Zinc coatings duplex

The benefits of a duplex coating are achieved only if there is good, long-lasting adhesion of the paint to the zinc coating. The NMP (A(-methyl-pyrrolidone) test has been described by Van Ooij et al. (1982). The test panels are immersed in the heated solution at 60 C (Fig. 1.16) and the time before the paint lifts is measured. Figure 1.17 shows the use of this test to demonstrate how silanes increase adhesion of an automotive epoxy-polyurethane primer to galvanized steel. 

The following examples illustrate the benefits of zinc or duplex coatings, including such aspects not directly related to corrosion resistance as... 

DSS Duplex stainless steel lOZ Inorganic zinc coating... 

The corrosion rate of a bare sprayed coating is comparable to that of solid zinc or aluminium, although the greater surface area exposed may cause apparent corrosion rates to be a few per cent higher. For most uses, however, the sprayed coating is sealed or painted and achieves the much higher corrosion resistance associated with duplex coatings. The extra life depends on the sealer or on the thickness and type of paint used, and on the environment. 

Paint and galvanized steel (steel coated with zinc) can be used together to provide superior corrosion control than either method used alone. This technique, known as a duplex system, ccmsists of painting steel that has been hot-dip galvanized after fabrication [12], The paint serves as a barrier protecting the galvanized steel from the atmosphere. It slows down the rate at which zinc is consumed and therefore extends the life of the galvanized steel. 

Duplex Coatings (two metal layers). A layer of zinc followed by a layer of another metal (usually aluminum) is occasionally advocated to provide a barrier or more slowly corroding surface initially or to give specific properties such as reflectivity. 

In the thermal spraying industry, a processing benefit of applying zinc first and then aluminum is that the quality of blasting pretreatment required for zinc spraying is less critical than for aluminum, especially when gas pistols are used. Such duplex coatings have been used on Royal Navy ships decks, for example, and also in hot areas, such as near coke ovens, where aluminum helps to reflect heat. 

Vapor deposition remains only a possible commercial alternative for coated steel for cars. High deposition rates and the ease of producing duplex coatings (zinc followed by aluminum or vice versa) with improved coating resistance are potential advantages, but engineering for high speed line use remains to be developed. 

The cathodic protection on damaged galvanized items depends on (a) the coating thickness, the width of the bared area or scratch, and (c) the local climate. Allowance must be made for any extra loss of zinc due to cathodic protection effects or due to exposure of more zinc surfaces to the environment. It also has been shown that coatings of Al/Zn alloys alone offer no or only poor cathodic protection. More or less the same applies to duplex systems, where the paint coat covers the zinc (-q) layer, thus preventing or retarding the liberation of the necessary ions. 

Some short-term trials should also be mentioned because they include comparison with other metallic coatings. B. A. Shaw and Moran (1985) found that a zinc-15% aluminum pseudoalloy was best after 6 months of atmospheric, splash, and spray exposure (compared with the pure metals and duplex coatings of two metals), but for some unexplained reason there was flaking and blistering of some of the prealloyed zinc-15% aluminum alloy. 

The flat cell is illustrated in Fig. 8.5. In this construction, a duplex electrode is formed by coating a zinc plate with either a carbon-filled conductive paint or laminating it to a carbon-filled conductive plastic film. Either coating provides electrical contact to the zinc anode, isolates the zinc from the cathode of the next cell, and performs the function of cathode collector. The collector function is the same as that performed by the carbon rod in cylindrical cells. When the conductive paint method is used, an adhesive must be placed onto the painted side of the zinc prior to assembly to effectively seal the painted surface directly to the vinyl band to encapsulate the cell. No expansion chamber or carbon rod is used as in the cylindrical cell. The use of conductive polyisobutylene film laminated to the zinc instead of the conductive paint and adhesive usually results in improved sealing to the vinyl however, the film typically occupies more volume than the paint and adhesive design. These methods of construction readily lend themselves to the assembly of multi-cell batteries. 

In flat cells a zinc plate is coated with carbon to form a duplex electrode - a combination of the zinc of one cell and the carbon of the adjacent one. The Union Carbide Mini-Max cell contains no expansion chamber or carbon rod as does the round cell. This increases the amount of depolarizing mix available per unit cell volume and therefore increases the energy content. In addition, the flat cell, because of its rectangular form, reduces waste space in assembled batteries. 

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