Aiming to simulate the underground corrosion of pipeline steels in tropic soils, the electrochemical behavior and corrosion morphology of API 5LX65 steel were studied in 0.01molL−1 Na2SO4 solutions with additions of humic (HA) or fulvic (FA) acids extracted from a peat soil. These humic substances have a higher carboxylic and nitrogenated character for HA, and a higher concentration of oxidized groups for FA, as determined by infrared (FTIR) and Raman spectroscopies. The corrosion tests showed that HA and FA enhance the pitting corrosion of the API 5LX65 steel. The observed pits are always associated to the presence of the rounded calcium aluminate inclusions of API 5LX65. Raman spectra of the corrosion products partially covering pits and inclusions suggest the formation of organic Fe compounds. The influence of humic substances on pit nucleation was attributed to passivity breakdown on the interface between Fe and inclusions, by the formation of partially soluble compounds of Fe and humic substances.

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