When boiling 65% nitric acid was first proposed in 1930 for evaluation testing of stainless steels by W. R. Huey (1), a glass flask fitted with a reflux condenser via a ground glass joint was specified; i.e., the apparatus was similar to the Allihn arrangement of Fig. 1. Five 48-hour periods, each with fresh acid solution, were proposed. Later, the less costly cold finger condenser was introduced at many laboratories. Because of the large number of acceptance tests required by Du Pont, W. B. DeLong (2) developed a large multi-sample apparatus in 1949 in which as many as 75 specimens can be tested simultaneously. The design of DeLong's multi-sample apparatus was based on the recognition that chromium corrosion products increase the rate of attack on the stainless steel test specimens. To prevent accumulation of chromium corrosion products, the design provided for a large inflow of "clean" nitric acid condensate into the specimen chamber. As a result, there is a continuous overflow of test solution and consequent removal of corrosion products into the nitric acid solution surrounding the large cup in which the specimens are held. Because the concentration of the 65% nitric acid solution is near that, 68.5%, of the constant boiling composition, the changes in acid concentration in the testing cup and in the acid outside the cup are small. In 1954, J. E. Truman (3) showed that the chromium ion corrosion product is oxidized in the nitric acid solution from the divalent to the trivalent and then the hexavalent state. Each of these valence states has a different color, Table I. Only when chromium is in the hexavalent state does it accelerate corrosion of stainless steels. There is an increase in grain-face corrosion and significant intergranular attack, even on solution-annealed material (4). J. E. Truman (3) also reported differences in corrosion rates in tests made in flasks fitted with Allihn-type condensers and a type of cold finger, semi-closed system. He attributed the lower rate of corrosion in the latter system to the retention of nitrogen peroxide (NO2) corrosion product, which acted as a reducing agent, tending to keep chromium in the trivalent state.
