Elevated Temperature Oxidation Resistance of 310S Stainless Steel

Metallic alloys will react with their surroundings to some degree under most conditions. The most common reaction is oxidation – metallic elements combining with oxygen to form oxides. Stainless steel are resistant to oxidation through selective oxidation of chromium, which forms a slow-growing, very stable oxide (Cr2O3 or chromia).

Given enough chromium in the underlying alloy, a compact and adherent surface layer of chromium oxide is established which prevents the formation of other, faster growing oxides and serves as a barrier to further degradation. The rate of oxidation is controlled by transport of charged species through the external chromia scale. As the surface scale thickness, the rate of oxidation decreases dramatically because the charged species have to travel farther. This process, the high temperature analogue of passivation during corrosion at low temperature, is known as protective scale formation.

The oxidation resistance of austenitic stainless steel can be approximated by the chromium content of the alloy. True heat resistant alloys generally contain at least 20% (by weight) chromium. Replacing iron with nickel also generally improves an alloy's high temperature behavior. Alloys 309/309S and 310/310S are highly alloyed materials and are, therefore, very resistant to oxidation.

An oxidized metal sample will increase in weight corresponding to the amount of oxygen incorporated into the scale and any internal oxidation. Measuring the change in weight of a sample which has been exposed at high temperature for a set period of time is one way to determine the oxidation resistance of an alloy. Greater weight gain typically indicate more severe oxidation.

Oxidation is more complex than simple scale thickening. Spallation, or the detachment of the surface oxide scale, is the most common problem encountered during the oxidation of stainless steel. Spallation is typically manifested by rapidly accelerating weight loss. A number of factors can cause spallation, chief among them thermal cycling, mechanical damage, and excessive oxide thickness.

During oxidation, chromium is tied up in the scale in the form of chromium oxide. When the oxide scale spalls off, fresh metal is exposed and the local rate of oxidation temporarily increases as new chromium oxide forms. Given sufficient scale spallation, enough chromium may be lost to cause the underlying alloy to lose its heat resistant properties. The result is the formation of rapidly growing oxides of iron and nickel, known as breakaway oxidation.

Very high temperature oxidation can lead to scale volatilization. The surface chromium oxide scale formed on heat resistant stainless steel is primarily Cr2O3. At higher temperatures, the tendency is for further oxidation to CrO3, which has a very high vapor pressure. The rate of oxidation is then split into two parts – scale thickening by formation of Cr2O3 and the thinning effect of CrO3 evaporation. The tendency is for eventual balance between growth and thinning with the scale remaining at a constant thickness. The result is continuous recession of the surface and consumption of the metal beneath. The effect of scale volatilization becomes a significant problem at temperatures above approximately 2000°F (1093°C) and is exacerbated by rapidly flowing gases.

SA 213 TP 310S