Types of Corrosion

Uniform Corrosion

A characteristic feature of uniform corrosion is the uniform abrasion of the surface. Here, specific anodic and cathodic areas develop in many places on the metal surface. These can change their position (change of location), i.e. occur continuously in another place. If the abrasion of the surface occurs only locally, this is known as shallow pit corrosion.

Uniform corrosion primarily affects the surface properties and leads to optical impairments. It can be recognised at an early stage and only restricts the load-carrying capacity if the loss of the metal is significant.​​​​​​​

Stress Corrosion Cracking

Stress corrosion cracking, on the other hand, is a particularly critical form of corrosion because it damages the material unnoticed, which means that sudden failure can occur. A combination of mechanical and chemical attack can cause cracks in the structure of the material or completely break screws. There are three main factors that lead to stress corrosion cracking: the raw material is sensitive to stress corrosion cracking, the raw material is subjected to tensile stress, a specific attacking agent (e.g. chloride) is present. The sensitive raw materials here include all copper-zinc alloys (brass), some wrought aluminium alloys and some rust and acid-resistant steels. The attacking agents depend on the raw material. Chlorides affect the rust and acid-resistant steels and aluminium (seawater) and ammonia, sulphur dioxide and nitrate influence the copper-zinc alloys.

There are two different types of stress corrosion cracking: Anodic stress corrosion cracking occurs primarily on many stainless steels. Upon contact with oxygen, these form a thin oxide film - so-called passive coating - on the surface, which actually protects against further corrosion. If stainless steel is used in highly corrosive atmospheres such as swimming pools (containing chlorine), the passive coating can be damaged. This means there is no longer any inherent protection, so that the basis material can be attacked. Existing preinduced cracks can expand and reduce the load-carrying capacity of the screw until it falls below the critical cross-section. The screw joint is no longer secure. In contrast, there is cathodic stress corrosion cracking (hydrogen induced embrittlement), which occurs in particular on case-hardened screws. Due to the accumulation of hydrogen in the screw material, the load-carrying capacity of the screw can be exceeded when tensile stress is applied, causing the fastener to break spontaneously.​

Crevice Corrosion

Crevice corrosion occurs in narrow, open crevices without sufficient air and fluid exchange. The differences in the concentration of oxygen in an electrolyte in the gap result in corrosion elements. Then, anodic and cathodic areas develop locally. Due to the lack of oxygen supply, the medium can become more aggressive at the corrosive point and thus locally accelerate the corrosion. Even stainless steel can corrode in crevices if there is no oxygen available to restore the attacked oxide coating. The connection of screws and nuts as well as open welding seams can promote crevice corrosion if the material is unsuitable. Crevices that occur with overlapping surfaces and ledges are also particularly at risk.

We will deal with how crevice corrosion can be avoided in a separate guide part in the near future.

​​​​​​​Bimetallic Corrosion

Bimetallic corrosion occurs when two or more metallic materials with a certain potential difference come into contact. Due to the different potentials of the two raw materials in connection with a conductive electrolyte, e.g. water, the surface of the less noble metal corrodes.

Prerequisites for bimetallic corrosion are​​​​

• the different position in the associated electrochemical series (potential difference)
• direct contact of the raw materials
• the connection of the two by an aqueous corrosion medium.

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This type of corrosion becomes noticeable in everyday life when components made of, for example, stainless steel, brass or copper are combined with screws, nuts or rivets made of less noble metallic materials. The so-called galvanic corrosion can therefore be avoided through professional selection of raw materials or appropriate galvanic isolation.​​​​​​​​