Surface “Roughness” Effect on Marine Immersion Corrosion of Mild Steel Available to Purchase

For materials such as stainless steel, it is well known that the surface condition (roughness) can have an effect on corrosion loss or pitting as a function of exposure time. Although some isolated observations exist of this effect for mild steel under marine immersion, there appears to have been no systematic investigation. The present paper describes results obtained in field tests carried out over a period of 13 months in temperate waters on the Eastern Australian seaboard. Surface conditions considered were mill scale, surface ground, grit blasted, polished, acid cleaned, and pre-rusted. The effect of surface roughness on corrosion was found to be approximately proportional to the surface roughness index. Surface roughness has the most effect in the first few weeks of exposure, with the net effect then remaining approximately constant with time. The acid-cleaned coupons show only a low value of roughness index, but on the basis of scanning electron microscope (SEM) images and high early instantaneous corrosion rates, it is postulated that the actual surface roughness is actually much higher.