Marine corrosion Prevent of steel with Thermal Sprayed Zinc and Aluminum Coatings Resuits of 18 Years Exposure
Authors: Schrieber C.F. , Longo F.F. , Durman G.J. 
Published: October 2008

The Committee on Metallizing of the American Welding Society sponsored a test program to study the corrosion protection afforded by thermal sprayed aluminum and zinc coatings applied to structural at eel. The first panels were exposed in January, 1953. Inspections were made at 1, 3, 6, and 12 year intervals. Reported results examined the performance of the subject metallized coatings in marine atmospheric and seawater environments. Seawater tests included full and half tide immersion in quiet seawater and full immersion in flowing seawater (approximately 3 knots). The subject conditions are highly representative of those found on off-shore structures.

The results indicate that low carbon steel can be protected from the corrosive effects of these environments for 18 years or more by the application of flame sprayed zinc or aluminum coatings.

INTRODUCTION

This paper reports the results of an l8-year study of the corrosion protection afforded by metallized aluminum and zinc coatings applied to low carbon steel. Test sites were selected throughout the United States to provide a wide variety of environmental conditions. These included urban, industrial, salt air, and severe marine atmospheric exposure. Also below mean tide, mean tide, and total seawater immersion conditions were tested. This paper will only be concerned with the marine atmospheric and seawater environments.

There were three major program objectives:To determine the life of any given thickness of coating in a specific environment (and the protection afforded the base metal).To determine the effect of various methods of steel preparation on the corrosion protection properties of the flame sprayed coating.To determine the increase in life of the aluminum and zinc flame sprayed coatings with the addition of seal coats.

Steel panels, numbering 4,248, were employed in the study; half were coated with commercially pure zinc and the balance coated with commercially pure aluminum. Steel panels for atmospheric exposure were 4 × 6 × 1/8 inch and 4 × 12 × 1/8 inch for seawater exposure.

Aluminum and zinc coatings for atmospheric exposure tests were 0.003, 0.006, 0.009 inch. In addition, a coating thickness of 0.018 inch was included in seawater exposure tests. Originally, exposure periods of 1, 3, 6, and 12 years were scheduled. At the end of each of the above periods, a complete set of panels, consisting of three identical panels of each type, was to be removed from the exposure racks for laboratory examination. At the same time, the panels of each type remaining on site were to be inspected and their condition recorded. However early inspections indicated that nearly all coatings would last for more than 12 years, at all exposure sites. Accordingly, the panels intended for removal at 6 years were left on exposure for 12 years, and the 12year set was left exposed for 18 years. Results of the 6- and l2-year inspections are reported in American Welding Society publications C2.8-62 and C2.ll-67, respectively.