Zinc coating continued sprayed

In the Forth, Severn and many other suspension bridges, zinc coatings have an important function. The whole main structure is of steel and has been zinc-sprayed on the external surfaces, while the main cable and hanger ropes have been coated by continuous hot-dip galvanising. Case histories of galvanised multi-truss bridges cover more than 30 years. 

PROTECTIVE COATING. A film or thin layer of metal glass of paint applied to a substrate primarily to inhibit corrosion, and secondarily for decorative purposes, Metals such as nickel, chromium, copper, and tin are electrodeposited on the base metal paints may be sprayed or brushed on. Vitreous enamel coatings are also used these require baking. Zinc coating are applied by continuous bath process in which a strip of ferrous metal is passed through molten zinc. 

More than 10 years experience with thermally sprayed coatings of zinc-15% aluminum is now available. As a result of the two-phase structure (a zinc-rich and an aluminum-rich phase) of such coatings, the oxidation of the active zinc-rich phase occurs within the more inert aluminum-rich phrase. Oxidation products are encapsulated in the porous layer formed by the latter and do not build up a continuous surface layer as with pure zinc coatings. Consequently, no thickness or weight loss is observed even after several years of exposure in atmospheric field testing. 

The anode system may be a single component, such as flame sprayed zinc, or multiple component such as a titanium mesh with a cementitious overlay. It may consist of a single continuous anode such as mesh or coatings, or multiple anodes connected together such as ribbon anodes or the discrete rod anodes. All anodes require electrical connections to the power supply. As for the rebar connections these should be duplicated for redundancy. 

Since most galvanized and thermally sprayed coatings are around 100 p,m thick, continuous contact with any material that gives a corrosion rate in excess of 30 ixm/year is likely to result in unsatisfactory or uneconomic service unless supplementary coatings are provided. However, zinc and zinc alloy coatings and wrought zinc may well be suitable in many uses, even with rather higher rates of attack. 

In the process droplets of semimolten zinc are sprayed from a special gun that is fed with either wire or powder onto a grit-blasted surface. The semimolten droplets coalesce with some zinc oxide present at each interface between droplets. Electrical continuity is maintained both throughout the coating and with the iron substrate so that full cathodic protection can be obtained since the zinc oxide forms only a small percentage of the coating. 

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