Combined effect of hydrostatic pressure and dissolved oxygen on the electrochemical behavior of low-alloy high-strength steel

With the development of marine industry, the performances of metal materials in marine environment have gathered much attention of scientists. Seawater, as a Cl^--containing electrolyte, degrades the properties of steel structures and limits their service life due to its erosion to steel surface. The corrosion phenomena of low-alloy high-strength steels in surface seawater are well known but not sufficiently understood in deep-sea environment. The effect of hydrostatic pressure on the corrosion behavior of low-alloy steels is a focus in this aspect. However, the results from the laboratory study cannot well illustrate the ones from the field test, because some factors change simultaneously with the increase of ocean depth. Therefore, it is necessary to study the corrosion behaviors of steels in a multi-factor coupled environment. In this report, the combined effect of hydrostatic pressure and dissolved oxygen on the electrochemical behavior of low-alloy high-strength steel in 3.5% (mass fraction) NaCl solution was investigated using potentiodynamic polarization tests and scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) measurements. The results show that the corrosion potential increases at first and then decreases with the increase of both hydrostatic pressure and dissolved oxygen. The corrosion current density exhibits a nonlinear increasing tendency with the increase of these two factors. The ideal polarization curve method was used to analyze the interaction of hydrostatic pressure and dissolved oxygen in the corrosion process. The results indicate that there is a competitive inhibition relationship between hydrostatic pressure and dissolved oxygen. With the increase of both hydrostatic pressure and dissolved oxygen, dissolved oxygen first accelerates the cathodic reaction process and inhibits the anodic reaction process. Afterwards, hydrostatic pressure starts accelerating the anodic reaction rate and inhibits the acceleration of the cathodic process caused by dissolved oxygen. The corrosion films on the steel surface significantly inhibit the acceleration to corrosion process given by the combined effect of hydrostatic pressure and dissolved oxygen. Moreover, these two combined factors encourage the growth of corrosion films and increase the number and sizes of corrosion pits forming on the steel surface.