Direct current-sulfuric acid technique

With the direct current-sulfuric acid technique (GS), final hardness values are between 500 and 600 HV. Lower hardness values of 250-400 HV are reached by a special anodizing technique (NS). The latter procedure is preferred in practice, because, contrary to the GS process, it prevents attack on the base metal of pieces that are not completely aluminum plated, such as a hollow form. 

For compounds that nitrate rapidly at room temperature, two methods may be used. The substrate may be dissolved in sulfuric acid of appropriate strength and introduced into one limb of a two-limbed flask. A solution of nitric acid in sulfuric acid is introduced into the other limb, the flask is thermostatted, and the reaction started by vigorously shaking the flask. Aliquots are then withdrawn and quenched as before. However, by far the most convenient method is to carry out the kinetics directly in the thermostatted spectrophotometric cell and this would now be the method of choice in view of the automatic monitoring facilities currently available on spectrophotometers. For very fast reactions, stopped-flow techniques have been used. 

The anodic protection technique now enables air coolers and tube bundles in sulfuric acid plants to be protected from corrosion reliably and economically. Anodic protection was provided for air coolers of sulfuric acid production plants for the first time in 1966. Since then, a combined cooler surface area exceeding 10,000 m in air-cooled and water-cooled sulfuric add plants has been protected in this way worldwide. The installed initial electrical direct current output of the potentiostats is >25 kW, corresponding to an energy requirement of 2.5 W/m for the surface needing protection (Kuron and Grafen 1988). 

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