Corrosivity Survey Studies Under The Arctic Ice Sheet

ABSTRACT The need for preconstruction on-site corrosivity surveys is discussed. Instantaneous corrosion rate determination techniques based on electrochemical principles are available which could be used for preconstruction corrosivity surveys. This paper discusses an effort to apply these techniques to a corrosivity survey under the Arctic ice sheet during spring 1978. INTRODUCTION Cathodic protection design for offshore structures is normally accomplished prior to construction and placement of the structure. Resistivity surveys and other pre-installation surveys are commonly used in designing on-shore cathodic protection in installations, but they are not commonly used in the design of corrosion control systems for offshore structures. Current practice is to rely on data available from other structures in operation in similar geographic locations. Design guidance is available for cathodic protection designers which lists current density requirements for the North Sea, Gulf of Mexico, Cook Inlet, etc. This guidance is based on previous experience in the area. Applications of design procedures that have worked well elsewhere have led to notably inadequate corrosion control designs in Alaskan and North Sea applications. The high corrosion rates, and the high cathodic protection current requirements, experienced in colder waters is thought to be due to high dissolved oxygen concentrations in these waters. Water resistivity surveys would not indicate the high corrosivity of colder waters. The need exists for a relatively short-term survey technique which can be used in site surveys prior to cathodic protection design and installation. While on-site potential surveys and upgrading of designs is possible, it normally requires the use of divers and is extremely expensive. PURPOSE The purpose of this investigation was to develop on-site survey techniques for use in marine cathodic protection design. The technique must be relatively short-term so that it can be performed during oceanographic site surveys, which are routine procedures prior to marine construction in a new area. While research can be performed to correlate short-term survey results with more conventional long-term corrosion tests, there would be no need for short-term data in areas where more conventional experience is already available. The basic premise upon which this program was founded is that present design procedures for cathodic protection in the Gulf of Mexico are adequate for that location. The problem then became one of measuring quantitatively how much more corrosive a given Arctic water location may be. Hopefully, if this could be done reliably, correlations then could be made between corrosion rates in a new location and accepted design procedures based on long-term corrosion experience in other locations. EXPERIMENTAL PROCEDURE A number of electrochemical methods exist for in situ determination of corrosion rates. They all involve changes in electrode potential as electric current is supplied to the metal in question. Controlled changes in current, or potential, produce measured changes in the other variable. The slopes of the potential-current lines can then be used to obtain an average corrosion rate for the sample in question.