Blog / Stainless steel
26 July 2024 | Stainless steel

Optimizing Stainless Steel Weldability Against Chromium Oxide Effects

The stainless steels owe their oxidation resistance properties “stainless character” with their specific chemical composition which requires important contents of chromium, 12% at least generally, in order to ensure the formation of a fine surface protection. Chromium oxidizes instantaneously and forms a continuous chromium oxide film (Cr2O3) that is hard, compact and very stable chemically to protect steel against the chemical action from the surrounding area. Let us recall however that the stainless character relates only to a certain degree of chemical aggressiveness. Indeed, in the presence of chlorides (water salt, sea water, etc), this type of steel undergoes a form of corrosion localized by pitting.

In the welding field, the effect of chromium oxide on the weldability of stainless steel is of another nature and one wants to search to clarify some aspects of it. First of all, the procedures inherent to the welding of this class of materials have a common approach to prevent the formation of chromium oxide in the weld pool. As already indicated, chromium is very reactive with oxygen (great chemical affinity with O2). The introduction of oxygen into the weld pool can have multiple origins: ambient air, moisture and contaminants on the base metal surface, moisture in flux or coating, escapes in the gases pipe protection, etc.

In fact in contrast to iron oxide, the refractory chromium oxide formed remains liquid at the pool weld temperature. As a direct consequence, this has to affect the wetting degree of weld bead and its shape. Moreover, the arc transfer and its stability will be affected by the presence of this oxide which behaves like an electrical insulator. Concerning the integrity of the welded joint, this last incurs degradations in its own mechanical resistance and corrosion resistance primarily because of its chromium impoverishment. Also let us recall that the enrichment of the slag’s composition in chromium oxide makes it not easily removable.

As palliative solutions of chromium oxide detrimental effects on the stainless steel weldability, the filler metals must to be in conformity with rigorous specifications of chemical composition. Materials used in the coating of electrodes for SMAW and flux for SAW must contain compounds slightly oxidizing. Furthermore, the addition of fluorides in the composition of the latter allows also to reduce the chromium oxide which can form. Concerning flux cored wire used for GMAW and FCAW, with like active gases shielding the pure CO2 or mixed with inert gases, contents of elements more easily oxidizable than chromium such as titanium and aluminum are intentionally added in their chemical composition to seize the oxygen coming from carbon dioxide decomposition.