Bath chromic acid

Because the thickness of the plate deposited from trivalent baths is limited, these have only been employed for decorative appHcations. However, the bluish white deposit obtained from chromic acid baths can be closely matched by trivalent chromium baths (173). 

In 1979, a viable theory to explain the mechanism of chromium electroplating from chromic acid baths was developed (176). An initial layer of polychromates, mainly HCr3 0 Q, is formed contiguous to the outer boundary of the cathode s Helmholtz double layer. Electrons move across the Helmholtz layer by quantum mechanical tunneling to the end groups of the polychromate oriented in the direction of the double layer. Cr(VI) is reduced to Cr(III) in one-electron steps and a colloidal film of chromic dichromate is produced. Chromous dichromate is formed in the film by the same tunneling mechanism, and the Cr(II) forms a complex with sulfate. Bright chromium deposits are obtained from this complex. 

Black and colored plates can also be obtained from chromic acid baths. The plates are mostly oxides (177). Black chromium plating bath compositions are proprietary, but most do not contain sulfate. The deposit has been considered for use in solar panels because of its high absorptivity and low emissivity (175). 

Plate Thickness. Thickness of the plate should always be specified as should the locations on the work where the thickness is to be measured. Generally, thicker deposits perform better, but there are notable exceptions. Mating parts, eg, fasteners having fine machine threads, are not usable if over plated. Machine-threads are usually plated to 10 p.m or less, depending on tolerances. Additionally, gold-plate over nickel does not solder well if too thick thus, gold is usually 1—2 pm or less. Chromium, plated for decorative purposes from the conventional chromic acid bath, tends to macrocrack above about 0.7—1.0 pm. 

Specialty Chromium-Plating Baths. Chromic acid baths using sodium chromate and sodium hydroxide to form a tetrachromate (92) have had limited use. Porous chromium is used in lubricated wear applications, and is made by chemically etching regular chromium plate, sometimes with light grinding after the etch. Black chromium is used on solar collector surfaces (see PHOTOVOLTAIC CELLS Solarenergy). Baths are sulfate-free, and include fluosilicic acid or acetic acid (91). 

Fig. 8-33. Analysis of sulfate and chromate in a chromic acid bath. - Separator column IonPac CSS eluent 0.001 mol/L NaHC03 + 0.005 mol/L Na2C03 flow rate 1.5 mL/min detection suppressed conductivity injection 50 pL sample (1 2000 diluted).

Ion chromatography provides an alternative to atom spectroscopic methods for the determination of heavy and transition metals. Its main advantage is the simultaneousness of the procedure and the possibility to distinguish between different oxidation states. For example, the determination of iron(III), copper, and zinc in a chromic acid bath (Fig, 8-41) may be performed free of interferences despite the high chro-mium(VI) load. Fig. 8-42 illustrates the determination of heavy and transition metals in a nickel/iron plating bath. Both oxidation states of iron can be clearly distinguished via ion chromatography. 

Special cleaning of glass - for an acid wash use dilute acid, rinse with tap water (three times) to remove the washing solution and then rinse thoroughly at least three times with distilled or deionized water. To remove acidic deposits, wash with KOH/ethanol solution followed by rinsing with deionized water. Glassware, which must be exceptionally clean, should be washed in a chromic acid bath, but this must only be made up and used under supervision. 

The good solvent resistance of polyethylene and other olefins precludes the use of solvent-type cements. Several commercial rubber-type adhesives produce moderate adhesion with polyethylene that has been surface treated. One technique for surface treatment is to dip polyethylene in a chromic acid bath (made up of concentrated sulfuric acid 150 parts by weight, water 12 parts, and potassium dichromate 7.5 parts) for about 30 sec at 70°C. The parts are rinsed with water after this treatment. Still another effective surface treatment for producing cementable surfaces on polyethylene is electrical discharge. The open oxidizing flame method is also used extensively for this purpose. 

Chromium plating from chromic acid baths is more sensitive to the source of current than most other processes, sufficiently so for commercial operators to use at least three-phase rectifiers as a rule, and to take precautions against any temporary break of current during voltage regulation. A recent investigation showed that the ripple introduced by thyristor control of rectifiers was detrimental to chromium electrodeposits. 

L Electroplating of chromium from chromic acid baths leading to spent or contaminated baths and rinsewaters. 

---