Study of the metal structure of a pipe operated under the influence of salt mist

Products intended for use in coastal and marine environments are constantly exposed to a harsh environment. The aim of this study is to investigate the microstructure of pipe metal after 15 years of use in a salt-mist removal system (5% NaCl, pH 6.5 – 7.2, 35°C). The chemical composition of the metal in different parts of the pipe was analyzed using optical emission spectroscopy. It was found that the chemical composition corresponds to austenitic steel AISI 304. When measuring the microhardness with a load of 9.81 N, a 34% increase was observed in the curved parts of the pipe compared to the straight parts due to riveting and the formation of deformation twins. To identify the microstructure of the metal, chemical etching was performed using a boiling solution of acids (HNO₃ and HCl) and a Marble reagent. The results showed that intercrystalline corrosion was the most common type, observed on both the inner and outer surfaces of the pipe, near the welding joint. Additionally, there were pockets of knife-shaped corrosion. The maximum depth of corrosion damage, which was 55% of the wall thickness, was detected at the intersection of factory and installation welds. These results can be used to improve the method of non-destructive testing for critical sections of stainless steel pipes.

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