The results of coulometric and mass balance analyses have been used to elucidate the mechanism of this demineralization process. Essentially, undissociated fixed groups (e.g., weak acid or weak base) that are ionized as a result of the electrochemical reactions partake in an ion exchange process. The properties of weak acid and weak base ion exchange resins are incorporated into the electrodes. The simultaneous removal of anions and cations is undertaken in neutral solution. Regeneration of the electrodes is accomplished by polarity reversal.

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Tafel‐type cathodic polarization curves were obtained for aluminum alloys corroding in anaerobic acid and food media. The overvoltage‐intercept method was found to give corrosion currents equivalent to the measured rates of hydrogen evolution. Tafel‐type anodic polarization curves could not be obtained. Anodic polarization curves calculated from cathodic polarization data in the vicinity of the corrosion potential gave Tafel slopes which were identical to the cathodic Tafel slope, within 20%, in each individual case. On using suitable values for the Tafel slopes in the Stern and Geary equation, the polarization resistance method likewise gave corrosion currents in good agreement with measured rates of hydrogen evolution.

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At present, the highest current density that has resulted in the efficient removal of ions is approximately 1.5 ma/cm2. Chemical ion exchange rates were determined at ion exchange sheets prepared in a manner similar to electrode fabrication. It was concluded that transport through the binder limited the rate of exchange. A simple model was used to predict the critical current density for the process.

Corrosion of 3003 aluminum in citrate buffer solution in the presence of air may be by pitting or by uniform attack. Polarization data indicate that uniform attack is controlled by the rate of chemical dissolution of the oxide film. There is a simultaneous thickening of the film by a tarnishing-type reaction at the same rate to maintain the film at constant thickness.

New sacrificial anode protection criteria were developed and applied to the cathodic protection system of Platform "JJ" which was installed in the Arabian Gulf in September 1982. The new design resulted in a 40 percent decrease in anode weight, and with 15 fewer anodes than required by conventional accepted design. Potential surveys carried out one month and nine months after installation showed potentials of the order of -1.0 volt with respect to the silver-silver chloride reference electrode. The calculated lifetime for this 20-year design is 26 years. The basis for the new design criteria will be presented and discussed. INTRODUCTION The cathodic protection design chain used for Platform "JJ" is primarily based on a high initial platform current density that insures the rapid development of a protective calcarious deposit. As a consequence the current requirement for protection is significantly reduced. The design current density for cathodic protection in the Gulf of Mexico is 6 MA/ft. In sea water at Kure Beach, North Carolina, a current density of 3MA/ft was found to protect steel after the initial deposition of a calcareous coating. Subsequent work has confirmed that after the formation of a calcarious deposit, a current density between 3 and 3.5 MA/ft protects steel structures in the Gulf of Mexico. ' Utilizing present design criteria, it has been shown that platforms designed for a 20-year life will last for 33 years in the Gulf of Mexico. It is well established that high initial current densities result in the rapid development of a calcareous deposit. In recent field experiments in the North sea,7 high initial current densities resulted in protective potentials within a few days after immersion. While the recommended design value for the Northern North Sea is 11 MA/ft, the current density determined after the calcareous deposit had formed was less than 5 MA/ft. Retrofitting of sacrificial anodes was recently completed on three platforms in the North Sea. These platforms were previously protected with an impressed current system. The recorded current densities on each of the three platforms were less than 5 MA/ft DESIGN BASIS Recently, a comprehensive study was made of the cathodic protection systems of seven platforms that have been installed in the Arabian Gulf within the last six years. The cathodic protection systems for these platforms were consistent with accepted design criteria. Field measurements clearly showed that the accepted cathodic protection design resulted in polarizing the platform beyond the minimum structural protective potential of -0.80 volt with respect to the silver-silver chloride electrode. Representative potentials determined one month and one year after installation are shown in Tables 1 and 2. It was concluded that, in the Arabian Gulf' an initial platform current density of 29 ± 2 MA/ft resulted in platform potentials of the order of -1 volt with respect to the silver-silver chloride electrode, within one month after installation. The lifetime of the 20-year design anodes was calculated to be 39 years.

The sub sea corrosion protection system of the tension leg well platform which incorporates coatings, cathodic protection, and high initial current densities, is detailed. The results of surveys undertaken after installation of the TLWP reveal potentials at least 150 millivolts more protective than the minimum potential required for protection. The system life will exceed the design life of 20 years. The TLWP protection system weight is 434,000 pounds less than that associated with conventional cathodic protection design, with 286,000 fewer pounds on the floating portion of the TLWP. INTRODUCTION In late 1989, Conoco installed the tension leg well platform (TLWP) in 1760 feet of water in the Jolliet Field in Green Canyon Block 184 (Figure 1).1 The TLWP is comprised of 4 main elements:2 drilling template, foundation template, tension legs (or tendons), and the TLWP itself (Figure 2). The objectives of the sub sea protection system, which combined coatings and cathodic protection (CP), were: to provide a safe, low maintenance protection system with a life of 20 years, to minimize the anode weight on the floating components (hull, tendons, risers), and to eliminate risks that could result in premature retrofits in deep water. BACKGROUND The CP design philosophy involves minimizing anode weight by coating all of the components of the structure, and by ensuring a high initial current density on the structure such that a low maintenance current density is required. The combination of coatings and cathodic protection has resulted in significant weight and cost savings with two Conoco fixed platforms recently installed in 617 feet of water at Green Canyon Block 52.3 The synergistic combination of coatings and CP is reflected in several technically and economically successful applications in the Gulf of Mexico, Abu Dhabi, the southern North Sea, and the northern North Sea4,5,6,7. In addition to anode weight reduction, it is important to strategically place the anodes such that much of the anode weight required to protect the tendons and risers is concentrated on the templates, located on the sea floor. Our experience in the Gulf of Mexico and the Arabian Gulf is that CP is markedly enhanced on well coated pipes, with an individual anode having a throwing power to at least 3,000 feet. Exxon has reported similar observations for coated guy lines on its Lena structure.8 Numerical modeling, with the computer program SEACORR, 9 has also shown CP to be effective over large distances for coated cathodes. The TLWP CP design is based on achieving an initially high structure current density (e.g., 30 Ma/ft2) that will result in a structure potential at least as negative as -0.950 V with respect to the silver- silver chloride reference electrode (Ag/AgCl). Meeting this initial polarization criteria enables the safe use of a maintenance current density of 3.5 Ma/ft2 - The use of high initial current densities along with lower maintenance current densities has been well documented for platforms in the Gulf of Mexico, the Arabian Gulf and the North Sea. 10,11,12 The use of lower maintenance current densities results in lowering the anode weight requirement. Initial current densities of this order have been found to be effective in deep waters in both the Gulf of Mexico and the North Sea. 3,13