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Title: Remedial methods for intergranular attack of alloy 600 tubing. Volume 3. Boric acid and acetic acid remedial methods. Final report

Abstract

An important cause of recent tube degradation in recirculating pressurized water reactor (PWR) steam generators with open tube/tubesheet crevices is intergranular attack (IGA) of alloy 600 tubing in the crevice region. The attack appears to occur on the hot leg tubing because of high concentrations of caustic species formed from remnants of past phosphate water treatment, combined with materials from inleakage from freshwater-cooled condensers. The concept of using neutralizers to modify the aggressiveness of the crevice environment was examined. It appears that this can be accomplished by neutralizing the caustic species with an acid. Two ways to apply the acid are by off-line flushing during plant shutdown and by on-line treatment during operation. The substance that appears to be most suitable for off-line flushing is acetic acid, with boric acid as a second choice. Concentrations should be in the range of from 1000 to 5000 ppM. The addition of 1000 to 5000 ppM of a non-ionic detergent in the flush solution should improve penetration of the crevice. Use of preflush lancing to remove sludge on the tubesheet will also help by reducing acid consumption. The requirements for materials to be used in on-line treatment are more stringent because of possiblemore » interaction with other components in the secondry system. Boric acid is the only substance that has operational experience. A series of tests are proposed to investigate the behavior of acetic acid and boric acid on tubesheet sludge, on tubesheet/support plate material, and on alloy 600/tubesheet couples. Similarly, areas of uncertainty of on-line treatment with boric acid are its effect on tubesheet/support plate materials and on the rest of the secondary system. 23 refs.« less

Authors:
Publication Date:
Research Org.:
Babcock and Wilcox Co., Alliance, OH (USA). Research and Development Div.
OSTI Identifier:
5708583
Report Number(s):
EPRI-NP-4635-Vol.3
ON: TI86920389; TRN: 86-021995
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS; 36 MATERIALS SCIENCE; INCONEL 600; CREVICE CORROSION; INTERGRANULAR CORROSION; REACTOR MATERIALS; PWR TYPE REACTORS; STEAM GENERATORS; TUBES; ACETIC ACID; BORIC ACID; CORROSION INHIBITORS; WATER CHEMISTRY; ALLOYS; BOILERS; CARBOXYLIC ACIDS; CHEMICAL REACTIONS; CHEMISTRY; CHROMIUM ALLOYS; CORROSION; HYDROGEN COMPOUNDS; INCONEL ALLOYS; INORGANIC ACIDS; IRON ALLOYS; MATERIALS; MONOCARBOXYLIC ACIDS; NICKEL ALLOYS; NICKEL BASE ALLOYS; NIOBIUM ALLOYS; ORGANIC ACIDS; ORGANIC COMPOUNDS; REACTORS; VAPOR GENERATORS; WATER COOLED REACTORS; WATER MODERATED REACTORS; 210200* - Power Reactors, Nonbreeding, Light-Water Moderated, Nonboiling Water Cooled; 360105 - Metals & Alloys- Corrosion & Erosion

