We obtained a permission to do an excursion into a paint facility of the Škoda car factory a.s., in Mladá Boleslav. This large enterprise with 13 000 employees is part of the Volkswagen Group. Thanks to investments provided by Volkswagen, Škoda is today a thriving top-level enterprise. The factory premises house also a large chemical unit in which the strictest cleanliness and technological discipline are an absolute must.

Iron corrosion
Iron corrosion causes vast economical losses in all branches of industry. It is an electrochemical process that requires the presence of oxygen and water (more precisely an electrolyte). In dry air and in pure oxygen-free water, corrosion is virtually absent. The conditions under which cars usually operate are conducive to corrosion. An occluded moisture layer contains dissolved atmospheric CO₂, which alone suffices to increase the concentration of H⁺ ions. In addition, the air in industrial areas contains SO₂ and NO_x, which cause acid rains. Conditions highly favourable for corrosion are created by road salting in winter. The mechanism of corrosion is complex. It can be basically described by the equations:

    Anodic oxidation 2 Fe (s) ® 2 Fe²⁺ (aq) + 4 e^-
    Cathodic reduction O₂ (g) + 4 H⁺ (aq) + 4 e^- ® 2 H₂O (l)
    Overall reaction 2 Fe (s) + O₂ (g) + 4 H⁺ (aq) ® 2 Fe²⁺ (aq) + 2 H₂O (l)
    The Fe²⁺ ions are further oxidised   4 Fe²⁺ (aq) + 3 O₂ (g) + (4 + x) H₂O (l) ® 2 Fe₂O₃. xH₂O (s) + 8 H⁺ (aq)
Hydrated Fe₂O₃ is known at rust. The layer of rust flakes off the surface and the corrosion proceeds to the depth of the iron. In this iron differs from other metals whose electrode potential is more negative, and which resist corrosion, such as Al or Cr. In these metals, the oxide layer firmly adheres to the surface and thus protects the metal from further corrosion (passivation).

Protection against corrosion
There are several methods for protection against corrosion. They include mechanical, chemical and electrochemical methods. Let us briefly mention a method, which we saw during our excursion. This procedure includes 52 operations, which are illustrated in the enclosed scheme.

The steel sheet car body comes from the welding shop into the priming paint bay. It is already zinc-plated either from both sides (Škoda Octavia) or from one side (Škoda Felicia). Zinc plating is the first protection against corrosion (Zn functions as a sacrificed electrode). The body is covered with mineral oil for protection, and has to be defatted. The defatting has three steps and is carried out at 50 - 60 ^oC using an alkaline solution containing borates, carbonates, silicates and tensides. Rinsing in tap water follows the defatting, all other washes being done by demi water.

The next operation is the so-called activation of titanium phosphate, Ti₃(PO₄)₄ and phosphating, which is done with the aid of a solution of Zn²⁺, Ni²⁺ and Mn²⁺ dihydrogen phosphates. During phosphating, a micro-crystalline structure of very poorly soluble phosphate is formed on the metal surface. Phosphating is carried out at high temperature and in the presence of accelerators (sodium nitrite, hydroxylamine sulphate). This step is followed by a system of washes. The precipitated low-soluble Zn, Ni and Mn phosphates are separated from the wash waters on pressure filters. They represent a dangerous waste and are taken to a special dump. The next step is a chromium-less passivation involving the use of hexafluorozirconic acid H₂ZrF₆.

After the washing, the body is placed into a large tub containing a colloidal dispersion of the priming paint (modified epoxy resin + pigment paste + solvents butylglycol and butoxy ethanol + fillers). The paint is layered onto the body electrophoretically - the micelles have a positive charge and travel to cathode, i.e. to the body. When the micelle charge has been neutralized, the colloid coagulates and forms a whitish film of the priming paint on the body. The operation is denoted cathaphoresis (deposition of a colloid on cathode). There follows again a system of washes - the wash waters undergo ultrafiltration and the ultrafiltrate is used for washing while the paint is returned back to the cathaphoresis. The final wash is done by demi water and the body is taken to a drying room where it stays for 20 min at 175 ^oC. The resin undergoes cross-linking and the colour changes from white to yellow.

The lower part of the body is then protected by so-called plastisol, in fact PVC with dioctyl phthalate as softener. It is a highly viscous gray matter, which is forced into the spray pistols by a large high-pressure pump.

The wash waters are divided into two streams - inorganic (8 m³/h) and organic (4 m³/h). The organic stream is taken to an incinerating plant, the inorganic to a wastewater treatment plant.

The top varnish plant is the place where the body receives its final beauty. Paint deposition is electrostatic and proceeds in two steps. Before entering the deposition chamber, the body has to be freed of any randomly acquired electric charge. This is done by rotating ostrich-feather brushes (the operation is therefore called EMU). The paint is sprayed by rapidly rotating small bells in which the fine paint particles acquire a charge and settle on the surface of the body, which has been given an opposite charge. All paints used are water-soluble, with the exception of the final colourless varnish, which is soluble in organic solvents.