Influence of Nb Additive on Alkaline Corrosion Behavior of Slag-Free Self-shielded Metal-Cord Welding Overlay

The high chromium iron base welding overlays with different niobium additives were fabricated using slag-free self-shielded metal-cored wire. Their corrosion resistance in 1 mol/L NaOH + 3.5 wt.% NaCl alkaline solution was investigated. The results show that in the alkaline solution (pH = 14), the corrosion occurs in primary M₇(C, B)₃ carbide (M = Cr, Fe, and Mn) and eutectic M₃(C, B) carbide, and the corrosion resistance depends on the content of carbide in the structure. Since the self-corrosion potential of carbides in an alkaline environment is lower than that of the matrix, the micro galvanic couple can be formed between carbides and matrix due to the difference of potential. When austenite matrix is used as cathode, eutectic and primary carbides are used as anode due to low self-corrosion potential, carbide will corrode prior to a matrix structure. With the increase in Fe-Nb content, the proportion of primary NbC phase in the microstructure increases continuously. Due to the lack of Fe element to form a stable passivation layer in NbC structure, NbC reacts prior to the primary M₇(C, B)₃ carbide and eutectic M₃(C, B) carbide. The corrosion resistance of the welding overlay decreases with the increase in the content of Fe-Nb. The Nb-free slag-free self-shielded metal-cored wire exhibits best corrosion resistance.