Shop Problems Plating and Anodizinrz

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Use the Shop Problem Card in this issue for free expert advice.

CORRECTION

The following question and answer were published on page 120 in the June issue. Some of the values given in the test procedure were incorrect. The cor- rected values along with the original question are provided below.

Analyzing Nickel Seals

Q . Can you provide a procedure . for analyzing the nickel content

of our nickel seal tank? T.C.

A . A procedure based on titration . with ethylene diamine tetra

acetic acid (EDTA) in the presence of Murexide indicator should prove ade- quate. The details are as follows:

1.

2. 3.

4.

5.

6.

Pipette 5 ml of the sample into a clean 125 ml Erlenmeyer flask. Add 500 ml of demineralized water. Add 5 ml of concentrated ammo- nium hydroxide. Stir contents and add 0.1 gram (or 2 tablets) of Murexide indicator. Titrate while stirring with 0.1 M EDTA to a purple endpoint. Determine the nickel content (in g/L as nickel) by multiplying the number of mls of EDTA used by the factor 1.18.

Replating Stripped Parts

Q . We sometimes have trouble . plating hexavalent chromium

on smal1 (3/4 to 1 in.) high-carbon steel parts. These parts are always being chrome plated and stripped. The strip tank is made up of sodium hydroxide and water. Some parts work fine, but others can’t be plated again. Passiva- tion is probably the problem. We need a method to depassivate. Hydrochloric acid doesn’t work.

K.L.

A . On the parts that won? replate, . first be sure that al1 the chro-

mium has been removed. If it has, try a

reverse etch in the plating bath (or using the same formulation in a sepa- rate tank to avoid iron buildup). You wil1 have to experiment to find the best conditions. Begin at a current density of about 0.25 A/in2.

If this doesn’t solve your problem, it may be necessary to include some ad- ditional steps. High-carbon steels are prone to formation of smut. This can be removed by anodic treatment in an electrolytic cleaner prior to the reverse etch. You might be able to avoid pas- sivation by stripping in hydrochloric acid.

Blackening Electroless Nickel

a

. Can you recommend a process

. for blackening electroless nic el? The deposit contains 10% by weight phosphorus.

S.L.

A . A simple process for blacken- . ing electroless nickel was de-

scribed by C.E. Johnson in the July, 1980 issue of Metal Finishing on pages 21 to 24. It consists of etching the dry electroless nickel coating in a nitric acid solution and rinsing in tap water, then distilled or deionized water, and finally ethyl alcohol followed by dry- ing. The time of immersion depends upon the concentration of the nitric acid, the solution temperature, and the composition of the electroless nickel deposit. The nitric acid concentration can range from a 1:5 ratio by volume with distilled or deionized water to concentrated, with a nomina1 1: 1 ratio by volume. The temperature can range from 20 to lOO”C, nominally 50°C.

You would need to experiment to determine the optimum conditions and immersion time, but using the nomina1 conditions of a 1:l acid concentration and 50°C it would appear that an im- mersion time in the range of 30 to 60 seconds should be adequate to obtain an ultrablack surface on a 10% phos- phorus coating.

Cloudiness in Recycled Rinsewater

Q . We are attempting to recirculate . the rinsewater from our clean-

ing and pickling operations. The sys- tem consists of a three-stage counter- current rinse, which then flows to a holding tank. The water is then pumped through a bag filter, pH ad- justed, carbon filtered, and then passes to ion exchange. Sometimes a grayish color and cloudiness are observed in the rinse. Can you help to identify a source for this cloudiness?

R.T.

A . Cleaners often contain wetting . agents. These can react with ac-

ids to form oily compounds that farm cloudy emulsions. Circulating the cleaner and pickle rinses separately would eliminate the problem.

Refinishing Zinc Die Castings . A portion of our business, which

4 . is constantly growing, is the re-

finis ing of antique car parts. The prob- lem we’re having is with some zinc die castings. After conventional stripping, we glass bead blast and polish before copper/nickel/chromium plating. On oc- casion, after polishing, spots wil1 begin to appear. Without fail these spots al- ways result in blisters. Thus far we have not been able to solve the problem or predict on what pieces this wil1 occur so that we can avoid them. Any help would be appreciated.

J.M.

A . What you are seeing are corro- . sion pits and there isn’t much

that can be done to avoid them. This is a common problem encountered when refinishing old die castings. In some cases car buffs have resorted to having the parts recast. Another possibility is to encapsulate the part and then go through metallizing procedures as would be done with nonconductors such as plastics. MF

METAL FINISHING . OCTOBER 1996 63