Types of Metals That Don’t Rust

Introduction

Metals are ubiquitous in modern industry and daily life, but many metals are prone to rust when exposed to moisture or oxygen-containing environments, affecting their appearance and service life. However, not all metals are troubled by rust. Some metals are naturally resistant to oxidation and corrosion due to their unique chemical properties, making them ideal for durable applications.

This article will explore the reasons why metals rust, as well as several metals that are not easy to rust, including their characteristics, common uses, and why they can resist rust. HYT is confident that after reading this article, you can choose the right anti-rust metal material for your project.

If you are still hesitant about choosing metal materials for your project, you are also welcome to contact HYT’s solution team directly. We can provide you with professional metal material solutions based on your project needs.

Why Metals Rust

When we talk about the rust problem of metals, we are actually referring specifically to the rust problem of ferrous metals (for the distinction and characteristics of ferrous and non-ferrous metals, please refer to this article). Ferrous metals mainly refer to iron and various iron-based alloys, while the more common iron-based alloys are cast iron and steel. Their common feature is that they all have a high iron content.

When ferrous metals are exposed to oxygen and moisture, the iron content of the ferrous metals undergoes oxidation reaction, and the main product is hydrated iron oxide (Fe₂O₃·nH₂O), which is usually a loose substance with a reddish brown or orange color, which is also commonly known as rust. Although steel structures appear strong to the naked eye, water molecules can penetrate tiny gaps in the metal. This triggers the corrosion process. If salt is present, such as seawater, corrosion will be faster. Exposure to sulfur dioxide and carbon dioxide will also accelerate the corrosion process.

Rust on steel will have many negative effects on the material itself. Rust not only reduces the strength and toughness of steel, reducing its load-bearing capacity, but also forms corrosion pits on the surface, causing stress concentration and accelerating the generation of fatigue cracks. In addition, rust will destroy the flatness and dimensional accuracy of the steel surface, affecting the matching performance of mechanical parts. The rust layer will also hinder the adhesion of anti-corrosion coatings, further exacerbating corrosion. In a humid or corrosive environment, rust may continue to expand, leading to structural failure or loss of function, and increasing maintenance and replacement costs.

What is Corrosion?

People often don’t pay much attention to the difference between metal rust and metal corrosion. In fact, metal rust is a type of metal corrosion. Metal corrosion is a broad term that refers to the process in which all metals are destroyed by chemical or electrochemical reactions, including oxidation, dissolution, etc. Metal corrosion is mainly divided into chemical corrosion (dry corrosion) and electrochemical corrosion (wet corrosion) according to the corrosion mechanism. The rust problem of iron and iron alloys is a type of electrochemical corrosion.

Types of Metals That Don’t Rust

Dealing with the rust problem of iron and iron alloys is a time-consuming, labor-intensive and costly task. Therefore, the best solution is to choose a suitable surface treatment process or use metals that will not rust. Although metal rust mainly occurs on ferrous metals, there are also some types of ferrous metals that are not easy to rust. Many non-ferrous metals also have good corrosion resistance.
Common metals that won’t rust include:

Ferrous Metals and Surface Treatments:

Corten or Weathering Steel
Stainless Steel
Galvanized Steel

Nonferrous Metals:

Aluminum
Copper
Bronze
Brass
Gold, Silver, Platinum
These metals are chosen not only because they resist rust, but also because they are durable, easy to maintain, and can withstand harsh environmental conditions. Choosing the right metal can extend the life of a structure or product and reduce the long-term costs associated with repair, replacement, or maintenance.

Corten or Weathering Steel

Weathering steel, usually we also call it COR-TEN steel or corten steel. COR-TEN refers to the two significant characteristics of this steel, namely corrosion resistance (COR) and tensile strength (TEN). Weathering steel has a variety of alloy components, such as copper, chromium, manganese, chromium, etc., among which the most important alloy component is copper.

