In November 1978, Products Finishing magazine published “The Burn That Burns,”1 which examined true burn and false burn from bright nickel and chromium systems, as well as some of the remedies. Twenty-six years later, plating practitioners continue to be challenged and frustrated by this aspect of the process, prompting this updated and expanded review. Here, in part two of this three-part series, our authors analyze three of the four major sources of false burn…
Last month, we examined three of the four sources of false burn—electrical problems, false burns originating in the bright nickel solution and those originating between nickel and chromium stations. The fourth source—issues with the chromium plating solution itself—is a complex one, to say the least. Discussed below are 14 conditions that relate to false burn as a consequence of problems originating in the chromium bath.
1. High chromic acid to sulfate ratio. In order to obtain the maximum throwing power, some practitioners use a higher ratio than that considered normal. In full automatic lines, a higher-than-normal entry voltage and a narrower range of voltage for the live lead may be required if the ratio is too high. Hand tanks and hoist lines are more susceptible to these high-ratio problems, since live leads are seldom used here.
2. Low level of proprietary catalyst. Suppliers of proprietary catalysts specify concentrations and proper working conditions in their literature. It is prudent that these recommendations be followed. In general, a low proprietary catalyst level will produce problems similar to those resulting from low sulfate (high ratio): streaking, staining or spotting of deposits on parts anywhere on the rack true burn. Severe cases may produce “rainbow,” a pleasant name for one unpleasant problem.
3. Low sulfate. Results are the same as those from high-ratio baths, as discussed above.
4. Low current on live-lead entry to chromium bath. Again, this may result in low- and medium-CD staining and spotting, along with the false burn. This is more prevalent if the bath has an abnormally high total chromic acid/sulfate ratio and proprietary catalyst. Above-normal bath ratios almost always seem to demand higher live-lead voltage in automatics and vice versa. This can be one method of helping to identify the problem. Live leads for chromium are best constructed with a separate rectifier for the live lead. Installations that economize by using fixed resistors are not favored for best operation, since they do not allow for any flexibility in amperage from small to large racks.
5. High current on live-lead entry to chromium. This results in what might be called a “flash burn”. Appearance is very similar to that of a false burn but the marred area will be on that first portion of the part to enter the chromium solution. It’s really the result of excessive current on this small area. Very cold parts also may respond in this manner. The condition becomes more critical as the ratio drops below normal (higher sulfate) or if the chloride content is too high. Chlorides are about eight times as effective as the same amount of sulfates. This value when added to the amount of sulfates and the composite figure used to calculate a new ratio will not only reduce the actual ratio, but usually the chromium covering power as well.
Hand tanks and hoist lines are also subject to this flash burn if instantaneous high CD’s are applied to the rack immediately after setting down in the chromium tank. Better results will be obtained if the rectifier is turned on at an intermediate voltage and then raised (“ ramped”) to the operating plating voltage.
6. Scaled anodes in the first chromium station. The electrical insulation resulting from scale on the anodes reduces the current on the live lead (in full automatics), and thus current density is lowered. Additionally, low- and medium-CD staining and spotting3 may be experienced.
7. Bipolar problems. As complex as they can be, they are presented in great detail in references.1,3-6
8. Double contact in chromium tank. This usually results in widespread white deposits covering a larger area than the normal burn, depending upon severity of conditions. The resulting effect could be described as a more massive true burn. The same problems may occur if rectifiers exhibit high ripple, either by design or malfunction.
9. High trivalent chromium content. Trivalent chromium, Cr (III), is present in practically any chromium bath that has been operating for more than a few days. Some authors claim that in small amounts, Cr (III) is desirable and that such a bath will plate better at low CD’s. Others disagree. However, as concentration of Cr (III) is increased, the conductivity of the solution is decreased. Therefore this is not a case where “a little is good, and a lot is better.” High Cr (III) can add to the miseries of false burn and low- and medium-CD staining, spotting and streaking. To lower trivalent content, dummying at high cathode current density to reoxidize Cr(III) to Cr(VI) is in order. The ideal cathode/anode surface ratio is 1:30. Raising the temperature to maximum allowable for a tank lining will speed up the reaction. Also, agitation of the solution is helpful, since it moves the solution past the anodes, where the oxidation of Cr(III) to Cr(VI) takes place.
10. Incorrect anode area. Too-high or too-low anode area will cause not only false burn, but also a whole array of other problems, in both proprietary and conventional sulfate baths.4
11. Loose parts on the bottom of the tank. These can also cause “bipolar burns” and true burns by way of stray currents and metallic contamination of the chromium bath. Consult parts two and three.
12. Metallic impurities. Metals in solution normally increase the resistance in all chromium baths.7 In proprietary baths, some metals and chemicals react with the proprietary catalyst to reduce its level, producing the same results listed under two.
