Metal Exposures at Tropical and Marine Sites

Abstract

ABSTRACT Eleven aluminum alloys, electrolytic copper, commercial zinc, low-alloy steel, and type 304 stainless steel were exposed for 7 years at atmospheric test locations at Kure Beach, N. C.; Manila, Philippine Islands; Denge Marsh, England; and Aruba, Netherland Antilles. After exposure for 1, 2 and 7 years, corrosion rates were determined from measurements of panel weight losses, and mechanical properties and pitting depths were measured Metals affected most seriously by the severe Aruba environment included low-alloy steel, zinc, aluminum-copper, and aluminum-zinc alloys. All metals were affected to a lesser degree by the other atmospheres. INTRODUCTION The requirements for a strong, wieldable, corrosion-resistant metal for offshore structures and equipment has made aluminum (and its alloys) a very attractive candidate material. A number of workers have reported the performance of ferrous and nonferrous materials exposed for long periods of time at marine sites in this country and abroad. Generally, however, the studies have stressed one metal system, or one or two test locations. In this paper we shall compare the performances of aluminum and its alloys, copper, zinc, copper-bearing or low-alloy steel, and an 1$-8 type stainless steel at a severe tropical marine location on the island of Aruba, Netherland Antilles, with those at an English Channel coastal marine site; a Manila, Philippine Island, subtropical industrial location; and the well known East Coast Kure Beach, N. C. 80-ft marine lot. LITERATURE REVIEW--GORROSION IN TROPICAL AND MARINE ENVIRONMENTS The following review, restricted to metal systems discussed in this paper, will enable the reader to pursue his interest more thoroughly. Literature citations are listed in the references. These references represent a brief but judicious selection of the world's publications covering the last 20 years. Aluminum Prior to World War II, aluminum alloys had been used extensively for marine applications including hulls and superstructures of boats and ships. The performances of both bare and coated aluminum for a number of alloys have been tabulated. 1 The aluminum alloys having 3 to 5 percent magnesium (AA 5000 series) were reported as having the best resistance to marine corrosion. 2 The aluminum-magnesium-silicon alloys (AA 6000 series) were nearly comparable in corrosion resistance with the 5000 series alloys. Cladding the aluminum-copper alloys (2000 series) with a pure aluminum made it possible to have satisfactory corrosion performance even in severe environments. Architectural and structural applications for aluminum were enhanced as a result of studies by the National Bureau of Standards. 3 The necessity for preventing dissimilar metal contacts through use of insulation was emphasized. Some service tests lasted for periods as long as 52 years.4 Based on relative corrosion rates,7 Godard found that costs for aluminum roofing sheet and copper roofing were roughly equivalent in marine atmosphere in tropical marine locations aluminum was similar to lead. Steel showed the highest cost figure in all marine environments, with galvanized steel in second place.