Galvanic alloys Zn–Mn—composition of the corrosion products and their protective ability in sulfate containing medium

The composition of the corrosion products of non-chromated and chromated galvanic coatings of Zn, Zn–Mn (6 wt.%) and Zn–Mn (11 wt.%) in a model free aerated solution of 1 N Na2SO4 (pH 6) is determined using X-ray powder diffraction analysis (XRD). Surface morphology and its changes are registered with scanning electron microscopy (SEM). Microprobe analysis is applied to specify the amounts of Zn, Mn and S after the corrosion treatment. The registered compounds on Zn and Zn–Mn alloys during the experiments—Zn4(OH)6SO4·5H2O, Zn4(OH)6SO4·4H2O, Zn4(OH)6SO4·3H2O, Zn4(OH)6SO4·H2O, Zn4(OH)6SO4·0,5H2O, Zn7(OH)12SO4·4H2O, Zn4(OH)6SO4—are known as zinc hydroxide sulfates (ZHS) hydrates and distinguish with low product of solubility (Ps). The relative amount of these compounds varies and depends on the nature of the galvanic coating as well as on the experimental conditions. Using potentiodynamic (PD) polarization curves as well as polarization resistance (Rp) measurements it is established that the appearance of ZHS lead to increased protective ability of the investigated galvanic coatings towards the steel substrate.