Creep, Corrosion and Heat Resistant Nickel Alloys

Superalloys, and Creep, Corrosion, Heat Resistant Nickel and Cobalt Alloys – Grades

Creep, Corrosion and Heat Resistant Nickel Alloys

Why is this category so broad?

The alloys presented here are difficult to divide into further subcategories, due to the unique properties of nickel. Its alloys are highly universal and usually have more than one application.

For example, it’s hard to classify those alloys as either resistant to corrosion resulting from an aggressive environment or resistant to increased oxidation at higher temperatures. Chemical industry exploits both of the above unique properties in nickel-based fixtures and fittings.

Similarly, there are no strict criteria, allowing us to classify these alloys as either heat-resistant or creep-resistant.

In result, some alloys are sometimes classified into different categories, while in fact, they are very similar.

Of course, particular alloys are often designed to meet specific needs.

For example, Hastelloy N can be considered a corrosion-resistant alloy, a heat-resistant alloy or a high-performance, creep-resistant superalloy. However, it has a distinctive feature, an excellent resistance to hot fluoride salts.

A kind of distinctive category is superalloys, with their most important feature, high creep-resistance.

All alloys described here are creep-resistant, resistant to aqueous corrosion and/or high-temperature corrosion.

Creep, Corrosion, and Heat Resistant Nickel Alloys – Applications

Nickel alloys listed in this category are used, among others, in the following industries:

• In power industry: blades, nozzles, steam turbines superheaters
• In automotive: turbo-chargers, ignition plugs, and piston engines exhaust valves
• In metal processing plants: furnace muffles, conveyor belts and other heat treatment equipment
• In aviation: elements of gas turbines, including nozzles, combustion chambers, exhausts, blades, discs, afterburners, and thrust reversers
• In dental medicine: dentures and dental tool
• In cosmonautics: rocket engine components and covers
• In chemical, petrochemical and papermaking industries: pipes, pumps, tanks, fans, fittings, and fixtures

Alloy additions for heat-resistant and creep-resistant nickel alloys

Nickel alloys differ significantly in composition. This group includes commercially pure nickels as well, as complex alloys made of 10 and more alloying additions.

How individual additions may affect nickel alloy?

• Titan – Stabilizes γ phase. Forms intermetallic phase γ’, but also causes to form carbides, which are unstable when held at a high temperature. Age-hardening component.
• Tantalum - Stabilizes γ phase. Forms intermetallic phase γ’, but also causes to form carbides, which are unstable when held at a high temperature.
• Iron – It’s cheaper than nickel and can replace it to some degree, even in creep-resistant alloys, but it causes heat-resistance to decrease. It can serve as a controlled thermal expansion addition. Has nearly complete solubility with nickel.
• Cobalt – Improves creep-resistance, positively affects γ solid solution. Reduces the solubility in aluminum and titanium matrix. It can serve as a controlled thermal expansion addition.
• Chromium – Strengthens matrix, improves creep and heat resistance, by enhancing mechanical properties and improving oxidation and sulfidation resistance. At low concentrations, it can improve halogens or high-temperature halides resistance.
• Aluminium – Improves oxidation and sulfidation resistance and forms strengthening intermetallic phase γ’ (often in tandem with titanium). Age-hardening component.
• Copper – Improves sulfuric acid, as well as reducing acids and salts resistance. Has complete solubility with nickel.
• Molybdenum – Strengthens solid solution. Improves reducing acids resistance and pitting corrosion resistance. On the downsides, it may reduce oxidation resistance.
• Wolfram - Strengthens solid solution. Improves pitting corrosion resistance.
• Niobium – Strengthens solution, positively affects creep-resistance, improves pitting corrosion resistance. Age-hardening component. Forms intermetallic phase γ’, but also causes to form carbides, which are unstable when held at a high temperature.
• Vanadium – Forms intermetallic phase γ’, but also causes to form carbides, which are unstable when held at a high temperature.
• Boron – At low concentrations, it improves creep-resistance.
• Hafnium - At low concentrations, it improves creep-resistance and reduces high-temperature brittleness.
• Zirconium – At low concentrations, it improves creep-resistance and reduces high-temperature brittleness.
• Cerium – At low concentrations, it improves heat-resistance, by reducing high-temperature oxidation.
• Sulfur – Improves machinability
• Silicon – Improves oxidation and carburization resistance
• Nitrogen – Improves pitting resistance. It can serve as an austenitizer.

