A Brief History of Environmental Cracking in Hot Water

Stress corrosion cracking (SCC) and corrosion fatigue became a widespread issue during the industrial revolution, with several sources reporting 50,000 deaths and 2,000,000 injuries from steam boiler failures in a single year in the United States. While this eventually led to the development of pressure vessel and boiler codes to provide design margins against overload and fatigue, even now, 165 years later, the design codes transfer to the individual designer the job of ensuring that SCC will not occur. This simplistic requirement of immunity to SCC has generally been assessed using simple, short-term tests that would only identify conditions of severe susceptibility and focus on crack initiation with its inherent ambiguity and reliance on near-surface conditions that are not understood or well-characterized in hot water environments, the focus of this paper. With increasing plant operating time and more sophisticated laboratory experiments, the reality has emerged that SCC in hot water can occur over a very broad spectrum of conditions, although sometimes at moderately low SCC initiation and growth rates that might not lead to problems after 30+ years. The lore has been that SCC occurs only when specific ions are present, sensitization or segregation exists at grain boundaries, or the corrosion potential, stress intensity factor, temperature, etc. are above some threshold value. Better laboratory data and extended plant experience have altered our understanding of environmental cracking and eroded much of the lore surrounding immunity and thresholds. Environmental cracking merits more sophisticated efforts, both in R&D and in design codes, to understand the vulnerabilities to and dependencies of SCC, and identify conditions that lead to adequate lifetime based on both SCC initiation and growth. The issues identified in hot water environments-the conditions of the original industrial problem and a system studied extensively-are likely to be broadly relevant to other environmentally assisted cracking systems.