metal alloy selection guide

Corrosion Resistant Metal Alloy Selection Guide

Metal alloy selection is easy when you understanding what properties matter to your application. The most common properties considered include corrosion resistance, strength, weight, machinability and formability. When it comes to metal materials selection, corrosion-resistant metals are widely used in a variety of applications ranging from the culinary industry to aerospace. They provide structural strength and heat resistance like most metals, but the ones on this list also resist corrosion better than others. This article will cover some of the more common corrosion-resistant metals and alloys.

1. Stainless Steel

Stainless steel is a material that has been around for over a century. It was originally used in the World War I era as a means of preventing corrosion from water and other elements. Today, stainless steel is used in many medical and industrial applications. The specific chemical resistance varies with the chemistry of the metal. Stainless steels are iron alloys with a minimum of about 10.5% chromium that make it resistant to corrosion (stainless steel rusts only under very specific conditions). Of the tens of thousands of types of steel in the world, most fall into one of the following groups: carbon steels, low-alloy steels, high-alloy steels, tool steels, and stainless steels. The latter two types are what we call specialty steels. They have characteristics allowing them to be used in highly specialized applications.

Stainless steels are used extensively in modern manufacturing because they not only provide required properties (e.g., strength and toughness), but also deliver properties that go well beyond standard requirements. Stainless steels are typically used for their corrosion resistance properties, which far exceed those of standard alloy and carbon steel grades. There are different grades of stainless steel for different applications. For example, there is a grade of stainless steel that is used for making surgical instruments and another grade of stainless steel that is used for making knives. There are also many other grades of stainless steel that have various uses.

Based on its microstructure, stainless steel can be divided into three major categories:

Austenitic Stainless Steel: This alloy family is also known as 300-series stainless steel. 304 and 316 stainless steel are common grades, with 316 being the most corrosion-resistant. The elemental composition contains approximately 18% chromium and 8% nickel, depending on the grade. In addition, trace amounts of nitrogen and manganese are present. The most common corrosion-resistant metal on the market is austenitic stainless steel.

Martensitic Stainless Steel: This alloy family is also known as 400-series stainless steel, and the most common grade is 420A. This steel alloy contains 18% chromium and no nickel, which also contains more carbon, making it more challenging than other corrosion-resistant metals in the stainless steel family. This alloy, however, is not as corrosion-resistant as steel from the 300 series.

Ferritic Stainless Steel: This alloy is also part of the stainless steel 400 series. 430A is one of the most common grades. On the other hand, Ferritic stainless steel has a much higher chromium percentage (up to 27 percent) but a lower carbon content.

Duplex Stainless Steel

Duplex steel is a type of steel alloy with two distinct phases, hence the name. Ferritic and austenitic phases are common. Duplex steels combine the best properties of both phases to create an advanced corrosion-resistant metal that can be used in the most demanding applications. Typical grades include 2205, which contains 22% chromium, 5% nickel, and 3% molybdenum, and S32750, which contains 25% chromium, 7% nickel, and 4% molybdenum.

2. Superalloys

Superalloys, or high performance alloys, are designed to deliver outstanding mechanical strength and creep resistance at elevated temperatures. Superalloys are mixtures of nickel, iron, cobalt and other metals that can be strengthened by solid-solution hardening, precipitation hardening and work hardening methods.

Superalloy are specially formulated to provide excellent mechanical and corrosion-resistance properties at high temperatures. They are typically used between 500°C and 1200°C, and offer higher strength than standard austenitic stainless steels, but at a significantly higher cost. Common uses include components for furnace applications, aircraft engine parts, nuclear reactors, chemical plant equipment and pressure vessels. As a result, these corrosion-resistant metal grades are frequently used in aerospace and energy applications. The main matrix element distinguishes superalloys.

Nickel Superalloy: Nickel-based alloys such as Hastelloy are commonly used in chemical processing environments because they resist pitting and crevice corrosion and can tolerate chloride-contaminated fluids. The majority of nickel alloys are non-magnetic and have good thermal conductivity properties. These materials also have excellent weldability. It is the most common superalloy and is generally less expensive than cobalt versions.

Cobalt Superalloy: Compared to other superalloys, cobalt superalloys have a higher melting point and thus superior hot corrosion resistance.

Iron Superalloy: Iron superalloys have high strength at room temperature and high corrosion resistance. They are also significantly less expensive than the other two types of superalloys.

3. Aluminum Alloys

Aluminum alloys are widely used in engineering structures and components where light weight or corrosion resistance is required. Aluminum alloys are among the most popular materials selected for a wide range of products, including aircraft components, automotive systems, and architectural applications. Aluminum’s unique characteristics—lightweight, corrosion resistance, high thermal and electrical conductivity, low density, and natural ease of fabrication—make it the material of choice for a wide range of applications in industries ranging from transportation to packaging to building and construction.. Aluminum is commonly alloyed with copper, manganese, silicon, magnesium and zinc. This results in a number of different grades that offer a variety of properties specific to the application.

