Zinc alloys anodes

In spite of a low driving voltage of about 0.2 V, about 90% of all galvanic anodes for the external protection of seagoing ships are zinc anodes (see Section 17.3.2). Zinc alloys are the only anode materials permitted without restrictions for the internal protection of exchange tanks on tankers [16] (see Section 17.4). 

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

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

Anode efficiency is of little practical significance and can be misleading. For example, magnesium alloy anodes often have an efficiency ca. 50% whilst for zinc alloys the value exceeds 90% it does not follow that zinc alloy anodes are superior to those based on magnesium. Efficiency will be encountered in many texts on sacrificial anode cathodic protection. 

ZnO displays similar redox and alloying chemistry to the tin oxides on Li insertion [353]. Therefore, it may be an interesting network modifier for tin oxides. Also, ZnSnOs was proposed as a new anode material for lithium-ion batteries [354]. It was prepared as the amorphous product by pyrolysis of ZnSn(OH)6. The reversible capacity of the ZnSn03 electrode was found to be more than 0.8 Ah/g. Zhao and Cao [356] studied antimony-zinc alloy as a potential material for such batteries. Also, zinc-graphite composite was investigated [357] as a candidate for an electrode in lithium-ion batteries. Zinc parhcles were deposited mainly onto graphite surfaces. Also, zinc-polyaniline batteries were developed [358]. The authors examined the parameters that affect the life cycle of such batteries. They found that Zn passivahon is the main factor of the life cycle of zinc-polyaniline batteries. In recent times [359], zinc-poly(anihne-co-o-aminophenol) rechargeable battery was also studied. Other types of batteries based on zinc were of some interest [360]. 

Tin—Zinc, Baths for tin—zinc alloys stem from work done after World War II in efforts to find a substitute for cadmium. Although alloys of all concentrations are possible, 80% tin—20% zinc gives the best combination of properties. This alloy has a low coefficient of friction, low electrical contact resistance, is solderable, slightly anodic to steel, and does not form voluminous corrosion products. In addition, the tin—zinc alloy has good paint adhesion qualities, good ductility, and is easily spotwelded. 

However, if the initial ammonium chloride concentration is depleted, the reaction, as indicated above, becomes more complex and formation of hydroxochloro complexes plays an increasingly important role. The zinc alloy sheet used for the anode manufactme contains small quantities of lead and cadmium, and various corrosion inhibitors, such as a soluble mercury salt, are added to improve shelf life. 

Arsenic is sometimes used in the manufacture of its compounds, but more often in alloys. Small quantities, o-i to o 2 per cent, are added to lead for the production of shot (p. 196). Arsenical lead anodes are used in the electrolytic production of zinc. Alloys with antimonial lead containing 1 to 2 per cent of arsenic and sometimes other elements are used for sheaths for electric cables, etc. Arsenical coppers and bronzes are used for high temperature work such as locomotive fireboxes, etc. 

Sulphates derived from nonylphenol have been employed as corosion inhibitors in the electrolyte of batteries having zinc alloy anodes (ref. 41)... 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

Ship hulls Painting cannot always protect hostile marine conditions, in ships and, areas above keel blocks. Stem and mdder areas suffer erosion and corrosion due to the high turbulence caused by the propeller coupled with the galvanic effects of the noble bronze propeller. Effective cathodic protection of ship hulls and similar marine structures in seawater against corrosion can be apphed using either aluminum or zinc alloy sacrificial anodes. Twenty percent of the anodes required for full hull protection are required for stern protection only. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

Exchange current density for zinc alloy anode (z Mn = 10 A/cm ... 

Aluminum and chromium are both passive metals in the true sense. Zinc exhibits good corrosion resistance, which is attributed to the formation of an adherent protective layer of corrosion products (this may also be considered as a form of passivity). Chromium, at concentrations >12%, confers passivity to its alloys with iron, and these alloys are cathodic to steel. Under most conditions, both aluminum and zinc are anodic to steel, as are iron-zinc alloys. Iron-aluminum alloys, however, are cathodic and in this respect are similar to iron-chromium alloys. The corrosion resistance of steel is increased by alloying with aluminum or chromium, such increase being marked with the latter. The addition of zinc decreases the corrosion resistance of steel, although, in many cases, a protective layer of corrosion products leads to an apparent decrease in the corrosion rate. 

---