Abstract
The effect of W and Mo on the corrosion behavior of low alloy steels exposed to acid humidity corrosion environment containing O2, H2S, and CO2 was investigated using weight loss, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. The results indicate that both addition of W and Mo can improve corrosion resistance of low alloy steels, and the enhancement effect of W is better than Mo. Surface layers are mainly composed of FeOOH, Fe3O4, and small amount of S and Fe2 (SO4) 3·nH2O. Mo and W element in steels can be oxidized into MoO2, MoO3, and WO3, respectively, which can act as corrosion inhibitor during corrosion process. Mo steel have lower corrosion resistance than W steel, because the chemical transformation from MoO2 to MoO3 may form defects inside surface layers, and the smaller molecular mass and volume of MoO3 than WO3 makes MoO3 easier to move in surface layers and lower the resistance of anodic dissolution.
Introduction
The inner gas in vapor space of crude oil tankers (COTs) contains oxygen (O2, 1~5%), carbon dioxide (CO2, 1~13%), hydrogen sulfide (H2S, 500~3000ppm) and water vapor originating from transported crude oil. 1, 2 Electrolyte film have been observed to form on the backside of upper deck due to the day/night temperature change, and dissolution and chemical reaction of O2, H2S, and CO2 lowers the pH of the electrolyte film. Thus, severe corrosion occurs. 3, 4 Low alloy corrosion resistant steels has been developed by many researchers and steel corporations to provide high service stability and reliability for the upper deck in such corrosion circumstances due to its high economic values and corrosion resistant properties. 5, 6 A small amount of alloy elements, such as Cu, Ni, Cr, Mo, and W, are added into low alloy steels to form a dense and protective surface layers preventing the steel plates from further corrosion. 7, 8 The effect of Cu, Cr, and Ni on the corrosion behavior of low alloy steels in various corrosion environments containing O2, H2S, and CO2 have been reported by lots of researchers, 7, 8 but the effect of W and Mo was rarely reported. The aim of present investigation is to illustrate the effect of W and Mo on corrosion behavior of low alloy steels exposed to an O2-H2S-CO2 humidity corrosion environment.
