Effect of Rust Layer on the Corrosion Behavior of Carbon Steel in Reverse Osmosis Product Water

Abstract

The morphologies and electrochemical characteristics of rust layers generated in seawater and reverse osmosis(RO)product water were investigated by scanning electron microscopy(SEM),infrared spectroscopy(IR),X-ray diffraction(XRD)and electrochemical tests.Experimental results revealed obvious differences in the components,structures and functions of rust layers in two types of solutions,so that the corrosion rates of carbon steel were markedly different.The reduction potential of γ-FeOOH is higher than the corrosion potential of carbon steel in RO product water,such thatγ-FeOOH is easily reduced to Fe 3 O 4 and two layers emerge in the rust structures.The outer layer(γ-FeOOH layer)is too thin to inhibit oxygen diffusion,and tends to accelerate the cathodic process via reduction ofγ-FeOOH.Because Fe 2+ and electrons can pass through the Fe 3 O 4 layer,oxygen can be directly reduced on the surface of the inner layer(Fe 3 O 4 layer).Therefore,the inner rust layer(Fe 3 O 4 layer)can provide a large cathode area on which to promote oxygen reduction.As a result,the corrosion process of carbon steel in RO product water can be accelerated greatly.The corrosion rate of carbon steel is determined by the limiting diffusion rate of oxygen from solution to the inner rust layer.Anti-corrosion measures should inhibit the reduction reaction ofγ-FeOOH.