Influence of Humic Substances on the Corrosion of the API 5LX65 Steel

Abstract

Aiming to simulate the underground corrosion of pipeline steels in tropic soils, the electrochemical behavior and corrosion morphology of the API 5LX65 steel in 0.01 mol/L sodium sulfate (Na2SO4) solutions were studied, with additions of humic (HA) or fulvic (FA) acids, extracted from a peat soil. The corrosion tests showed that FA additions improve the passivity of the steel, whereas HA has the opposite effect. The nucleation of pits is always associated with the presence of rounded complex inclusions, containing sulfides and oxides. Raman spectra of the corrosion products covering the pits and inclusions suggest the formation of organic Fe compounds. The influence of humic substances (HS) on pit nucleation was attributed to a passivity breakdown on the interface between Fe and inclusions, through the formation of partially soluble compounds of Fe and HS.