Nickel has an extensive solubility with other elements. For example, it has complete solubility with copper, and very good with iron. It can also dissolve notable amounts of chromium, molybdenum, and .wolfram, and many alloys contain those elements. It can also dissolve smaller amounts of aluminium, titanium, manganese, and vanadium.

Superalloys

SUPERALLOYS are nickel-, iron-nickel-, and cobalt-base alloys produced for high creep-resistance.

Creep-resistant alloys keep very good mechanical properties in high temperatures. Superalloys are supposed to work under stress, in severe conditions, at temperatures above approximately 540 °C.

Some superalloys can work at temperatures exceeding 800 °C. Max operating temperature does not exceed 1100 °C. Special protective coatings improve superalloys’ performance, allowing further growth of temperature.

Properties can be controlled by adjustments in composition and by processing (including heat treatment), and excellent elevated-temperature strengths are available in finished products.

Basic features of superalloys:

• Perfect creep resistance in high temperatures
• Good strength
• Good heat resistance (resistance to corrosion due to oxidation and sulfidation in high temperatures)

Why are these features so important?

Superalloys are supposed to keep their strength and corrosion resistance in the presence of hot aggressive chemicals. More on this subject in the tab „superalloys – applications”

Superalloys – applications

Applications of superalloys continue to expand, but at lower rates than in previous decades. Although developed for high-temperature use, they are used also at cryogenic temperatures and at body temperature. Aerospace usage remains the predominant application on a volume basis.

Superalloys – applications:

• In power industry: blades, nozzles, gas turbines, steam turbines superheaters, nuclear reactor components
• In aviation: elements of gas turbines, including nozzles, combustion chambers, exhausts, blades, discs, afterburners, and thrust reversers
• In medicine: orthopedic and dental prostheses
• In cosmonautics: rocket engine components and covers
• In chemical, petrochemical and papermaking industries: fittings and fixtures

It's worth discussing the typically severe work conditions of superalloys. It helps to appreciate these extraordinary technological achievements. Superalloys are supposed to withstand:

• Mechanical Loads
• High-temperature
• Aggressive chemicals

It should be noted, that these factors occur all at once, which intensifies their impact even more. High-temperature softens metal and accelerates corrosion. Aggressive chemicals further deteriorates mechanical properties. Corrosion creates oxides on surfaces of superalloys, weakens, and eventually destroys an attacked element.

As mentioned, gas turbines are one of the main and most demanding applications of superalloys. What accelerates corrosion? How to prevent it? More on this topic in the tab „Corrosion in gas turbines.”

Corrosion of gas turbines

Gas turbines are an inseparable part of turbojets, turbochargers and more. Corrosion is the main threat to these devices.

Gas turbines are affected by two basic types of corrosion:

• Hot corrosion
• High-temperature oxidation

What's the difference?

High-temperature oxidation is oxidation accelerated by high temperature. Hot corrosion is oxidation and sulfidation, which occurs in lower temperatures and in presence of aggressive chemicals.

High-temperature corrosion of superalloys

Without other aggressive chemicals present and at temperatures reaching approximately 880 °C, superalloys generally retain high oxidation resistance. At higher temperatures, they are attacked by oxygen. Suitable alloy additions (most notably chromium and aluminium) may increase oxidation resistance above 1000 °C. The higher the temperature, the more important is aluminium, which forms an important protective oxide Al2O3. Chromium also forms a protective oxide Cr2O3 and reduces the amount of aluminium required to form an Al2O3 oxide.

However, aluminium level of many superalloys is insufficient to provide long-term oxidation resistance. Protective coatings are applied to cope with this problem.

Hot corrosion of superalloys

Hot corrosion results from hot vapors and gases. It is the most aggressive method of degradation of metals, alloys, and ceramics, especially when they operate at very high-temperatures. Oxygen mixed sulfur's compounds is responsible for most cases of hot corrosion.

Hot corrosion and reactive chemicals

Hot corrosion may occur in lower-temperature operating conditions, far below 880 °C and that's through the operation of selective fluxing agents.

