Luciana Machado Rodrigues

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

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 (Na 2 SO 4 ) 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 ofFe and HS.

AFM Study of the Corrosion of Pipeline Steel in Organic Compounds Extracted from Soil

Aiming to simulate the external corrosion of pipelines, the first stages of the inclusions attack and nucleation of pits were studied by AFM on the API 5LX56 steel in pure 10 mM Na2SO4 solutions with additions of humic and fulvic acids, both extracted from a Brazilian peat soil. For this, selected inclusions were first marked with micro indentations, analyzed in the SEM in the backscattering mode and chemically mapped with EDS. Pit formation was then investigated by scanning Kelvin probe force microscopy (SKPFM). The SKPFM evidenced that HA added to sulfate solutions promotes the fast dissolution of the sulfides of complex inclusions, resulting in pit nucleation, while FA additions slow this attack, acting as a natural corrosion inhibitor.