8. Zn-X and hydrogen codeposits

Wet surface treatments generate hydrogen. Hydrogen, derived from the reduction of water (or the hydronium ion), can be produced during :

• chemical" treatments (chemical pickling, conversion, etc.) involving redox reactions between the metallic material to be treated and the solution;

• electrolytic treatments in which the part, in the cathode position, is subjected solely to water reduction (e.g. cathodic degreasing);

• several concomitant reactions (electroplating proper, involving competition between the reduction of water and metal ions).