The invention relates to a process and a moveable container for pickling a metal piece after welding, in which process, the metal piece is brought into contact with an acid surface-treatment agent in the area of the welded joint and the heat-affected zone, the surface treatment agent is allowed to act on the said area for a set period of time, the surface agent is removed from the surface of the metal piece with the aid of a neutralizing wash fluid, the wash fluid is collected and taken to be cleaned, the wash fluid is chemically treated with an alkalizing agent and an agent that increases the floc size, the chemically treated wash fluid is taken for clarification to form a clarified liquid and a sediment, the heavy-metal waste accumulated during clarification is taken to a hazardous waste treatment plant.