Spray-on Zinc Expected to Grow

Hugh Morrow
Zinc Metalizers Task Group

Reprinted from the October, 1988 issue of "Metal Bulletin Monthly"

Zinc and zinc-aluminum thermally sprayed coatings will compete with those coating systems which are utilized for the long-term corrosion protection of large steel structures. The competing systems include galvanizing, galvanizing plus painting, and the premier three-coat paint systems which are usually based on a zinc-rich primer paint. In marine applications, the thermally sprayed zinc- aluminum alloy coating may compete with thermally sprayed aluminum coatings which are now well accepted.

Five years ago the Zinc Institute and Platt Bros. & Co in the USA formed the Zinc Metalizers Task Group (ZMTG), and this has been promoting zinc thermal spraying in the North American market ever since. To date, most of its efforts have centered on the bridge market, but other application areas such as railroad cars, water towers, industrial plant steelwork, underground piping and storage tanks, as well as many applications in the marine industry are beginning to establish themselves.

In Canada, the province of Quebec has pioneered the North American zinc thermal spraying market. The world's largest on-site thermally sprayed structure is the Pierre LaPort bridge over the St. Lawrence River. The province has also zinc thermally sprayed many other smaller highway overpass bridges and is quite pleased with the corrosion protection provided. British Columbia has also zinc thermally sprayed a number of bridges in the past few years.

In the USA, the state of Ohio has embarked on an ambitious program to evaluate thermal spraying. Thus far it has zinc sprayed three entire bridges, and is planning more for the future. New York, New Jersey, Connecticut, Rhode Island, Massachusetts, Pennsylvania and Virginia have all thermally sprayed entire bridges or test sections of bridges in the past four years, and thus far, all have shown good results. Interest is spreading to other states which have requested information, have evaluated corrosion coupons, or have already been using zinc thermal spraying in some critical applications. Vermont, for instance, has been using zinc thermal spraying to protect highway bridge bearings.

In the past four years too, committees have been formed in a number of the major US societies to consider specifications, corrosion testing, quality control and inspection of thermally sprayed coatings.

Even though zinc thermal spraying is touted because it is a thick zinc coating, it is also challenging some of the thin zinc coatings as well. In a process known as "hot spray galvanizing", coating from 1-3 thousandths of an inch of zinc can be successfully and uniformly applied to small hardware items such as nails, bolts, nuts, screws and washers. These items are normally zinc barrel plated, mechanically plated, or hot-dip galvanized, with each of those processes having advantages and disadvantages. Thus, it is expected that thermal spraying may compete with the mechanical plating and electroplating processes as well as the hot-dip and zinc paint coating methods.

The experimental development of continuously thermal sprayed steel sheet in Japan, the UK and the Netherlands suggests that thermal spraying could even compete one day with hot-dip continuous galvanizing and electro-galvanizing.

While the implications of the interest of zinc thermal spraying on the total zinc market may not be large, this area could develop into a significant niche market for zinc coatings. Of the 25,000 tons of zinc consumed in thermal spraying in 1982, about 20,000 tons was in the European market, the remaining 5,000 tons coming from applications in the USA, Canada, Japan, and India. It is expected the use in the USA and Canada could easily match that in Europe in the future, particularly in view of the critical state of many of the bridges in the USA, their need for constant maintenance, the rapidly increasing cost of that maintenance, and in some cases a completely unsafe condition in the existing bridge.

One of the large advantages of the thermally sprayed coatings from the zinc consumption point of view is that, because a thermally sprayed coating is so much thicker than other zinc coatings, use of this coating will result in far greater zinc consumption than if the same structure had been galvanized or painted with zinc rich paint. Initial consumption of zinc in a thermal spraying operation would be at least twice that in a galvanizing or painting operation on the same bridge.

It should also be noted that thermal spraying is widely accepted as probably the best method of repairing damaged galvanized coatings, and that zinc thermal spraying is often the only alternative for very large structural members which cannot be galvanized even by the double dipping technique because of kettle size limitations.

In the final analysis, the growth of the zinc thermal spraying market in North America will depend on whether or not the end users continue to embrace the policy of adopting the lowest cost method on an initial basis regardless of the long term implications. If they begin to be more far-sighted, and to evaluate coatings on total cost (both initial costs and future costs) then the zinc and zinc- aluminum thermally sprayed coating should grow impressively in the next 20 years, and with it, the demand for zinc.
Thermal spraying (Article Inset)

ZINC THERMAL SPRAYING is an application technique whereby zinc wire or powder is fed into a thermal spraying gun which converts the solid wire or powder, using either a gas flame or electric arc heat source, into tiny drops of molten zinc which are then rapidly propelled against the suitably prepared steel surface to be protected. There are advantages and disadvantages to each of the input material forms and to each of the heat sources. The most common technique of thermal spraying in use today is the wire-fed, gas-flame gun.

In many ways, the thermally sprayed zinc coating contains elements of both the hot-dipped coatings and the mechanically applied coatings. When the thermally sprayed zinc particle reaches the steel substrate it freezes instantly and deforms around the profile of the surface, forming an adherent layered coating on a very fine scale. Because of the rapid heating, melting, and freezing of the thermally sprayed alloys, the microstructures and phases present are not those normally found in the more slowly cooled materials. The structures are very fine grained, and often contain considerable amounts of finely divided eutectic phases, the phases which are formed immediately after solidification, or just below the alloy's melting temperature.

The thermal spraying method of applying zinc coatings requires some special conditions, however, and therefore may not always be the ideal coating even though long term protection from corrosion is required. (The steel substrate to be coated must be properly cleaned and skilled operators are also needed to apply the coating successfully. Most applications require that the thermally sprayed coating be sealed with a low viscosity sealer on the same day as the thermal spraying is performed.)

Because of some of these conditions, thermal spraying has been viewed in the past as being a very expensive coating system. On an initial cost basis, it may be somewhat more expensive than other zinc coating systems, depending on the particular comparisons made, but in the long term, it is probably the most cost-effective of the zinc coating techniques. Studies in both the UK and the USA have shown that, for the long term corrosion protection of large steel structures having a low surface- area-to-weight ratio, zinc thermal spraying is the most cost-effective technique.

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