Factors affecting the testing of stainless steels in boiling concentrated nitric acid

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.

Austenitic and Ferritic Stainless Steels

“…Later investigators [53][54][55] showed that acceleration is due to the action of chromium in the hexavalent state. When chromium dissolves in acids, it enters the solution as divalent, blue ions, which are rapidly converted to the trivalent state, green.…”

Failure prevention by short-time corrosion tests

“…The presence of fluoride ions in the 4M HNO 3 test solution resulted in an increased chromium ion concentration in the solution as shown by the blue/green color of the test solution [4]. In the 14 M HNO 3 test solution, the solution became brown quickly, which was associated with aggressive dissolution of all steel constituents.…”

“…The 4 M solution was noted to have a blue/green coloring, whereas the 14 M solution without fluorides remained clear. The color change is typical for solutions enriched in Cr +3 [4], which would be associated with the corrosion products from the stainless steel.…”

“…For Series 2, samples were exposed for the entire period with out intermittent weight loss measurements. Truman has shown that nitrogen peroxide, a reducing gas product, would be present and lead to lower corrosion losses [4]. With the frequent sample removal during Series 4, these gases could escape and result in higher corrosion rates.…”

Improvement of Transpassive Intergranular Corrosion Resistance of 304 Austenitic Stainless Steel by Thermomechanical Processing for Twin-induced Grain Boundary Engineering

“…11,15,16) Meanwhile, intergranular corrosion possibly occurs for stainless steel due to the preferential dissolution of phosphate and/or phosphorus segregated at grain boundaries under corrosion environment at transpassive potentials. [21][22][23][24][25][26][27] GBE may probably be also effective to suppress the transpassive intergranular corrosion of austenitic stainless steels. The present study aimed to examine the effect of GBE on the transpassive intergranular corrosion of type 304 austenitic stainless steels containing different concentrations of phosphorus.…”