Influence of Chromium on Corrosion Behavior of Low-alloy Steel in Cargo Oil Tank O_2-CO_2-SO_2-H_2S Wet Gas Environment

As international maritime organization(IMO)draft 289 was adopted to develop a low-alloy anti-corrosion steel for the deck of cargo oil tank and to understand corrosion mechanism,corrosion behavior of a low-alloy steel with chromium contents was studied in O2-CO2-SO2-H2 S wet gas environment.Corrosion rate was measured,and the microstructure and morphology of corrosion product film were characterized by scanning electron microscopy(SEM).The phase and chemical composition of the corrosion product film were investigated by X-ray diffraction(XRD)and energy dispersive spectroscopy(EDS),respectively.The effect of misorientation distribution on corrosion resistance of steel was evaluated by electron backscattered diffraction(EBSD).The results showed that corrosion rate decreased with increasing chromium content in the low-alloy steel,and the corrosion type was general corrosion.The phenomenon of chromium enrichment was found in corrosion product film consisting ofα-FeOOH,γ-FeOOH,sulphur,FeS2 and Fe1-xS.The increase of chromium content decreases the amount of high-angle grain boundaries,thus resulting in the improvement of corrosion resistance.

Influence of Inclusion on Corrosion Behavior of E36 Grade Low-alloy Steel in Cargo Oil Tank Bottom Plate Environment

Corrosion behavior of low-alloy steel was investigated in simulated cargo oil tank(COT)bottom plate service environment(10% NaCl solution,pH=0.85).The corrosion behavior of inclusion was studied by in-situ scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).It was found that pitting corrosion was inclined to occur around the place where inclusions exist.After initial corrosion,an area of 10-20μm in diameter was formed as a circinate cathode around the edge of inclusion.MnS inclusion dissolved in the simulated COT corrosion solution before low-alloy steel matrix,and pitting was formed at the place where MnS dissolved.TiO2 inclusion dissolved in the simulated COT corrosion solution after low-alloy steel matrix,and pitting was formed at the place where steel matrix dissolved.The corrosion tended to occur at the area where the curvature radius of inclusion is smaller.The size of round TiO2 inclusions had little influence on corrosion behavior in this certain environment.

Influences of Alloying Elements on Oxidation Behavior of Steels and Microstructure of Oxide Scales

In order to figure out the oxidation behavior of steels during heating,five micro-alloyed steels were subjected to continuous and isothermal oxidation using the thermo gravimetric analyzer and the Gleeble-3500thermo-mechanical simulator.The microstructure of oxide scales,especially the thickness fractions of Fe2O3,Fe3O4 and FeO layers,was analyzed using the scanning electron microscope(SEM),electron probe microanalyzer(EPMA)and electron backscattered diffraction(EBSD)techniques.The micro-alloyed steels containing alloying elements(Si,Cr,Ni and Cu)show a higher oxidation resistance compared with the low carbon steel.It is found that alloying elements accumulated at scale/substrate interface during high temperature oxidation.Alloying elements function in two ways in the oxidation of steels:one is enhancing the scale/substrate interface and consequently suppressing the blister of scales;and the other is impeding the outward diffusion of iron cations from substrate to scales,resulting in the decrease of oxidation rate.As the diffusion of iron cations is impeded,the thickness fractions of Fe2O3 and Fe3O4of micro-alloyed steels are more than those of low carbon steels.