When stainless is heated (beginning at about 500 degrees F) the oxidation reaction is enhanced, and the layer becomes thicker. As the thickness of the layer increases, the wavelength of light transmitted changes, and hence the color we see. The surface discoloration on stainless steel after grinding or welding is surface oxidation. ... Heating the stainless steel to temperatures in excess 800 °F can cause metallurgical changes and reduce corrosion resistance in a manner that is not corrected by surface cleaning or chemical passivation. Typically, stainless steels contain less than 30% chromium and more than 50% iron. ... While stainless steels are resistant to rusting at room temperatures, they're prone to discoloration by oxidation at elevated temperatures due to the presence of chromium and other alloying elements such as titanium and molybdenum. Passivation is the process of treating or coating a metal in order to reduce the chemical reactivity of its surface. In stainless steel, passivation means removing the free iron from the surface of the metal using an acid solution to prevent rust.
I have seen some of the stainless steel weld joints cleaning with laser to remove surface oxide layers. After this cleaning, surface appears to be nice and glittering as of that the of parent material surface. It indicates chromium oxide has formed on the surface. It seams this technique is better than conventional pickling.
Dear Wayne Tupuola,
Laser surface treatment removes the surface oxide layer. Of course, this is not an indication of a more corrosion-resistant surface. If stainless steel is white after machining then we have a condition for corrosion of the surface. If we have a darkening (surface) then a protective layer is created. This may be oxidized or otherwise depending on the medium. The environment is oxide thanks to O2 and N in the atmosphere. Under special conditions, it may be nitrite or carbide. All these layers create a surface protection against corrosion and in some cases wear resistance.
Dear Wayne Tupuola,
Laser treatment is an ablation, so according to Mr Yankov answer it remove a surface layer by locally sublimating the topmost layer of the material. This leads to the generation of a fresh metal surface free of any surface contamination layer and native oxide. Once laser is shifted to another position, the steel surface promptly reacts with environment to form a new surface layer. The nature of this layer will depend on the surface temperature (laser treatment type and parameters), gas atmosphere and as well steel composition. Both protecting passivating layers based on Fe3O4 or Cr oxide (Nitridation should be induced in nitrogen richer atmosphere if nitriding elements like Cr, Ti, Nb are suffiently present in the steel) should be formed as well non protective discontinuous patchy layers. Hope to be useful, best regards, Simone
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