The Effects of Alloying Elements on the Corrosion of Steel in Flowing Sea Water

Abstract

The corrosion of steels in flowing 3% NaCl solution was studied by means of a loop-test at velocities up to 2m/sec and at temperatures of 20, 35 and 55°C. The test period was 15 days. Some corrosion tests in tap water were also carried out for comparison. Corrosion rates and corrosion potentials were measured, and corrosion products were analysed in order to investigate the effects of alloying elements on the corrosion of steel in flowing sea water.
The results obtained are summarized as follows:
1) The fully annealed steels show comparatively higher corrosion rate than normalized steels in 3% NaCl solution. Al, Si, P, Cr and Mn followed by Mo and Cu are beneficial, while C is detrimental in the former steels, whereas Al, Cr and Cu followed by Mn and C are beneficial in the latter.
2) Al and Cr followed by Si are beneficial, but S and Cu are detrimental in tap water.
3) Some steels containing appropriate contents of Al, Si and P show good corrosion resistance both in laboratory and field tests.
4) The corrosion potentials of steels are constant at about -600mV (vs. SCE) in 3% NaCl solution, while they change in the noble direction continuously with time in tap water. It means that the oxide film formed on the surface gradually increases protected area in tap water.
5) The corrosion products produced in 3% NaCl solution were identified as α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄. The amount increases with increase in corrosion rate.