EFFECT OF BORIC-ACID ON PIT GROWTH IN ALLOY-600 STEAM-GENERATOR TUBING

An experimental program was conducted to determine the role of boric acid on the pitting behavior of alloy 600 steam generator tubes. The test solution consisted of 5620 wppm Cl-, 3984 wppm SO4=, and 500 wppm Cu++ yielding a Cl-/SO4= ratio of 1.4. Boric acid was added in amounts of 0, 50, 100, 200, 500, 1000, and 2000 wppm BO33-. Exposure tests were conducted at 75-degrees-C in aerated and deaerated solutions and at 288-degrees-C in deaerated solution. Results show that pit growth at 75-degrees-C is greater than at 20 or 288-degrees-C regardless of BO33- concentration. Pit growth rate in deaerated tests at 75-degrees-C is greater than that in aerated tests by four to six times. A clear effect of borate on pit growth was not observed, but the presence of borate did have several effects. With increasing borate levels, pit coalescence increased, pit morphology changed from crystallographic etching to intergranular attack and micropitting, and the level of general corrosion increased.