1/2/2023 0 Comments Primer paintJarman, in Shreir's Corrosion, 2010 3.35.6 Protection of Welded Joints The silane improves the adhesion of the primer to the metal, the Cr-free inhibitors incorporated in the primer protect the metal from corrosion and the functionality of the silane-containing polymer matrix ensures good adhesion to the subsequently applied topcoat. The VOC problem is solved by dispersing the resin, the crosslinker, the silane and the Cr-free inhibitors into a water-based solution. The future system (scheme C) involves only two layers, the primer and the topcoat. Therefore, for the Cr-free primers to perform equally to Cr-containing primers they need to contain a mixture of Cr-free inhibitors to cover the entire pH-range and the paint layer matrix needs to be designed so that they are able to leach out on demand to protect the base metal. It is, however, not so easy to just replace the chromate in an existing paint with a Cr-free inhibitor, because none of the Cr-free inhibitors are active over the whole pH range as chromate is and they also do not possess similar optimal solubility properties as, e.g., strontium chromate does. These types of Cr-free inhibitors are incorporated into both solvent- and water-based primers, because simultaneously as legislation is pressing paint producers to discontinue the use of chromates, there is pressure to reduce the amount of volatile organic compounds (VOCs) in paints as well. Hybrid inhibitors possess inhibition action of both the types of inhibitors. Organic inhibitors such as azoles typically function by forming a film on the metal and providing protection by sealing the location of corrosion. Inorganic inhibitors work by leaching and dissolving out from the coating and acting as anodic or cathodic inhibitors. Inorganic inhibitors studied for paint applications include nitrites, phosphates, molybdates, metaborates, silicates and cyanamides, but actually very few of these possess inhibitor properties suitable for use in paints. These can be classified as inorganic, organic and hybrid inorganic–organic inhibitors. Several Cr-free inhibitors have been investigated as replacements for chromates in organic coatings. In summary, it is the chromate in the primer that actively inhibits corrosion of the metal. Remarkably for chromates they work in the whole pH region providing both anodic as well as cathodic protection to metals. In order to protect the metal the hexavalent Cr in CrO 4 2 − gets reduced to trivalent state and passive trivalent Cr(OH) 3 is formed on the metal where the corrosion started. The chromate CrO 4 2 − ion is a powerful oxidizing agent. Due to its low and selective solubility, it leaches out from the primer only on-demand, when the painted metal gets scratched or torn. When the primer is loaded with chromates (scheme A) then the chromate in the primer coating protects the metal in the same way as in the thinner CCC layer. It is well known that in typical duplex systems, the most important layer for active corrosion protection of the metal is the primer layer. 3 is to protect the metal by providing a good barrier against, e.g., water, electrolytes and pollutants. The function of the duplex paint coatings on all systems shown in Fig. 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. 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. 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. 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. Still in many applications the primer coating is loaded with chromate, typically strontium chromate, because its solubility is optimal in primer coatings. For instance in aerospace applications the primer coating is usually 25 μm thick, in coil coatings it is around 8 or 20 μm depending on the type of coating. The primer paint layer is, however, much thicker than the CCC layer. Puomi, in Thermodynamics, Solubility and Environmental Issues, 2007 3.3 The Primer Layer Containing the Corrosion Protective Inhibitors
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