The Unexpected Role of Benzotriazole in Mitigating Magnesium Alloy Corrosion: A Nucleating Agent for Crystalline Nanostructured Magnesium Hydroxide Film

Benzotriazole (BTA), an effective corrosion inhibitor for Cu or Cu-containing Al alloys, was added to 0.1 M NaCl to study its potential role in suppressing corrosion of the commercial Mg alloy, AMlite. In particular, the impact of pH and BTA concentration was investigated, indicating that BTA was effective at restricting corrosion of AMlite in weakly alkaline NaCl solution, i.e. pH 10.0. The degree of the protection afforded was a function of BTA concentration. The best corrosion inhibition was found in the NaCl containing 15 g/L BTA, however, the mechanism of inhibition was not a classical function, nor related to the inhibition mechanism of BTA for Cu-bearing alloys. Instead it is posited that BTA− anions act as a nucleating agent to stimulate the formation of a dense and highly crystalline Mg(OH)2 surface film with a uniform nano-structure capable of passivating the Mg-alloy, in contrast to say, the insoluble Cu-BTA complex via Cu-N coordination bonds for Cu-alloys. The magnitude of this effect of BTA on Mg corrosion was not anticipated, but effective, with beneficial implications to utilization of Mg alloys as anode materials, where in such cases, when dissolution occurs, the anode can passivate with little change in its potential.