Acids → Protons, counterion of the protons → e.g. Cl– (catalyst and / or complexing agent), complexing agent (e.g. tartaric acid)
Main reaction (desired): MexOy + 2yHA → MexA2y + yH2O Me = Metal A = Anion
Alkalis → OH- ions as ligands (applies only to metals that can form soluble hydroxocomplexes)
ZnO + 2NaOH + H2O → Na2[Zn(OH)4] Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4]
In addition to the oxide, the mordants also attack certain metals (reasons: porous oxide layers, different thickness of the oxide layer).
It can be said that all metals whose electrode potentials in the voltage series of the metals are less noble than the hydrogen electrode are attacked by the protons of the acid. The greater the potential difference, the greater the driving force of this reaction (but not always the reaction speed!).
The resulting H2 has a pickling supporting effect in that if it is formed (e.g. by pores in the top layer) below the layer, the layer is mechanically blown off. Before H2 can form, atomic hydrogen is formed: H+ + e– → Hat is more or less capable of diffusing into the base material, depending on the base material. Here it preferably accumulates on so-called interstitial sites. This leads to lattice distortions (compressive / tensile stresses) which ultimately lead to a minimization of the compressive strength. This process leads to so-called “hydrogen embrittlement”.
Prevention of the formation of atomic hydrogen by base metal protecting pickling additives. (Inhibitors)
Subsequent tempering at elevated temperature approx. 180-220°C to expel the hydrogen again.
Note: Only atomic H is diffusible, molecular (i.e. H gas) no longer! I.e., should be mol. H, tempering is nonsense, on the contrary, due to the heat treatment of the gas, pressures of up to approx. 1000 bar occur, which can have a rupture of the surface. (pickling blowing)
Application for very sensitive materials (mainly spring steels), where hydrogen embrittlement must be excluded for reasons of the area of application of the workpieces.