In some cases, even the strongest metal parts can suffer from corrosion, which occurs when the metal reacts with the environment. Corrosion turns the surface of metal parts into metal oxides, hydroxides or sulfides.

Any metal can suffer from some degree of corrosion, although some metals react differently than others. Importantly, however, corrosion can often be prevented or minimized by following certain steps. These steps involve good product design, material selection and the application of surface treatments.

This guide looks at the five best ways to reduce corrosion on metal parts made using techniques such as CNC machining.

What Is Corrosion?

Corrosion is a natural process in which environmental conditions transform refined metals into their oxide, hydroxide or sulfide forms.

In engineering, electrochemical oxidation of metals (reaction with oxidizing agents such as oxygen or sulfates) is the most common and worrisome form of corrosion, and the rusting of iron falls into this category.

Some metals corrode readily when exposed to moisture in the air, while others only corrode when exposed to other, more corrosive substances.

Corroded metals are usually identified by their orange color and powdery texture.

How to prevent corrosion?

Steps can be taken to prevent corrosion of metal parts. Most of these steps are taken before and during the manufacturing process, but it is also possible to reduce corrosion on parts after they have been used.

Following these steps does not guarantee that your parts will have an unlimited life, but it will go some way toward reducing potential corrosion while helping to maintain the proper function and appearance of metal parts.

1. Choose a Corrosion-Resistant Metal

No metal is indestructible, but if your parts will come into contact with moisture or corrosive substances, it makes sense to choose a corrosion-resistant metal.

Stainless steel: Stainless steel contains iron, which oxidizes and rusts. However, steels with high chromium content (such as stainless steel 316) are less likely to rust because the chromium forms a protective layer of chromium oxide on the surface of the part.

Aluminum: Aluminum does not contain iron and therefore will not rust. When parts are exposed to moisture, a protective layer of aluminum oxide is formed on the surface of the part, which prevents corrosion.

Copper, bronze and brass: Red metals such as copper, bronze and brass contain little or no iron and will not rust. Copper oxide has a greenish sheen, and while this may not be aesthetically pleasing, it protects copper parts from corrosion.

2. Design Parts To Minimize Damage To The Environment

The material of a part affects its corrosion resistance, but so does its shape and texture. Therefore, engineers should consider whether certain design features will encourage or discourage corrosion when creating CAD designs.

Features that may trap moisture or air should be avoided. Parts should not contain deep cracks that tend to accumulate moisture and, if possible, should encourage airflow. If the part absolutely must contain cracks, tunnels, or other vulnerable points, these areas should, at a minimum, be made easy to clean and maintain.

3. Apply a Corrosion-Resistant Coating

Applying a coating to a metal surface prevents corrosion of the metal because air, moisture and other corrosive substances cannot come into direct contact with the metal unless the coating itself wears off. The coating can be reapplied after a period of time to maintain the condition of the part.

Paint and/or primer is a straightforward and inexpensive way to prevent corrosion and may be necessary anyway for aesthetic reasons. Powder coatings can also form a protective layer of plastic material (epoxy, nylon, etc.) on metal surfaces.

For some metals, it may be desirable to add hard chrome plating (affordable, good corrosion resistance) or black chrome plating (more expensive, good corrosion resistance).

4. Application Of Sacrificial Coatings

Another way to make parts corrosion resistant is to apply a sacrificial coating to the metal rather than a protective coating.

The purpose of a sacrificial coating is not to completely prevent corrosion, but to allow the coating to corrode rather than the underlying material of the part. This is why the coating is called a "sacrificial" material.

An example of a sacrificial material is zinc or magnesium on a stainless steel part. Steel that is completely coated with zinc is called galvanized steel, and galvanization is an example of cathodic protection: the steel becomes the cathode of an electrochemical cell.

However, the sacrificial material does not have to be applied as a coating; zinc and magnesium blocks placed against the protected part are sometimes used in the marine industry to achieve similar results.

5. Control The Environment Around The Part

Post-manufacturing steps may be more difficult to implement than pre-manufacturing steps, but once the part is in service, corrosion can still be reduced.

In some cases, it is possible to control the amount of substances such as oxygen or chlorine in the surrounding environment. For example, parts can also be prevented from coming into contact with excess moisture by adjusting their location or introducing barriers.