13. Organic impurities. Most of the organic materials in contact with a chromium bath will eventually be oxidized and decomposed by the solution. Oxidation of organic matter with hexavalent chromium from chromic acid produces unwanted trivalent chromium. Consult section nine.
14. Ripple. Usually causes a more widespread burn, similar to that from double contact. Covering power is also narrowed and there is a tendency to aggravate any other latent problems in nickel or chromium. Consult section eight.
After reading what has been presented, the readers may feel they have a lot in common with Christopher Columbus, who knew where he wanted to go, didn’t know how to get there, and didn’t know where he was when he arrived. Nevertheless, the essence of locating problems in a bright nickel-chromium system is not necessarily in being a nickel expert or a chromium expert. The key is in having an overall awareness of the singular or multiple interweaving effects that each bath or deposit has on the other; and how the electrical, chemical, physical (including people) and mechanical conditions interlock to produce problems.
It should be remembered that bad nickel with good chromium has the same results as good nickel with bad chromium—rejects! And one final comment. It is the writer's opinion that whatever defect appears after chromium plating, more than likely occurs within the first few seconds after entering the chromium solution. Exception is the true burn, which is time, CD, temperature and bath composition related.
Problem solving on paper, as presented here, is relatively simple. In practice, the ability to locate the point at which the problem is occurring may not be so simple. Important locations on the plating machine may be visually and virtually inaccessible. In addition, production and physical conditions may interfere with the discriminating tests that should have been devised to prove where the problem originates. For those who have gone this route, no further explanation is needed.
For the conscientious plater and engineer, this is of little concern, since the interesting part of this business has always been the solving of unsolvable problems and probably always will be.8
References 1. V.E. Guernsey, “The Burn that Burns,” Products Finishing, 42, no. 11 (1978). 2. V.E. Guernsey, “Those Elusive Little Amperes,” Plating & Surface Finishing 63, no. 2 (1976): pg 38; 63, no. 3 (1976): pg 44. 3. G.R. Davies, “The Effect of Current Source on the Properties of Chromium Electroplate,” Trans. Inst. Metal Finishing 51, no. 3 (1973): pg 47. 4. V.E. Guernsey, “Strangers in the Tank,” Plating & Surface Finishing, 64, no. 5 (1977). 5. N.V. Mandich, “Understanding & Troubleshooting Electroplating Installations: Stray Currents & Bipolar Effects,” Plating & Surface Finishing 89, no. 3 (2002): pg 40. 6. N.V. Mandich, “Practical Considerations in Bright and Hard Chromium Plating-part IV,” Metal Finishing 97, no. 9 (1999): pg 79. 7. N.V. Mandich, “Toward Understanding of Nickel Activations & Chromium Reverse Etching,” Plating & Surface Finishing 85, no. 12 (1998): pg 91. 8. N.V. Mandich, “Practical Considerations in Bright and Hard Chromium Plating- part III,” Metal Finishing 97, no. 8 (1999): pg 46. 9. N.V. Mandich, “Practical & Phenomenological Aspects to Troubleshooting of Electroplating Systems,” Plating & Surface Finishing 88, no. 12 (2001): pg 49.
About the Authors
V.E. Guernsey obtained his BS in chemistry from Alma College, Alma, Michigan. He furthered his education in chemical engineering and completed a two-year course in electronics. He spent three years in research and process development on continuous wire plating for Western Electric, Baltimore, Maryland, followed by 11 years in job shops. Guernsey was technical service engineer and technical service manager for 16 years at M & T Chemicals, Inc. He subsequently served for three years in Latin America as Plating Manager for M & T Chemicals, Inc., residing in Sao Paulo, Brazil. In 1972 he joined the Udylite Company as Product Manager. He then moved on to McGean Chemical Company, Inc., as Marketing Manager. Guernsey was an active member of the Detroit Branch of the AESF and upon moving to Broken Arrow, OK became a member of the Tulsa Branch. In 1983 he started in business as Electroplating Consultants International and was joined by his son, Jeff, in 1987. Mr. Guernsey serves as the technical advisor for the business.
Dr.-Ing N.V. Mandich, CEF, AESF Fellow, Fellow IMF, is founder, president and research director of HBM Electrochemical & Engineering Co., 2800 Bernice Road, Lansing, IL 60438 since 1982. He holds the Dipl-Ing degree in chemical engineering from University of Belgrade, Yugoslavia, M.Sc. in theoretical chemistry, from Roosevelt University, Chicago, and a Ph.D. in applied electrochemical engineering at Aston University, England, under Professor J.K. Dennis. He is an AESF-certified instructor and certified electroplater finisher. He teaches, consults and lectures in the United States and abroad. He received an AESF Special Award and six Abner Brenner’s medals for best-published research papers. He is listed in “Who’s Who In Science and Engineering” and holds 12 U.S. Patents and patent applications and has more than 100 technical and research papers either published or submitted. He authored a number of chapters in technical books.