Study on the corrosion behavior of Q235A carbon steel in RO product water of seawater

Some power plants in China that adopt reverse osmosis (RO) product water as their fresh water source face serious metal corrosion of their water distribution system. The corrosion process of carbon steel in RO product water is still not clear and there is no suitable anti‐corrosion method for the power plant to employ. The purpose of this paper is to study the corrosion behavior of carbon steel in RO product water, determine the factors leading to the high corrosion rate of carbon steel, and then suggest appropriate anti‐corrosion measures.

By measuring polarization curves and AC impedance values of the corrosion system and analyzing corrosion products using scanning electron microscopy (SEM), infrared spectroscopy (IR) and X‐ray diffraction (XRD), the corrosion behavior of Q235A carbon steel in the RO product water derived from seawater was studied.

The experimental results showed that the corrosion process of carbon steel in RO product water is controlled by the diffusion process of oxygen, and the corrosion products contain γ‐FeOOH, Fe₃O₄ and small amounts of α‐FeOOH. Although rust formed had a double layer structure, the outer rust layer, which contained γ‐FeOOH and a little α‐FeOOH, was thin. The inner rust layer, containing Fe₃O₄, was the main component of the rust layer. Due to the weak acidity of RO product water, γ‐FeOOH can be transformed to Fe₃O₄ very quickly and Fe₃O₄ will accumulate on the metal surface. Because of the electrical conductivity and fractured surface of the Fe₃O₄ layer, the corrosion product layer cannot inhibit the corrosion process by hindering the diffusion process of oxygen, and hence the corrosion rate of carbon steel is always high.

The paper describes the first systematic research to be carried out on the corrosion behavior of carbon steel in RO product water. It was found that the generation and accumulation of Fe₃O₄ on the metal surface was the primary reason leading to the high corrosion rate of carbon steel, and anti‐corrosion measures can be chosen following the following rules: deoxygenation, raising of the pH of the solution, or addition of corrosion inhibitors to the solution.