In the early stage of material use, when weathering steel is exposed to air and moisture, it will still oxidize and rust like ordinary carbon steel. However, as weathering steel continues to be exposed to various environmental cycles, a stable and dense protective oxide layer, namely patina, will form on the surface of the material. Patina prevents moisture and oxygen in the environment from continuing to penetrate into the steel, thus slowing down the rusting of the material.

In addition, the formation of weather steel’s Patina can be accelerated by pre-rusting treatment of weather steel in the factory to reduce the impact of environmental factors on the steel after the material is put into use.

Stainless Steel

Stainless steel is a very common material and also an alloy steel material. The alloy composition of stainless steel includes at least 11% chromium. The chromium element in stainless steel has higher metal activity and is more likely to react with oxygen than iron. Therefore, before iron comes into contact with air, chromium will react with oxygen first to form a stable and dense passive film on the surface of the material. The main component of this film is Cr₂O₃. Chromium oxide can prevent oxygen, moisture, and chloride ions from further penetrating and corroding the material.

Chromium oxide has a certain self-repair ability in most conventional corrosion environments. However, in some more severe working conditions, such as high-chlorine environments, the chromium oxide on the surface of some stainless steel materials will still be destroyed, causing the base material to rust. At this time, we need to choose stainless steel materials with higher chromium content, such as 316, 321, duplex stainless steel 2205, etc., which have alloy elements such as molybdenum and nickel added to their composition. These new alloy elements make these stainless steel materials have stronger rust resistance.
Stainless steel materials are widely used in many high-demand industries such as energy, ocean, and drilling because of their high biological cleanliness, strength, corrosion resistance, and structural strength.

If you are in demand of  corrosion-resistant and high-strength stainless steel pipes, welcome to visit our stainless steel pipe product page for solutions and more customized options.

Galvanized Steel

Galvanized steel is a steel with a special coating on the surface. By plating a layer of zinc on the surface of steel or iron, the rust resistance of metal products can be improved.

Galvanized steel is a type of steel with a special coating on the surface. By coating the surface of steel or iron with a layer of zinc, the rust resistance of metal products can be improved.

Zinc has a higher metal activity than iron. In a humid or corrosive environment, the zinc layer on the surface will be oxidized first, forming a protective layer. The main component of the protective layer is ZnO or ZnCO₃. This protective layer can prevent steel and iron from being corroded by oxygen and moisture. This mechanism is called sacrificial anode protection.

In comparison, although copper, nickel and other metals can also be used as coating materials, their metal activity is lower than that of iron. Once the coating is damaged, iron will react with oxygen before copper, nickel and other materials. The base material still faces the problem of rust, and the cost of these metal coatings is relatively high.

Even if the zinc layer is damaged, the remaining zinc will still react with oxygen first to produce a protective layer, thereby preventing the material from rusting, so the rust prevention time is longer. Therefore, considering factors such as cost and use environment, galvanizing is a very cost-effective and widely used anti-rust surface treatment process.

Galvanized steel is mainly produced by hot-dip galvanizing (HDG) and electrogalvanizing (EG).

The main difference between these two galvanizing processes is that hot-dip galvanizing directly immerses the material in molten zinc liquid (over 450°C) to form a thicker zinc layer, so it has strong rust resistance and can ensure long-term rust resistance, which is suitable for harsh outdoor environment applications;

Electrogalvanizing forms a thin but uniform zinc layer on the metal surface through electrolytic deposition. Although this zinc layer is relatively thin and has poor rust resistance, it is relatively smooth and has a higher aesthetics, so it is suitable for the production of some small precision parts.

Under the same unit area, the cost of hot-dip galvanizing process is lower than that of electrogalvanizing, so it is more suitable for large-scale production.

