Abstract: Metal articles are sequentially treated in a surface-treatment tank containing a solution of surface-treating agents, then rinsed in a plurality of series-connected water-washing tanks.The contents of the surface-treatment tank and water-washing tanks are cycled to a concentrator by way of a heat exchanger and concentrated solution is returned to the surface-treatment tank. Steam is condensed in the heat exchanger to cause evaporation of water in the concentrator and the steam condensate is transferred to the water-washing tanks.