“The element iron is the most abundant transition metal on Earth and has been used by mankind for over 5,000 years. Despite its strength, iron is prone to corrosion in the presence of water and oxygen. This rusting process proves costly in daily life — to the tune of about 1 percent of the value of the world’s economy”, reports Seattle PI which is a metro daily newspaper in Seattle.
In order to understand which metals do not rust, we need to address what the terms “rust” and “corrosion” mean.
What Causes Metal to Rust?
Rust is a form of iron oxide. It occurs when iron combines with the oxygen in the air causing it to corrode. Rust is the orange-brown discoloration that builds up on metal. Rust can affect iron and its alloys, including steel. Whenever you have iron, water and oxygen together, you get rust. The main catalyst for rust to occur is water. Although iron and steel structures seem solid to the eye, water molecules are able to penetrate microscopic gaps in the metal. This starts the process of corrosion. If salt is present, for example in seawater, the corrosion will be more rapid. Exposure to sulfur dioxide and carbon dioxide will also hasten the corrosive process.
Rust causes the metal to expand, which can place great stress on the structure as a whole. At the same time, the metal will be weakened and become brittle and flaky. Rust is permeable to air and water, so the metal beneath the rust layer will continue to corrode.
What is Corrosion? The Difference Between Rust and Corrosion
Some people believe that corrosion and rust are the same. Corrosion is a broad term that covers the destruction of any metal. In general, corrosion is the deterioration of a metal by oxidation or other chemical action. Rust is a term used to describe the actual corrosion of iron and iron alloys.
10 Metals That Don’t Rust
Aluminum is one of the most commonly used metals on the planet, and it’s arguably the most famous for not rusting. Aluminum doesn’t rust, only iron’s specific oxidation is called rust and there is no iron used when creating aluminum. However, like all metals, Aluminum is susceptible to corrosion.
Leland Stanford Junior University explains Aluminum in this manner, “the quick answer is that most aircraft are made of aluminum — a chemical element that seems to resist corrosion even when exposed to air and water. But the fact is that pure aluminum reacts so readily with water that, according to the laws of chemistry, the aluminum shell of an airplane should actually dissolve in the rain. Fortunately for the airline industry, when aluminum metal is placed in the atmosphere, a thin layer, known as aluminum oxide, forms on the metal’s surface and acts like a protective, rust-resistant shield”.
Brass does not rust for the same reason as aluminum. It has a negligible amount of iron in it. Therefore, no iron oxide, or rust, can form. However, copper can form a blue-green patina on its surface when exposed to oxygen over time.
Bronze does not rust for the same reason as aluminum. It has a negligible amount of iron. Therefore, no iron oxide, or rust, can form.
Copper does not rust; however, it can corrode. Copper is naturally brown and turns a shade of bright green as it corrodes. While some consider copper’s reaction to be tarnish rather than oxidation, the metal still undergoes a similar rusting process.
5. Corten or Weathering Steel
Use a weathering steel, also known as “COR-TEN” steel which contains up to 21% of alloying elements such as chromium, copper, nickel, and phosphorous. The alloys form a protective rust patina which reduces the corrosion rate with time. COR-TEN steel tends to be cheaper than stainless steel.
6. Galvanized Steel
Galvanizing is a method of rust prevention. This is accomplished through hot-dip galvanizing or electroplating. The iron or steel object is coated in a thin layer of zinc. This stops oxygen and water reaching the metal underneath but the zinc also acts as a sacrificial metal. Zinc is more reactive than iron, so it oxidizes in preference to the iron object. The zinc oxide layer prevents the formation of iron oxide, thus eliminating the possibility of rust forming.
Gold is a pure metal which doesn’t rust because it doesn’t contain iron. “Gold is the most non-reactive of all metals and is benign in all natural and industrial environments. Gold never reacts with oxygen (one of the most active elements), which means it will not rust or tarnish. Gold tarnish is very thin and shows up as a darkening of reflecting surfaces”, states Corrosion Doctors.
Platinum is a pure metal which doesn’t rust because it doesn’t contain iron. Platinum doesn’t rust, corrode, tarnish, or change color. It’s dense, malleable, (moves easily) and at the same time, very strong.
Silver is a pure metal which doesn’t rust because it doesn’t contain iron. Corrosion Doctors explains that “silver is a brilliant gray white metal that is quite soft and malleable. It is quite resistant to corrosion and does not oxidize easily, although it readily forms a surface tarnish of silver sulfide. Of all the metals, it is the best conductor of electricity. Due to these qualities (and its relative scarcity), it is often classified along with gold and platinum as a precious metal”.
10. Stainless Steel
Rust resistant alloys. Stainless steel is an alloy and contains a minimum of 11% chromium. This allows the formation of a protective film of chromium oxide which acts as a shield against rust. The protective film will re-form if damaged. Corrosion resistance can be further enhanced with the addition of nickel.
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