The present invention relates to an aqueous alkaline composition for alkaline passivation of metallic components comprising at least in part surfaces of zinc or zinc alloys, containing iron ions, phosphate ions, and one or more complexing agents, where a free alkalinity ranges from 1 to 6 points, and the pH is at least 11. A plate-shaped member is detachably mounted on the head and includes first and second fingers. In this manner, larger effective bubble volumes can be achieved, thus increasing low frequency output.