Effect of dissolved oxygen on pitting corrosion behavior of low-alloy steel under hydrostatic pressure in neutral 3.5 wt.% NaCl solution was investigated by scanning electron microscope (SEM), 3-D measuring microscope and so on. The results show that the combined effect of hydrostatic pressure and dissolved oxygen increases the overall growing rate of corrosion pits and causes deeper corrosion pits but decreases their sizes in the horizontal direction. Electrochemical measurement results reveal that dissolved oxygen dominate the corrosion kinetics of low-alloy steel in the environment containing high hydrostatic pressure and high dissolved oxygen by accelerating cathodic process, slowing down the anodic process and promoting open circuit potential (OCP). Consequently, the increased charge transfer process makes the pits have a larger volume and depth. Meanwhile, the decreased initial potential energy difference based on energy theory calculation at the initiation site by dissolved oxygen possibly inhibits the pitting growth in the horizontal direction.

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In harsh environments, the corrosion damage of steel structures and equipment is a serious threat to the operational safety of service. In this paper, a Zn-Al diffusion layer was fabricated on 45 steel by the Mechanical Energy Aided Diffusion Method (MEADM) at 450 °C. The microstructure and composition, the surface topography, and the electrochemical performance of the Zn-Al diffusion layer were analyzed before and after corrosion. The results show that the Zn-Al diffusion layer are composed of Al2O3 and Γ1 phase (Fe11Zn40) and δ1 phase (FeZn6.67, FeZn8.87, and FeZn10.98) Zn-Fe alloy. There is a transition zone with the thickness of about 5 μm at the interface between the Zn-Al diffusion layer and the substrate, and a carbon-rich layer exists in this zone. The full immersion test and electrochemical test show that the compact corrosion products produced by the initial corrosion of the Zn-Al diffusion layer will firmly bond to the Zn-Al diffusion layer surface and fill the crack, which plays a role in preventing corrosion of the corrosive medium and reducing the corrosion rate of the Zn-Al diffusion layer. The salt spray test reveals that the initial corrosion products of the Zn-Al diffusion layer are mainly ZnO and Zn5(OH)8Cl2H2O. New corrosion products such as ZnAl2O4, FeOCl appear at the middle corrosion stage. The corrosion product ZnAl2O4 disappears, and the corrosion products Zn(OH)2 and Al(OH)3 appear at the later corrosion stage.

Hydrostatic pressure (HP) and dissolved oxygen (DO) both change with the increase of ocean depth. In order to unmask the corrosion phenomena of metals in deep sea environment, studying their combined effect on corrosion is necessary. In this report, couple effect of HP and DO (HP + DO) on corrosion behaviour of low-alloy high strength steel in 3.5 wt-% NaCl solution was investigated by Electrochemical measurements, SEM observations, XPS characterisations and 3-D size measurements. The results show that the corrosion rate of low-alloy high strength steel increases with increasing HP + DO under each immersion period, appearing an exponential decay growth. As HP + DO increases to a critical value, 30 ATO, its acceleration to corrosion process becomes limited. Moreover, HP + DO remarkably increases the growth of corrosion film on steel surface and leads to more loose structure in corrosion film. Finally, HP + DO stimulates the propagation of corrosion pits in 3D direction.