Effect of alloy element on corrosion behavior of the huge crude oil storage tank steel in seawater

Abstract

Corrosion behaviors of the high strength low alloy (HSLA) steel and the carbon steel for a huge crude oil storage tank in seawater were studied by electrochemical methods and microanalysis techniques. The research found the alloy element had decreased corrosion rates for a long time but slightly increased corrosion rate at the beginning of experiments. Potentiodynamic tests results showed that all specimens exhibited active corrosion behavior, and corrosion rate tended to increase as a result of adding alloying elements. However, the loss weight results showed the corrosion rates of the HSLA steel were much smaller than those of carbon steel. Furthermore, more alloying elements led to a remarkable decrease in the corrosion rate. The reasons for these by electrochemical impedance spectroscopy (EIS) showed that the corrosion process of all samples showed two stages. At the first 144h, EIS of all samples showed one increasing capacitance arc. The carbon steel showed the least corrosion rate, which was due to that the corrosion rates depended on the active site on the sample surface and the alloy effects were minor. After 144h of immersion, EIS of the HSLA steels showed two capacitance arcs and the EIS of the carbon steel always showed one capacitance arc, which indicated the HSLA steel can form a compact corrosion scale. The HSLA steel showed the smaller corrosion rate than carbon steel, which was due to that the corrosion rates depended on the protective ability of corrosion scales and the alloy elements were help to form a compact corrosion scale on sample surface. The EDS results indicated that Cr, Mo and Al were distributed densely at the interface between the rust layer and the steel surface which helped to improve corrosion resistance for the HSLA steel. Corrosion of the HSLA steels was suppressed by insoluble compound formed near the surface.