Cathodic Protection Of Stainless Steels Against Crevice Corrosion

ABSTRACT Crevice corrosion on stainless steel instrument components has been a problem affecting designers and operators of marine instrumentation packages for many years. A cathodic protection study of preventing crevice corrosion on stainless steel Types 316 and 17-4 PH was performed in flowing filtered seawater. Aluminum and low carbon steel anodes were used on the stainless steel panels installed with Delrin crevice-nut assemblies with both bare and partially painted panels. Results are presented in terms of percentage of crevice sites attacked, crevice pit depth, and separate metal rest potentials. Stainless steel Types 316 and 17-4 PH are frequently used in many sections of the marine environment. They are susceptible to both crevice corrosion and hydrogen embrittlement. This paper presents information to the designer about effective methods of preventing crevice corrosion in seawater without the risk of hydrogen embrittlement. Both the aluminum and steel anodes prevented crevice corrosion at all sites. Rest potentials indicate that the aluminum anode samples present the risk of overprotection and hydrogen embrittlement. Comparison with previous data demonstrates the effects of dissolved oxygen on the crevice corrosion severity of attack. INTRODUCTION Stainless steel Type 17-4 PH is a high strength heat treatable alloy frequently used in high stress applications in seawater. 1 17-4 PH has excellent general corrosion resistance characteristics in seawater, but is susceptible to both crevice corrosion 1 and hydrogen embrittlement. 2, 3 Type 316 stainless steel is a conventional austenitic stainless steel which is also frequently used in marine applications. It cannot be heat treated and obtains its strength through a combination of chemistry and work hardening. While it is generally considered to be immune to hydrogen embrittlement at normal temperatures, it is subject to crevice and pitting corrosion, although to a lesser extent than some other austenitic stainlesses such as Types 301 or 304. Table 1 lists the compositions for both 17-4 PH and 316 steels. The chromium and nickel contents of 17-4 PH are lower, and thus it is logical that the corrosion resistance may also be lower. Crevice corrosion is an electrochemical process that may be retarded or accelerated by altering the local electrolytic environment. Examples of crevices include surfaces with metal-to-metal contact or inert material and metal contact, such as underneath washers of electrical penetrators. Detailed explanations of crevice corrosion are available, but in general it occurs on stainless steels when the crevice prevents a passive metal oxide film from forming on the metal surface. This passive film is necessary to prevent corrosion. One method of preventing crevice corrosion is by the use of cathodic protection, an electrochemical means of corrosion control. Cathodic protection can be achieved by coupling the metal to be protected to a more active sacrificial anode. Aluminum, magnesium, and zinc are the sacrificial anode metals normally used for this purpose. 4 However, too much cathodic protection can be harmful and lead to the loss of strength and possible premature failure, especially in steels subject to hydrogen embrittlement such as 17-4 PH. 5