Stress corrosion cracking (SCC) of discs (at keyways, bores, and blade attachments) is caused by a combination of high surface stresses, a susceptible material and operational and shutdown environments. Design-related root causes are the most important and prevalent. They include high surface tensile stresses and stress concentrations, and use of high strength materials.
This article contains excerpts from the paper, “Steam turbine corrosion: Problems and solutions” presented at the 37th Turbomachinery Symposium by Otakar Jonas and Lee Machemer of Jonas, Inc.
Sources of stresses that contribute to SCC of discs include:
Basic centrifugal load caused by rotor rotation. Locally high concentration of centrifugal loads caused by variation in the gaps (gauging) between blade and disc rim attachment.
Residual machining stresses.
Vibratory stresses—interaction of SCC and corrosion fatigue. Also, vibratory stresses reduce the life of the cracked disc when the flaws reach a sufficient size that fatigue becomes a dominant mechanism.
Steam chemistry root causes of SCC and CF cracking include:
Operating outside of recommended steam purity limits for long periods of time; sometimes caused by organic acids from decomposition of organic water treatment chemicals.
Condenser leaks—minor but occurring over a long period of time.
Condenser leaks—major ingress, generally one serious event, and the system and turbine not subsequently cleaned.
Water treatment plant or condensate polisher regeneration chemicals (NaOH or H2SO4) leak downstream.
Improperly operated condensate polisher (operating beyond ammonia breakthrough, poor rinse, etc.).
Shutdown environment: poor layup practices plus corrosive deposits. Sodium hydroxide is the most severe SCC environment encountered in steam turbines. The sources of NaOH include malfunctioning condensate polishers and makeup systems and improper control of phosphate boiler water chemistry combined with high carryover. Many other chemicals can also cause SCC of low alloy steels. The chemicals used in turbine assembly and testing, such as molybdenum disulfide (lubricant) and Loctite™ (sealant containing high sulfur), can accelerate SCC initiation (Turner, 1974; Newman, 1974).