Magnetic particle testing (MT) is the most sensitive method to detect surface cracks in ferromagnetic materials [2]. This method can be executed by mobile equipment or stationary systems [3]. Magnetic particles are used for the visualisation of the surface cracks. The magnetic particles are an iron oxide powder coated with visible or fluorescent paint. They are also called test agent or magnetic ink. For mobile test tasks, the magnetic particles are normally applied as a wet suspension from aerosol cans. The magnetic particles are only used once. Therefore the longterm stability is not an issue. On the other hand, for stationary systems the magnetic particle is contained in a closed circuit and is used for many hours. A suspension of magnetic particles diluted in water (or sometimes oil) is kept in constant circulation. The properties of the magnetic particles degrade over time and have to be controlled in regular intervals.
