SYMPOSIA PAPER Published: 01 January 1950

The Formation of Sigma and Its Influence on the Behavior of Stabilized 18 per cent Chromium — 8 per cent Nickel Steels in Concentrated Nitric Acid


It was found that the presence of sigma phase, rather than chromium carbide precipitation, is responsible for the low corrosion resistance of titanium-stabilized austenitic stainless steels in boiling 65 per cent nitric acid, when tested after a “sensitizing” treatment. The factors influencing the formation of sigma phase were studied, and it was found that its quantity could be minimized by rather close control of the aluminum content and of the chromium and nickel contents. A slight increase in the aluminum content increases the amount of delta ferrite to a moderate degree, but markedly increases the amount of sigma phase transformed from the ferrite present when the steel is reheated for 2 hr. at 1250 F. Data is presented to show the relation between these factors and corrosion resistance in boiling 65 per cent nitric acid of steels annealed at 1950 F. and “sensitized” 2 hr. at 1250 F. A study of the effect of annealing temperature indicates that as the temperature is increased from 1800 F. to 2300 F. the amount of sigma formed on “sensitizing” is increased and the corrosion resistance is decreased. It was found that a precipitation treatment of 4 hr. at 1700 F. to 1800 F. markedly improves the resistance to nitric acid resulting from a high-temperature anneal (2100 F. for example) by reducing the amount of sigma formed on “sensitizing.” It is shown that commercial type 321 steels can be made to yield acceptably low corrosion rates in the Huey test by means of such a precipitation before “sensitizing” and testing.

Author Information

Stewart, Raymond, S.
National Lead Co., Niagara Falls, N. Y.
Urban, Stephen, F.
National Lead Co., Niagara Falls, N. Y.
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Developed by Committee: E04
Pages: 128–145
DOI: 10.1520/STP46109S
ISBN-EB: 978-0-8031-6741-4
ISBN-13: 978-0-8031-6506-9