Citation Formats

Rubright, M.M. Remedial methods for intergranular attack of alloy 600 tubing. Volume 3. Boric acid and acetic acid remedial methods. Final report. United States: N. p., 1986. Web.
Rubright, M.M. Remedial methods for intergranular attack of alloy 600 tubing. Volume 3. Boric acid and acetic acid remedial methods. Final report. United States.
Rubright, M.M. 1986. "Remedial methods for intergranular attack of alloy 600 tubing. Volume 3. Boric acid and acetic acid remedial methods. Final report". United States. doi:.
@article{osti_5708583,
title = {Remedial methods for intergranular attack of alloy 600 tubing. Volume 3. Boric acid and acetic acid remedial methods. Final report},
author = {Rubright, M.M.},
abstractNote = {An important cause of recent tube degradation in recirculating pressurized water reactor (PWR) steam generators with open tube/tubesheet crevices is intergranular attack (IGA) of alloy 600 tubing in the crevice region. The attack appears to occur on the hot leg tubing because of high concentrations of caustic species formed from remnants of past phosphate water treatment, combined with materials from inleakage from freshwater-cooled condensers. The concept of using neutralizers to modify the aggressiveness of the crevice environment was examined. It appears that this can be accomplished by neutralizing the caustic species with an acid. Two ways to apply the acid are by off-line flushing during plant shutdown and by on-line treatment during operation. The substance that appears to be most suitable for off-line flushing is acetic acid, with boric acid as a second choice. Concentrations should be in the range of from 1000 to 5000 ppM. The addition of 1000 to 5000 ppM of a non-ionic detergent in the flush solution should improve penetration of the crevice. Use of preflush lancing to remove sludge on the tubesheet will also help by reducing acid consumption. The requirements for materials to be used in on-line treatment are more stringent because of possible interaction with other components in the secondry system. Boric acid is the only substance that has operational experience. A series of tests are proposed to investigate the behavior of acetic acid and boric acid on tubesheet sludge, on tubesheet/support plate material, and on alloy 600/tubesheet couples. Similarly, areas of uncertainty of on-line treatment with boric acid are its effect on tubesheet/support plate materials and on the rest of the secondary system. 23 refs.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1986,
month = 6
}

