Spraying with zinc sealing

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. 

Thermal sprayed Aluminium/Zinc/Indium - A proprietary variation on thermal sprayed zinc that uses an alloy to enhance the current rather than a humectant. A 300 micrometre thick coating is applied by arc spray. Approrimately 35,000 m had been applied up to May 2003. Adhesive zinc sheet - A proprietary system consisting of rolls of zinc 0.25 mm thick, 25 mm wide with a conductive gel adhesive on one side. The installed system can be painted. The edges must be sealed against water ingress as the gel can swell and leak. 

In industrial atmospheres (New York City Columbus, Ohio East Chicago, Indiana), 230 (xm of unsealed zinc provided 19 years of maintenance-free protection 150 p.m zinc showed slight rust on the upward-facing surface in New York and at edges in Columbus. 80 (xm sprayed zinc sealed with aluminum-vinyl was in generally good condition deterioration of the aluminum-vinyl often was associated with excessive application. A chlorinated rubber sealer was generally less effective than the aluminum-vinyl. 

The welded decks of 10 barges, heavily pitted and totally perforated in places, were repaired in 1963-1964, sprayed with 250 pm zinc, and sealed with one coat of aluminum-pigmented unmodified phenolic resin sealer. At 3-4 year intervals, one coat of ordinary red oxide paint is applied for decorative purposes only. 

Zinc or aluminum metal spray Sprayed metal coatings are porous and should be sealed after application by applying a sealer coat (i.e. a thin coat such as an etch primer) or a thinned version of the final coating system. Oil-based systems should not be used. Metal spray coatings can have excellent durability without overcoating with paint and, particularly for aggressive conditions it is preferable to leave them with sealer only. 

Flammable gas. Very dangerous fire hazard when exposed to heat, flame, or powerful oxidizers. Moderate explosion hazard when exposed to flame and sparks. Explodes on contact with interhalogens (e.g., bromine trifluoride, bromine pentafluoride), magnesium and alloys, potassium and alloys, sodium and alloys, zinc, Potentially explosive reaction with aluminum when heated to 152° in a sealed container. Mixtures with aluminum chloride + ethylene react exothermically and then explode when pressurized to above 30 bar. May ignite on contact with aluminum chloride or powdered aluminum. To fight fire, stop flow of gas and use CO2, dry chemical, or water spray. When heated to decomposition it emits highly toxic fumes of cr. See also CHLORINATED HYDROCARBONS, ALIPHATIC. 

This process is applied mainly to steel components which are sprayed or dipped in a phosphoric acid solution containing iron, zinc or manganese phosphate. The most common processes use zinc or manganese phosphate and are employed in the treatment of car bodies and domestic appliances before painting. Where long-term corrosion protection is required, the phosphate coating must be sealed with oil lacquer or paint... 

Sprayed coatings are commonly sealed with organic lacquers or paints to delay eventual formation of visible surface rust. The usual thickness of sprayed aluminum is 3-8 mils (0.08-0.2 mm). In one series of tests conducted in an industrial atmosphere, sprayed coatings 3 mils (0.08 mm) thick showed an average life of 12 years in comparison with 7 years for zinc, with the latter coatings being sprayed, electrodeposited, or hot-dipped [38]. 

A further examination in 1983 showed some areas bereft of zinc, although about 75% of the roof surface had some zinc present. Because roof maintenance normally occurs only every 25 years, it was decided to renew the zinc and, to facilitate on-site work, zinc spraying (300 p.m) was specified, followed by sealing with a dark gray vinyl copolymer sealer. Five years later, the roof was in perfect condition, and the same treatment was proposed for the Victoria Tower at the other end of the Houses of Parliament, which was erected in 1860 and has a similar history. 

---