Corrosion Products of Zinc-Manganese Coatings―Part Ⅲ: Double-Protective Action of Manganese

摘要

The electrodeposition conditions of the galvanic alloys Zn-Mn as well as their elemental and phase composition were described, and corrosion resistance, anodic behavior, and protective ability of these alloys were investigated in a model corrosion medium of free aerated 5% NaCl and in a salt-spray chamber (NSS test). The results obtained by X-ray diffraction (Part Ⅰ, Sept. 2001, pgs. 56-60) and X-ray photoelectron spectroscopy (Part Ⅱ, May 2002, pgs. 14-20) cleared up the elemental and phase composition of the corrosion products on the sample surface. The appearance of these products is related to the protective mechanism of the alloys Zn-Mn or, in other words, to the presence of the compound zinc hydroxide chloride (ZHC) (see Part Ⅰ). ZHC is formed in two consecutive steps at pH values 7.0 to 7.5. The first step consists of the formation of Zn(O-H)Cl as a result of the zinc dissolution in neutral or slightly acidic medium, and the second step involves partial hydrolysis of Zn(OH)Cl and formation of hydrated Zn(OH)_(2-x)Cl_x (x = 0.4), or more correctly, Zn_5(OH)_8Cl_2·H_2O.