Zinc alloys, alkaline corrosion

Wilde, B. E. and Teterin, G. A., Anodic Dissolution of Copper-Zinc Alloys in Alkaline Solutions , Brit. Corrosion J., 2, 125 (1967)... 

The general corrosion rate of zinc and zinc alloys in practice often have been shown to be much less than in simulated conditions this is because many naturally occurring substances act as inhibitors. Figure 4.42 is a good example of this. The diagram is valuable for the qualitative relationship between acid, neutral and alkaline conditions but, in practice, the corrosion rates are usually very much lower than indicated by the pH because of the effect of other dissolved constituents and the barrier effect of corrosion products. Seawater around the British Isles is much less corrosive to zinc than tropical seawater. 

Zinc—Iron. The Zn—Fe alloy is plated from an alkaline bath. Deposits are 0.3—0.8% iron and can be given attractive, resistant, black, silver-free chromate coatings. Corrosion protection requires the heavier, darker chromates. Zinc—iron baths are the most economical of the zinc alloys. 

Mildly alkaline solutions of ordinary laundry soaps develop a protective coating on zinc and its alloys. They are satisfactory for both warm and cold applications. Detergent solutions are more corrosive however, the better grades include inhibitors, which reduce corrosion to allow reasonably satisfactory service for zinc alloys. Strong alkalies (e.g., hypochlorite bleach solutions) or acid solutions (e.g., Harpic, which contains trisodium hydrogen disulfate) are to be avoided. 

Acidity and alkalinity affect the corrosion rate. Generally, alkaline conditions favor lower corrosion rates however, some metals, such as aluminum and zinc, are amphoteric and show increased corrosion at pH values above 9. For iron alloys, the corrosion rate is relatively steady between a pH of 4 and 10 at ambient temperature and where oxygen reduction is the primary cathodic corrosion reaction however, the corrosion rate increases rapidly below pH 4 [4]. Iron passivates and the corrosion rate decreases rapidly above a pH of 10, except at very high pH levels where the corrosion rate again can increase. 

A typical example of the application of EIS is the investigation of passive films on Zn, Zn-Co, and Zn-Ni (Fig. 7-18), which were carried out to explain the difference in the corrosion behavior of pure and low-alloyed zinc by the possible formation of electron traps through the incorporation of cobalt or nickel into the oxide film (Vilche et al., 1989). Passive films of zinc in alkaline solutions are known to be n-type semiconductors with a band gap Eg = 3.2 eV (Vilche et al., 1989). The n-type character arises from an excess of zinc atoms in the nonstoichiometric oxide. The impedance measurements in 1 N NaOH solution were carried out at potentials at which Faraday reactions like transpassive dissolution and oxygen evolution do not interfere. The passive layer was formed for 2 h at positive potential before the potential was swept in the negative direction for the impedance meas-... 

The most harmful deposits are those that are water permeable. Truly water-impermeable material is protective, since without water contacting metal surfaces corrosion cannot occur. Innately acidic or alkaline deposits are troublesome on amphoteric alloys (those attacked at high and low pH—e.g., aluminum and zinc). 

Most simple inorganic salt solutions cause virtually no attack on aluminium-base alloys, unless they possess the qualities required for pitting corrosion, which have been considered previously, or hydrolyse in solution to give acid or alkaline reactions, as do, for example, aluminium, ferric and zinc chlorides. With salts of heavy metals —notably copper, silver, and gold —the heavy metal deposits on to the aluminium, where it subsequently causes serious bimetallic corrosion. 

Zinc—Cobalt. Alloys of Zn—Co usually contain 0.3—0.8% cobalt. Higher cobalt alloys, from 4—8%, have shown better salt spray resistance (156), but the commonly plated alloy is 0.3—0.8%. One automotive company specifies 0.3—1.0%. Cobalt is expensive, and economics favor the lower alloys. Costs have been quoted for zinc—cobalt at 1.2 times the cost of chloride zinc, with zinc—nickel alloys at 1.5—1.6 times the chloride zinc. Deposits can be very bright, but the improved corrosion resistance advantage requires yellow or bronze chromates. Alkaline baths give fewer problems in plating components with lapped, spot-welded seams. 

Lead is sometimes still used in both battery systems. In zinc-carbon batteries it is employed chiefly as an alloying addition to improve the forming characteristics of the zinc can, and additionally acts as a corrosion inhibitor. In alkaline-manganese it has found use as a plating alloy on the brass nail to reduce gassing. In zinc-carbon cells, the lead content is in the order of 0.02% and in those alkaline-manganese batteries where lead is still used, the addition is at a level of a few parts per million. 

Zinc-nickel Zn-Ni alloys with 5 to 15 wt% Ni offer excellent corrosion resistance and are mainly used in the automotive, aerospace, and electronics industries. Above 15% Ni, the alloy coating becomes more noble than steel, and the corrosion-protection mechanism changes from a sacrificial to a pure physical one (comparable to pure Ni coatings, see Sect. 5.5.4.2.2). They can be electrode-posited from acid or alkaline baths. The acid baths are usually based on sulfate, chloride, sulfate-chloride, pyrophosphate, or acetate (Table 15). The system shows anomalous codeposition (see Sect. 5.5.1.2), which has been explained by a hydroxide suppression mechanism [47]. As in the case of Ni-Fe, the alkaline baths must contain complexing agents (see Sect. 5.5.4.6.2). The alloys electroplated from add haths contain approximately 10 to 14% Ni, whereas the alkaline Zn-Ni... 

While being similar in corrosion resistance, alnminnm coatings are preferred to zinc coatings for marine and indnstrial environments they are, however, less snited to alkaline conditions. Chromium is only nsed decoratively in conjunction with nickel or copper undercoats. The general properties of the diffnsion (alloy) coatings are summarized in Table 10.9. 

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