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  • This report evaluates the occurrence of intergranular attack (IGA) and stress corrosion cracking (SCC) of Alloy 600 tubing in PWR steam generators and reviews candidate remedial actions. Plant eddy current and tube pull data were examined to determine the morphology and location of corrosion. These data suggest that the affected plants fall into five categories: IGA occurring over a long axial length of the tube to tubesheet intersection; IGA and SCC localized at the top of the tubesheet; localized SCC within the tubesheet; SCC/IGA within tube support plate crevices; and SCC or IGA above the top of the tubesheet. Eachmore » category must be considered when selecting the inhibitor application mode. To determine which inhibitors might be effective, a review was performed of crevice deposit and crevice flushing solution analytical data taken from plants having SCC/IGA. From this review it was concluded that although there are some isolated instances of acidic sulfur induced SCC/IGA this form of corrosion is most often caused by an alkaline environment at specific electrochemical potentials. Thus SCC/IGA may be inhibited by neutralizing the alkaline environment, by moving the electrochemical potential out of the susceptible range, or by preventing the ingress of corrodent into the grain boundary and the egress of corrosion product out of the actively corroding grain boundary. Many potential IGA/SCC inhibitors are considered. However, the inhibitors identified have a significant disadvantage relative to boric acid because they are not plant qualified. On-line and off-line inhibitor application is discussed. A test plan for evaluating the inhibitors is outlined. 48 refs., 19 figs., 16 tabs.« less
  • A literature search was conducted to find potential remedies for caustic-produced intergranular attack (IGA) on alloy 600 tubes in the tube-tube crevice region of steam generators in a few PWR plants. Caustic-producing alkalies Na and K are concentrated on the tube side as phosphates and silicates in multi-component deposits which have accumulated in crevices in these plants. Off-line flushing of crevices, using low temperature water soak and drain cycles, is recommended as reasonably effective and the safest method of removing the alkalies. For a thorough cleaning of crevices, chelant-based formulations have been identified to remove the major components of deposits,more » namely, Fe/sub 3/O/sub 4/, Cu/CuO, and Ca-salts. Conventional inorganic or organic inhibitors were considered not suitable for use, because of their poor stability under steam generator crevice conditions and possible adverse effects on tubes and ancillary plant components. However, boric acid which is used frequently to combat other forms of corrosion in PWR plants, was identified as a potentially useful additive to the feed water or to the soak water for neutralizing caustic and also inhibiting IGA. 63 refs., 42 figs., 19 tabs.« less
  • The effect of boric acid applied on-line and via crevice flushing was evaluated in this program as a remedial action for intergranular attack on mill annealed Alloy 600 steam generator tubing in tubesheet and support plate crevices. Single tube model boilers were used to simulate the full length of a steam generator tubesheet crevice and the support plate crevices at hot leg conditions. The tubesheet crevices had preloaded inventories of 10% sodium hydroxide/2.5% sodium sulfate solution. All of the crevices were exposed to on-line additions of 0.1 ppM hydroxide and 0.04 ppM sulfate as the sodium salts. These baseline conditionsmore » caused pre-existing IGA beneath eccentrically mounted support plates to increase in depth at a rate of 2.40 ..mu..m/day. In the tubesheet crevices the pre-existing IGA increased in depth at a rate of 1.6 ..mu..m/day. The virgin mill annealed tubing in the tubesheet crevice developed IGA at a rate of 0.73 ..mu..m/day during the final 60 days of the 90 day test. The first 30 days were an incubation period during which no IGA occurred. The results of this work suggest that boric acid can inhibit IGA on unattacked tubing when applied continuously at the field specifiation of 5 to 10 ppM in the blowdown. It can also slow down propagation of pre-existing IGA if present in the proper ratio (which requires adequate treatment levels) and if it can access the corrosion sites. Corrosion may progress in areas where boric acid is not able to penetrate in sufficient excess to the corrodent present. 10 refs., 85 figs., 39 tabs.« less
  • Each model boiler was preconditioned in All-Volatile Chemistry (AVT). One model boiler was then faulted with chemicals typical of a fresh-water condenser leak and simulated plant sludge. The other model boiler was first faulted with caustic, sodium carbonate, and sludge, then sodium carbonate and magnetite. Each model boiler contained seven tubes; tubing included Alloy 600 mill-annealed, cold-drawn, thermally-treated, and sensitized materials. Two different widths of tubesheet crevices were used. The fresh-water-faulted boiler experienced minor pitting and no significant IGA or IGSCC. The caustic-carbonate-faulted boilers experienced throughwall IGSCC and shallow IGA of mill-annealed and mill-annealed plus cold-drawn A600. Plastically deformed A800more » (mill-annealed, cold-drawn glass bead peened) were susceptible to transgranular cracking in crevice regions. C-ring specimens in both boilers experienced no significant corrosion. Caustic denting of A508 tubesheets were observed.« less
  • The intergranular attack (IGA) that affects alloy 600 tubes in the tube sheet crevices of PWR steam generators is hard to simulate in laboratory studies. For this study, a special apparatus was designed with a range of representative materials, mechanical conditions and geometry. The design also took account of sludge piles, thermal fluxes and water chemistry. During a first series of seven model boiler tests, chemical parameters and test procedures were adjusted to finally obtain a field prototypical degradation of tubing over a significant length within the tube sheet crevice for the case of caustic pollution. IGA was not producedmore » for the river water in-leakage case. A second series of model boiler tests also showed the possibility of producing a representative IGA by initially filling the tube sheet crevices with concentrated caustic solutions. A third series of five model boiler tests aimed at studying in the effectiveness of remedial actions on either virgin of IGA affected tubing. Tube sheet crevice flushing operations using the natural circulation procedure showed a poor efficiency for moving concentrated contaminants, but they succeeded in forcing chemicals additives (acetic acid or boric acid) within the non-occluded portions of crevices. This off-line treatment resulted in a reduction in the progression rate of the pre-existing IGA by a factor of 2 to 3. Simulation of this treatment on virgin material showed that this result was obtained because acetate or borate shifted the cation-to-anion equivalent ratio to well under one. However this off-line treatment could not prevent the occurrence of intergranular stress corrosion cracking (IGSCC) near the top of crevices, since caustic continued to hideout under full power operation. The best remedial action consisted of a combination of off-line and on-line boric acid treatment. IGSCC was prevented both on virgin and IGA affected tubes. 3 figs., 3 tabs.« less