Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:email@example.com
PLATING ONTO "DIFFICULT-TO-PLATE" METALS
I receive phone calls and e-mails from time to time asking about how to plate on some of the more unusual metals and materials. This paper deals with some of the inquiries I have encountered. The procedures are identified as to the source. Included is leaded steel and leaded brass and bronze, piezoelectric ceramics containing lead. Zirconium, Niobium and beryllium copper, I hope one or more can be useful to you.
I Leaded steel ("free machining") and leaded brass
Leaded steel and brass are difficult because of the nature of lead. The use of more traditional acids for metal preparation is not good for leaded metals. Lead forms insoluble compounds with sulfate as in sulfuric acid dips. The same is true for hydrochloric acid. Lead chloride and lead sulfate are both insoluble. They leave a film tightly adherent to the lead. There can be some slight loss of adhesion, but another result is "stardusting’ and dulling of bright deposits. "Stardusting" can be from pits or small areas of insoluble salts causing rough plating. Lead is present in small globules on the surface of these metals. Particularly in brass since lead migrates to the surface when brass castings cool.
What to do about lead in the surface? Use one of three acids for the preparation instead of sulfuric or hydrochloric acid.
1. Use Fluoboric acid. This acid will deoxidize and leave no film. Note that there is an increased danger in using fluoride. With repeated exposure, fluorides tend to accumulate in the body. After repeated exposure or one larger exposure will adversely affect fingernails, bones and other parts of the body. Comment: a very small amount of fluoride is very beneficial in strengthening teeth, prevent cavities and preserve enamel, etc. Fluorinated drinking water is well below the toxic level, and is in the beneficial level. These very small amounts accumulate only in the teeth and fingernail area helping both. The toxic level will not be reached drinking fluorinated drinking water.
2. Sulfamic acid works very well. It is very activating to most metals. The sulfamate very slowly hydrolyzes to sulfate over time. It is good to test for sulfate and when the level becomes significant, change the bath. Small amounts of sulfate will not form the insoluble films because the sulfamate dominates; therefore the sulfamic acid dip is long lasting. Sulfamate activates many metals found in alloying constituents of steel and brass.
Fluoboric acid and sulfamic will attack lead leaving tiny pits that can cause some minor star dusting if left too long in the acids. A copper strike will cover these areas and protect the basis metal from reacting to nickel plating solutions. The sulfate in nickel-plating solutions will cause stardusting without the copper strike.
3. The third acid is citric acid. Citric acid is very activating and leaves no residue. It has the additional advantage that it deoxidizes the lead and steel or brass very well without any chemical attack. I have found it useful for high strength steels, stainless steels and numerous other alloys. The problem with citric acid is that it can grow mold. And therefore needs an antimicrobial material added to the citric acid solution to prevent mold from forming. There are proprietary citric acid solutions with the antimicrobial material included. It works even better if the pH is raised slightly to 3.6 using ammonia. Since citric does not attack lead there is no danger of stardusting from the process step. The copper strike may still be necessary for bright nickel plating to prevent the nickel solution containing sulfate from attacking the lead and result in star dusting.
II Piezoelectric Ceramics.
Piezoelectric ceramics are used for transducers, weight measuring and a number of other applications. Compressing these ceramics produces electricity. Piezoelectric materials contain lead and other similar materials as a part of the matrix. In this case we want to attack the lead at the grain boundaries to provide a tooth for adhesion of electroless nickel deposits.
Process 1. By Dr. Charles Baumgartner
Process 2. By Don Baudrand
Although all of the process work, I like chuck’s method best. It is gentler, has no chromium to waste treat and works just fine. Baumgartner did a lot of work on this to come up with the final process. It is a good idea to read his paper. Reference 1.
Process 3. X. Shi, Y. Liu, X. Li, Q. Zheng and J. Fang
These people published in "plating & Metal Finishing, March 1997, referencing Baumgartner’s paper. Their process is presented below in simplified form.
Zirconium is used where severe chemicals must be in contact with something that will withstand them. Zirconium resists attack by sulfuric acid at all concentrations up to 75%. It is not attacked by 40-60% boiling sulfuric acid. A common use is heat exchangers and heaters for sulfuric acid processes. There have been occasions where plating is needed on some part of a zirconium device. Two preparation processes are presented here. There is some efficacy to using them in tandem.
Process 2. Zirconium
The sulfamate nickel strike is made as follows: 10 oz/gal nickel added as liquid sulfamate nickel solution. (Nickel sulfamate is available having 24oz/gal Nickel. Use 42% by vol.)
4 oz/gal boric acid
Add sulfamic acid to pH 2-1.6, and then add a small amount of hydrochloric acid to
pH 1 –1.5. Maintain the pH at that level using sulfamic acid and a little HCl.
V Beryllium Copper
Beryllium copper is used instead of pure copper for applications requiring more strength and hardness. Its unusual properties make it desirable for many electronic applications. Beryllium copper can be heat-treated to increase hardness to a figure greater than many other copper alloys. Common products are Berylco 10 and Berylco 24.
Charles E. baumganrner J. Cheram. Soc., 72  890-95 (1989)
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