Passive film breakdown
Destruction of the protective oxide layer on corrosion-resistant metals by chemical or mechanical influences leads to pitting and crevice corrosion.
Physical-Chemical Mechanism of Action
The passive layer is a thin, stabilized film on the surface of corrosion-resistant metals like steel or nickel. It forms through the reaction of the metal with oxygen and water vapor in the air. If this layer is destroyed by chemical (e.g., hydrochloric acid) or mechanical (e.g., mechanical stress) influences, the underlying metal can corrode again.
Metallurgical Relevance & Welding Risks
Destruction of the passive layer can lead to increased corrosion susceptibility. Pitting and crevice corrosion can cause cracks and wear in components, compromising the safety and service life of machinery and structures. It's especially dangerous if the passive layer is not fully restored after welding.
Workshop Practice & Instructions
To protect the passive layer and avoid corrosion-prone areas, take the following steps:
• Use correct welding parameters (temperature, time, pressure) and appropriate filler materials.
• Keep the workshop clean and dry to avoid corrosive acids.
• After welding, apply a passivation method, e.g., immersion in an oxidizing acid or applying passivation coatings.