Calcium magnesium acetate (CMA) is a term used to describe the reaction product of acetic acid and dolomitic limestone. This product is a mixture of calcium acetate and magnesium acetate and has been identified as a possible deicing chemical to replace chloride-containing chemicals such as sodium chloride and calcium chloride. The Federal Highway Administration has sponsored work to determine the corrosive behavior of CMA on bridge structural materials and steel in concrete.
Electrochemical tests were conducted in commercial and reagent-grade CMA solutions of 0.5 to 20 wt%. Anodic and cathodic polarization behavior and corrosion rates were determined by the polarization resistance technique.
Polarization scans showed that steel can exhibit active-passive behavior in both commercial- and reagent-grade CMA solutions. Reverse polarization scans indicated an unusual electrochemical behavior occurred with three current reversals in commercial- and reagent-grade CMA solutions of 2 wt% and higher. A current reversal occurred, followed by a cathodic loop; then a second current reversal occurred. An anodic current loop followed this second current reversal, which was in turn followed by third current reversal.
The double-loop behavior was determined to be dependent on CMA concentration and upon the presence of carbon dioxide. It is proposed that carbon dioxide is formed as a consequence of an electrochemical reaction involving the acetate ion. The carbon dioxide dissolves in the solution and in turn results in precipitation of insoluble carbonates at cathodic sites on the steel surface. The total exchange current is lowered by the covering of cathodic sites and results in a shift in the corrosion potential in the cathodic direction. When the transient corrosion potential becomes more cathodic than the initial corrosion potential, then net anodic current will be drawn during the reverse anodic polarization scan. The double-loop behavior is then observed.
