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HYDROELECTRIC US ARMY CORPS USACERL REPORT ON HVOF
National Thermospray Inc,also offers Metallizing in shop and in the field. Metallizing is for long-term,low-cost corrosion protection. In the long run a sprayed coating is far less expensive than paint films. A sprayed and sealed coating should hold up for 10 to 20 years or more with little or no maintenance. In less severe exposure, the metallized system may add a few pennies per square foot to the first-time cost of a job, but this initial cost is far less than the cost of a single re-coating job when you include rigging, preparation and coating. For severe exposure, a paint system might require 4 to 6 applications to just one metallized application. These application are for all steel structures, piping and oil & gas platforms exposure to sea water. Shipping vessels, refineries operations. Type of equipment used to apply metallizing is Powder Gun System and Single Wire Metallizing Gun and Electric Dual Arc Gun. See Materials Page for list of the Metallizing Wires and Powders.Metallized coatings systems offer three-way corrosion protection.There are four ways to protect iron and steel against corrosion: 1 Apply a barrier coat of an impervious film such as paint. 2 Provide electrolytic protection.3 Use inhibitors such as zinc chromate. 4 Passivate by forming coversion films on the surface such as phosphate and black oxide coatings.Of all the methods commonly used for corrosion protection,metallizing coatings systems are the most versatile.Metallized aluminum or zinc coatings can be used to provide electrolytic protection...both metals are anodic to steel and therefore corrode preferentially to the steel. If greater protection is wanted, a primer that acts as an inhibitor, plus one or more seal coats can be added to the metallized coating.The sealing materials infiltrate the minute surface pores of the coating, establishing a permanent barrier against corrosion.
Aluminum and zinc are the two metals recommended for atmospheric protection of iron and steel and also for protection in salt and fresh water immersion. Zinc is usually 99.9% pure and is not contaminated in the spraying process. The result is that metallized zinc coatings are much purer than those applied by hot dip galvanising, for instance, since nickel used in galvanizing picks-up considerable iron as an impurity. Aluminium is usually 99.00% pure aluminium and is used for much corrosion protection work.
For the protection of articles on a competitive basis with methods such as painting or plating, thin coatings of sprayed zinc are used. Zinc, 25 microns thick provides excellent protection for small hardware items such as nuts, screws etc, which can be prepared and metallized in tumbling barrels. While the cost of aluminium coatings is slightly less than that of zinc of equal thickness, aluminium requires more thorough surface preparation. Also aluminum is not recommended in thicknesses less than 75 microns. Therefore, where the cost of blasting must be kept at a minimum or where very thin coatings are sufficient, zinc should be used.
Zinc has a higher electrolytic potential than aluminium, and offers better protection on work which is not readily accessible at all points around it's surface. Small uncoated areas such as scratches or voids on the underside of imperfectly headed rivets are protected electrolytically by zinc better than aluminium.
American Welding Society 19 Year Corrosion Tests AWS Tests Prove Superiority of Metallized Systems
In 1974 the American Welding Society completed a 19 year study of corrosion protection afforded by wire metallized aluminium and zinc coatings applied to low carbon steel. Here is a summary of the results:
Aluminium coatings 0.003 in to 0.007 in (0.08mm to 0.15mm) thick, both sealed and unsealed, gave complete base metal protection from corrosion for 19 years in sea water and in severe marine and industrial atmospheres. Unsealed zinc sprayed coatings required 0.012 in (0.30mm) minimum thickness to give complete protection in sea water for 19 years. In severe marine and industrial atmospheres 0.009 (0.23mm) of unsealed zinc or 0.003 in to 0.006 in (0.08mm to 0.15mm) of sealed zinc gave 19 years protection. In severe marine atmospheres, application of one coat of wash primer plus one or two coats of aluminium vinyl enhanced the appearance and extended the life of zinc coating at least 100%.
With aluminium, the sealing systems primarily enhance appearance, because both sealed and unsealed systems showed no base metal rust after 19 years. The only sealed zinc panels tested in sea water were coated with chlorinated rubber. This seal coat did not prove effective. Thin coats of metallized aluminium perform better; have less tendency to develop pits and blisters and therefore are expected to give extended life.Where aluminium coatings showed physical damage such as chips or scrapes, corrosion did not progress, suggesting the occurrence of galvanic protection. Results of AWS 19 Year Corrosion Test of Metallized Coated Steel
The below listed coatings afforded complete protection to low carbon steel panels.
Source: American Welding Society
Notes
(a) All sealed panels were sealed with a wash primer plus one or two coats of either aluminium vinyl or clear vinyl.
(b) Atmospheric panels with a wash primer plus one coat of aluminium vinyl.
(c) Both below low tide and mean tide level.
(d) Both severe marine and salt air.
Theory of Operation: Arc Wire Spray uses two metallic wires as the coating feedstock. The two wires are electrically charged with opposing polarity and are fed into the arc gun at matched, controlled speeds. When the wires are brought together at the nozzle of the, the opposing charges on the wires create enough heat to continuously melt the tips of the wires. Compressed air is used to atomize the now molten material and accelerate it onto the workpiece surface to form the coating. In arc wire spray, the weight of coating that can be deposited per unit of time is a function of the electrical power (amperage) of the system and the density and melting point of the wire. All Sulzer Metco Arc Wire Spray coating systems have unique synchronized "push-pull" wire feed motors and mechanisms, assuring consistent wire delivery.
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NATIONAL THERMOSPRAY, INC.
5120 Oak Ave.
Pasadena, Texas 77503
Contact Ken Norris
OFFICE: 281-373-0609 or 281-991-1133
CELL: 713-898-6844
FAX: 281-991-9911
