Chromate free pretreatment

Barbucci A., Delucchi M, Cerisola G. (1998), Study of chromate free pretreatments and primers for the protection of galvanized steels. Progress In Organic Coatings, Vol. 33, No. 2, pp. 131-138, ISSN 0300-9440... 

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). 

The investigations concerning chromate-free pretreatment methods have been funded by the Federal Ministry of Economics, procured by AiF (Arbeitsgemeinschaft industrieller Forschungsvereinigungen) and coordinated by DFO (Deutsche Forschungsgesellschaft fur Oberfl henbehandlung e.V.). 

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]. 

Aramaki, K (2002). Preparation of chromate-free, self-healing polymer films containing sodium silicate on zinc pretreated in a cerium (III) nitrate solution for preventing zinc corrosion at scratches in 0.1 M NaCl. Corrosion Science, Vol. 44 (August 2001), pp. 1375-1389, ISSN 0010-938X... 

An important chromate-free acid etching technique is also available, it is designated the P2 [24] process. Here, as ferrous sulphate replaces the dichromate salts used in the more conventional chromic-sulphuric acid pickling, it is seen as being more friendly to the environment. Although not currently specified as the principle etching process, specifications for pretreatments such as phosphoric acid... 

A chromate-free Ti—Zr-based pretreatment provided improved durability relative to an alkaline etch and deoxidation pretreatment but was clearly inferior to chromating in terms of adhesion and FFC resistance. Excessive Ti—Zr oxide deposition occurred in the vicinity of intermetallic a-Al(Fe,Mn) Si particles during pretreatment, which significantly... 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

There are essentially three main steps in a conversion coating process cleaning, conversion coating, and post-treating. These three different, but equally important, steps in the pretreatment of metal articles will be discussed in more detail for the purpose of providing a background for the main emphasis of this paper, the post-treatment part of the conversion coating process, and more specifically chromium-free polymeric post-treatments which have been developed in recent years to replace the environmentally unacceptable chromate systems. 

The effects of chromium(III) chloride and sodium chromate(VI) on the hepatotoxicity of carbon tetrachloride exposure to mouse hepatocytes were examined by Tezuka et al. (1995). Primary cultures of mouse hepatocytes were pretreated with 10 or 100 pM chromium for 24 hours followed by exposure to 1-5 mM carbon tetrachloride for up to 1 hour. Chromium(VI) pretreatment significantly reduced the cell toxicity as well as lipid peroxidation caused by carbon tetrachloride. Chromium(III) pretreatment did not have any effect on cell toxicity. About 50% of chromium(VI) was taken up and reduced in the cells by 90% to chromium(III) within 10 minutes. The initial uptake rate of chromium(HI) into cells was greater than 500-fold less than chromium(VI), and only about 5% was absorbed. The protection against carbon tetrachloride damage by chromium(VI) was attributed to its rapid uptake and conversion to chromium(III), and it was determined that chromium(III) acts as a radical scavenger for the free radicals generated by carbon tetrachloride within the cell. Furthermore, chromium(VI) pretreatment reduced the activity of NADPH cytochrome c reductase which metabolizes carbon tetrachloride to reactive species. 

---