Super alloys for corrosion prevention in aggressive application media

The alloys containing high carbon content are referred as super alloys that are named as Incoloys, A-286, Inconels, Hastelloys etc. The super alloys offer good high temperature strength and oxidation resistance. The super alloys are based on nickel are widely used materials, as they offer superior services than FeNiCr alloys and less costlier than cobalt based alloys.

For comparison objectives, use of high temperature strength in heat resistant alloys is recommended. Although for design purposes creep or stress rupture data should be used. A design engineer should often find if the component is bounded by crack or extent of deformation. Generally the alloys offering superior stress rupture characteristics offer the excellent creep strengths.

The alloys referred above are wrought and mechanical alloyed types. Mechanically alloyed materials consist of fine dispersion of oxide particles and developed by powder metallurgy methods. Many of powder metallurgy alloys have been substituted to forged alloys and utilized as turbine discs.

However the use of powder metallurgy methods have been implemented to these alloys, development has been basically limited to warm isotatic pressing processes and warm compaction followed by extrusion procedures.

Several of wrought alloys are also fit for investment casting procedures. Moreover, nickel and cobalt base alloys have been made for service as cast materials.

Applications of Superalloys

Incoloy 800	Catalytic cracking tubes, reformer tubes, aqueous attack applications, sulphuric and phosphoric acid conditions, heat exchangers, industrial furnaces, steam producers
Inconel 617	Gas turbines, petrochemical treatment, heat processing unit, nitric acid development
Inconel 718 and X-750	Gas turbines, rocket motors, spacecraft and pumps

Nickel base alloys contribute by 65% in the aerospace engineering. These are widely used in rocket engines offering excellent corrosion resistance, elevated temperature oxidation resistance, maintaining significant characteristics over the large temperature range and in several cases, offer exclusive set of physical characteristics. These alloys are categorized into three groups depending on their applications:

1.       Nickel base alloys utilized because of their outstanding corrosion prevention potential for example Nickel 200, Monel grades, alloy 600, alloy 625 and electroformed nickel. The corrosion resistant nickel is not based on the metastable oxide layer for security and acting as firm electronegative element, it is not sensitive towards galvanic attack when interacts with other metallic materials. Monel alloys offer supreme corrosion resistance, possess significant magnetic characteristics at cryogenic limits and offer supreme resistant to inflammation in oxygen. Inconel grades consisting of nickel, chromium and iron prevent oxidation at temperatures about 1800oF.

2.       Nickel containing super alloys that possess supreme strength at the high temperatures. Inconel 718, the leader of this group offer supreme strength up to 1300oF, supreme cryogenic ductility and excellent welding potential. Fine grained material must be mentioned for components that require to be electron beam welded.

3.       Special purpose materials such as Nichrome, Incoloys and Invar.

The whole of these nickel based superalloys prevent attack and stress corrosion and oxygen computability however are sensitive to conditions containing hydrogen due to embrtillemen at temperatures about -200oF. Hydrogen embrittlement in nickel based alloys can be avoided by discarding plastic strains or by offering a security shield for example electroplating with corrosion resistant alloy.

Special austenitic stainless steels

There are many commercial proprietary heat resistant materials that are a member of austenitic stainless steel group considering nickel and chromium concentrations however with inclusion of silicon offers good resistance to oxidation and other high temperature corrosion attack. For example Incoloy 800H that offers service up to 1093 to 1150oC.

FeCrAl grades

Aluminum is a strong alloying element that enhances resistance to oxidation and other types of corrosion at the elevated point. The alloy needs about 4% aluminium to develop a regular alumina scale. The alumina layer offers outstanding security from the corrosive attack of oxidation. When the alloy is heated up to 1200oC or above, a layer of Cr2O3 is formed that grows gradually and develops volatile CrO3, becomes non-secured. Alumina layer offers supreme protection from oxidation. Due to very small growth rates at low and moderate temperatures, alumina scale offers low security at such limits. So high temperature alloys are made to develop alumina scale for extremely high temperature services also consist of sufficient chromium content to develop chromium oxide layer for moderate temperatures.

Few commercial electrical resistance heating materials are constructed from FeCrAl alloys like heating elements that depend on development of alumina layer for service up to 1400oC. For instance these alloys are made in wire, strip, rod and mesh forms. As these wrought alloy forms are basically ferrite materials, they attain small creep rupture strengths when the temperature limit goes above 650oC or 1200oF and is not feasible for high temperature structural materials. So the heating elements made from such alloys need to be adequately supported to prevent creep deformation for example sagging. The heating wires are used in flame spray or arc to develop an oxidation resistant coating or in weld overlay cladding by using gas metal arc welding process. A powder metallurgy process was utilized to develop a  supreme heating element FeCrAl Cr25Al5 that have excellent creep rupture strengths.

