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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 microstructureon the corrosion resistance of an API 5L X42 steel was examined in a 0.1 M chloride environ-ment (pH ∼ 7). Energy Disperse Spectrometry analysis found aluminium oxide inclusionson as-rolled, annealed, normalised, quenched, and quenched plus tempered samples. Heattreatments 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 0.85percent. It was found that the percentage of aluminium oxides did not affect the corrosionrate but promoted pitting corrosion. The average corrosion rate showed the following orderin 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 heattreatments applied to result in pearlite/ferrite phases for annealed and normalised sam-ples, martensite/austenite for quenched samples and tempered martensite/austenite forquenched plus tempered samples. The higher dissolution rate of manganese and copperelements in as-rolled and annealed samples induced the hardness of ferrite grains, improv-ing their corrosion resistance and making ferrite grains less anodic. The corrosion resistanceof quenched and quenched plus tempered microstructures did not depend on the dissolu-tion of elements, but the distribution of phases in their microstructures. Quenched samplesshowed a single-phase microstructure that reduced the formation of micro-galvanic cou-ples, improving corrosion resistance. Quenched plus tempered samples had two-phasemicrostructures that increased the number of micro-galvanic cells, therefore reducing thecorrosion resistance of the steel.