Inhibition by zinc–1-hydroxyethylidene, 1,1-diphosphonic acid mixtures of galvanic corrosion of mild steel coupled to copper in dilute chloride solution and effect of benzotriazole

Abstract

The inhibition by a zinc-1-hydroxyethylidene, 1,1-diphosphonic acid (Zn-HEDP)mixture of the galvanic corrosion of mild steel coupled to copper with different cathode to anode area ratios has been investigated in neutral, aerated, chloride solutions. It was found that the Zn-HEDP mixture provides some inhibition of galvanic corrosion of mild steel, but only at relatively low area ratios of copper/steel (1 : 1 or less). However, this inhibition is significantly poorer than that provided for mild steel when not coupled to copper. At higher copper/steel area ratios (3 : 1 or greater) the Zn-HEDP mixture failed to show any inhibition of the galvanic corrosion of mild steel. This is considered to be due to an increase of the potential of mild steel to the pitting potential. Although the Zn-HEDP mixture is not capable of inhibiting the galvanic corrosion of mild steel coupled to copper at a 3 : 1 copper/steel area ratio or above, it provided good inhibition of the galvanic corrosion in the presence of benzotriazole (BTA). This inhibition is attributed to a significant reduction of the cathodic reaction on copper and of the anodic reaction of mild steel, i.e. it is due to a combined effect of both BTA and the Zn-HEDP mixture. It is believed that the former acts by blocking the copper surface, reducing the oxygen reduction reaction and hence allowing the Zn-HEDP mixture to inhibit the corrosion of mild steel.