Corrosion inhibitor control in aqueous solutions evaluated with galvanic couples

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An electrochemical method was evaluated for obtaining measurements to determine the effectiveness and the type of control exerted by corrosion inhibitors in galvanic systems. Anode and cathode potentials and corrosion currents were measured on zinc-platinum and iron-platinum galvanic couples in test solutions before and after addition of potassium chromate as an inhibitor. The measurements were then plotted as galvanic polarization diagrams which indicate a comparative corrosion rate and the type of inhibitor control, as anodic, cathodic, or mixed. Some of the results were also compared with weight loss data. Experimental variables such as inhibitor concentration, sodium sulfate concentration, corrosion current density, spacing between the anode and the cathode of the couple, and external resistance of the test circuit were investigated. The method gave a good evaluation of the behavior and the type of control of an inhibitor in galvanic systems in dilute aqueous solutions. The results show that it possibly could be used to identify the inhibitor behavior in ordinary corrosion provided the corrosion current generated by the galvanic couple is adjusted to approximate that generated by local cell action. The inhibiting effectiveness of chromate ions was lowered by the presence of the other anions such as the sulfate ion. When the initial corrosion current started at low values, the chromate tended to develop greater anodic polarization and better inhibition than higher initial corrosion currents.