Corrosion mechanisms of Zn(Mg,Al) coated steel: The effect of HCO3− and NH4+ ions on the intrinsic reactivity of the coating

• The effect of Mg on cathodic Al dissolution cannot be attributed only to pH buffering.
• Anodic reactivity of ZnMgAl (ZM) was higher compared to galvanized steel (GI).
• Zn2+ accumulation in insoluble film increased by HCO3− and decreased by NH4+.
• In NH4Cl + NaHCO3 anodically formed film on ZM prevented cathodic Al dissolution.
• The effect was not observed neither for ZM in NaCl nor for Zn - 5% Al in both electrolytes.

The effect of NH4+ and/or HCO3− ions on the reactivity of hot dip galvanized steel and Zn - 3.7 wt. % Mg–3 wt. % Al coated steel in aqueous solutions containing Cl− was studied by direct measurement of the elemental dissolution rates of Zn, Mg and Al by atomic emission spectroelectrochemistry at open circuit potential and under alternating anodic (0.95 V vs SCE) and cathodic (-1.15 V vs SCE) potentiostatic pulses. The anodic reactivity of ZnMgAl coating was higher compared to a standard hot dip galvanized coating in all electrolytes. Mg rich phases were selectively dissolved. NH4+ increased the anodic reactivity of Zn in both coatings, suppressed the accumulation of zinc corrosion products, and inhibited Al dissolution at open circuit. HCO3− increased the accumulation of insoluble zinc corrosion products as well as the anodic reactivity. The cathodic current on the fresh surface of ZnMgAl in 0.1 M NaCl and in the mixture of 0.1 M NH4Cl and 0.1 M NaHCO3 was the same but cathodic dissolution of Al was higher in the mixed electrolyte. A series of alternating anodic and cathodic potentiostatic pulses demonstrated that the preliminary anodic polarization modified the following response of ZnMgAl on the cathodic pulse: the cathodic Al dissolution was significantly delayed and the cathodic current increased in mixed electrolyte. The cathodic current in NaCl was not affected by the preliminary anodic polarization. The effect was interpreted using a hypothesis of the formation of low soluble Mg carbonates during anodic polarization. Layered double hydroxides were not detected on ZnMgAl coatings after short experiments in mixed electrolyte. In contrast, Mg(OH)2, which is thermodynamically less stable at slightly alkaline pH, was detected. This indicates the importance of kinetic factors for the formation of corrosion products during the first stages of corrosion.

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