Sodium Cholride, Corrsion of Reinforcing Steel, and the pH of Calcium Hydroxide Solution
H. A. Berman
Appears on pages(s):
bridge decks;calcium hydroxides;cholrides; concretes; corrosion; mortars (materials);pH;reinforcing steels; sodium cholride.
Electircal half-cell potential measurement is a valid technique for detecting the existance of corrsion in the steel reinforcement of cholride-containing concrete. The voltage measured is not directly affectd by the presence of sodium cholride. The salt affects the voltage only indirectly by its tendency to initiate steel corrosion. Corrosion in saturated calcium hydroxide solution exposed to air occurred in these tests at a sodium cholride concentration as low as 0.03 molar. This threshold concentration was raised to 1.0 molar when oxygen was excluded. To determine whether pH change might be a factor in corrosion, measurements were made in a carbon dixiode-free atmosphere on the pH change produced by addition of sodium cholride to saturated calcium hydroxide solution. The pH of saturated calcium hydroxide decreased with increasing NaCI concentration and increased with decreasing NaCI concentration. Small additions of NaCI up to a concentration of 0.008 molar actually produced a pH increase. The results indicate that the pH reduction may contribute to steel corrosion in concrete, but do not conclusively prove that it does. Nevertheless, they suggest the desirablity of investigating the addition of alkaline materials to concrete containing, or exposed to, cholride ions above the critical concentration, as a means of restoring a suitable pH and thereby preventing steel corrosion.