Which Metals/Alloys are Oxidation Resistant?
In the several industrial applications, a metal is exposed to high temperature air which results into their oxidation. In real, many metals burn when they are in hot state such as magnesium and titanium burn in the standard cases which cause severe fire in the processing units. Iron also catches fire.
For a metal to become oxidation resistant, two ways are possible:
- It is inert in nature and doesn’t chemically interact with oxygen present in air. For example platinum and gold. Due to extremely high melting point about 3217oF or 1769oC and oxidation resistance, Platinum is recommended for use in the lab equipments and other materials that withstand widely high temperature limits.
- Oxidation resistance. It is possible when a metal/alloy creates a secured oxide layer that avoids its further reaction with oxygen. Chromium is commonly used to produce this type of oxide layer (Cr2O3), called as chromia.
Capability of Chromium oxide layer to prevent oxidation
However chromium is extremely sensitive to oxidation and quickly oxides to produce fine layer which tightly joins with metal. This type of layer is rapidly formed at the elevated temperature however once it is produced, the layer secures the metal from additional oxidation. This layer also secures the alloy from carburization and sulfidation to some extent. But it is not an ideal scale. It contains flaws by which oxygen and other reactive agents may contact the alloy surface. Further thermal and mechanical strains also destroy the layer. For an elevated temperature alloy to offer good oxidation resistance the scale must be capable to recover itself by chromium diffusion to surface to produce a new layer.
Other elements also improve the alloy’s features to enhance the security nature of the oxide layer such as silicon. Silicon oxidizes to silica or SiO2. The availability of sufficient silicon content produces a sub-layer beneath the chromium oxide layer. The presence of silicon oxide offers carburization resistance and also enhances prevention of oxidation.
The suitability of chromium oxide layer may be further enhanced by nominal inclusions of rare earth elements for example cerium. It accelerates the production of thinner layer that is more secured against oxidation because it is more cracking resistant than thicker layers.
Aluminum has also a specific role in enhancing the oxidation resistance. A protective layer of alumina (Al2O3) only develops if the significant magnitude of aluminum is present. In Inconel 601, 1.4% aluminum is present that is sufficient to improve the oxidation resistance, it is recommended for use as Inconel 601 plate. On the other hand, Haynes 214 has 4.5% Al that makes it an outstanding oxidation resistant alloy for temperatures higher than 982oC or 1800of even it is better than alloy 601.
Factors that adversely affect the security oxide layer
Frequent heating and quenching of a metal or alloy causes extension and contraction in it that results into change in size of oxide layer at the variable rates. Higher frequency of heating and cooling results into more damage of the protective layer.
Many alloys offer tightly adhering oxide layers at the different temperature limits. Some alloys are resistant to oxidation at the specific temperatures but increasing the limits, the protection degree reduces significantly. For example SS 321 offers sufficient performance up to 870oC or 1600of but at 980oC or 1800of, its service is inadequate, on the other hand SS 309 is suitable for use up to 1040oC or 1900of.
Mechanical stress and creeping, for example extension of bar under applied load, also affects the oxide layer’s strength. For a ductile metal, the oxide layer is delicate and it spalls off.
The service media has also a main role in accelerating the damage of security layer by a chemical reaction. Till now we have seen several examples when the heat processed components were initially coated with ammonium chloride. This chloride caused a chemical reaction with the security oxide layer of the furnace components.
Another case of chemical damage is corrosion by welding fluxes. Fluoride bearing fluxes made of coated welding electrodes should be cautiously and completely eliminated. Otherwise the corrosion is not only of oxidation type but carburization also occurs.
When a metal or alloy is subjected to the oxidizing media, a secured oxide scale is produced as stated above. In the extremely reducing environments, nickel and other mildly stable oxides are reduced to produce pure metal that disappear soon. However chromium oxide is more stable and it doesn’t reduce.
It is a type of oxidation that occurs quickly that a rapid metal damage occurs in the small length of time. Specific elements including molybdenum, columbium, vanadium and tungsten produce oxides that are unstable at low temperature limits. The production of such oxides causes the damage of the security layer.
Metal Thickness: Thin materials catch fire more quickly than thick materials. Because thin parts have lower content of chromium to reproduce adherent oxide layer.
