The invention relates to a method for one-bath chemical cleaning and preservation of interior surfaces of hollow steel bodies, without any organic solvents, comprising controlled change of the liquid bath in that in a circuit of hoses running from a mobile tank equipped with a pump unit and steam generator there is circulated a liquid stream to a defined cavity of steel. The composition of the liquid is changed methodically by the addition of fresh chemical additives until the end of the process. Fresh water at 80.degree. C. has added thereto an alkaline degreasing liquid A containing chlorine-free tensides, which is circulated, whereupon cleaning liquid B, consisting of citric acid with added cationic tensides, is added to a pH of 4.5 and is circulated. There is then added a complexing derusting liquid C, with active modified aminocarboxykate made biologically degradable, with adjustment of the pH to 4.5 5.0 with cleaning liquid B, and extra addition of the complexing agent if derusting liquid C falls below 5% by weight, until the iron content of the liquid is stable. For passivation, a non-ionic passivating and neutralising liquid D, consisting of alkaline degreasing agent and non-ionic, chlorine-free tensides, is added to pH 7.0 7.5, after which there is added 2 4% by weight of inhibiting liquid E, consisting of a chain molecule of a chelate, active at pH>7, which functions as a supplement to the aminocarboxylate. A gluconate with heptonate and silicate provides a diffusion-proof microfilm on the steel surface.
