Erosion and corrosion in mechanical engineering

Corrosion and erosion affect the service life and performance of components. But what is actually the difference between erosion and corrosion? What types are there? And how can these forces be counteracted? The following article examines this in more detail.

What is the difference between erosion and corrosion?

As used in daily life, the two terms erosion and corrosion are frequently confused, but both stand for two entirely different concepts: erosion refers to material removal and thus to a physical change, while corrosion is primarily a change caused by a chemical reaction. In the context of mechanical engineering, erosion means that, for example, the mechanical effect of erosive particles on the surface of a component leads to damage or abrasion of surface material. The resulting damage can be microscopically small but also macroscopic in nature, i.e. visible to the naked eye. Abrasion is a form of erosion and denotes the process of removing material from a solid surface by friction with another friction partner. Friction partners can be small particles in a flowing gas but also flowing liquids or rubbing solids. The removed material is referred to as abrasion.

Corrosion, on the other hand, describes a chemical or electrochemical reaction of the material based on an interaction with the environment, which leads to a change in the material up to its destruction. Erosion can be a prerequisite for corrosion, such as erosive corrosion. In this special case, removal (erosion) of the protective oxide film on the surface of the metal causes a corrosive attack to occur on the now exposed surface of the affected areas.

Some metals and metal alloys, such as iron or steel, are susceptible to corrosion, which can lead to complete destruction. Other corrosive metal alloys corrode on their surface and form an oxygen-tight oxide film, which prevents deeper corrosion. For example, untreated aluminum or copper are susceptible to corrosion from moisture and salts. They react with the moisture and oxygen of the environment and form a protective oxide film on the material surface, which then protects against further corrosion.

Both corrosion and erosion are a major problem in many industries and lead to material wear and high maintenance costs.

Corrosion in detail

DIN EN ISO 8044 defines corrosion as the reaction of a material with its environment. This reaction causes a measurable change in the material and can lead to a functional impairment. This reaction is usually of an electrochemical nature, but can also have chemical or metal-physical causes. The standard further differentiates between corrosion damage and corrosion stress:

• Corrosion damage: material damage due to chemical or electrochemical reactions with the environment. Corrosion products are formed that weaken the material, e.g. surface corrosion or gap corrosion.
• Corrosion stress: Material contamination due to corrosive environmental conditions. This stress can be increased by factors such as humidity, salt content, temperature and chemical influences. Corrosion stress makes materials more susceptible to corrosion damage, especially with additional mechanical stresses.

Corrosion symptoms

There are different stages or appearances of corrosion:

• No changes can be seen on the surface: no corrosion by-products have formed or the incident surface deposits are in the nanometer range; further degradation is therefore unlikely.
• Surface is discolored, otherwise no changes: corrosion by-products can be seen on the surface, but the corrosion does not progress any further otherwise. The discoloration has a thickness several 10 nm of rust formation.
• Progressing corrosion: corrosion by-products do not adhere firmly to the surface, thus continuously exposing the surface to the corrosion-inducing environment. This is noticeable by flaking rust, e.g. in unalloyed steels exposed to rain or wind.
• No rust formation, progressing corrosion: The corrosion by-products have dissolved into the environment, but corrosion continues to progress, e.g. when the metal comes into contact with acid.

Types of corrosion

Corrosion can be classified according to the type of reaction mechanisms: Chemical corrosion, electrochemical corrosion and metal-physical corrosion.

• Chemical corrosion: Chemical corrosion occurs when metals and other materials react with their environment, in particular with oxygen, water or aggressive chemicals, and are then broken down and destroyed.
• Electrochemical corrosion: Electrochemical corrosion is triggered by the presence of an electrolyte, e.g. intercrystalline corrosion.
• Metal-physical corrosion: Metal-physical corrosion occurs when physical phenomena lead to corrosion.

Corrosion resistance and corrosion protection

Corrosion resistance is a material property that depends on the following factors, for example:

• Material composition
• Surface treatment
• Alloy elements

Ideally, the relevant components have high corrosion resistance. However, there are also other options for corrosion protection.

The following table shows the effects of the salt water spray test iaw. JIS H8502 on a simple linear ball bearing with flange:

Erosion in detail

Erosion is particularly important when machines and components are subjected to extreme loads. The shape and the angle of attack of the incident particles also influences the wear effect. The property of the material also plays a role. Brittle materials behave differently than ductile ones. For glass as a brittle material, for example, the wear sensitivity increases with the angle of attack, it literally splinters. For ductile materials, wear increases at an angle of attack of up to 25°, but then drops again quickly.

Here are some examples of possible failure modes of different materials:

• 1 - Ductile, soft
• 2 - Ductile, soft, coated
• 3 - Brittle

On brittle materials, various fracture modes can result depending on the material structure, shape and impact energy of the impacting particle and different impact angles:

• 1 - Conical rupture
• 2 - Radial rupture
• 3 - Lateral rupture

Erosion as material-removing manufacturing process

Erosion can also be used to positive effect, e.g. by spark erosion. Spark erosion is a removing, thermal manufacturing process. It is based on electrical discharge processes and is therefore only suited for conductive materials. An electrode tool submersed in a dielectric is supplied with DC voltage and then guided toward a conductive material. This results in discharges in the form of sparks, which result in high temperatures of up to 1200°C. The workpiece material is melted and the removed material particles are flushed away in the liquid. Complex geometric shapes with a high surface quality can be produced by spark erosion. Die-sinking, as a sub-type of spark erosion, uses a tool that represents a negative of the structure to be produced. This method is used primarily for various molded parts.

Particle erosion is yet another way to make purposeful use of erosion in mechanical engineering. Particle erosion occurs when small solid particles (e.g. sand) are propelled against the surface of a component. This results in abrasion and loss of material.

Corrosion and erosion protection

There are various types of corrosion and erosion protection. The basic principle is that properties are added to the material to be protected in order to make it more resistant to erosion and/or corrosion. This also extends the service life. However, additional erosion protection is often more expensive than simply replacing the component and must therefore be considered carefully. Erosion protection and corrosion protection can both be categorized into passive and active erosion or corrosion protection.

Passive corrosion protection is achieved, for example, by corrosion protection agents that protect the metal surfaces as a coating. A protective coating is often applied as a final operation. Common methods include thermal spraying and polymer coating:

• Thermal spraying: Thermal spraying involves spraying additive materials onto the surface as spray particles, which are then deposited on the surface in layers to form the spray layer. The main application for thermal spraying is corrosion and wear protection. The thermal load of this method is very low. Arc spraying, plasma spraying and flame spraying are subtypes.
• Polymer coating: Polymer coating involves coating the tool with a layer of polymeric material. Tribological polymer coatings are particularly adaptable. They reduce abrasion and are also used as protection against corrosion and wear. The following options are available: Powder coating, plasma coating for very thin coatings, wet coating, vacuum coating.

Active corrosion protection is mainly used on inaccessible workpieces, e.g. submerged or underground cables or pipes. Active corrosion protection can be achieved by adding a corrosion inhibitor or by electrochemical polarization. A distinction is made between anodic and cathodic protection:

• Cathodic: the metal to be protected is connected as a cathode with a positively charged anode (e.g. a non-royal metal, such as zinc) by means of external current. External current sources can even be omitted when magnesium is used as an anode. The electrodes are directed toward the material to be protected and are absorbed by the latter. A protective current is created that prevents rusting.
• Anodic: on metals that generate corrosion or oxidation products, these products are used as a protective layer to prevent further attack.