Chromate passivator

Zinc will initially react with cement-based materials with the evolution of hydrogen. This reaction can be controlled by the presence of soluble chromate either in the cement (over 70 ppm) or as a chromate passivation treatment to the zinc surface. Zinc can therefore be used to provide additional protection to steel in concrete. It is more effective in cmbonated concrete than in chloride-contaminated concrete. 

Properties of deposits These deposits invariably possess an appreciable number of discontinuities, which diminish their protective value. At such discontinuities there is an inherent susceptibility to corrosion because of the galvanic relationship that necessarily exists between A/, and. Corrosion resistance may, however, sometimes be improved by sealing (cf. anodising) or by chromate passivation. 

BS 6536 1985 refers to four grades of carbon steel strip coated with Al-Si alloys containing 5-11 wt.% Si. The availability of the steel grades allows account to be taken of the type and degree of forming which is to be applied to the coated steel. Various coating masses are supplied in the range 40-180 g/m which may be additionally protected by a chromate passivation treatment. 

Full chromate passivation (Section 15.3) improves the corrosion resistance of both zinc and cadmium towards all environments and is applied for a wide range of applications. Clear and olive-coloured chromate coatings can also be applied for certain purposes. The highest degrees of corrosion protection... 

In view of its susceptibility to sulphide tarnishing, silver may itself require some measure of protection in many decorative and industrial applications. Chromate passivation processes are commonly employed, but as an alternative, thin coatings of gold, rhodium or palladium may be used. 

The best protection is given by paint. An etch-primed paint scheme can be applied directly to the metal for other paints an inorganic treatment must be given to ensure good adhesion. Of the two classes of inorganic treatment, phosphate treatment has little protective value in itself, but chromate passivation gives appreciable protection and in mildly corrosive surroundings may be sufficient in itself. 

The most commonly used chromate passivation process is the Cronak process developed by the New Jersey Zinc Co. in 1936, in which the parts are immersed for 5-10 s in a solution containing 182 g/1 sodium dichromate and 6ml/l sulphuric acid. A golden irridescent film is formed on the zinc or cadmium surface. Many variants (all fairly acidic) have been developed subsequently all are based on dichromate (or chromic acid) with one or more of the following sulphuric acid, hydrochloric acid (or sodium chloride), nitric acid (or nitrate), phosphoric acid, formic acid and acetic acid. A survey by Biestek shows that several of these variants are as good as the Cronak process, although none is superior. 

Fig. IS.S Effect of sulphuric acid concentration on chromate passivation of zinc. Solution 182g/1 of Na2Cr207 -2H20 + H2SO4 as indicated temp. I8°C I 0 x 10 mg Zn/cm =...

Most recently, these same authors have employed an in situ cell (Fig. 14) for carrying out these experiments. Again they studied nitrite- and chromate-passivated films. The results obtained in this case are quite different from the ex situ measurements. In addition,... 

The Fourier transforms for both films are again quite similar, but as for the ex situ measurements, the chromate-passivated films appear to have a more glassy structure. It should be mentioned that these studies employed a rather limited data range which makes spectral differentiation difficult. 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

Chromates are seldom employed these days. When chromate passivation was commonplace, the chromate was often held at 50 to 100 ppm C1O4 for 8 to 16 hours at 100°F and pH 6 to 7. 

A similar coating system (two-coat coilcoating system on phosphated and chromate passivated galvanized steel) was exposed to a pure salt spray test (360 h). The... 

A chromate passivation usually used in coil-coating lines prior to the application of the organic coating leading to ultrathin... 

To retard weathering discoloration and improve atmospheric corrosion resistance, in addition to preventing wet storage staining, chromate passivation procedures have been developed. 

Most types of cement contain small quantities of chromates. These chromates passivate the zinc surface, which is then not attacked by fresh concrete. If the cement contains less chromate than will give at least 20 ppm in the final concrete mix, the hot dip galvanized bars can be dipped in a chromate solution or chromates can be added on the basis of twice the minimum required. Avoid handling the chromates directly, for this can cause dermatitis hexavalent chromate is a toxic substance. 

The white storage stains on zinc and aluminum coatings can be prevented via application of a chromate passivation treatment, which forms a hydrated chromium oxide film on the surface. 

The chromate passivating ion is among the most efficient passivators known. However, due to health and environmental concerns associated with hexavalent chromium, this class of anticorrosion pigments is rapidly disappearing. 

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