Protective oxide scale formation
The natural or thermally induced formation of a dense reaction layer on metal surfaces. It slows down oxygen diffusion, preventing further progression of corrosion.
Oxide layers are a natural protection for many metals – but they are the problem when welding. The heat destroys the protective top layer, and without shielding gas the hot surface oxidizes immediately. The result is temper colors and scale that have no protective effect and later promote corrosion. With stainless steel, the zone under the oxide layer becomes depleted of chromium; with aluminum, the hard oxide layer prevents clean melting. Therefore: always work with enough shielding gas, and for stainless steel also protect the root side. Remove temper colors and scale completely after welding – mechanically with a stainless steel brush or chemically with pickling paste. Then passivate so that the protective layer can rebuild. For aluminum, brush off the oxide layer shortly before welding and weld with AC so that the oxide layer breaks up.