stainless steel often includes passivation to restore corrosion resistance. Which acids are commonly used for passivation?

• A. Hydrochloric acid
• B. Citric acid
• C. Sulfuric acid
• D. Nitric or citric acid

Answer: (D)

Passivation after welding is about restoring stainless steel’s corrosion resistance by forming a uniform, chromium-rich oxide layer on the surface. This requires an acid treatment that removes surface iron and contaminants without damaging the protective oxide. Nitric acid is a classic choice because it oxidizes and dissolves iron on the surface, helping the passive film reform evenly. Citric acid is a safer, milder alternative that chelates iron and still promotes the same chromium oxide layer without the hazards of strong mineral acids. Hydrochloric acid tends to attack the protective oxide and can cause pitting or corrosion, so it’s not used for passivation. Sulfuric acid is also not suitable for passivation of stainless steel for similar reasons. In practice, acids used for passivation are nitric or citric acid.

Passivation after welding is about restoring stainless steel’s corrosion resistance by forming a uniform, chromium-rich oxide layer on the surface. This requires an acid treatment that removes surface iron and contaminants without damaging the protective oxide. Nitric acid is a classic choice because it oxidizes and dissolves iron on the surface, helping the passive film reform evenly. Citric acid is a safer, milder alternative that chelates iron and still promotes the same chromium oxide layer without the hazards of strong mineral acids. Hydrochloric acid tends to attack the protective oxide and can cause pitting or corrosion, so it’s not used for passivation. Sulfuric acid is also not suitable for passivation of stainless steel for similar reasons. In practice, acids used for passivation are nitric or citric acid.