Localized corrosion performance of laser surface cladded UNS S44700 superferritic stainless steel on mild steel

Abstract

UNS S44700 superferritic stainless steel has been successfully cladded on mild steel substrate by laser surface cladding (LSC) with pneumatic delivery of mixed Fe, Cr and Mo powders followed by laser surface melting (LSM) in order to obtain a homogeneous chemical composition and microstructure. The surface alloy has a two-layer structure. The inner layer is the nonremelted area of the LSC surface alloy. The chemical composition and microstructure in the layer are not uniform; semi-dissolved Cr and Mo particles are distributed randomly in it. The outer layer is the remelted area of the LSC surface alloy. It consists of fine cellular dendrites with ferrite structure. The chemical composition of the layer is quite uniform on the macroscale, but is nonhomogeneous on the microscale; Cr and Mo are slightly segregated at the dendrite boundaries. The localized corrosion performance of the surface steel was studied by potentiodynamic anodic polarization in sodium chloride, ferric chloride and hydrochloric acid solutions and long-term immersion tests in acidified ferric chloride solution. The surface alloy displays excellent passive performance and pitting corrosion resistance in anodic polarization tests in 0.5 M NaCl, 1 M FeCl 3 and 0.6 M HCl solutions. After immersion in 1M FeCl 3 (acidified to pH 0.4) for 2 months, the surface alloy suffers slight pitting corrosion.