Abstract: A water seal device for closing a tank containing a corrosive product, such as acid, closed by a cover having a side lip which is able to engage, on closing, in a U-shaped channel provided along the upper edge of the side wall. The channel is made up of monolithic shaped elements, each made from a natural rock of eruptive origin of the lava type, and arranged in sequence on the upper edge of the side wall. The invention applies particularly to installations for chemically pickling sheet metal.

Abstract: The invention relates to an acid pickling tank for metal strips with a width equal to or greater than 0.50 meter.The tank incorporates an external supporting structure (1) and a metal casing (3) coating internally with rubber (6). The rubber layer (6) is lined internally with large slabs (10a, 10b) of a natural rock of volcanic origin, which has a total SiO.sub.2 and Al.sub.2 O.sub.3 content greater than 60%, a porosity of between 6 and 30% and a relative density of less than 3. At most three slabs (10b) are placed side by side along the width of the tank and at most one slab (10a) is placed along its height. The minimum dimensions of a slab (10a or 10b) are 0.50.times.0.50 meter.The invention is applicable in particular to the pickling of strips of hot-rolled metal sheets.