Localized Corrosion of Carbon Steel by Electrolytes

The breakdown of passivity under the conditions of a changing electrolyte composition brought about by evaporation or deliquescence is of continuing interest in the management and disposition of radioactive liquid waste, among other industrial applications.  Inorganic aqueous salt solutions with predominantly sodium and aluminum cations can become highly concentrated as liquid evaporates, and highly concentrated solutions can form when residual dry salts absorb water from humid air by deliquescence.  Such solutions are the source of concern for localized corrosion in the form of either pitting or stress corrosion cracking.  Because of the uncertainty surrounding the electrolyte that may form on structures in contact with the vapor space above aqueous salt solutions, possible localized corrosion in the vapor space poorly understood and is an on-going challenge in assuring the structural integrity of the carbon steel liquid waste storage tanks at the U. S. Department of Energy’s Savannah River and Hanford Sites.  It has been shown that the composition of the electrolyte, whether formed by the evaporation of an existing solution or the re-formed by deliquescence or condensation, influences the degree of corrosion that can occur on the walls of the storage tanks.   The condition and composition of the steel surface also influences corrosion susceptibility.

To minimize pitting corrosion, an effort is under way to gain an understanding of the pitting response of carbon steel with various surface conditions simulated radioactive waste solutions.  Electrochemical testing has been used as an accelerated tool in the investigation of pitting corrosion. 

Cyclic potentiodynamic polarization testing has been used to evaluate the degree of pitting corrosion.  However, a difference in the CPP response can be seen in samples that form protective passive films.  The difference in the CPP response of the films suggests that they are of different chemical composition, yet they can both remain protective. 

While significant effort has been undertaken to evaluate the pitting susceptibility of carbon steel in various simulated waste solutions through evaluation of the degree of corrosion that occurred as a result of solution contact, an effort to evaluate the passive film that forms as a protective layer, preventing or slowing the corrosion process, is underway. 

Techniques such as Auger electron spectroscopy and x-ray photoelectron spectroscopy are being employed to determine the composition of the passive films that form while in contact with the solutions during electrochemical testing.  Furthermore, Raman spectroscopy is being used to evaluate the composition of the films.