Lochfraß

Lochfraß is a German term referring to a type of surface degradation that occurs on metal components. It is characterized by the formation of small pits or holes that penetrate the surface of the metal. This phenomenon is a form of corrosion, typically electrochemical in nature.

The process of Lochfraß often begins with a localized breakdown of the protective passive layer on the metal surface, such as the oxide film on stainless steel. This breakdown can be initiated by aggressive ions, most commonly chloride ions, which are found in many environments including seawater, de-icing salts, and industrial chemicals. Once the passive layer is breached, an electrochemical cell is established. The exposed metal at the pit site becomes anodic, while the surrounding passive surface becomes cathodic.

The corrosion process within the pit is self-sustaining. The accumulation of corrosion products and the generation of acidic conditions inside the pit further accelerate the dissolution of the metal. This localized attack can lead to significant material loss and structural weakening, even if the overall corrosion rate on the exposed surface appears low. Factors that influence the susceptibility to Lochfraß include the material's composition, the presence of contaminants, temperature, and the concentration of aggressive ions. Mitigation strategies involve material selection, surface treatments, and controlling the operating environment.