Boric acid inhibition of Alloy 600 intergranular corrosion

The environmental factors leading to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) are discussed. These factors include pH, stress and potential. Westinghouse used single tube model boiler tests to simulate the geometric, chemical and operating conditions found in nuclear steam generators. A schematic of the test facility is presented. Testing was conducted to reproduce the morphology and location of IGC found in the field. The simulated plant sludge used in the tests is described. A carbonate environment with an oxidizing sludge was used as a references environment. Boric acid was added to this environment to assess its effect. The conclusions of this test program were: boric acid prevented crack initiation in the references carbonate environment; the addition of boric acid after crack initiation reduced the crack propagation rate by a factor of 8 to 10; and boric acid inhibited IGA in the reference carbonate environment