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Plating and Anodizing Use the Shop Problem Card in this issue for free expert advice.
We repeat the question and answer for convenience.
Gold Bath Purification
Q . How would we go about purify- * ing a contaminated acid gold
A . Except for nickel and cobalt, ?? cupferron (ammonium salt of
N-nitrosophenyl hydroxylamine), available from laboratory supply houses, will precipitate heavy metals at room temperature when added at about seven times the metal concentration. Any excess will decompose. The rea- gent is stirred for about 20 min, followed by powdered activated car- bon and filtration through a OS-l.0 pm filter. According to A.J. Avila, with 100 ppm contamination, 90% is removed. If nickel and/or cobalt are also to be removed, dimethylglyoxime is the suggested reagent.
Considering the effort and the una- voidable losses of gold, it may be more practical to send the bath back to the supplier or to a refiner.
R . I read with interest the reply to ?? “Gold Bath Purification,” a
shop problem in the December 1994 issue on page 65, and believe that I have a better candidate than those mentioned.
The reagent is 8-hydroxyquinoline, which dissolves in most acids used on gold bath formulations. It is most effective when added to a neutral or slightly alkaline gold bath and will remove nickel, cobalt, iron, copper, zinc, cadmium, mercury, indium, and a host of others without loss of gold. It forms a bulky precipitate that is easily filtered and should be added to a warm solution (14O’F). The reagent require- ment is about eight times the contami- nation level by weight. Nickel and
cobalt are incompletely removed due to a tendency to form complex cya- nides, which do not end up in the deposit anyway.
The 8-hydroxyquinoline has a char- acteristic odor when present in the bath and carbon treatment is not recom- mended after treatment. In fact, a small amount is beneficial in 24karat gold baths, preventing a buildup of metallic impurities in the bath, and small amounts can be added if the odor is not detected.
Nonuniform Black Anodizing
Q . I have a conventional anodize ?? shop (170 g/L sulfuric acid
bath). Several customers have cast aluminum parts that they want dyed black. The cast parts come out with lighter spots in the material, which is not uniformly black as with 6061-T6 alloy. Can anything be done to im- prove the appearance of dyed anodized cast parts?
A . This is an age-old problem with ?? aluminum castings. The prob-
lem is the nonhomogeneous nature of the cast parts. There is a variation in the silicon content throughout the body of the material. Since silicon doesn’t anodize, the coating will be thinner in those areas higher in silicon, leading to less take up of dye and lighter color.
There is no method that can guaran- tee uniform coatings. One precaution that can be taken is to clean and deoxidize the parts, but eliminate any etching, which only makes matters worse by bringing up the silicon.
It may also be helpful to improve rinsing after anodizing. Warm rinses, neutralizing with sodium bicarbonate, or a nitric acid dip prior to dyeing will help to remove sulfuric acid from discontinuities in the casting and thus improve color to some extent.
Yellow Electroless Nickel
Q . We deposit electroless nickel ?? (4-6 pm) on steel watch parts in
a vibratory plating machine. After 0.4 metal turnovers, the pieces change color from white to dark yellow. We have tried several different baths (8- 10% phosphorus) and pretreatments. The dark color is not removed by either alkalis or acids and also occurs if we rinse directly with cold water.
Rack, barrel, and basket plating operations are absolutely okay, but it’s not possible to plate these pieces in a barrel. Can you offer any suggestions?
A . There are two areas that could ?? be investigated. First, better
rinsing before plating to remove sur- factants. Try cutting the concentration of the cleaner and improving the rinsing after cleaning. It might be better to clean the parts in a separate operation, then utilize a mild cleaner in the vibratory machine, rinse thor- oughly, and plate.
A second area to check is the loading factor. Do not underload the vibratory machine. Since the make-up compo- nents of the bath are designed for a full load, you might be getting out of balance.
Plating Silver on Chromium
Q . Kindly advise as to how to carry ?? out silver plating (O.OOOS-
0.0007 in. thick) as per QQ-S-365 over chrome plating (0.0005-0.001 in. thick) already done as per AMS 2406. The base material is aluminum alloy.
A . The following sequence should ?? be suitable:
1. Direct electrolytic alkaline clean 2. Rinse 3. Wood’s nickel strike 4. Rinse 5. Silver strike 6. Silver plate 7. Rinse and dry. MF
METAL FINISHING . APRIL 1995 57