Corrosion failure in biological fluids of laser melted surface of AISI 321 stainless steel

Abstract

The aim of the present study is to investigate the failure of laser melted surfaces of AISI 321 steel under corrosion in two types of biological fluids: artificial saliva (AS) with different acidity (pH 5,6 and 6,5) and Ringer’s solution. The surface of prismatic samples was melted by continuous CO₂ laser with different regimes. The melted surfaces were subjected to electrochemical corrosion test in biological fluids. The corrosion failure on the surfaces was investigated by OM, SEM and EDX analysis. It was found that regardless of the type and acidity of the environment, two types of corrosion processes took place on the samples’ surface - pitting and crevice. Corrosion pits of different shape and sizes were observed on the surface of all samples. In both corrosion media, the pits on the untreated steel surface were characterized with irregular shape, smooth walls and sizes between 10-300 µm. While these on the surface of the laser-melted layers had a rounded shape, sizes of 10-400 µm, less depth and rough walls. The corrosion failure in depth of the surface layer of wrought and laser treated steel was identical – selective, mainly by destruction of the corrosion non-resistant δ-ferrite phase. Its microstructure defined the corrosion pits morphology, characterizing with irregular shape and great depth (50-300 µm) in the wrought steel surface and shallow (40-100 µm) equiaxed pits on the laser melted layers. From the point of view of resistance to corrosion failure, laser surface melting of AISI 321 steel improves conditions for enhanced durability of the dental and implant constructions.