Exchange current density stainless steel

Fig. 10.34 Anodic protection of stainless steel by additions of noble metals which form local cathodes for hydrogen evolution. The alloying element (palladium or copper) has a high exchange-current density for hydrogen evolution which effectively raises Ecorr from an active to a passive region on the anodic curve of the metal. Note the shift in the corrosion potential to more positive values due to the passivation which occurs upon alloying.

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. 

Figure 15.4 Macrocell current density exchanged between a corroding bar of carbon steel in 3% chloride-contaminated concrete and a (parallel) passive bar of carbon steel in chloride-free concrete, 316L stainless steel in...

Roll cell. These are sandwich constructions consisting of a packed stainless steel or titanium mesh cathode, separator and a screen anode rolled up like a swiss roll. These cells can operate with fluid velocities of 1-10 cm s and with apparent current densities of 10-200 mA cm at the separator. This type of cell is a concentrator device for metal ions, the metal is recovered from the cell by leaching or by anodic dissolution. An economic analysis [23] showed that waste water treatment with this cell is highly competitive with ion-exchange technology. Typical applications for metal ion removal are recovery of copper and Hg from waste stream recovery of Ag from a used fixer solution down to a silver concentration of 0.1 ppm and the treatment of zinc cyanide plating bath rinse waters which contain the Zn(CN)5 complex ion. 

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