Evaluation of the corrosion protection of organic-coated zinc-alloy galvanised steels using novel, environmentally-friendly corrosion inhibitor pigments

The corrosion behaviour of steel coated with either: a primarily zinc coating comprised of 0.15 wt% Al (HDG) or a zinc-aluminium-magnesium coating, 1.6 wt% Mg, 1.6 wt% Al, and 96.8 wt% Zn (ZAM) are investigated in the presence of industry standard and emerging, environmentally-friendly corrosion inhibitor technologies. With this paper evaluating the inhibitory performance of a functionalised oxy-amino-phosphate-salt of magnesium (OPMG), a hydrotalcite carbonate clay loaded with 4-aminobenzoic acid (HT-PABA), a calcium ion exchange pigment (Ca-Ex), and an inhibitor based on 2-(1,3-benzothiazol-2-ylithio) succinic acid (BTSA) dispersed in model poly-vinyl-butyral (PVB) coatings and 3.5 wt% NaCl (aq) solutions. With their inhibitory performance evaluated against two corrosion-driven coating failure mechanisms: cathodic disbondment (CD), and filiform corrosion (FFC) as a function of inhibitor loading. In the case of CD, an inhibitor ranking order of BTSA > HT-PABA > OPMG > Ca-Ex is observed, while OPMG and HT-PABA are the most effective at slowing rates of FFC. Potentiodynamic and scanning vibrating electrode (SVET) experiments were conducted to evaluate the efficiency of the most promising inhibitors at slowing rates of corrosion on the bare ZAM alloy surface and the exposed cut edges immersed in chloride solutions. With OPMG and BTSA both shown to act as net anodic inhibitors, resulting in an increase in the polarisation resistance by over an order of magnitude. Both inhibitors produced a derived 67 % reduction in total (zinc) metal loss at the exposed ZAM cut-edges over a 24 h immersion period against the control. • Systematic assessment of inhibitor technologies for coated zinc alloy steels against corrosion driven coating disbondment • Inhibitors ranked (CD): BTSA > HT-PABA > OPMG > Ca-Ex; with OPMG showing the greatest inhibition to FFC on coated ZAM steel • BTSA and OPMG identified as effective net anodic inhibitors through potentiodynamic testing • BTSA and OPMG increased polarisation resistance by >10× on chloride immersed ZAM • Both inhibitors reduced SVET derived zinc metal losses for ZAM coated steel cut-edges by > 67% after 24 h immersion.