What is passivation and passive layers?

Treatment to strengthen protective layer

The term passivation can have various meanings. It can refer to the process where stainless steel spontaneously forms a protective oxide layer when in contact with oxygen. It can also involve cleaning a surface from substances that could cause corrosion – for example, iron particles.

When we at Calamo refer to passivation, we are referring to a treatment with a weak acid (oxidizing agent) to enhance the protective oxide layer.

Surface loses its passive properties

During mechanical polishing or grinding of stainless steel, the surface becomes active and loses its passive properties. To regain corrosion protection, the passive surface needs to be reestablished. If this occurs spontaneously with the presence of foreign particles on the surface, they will remain in the passive layer. This may provide less protection against corrosion than creating the oxide layer through a controlled process.

Different standards for passivation

• According to ASTM A380, cleaning, removal of scale, and weld oxides are not considered passivation, but they may be necessary to achieve satisfactory results.
• In ASTM A967, a pickled surface free from scale, free iron, and other foreign substances may not require further treatment to be considered passivated.
• ASTM B912-02 and SS-EN ISO 15730 include electropolishing as a form of passivation.

In simple terms, the measures taken to protect a stainless steel surface from corroding through a protective oxide layer constitute passivation.