The invention relates to a treatment device (1) for the single-stage treatment of a metal object (2) to be treated, wherein the treatment comprises at least the pickling and the phosphating of the object (2) to be treated, wherein the treatment device (1) comprises at least the following apparatuses: a treatment container (4) for holding the object (2) to be treated and for holding a flowable treatment substance (6), a pumping apparatus (10) for circulating at least a portion of the treatment substance (6), wherein the treatment substance (6) flows around at least part of the object (2) to be treated, in particular the entire object (2) to be treated, wherein the treatment substance (6) is a phosphorus- or phosphate-containing solution, in particular phosphoric acid, wherein the phosphorus- or phosphate-containing solution consists of water, in particular deionized water, and of a reaction substance, and the reaction substance consists of phosphorus or of a phosphate and of at least one additional substance that improves the treatment effect, in particular one or more inhibitors, wherein the fraction of the phosphorus or the phosphate in the reaction substance is between 75 vol% and 94 vol% and preferably between 80 vol% and 90 vol% and especially preferably between 83 vol% and 88 vol%, wherein the reaction substance has no fractions of hydrochloric acid and sulfuric acid and preferably also no fractions of fluorine, chlorine, bromine, iodine, lead, mercury, and selenium and wherein the reaction substance is mixed with water at a specified ratio, wherein the specified ratio lies between a lower limit and an upper limit, wherein the lower limit is defined by a mixture at a ratio of 1 kg of reaction substance to 4 liters of water and the upper limit is defined by a mixture of 1 kg of reaction substance to 12 liters of water, and in particular the mixture has a ratio of 1 kg of reaction substance to 6 liters of water.