Aluminum

Aluminum is the third most abundant element after oxygen and silicon. It is a metal element with very abundant reserves and wide applications. Aluminum does not contain iron, and in most cases, iron is not involved in its smelting and processing process, so aluminum will not rust. In addition, aluminum will react with oxygen to form a thin anti-corrosion layer, that is, aluminum oxide, which can protect the metal from corrosion to a certain extent. Unlike rust, which will fall off the base material, aluminum oxide will not. Aluminum oxide can protect the base material from corrosion by environmental factors for a long time.

Aluminum does not rust and has excellent corrosion resistance and strength-to-weight ratio, so many aircraft and automobile parts are made of aluminum. However, aluminum has a low tensile strength, so it is generally alloyed with other elements (such as copper, magnesium, silicon and manganese) to improve the mechanical properties of the material.

Red Metals: Brass, Copper, Bronze

Red metals refer to Brass, Copper, Bronze. The main chemical composition of red metals contains almost no iron, so they will not rust.

Copper

Copper is a natural metal and one of the few metals that exists directly in nature in metallic form. Copper does not contain iron, so it will not rust naturally, but it will gradually oxidize in the air. In the early stages of copper oxidation, the color of the copper surface will darken from pinkish-orange color to a brownish black or dark brown color. This is mainly because the initial products of copper oxidation are copper oxide and cuprous oxide, which are red and black respectively. They can protect the copper matrix material from corrosion and will not fall off from the matrix material. Over time, bronze will further oxidize with oxygen to produce Cu₂(OH)₂CO₃, which is green or blue-green, which is known as verdigris, so we often see a green film on old copper structures.

Bronze

Bronze is not a natural metal, but a copper alloy that usually contains 88% copper and 12% tin. Bronze does not contain iron and therefore will not rust. The oxidation reaction changes of bronze are similar to those of copper, so I will not repeat them here. However, if the working conditions are too harsh and there are acidic substances in the environment, it may cause “bronze disease”, which is because the chlorides in the acidic environment react with copper to produce CuCl₂, which makes the matrix material brittle and reduces the mechanical properties. Bronze is softer than pure copper and has good toughness. Bronze is often used to produce various sculptures, musical instruments, architectural components and marine applications.

Brass

Brass is an alloy of copper and zinc. Although it also contains some iron, it is very small, so brass will not rust either. The oxidation reaction of brass is similar to that of copper, so I will not elaborate on it. Brass is similar in color to gold, has good appearance, strength and ductility, and is therefore often used as a raw material for decorations, musical instruments, household items and other products.

Gold, Silver, Platinum

Gold, Silver, and Platinum are called precious metals. They are all pure metals and do not contain iron, so they will not rust. At the same time, these metals are not prone to oxidation reactions.

Conclusion

When selecting metal raw materials for a project, we often pay attention to whether it will rust first, but as explained above, rust is only one form of corrosion. Although the metals mentioned above will not rust, they are still affected by other environmental factors and will produce various corrosion phenomena. These corrosions will cause the structural strength and mechanical properties of metal products to decrease, bringing serious safety hazards.

Therefore, when we choose metal materials, we must consider multiple factors comprehensively. From the perspective of the application conditions of metal materials, we must comprehensively consider humidity, pH, temperature, electrochemical changes, physical stress conditions, etc.; from the perspective of the overall project in which the metal materials are located, we must comprehensively consider factors such as the maintenance cost and service life of the facilities.

For example, in marine applications, harsh corrosive environments are more common working conditions, so a material with excellent corrosion resistance such as 316 and duplex 2205 stainless steel materials would be a better choice; in the production of products such as automobiles and aircraft, we must take into account both fuel saving and structural strength, so we will choose titanium alloy, a material with excellent strength-to-weight ratio; and in high temperature and high pressure applications, we will give priority to nickel-based alloys, a material that can maintain good mechanical properties under high temperature and high pressure.

There are many working conditions in industrial applications. If you don’t know what material you should choose, please contact our HYT technical team. We are committed to providing customers with one-stop metal material solutions to help you reduce time and financial costs and improve the overall safety and economic benefits of your project.