CHEMICAL CONVERSION COATINGS

15.3 CHEMICAL CONVERSION COATINGS

Chemical conversion coatings are protective coatings formed in situ by chemical reaction with the metal surface. They include special coatings, such as PbSO 4 , which forms when lead is exposed to sulfuric acid, and iron fl uoride, which forms when steel containers are fi lled with hydrofl uoric acid ( > 65% HF).

Phosphate coatings on steel ( “ Parkerizing, ” “ Bonderizing ” ) are produced by brushing or spraying, onto a clean surface of steel, a cold or hot dilute manganese or zinc acid orthophosphate solution (e.g., ZnH 2 PO 4 plus H 3 PO 4 ). The ensuing reaction produces a network of porous metal phosphate crystals fi rmly bonded to the steel surface (Fig. 15.1 ). Accelerators are sometimes added to the phos-

phating solution (e.g., Cu 2+ , ClO − 3 , or NO − 3 ) to speed the reaction.

CHEMIC AL CONVERSION COATINGS

Figure 15.1. Scanning electron photomicrograph of phosphated type 1010 mild steel; acidic zinc phosphate + sodium nitrate accelerator applied at 65 °C for 1 min ( Symposium on Inter- face Conversion for Polymer Coatings, edited by P. Weiss and G. Cheever, Elsevier, New York, 1968).

Phosphate coatings do not provide appreciable corrosion protection in them- selves. They are useful mainly as a base for paints, ensuring good adherence of paint to steel and decreasing the tendency for corrosion to undercut the paint fi lm at scratches or other defects. Sometimes, phosphate coatings are impregnated with oils or waxes, which may provide some protection against rusting, especially if they contain corrosion inhibitors.

In corrosion - resistant automobile bodies, the fi rst — and probably the most important — layer of the paint system is the phosphate coating. Although it is the thinnest coating in the paint system [about 3 μ m (0.1 mil) thick], it is the anchor for the subsequent layers [1] .

Oxide coatings on steel can be prepared by controlled high - temperature oxidation in air or, for example, by immersion in hot concentrated alkali solutions containing persulfates, nitrates, or chlorates. Such coatings, blue, brown, or black

in color, consist mostly of Fe 3 O 4 and, like phosphate coatings, are not protective against corrosion. When rubbed with inhibiting oils or waxes, as is often done with oxidized gun barrels, some protection is obtained.

Oxide coatings on aluminum are produced at room temperature by anodic oxidation of aluminum (called anodizing ) in a suitable electrolyte (e.g., dilute sulfuric acid) at current densities of 100 or more A/m 2 . The resultant coating of Al 2 O 3 may be 0.0025 – 0.025 mm (0.1 – 1 mil) thick. The oxide so formed must be hydrated to improve its protective qualities by exposing anodized articles to steam or hot water for several minutes, a process called sealing . Improved

INORGANIC COATINGS

corrosion resistance is obtained if sealing is done in a hot dilute chromate solu- tion. The oxide coating may be dyed various colors, either in the anodizing bath or afterward.

Anodizing provides aluminum with some degree of improved corrosion resistance, but the additional protection is not spectacular, and it is certainly less than proportional to oxide thickness. Anodized coatings provide a good base for paints on aluminum, which is otherwise diffi cult to paint without special surface preparation.

Coatings of MgF 2 on magnesium can be formed by anodizing the metal at

90 – 120 V in 10 – 30% NH 4 HF 2 solution at room temperature. This coating has been recommended for surface cleaning or as a base for fi nishing treatments [2] . Chromate coatings are produced on zinc by immersing the cleaned metal for

a few seconds in sodium dichromate solution (e.g., 200 g/liter) acidifi ed with sul- furic acid (e.g., 8 mL/liter) at room temperature and then rinsing and drying. A zinc chromate surface is produced that imparts a slight yellow color and that protects the metal against spotting and staining by condensed moisture. It also increases the life of zinc to a modest degree on exposure to the atmosphere. Similar coatings have been recommended for Zn – Al coatings [3] and for cadmium coatings on steel.