Inhibition of Localized Corrosion of Hot Dip Galvanized Steel by Phenylphosphonic Acid

Abstract

Phenylphosphonic acid (H 2 PP) is investigated as a corrosion inhibitor of hot dip galvanized steel (HDG) fully immersed in a 5% (w/v) sodium chloride electrolyte. An in-situ Scanning Vibrating Electrode Technique (SVET) is used where concentrations of H 2 PP are systematically added to the electrolyte in neutral conditions. H 2 PP, at a concentration of 5 × 10 −2 mol dm −3 , is shown to effectively inhibit localized corrosion over a 24 h period with 96% efficiency. H 2 PP is compared with a sodium phosphate (Na 3 PO 4 ) inhibitor at the same concentration over a wide pH range. With current legislative pressure to eliminate Cr(vi)-based inhibitors from protective coating formulations, there is a pressing need to identify effective, environmentally-friendly alternatives. Chromatefree passivation treatments have been studied extensively 1-4 and the most widely used alternative to chromate is a phosphate passivating treatment. Such a system facilitates the precipitation of protective zinc compounds via the supply of soluble anions that react with the zinc ions generated in metal dissolution.5 A phosphate passivation treatment generates a Zn 3 (PO 4 ) 2 barrier to inhibit the anodic process, 4,5 the formation of which can act to extend the pH stability of the surface layer down to values of around pH 6. However, as zinc phosphates are 1000 times more soluble than Cr (III), inferior corrosion inhibition performance is observed. 7 The corrosion of a zinc surface generates a wide range of pH zones, whether in bulk electrolyte conditions, atmospheric conditions or, most notably, under a water droplet. 8,9 Corrosion-driven cathodic coating delamination phenomena produces an alkaline underfilm environment where values of pH 10-11.5 have been recorded previously on a coated zinc surface. 10In order to compete with an adsorbed chromium (III) oxide layer, an effective corrosion inhibitor system would have to be stable in a range of pH 4-14.7 It is therefore of great importance to examine potential chromate replacement inhibitors in non-neutral conditions.In a recent Scanning Kelvin probe study, we showed that in-coating additions of H 2 PP increased the initiation time for underfilm cathodic coating delamination by up to 20 h and reduced the delamination rate by up to 94% on hot dip galvanized steel (HDG) surfaces.11 An 'etch' effect was shown to be the dominant inhibition mechanism, whereby a reaction occurs at the point of first contact of the wet coating with the substrate producing an interfacial metal phosphate salt layer. We hypothesize that a second mode of inhibition occurs whereby the leaching of dissociated PP 2− ions from the coating into the substrate/coating interface forms a ZnPP layer, blocking the anodic reaction. H 2 PP has been shown previously to strongly adsorb onto surface oxide films, disfavoring chloride.12 The principal aim of this work is to determine the efficiency of H 2 PP as an inhibitor for HDG surfaces in bulk immersion conditions. The localized corrosion activity occurring over a commercial gr...

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