Metals are notorious for rusting. We have all seen that thick, flaky layer of rust on old shipwrecks and other metal relics. However, there are a host of metals that do not rust – read on to find out what they are and why they do not rust.
What is Rust?
In technical fields, rust is better known as corrosion. “Rust” refers specifically to corroded iron, which is iron oxide. When other metals rust, we call it oxidation or corrosion. Here, we focus on traditional iron oxide rust.
Under favorable conditions, iron combines with oxygen to form iron(III) oxide, better known as rust. Here, a chemical reaction known as oxidation takes place in the presence of water, which acts as a catalyst. The presence of chloride speeds up this reaction, hence the high rate of rust near the ocean.
Since this oxide is less dense than the parent metal, it forms a flaky, orange-brown layer on the metal’s surface. When rusting occurs in the wrong place, this could have a severely adverse effect on the metal’s structural integrity. The oxidation reaction depletes the ions in the parent metal, explaining why rusted metals become thin and worn over time.
Unfortunately, rust affects most forms of iron and its alloys, except for some grades of stainless steel.
List of Metals That Don’t Rust
1. Stainless Steel
Steel is highly corrosive. Adding 11% or more chromium to this carbon-iron alloy renders it relatively rust-resistant. Chromium is more reactive than iron and thus oxidizes preferentially. Here, a thin (5 nanometer) layer of chromium oxide forms on the metal surface, protecting it from further corrosion. This protection works in two ways: firstly, the chromium oxide is chemically inert, preventing further reaction. Secondly, this physical barrier prevents access to the metal underneath.
Adding nickel, manganese, and molybdenum to this alloy will further enhance the corrosion resistance.
Chromium oxide formation happens only in the presence of oxygen. In the absence of oxygen, the protective chromium oxide layer will not form, and thus the steel would still be susceptible to corrosion when exposed to other corrosive materials. Partial covering of the stainless steel’s surface could also lead to the formation of oxygen-depleted areas, resulting in localized corrosion or rusting.
Note that only some grades of stainless steel are actually corrosion resistant. If the proportion of chromium included in the alloy is too low, the steel will still rust at a rate similar to that of ordinary steel. An absolute minimum of 10.5% chromium must be present in the allow for adequate rust protection.
2. Alloy Steel
Often, manufacturers alloy steel with a variety of other elements, such as chromium, copper, nickel, and phosphorous. This alloying improves mechanical and chemical properties and often lowers costs. A 21% portion of these alloying elements forms COR-TEN steel, also known as weathering steel.
As with stainless steel, the chromium present in the alloy reacts with oxygen, forming a thin layer, or patina, on the metal’s surface. This patina protects the metal from further corrosion.
3. Galvanized Steel
Galvanizing, or electroplating, steel means covering it in a thin layer of zinc. This thin layer keeps water and oxygen away from the underlying steel. Since iron is less reactive than zinc, zinc reacts to oxygen preferentially, thus acting as a sacrificial layer. Over time, this zinc layer is depleted, exposing the underlying steel to corrosion, hence the limited useful lifespan of galvanized steel.
While aluminum does not rust, it can still corrode. Aluminum reacts readily and rapidly with water, forming the aluminum oxide. This oxidation product forms a layer on the metal’s surface, effectively protecting it against further corrosion since aluminum oxide is extremely dense and does not allow more water to penetrate to the aluminum layer underneath. So, unless a scratch damages this surface layer, allowing water (even the little bit of atmospheric water) access to the underlying aluminum, your metal is safe from further corrosion.
Aluminum alloys usually incorporate copper, magnesium, manganese, silicon, tin, and zinc. Since there is no iron in aluminum alloys, they cannot technically rust.
5. Copper & Copper Alloys (Red Metals)
Copper, known as a red metal along with all its alloys, does not rust since it contains no iron.
Over time, it does corrode in the presence of oxygen. This oxidation reaction creates a bright green layer on the copper’s surface, usually called tarnish.
Brass does not contain iron, and thus cannot technically rust. Some grades of brass contain trace amounts of iron, but this is so small that it does not affect the metal’s chemical properties.
When exposed to oxygen for long periods, brass forms a blue-green layer of tarnish, better known as patina. Technically, this is a corrosion product. Luckily, it is easy to remove this patina layer with a bit of brass polish, although some people prefer to leave it there to give the brass an aged look.
Bronze is a copper alloy typically containing around 12% tin. Some grades of bronze contain various other metals and even non-metals, such as arsenic, phosphorus, and silicon. When treated well, bronze is highly resistant to corrosion. Since bronze does not contain any iron, it cannot rust.
Bronze oxidation is usually only superficial, forming a thin layer of copper oxide. Over time, this oxide reacts further to form copper carbonate, which forms a protective layer over the parent metal. This copper carbonate layer protected historic bronze coins over centuries, creating a treasure trove for archaeologists to find.
Any form of chloride will, however, annihilate bronze through a series of corrosion reactions known as “bronze disease.”
Titanium is the strongest metal known to man and also incredibly light, at only 45% of the density of steel. When in contact with oxygen, a thin layer of titanium dioxide forms on the metal’s surface. This thin white layer is inert and thus protects the underlying metal from further corrosion.
7. Precious Metals (Gold, Silver, Platinum)
Gold is a highly non-reactive pure metal. In pure form, gold will not tarnish or corrode at all. When it contains a small portion of base metals, these impurities react with oxygen or sulfur to form a dark discoloration, called tarnish.
Silver does not contain iron, and thus cannot rust. It also does not oxidize easily, but it does react with sulfur and sulfides, forming silver sulfide. Silver sulfide forms a dark tarnished layer on silver. Interestingly, silver is the best known electrical conductor, with an even higher conductivity than copper.
When used in its pure form, platinum will not corrode, tarnish, or change color. Its high density and malleability render it quite strong.
Since platinum does not contain any iron, it cannot rust.
Marine Grade Metals
Seawater is notorious for causing corrosion in metals. The high concentration of chloride in seawater creates an environment that annihilates most metals. For this reason, manufacturers take special care when building ships and other types of equipment used in or near the ocean. Most metal alloys have a select marine-grade, specially designed to withstand the corrosive effects of seawater.
Stainless steel, specifically 216 steel, is used in most marine applications. Alloyed with molybdenum, it is highly resistant to rust and pitting corrosion, compared to other grades of steel.
Aluminum is an excellent marine grade metal since it is lightweight and highly corrosion-resistant. The hulls of combat ships often incorporate aluminum, where lightweight, durable, and corrosion-resistant materials are essential.
Copper alloys, specifically those containing nickel and beryllium, are widely used in marine applications, along with brass and bronze. An alloy of bronze and aluminum is especially famous for its high corrosion resistance. In addition to being highly corrosion-resistant, copper alloys have the added benefit of being highly resistant to biofouling. Using these alloys eliminates the need for expensive antifouling coatings and water treatment.
While rust technically only affects iron and its alloys, metal corrosion is common and widespread. The above collection of metals that do not rust are used in a variety of different applications, ranging from industrial to naval and decorative, where their corrosion-resistant properties are a marked benefit. Armed with this knowledge, we trust that you will look at metals and their applications in a new way.