The Na2SO4-induced accelerated oxidation of nickel-base alloys containing elements such as Cr, Al, Mo, W, and V has been studied in 1.0 atm O2 in the temperature range of 650° to 1000°C. It has been found that the hot corrosion behavior of these alloys can usually be characterized according to one of two types of attack: 1) Na2SO4-induced accelerated oxidation; 2) Na2SO4-induced catastrophic oxidation. In both types of hot corrosion, accelerated oxidation occurs as a result of the formation of a liquid flux based on Na2SC>4 which dissolves the normally protective oxide scales. Catastrophic, or self-sustaining rapid oxidation can occur in alloys which contain Mo, W, or V, because solutions of oxides of these elements with Na2SO4 decrease the oxide ion activity of the molten salts, producing melts which are acidic fluxes for oxide scales. The accelerated oxidation type of attack which was observed with most alloys which did not contain Mo, W, or V, was more severe than for normal oxidation, but much less severe than catastrophic oxidation. Na2SO4-induced accelerated oxidation occurs because the oxide ion activity of the Na2SO4 increases to the point where oxide scales can partially dissolve in the basic melt. Generally, this basic fluxing results from the diffusion of sulfur from the Na2SO4 into the alloy. In some alloys, the formation of sulfides during basic fluxing is a sufficient condition to cause accelerated oxidation. In other alloys, changes in the oxidation mechanism occur because of depletion of the alloy surface, concomitant with basic fluxing, of those elements needed for protective oxide scales, such as aluminum and chromium.

Mechanisms for the hot corrosion of nickel-base alloys

The Na2SO4-induced accelerated oxidation of nickel has been studied at 1000°C. It has been found that low oxygen activities in the Na2SO4, which are produced by the formation of NiO, cause the sulfur activity of the Na2SO4 to be increased. Nickel and sulfur from the Na2SO4 combine to form nickel sulfide and the oxide ion activity of the Na2SO4 is increased. The accelerated oxidation of nickel occurs because oxide ions react with NiO to form a nonprotective oxide scale. The accelerated oxidation of nickel is not self-sustaining since oxide ions are not produced when conditions in the Na2SO4 are no longer favorable for the formation of nickel sulfide.

Na 2 SO 4 -induced accelerated oxidation (hot corrosion) of nickel

The attack of nickel, cobalt, iron, and alloys of these metals containing chromium and aluminum, by gases containing sulfur-oxygen, carbon-oxygen, and nitrogen-oxygen has been studied at temperatures of 600 and 900°C. The degradation of these metals and alloys was characterized by using standard analytical techniques with emphasis on optical metallography. Three types of accelerated degradation were identified for the attack of alloys by gases containing another oxidant in addition to oxygen. One type of degradation occurred because of the formation of reaction products composed of mixtures of phases involving both of the oxidants. Another type resulted from the reaction of second oxidant phases with oxygen. The third form of degradation involved the development of less protective phases due to thermodynamic instabilities. Thermodynamic stability diagrams are used to help account for the effects produced by different elements in the alloys.

Corrosion of metals and alloys in mixed gas environments at elevated temperatures

Internal oxidation and carburisation of heat-resistant alloys

Scaling reactions between pure metals and multiple oxidant gases are reviewed briefly. It is recognized that elemental oxidant activities are usually so low that the actual reactant species are heteronuclear molecules such as SO2, CO2, etc. The formation of duplex, sulfide-oxide scales on iron and manganese, even when sulfide is unstable with respect to oxide, is attributed to direct reaction with SO2. The persistence of the metastable sulfide is due to its preservation by the rapidly growing scale. The reaction of pure chromium with a number of mixed gases is also discussed. The continued formation of carbides and nitrides beneath an external Cr2O3 scale layer indicates that the latter material is permeable to gas species. Interaction among different gas species is observed, and is attributed to selective adsorption on internal surfaces within the chromium oxide. New work on the reaction of alloys with mixed gases is reported. Several austenitic heat-resistant alloys were exposed at 1000°C to gases containing one, two or all of the oxidants carbon, sulfur and oxygen. Gases containing two or more oxidants produced multiple zones of internal precipitation. The precipitates were chromium-rich oxides, sulfides and carbides arranged in order of thermodynamic stability: oxides beneath the external scale, carbides deepest within the alloys and sulfides in an intermediate zone overlapping the oxide zone. Each precipitate zone widened according to parabolic kinetics. This finding confirms the as yet untested prediction made by J. L. Meijering in 1971. However, the rate at which a particular zone grows changes according to presence of other oxidants. Interactions between the oxidants can be large and reaction rates are currently not predictable.

F.S. Pettit

Papers

Thermodynamic analysis of the simultaneous attack of some metals and alloys by two oxidants