Your Guide to Corrosion-Resistant Metals
Here's a handy guide to understanding corrosion-resistant metals, including the applications where each is appropriate, how they are used and their relative cost.
There are several different corrosion-resistant metals available to choose from, each with their own advantages and disadvantages for a particular application. Here's a handy guide to corrosion-resistant metals, including the applications where each is appropriate, how they are used and their relative cost.
Stainless steel is perhaps the most commonly thought of metal when mention of a corrosion-resistant material is made. However, many people do not realize what a stainless steel is comprised of, what types are available, the environments it is effective and the environments in which it is less effective.
Stainless steels are carbon steels, typically a low carbon steel, that have higher amounts of chromium. The addition of chromium is part of the reason for their corrosion resistance. Stainless steels also frequently have other corrosion-resistant alloying elements, such as nickel or molybdenum, added to them. Not only do these elements have an effect on corrosion resistance, but they also play a role in the mechanical properties, thermal properties and even the magnetism of a stainless steel. The measure of alloying for resistance to corrosion (MARC) is an equation used to measure the susceptibility of various alloys to corrosion.
Because there are so many different combinations of alloying elements that can be used in stainless steel, there are many different grades and types. Stainless steels can be divided into three main families: austenitic, ferritic and martensitic. While there are other groups, these three are typically considered the most widely used.
Austenitic stainless steels are extremely corrosion-resistant. Like all stainless steels, they are comprised of a carbon steel make up with a high addition of chromium. What makes austenitic stainless steels different than other stainless steels is that they contain high amounts of nickel, roughly 8% of their composition or greater. This nickel acts as an austenite promoter, meaning that it causes the steel to have an austenitic (face-centered cubic) crystalline structure. They are generally the most expensive of the three. Common applications include food preparation equipment, medical devices and process vessels subjected to immersion in water.
Ferritic stainless steels, while still corrosion-resistant, are less so than their austenitic counterparts. They too have high amounts of chromium, but do not have the nickel that austenitic stainless steels possess. This reduces their ability to reduce corrosion, but it does give them an edge with some mechanical properties such as formability. Ferritic stainless steels are also more affordable than austenitic stainless steels. Ferritic stainless steels are commonly used for automotive components such as exhaust systems and for heat exchangers.
Martensitic stainless steels are also generally less corrosion-resistant than austenitic stainless steels. Martensitic stainless steels have high amounts of chromium, sometimes have additions of nickel, and have a higher carbon content than austenitic or ferritic stainless steels. This allows them to be heat treatable and have a much higher strength and hardness than ferritic or austenitic stainless steels. Martensitic stainless steel is used for tooling and cutlery.
As with all of the metals listed in this guide, corrosion-resistant does not mean corrosion-proof; under the right conditions, stainless steel corrosion can occur. (For more about stainless steels, read An Introduction to Stainless Steels.)
Aluminum is another corrosion-resistant metal that is frequently used in applications subject to harsh environments. Aluminum is corrosion resistant because when its surface is exposed to oxygen, it forms an aluminum oxide layer on the aluminum's surface. The aluminum oxide is actually more durable than the aluminum itself, thus protecting the remaining aluminum. Compare this to iron oxide, or rust, which flakes off and allows more iron oxide to form.
Aluminum is frequently used in the aerospace industry for airplane body components. The ability of aluminum to resist corrosion makes it attractive to the aerospace industry because it reduces the amount of overhaul that must be performed due to corrosion. Aluminum is also used extensively in the food and beverage industry, where its corrosion resistance makes it less likely to contaminate food. Other industrial uses include marine, automotive, electrical systems and structures.
Aluminum is generally more expensive than normal carbon steel, but more affordable than some stainless steels. There are many different types of aluminum grades, and some are cheaper than others are. Aluminum is also roughly one-third the weight of stainless steel and carbon steel, and a much better conductor, thus giving it other benefits apart from solely corrosion resistance.
Copper, brass and bronze are also corrosion-resistant metals. Brass is a copper alloy containing a large amount of zinc. Bronze is a copper alloy that includes a large amount of tin and smaller amounts of other alloying elements. Similar to aluminum, copper forms a protective oxide layer when exposed to oxygen. The copper oxide layer protects the underlying base material, thus preventing it from degradation as a result of oxidation.
Copper, brass and bronze are very popular because of their corrosion resistance, and are frequently used for electrical and plumbing equipment and supplies. Their ability to resist corrosion makes them an excellent choice for carrying and transporting liquids. Copper is also used in artwork, structures and industrial products.
Copper and stainless steel are generally more expensive than aluminum. Copper is more ductile than stainless steel and aluminum, thus making it appealing for other reasons besides corrosion resistance, such as formability. Copper also has the highest conductivity of any of the metals mentioned in this article.
Titanium is an extremely useful corrosion-resistant material. Titanium obtains its corrosion resistance in the same way as aluminum and copper — through a passive oxide film that forms on its surface, protecting the remaining interior of the material. As a result, titanium can endure very harsh conditions for long periods of time. (The limitations of titanium is discussed in 5 Things to Know and Understand About Titanium Corrosion.)
Many industries rely on titanium to enable their components to outlast harsh, potentially corrosive conditions. Aerospace is perhaps the most abundant user of titanium, with common instances of titanium use including structural components and engine parts. Since titanium doesn’t tarnish quickly, it is also popular in the jewelry industry. Titanium is often used for marine applications because salt water is very corrosive. Titanium is nontoxic in the human body, so it is also frequently used to manufacture medical devices.
Titanium use is so prevalent due to several advantages it holds over other materials. First and foremost is its corrosion resistance. The passive titanium oxide layer that forms on its exterior makes it advantageous for use in corrosive environments. Also noteworthy is its strength-to-density ratio, which is one of the highest of any metal. Although having only about 60% the density of steel, it still has incredible strength. This makes it very useful for applications that require lightweight components placed under high loads. However, titanium is generally the most expensive metal highlighted in this article.