Grain Size: With the loss of security oxide scale, chromium diffusion with metal surface occurs to recovers the layer. The diffusion rate of chromium is greater along the grain boundaries than it is across the grain. Small grain size enhances the scale potential to recover the loss.
Recommended Oxidation resistant Metals/Alloys
To find the advanced and competitive alloys, a laboratory oxidation test is conducted at the temperature limits of 1232oC or 2250oC. The weight gain is total magnitude of oxygen or nitrogen that has combined with the test samples. The samples are generally plate gages and cyclic tests are conducted. The specimens are heated for 160 hours at the given temperature and then allowed to cool down naturally to room temperature. Among all metals Stainless Steel 330 and 333 grades offered good results. Similarly SS 309 is only partial of heat resistant alloys that generally offer rarely poor service, normally at 1040oC or higher.
It is considered that for alloys performing adequately in a test, they will also perform good during service. A simple coupon test does not simulate the entire factors that can happen in an application. Hence it is feasible for an alloy to offer good service in lab however not overall satisfactory in the production system.
There are numerous conditions used in the high temperature systems such as:
- Heat cycling: It is usually referred by weekly cycling of a sample to room temperature. Quick cycling refers more scale spalling, improving oxidation rates for few alloys than others. For instance, in constant 1000 hour oxidation analysis, Stainless steel 310 offers greater oxidation resistance than Stainless steel 330. However by inclusion of heat cycling, Steel grade 330 better sustains its security scale.
- Creep Strain: It cannot be made completely practical in the laboratory. The creep strain and heat cycling significantly increase the damage of specimen layer.
- Stagnant Environments: there is mild or no environments in specific areas of the electrically heated systems and below insulation or solid accumulations. Molybdenum enriched alloys attain catastrophic oxidation in these conditions, however they may offer better performance than in the open air testing.
- Environments beside of dry air: water magnitude of atmosphere affects oxidation rates. Large water content increases the metal damage in the family of low of nickel alloys more quickly than high nickel alloys.
The test is conducted for four months only so it cannot be related to the practical operations where the processes last regularly for one to two or ten years. However our test results can be taken as reference to rate the alloy’s performance after a specific time duration. The samples regularly change their composition during the test. Fine samples possessing low magnitude of chromium may exhibit greater oxidation rates as compare to thick samples.
Among alloys used such as SS 304, SS310, 353 Ma, 601 and 602, Stainless steel 330 shows better performance in retaining its security layer. Here it is essential to mention that high nickel based alloys offer superior oxidation resistance than their low nickel counterparts.
Oxidation resistant Alloys’ Properties and Applications
Inconel 625– High strength, prevents pitting and crevice attack, stress corrosion cracking (SCC) and fatigue.
Applications: Chemical processing, waste processing, marine water treatment, nuclear treatment, oil and gas units
Hastelloy C-276- Prevention of pitting and crevice attack and wide range of severe corrosive conditions
Applications: chemical and petrochemical units, waste treatment, paper mills
Inconel 600: Resistant to high temperature oxidation, SCC due to chlorine gas, nitridation and carburization.
Applications: nuclear plants, chemical processing, food processing, paper mills and furnace parts.
Inconel 601: Extremely high oxidation resistance, good mechanical characteristics and carburization resistance.
Applications: Furnace parts, conveyor belts, petrochemical plants
Inconel 617: Elevated temperature strength up to 950oC, outstanding resistance to extensive range of elevated temperature corrosive media.
Applications: combustion cans, transition liners, nitric acid preparation and catalyst support.
Inconel 718: Resistance to corrosion, it has great creeping strength at the elevated temperatures.
Applications: Nuclear engineering, gas turbines, cryogenic industry, bolts, oil extraction units.
Incoloy 825: High corrosion and oxidation resistance, great tensile and creep rupturing characteristics at the high temperatures.
Applications: Fasteners, bolts, springs, nuclear engineering and gas turbines
Inconel A-286: Supreme mechanical characteristics and resistance to corrosion at the elevated temperature.
Applications: fasteners, bolts, springs for high temperature application, suitable stainless characteristics and large yield strength at room temperature.
SS 330: Carburization and oxidation resistance
Applications: Furnace parts, conveyor belts and catalytic equipments.
Monel 400: Extensive corrosion resistance in the sea water, hydrofluoric acid and sulfuric acid and alkaline media.
Applications: fasteners, marine engineering, chemical processing, sea water processing.