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Corrosion Tests
There have been numerous tests of metallized coatings over the years. Many of these tests are repeated by owners before using metalizing on a bridge. This testing is often duplicative of earlier work. Here are test reports that establish the effectiveness of metalizing.
Corrosion Tests of Flame-Sprayed Coated Steel, 19-Year Report (AWS 19-Year Report)
This is the single most important test report available. It is internationally recognized as thorough and definitive. It is one of those essential documents for all who use or apply metalized coatings for atmospheric service.
From the Abstract
This report presents the results of a 19-year study of the corrosion protection afforded by wire-flame-sprayed aluminum and zinc coatings applied to low-carbon steel. It contains the results of an inspection of the flame-sprayed coated steel panels made after all panels had been exposed for 19 years.
Exposure Sites
- Industrial Atmosphere New York City Area; Columbus, Ohio; East Chicago, Indiana.
- Marine Atmosphere Kure Beach, North Carolina; Brazos River, Texas; Point Reyes, California.
- Seawater Exposure Wrightsville Beach, North Carolina; Freeport, Texas.
Conclusions
- Aluminum-sprayed coatings 0.003 in. to 0.006 in. (0.08 mm to 0.15 mm) thick, both sealed and unsealed, gave complete base metal protection from corrosion in sea water and also in severe marine and industrial atmospheres.
- Unsealed zinc-sprayed coatings required 0.012 in. (0.30 mm) minimum thickness for complete protection in seawater for 19 years. In severe marine and industrial atmospheres, 0.009 in (0.23 mm) of unsealed zinc or 0.003 in. to 0.006 in. (0.08 mm to 0.15 mm) of sealed zinc are needed for 19-year protection.
- In severe marine atmospheres, the application of one coat of wash primer plus one or two coats of aluminum vinyl enhanced the appearance and extended the life of zinc coatings at least 100%. With aluminum, the sealing system primarily enhances appearance, because both systems showed no base metal rust after 19 years.
- Thin coats of aluminum perform better because they have fewer tendencies to develop pits and blisters, and, therefore, extended life is expected. Vinyl also seems to perform better because it must penetrate to the base metal.
- Where aluminum coatings showed damage such as chips or scrapes, corrosion did not progress, suggesting the occurrence of galvanic protection.
- The corrosion protection afforded by zinc and aluminum coatings is not affected by the method of surface preparation used for this test. Specifically, a steel-flash-bond coat is not essential. For large parts or where coating thickness will exceed 0.006 in (0.15 mm), a coarse abrasive is recommended. (See Table 2.)
- The use of flame-sprayed aluminum and zinc coatings is recommended as a means to extend the life of such iron and steel structures as bridges.
- American Welding Society 19-Year Report, 1974, pp. 4-7
The report "Corrosion Tests of Flame-Sprayed Coated Steel, 19-Year Report" is known worldwide as the definitive work on the effectiveness of metalized coatings. It is available from the American Welding Society, Miami, Florida.
Marine Atmospheric Corrosion Museum Report on the Performance of Thermal Spray Coatings on Steel
Another thorough evaluation of metalizing is detailed in Marine Atmospheric Corrosion Museum Report on the Performance of Thermal Spray Coatings on Steel, Robert M. Kain and Earl A Baker, ASTM 1989.
From the Abstract
In the early 1950s, a number of zinc-aluminum compositions were applied to carbon steel using then-current practice and exposed at several test sites in England and the United States. The performance of these test panels was described by Hoar and Radovici following 10-½ years of exposure. In the ensuing years, a number of the original panels remained on exposure at Kure Beach, NC, in the LaQue Center for Corrosion Technology, Inc., marine atmospheric corrosion site.
This paper describes the protection afforded by the various coatings after 34 years of continuous exposure in the moderately aggressive marine location situated 250 m from the Atlantic Ocean. Several electrochemical methods presently in use are reviewed in a reflection of how accurate they might be in forecasting the effectiveness of these coatings.
From the Summary
Examination of the specimens for detectable base metal rust and stain through the coating revealed a broad range in the degree of protection. This varied considerably with alloy content, type of powder, and the method of application.
Although a number of zinc coatings and high zinc containing, mixed and alloy powder, coatings performed well, the greatest number of resident coatings were generally those of high aluminum content. The latter materials typically exhibited less variability as a function of the applicator gun .
Other investigators are presently studying the protection characteristics of thermal sprayed alloy wire coatings with compositions approximating those which have performed well in 34-year tests.
- Performance of Thermal Spray Coatings on Steel, ASTM, 1989
The coatings evaluated in this series of tests were applied by flame-sprayed powder. Todays standard is wire flame-spray or twin wire electric arc spray. These tests evaluated coatings of Zinc-20 Al and Zinc-30 Al mixed powder and other compositions. Today the standard zinc/aluminum coating material is Zinc-15 Aluminum pre-alloyed wire. One could predict even better results using todays standard flame or arc wire equipment and the 85% zinc / 15% aluminum wire.
NOTE: The fiftieth anniversary of this test is in the year 2002. The LaQue Center is planning to evaluate the condition of the metalized coupons still exposed on this anniversary.
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