Commonly used aluminum alloy families include:

1xxx – Pure Aluminium Series: 1000 series is 99 percent pure aluminum and has the best corrosion-resistant properties in regular applications.

2xxx – Aluminum Copper Alloy. 2000 series are alloyed with copper, can be precipitation hardened to strengths comparable to steel. Formerly referred to as duralumin, they were once the most common aerospace alloys, but were susceptible to stress corrosion cracking and are increasingly replaced by 7000 series in new designs.

3xxx – Aluminum Manganese Alloy. 3000 series are alloyed with manganese, and can be strain hardening.

4xxx – Aluminum Silicon Alloy. 4000 series are alloyed with silicon. Variations of aluminium-silicon alloys intended for casting (and therefore not included in 4000 series) are also known as silumin.

5xxx – Aluminum Magnesium Alloy. 5000 series are alloyed with magnesium, and offer superb corrosion resistance, making them suitable for marine applications. Also, 5083 alloy has the highest strength of not heat-treated alloys. Most 5000 series alloys include manganese as well.

6xxx – Aluminum Magnesium and Silicon Alloy. 6000 series are alloyed with magnesium and silicon. They are easy to machine, are weldable, and can be precipitation hardened, but not to the high strengths that 2000 and 7000 can reach. 6061 and 6063 alloy is two of the most commonly used general-purpose aluminium alloys.

4. Copper Alloys

Copper-based alloys have been used for thousands of years and today are found in many diverse applications. While copper is the base metal, other metals such as iron, nickel, and chromium can be added to create a wide range of alloys that exhibit superior corrosion resistance in comparison to plain copper alloys.

Pure copper (Cu) is rarely used for engineering applications due to its relatively poor mechanical properties. Instead, copper-based alloys are utilized due to their unique combination of properties including superior strength, hardness, good electrical conductivity, high thermal conductivity and resistance to corrosion. Common copper-based alloys include:

Bronze – Bronze is an alloy consisting primarily of copper with the addition of other ingredients such as aluminum or silicon. This alloy contains 8 to 12 percent tin along with other metals such as aluminum or manganese. Bronze was one of the first alloys ever created and has been used by humans for tools and weapons since ancient times. Bronze is typically harder than brass while having similar ductility, malleability and machinability. Bronze alloys are often used in thruster propellers due to their excellent wear resistance in seawater environments. 

Brass – Brass is a metal composed primarily of copper and zinc. The color of brass varies from a dark reddish brown to a light silvery yellow depending on the amount of zinc present; the more zinc, the lighter the color. Brass is stronger and harder than copper, but not as strong or hard as steel. It is easy to form into various shapes, a good conductor of heat (but a poor conductor of electricity), and resistant to corrosion by water and most acids. There are two basic types of brass alloys: casting brass and wrought brass. Casting brass is used in applications such as gears, bushings, valves and plumbing fixtures. Wrought brass is used in applications such as screws, nuts, bolts, electrical components and springs.

5. Titanium Alloys

Titanium is a strong metal with low density that is quite ductile (especially in an oxygen-free environment), lustrous, and metallic-white in color. The relatively high melting point (more than 1,650 °C or 3,000 °F) makes it useful as a refractory metal. It is paramagnetic and has fairly low electrical and thermal conductivity. Commercial grades of titanium have ultimate tensile strength of about 434 MPa (63,000 psi), equal to that of common, low-grade steel alloys, but are less dense.

Titanium alloys are metals that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness. They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, medical devices, connecting rods on expensive sports cars and some premium sports equipment and consumer electronics.

Which metal is suitable for my application?

This is a very common question we receive from customers. However, there is no definitive answer to this question. There are many factors that influence the choice of a specific metal for your application.The first thing to consider when choosing a metal for an application is the required strength. 

In order to make the right choice, you need to consider the following points:

  • What is the design of your application?
  • What is the function of your application?
  • What type of forces will be acting on the metal? (Pulling forces, pushing forces, etc.)

When choosing a metal for a particular application it is important to consider cost and availability in addition to properties. For example, aluminium is suitable for many applications because it has good strength-to-weight ratio but this comes at an increased cost. If you need to make a structure that will withstand high winds, then high tensile strength and low weight (such as aluminium) may be appropriate. If the structure will be exposed to harsh environments, such as strong acids or seawater, then corrosion resistance may be important (such as stainless steel).  A similar requirement could potentially be met by steel at a lower cost. In addition, some metals are more readily available in specific sizes or grades than others.

Considerations when choosing a metal:

Cost: price per unit volume of material including purchase price and processing costs

Availability: ease of sourcing material in required sizes and grades

Strength: ability of the material to support load without failure; may include stiffness (Young’s modulus), yield stress/strength

If you need a specific material for your application, please let us know. LEADRP could offer many different materials and surface finishes options.

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