Precipitation Reactions in Inconel Alloy 600 and Their Effect on Corrosion Behavior

Precipitation reactions that occur in Inconel Alloy 600 were studied and correlated with corrosion behavior in nitric acid and stress corrosion cracking (SCC) susceptibility in polythionic acid and sodium hydroxide. It was found that exposure of the alloy to times and temperatures near the boundaries of the precipitation region on the TTT diagram results in continuous grain boundary carbides with continuous chromium depleted areas near the boundaries. Such a sensitized condition leads to high corrosion rates in nitric acid. Exposure of the alloy to conditions well within the precipitation region results in intragranular carbide precipitation, restructuring of grain boundary films into discrete particles, and replenishment of chromium depleted areas. The alloy is then considerably less susceptible to intergranular corrosion in nitric acid. The sensitized condition can be either beneficial or detrimental to the alloy's resistance to SCC, depending on the environment involved. If the environment tends to cause true SCC and does not attack chromium depleted areas, the continuous grain boundary films of sensitized material may block the progress of a crack. However, if the environment corrodes chromium depleted areas, cracking could be accelerated by grain boundary films and consequent chromium depletion.