Duplex paint

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. 

The metallurgy selected for construction of a firewater pump is dependent on the properties of the water source to be used. For fresh water sources (i.e., public water mains), cast iron is normally adequate although bronze internals may be optional. Brackish or sea water utilization will require the use of highly corrosion resistance materials and possibly coatings. Typically specified metals include alloy bronze, monnel, ni-resistant, or duplex stainless steels sometime combined with a corrosion resistant paint or specialized coating. 

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. 

Exposed parts of bolts require supplementary protection. Goodwin and Piessen (1988) in 10-year tests in an industrial atmosphere showed that the best duplex systems were galvanized plus either 80 20 zinc dust/zinc oxide paint, polyurethane paint, or anodizing. 

Table 1.7 Comparative Costs of Galvanizing, Painting, and Duplex Systems Netherlands...

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. 

However, it is desirable (especially for paint systems and duplex systems) to carry out corrosion tests in the atmosphere on panels on other specimens, which are exposed in spring and another series in autumn. 

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. 

Claims that weathering steels are equal to galvanized steel have proved to be unjustified. Even when coated with paint systems, such steels do not offer the supreme corrosion resistance of duplex (i.e., galvanized and over-coated) materials. 

Two television/radio towers in the Netherlands were built and galvanized in 1959. After 13 years they were painted with a polyurethane-based system. In 1984, when both towers were inspected, there was still no need to recondition this duplex system. One tower stands near Utrecht in a mildly industrial atmosphere. The other stands near Groningen, in the northern part of the country, in a light marine-industrial climate. 

On offshore platforms, duplex systems (galvanizing plus a three-coat non-saponifiable paint system) have protected against rust for 10-20 years. A test site on Heligoland run by a German steel association provides more data (Schwenk and Friehe, 1972). 

Van Eijnsbergen, J. F. H. (1978). The reliability of galvanizing versus two-coat paint systems and a duplex system (in Dutch). SDV, the Hague, Netherlands. 

Kim and her family moved into a Little Italy-type neighborhood in Cleveland where a friend offered them a good deal on an apartment in an old duplex. It appeared to be in good condition. Only the window wells and threshold drew their attention as in need of repair on the initial walk-through of the apartment. They asked the landlord to repair the wells, but he did not, feeling that his tenant was just as able to take care of it, so the matter slid. In the meantime, the house next door (about a dozen feet away) had exterior paint falling off it and onto the driveway area (composed of brick and soil). 

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