OXIDATION - RESISTANT ALLOYS

11.13 OXIDATION - RESISTANT ALLOYS

11.13.1 Reactive Element Effect ( REE )

The reactive element effect (REE) is obtained when 1 wt.% or less of a reactive element, such as yttrium, hafnium, lanthanum, zirconium, or cerium, is added to

OXIDATION-RESISTANT ALLOYS

high - temperature alloys containing chromium or aluminum. The effect can be obtained when the reactive element is added as an alloy addition or when a coating of the reactive element oxide is applied to the surface of the alloy. The useful life of high - temperature alloys is typically limited by spalling of the oxide from the metal surface. Reactive elements greatly improve the spalling resistance of both chromia - forming and alumina - forming alloys. Commercial chromia -

forming heater alloys now all contain small amounts of one or more reactive elements, and yttrium has become a commonly used reactive element addition to high - temperature alloys [55, 56] .

Applying a 4 - nm - thick coating of ceria (CeO 2 ) or yttria (Y 2 O 3 ) has been found to increase the oxidation resistance of chromium and iron – chromium alloys. By using secondary ion mass spectrometry (SIMS) to establish the location of the reactive element in the oxide scale, the reactive element was found in the middle of the oxide scale, suggesting that inward oxygen transport in chromium oxide is the dominant transport mechanism. Addition of the reactive element inhibits the diffusion of Cr and results in growth primarily by the inward diffusion of oxygen [56, 57] .

11.13.2 Chromium–Iron Alloys

The 4 – 9% Cr alloys are widely used for oxidation resistance in oil - refi nery construction. The 12% Cr – Fe alloy is used for steam turbine blades because of excellent oxidation resistance and good physical properties. The 9 – 30% Cr alloys are used for furnace parts and burners; when combined with silicon and nickel, and sometimes other alloying elements, they are used for valves of internal - combustion engines. The approximate upper temperature limits for exposure to air are presented in Table 11.4 .

Addition of 1% yttrium to a 25% Cr – Fe alloy was reported to extend the upper limit of oxidation resistance to about 1375 ° C (2500 ° F) [58, 59] . Rare - earth metal additions, such as yttrium, in general, are benefi cial to oxidation resistance of chromium and chromium alloys, including gas turbine alloys [60] .

T A B L E 11.4. Approximate Upper Temperature Limits for Exposure of Cr – Fe Alloys to Air

% Cr in Cr – Fe Maximum Temperature in Air Alloys

°F 4–6

°C

OXIDATION

11.13.3 Chromium–Aluminum–Iron Alloys

The chromium – aluminum – iron alloys have exceptional oxidation resistance, combining the oxidation - resistant properties imparted by chromium and alumi- num; for example, the 30% Cr, 5% Al, 0.5% Si alloy (Trade name: Megapyr) resists oxidation in air up to at least 1300 ° C (2375 ° F). Similarly, the 24% Cr, 5.5% Al, 2% Co alloy (Trade name: Kanthal A) is resistant up to 1300 ° C. They are used, among other applications, for furnace windings and parts and for electric - resistance elements. Drawbacks in their properties are poor high - temperature strength and tendency toward embrittlement at room temperature after prolonged heating in air, caused in part by aluminum nitride formation. For this reason, furnace windings must be well - supported and are usually corrugated in order to allow for expansion and contraction.

11.13.4 Nickel and Nickel Alloys

The good oxidation resistance of nickel is improved by adding 20% Cr; this alloy (one U.S. trade name: Nichrome V) is resistant in air to a maximum temperature of about 1150 ° C (2100 ° F). It is a heat - resistant alloy that combines excellent oxidation resistance with good physical properties at both low and elevated temperatures. Oxidation resistance of the commercial alloy is considerably improved by the addition of calcium metal deoxidizer during the melting process, which is said to avoid oxidation of the alloy along grain boundaries. Small amounts of zirconium, thorium, and rare - earth metals (e.g., cerium) are also benefi cial, probably in part because they decrease the tendency of protective oxides to spall. This explanation was also mentioned earlier as applying to the benefi cial effect of the rare - earth yttrium when alloyed with the chromium – iron alloys.

The 16% Cr, 7% Fe, 76% Ni alloy (Trade name: Inconel 600) is slightly less resistant to oxidation than the 20% Cr – Ni alloy, but similarly has excellent physi- cal properties, is readily fabricated or welded, and can be used in air up to a maximum temperature of about 10 ° C (2000 ° F). Electrical heating units for some stoves are fabricated of this alloy in tubular form. An electric heating wire of the 20% Cr – Ni alloy inside the tube is insulated from the outer sheath by powdered MgO. Because of its high nickel content and good strength (nickel does not readily form a carbide or nitride), this alloy is often used for construc- tion of carburizing or nitriding furnaces.

Some heat - resistant thermocouple wires consist of nickel alloys. The 10% Cr – Ni alloy (Chromel P) and 2% Al, 2% Mn, 1% Si, bal. Ni alloy (Alumel) can

be exposed to air at a maximum temperature of about 10 ° C (2000 ° F). Nickel and high - nickel alloys tend to oxidize along grain boundaries when subject to alternate oxidation and reduction. Alloying with chromium reduces this tendency. Also, in contact with sulfur or sulfur atmospheres at elevated tem- peratures, nickel and high - nickel alloys are subject to intergranular attack. Con- sequently, nickel is not usefully resistant to such atmospheres above about 315 ° C

REFERENCES

(600 ° F). For best resistance to sulfur - containing environments, iron - base alloys should contain high chromium and low nickel.

Gas turbine blades are essentially nickel - base or cobalt - base alloys contain- ing substantial amounts of chromium, several percent aluminum, and a few hundredths percent yttrium. Their susceptibility to hot corrosion and sulfi dation has already been discussed. Applied coatings of aluminum or of aluminum – chromium – yttrium increase resistance to attack.

11.13.5 Furnace Windings

Twenty percent chromium – nickel and various chromium – aluminum – iron alloys are in common use as furnace windings. To achieve higher temperatures in air, a 10% Rh – Pt alloy can be used up to at least 1400 ° C (2550 ° F). This alloy performs better than pure platinum because of higher strength and a lower rate of grain growth. A single crystal of the same dimensions as the resistance wire cross section tends to shear easily and cause failure.

Molybdenum - wound furnaces are operated to at least 1500 ° C (2725 ° F). Because molybdenum oxidizes in air, such furnace windings are blanketed in a protective atmosphere of hydrogen.