Passivity of zinc

Fig. IS.S Effect of sulphuric acid concentration on chromate passivation of zinc. Solution 182g/1 of Na2Cr207 -2H20 + H2SO4 as indicated temp. I8°C I 0 x 10 mg Zn/cm =...

The ways in which inhibitive anions affect the corrosion of zinc are mainly similar to those described above for iron. In inhibition by chromate, localised uptake of chromium has been shown to occur at low chromate concentrations and in the presence of chloride ions Thus under conditions unfavourable for inhibition, pore plugging occurs on zinc. Inhibitive anions also promote the passivation of zinc, e.g. passivation is much easier in solutions of the inhibitive anion, borate , than in solutions of the non-inhibitive anions, carbonate and bicarbonate , A critical... 

Addition of sodium dodecyl benzene sulfonate to dilute alkaline electrolyte depresses the passivation of zinc surface [275]. Owing to the dodecyl benzene sulfonate adsorption, the passive layer on zinc has a loose and porous structure. Zinc electrodissolution was inhibited by the presence of sodium metasdicate [276] and some acridines [277]. The protection effect was described by a two-parameter equation. 

The passivation of zinc depends on the pH of the pore solution. In contact with alkaline solutions, as long as the pH remains below 13.3, zinc can passivate due to formation of a layer of calcium hydroxyzincate. Figure 15.6 shows the typical corrosion rate of zinc as a function of pH however, even at pH values higher than 12, in the presence of calcium ions, such as in concrete pore solution, zinc can be passive and has a very low corrosion rate. In saturated calcium hydroxide solutions it was found that for pH values up to about 12.8 a compact layer of zinc-corrosion products forms, which will protect the steel even if the pH changes in a subsequent phase. For pH values between 12.8 and 13.3, larger crystals form that can still passivate the bar. Finally, for values above 13.3, coarse corrosion products form that cannot prevent corrosion. 

R.W. Powers, M. W. Breiter, The anodic dissolution and passivation of zinc in concentrated potassium hydroxide solutions, J. Electrochem. Soc. 116 (1991) 719—729. 

Laboratory and field studies by Crennell and Wheeler (1956, 1958) showed that an iron content level of more than a few parts per million caused passivation of zinc and that no other common impurities in zinc were of importance to anode performance. These studies showed that the addition of 1% aluminum was beneficial and that 0.5% aluminum plus 0.5% silicon significantly increased a zinc anode s tolerance for iron and also produced a finer grain structure than was obtained with SHG zinc with 0.0014% maximum iron. 

Bianchi, G. (1954). Passivity of zinc in relation to conditions of aeration. Metall. Ital., 46, special issue, 29-30 (in Italian). 

Cerveny, L. (1958). Passivierung des Zinks mittels 1,1-Dimethylathan-l-ol-Chromats in der Gasphase (Passivation of zinc in the gaseous phase by means of 1,1-dimethylethane-l-ol-chromate). Werkst. Korros., 8(8/9), 543-546 (in German). 

Goff AH-L, Joiret S, Sai dani B, Wiart R (1989) In-situ Raman spectroscopy applied to the study of the deposition and passivation of zinc in alkaline electrolytes. J Electroanal Chem Interfacial Electrochem 263 127-135. doi 10.1016/0022-0728(89)80129-9... 

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