An Aspect of the Internal Corrosion of DEFA Cases

摘要

The internal corrosion of zinc-plated steel DEFA cases was due to reaction between the zinc coating and internally derived nitric and formic acids in the vapour phase. These gave the corrosion products basic zinc nitrate and zinc formate. The predominance of formate in the outermost layers of the corrosion product and of nitrate in the lower layers suggests that corrosion processes changed at one stage. This was presumably associated with changes in the relative amounts of the two corrodents with time. These observations are in conformity with the products formed during the thermal decomposition of nitrocellulose. These products include nitrogen dioxide, water, and formaldehyde, where nitrogen dioxide is observed in the early stages of the reaction but disappears as the reaction proceeds. Formaldehyde, on the other hand, appears to build-up to a steady concentration. The corrodent nitric acid is explained by the presence of the products nitrogen dioxide and water, while formic acid could arise from the oxidation of formaldehyde by nitrogen dioxide. (Author)