The Successful Use of Austenitic Stainless Steels in Sea Water

Abstract

ABSTRACT Austenitic stainless steels are providing excellent trouble-free service in sea water for pumps, propellers, valves and other items of marine equipment. Once in a while, a failure occurs as the result of deep localized pitting in a crevice. Data is given showing that austenitic, ferritic and martensitic stainless steels suffer pitting in crevices and under deposits in quiescent seawater. Austenitic stainless steels remain free from attack in high velocity sea water. Low purity ferritic and the martensitic stainless steels frequently pit in high velocity sea water. Crevice corrosion can be effectively controlled with cathodic protection from iron, zinc, aluminum or magnesium galvanic anodes or impressed current cathodic protection by polarization to -0.6 volts versus Calomel. Austenitic stainless steel, in many situations, performs well because it is a component of a multialloy assembly utilizing iron or steel. Examples from field experience are given. INTRODUCTION In the past decade, there has been a growing usage of austenitic stainless steel in marine equipment. Most of the applications have been successful but on occasion an unexpected failure has been observed. It is the purpose of this paper to describe when and how to use austenitic stainless steel with success. The selection of stainless steels appears to result from the engineering requirements of new advanced high speed, high reliability commercial, pleasure and military craft. Ocean science and engineering, offshore oil production, fishing and ocean mining are also contributing to the selection of stainless steels for sea water applications. The increasing usage of stainless steel in the marine environment is found in work boat propellers, pump components, bow thrusters, valves, shafting and shaft components, through hull fittings, parts on data gathering buoys, fasteners and housings of oceanographic instruments. When austenitic stainless steel has given good corrosion free service, it is most often found to be utilized as a key component in a multi component, multi-alloy assembly or system therapy receiving the benefit of built-in cathodic protection. For example, in Figure 1, a cast 304 (Alloy Casting Institute CF-4) propeller is being used on a steel seagoing tugboat with zinc anodes attached to rudder. In Figure 2, a cast ACI CE-30 (1) (2) power plant sea water circulation pump impeller free of any corrosion after six years of service, has been used in combination with an austenitic cast iron suction bell and diffuser. BEHAVIOR OF THE STAINLESS STEELS IN SEA WATER QUIET SEA WATER: A brief review of the behavior of stainless steels in sea water is in order before discussing the criterion to successfully use stainless steels in marine applications. The stainless steels are passive active alloys. In the passive state, stainless steels are highly resistant to corrosion, showing little or no corrosion depending on the exposure. In the active state, when passivity is destroyed, these alloys display localized attack, with little total metal loss, in the form of serious pitting properly identified as crevice corrosion.