Anomalous corrosion phenomena observed on electrovalves of coffee espresso machines

The domestic coffee machines market has widely expanded in the last five years. The core of these machines is an electro-pump that allows the correct water distribution. In this paper a root cause failure analysis was conducted utilizing some electro-pumps samples taken from a corroded batch. Several examinations were carried out in order to identify the root cause of the failure: visual examination, SEM-EDS analysis and microstructural characterization. The corrosion phenomenon is located at the piston-spring interface. As result of the examinations carried out, it has been possible to identify that the corrosion mechanism is due to different interacting factors: high chlorine concentration, potential dynamic difference between piston and spring materials, material sensitization and iron-manganese bacteria growth.