Destruction Mechanism of Laser Melted Layers of AISI 321 Austenitic Stainless Steel After Electrochemical Corrosion in Ringer’s Solution

The aim of the present study is to investigate the mechanism behind corrosion destruction in laser-melted layers (LMLs) of AISI 321 austenitic stainless steel after electrochemical corrosion in Ringer’s solution. Surface morphology, microstructure, chemical composition, grain sizes, and orientation are studied using OM, SEM, EDS, and EBSD. It was confirmed that (1) the main mechanism behind corrosion destruction is identical between untreated and laser-melted steel, i.e., the selective destruction of the lower corrosion resistance phase (δ-ferrite) in the form of pits, and (2) the morphology and size of corrosion pits are different, as determined via δ-ferrite morphology, with narrow deep pits of uneven shape observed on the surface of wrought steel and rounded shallower pits seen in LML. The following mechanism is proposed with regard to corrosion destruction in LML: (1) the initial destruction of δ-ferrite; (2) the formation of an austenitic dendrite network; (3) the mechanical fracture of austenitic dendrites and pit formation; and (4) the growth of pits inside the grain. The following relationship between corrosion pit development and dendrite orientation in the LML is observed: (1) In the melted zone, with dendrite axes perpendicular to or inclined toward the surface, the corrosion pit grows within the grain. (2) At the melted zone/base metal (MZ/BM) boundary, with dendrite axes parallel to the surface, the corrosion pit develops in the heat-affected zone, along the MZ/BM boundary.