Лако-красочные материалы — производство

Corrosion inhibition by protective coatings can trace its origins to the earliest days of civilization. Red lead, as an anticorrosive pigment used in linseed oil coatings, estab­lished itself as the standard for corrosion inhibitive coatings [5.57]. Over the years, lead pigments have been proven to be outstanding anticorrosives, which perform par­ticularly well when used in coatings for application on insufficiently prepared surfaces [5.53]. However, the increasing awareness for human health protection (see Section 5.2.16) has led to a distinct curtailment concerning the usage of lead pigments.

Red lead (Pb3O4) was the most popular lead-based anticorrosive. It was produced industrially by oxidizing lead monoxide (PbO) at about 480 °C with agitation in a stream of air for 15-24 h [5.56]. Red lead is a cathodic passivator (reduction of Pb(IV) to Pb(II)), that means an oxidizing agent, but its inhibitive mechanism is much more complicated. The main use ofred lead has been in linseed oil based paints. Red lead can form soaps with the fatty acids present in the oil. These soaps can also inhibit rust formation. In addition, the lead soaps can improve the mechanical properties of the paint film and give good mechanical strength, water resistance and adhesion to the substrate. Furthermore, it is discussed in the literature that red lead and other lead-based pigments precipitate corrosion, promoting chloride and sulfate ions by Pb(II) ions [5.55, 5.56].

Basic lead silicochromate (4(PbCrO4 • PbO) + 3(SiO2 • 4 PbO)) reached certain industrial importance. It was developed to replace red lead. The pigment is a so — called core pigment, in which the active pigment substance (PbCrO4) is precipitated onto an inert core (SiO2). Because this anticorrosive contains both lead and hexavalent chromium, it has lost its previous economic importance as well [5.53, 5.56].

For very specific and limited applications the following lead-based products have been used: lead suboxide, lead carbonate and lead cyanamide (mirror coatings), lead silicate (electro disposition primers), dibasic lead phosphate, tribasic lead phosphate silicate, and calcium plumbate [5.51, 5.53].

212 I 5 Specialty Pigments

5.2.4.2