Several more FeCrAl grades are made for use as resistance heating elements such as foil to different temperature limits for 2 minute as long as it failed. The failure occurs when the foil was oxidation penetrated. The use of rare earth elements such as cerium is essential for improvement in alumina scale. A nominal studies have been performed on the suitability of cerium on adhesion of alumina layer. Many more analyses are performed on the influences of yttrium, zirconium and other reactive elements. In the FeCrAl alloy, the rare earth element such as yttrium is included to enhance adhesion of the alumina layer developed on the FeCrAl alloys hence enhancing the oxidation resistance of the alloy. A FeCrAl alloy is reinforced by oxide-dispersion strengthening mechanism to significantly enhance its high temperature strengths by mechanical alloying.

Iron-Nickel-Chromium Alloys

With increase in nickel concentration in the FeNiCr system from austenitic stainless steel grades to iron base alloys, the materials attain more stability such as metallurgical structure and good resistance to creep deformation. Normally these alloys offer superior oxidation prevention. For example wrought Incoloy 800H/800HT that resist corrosion in the prolong oxidizing media.

Ni-Cr/Co-Cr Super alloys

In various Nickel-Chromium alloys, the composition elements for example the solid solution reinforcing elements like molybdenum and tungsten, and precipitation reinforcing elements for example aluminium, titanium and niobium are included in to the alloys to offer reinforcement at the high temperatures. Most of these alloys are preferred as super alloys that involve oxide dispersion strengthened alloys.

Similar to FeCrAl alloys, aluminium acts as a composition element in the Nickel-Chromium alloys to enhance the oxidation resistance. However it usually needs least 4% in the Ni-Cr matrix to develop alumina scale, the inclusion of aluminium enhances oxidation resistance of alloy.

Inconel 601 contains just 1.3% aluminium and offers supreme oxidation resistance. However alloy 601 contains 1.4% aluminium that improves its oxidation resistance, the adherent oxide layers developed on this metal are usually enriched of chromium. But at high temperatures above 1100oC, these oxides become sensitive to failure, receiving scaling, deformation and spalling.

The oxidation resistance can be increased by modifying the concentration of chromium, aluminium or silicon, meanwhile many alloys are developed to offer sustained high temperature strengths by alloying with other elements. A big count of super alloys are developed to meet the challenging needs of gas turbine engines for critical service media including high stress and elevated temperatures. To meet the demands of high stresses at moderate temperatures, a group of wrought super alloys is reinforced by precipitation strengthening with Ni3X precipitates along with solid solution strengthening by using molybdenum or tungsten. These alloys include Inconel 718 and X750. Few applications are gas turbines components such as compressors, diffusers, turbine disks, cases, heat shields, exhaust units, thrust reversers and turbine shroud rings. Many alloys of this category are utilized in the heat processed conditions to get the benefit of precipitation strengthening. Many heat processing methods are followed at the moderate temperature limit. So the applications of these alloys are referred to be in the moderate temperature limits to avoid overaging of the reinforced precipitates. The oxidation of these alloys at moderate limits does not show a major problem in their service.

The alloys containing none or nominal chromium level for example Hastelloy B can only perform in the reducing media. Stainless steel type 304 and 316 offer good corrosion resistance in the oxidizing media. Austenitic stainless steel grade for example type 304 and SS 321 contain borderline limit of chromium content, these are susceptible to chromium concentration for heavy composition and surface composition in the material. When the surface penetration of chromium happens in stainless steel product when excessive chromium concentration is the bay of the specification, cracking oxidation occurs possibly so causing terrible oxidation corrosion.

Most of the oxidation attack is noticed in the form of weight change over the time or temperature. Although, it is possible to use the weigh change information to assess the service life of the component because of oxidation attack. The oxidation analysis is significant for engineering purposes that includes metal loss and depth of internal oxidation corrosion. The overall depth of the oxidation corrosion is responsible for loss in load bearing property of the material.

Nickel and cobalt base alloys comprising of molybdenum or tungsten or both also cannot withstand oxidation at the excessively high temperatures. The samples of nickel base alloys that are used at 1200oC were Hastelloy X and Inconel 625. Few of the nickel base alloys comprising of molybdenum or tungsten or both were not attacked up to 1200oC or 2200oF for example Inconel 617 so it is estimated that nickel base alloys comprising of molybdenum and tungsten can be used for high temperature reinforcing to prevent oxidation highly elevated points by modifying contents of other elements.

In the nickel base alloys comprising of high concentrations of molybdenum and tungsten or both, it is trusted that increasing chromium is certainly the most significant decision to prevent quick oxidation. Titanium is found to be very effective in the development of oxide layer.