The role of non-metallic Al2O3 inclusions, heat treatments and microstructure on the corrosion resistance of an API 5L X42 steel

Abstract

The role of heat treatments, non-metallic aluminium oxide inclusions, and microstructure on the corrosion resistance of an API 5L X42 steel was examined in a 0.1M chloride environment (pH∼7). Energy Disperse Spectrometry analysis found aluminium oxide inclusions on as-rolled, annealed, normalised, quenched, and quenched plus tempered samples. Heat treatments changed the percentage of aluminium oxide inclusions in X42 steel. Annealed, normalised, quenched, and quenched plus tempered samples reached mean values of 0.15, 0.15, 0.12 and 0.07 percent respectively, whereas, rolled samples reached a mean value of 0.85 percent. It was found that the percentage of aluminium oxides did not affect the corrosion rate but promoted pitting corrosion. The average corrosion rate showed the following order in millimetres per year: as-rolled (0.067)<annealed (0.070)<quenched (0.078)<normalised (0.091)<quenched and tempered (0.103). The microstructure varied depending on the heat treatments applied to result in pearlite/ferrite phases for annealed and normalised samples, martensite/austenite for quenched samples and tempered martensite/austenite for quenched plus tempered samples. The higher dissolution rate of manganese and copper elements in as-rolled and annealed samples induced the hardness of ferrite grains, improving their corrosion resistance and making ferrite grains less anodic. The corrosion resistance of quenched and quenched plus tempered microstructures did not depend on the dissolution of elements, but the distribution of phases in their microstructures. Quenched samples showed a single-phase microstructure that reduced the formation of micro-galvanic couples, improving corrosion resistance. Quenched plus tempered samples had two-phase microstructures that increased the number of micro-galvanic cells, therefore reducing the corrosion resistance of the steel.