Corrosion and Corrosion Inhibition of Reinforcing Steel: I. Immersed in Alkaline Solutions

Abstract

The electrochemical behaviour of reinforcing steel in stagnant alkaline solutions, especially calcium hydroxide, has been investigated. It was found that the oxidation processes that take place on steel are determined by the degree of surface oxidation of the sample and by dissolved oxygen, but not by the type of cation present.

In aerated solution, ferrosoferric oxide (Fe₃O₄) is the intermediate oxidation product on the steel surface while ferrous hydroxide is the intermediate product in de-aerated solution.

In pure alkaline solutions, the critical pH above which stable passivity occurs is 11·5. This value increases when the alkaline solution is contaminated with aggressive ions or when reinforcing steel having mill-scale on its surface is used. A linear relationship holds between the pH of the alkaline solution and the highest concentration of sodium chloride that can be tolerated (CC1-): pH = n logCcc⁻ + K where nand K are constants.

Calcium hydroxide solutions prepared fronl water of different local sources act differently towards steel. Thus, in saturated calcium hydroxide solution prepared from either distilled or tap water, the steel becomes passive. In the corresponding solution in mineral or sea water, the metal corrodes. On the other hand, mineral water becomes inhibitive when the pH is increased to 13. Sea water could not be tolerated by mere increase in pH.