The ingress of oxygen into pressure vessels used in oil & gas production and transportation could easily result in serious corrosion. In this work, the corrosion behaviors of Q345R steel at the initial stage in 1 wt.% NaCl solution were investigated using electrochemical techniques. The effects of oxygen concentration, temperature and pH on corrosion behaviors were discussed. Simultaneously, a numerical model based on the mixed potential theory was proposed. The results show that the proposed model accords well with the experimental data in the pH range from 9.0 to 5.0. In this pH range, the oxygen reduction reaction, H⁺ reduction, water reduction, and iron oxidation can be quantitatively analyzed using this model. However, this model shows a disagreement with the experimental data at lower pH. This can be attributed to the fact that actual area of reaction on the electrode is much smaller than the preset area due to the block effect resulted from hydrogen bubbles adsorbed on the electrode surface.

The effects of HSS (heat-stable salts) on the corrosion behaviours of 20 steel were explored using self-designed HTHP autoclave. The charateristics of corrosion scales were investigated by SEM, EDS and XRD. Corrosion scales of 20 steel formed in the liquid phase environment were mainly composed of FeS₂, Fe₃O₄, and Fe. HSSs affected the corrosion rate through changing the morphology of corrosion scales of 20 steel. Inorganic salts enhanced the attractive force between ions and the hydrolysis of CH₃COO⁻ in organic salts promoted the ionisation of HS⁻, which accelerated the deposition of FeS on the surface of 20 steel to form a dense corrosion scales, thereby significantly reducing the corrosion rate of 20 steel. Inorganic–organic salts reduced the bonding opportunity between Fe²⁺ and S²⁻ and allowed the formation of porous corrosion scales on the surface of 20 steel, thus increasing the corrosion rate of 20 steel.