What Are Corrosion Resistant Alloys?

Corrosion Resistant Alloys?

A Note From Our Metallurgist

Corrosion-resistant alloys are metals engineered to resist degradation by oxidation or other chemical reactions. The most common corrosion resistant metals, used for mild to moderate corrosion resistance, are stainless steels, often referred to as rust resistant steel.

Key Takeaways

CRAs are metals engineered to resist degradation from oxidation or chemical reactions.
Stainless steels are common CRAs, containing at least 10.5% chromium to prevent rust in typical conditions.
Nickel Base Alloys contain more nickel than iron and are used in extremely corrosive conditions. Specialty metals like titanium, molybdenum, zirconium, and tantalum base alloys are also used.

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What Is Corrosion Resistant Metals?

Corrosion-resistant steel is a type of steel that protects against corrosion, making it essentially rust resistant steel. Stainless steels are iron-based alloys containing a minimum of 10.5% chromium, which is sufficient to prevent rust under typical room-temperature atmospheric conditions. These are some of the most widely used corrosion resistant materials for oil and gas along with various industries.

Types of Corrosion-Resistant Alloys

Ferritic Stainless Steels

Stainless steels simply alloyed with Chromium, such as Type 430, are referred to as Ferritic Stainless Steels. These are a type of rust resistant steel. This family of alloys can’t be strengthened by heat treatment, however, with the addition of carbon and other elements, they become Martensitic Stainless Steels.

Martensitic Stainless Steels

The most common Martensitic Stainless Steels, Types 410 or 13 Chrome, are strengthened by quench and temper heat treatment. There is also a family of Precipitation Hardened Martensitic Stainless Steels which includes the widely used Type 17-4. Martensitic Stainless Steels may also contain additions of Nickel and Molybdenum for improved corrosion resistance.

Austenitic Stainless Steels

With sufficient Nickel, Austenitic Stainless Steels, such as types 304 and 316, are formed. These steels are known for being highly rust resistant steel. Highly alloyed Austenitic Stainless Steels include Types 28 Chrome and 2535, widely used in oil & gas production. Most Austenitic Stainless steels are not heat treatable, however, they can be cold worked to achieve high strengths. An exception to this is the Precipitation Hardened Austenitic Stainless Steel, Type A286.

Duplex Stainless Steels

Duplex Stainless Steels are formed with a balance of Chromium, Nickel, and Molybdenum between that of Ferritic and Austenitic Stainless Steels, named so because their microstructure is a mix of Ferrite and Austenite. These alloys may be cold worked to achieve very high strength, and are most commonly used where pitting or crevice corrosion is a problem, such as environments with water high in chlorides or dissolved oxygen.

Super Duplex Stainless Steels

The most highly alloyed of this family are referred to as Super Duplex Stainless Steels . In addition to the Chromium, Nickel, and Molybdenum found in all Duplex Stainless Steels, Super Duplex Stainless steels may include alloying elements such as Copper and Tungsten to improve corrosion resistance for specific environments.

Nickel Base Alloys

Alloys containing more Nickel than Iron are considered Nickel Base Alloys. This group of alloys includes Types 825, 625, and 2550, which may be cold worked to achieve high strength. Precipitation Hardened Nickel Base Alloys include Types 718 and 925. Nickel Base Alloys are included in a class of materials referred to as specialty metals. Used in extremely corrosive conditions, these specialty metals also include titanium, molybdenum, zirconium, and tantalum base alloys.

Frequently Asked Questions

What is the minimum chromium content required for stainless steel?

Stainless steels are iron-based alloys that require a minimum of 10.5% chromium. This chromium content is what gives stainless steel its corrosion-resistant properties, forming a passive oxide layer that protects the material from rust under typical atmospheric conditions.

What are corrosion-resistant alloys (CRAs)?

Corrosion-resistant alloys (CRAs) are metallic materials specifically engineered to withstand degradation caused by chemical reactions with their environment. Unlike standard carbon steel, CRAs maintain their structural integrity in harsh conditions involving moisture, acids, high temperatures, and corrosive gases. They are widely used in demanding industries such as oil and gas, chemical processing, aerospace, and marine applications where material failure due to corrosion can lead to costly downtime or safety risks.

What is the difference between austenitic and martensitic stainless steel?

The primary difference lies in how each type is strengthened. Martensitic stainless steels contain chromium along with carbon and other elements, and can be hardened through quench and temper heat treatment. Common examples include Types 410 and 13 Chrome. Austenitic stainless steels, on the other hand, are formed with sufficient nickel content and are generally not heat treatable, instead, they achieve high strength through cold working. They tend to offer superior corrosion resistance and include common grades like Types 304, 316, 28 Chrome, and 2535.

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*While every effort has been made to ensure the accuracy of the above review, assessment, conclusions, and report, the appropriateness of their application and their interpretation remain the sole responsibility of the user.

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