No abstract

No abstract

No abstract

The rates of corrosion obtained when testing stainless steels in concentrated boiling nitric acid can be very markedly affected by the method of testing. The main source of error is the contamination of the test acid by the dissolved steel. If the acid can be maintained in an essentially pure state, corrosion is linear with time, but, if corrosion products are allowed to accumulate, acceleration of attack occurs, and thus the volume of acid used for testing has a serious effect on the rates of attack obtained. The prime cause of this acceleration is the chromium dissolved in the acid. This affects the rate of corrosion only if it is in the sexivalent state, and the degree of acceleration is roughly proportional to the amount of dissolved chromium in this condition. Chromium can be oxidized from the terto the sexi‐valent state by boiling concentrated nitric acid, but the extent to which this reaction proceeds is affected by several factors, which can consequently have a marked effect on the rate of corrosion of the steel. These include the type of condenser used and the strength of the test acid. Cyclic tests, in which the acid is changed at regular intervals, are not necessarily the answer to errors introduced by this process, as the mechanism of attack in sexivalent chromium‐bearing nitric acid is such that the stainless steel is left in a condition susceptible to heavier attack, even in pure nitric acid, and this can lead to earlier serious contamination of the acid used in later cycles. Unless these factors are taken into account when designing testing procedures, less satisfactory materials may be selected, since the steels most resistant to pure nitric acid are not necessarily those most resistant to nitric acid contaminated with sexivalent chromium.

No abstract

No abstract

No abstract

No abstract