Cleaning medium for structural steel surfaces coated with a crystallised iron phosphate layer, and process for cleaning said surfaces with such a cleaning medium

摘要

A cleaning medium for structural steel surfaces which are coated with a crystallised iron phosphate layer and which have been obtained by pickling a scale-covered and/or rusted surface of a structural steel with a phosphoric acid pickle and have become contaminated during storage and/or in the course of mechanical processing steps, such as drilling, cutting, welding and the like is described, which medium has the following composition: 0.5-1.5% by weight, preferably about 1% by weight, of formic acid, 1.5-6% by weight, preferably about 4% by weight, of orthophosphoric acid, 0.2-2.5% by weight, preferably about 0.75% by weight, of oxalic acid, 0.2-1.5% by weight, preferably about 0.5% by weight, of anionic, biodegradable wetting agent, for example a mixture of secondary n-alkanesulphonates ("Hostapur SAS 60"/Hoechst), 0.2-2.5% by weight, preferably about 0.5% by weight, of hydrofluoric acid, and 0.05-1.5% by weight, preferably about 0.1% by weight, of an amino alcohol, in particular triethanolamine, the remainder being water. A process for cleaning structural steel surfaces coated with an iron phosphate layer with a cleaning medium is furthermore described, in which process the medium, at a temperature of 55, to 140 DEG C and finely distributed in water vapour in a ratio of 1 part by weight of medium to about 2.3 parts by weight of water vapour, is sprayed onto the iron phosphate layer at a pressure of 5 to 15 bar.