NR198002 Evaluation of Chelating Agents for Controlling Stress-Corrosion Cracking

Abstract

Studies to develop methods for preventing stress-corrosion cracking have been concentrated in three general areas: (1) modifying the steel from which pipes are made, (2) modifying the operating practices involving stress, temperature and cathodic protection, and (3) modifying the environment. In the latter category, various approaches have been considered, and experiments have been performed or are currently underway. One approach is to modify the soil chemistry at the soil/pipe interface by using interrupted cathodic protection. Another approach is the use of inhibitors to modify the environment known to promote stress-corrosion cracking. Yet another approach under the category of modifying the environment is the possibility of using chelating agents to affect the film-forming properties of line-pipe steel. In this final report, the rationale behind this approach is described. The chelating agents chosen are identified and the methods for their evaluation delineated. In this study, the use of chelating agents to modify the film-forming properties of line-pipe steel is discussed as one means of changing the pipe/coating/soil environment. The rationale behind this approach was that if chelating agents could tie up iron cations as they dissolved in the liquid environment in contact with a bare pipe surface (e.g., at a holiday or under a disbond) then films would not form. Film cracking and repair then would not be possible, hence this proposed mechanism for SCC could not occur and the pipe would be protected from this phenomenon. Filming experiments, potential shift measurements, corrosion experiments, and crevice cell tests were conducted with seven selected chelating agents and compared with the corresponding behavior of line-pipe steel in control experiments. In preliminary filming and general corrosion attack experiments only triethanolamine (TEA) was found to have any desirable characteristics at the 10 weight percent addition level. Some chelating agents promoted excessive general corrosion attack. TEA, therefore, was selected for the potential shift measurement and crevice cell tests. The results of these experiments were not as encouraging, although it is possible that TEA could act as a weak inhibitor for SCC.