The effect of synthetic zinc corrosion products on corrosion of electrogalvanized steel. II. Zinc reactivity and galvanic coupling zinc/steel in presence of zinc corrosion products

• Anodic inhibition of zinc dissolution by hydrozincite and sodium zinc carbonate (SZC).
• Modified galvanic coupling zinc/steel in presence of precipitated zinc corrosion product.
• Initial decrease of galvanic current by simonkolleite and zinc hydroxysulphate (ZHS).
• Transformation of ZHS into Zn(OH)2 during galvanic coupling steel/patina/NaCl aq/zinc.
• Insignificant difference between patinas in confined zone probably due to pH instability.

The reactivity of zinc under synthetic zinc patinas and the galvanic coupling in steel/patina/Zn are studied. Zn5(OH)6(CO3)2 and Na2Zn3(CO3)4⋅3H2O inhibit zinc anodic dissolution in NaCl, while Zn5(OH)8Cl2 H2O and Zn4(OH)6SO4nH2O do not. The galvanic current in steel/patina/NaCl/Zn is smaller as compared to steel/NaCl/Zn. The inhibiting effect decreases with time for Na2Zn3(CO3)4⋅3H2O or Zn4(OH)6SO4 nH2O due to the transformation into Zn(OH)2. In NaHCO3, the polarity between zinc and steel can reverse. The effect of confinement on the cathodic current is stronger than the initial effect of patina which is explained by the instability of the patinas under rapid pH-increase.