Zinc silicate films

The zinc silicate film is porous and readily absorbs corrosive electrolyte. The film is harder, stronger than organic zinc-rich films. These films have better resistance to solvent and heat than the organic zinc-rich primers and may be used for tank linings and other applications up to 400°C. 

Figure 1.66 Stylized representation of postulated structure of inorganic zinc-rich film in which silicate vehicle is primary valence bonded to zinc atoms on particles of zinc dust pigment. There is no encapsulation, and the film is porous to ingress of electrolyte. This affords good film strength, adhesion, electrical conductivity and cathodic protection...

Typical fillers carbon black, calcium carbonate, dolomite, clays, calcinated clays, talc, soapstone, zinc oxide, filmed silica, borates, iron oxide, zinc oxide, magnesium carbonate, pulverized polyurethane foam, barium and strontium ferrites, magnesium aluminum silicate, nylon fibers, quartz in EMI shielding field silver plated aluminum, silver plated nickel, silver coated glass spheres, silver plated copper, silver, nickel and carbon black... 

Zinc silicate outstanding neutral solvent resistance poor acid and alkali resistance poor film formation at low humidity... 

In some of the more corrosive soils, corrosion of the steel (after the zinc has been completely destroyed) is considerably less than that observed on uncoated steel control specimens. Also, it appeared in some soils that the protection afforded to the steel was not due solely to sacrificial corrosion of the zinc but was supplemented by some other mechanism. A few tests indicated that a film, consisting primarily of zinc silicate formed by galvanic action between the zinc and the underlying zinc-iron alloy or bare steel, was responsible for the additional protection. 

The above-mentioned characteristic explains the great porosity of zinc-rich nano lithium silicate films and their high cathodic protective activity as comparing with zinc-rich tetraethyl orthosilicate films. 

Firstly the silicate film is concentrated as the solvent is evaporated. The binder (tetraethyl-silicate) hydrolyzes to form silicic acid which reacts with zinc to form silica-oxygen zinc polymer. Zinc reacts and combines with the matrix. The unreacted zinc surrounds the large molecules. A film of silica matrix will surround zinc polymer. 

Szab6, T., N6meth, J., and Dekany, 1. (2004) Zinc oxide nanoparticles incorporated in ultrathin layer silicate films and their photocatalytic properties. Colloids Surf. Physicochem. Eng. Asp., 230, 23-35. 

The flow properties are not as good as those of zinc phosphate cement (Eames et al., 1978 Hembree, George Hembree, 1978) and film thickness is greater (Table 6.11). Moreover, it does not have the translucency of dental silicate cement (Wilson, 1975c). 

Inorganic zinc-rich primers consist of zinc and a reactive binder such as an alkali metal silicate (sodium, potassium, lithium or quaternary ammonium silicate) or hydrolyzed ethyl silicate as binder. On mixing hard and cohesive films of silicate are formed. The structure of the inorganic zinc-rich silicate may appear as shown in Figure 1.66. 

Corrosion control is one of the demonstrated uses of silicates in detergents since builders may be imagined as breaking the structural integrity (and increasing the rate of corrosion) of the hydrous oxide films that protect metals such as aluminum in contact with water. Other metals (e.g. zinc) and combinations of materials are sometimes attacked by alkaline detergents. 

On application, the paint layer hardens within a short time due to evaporation of the solvent, progressive hydrolysis of ester groups after uptake of atmospheric moisture, reaction of silanol groups with the zinc and substrate, and condensation of silanol groups to form sparingly soluble, cross-linked silica gel [2.186]. Conditions are generally chosen such that the film is hard enough to allow transportation after 4 h and a topcoat after 24 h. Alkyl silicate zinc dust paints are particularly suitable... 

The zinc oxidation products form a hard cementlike layer in the silicic acid film. Resistance to diffusion is increased and attack of atmospheric oxygen on the substrate is thus slowed down (barrier protection). 

CS-100. See Dimethicone CS-420. See Silicone emulsion CS-922. See Calcium sodium caseinate CSA. See Chlorosulfuric acid CSC. See 4-Cyanobenzenesulfonyl chloride CSE-6000 Series. See Epoxy resin CSet. See Starch CS Film. See Polyethylene CS gas. See o-Chlorobenzylidene malononitrile CSM. See Polyethylene, chlorosulfonated C Sodium Silicate. See Sodium silicate CSorbidex C CSorbidex NC] CSorbidex P, CSorbidex S. See Sorbitol CSP. See Cupric sulfate pentahydrate CSPE. See Polyethylene, chlorosulfonated CStabiTex 06301] CStabiTex 06305] CStabiTex 06307. See Food starch, modified CStabiTex-lnstnat 12631] CStabiTex-lnstnat 12632. See Starch, pregelatinized CSX-240. See Carbon black CT-58. See Zinc phosphate CT-62. See Zinc chloride CT-70. See Sodium silicate CT-708 Potable Water Treatment. See Sodium hexametaphosphate CT-781] CT-788. See Zinc phosphate CTA. See 4-(Methylthio) benzonitrile CTA. See 2-Ethylhexyl thioglycolate... 

Aramaki K. Self-healing mechanism of an organosiloxane polymer film containing sodium silicate and cerium (111) nitrate for corrosion of scratched zinc surface in 0.5 MNaCl. Corrosion Science, 44(7), 1621-1632 (2002). 

Smith, R.K., Lewis, P.A., and Weiss, P.S. 2004. Patterning self-assembled monolayers. Prog. Surf. Sci. 75 1-68. Socha, R.P. and Fransaer, J. 2005. Mechanism of formation of silica-silicate thin films on zinc. Thin Solid Film. 488 45-55. 

Immersion test in 0.1 M sodium chloride solution allowed to observe, particularly in those panels with X-cut, that coatings based on nano-structured film-forming material as binder showed greater amount of white products from corrosion of metallic zinc than in those panels protected with primers made with partially hydrolyzed ethyl silicate as binder. This performance would be supported in the less zinc dispersion ability that displays the first binder, which would generate films more porous. 

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