On the Mechanism of Corrosion of Steel Immersed in Saline Water
Authors: Cleary Harold J. 
Published: October 2008

The corrosion rates of iron and steel completely immersed in sea water are well documented in the literature for a variety of ambient conditions. A great quantity of published discussion has also been devoted to possible specific reaction mechanisms which cause this corrosion, but uncertainties still exist in this area concerning oxygen transport, the role of the complex corrosion products in the corrosion processes, the extent of hydrogen evolution, etc.

This paper reviews the voluminous literature relating to the mechanisms of iron and steel corrosion in saline, particularly chloride-bearing waters. Some preliminary measurements of pH and dissolved oxygen content variations across natural rust layers are also presented. These data were obtained using recently developed microelectrode techniques, and their significance with respect to corrosion reaction mechanisms is discussed.

INTRODUCTION

This paper is concerned with mechanisms of corrosion of steel after long periods in ambient natural waters. These mechanisms evidently differ from short term behavior because of the demonstrated failure of short term tests to predict long term Corrosion rates1,2. Since environment has been Shown to play a more important role in this corrosion than alloy composition or structure 1,2, it is likely that the corrosion mechanisms involved are very similar for the entire family of low alloy steels

CORROSIVITY OF NATURAL WATERS

It might be well to introduce the complex subject of corrosion mechanisms in saline waters by giving first a brief description of the natural environments of interest here and their effects on steel corrosion. Of the naturally occurring corrosive environments, one of the most severe is sea water. The relative proportions of the major constituents of open ocean water (listed in Table 1) are about the same in the major oceans throughout the world. Sea water is approximately equivalent in strength to 3.5% (w/v) NaCl (?0.6 N NaCl), but the actual composition is very complex and corrosion behavior may be significantly different in the two solutions. Marked variations can be produced by local conditions. Much more appears to be known about open ocean waters than coastal and harbor waters which are more variable in composition and more corrosive3. The pH is normally 8.1 – 8.3; in stagnant basins it may drop to 7. Of great importance to corrosion, the oxygen content ranges from saturation values in surface ocean waters (approximately 5.4 ml/l at 25°C and normal salinity) to zero in some estuarial waters. Oxygen content decreases with increasing temperature, and with depth to about 20003000 feet; the concentration has been found to increase gradually at greater depths4. Corrosion properties of sea water are also enhanced by the presence of marine animals and plants5. Temperature varies from-2°C to 35°C on the surface and seldom exceeds 10°C at great depths3. Water velocity varies widely from stagnant conditions to very high values at propeller surfaces.

Corrosion rate data for mild steel continuously immersed in sea water (near the surface) are shown in Figure 1. Corrosion rates are remarkably similar for widely separated geographical locations.