Chromate Pretreatment Layer

The chromate pretreatment layer, which is also called the chromate conversion coating (CCC) varies in thickness depending on the chemistry of the process and the application method used. The CCC layer is, however, usually not thicker than a few microns, which in coating weight is somewhere between 5 and 25 mg/m, expressed as Cr [19], This CCC layer improves the adhesion between the metal and the primer, it aids in the protection of scratches and defects and it also protects cut edges of the metal to some extent [20]. The hexavalent chromate in the CCC layer is known for its low solubility and the self-healing effect, which means lliat it only leaches out on demand when the base metal has been scratched [21]. 

Naturally, when comparing the film thicknesses of the CCC and primer layer it is obvious that the replacement of the chromate in the paint layer has a more substantial effect on reducing the amount of chromate that may leach/ dissolve out into the environment than replacing the pretreatment layer. The first step in improving the conventional three layer paint system shown in scheme A was to replace the CCC layer with a Cr-free pretreatment and the Cr-containing primer with a primer loaded with Cr-free inhibitors. This resulted in scheme B [25, 26]. In recent years attempts have been made to incorporate the silane into the primer, which results in 2-in-1 primers, where the pretreatment layer is built in the primer paint layer. This idea was first introduced by van Ooij et al., who have investigated these types of silane-containing Cr-free primers on aluminum alloys, HDG steel and CRS [7-11, 27-30]. 

Figure 31.14 shows the Rp values of [2A] with three different chemical pretreatments and with a TMS plasma polymer on each of the three pretreated surfaces, as well as on the control [2A]CC surfaces (chromate conversion-coated 2A). It can be seen that the Rp values of [2A] were decreased to some extent by pretreatment of alkaline cleaning and were drastically reduced by alkaline cleaning plus deoxidization. As observed in the XPS results, the accumulation of Cu elements and removal of oxide layer on [2A] surfaces were presumed responsible for the reduction in corrosion resistance of these chemically pretreated [2A] panels. 

Studies in the 1990s revealed the good corrosion protection properties of silicon-based plasma polymers on steel substrates and the cmcial influence of the pretreatment process on the stability of the resulting interface [92-101]. The pretreatments for trimethylsilane-based films may consist of an oxidative step (02-plasma) to remove organic contaminations from the substrate and a second reductive step (Ar/H2-plasma) to remove the metal oxide layer. Although the successive application of both steps provides the best corrosion protection of various plasma treatments for steel in combination with a cathodic electrocoat, little is known about the chemical structure of the interface. Yasuda et al. [101] and van Ooij and Conners [97] in particular have shown that the deposition of plasma polymers on steel and galvanized steel might even substitute the chromatation process. 

Some alternative chromate free pretreatment methods for aluminium surfaces have already been described [14 16], Commercially available products mainly consist of titanium and zirconium compounds. Hybrid polymer sol-gel materials are potential substitutes for hexavalent chromium-based surface treatments as well. Due to the chemical characteristics, in particular, the presence of hydroxy and alkoxy groups, the hybrid materials are qualified to coat metal as well as metal oxide surfaces. These groups can react with OH-groups on the surfaces of both metals and metal oxides. Water and alcohols are eliminated, while bonds between the hybrid polymer and the metal surface are created (Fig. 6.6), thus leading to good durable adhesion of the layers to the metal substrates [17,18]. Similar to silane adhesion promoters, the hybrid sol-gel materials can also link to organic polymer paint systems (Fig. 6.7). 

---