Oxidation Resistance Properties of Stainless Steels: The Essential Role of Chromium
Stainless steels owe their oxidation resistance properties, also known as“stainlessness,” to their specific chemical composition. To ensure theformation of a protective surface layer, these steels require a chromiumcontent of at least 12%. This chromium oxidizes instantly, forming a thin layerof chromium oxide (Cr2O3) that is hard, compact, andextremely stable, effectively protecting the steel against corrosion. However,this protection is limited to certain levels of chemical aggressiveness; inchloride environments, such as saltwater, stainless steels can experiencelocalized corrosion through pitting.
The Effect of Chromium Oxide on the Weldability of Stainless Steels
When welding stainless steels, chromium oxide presents unique challenges.Welding processes for this type of material generally aim to prevent theformation of chromium oxide in the weld pool, as chromium has a high reactivitywith oxygen. Oxygen sources include ambient air, moisture, surfacecontaminants, as well as moisture in the flux or coating. Leaks in theshielding gas lines can also introduce oxygen.
Unlike iron oxide, chromium oxide, which is refractory in nature, does notliquefy at the temperature of the weld pool, reducing the wettability of thepool and altering the weld bead shape. The presence of undissolved oxide canaffect arc stability and the quality of metal transfer, directly impacting themechanical strength and corrosion resistance of the welded joint due tochromium depletion. Additionally, chromium oxide enrichment in the slag makesit harder to detach post-welding.
Solutions to Improve the Weldability of Stainless Steels
To counteract the adverse effects of chromium oxide in the weld pool, fillermetals must meet strict chemical specifications. Materials in the coating ofSMAW electrodes and the flux for submerged arc welding (SAW) should be low inoxidizing compounds. Adding fluorides to their composition helps dissolvechromium oxide. In FCAW and GMAW processes using flux-cored wires under Ar/CO2or CO2 gas, more oxidizable elements than chromium, such as titaniumand aluminum, are included to capture oxygen released by dissociated CO2.
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