The first reaction that occurs when the zinc phosphate solution comes into contact
with the metal surface is the pickling reaction, which will dissolve some
metal from the surface. On steel, depending on the process solution used, the pickling
rate is in the range of 1-3 g/m' of surface area treated; however, it can be much
higher for higher coating weights.
This pickling reaction is essential for the coating formation since this can be
said to be a chemical cleaning of the surface and, as such, will affect the adhesion
of the coating to the base metal.
During the pickling reaction, for free acid of the solution close to the metal surface
is consumed because of the dissolution of the metal surface. Metal ions are transferred
into the process solution. The type of metal ion depends on the type of sub- state mix being treated. Minimization of hydrogen development occurs by the use
of oxidation agents (Ox) (accelerators).