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Reprint from "The American City"

...with an insoluble seal or color coat to provide longer-lasting protection of steel structures.

Test results and field experience show that combination coating systems (flame-sprayed zinc or aluminum coatings followed by organic sealing systems) provide the least expensive and most effective long-term corrosion protection in many severe environments.

To protect steel from corrosion, one can use barrier coats, which exclude moisture; passivating treatments, which render the surface less reactive; rust inhibitors; and electrolytic protection. A good protective system may involve two or more of these. A superior system involves all four - and that is what combination sprayed metal/paint systems offer.

A typical combination system consists of:

	a sprayed zinc or aluminum coating to provide galvanic protection.
	a wash-prime coat containing phosphoric acid and zinc chromate that acts as a passivator and rust inhibitor, besides filling the pores and providing some barrier protection.
	one or more insoluble seal or color coats to fill the pores of the sprayed metal completely and create an effective barrier against contaminants and moisture.

Besides providing superior corrosion protection, a combination system offers other advantages over all-paint systems. It bonds with greater strength to steel and it offers greater resistance to mechanical abuse. In addition, if mechanical damage should occur, the sprayed metal provides electrolytic protection at points where the bare steel is exposed.

In a recent test conducted by the U.S. Navy Laboratories, technicians applied 23 coating systems to marine pilings. Among the eight best systems selected for further testing, a sprayed zinc coating of six mils cost only 1/3 the price of the most expensive coating. (A pure aluminum coating costs even less, and for marine service, properly sealed aluminum is far superior to zinc.)

People often associate the expense of base preparation by abrasive blasting with the cost of sprayed metal coatings. But many modern paint systems also require abrasive blast preparation. Even some widely used paint systems, formulated years ago, fail prematurely unless surfaces are prepared by blasting. Following blasting, the application of a few mils of aluminum or zinc adds little to the total cost - usually not over ten cents per square foot.

Initial Coating

Often, the initial coating of structural steel costs less than a recoating because parts can be coated before erection, where they are readily accessible and work can be performed under ideal conditions. Should re-coating become necessary after a period of service the work may require elaborate pre- cleaning and expensive scaffolding. It may even entail shutting down a facility.

Repainting that simply restores color and appearance may be greatly simplified if the original coating system included sprayed metal. There is, for instance, no blistering, peeling or rusting to contend with. The painters need only spray on a light paint coat to restore brightness and color.

In 1950, the Committee of Metalizing of the American Welding Society (AWS) set out to evaluate the corrosion protection provided by metalized zinc and aluminum coatings applied to low-carbon steel. Test panels placed in six different locations provided a variety of corrosive environments. For atmospheric corrosion tests, the researchers used the zinc and aluminum coatings applied in thicknesses of from 0.003 to 0.015 of an inch. For sea-water immersion, they used an additional panel coated with 0.018 of metal.

Test conditions included:

	One group of panels left as sprayed.
	A second group sealed with (a) a wash primer, plus a vinyl or (b) chlorinated rubber.
	A third group sealed the same as group two but with a second application of vinyl or chlorinated rubber.

The six-year report, published by AWS in 1963, contained these findings:

	In the heavily industrial environment of East Chicago, Indiana, the sealed aluminum- and zinc-coated panels showed no evidence of base-metal corrosion. While unsealed panels of aluminum did not show base-metal corrosion, the thinnest (3 mil) unsealed panels of zinc did exhibit a slight indication of base-metal corrosion.
	Sealed aluminum- and zinc-coated panels in the salt-spray environment of Kure Beach, North Carolina, showed no evidence of base-metal corrosion. The unsealed 0.003-inch-thick aluminum-coated panels contained a slight red rust stain on the groundward surfaces. A tightly adherent layer of zinc corrosion product covered the unsealed and chlorinated-rubber sealed zinc-coated panels. The panels were located only 80 feet away from the Atlantic Ocean.
	Aluminum-coated panels located 800 feet from the ocean showed no evidence of base-metal corrosion. However, a slight amount of rust stain did appear on the groundward surfaces of the unsealed panels coated with 0.003-inch aluminum. The zinc-coated panels also showed no base-metal corrosion; nor did the zinc sprayed-aluminum vinyl sealed panels show any indication of a zinc corrosion product. The chlorinated-rubber-sealed panels had some pinpoints of zinc corrosion product while the unsealed panels had greater amounts.
	One group of panels at Wrightsville Beach, North Carolina, was located in sea water at a level below low tide. The sealed aluminum-coated panels exhibited no evidence of base- metal corrosion; unsealed panels indicated some corrosion of the base metal. Zinc-coated panels indicated base-metal corrosion only on 0.003-inch-thick zinc coatings. Panels coated with one coat were worse than those sealed with two coats of chlorinated rubber.
	Other panels at this same site but located at the mean-tide level were alternately exposed to sea water and the atmosphere. Sealed aluminum-coated panels showed no evidence of base- metal corrosion while unsealed panels had slight indications of base-metal corrosion in the blisters present in the aluminum. The zinc-coated panels gave evidence of base-metal corrosion on all but those panels coated with 0.012-inch or more zinc. Some sealed panels exhibited more corrosion than did the unsealed panels of the same zinc-coating thickness. Zinc coatings on all panels were heavily corroded.

Conclusions

The six-year test provides insufficient exposure time to determine the longevity of the corrosion- protection abilities of metalized coatings. However, we can draw the following general conclusions at this time:

	Thin coats of aluminum sealed with a wash primer and one or two coats of aluminum vinyl or clear vinyl give excellent protection. A nine-year unofficial inspection recently made of aluminum-coated panels exposed to severe marine water immersion environments showed no significant change in the condition of aluminum coatings as outlined in the six-year report.
	Unsealed aluminum coatings definitely last longer than comparable zinc coatings for thinnest coating thickness.
	In all environments the sealed aluminum-coated panels show the least amount of coating dissipation and the best appearance. Those sealed with two seal coats remain brighter and cleaner than those with one seal coat.
	For sea-water exposure applications, metalized aluminum sealed with clear vinyl proved to be the best coating system tested in the AWS program.

The AWS tests indicate that sealed sprayed metal coating systems offer the best protection at the lowest cost in severe marine or industrial environments. Such systems survive for extremely long periods before any maintenance is required, and if a paint coat should be eventually needed to restore the color or the appearance, the job is simple and it is relatively inexpensive.

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