Chemical passivation
Chemical passivation removes free iron from the surface and accelerates oxide film formation, creating a homogeneous corrosion-resistant layer. This prevents cracks and porosity.
Physico-Chemical Mechanism
Chemical passivation is a process where free iron on the metal surface is converted into iron sulfate (FeSO4) through a chemical reaction with oxygen or other oxidizing agents. This process forms a thin, homogeneous oxide layer on the surface that prevents contact between the iron and further corrosive substances.
Metallurgical Relevance & Risks in Welding
Passivation increases the toughness and crack resistance of the weld. It prevents free iron from reacting with oxygen and forming a corrosive layer, which could lead to cracks and porosity. However, an incomplete or faulty passivation process can result in uneven oxide layer formation, leading to localized corrosion and damage.
Workshop Practice & Procedures
To ensure effective chemical passivation, the following measures should be taken:
• Use a suitable deoxidizing agent before welding to remove free iron from the surface.
• Keep the weld in a low-oxygen environment or under a shielding gas after welding to promote rapid oxide layer formation.
• Monitor the cooling temperature and time to ensure uniform passivation. Rapid cooling can cause incomplete oxide layer formation.