Corrosion Performance of Constructional Steels in Marine Application

Abstract

Abstract Corrosion is one of the most important factors to be considered in the design of offshore steel structures. In many instances, relatively simple design changes may prevent the occurrence of serious corrosion. Also, careful consideration should be given to the selection of methods for preventing corrosion to ensure that long term, economical protection is obtained. High-strength low-alloy and alloy steels are available that have improved corrosion resistance over that of structural carbon steel in marine atmospheres and in the splash and tidal zones. Under total immersion conditions, however, the lowalloy and alloy steels offer no advantage over carbon steel from a corrosion standpoint. Long-term corrosion data on various constructional steels under different exposure conditions will be reviewed. Galvanic corrosion, stress-corrosion cracking, and corrosion fatigue are discussed with emphasis on means for minimizing these forms of attack. Cathodic protection, metal sheathing, concrete encasement, and organic and metallic coatings are methods to be considered for preventing corrosion in marine applications. Introduction Exploration of the ocean both by industry and government has required either the temporary or permanent installation of many types of structures and vessels in marine environments. Carbon steel and the high strength low-alloy and alloy steels have been the basic construction materials for these structures and vessels and will continue to be used in the future because of their relatively low cost, high strength, good fabricability, and good corrosion resistance. The purpose of this presentation is to review the corrosion behavior of the constructional steels in marine environments and to discuss the specific types of corrosion that these steels will encounter. Important factors to be considered in the selection and design of steel structures are emphasized. The information given here will be primarily concerned with the performance of the standard grades of constructional steels in marine atmospheres and in sea water. The properties of the various constructional steels discussed are shown in Table I. The stainless steels will not be discussed; however, it should be recognized that they also have wide application in marine environments. Corrosion Performance Marine Atmospheres After World War II considerable emphasis was placed on developing quantitative corrosion data from tests in marine atmospheres. It has been learned from these studies that the rate of atmospheric corrosion of steel is dependent on 1) the length of time that moisture is in contact with the surface, 2)the extent of contamination or pollution of the atmosphere, and 3) the chemical composition of the steel. The results of atmospheric-corrosion tests conducted by various American Society for Testing Materials (ASTM) technical committees1, 2) and other societies and associations over the years have shown that the time of wetness of the corroding surface and the amount of chloride contamination of the atmosphere are the critical factors in controlling the corrosivity of marine environment. Data from tests in marine atmospheres reveal that large differences in corrosivity can exist at locations only a few hundred feet apart. The results of tests conducted both 80 and 800 feet from the ocean are given in Table 11.