The corrosion resistance of stainless steel mainly depends on the isolation of passive film. The passivation film on the surface of stainless steel consists of two parts, the inner layer is mainly composed of chromium oxide, which has strong blocking effect on corrosive ions, while the outer layer is mainly composed of iron oxide and hydroxide.
The corrosion resistance of stainless steel depends on the formation and protection of passive film. The concentration of Cl – and SO2-4 in seawater is very high. Cl – has corrosive effect on the passivation film. Therefore, the higher the concentration of Cl – in seawater, the greater the corrosion dissolution rate of the passivation film. The worse the stability of the passivation film is, the more loose the passivation film formed on the surface of stainless steel becomes, the worse the uniformity, and the passivation film is damaged. Therefore, stainless steel is eroded by seawater.
According to the research, the marine environment can be divided into five corrosion areas according to its nature, which are the seabed soil area, the whole immersion area, the tidal range area, the spray splash area and the marine atmosphere area. Due to the different temperature, pH and dissolved oxygen in each area, stainless steel has different corrosion types in different areas. Among many corrosion types, pitting corrosion and crevice corrosion are the main corrosion types. By measuring the corrosion potential, the corrosion resistance of stainless steel in seawater can be understood, and the addition of some alloys can improve the corrosion resistance of stainless steel to a certain extent. It is believed that the concentration of dissolved oxygen in seawater is the main factor affecting the corrosion of stainless steel. The corrosion of carbon steel in the deep sea is mostly uniform, while the corrosion of stainless steel in the deep sea is mostly crevice. The method of sacrificial anode is often used to prevent metal corrosion in deep sea.