Shop Problems Plating and Anodizing

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Use the Shop Problem Card in thi\ isue for free expert advice.

Brown Spots on Lacquered Brass

Q . We strip brass castings. mostly . antiques, using a soap solution

with a rectifier. After rinsing, bright dipping, polishing, and lacquering brown spots appear under the lacquer finish. Can you provide some assis- tance with this problem?

H.A.

A . The problem is related to po- . rosity in the castings. Solution

gets trapped in the pores during pro- cessing and leaches out after lacquer- ing. Since you’re dealing with an- tiques, the problem will show up in some cases but not others depending on the quality of the piece. In order to reduce the occurance of this bleeding out, you should improve the rinsing in your operation. In some cases, alternat- ing of hot and cold rinses to squeeze out the solution in the pores has proven helpful. Another approach is to fully dry out the parts prior to lacquering by placing them in an oven.

Bonding Solid-Film Lubricants to Aluminum

Q . I need to find a procedure to . bond a solid-film lubricant to

aluminum alloys. Would a porous an- odized layer work?

R.L.

A . A solid-film lubricant is basi- . tally a paint that utilizes a pig-

ment to provide lubricity instead of hiding power or coloration. Thus, pro- cedures for paint pretreatment would be ideal for preparation of aluminum alloys for bonding of solid-film lubri- cants to aluminum or aluminum alloys. The choices would include iron or zinc phosphate, anodize, chromate, chrome phosphate, or alkaline chromate oxide. A porous anodized coating would probably be suitable providing paint adhesion and corrosion resistance properties in the middle of the range of possibilities. Details on all these paint

pretreatment processes can be found in the Pretreatment Section of the Or- gunic Finishing Guidebook and Direc- tor-y Issue.

Poor Solderability

9

. We are having a problem with

. our RF shields involving poor sol erability. Currently the shields are plated with 30 microinches of matte tin over a low carbon steel base. Some of these shields don’t get assembled for up to 4 to 6 months after plating. The iron-tin intermetallic surface growth and thus oxide coating on these inter- metallics causes poor solderability. I have read a little on tin whisker growth. Would increasing the tin de- posit to 125 microinches or adding a nickel underplate help? Any informa- tion on tin-plated steel in regards to solderability would be of assistance.

M.D.

A It is unlikely that you are expe- 1 riencing iron-tin intermetallic

growth, but there will be an oxide coat- ing formed. This can be removed with a mild abrasive cloth such as Scotch Brite. Tin whisker growth, although an important consideration, is not related to solderability problems. Addition of a nickel underplate would be helpful; however, increasing the tin deposit thickness would be most effective. The 125 microinch thickness you men- tioned would be an improvement, but 250 microinches would be recom- mended.

Further information on tin plating and solderability can be found in the excellent book by Manfred Jordan en- titled “The Electrodeposition of Tin and Its Alloys.” This is available from the Metal Finishing Books Depart- ment, which can be contacted at 914/ 333-2578; fax 9141333-2570 for de- tails.

White Scale After Caustic Etch

R

In my anodize shop the rinse : tank after the sodium hydroxide

etc tank has developed a white scale

along with the overflow rinse drain line. The tank ih mild steel and the drain line is PVC. Is there any way to remove the scale short of hammer and chisel?

L.C.

A . In this case an ounce of preven- . tion is certainly worth a pound

of cure. Based on the location of your plant (Southwestern Texas), I would venture to say that the problem is caused by water hardness. The in- creased alkalinity following the etch is enough to promote scale formation. You should consider installation of a softening or deionization system to eliminate the cause of your scaling problem.

Smut on Brazed Copper Parts

Q . We have heavy copper brazed . parts that build up smut causing

poor coverage in our chloride zinc bath. The cleaning/acid cycle on the automatic line is short, but something could be added to our off-line pickling system to preclean them. The pickling system includes 30% hydrochloric acid and a proprietary acid salt. Please rec- ommend what to use without messing up the waste treatment.

J.K.

A . If the smut is caused by the heat . from brazing, hydrochloric acid

will not remove it. A persulfate-type acid or proprietary oxidizing acid will be required to do the job.

It’s also possible that the acid on your automatic line may be building up in copper. This could lead to immer- sion copper deposits, which will also cause poor coverage. To test for this possibility, conduct a test by immers- ing a steel panel into the acid. If copper is present, it will plate out and you will see a reddish deposit on the steel panel.

MF

METAL FINISHING ?? APRIL 1997 75