Abstract: The invention concerns a method for chemical attack of the surface of a plastic material with a solution based on sulphuric acid and chromic anhydride, characterised in that (i) the sulphuric acid concentration ranges between 500 and 760 g/l and (ii) the chromic anhydride concentration ranges between 30 and 200 g/l and (iii) the resulting acidity is at least equal to 13 and at most equal to 16.

Abstract: The invention relates to a dip comprising an acid aqueous solution having no ammonium ion, fluoroborate ions or citrate ion and containing per liter: between 10 and 60 g of Zn2+ ions and between 20 and 100 g of Mn2+ ions. The invention is characterised in that it comprises a buffer agent that maintains the pH at a value of between 3 and 7, preferably between 4.5 and 6 or better still between 4.8 and 5.5, and another agent, different from the buffer agent, which is used to bring together the deposition potentials of couple Zn/Zn2+ and couple Mn/Mn2+. The inventive dip used to deposit a Zn and Mn alloy by electrolysis.