Galvanized steel adherents

Buses have a profiled steel framework that is clad on the sidewalls and roof with steel, galvanized steel, and aluminum alloys. The anticorrosive primer must therefore adhere well to all of these metals, and to glass-fiber-reinforced plastics. 

Before applying the first layer of resin, the surface can be treated with a primer that has the task of promoting adherence. This first layer also creates a barrier against the penetration of humidity and ensures the durability of the reinforcement system in the case of galvanized steel or stainless steel the oxides formed on the surface must be removed wifli an appropriate surface treatment. 

This specification covers an alkyd painting system for new, unrusted, untreated, galvanized steel. This system is suitable for use on parts or structures exposed in Environmental Zones lA (interior, normally dry) and IB (exterior, normally dry). The primer has good adhesion to clean galvanized steel but does not adhere properly to rusted galvanized steel. Painting System No. 1.04 should be specified for this condition. The finish paint allows for a choice of durable, fade-resistant colors. 

Values determined for the five test sites showed that rust layers formed are not completely adherent and there is a partial control of corrosion at the metal/oxide interface. Examination showed that the corrosion rate of galvanized steel was much higher in the marine atmospheres than in any type of atmosphere. 

There is a major fossil fuel power plant next to the exposure site which was about 1 kilometer from the seashore. The effect of SOj and chlorides on galvanized steel corrosion was found to be detrimental. The surface of the galvanized steel sheets were sparsely covered with white corrosion products that was found to be adherent to the surface. 

Galvanizing is another form of cathodic protection. A galvanized pipe is a steel pipe coated with a thin layer of zinc. Since zinc is anodic to steel (Table 7-4), the zinc will corrode preferentially to the iron and so protect it. Moreover, the corrosion products of zinc, the carbonates and hydroxides, adhere to the galvanized surface and render it passive. 

Hot in hot hard waters, the scale that forms at water temperatures above about 55°C has a coarse-grained structure and adheres less well to the zinc surface. Corrosion of the zinc will still occur locally, as a result of discontinuities in the scale or local electrochemical action. Above about 60°C, zinc usually becomes cathodic to steel, and in bimetallic contacts the steel will then corrode first. Pouibaix (1970) has performed success-fill laboratory simulations of the corrosion of galvanized water distribution pipes. 

Industrially, the most important property of zinc metal is its ability to form an alloy with iron so that a thin adherent layer can be applied to the surface of iron workpieces. The zinc coating, obtained either by hot-dip galvanizing or aqueous electrogalvanizing, considerably extends the service life of steel constructions. In addition, zinc alloys can be easily cast into intricate shapes that are used extensively in household goods and automotive equipment, building, and construction utilities. Other important areas are brasses, batteries, tires, and other rubber goods. Furthermore, zinc is now known to be an essential nutrient that is indispensable for human health as well as for many species of animals and plants, and it is therefore used in pharmaceuticals, fertilizers, and animal feed. 

Since copper is very noble it causes extreme galvanically induced local corrosion of steel and aluminum substrates. Because of this, extreme care must be taken to produce weU-adhering nonporous layers. 

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