Silver chromate treatments

Where the corrosion resistance of a coating depends upon its passivity, it is common to follow plating with a conversion coating process to strengthen the passive film. Zinc, cadmium and tin in particular are treated with chromate solutions which thicken their protective oxides and also incorporate in it complex chromates (see Section 1S.3). There are many proprietary processes, especially for zinc and cadmium. Simple immersion processes are used for all three coatings, while electrolytic passivation is us on tinplate lines. Chromate immersion processes are known to benefit copper, brass and silver electrodeposits, and electrolytic chromate treatments improve the performance of nickel and chromium coatings, but they are not used to the extent common for the three first named. 

Another treatment consists of making the silver parts cathode in an alkaline chromate solution. 

Arsenites may also be determined by this procedure but must first be oxidised by treatment with nitric acid. Small amounts of antimony and tin do not interfere, but chromates, phosphates, molybdates, tungstates, and vanadates, which precipitate as the silver salts, should be absent. An excessive amount of ammonium salts has a solvent action on the silver arsenate. 

Cations forming insoluble chromates, such as those of silver, barium, mercury (I), mercury(II), and bismuth, do not interfere because the acidity is sufficiently high to prevent their precipitation. Bromide ion from the generation may be expected to form insoluble silver bromide, and so it is preferable to separate silver prior to the precipitation. Ammonium salts interfere, owing to competitive oxidation by bromate, and should be removed by treatment with sodium hydroxide. 

Chlorides, bromides, and iodides can be quantitatively determined by treatment with silver nitrate, and, with suitable precautions, the precipitated halide is washed, dried, and weighed. Chlorides in neutral soln. can be determined by F. Mohr s volumetric process 27 by titration with a standard soln. of silver nitrate with a little potassium chromate or sodium phosphate as indicator. When all the chloride has reacted with the silver nitrate, any further addition of this salt gives a yellow coloration with the phosphate, and a red coloration with the chromate. In J. Volhard s volumetric process, the chloride is treated with an excess of an acidified soln. of silver nitrate of known concentration. The excess of silver nitrate is filtered from the precipitated chloride, and titrated with a standard soln. of ammonium thiocyanate, NH4CN8—a little ferric alum is used as indicator. When the silver nitrate is all converted into thiocyanate AgN03-fNH4CNS=AgCNS +NH4NOS, the blood-red coloration of ferric thiocyanate appears. 

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. 

Upon treatment of the filtrate from 1 with sodium nitrite solution, chlorate and bromate are reduced to the simple halides, the presence of which is revealed by the separation of silver chloride and silver bromide respectively. Chromates (which, of course, yield a coloured solution) are simultaneously reduced to chromium(III) salts. 

The treatment of the Group I ions is summarized in Fig. 8.14. Note that the presence of Pb2+ is confirmed by adding Cr042-, which forms bright yellow lead(II) chromate (PbCr04). Also note that H+ added to a solution containing Ag(NH3)2+ and Cl- reacts with the NH3 to form NH4+, thus destroying the Ag(NH3)2+ complex. Silver chloride then re-forms ... 

As reported by McKenzie et al. (1948), catalytic hydrogenation of I with platinum oxide leads to an allylic alcohol (II), which upon treatment with methanolic hydrochloric acid and then with concentrated hydrochloric acid proceeds through a 12-methoxy derivative to the 12-chloride (III). This substance is not isolated but rather is treated directly with bicarbonate solution, whereupon the key intermediate, methyl 3a,9a-oxido-A -cholenate (IV), can be obtained in excellent yield. The bromination product (V) (Turner et al., 1946) of the oxide is oxidized with silver chromate to the 1 l-keto-12a-bromo oxide VI, which by successive debromination, hydrogen bromide opening of the oxide to VII,... 

The process can be repeated, and on each treatment with BaCl2 an amount of silver chloride equivalent to twice the original silver chromate is produced after n cycles l n + 1) moles AgCl can be found. 

More methods are available for chlorine. As free chlorine, the element may be analyzed by reduction to chloride using iodide, arsenite, alkaline hydrogen peroxide, sulfur dioxide, or sodium thiosulfate (Na2S203), or determined colorimetrically by treatment with ort/to-toluidine in hydrochloric acid. Chloride ion may be precipitated as silver chloride or titrated with silver nitrate in the presence of potassium chromate (K2Cr04). 

If the arsenic is present in the form of arsenious acid, arsenite, sulfide, alkali sulfoarsenite, or sulfoarsenate, it should be converted into arsenate by treatment with ammonia and hydrogen peroxide. The test is only specific in the absence of chromates and ferricyanides which also give colored silver salts, insoluble in acetic acid. 

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