Anodizing of zinc

The cathodic protection of plain carbon and low-alloy steels can be achieved with galvanic anodes of zinc, aluminum or magnesium. For materials with relatively more positive protection potentials (e.g., stainless steels, copper, nickel or tin alloys), galvanic anodes of iron or of activated lead can be used. 

The impressed current method with metal oxide-coated niobium anodes is usually employed for internal protection (see Section 7.2.3). In smaller tanks, galvanic anodes of zinc can also be used. Potential control should be provided to avoid unacceptably negative potentials. Pure zinc electrodes serve as monitoring and control electrodes in exposed areas which have to be anodically cleaned in the course of operation. Ag-AgCl electrodes are used to check these reference electrodes. 

The major harmful impurity is iron, and by keeping the iron content to less than 15p.p.m. it became possible to produce perfectly satisfactory anodes of zinc d . Alternatively the effect of the iron can be neutralised by alloying the zinc with certain metals, among which aluminium and silicon or cadmium have been found to be particularly effective. The presence of cadmium causes the corrosion product to fall away evenly, leaving an active surface (see Section 10.2). 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

Galvanic cathodic protection systems have been used extensively since the early 1990s in Florida on prestressed concrete bridge support piles in the sea. One of the reasons the galvanic system is used there is because concrete resistivity is low due to the marine exposure conditions. The Florida systems frequently incorporate a distributed anode of zinc fixed on the atmospherically exposed concrete and bulk zinc anodes in the water which pass current through the low resistance sea water to protect the submerged area as shown in Figure 7.4. 

Galvanic Probes. In contrast to the polarographic electrode, a galvanic probe utilizes an anode of zinc, lead, or cadmium and a cathode of silver or gold, where a silver cathode and lead anode are the most common (Linek et al., 1985). Figure 4.3b shows a schematic representation of a typical galvanic probe. The electrochemical reactions that take place at the probe surface are as follows (Linek et al., 1985 Turner and White, 1999 van Dam-Mieras et al., 1992) ... 

Glayman, J. (1970). L oxydation anodique du zinc (Anodizing of zinc). Galvano, 39(397), 147-149 (in French). 

Electrochemically, the Fe04 species have a high reduction potential, on the order of 0.9 V. Against an anode of zinc, the open circuit potential was found to be 1.75 V and 1.85 V for the K2Fe04 and BaFe04 cell, respectively. The proposed discharge reaction mechanism is as follows ... 

Several examples of cathodic protection of aluminum equipment in chemical plants, as well as a preference for saoificial anodes of zinc or alununum-zinc alloy, are discussed in Ref 17. Such protection is most successfiil in electrolytes in the pH range of 4 to 8.5— the so-called neutral range (refer to Chapter 2). The cathodic protection of aluminum stractures is reviewed in Ref 18, which supports general experience that cathodic protection is effective in preventing or greatly reducing several types of corrosion attack. 

---