In a gas turbine, for example, sodium chloride from the air reacts with sulfur from the fuel to form sodium sulfate. The sulfate then comes to contact with hot sections like the rotor blades, which in turn results in accelerated oxidation attack. This type of corrosion may initiate at very low temperatures, as low, as 620 °C.

Na2SO4 is one of the main corrosion factors. Although the sulfur in the fuel on commercial jet engines and the marine gas turbines is very low in percentage, it is capable of corroding a good portion of the metal surface.

Chromium plays an important part in counteracting hot corrosion. When a superalloy is exposed to it, first corrodes the outer layer of oxides. This continues until the outer layer run out of chromium, which results in a rapid acceleration of oxidation. A dense layer of scale forms on the surface of an attacked element.

Titanium and aluminium also counteract gas corrosion. More on this topic in the tab alloying additions.

Use of protective coatings proved to be an effective countermeasure, reducing corrosion and increasing strength in high-temperature. More on this topic in the tab protective coatings.

Alloy base and strength

Most of iron-nickel based superalloys are strengthened by intermetallic compound precipitation in an fcc matrix.

Similarly, most important nickel-base superalloys are strengthened by intermetallic compound precipitation in an matrix. For nickel-titanium/aluminum alloys the strengthening precipitate is γ ′.

The cobalt-base superalloys Are strengthened by a combination of carbides and solid-solution hardeners.

Mechanical properties and corrosion resistance

Higher chrome, titanium and aluminium ratio improves the resistance of uncoated superalloys on oxidation and sulfidation. However, optimizing an alloy's composition for corrosion resistance may reduce its strength.

Therefore, it is often the best solution to cover durable superalloys with protective layers. More on this topic in the tab "protective covers."

Superalloys classes by base metal

Superalloys may be divided into three categories by their base metal:

What's the difference?

First, they have different temperature resistance. The melting temperatures of the pure elements are: nickel 1453 °C, iron 1537 °C, cobalt 1495 °C. Generally, cobalt superalloys may operate in higher temperatures, than the others, but some advanced nickel superalloys outperform cobalt superalloys in this respect.

Nickel- and iron-nickel-base superalloys are notably less weldable than cobalt-base superalloys.

• Iron-Nickel-Base superalloys
• Nickel-Base superalloys
• Cobalt-Base superalloys

All superalloys are relatively ductile, although the ductilities of cobalt-base superalloys generally are less than those of iron-nickel- and nickel-base superalloys.

High-temperature oxidation resistance does not always go hand in hand with resistance to other types of corrosion. For example, some nickel-base superalloys with high aluminium and fairly low chromium content are strong and resistant to high temperature oxidation, but very poor in hot corrosion resistance.

Furthermore, optimizing an alloy's composition for corrosion resistance may reduce it's strength.

Various protective coatings proved to be the solution:

TBC protects against high-temperature

TBC's (Thermal barrier coatings), are ceramics. They effectively insulate superalloys, which allows them to operate in much higher temperatures, more than 150 °C above their customary level, in case of some TBC's.

Thermal barrier coatings do not protect against oxidation, thus they are often used together with one of below coatings:

Corrosion resistant coatings

There are two types of corrosion resistant coatings:

• Diffusion coatings - It's the most common type of protective coatings. High oxidation-resistant aluminide outer layer (usually CoAl or NiAl) is diffused on the element.
• Overlay Coatings - McrAlY Overlay Coatings are thicker than diffusion coatings and can be tailored to balance for optimal performance in given circumstances. More on this topic in the tab ”corrosion”.

Overlay Protective Coatings by composition:

• FeCrAlY – Iron, Chromium, Aluminum, Yttrium
• NiCrAlY – Nickel, Chromium, Aluminum, Yttrium
• NiCoCrAlY – Nickel, Cobalt, Chromium, Aluminium, Yttrium (more Nickel)
• CoNiCrAlY – Cobalt, Nickel, Chromium, Aluminium, Yttrium (more Cobalt)

Overlay Coatings' pros - Overlay coatings can be tailored to cope with the anticipated corrosive environment. Therefore, superalloys with better mechanical properties, or simply cheaper may be used. In addition, The coatings can be applied to renew worn surfaces, which of superalloy parts.