Zinc corrosion/passivation

Amalgamated zinc powder has been used as the negative material to prevent zinc corrosion and zinc passivation. Recently, from the viewpoint of environmental problems, mercury-free alkaline-manganese batteries were developed by using zinc powder with indium, bismuth and other additives [2-4]. Adding indium to zinc powder is the most effective way to improve the characteristics of the cells [3]. Figure 3 shows the variation in the internal impedance of the cells according to the additive content of the zinc powder. 

A great research interest concerns zinc corrosion inhibition. Chromate solutions were used [289-293] for the inhibition of zinc corrosion. The surface film contained a mixture of zinc oxide and hydroxide of chromium(III). Different behaviors of the passivated zinc electrode was observed depending on the cation of the used chromate [293]. 

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. 

The classical zinc-corrosion inhibitor has been mercuric or mercurous chloride, which forms an amalgam with the zinc. Cadmium and lead, which reside in the zinc alloy, also provide zinc anode corrosion protection. Other materials like potassium chromate or dichromate, used successfully in the past, form oxide films on the zinc and protect via passivation. Surface-active organic compounds, which coat the zinc, usually from solution, improve the wetting characteristic of the surface unifying the potential. Inhibitors are usually introduced into the cell via the electrolyte or as part of the coating on the paper separator. Zinc cans could be pretreated however, this is ordinarily not practical. 

Amalgamated zinc powder has been used as the negative material to prevent zinc corrosion and zinc passivation. Recently, from the viewpoint of environmental problems, mercury-free alkaline-manganese batteries were developed by using zinc... 

For example,copper has relatively good corrosion resistance under non-oxidizing conditions. It can be alloyed with zinc to yield a stronger material (brass), but with lowered corrosion resistance. Flowever, by alloying copper with a passivating metal such as nickel, both mechanical and corrosion properties are improved. Another important alloy is steel, which is an alloy between iron (>50%) and other alloying elements such as carbon. 

Precipita.tingInhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes that are insoluble at this high pH (1—2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically 7—9 pH). A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. 

The rate of self-corrosion of zinc anodes is relatively low. In fresh cold water, it amounts to about 0.02 g m h , corresponding to a corrosion rate of 25 /rm a. In cold seawater, the value is about 50% higher [10]. These figures refer to stagnant water. In flowing water the corrosion rates are significantly greater. Zinc is not practically suited for use in warm waters because of its tendency to passivate. 

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

Little information is available about the corrosion of metals in concrete, although it seems likely that all Portland cements, slag cement and high-alumina cement behave similarly Concrete provides an alkaline environment and, under damp conditions, the metals behave generally as would be expected e.g. zinc, aluminium and lead will react, copper is unaffected, while iron is passivated by concrete. 

Table 10.9 lists some common zinc anode alloys. In three cases aluminium is added to improve the uniformity of dissolution and thereby reduce the risk of mechanical detachment of undissolved anode material . Cadmium is added to encourage the formation of a soft corrosion product that readily crumbles and falls away so that it cannot accumulate to hinder dissolution. The Military Specification material was developed to avoid the alloy passivating as a result of the presence of iron . It later became apparent that this material suffered intergranular decohesion at elevated temperatures (>50°C) with the result that the material failed by fragmentation". The material specified by Det Norske Veritas was developed to overcome the problem the aluminium level was reduced under the mistaken impression that it produced the problem. It has since been shown that decohesion is due to a hydrogen embrittlement mechanism and that it can be overcome by the addition of small concentrations of titanium". It is not clear whether... 

Although aluminium is a base metal, it spontaneously forms a highly protective oxide film in most aqueous environments, i.e. it passivates. In consequence, it has a relatively noble corrosion potential and is then unable to act as an anode to steel. Low level mercury, indium or tin additions have been shown to be effective in lowering (i.e. making more negative) the potential of the aluminium they act as activators (depassivators). Each element has been shown to be more effective with the simultaneous addition of zinc . Zinc additions of up to 5% lower the anode operating potential, but above this level no benefit is gained . Below 0 9 7o zinc there is little influence on the performance of aluminium anodes . Table 10.10 lists a number of the more common commercial alloys. 

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