corrosion resistant metals

4 Types of Metal That Are Corrosion Resistant or Don’t Rust

We usually think of rust as the orange-brown flakes that form on an exposed steel surface when iron molecules in the metal react with oxygen in the presence of water to produce iron oxides. Metals may also react in the presence of acids or harsh industrial chemicals. If nothing stops the corrosion, flakes of rust will continue to break off, exposing the metal to further corrosion until it disintegrates.

Not all metals contain iron, but they can corrode or tarnish in other oxidizing reactions. To prevent oxidation and breakdown of metal products, such as handrails, tanks, appliances, roofing or siding, you can choose metals that are “rust-proof” or more accurately, “corrosion-proof.” Four basic types of metals fall into this category:

  • Stainless steel
  • Aluminum metal
  • Copper, bronze or brass
  • Galvanized steel

Stainless Steel

Stainless steel types, such as 304 or 316, are a mix of elements, and most contain some amount of iron, which easily oxidizes to form rust. But many stainless steel alloys also contain a high percentage of chromium – at least 18 percent – which is even more reactive than iron. The chromium oxidizes quickly to form a protective layer of chromium oxide on the metal surface. This oxide layer resists corrosion, while at the same time prevents oxygen from reaching the underlying steel. Other elements in the alloy, such as nickel and molybdenum, add to its rust-resistance.

Aluminum metal

Many aircraft are made from aluminum, as are car and bike parts. This is due to its light weight, but also to its resistance to corrosion. Aluminum alloys contain almost no iron and without iron, the metal can’t actually rust, but it does oxidize. When the alloy is exposed to water, a film of aluminum oxide forms quickly on the surface. The hard oxide layer is quite resistant to further corrosion and protects the underlying metal.

Copper, Bronze and Brass

These three metals contain little or no iron, and so do not rust, but they can react with oxygen. Copper oxidizes over time to form a green patina, which actually protects the metal from further corrosion. Bronze is a mixture of copper and tin, along with small amounts of other elements, and is naturally much more resistant to corrosion than copper. Brass is an alloy of copper, zinc, and other elements, which also resists corrosion.

Galvanized Steel

Galvanized steel takes a long time to rust, but it will eventually rust. This type is carbon steel that has been galvanized, or coated, with a thin layer of zinc. The zinc acts as a barrier preventing oxygen and water from reaching the steel, so that it is corrosion protected. Even if the zinc coating is scratched off, it continues to protect nearby areas of the underlying steel through cathodic protection, as well as by forming a protective coating of zinc oxide. Like aluminum, zinc is highly reactive to oxygen in the presence of moisture, and the coating prevents the iron in the steel from further oxidation.

Industrial Metal Supply carries a wide range of rust-resistant metals for a variety of applications.


What is Galvanized Steel?

Galvanized steel is coated with zinc oxide to prevent rusting, since the chemical compound takes far longer to corrode than steel. It also changes the steel’s appearance, giving it a rugged look that some people prefer. All in all, galvanization makes the steel stronger and harder to scratch, so it’s ideal for outdoor use.

How does galvanization work?

Typically, manufacturers will dip the steel into molten zinc, which bonds itself to the steel like any other alloy would. This makes zinc more than just a protective coating, because it’s actually becoming part of the steel’s chemical composition. The interior may be steel, and the exterior may be zinc, but in between are gradient mixtures of steel and zinc that combine both metals’ properties. This dipping process is the most common form of galvanization, called hot-dipped galvanization. It’s also possible to spray zinc onto the steel, but this method creates a weaker layer of zinc.

Advantages

  • Rust Resistance: Iron in steel is incredibly prone to rusting, but zinc acts as a protective buffer between moisture, oxygen, and the steel.
  • Easy Inspection: It’s fairly simple to tell how strong a galvanized coating is, just by looking at it. There are also quick stress tests that can tell you how thick the zinc is.
  • Sacrificial Anode: This ensures that any damaged steel is protected by the surrounding zinc coating. It doesn’t matter if the steel section is completely exposed; the zinc will still corrode first.
  • Longer Life: With galvanization, a piece of industrial steel is expected to last more than 50 years in average environments, and can last over 20 years with severe water exposure. No maintenance required.

Disadvantages

  • Impractical to Dip Certain Items: Some steel pieces are too tiny or too huge to be hot-dipped, and it doesn’t make sense to galvanize them using other methods.
  • Zinc Can Be Temperamental: It’s important to take enough time to let galvanized steel cool down and settle in, so that the zinc doesn’t peel off. Galvanization isn’t nearly as effective if the zinc is not binding to the steel. The right coating thickness must be applied.