| abstract |
In chromium-free aqueous acidic washing solutions for cleaning the surfaces of aluminum and aluminum-alloy particles, so as to remove therefrom smut and lubricating oil left on the surfaces thereof after metal forming operations, which solutions contain from 0.2 to 4 g/I ferric ion and sufficient sulfuric and/or nitric acid to impart a pH of 2 or less to the solution (and which optionally may also contain fluoride ions up to a concentration of 0.5 g/I) there is provided a method of controlling the effectiveness of the washing solution in which the ferric ion concentration therein is monitored, conveniently by the oxidation-reduction potential of the washing solution, as shown in Figure 3, and is controlled with the desired limits by adding when appropriate suitable amounts of oxidant capable of' oxidizing ferrous ions to ferric ions and, separately or in conjunction therewith, a replenisher containing a source of iron ions. |