Anticorrosive coatings chromates

The change in the toxicological classification of lead- and chromate-based anticorrosive pigments has resulted in reappraisal of the formulation of anticorrosive coatings. 

Emulsion systems for anticorrosive coatings are usually copolymers of vinylidene chloride, vinyl chloride, and an alkyl acrylate or methacrylate. Relatively small amormts of surfactant and colloid are used in order to minimize water sensitivity in the dried coating. Flash rusting inhibitors such as ammonium chromate/dichromate or alkali metal nitrate/nitrites may also be added. 

Primers. The primer is a universal component of all anticorrosive coatings and is considered to be one of the most important element of a protective system. A good primer generally provides the ability to stifle or retard the spread of corrosion discontinuities such as pinholes, holidays, or breaks in the film. To perform satisfactorily they must themselves adhere well to the base metal or any surface conversion coating that might be present. They should also contain an adequate concentration of a leachable inhibitor, where this is considered an important feature of the protection system, and this is usually a chromate pigment. The primary functions of a primer are... 

A primer On metal, the purposes of a primer are to enhance corrosion protection and to give excellent adhesion. The primer will contain anticorrosive pigments, such as strontium chromate or zinc phosphate, which will slowly release ions that can repair damage or faults in the underlying conversion coating. 

Surface Preparation of the Substrate. This is extremely important for all methods of paint and coatings application. The failure of a paint system is often due not to the paint itself, but because of a failure in surface preparation. For example, an anticorrosive paint applied to a rusty surface will not be effective if the rust falls off taking the new paint with it. For wood and plastic surfaces, old paint or a weathered surface layer may have to be removed. For older metal objects, the removal of corrosion is often required. Sandblasting is one method to remove both the old paint and any corrosion. For new metal objects, a phosphate or chromate layer is often chemically bonded to the metal to provide a surface to which a coating can easily adhere. 

Strontium chromate is used in the following applications as a pigment, as an anticorrosive agent in aluminum and magnesium alloys, in vinyl sheeting, and in chemical-resistant coatings. 

Zinc chromate is used in the following applications in pigments in paints, varnishes and oil colors, as an anticorrosive in primer coatings, as metal conditioners prior to priming, and as a catalyst. 

Water-soluble Cl significantly improve the protective ability of oil, alkyd, alkyd-styrene and alkyd-nitrocellulose coatings (Fig. 3.41), for example, 3wt% of water-soluble potassium chromate guanidine. Cl has been introduced into the lacquer base, after which a 20-pm-thick single-layer coat has been applied on a plate of low-carbon steel (St 2). The time until the appearance of corrosion traces under the coating has been recorded on exposure to atmosphere at 100% humidity and a temperature of T = 18-25°C. The results presented in Fig. 3.38 prove that the introduction of this Cl has perceptibly improved the anticorrosion properties of lacquer coatings [1]. 

It has been established by express tests that the introduction of chromate inhibitors at amounts below 3 wt% amounts in alkyd paint compositions does not improve their anticorrosion properties as compared to non-inhibited coatings. 

To obtain a coating with good corrosion protection, anticorrosion pigments have to be used (e.g., red lead, zinc chromate, zinc phosphate) and/or the paint must act as a barrier against the aggressive media. 

A. Seth, W.J. Van Ooij, P. Puomi, Z. Yin, A. Ashirgade, S. Bafna, and C. Shivane, Novel, one-step, chromate-free coatings containing anticorrosion pigments for metals - An overview and mechanistic study Prog. Org. Coatings, 58, 136-145 (2007)... 

Primer - applied to the dean surface and designed to adhere strongly and prevent corrosion. These can be chromate or phosphate coatings, as previously described, or etch-primer solutions and anticorrosive primers, containing zinc phosphate or iron oxide, and can be single or two pack. Some waterborne primers are available. 

Inorganic inhibitors incorporated into coatings include borate, chromate, molybdate, phosphate, phosphite, or silicate, typically as anticorrosive pigments (salts) of the metals aluminum, barium, calcium, strontium, or zinc. It should be noted that many of the inorganic inhibitors are anions and are, therefore, candidates for incorporation as dopant anions into CP coatings for controlled release. 

A. Ashirgade, P. Puomi, W.J. van Ooij, S. Bafna, A. Seth, C. Shivane and Z. Yin, Novel, One-Step, Chromate-Free Coatings Containing Anticorrosion Pigments for Metals that can be Used in a Variety of Industries , paper pubhshed on a conference CD, Eurocorr 2006 in Maastricht, Sept. 24-28, 2006. 

This yellow anticorrosive pigment is among the most expensive chromate-based pigments. It is mainly used to protect aluminum from corrosion. Epoxy primers containing strontium chromate are well-accepted in the aircraft industry. They are effective at low pigment levels and are also used in some coil coating applications. 

Thus, primers for new wood and certain other surfaces must provide for exceptional absorption of the medium. Primers for steelwork contain special anticorrosive pigments, such as red lead, zinc chromate, zinc powder, etc. See Metal Primer and Plaster Primer. A coating applied to a surface, prior to the application of an adhesive, to improve the performance of the bond. (Martens CR (1968) Technology of paints, varnishes and lacquers. Reinhold, New York Paint / coatings dictionary. Compiled by Definitions Committee of the Federation of Societies for Coatings Technology, 1978)... 

For many decades, chromate compounds have been successfully used as anticorrosive inhibitors in the surface treatment of aluminium and others alloys. The use of chromates is however restricted worldwide, as they are considered highly toxic and carcinogenic [1], This has stimulated research aimed at the development of effective and environmentally acceptable alternatives to chromates. To obtain the beneficial properties of chromatation, two approaches can be utilised passive [2] and active [3,4] corrosion protection. Passive protection is normally provided by a barrier film that prevents contact between the corrosive species and the metal surface and therefore hinders a corrosion process. However, when a defect is formed in the barrier layer, the coating cannot stop corrosion in that place. The second approach is active corrosion protection, which employs inhibitive species that can decrease corrosion activity. An important point is that both strategies must be used together to protect the metallic substrate adequately. 

Scratching was performed on galvanised steel samples coated with anticorrosion layers such as ppHMDSO or chromated layers. This simulates severe damage caused to these layers. 

---