Corrosion fatigue: Recent developments and future needs

Exposure to water has recently been found to have a much greater deleterious effect on the fatigue strength of the most commonly used carbon and low alloy and stainless steels than previously believed. The ASME Boiler & Pressure Vessel Code and other Codes and Standards for evaluating the fatigue design life of engineered components including power plants have commonly assumed that such exposure would not reduce the fatigue life by more than a factor of 2 below that measured in air Accordingly, a factor of 2 on fatigue cycles has been used to account for corrosion effects when developing the current fatigue design life curves using the data obtained in air. Recent environmental data independently developed in several major laboratories world-wide have shown that exposure to water temperatures above about 150°C (303°F) for carbon and low alloy steels and about 180°C (360°F) for stainless steels can reduce the cycles to failure (thru-wall cracking) or fatigue life by no more than a factor of 10 in the low and intermediate cyclic life regimes. Cyclic crack growth rates have been found to be 10 to 50 times faster in elevated temperature water than in air, a phenomenon referred to as Environmentally Assisted Cracking (EAC).The impact of these corrosion fatigue effects in both conventional and nuclear components is enormous, accounting for what were presumed to be premature failures due to material or fabrication defects. Efforts are continuing to quantify strain rate, hold time, temperature, corrosion potential, mean stress, strain amplitude, cyclic history, and flow velocity effects on the S-N fatigue life and da/dN cyclic crack growth rate behaviour of a whole range of materials now believed to be susceptible to this phenomenon.This paper presents compilations and analyses of the currently available world-wide database. Methods of accounting for such environmental effects on the fatigue design lives of components are presented. ASME Code Technical Committees, including the Subgroup on Fatigue Strength, have made a comprehensive study of the phenomena and the available data.They have made a series of technical determinations which are being used to guide the development of new corrosion fatigue design criteria for codes and standards such as the ASME Boiler and Pressure Vessel Code. The interdisciplinary nature and complexity of corrosion fatigue phenomena will continue to challenge engineers and materials scientists for decades.