Corrosion of iron alloys in supercritical

Thermodynamic Anafysis of Corrosion of Iron Alloys in Supercritical Water... 

A thermodynamic analysis was conducted for corrosion of iron alloys in supercritical water. A general method was used for calculation of chemical potentials at elevated conditions. The calculation procedure was used to develop a computer program for display of pH-potential diagrams (Pourbaix diagrams). A thermodynamic analysis of the iron/water system indicates that hematite (Fe203> is stable in water at its critical pressure and temperature. At the same conditions, the analysis indicates that the passivation effect of chromium is lost. For experimental evaluations of the predictions, see the next paper in the symposium proceedings. 

For thermodynamic analysis of the corrosion of iron alloys in supercritical water, the above computer program was modified based upon standard thermodynamic property extrapolation methods. 

Most of the data available in the literature are for subcritical conditions. Corrosion studies of iron alloys in supercritical water have not been reported. For supercritical fluid extraction and corrosion studies, a supercritical fluid reactor system for temperatures up to 530 C and pressures up to 300 atm was constructed. This system was used to determine the electrochemical behavior of type 304 stainless steel (304 S.S.), 316 S.S., 1080 carbon steel (1080 C.S.), and pure iron in supercritical water. 

Corrosion of Iron Alloys. Other recent work Q) presents experimentally measured values of the open circuit potentials of iron alloys in water at ambient and supercritical conditions. For iron and its alloys (1080 carbon steel, 304 stainless steel and 316 stainless steel), the open circuit potentials varied from -0.112 to +0.055 volt in water at its critical point, and varied from -0.138 to -0.060 volt in water at ambient conditions. These values were... 

High temperature and high pressure processing of materials often involves the use of supercritical fluids. Corrosion studies are quite essential for evaluation of the equipment in supercritical fluid operations. Previous electrochemical measurements for alloys in supercritical fluids are rare (1-1). The reported measurements (3) show that passivation of iron alloys is different at supercritical conditions compared to ambient conditions. The study of the electrochemistry of iron alloys can lead to control of corrosion of equipment utilizing the alloys. Thermodynamic analysis provides the information about stable species, i.e. corrosion products under given temperatures and pressures. 

Thermodynamic predictions were consistent with experimentally measured corrosion rates and open circuit potentials. The results indicate enhanced corrosion of stainless alloys containing chromium may be expected in supercritical water. These corrosion rates appear comparable to those for mild steel or iron. 

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

S. Kasahara, J. Kuniya, K. Moriya, N. Saito, S. Shiga, General Corrosion of Iron, Nickel and Titanium Alloys as Candidate Materials for the Fuel Claddings of the Supercritical-water Cooled Power Reactor, in GENES4/ANP2003, Paper 1132, September 15—19, 2003, Kyoto, Japan. 

G. Was, S. Teysseyre and J. McKinley, Corrosion and Stress Corrosion Cracking of Iron-and Nickel-base Austenitic Alloys in Supercritical Water, Proc. NACE s Annual Conference, Corrosion 2004, New-Orleans, LA, USA, March 28-April 1, 2004, Paper No. 04492 (2004)... 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

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