False Burns Caused by Electrical Problems
Some of the electrical problems that may be partial or total causes of false burns include bipolarity, stray currents, DC current ripple and live leads. These electrical problems are subjects of separate studies and were covered in previously published papers 2-5 in great detail. Electrical problems, although important and always challenging are too complex to discuss or even summarize in the scope of this paper.
False Burns Originating in the Bright Nickel Solution
The chromium false burns caused by nickel solutions generally result from one of several undesirable factors, combined with poor maintenance of a bright nickel bath. Some poor practices include:
- Allowing pH of the nickel solution to rise well beyond acceptable working limits
- Failure to keep fresh carbon on the filter
- High-brightener concentration and blind, ad hoc additions as a means of correcting a problem. Individually, one of these may or may not cause a problem, but coupled with external influences (mentioned earlier) each of these can be sufficient to produce a false burn.
For those installations employing full automatic systems with live leads on the nickel and the chromium tanks, one should pay particular attention to mechanical conditions that can cause a make-or-break contact on the hot lead6. This can lead to the problems of double contact and/or bipolarity. Voltage on the live-lead on the exit of the nickel plating tank can become critical if the anode area is low or if metallic or organic levels in the nickel are too high. In one particular case, the exiting live-lead voltage on an automatic line had to be reduced to about three-fourths of a tank voltage. In another case where there was a low-exiting voltage on the nickel, it had to be raised. Conditions do change, and under these circumstances, some experimenting is always in order.
False burn along the top row of parts on the rack is a frequent problem. Often this is attributed to “bipolarity.” Actually, this is almost an electrical and physical impossibility, except for loosely racked parts. The authors have seen and used many methods to try to correct this kind of false burn. Putting shields part of the way down on the anodes in a chromium bath sometimes will relieve this condition for a short time. Generally, this serves to reduce the current at the top of the rack after entry into the chromium, and the longer period of gassing, which always precedes the deposition of chromium, acts as an activator. The depth of the chromium solution above the rack also may affect the results.
In trying to correct the problem, concerned electroplaters should consider the following:
- Fast-leveling nickel brighteners are common today. In order to produce quick leveling and maximum luster, the brightener system must either be stronger and/or be present at higher concentrations than normal.
- As the amount of brightener or organic compounds in the nickel deposit increases, the deposit is more susceptible to adverse physical, electrical and/or chemical conditions.
- Air bubbles expand rapidly as they rise. As a result, the maximum agitation is at or near the surface of the nickel solution.
- The higher the agitation rate, the more brighteners and/or impurities are incor- porated in the deposit, since they are being diffusion controlled.
- The closer the top ends of the parts are to the top of the solution, the less the cross- sectional area of solution is available to carry the current in that critical area. The outcome is a lower current density than we might suspect. This results in a higher brightener/nickel ratio, since the nickel deposition rate and thickness is controlled by available CD.
- Anode baskets that are not reasonably full also will reduce the CD on the top parts.
At its worst, the condition that can arise as a consequence of putting these all together would be as follows:
The top row of parts is in an area of higher agitation and lower current density as a result of the reduced current-carrying capacity of the solution above these top parts. More brightener or impurities are plated out on top rows of parts than if parts were lower on the rack, magnifying chromium plating problems. Now we have the conditions most likely to produce at least one type of chromium false burn (high-CD whitewash) on parts at the top of the rack.
Now that we have this condition, what can be done to correct it? The answers usually lie in one or more of the following steps:
- Adjust pH to proper value. Higher pH in the nickel encourages false burning during chromium plating.
- Raise the level of the nickel solution, if possible, to increase the current-carry- ing capacity of the solution above the top parts, thus increasing local CD. Exercise common sense, since a solution level higher than the basket will cause roughness.
- Lower the brightener content of the nickel bath, if possible, by withholding bright- ener additions or by low-CD electrolysis (“dummying”).
- Anode baskets have to be kept full, especially in the last four to five stations in full automatics. In hoist lines, or on hand lines, individual baskets have to be checked for electrical contact and anode levels in the area of occurrence.
- Put fresh carbon on the filter.
- Use a chromium pre-dip composed of two to 22 g/liter (1/4 -3 oz/gal) of CrO3.
- Lower agitation rate in the nickel, if pos- sible. For full automatics, this would apply only to the last four or five stations.
- Consider use of proprietary additives, which in a pinch may temporarily alleviate this problem.
- Increase CD in the nickel bath, if possible.
- Change rack design to provide deeper immersion.
- Bring solution level of chromium solution up to normal.
If these suggestions cannot be put into practice, or if they fail to eliminate the problem, then the only recourse is the cathodic activation of the nickel, one more involved step.
False Burns Originating Between Nickel and Chromium Stations
Some causes for false burns that originate in processing between nickel and chromium plating are listed below. Please remember that false burn may be accompanied by low or medium CD white staining, spotting or streaking and/or poor chromium coverage. As a result, the plater should not be surprised if one, some, or all of these occur simultaneously, along with the false burn.
1. Contaminated rinses prior to chromium plating stage. Usually it is a matter of judgment and experience as to how much contamination will affect the subsequent chromium deposit. However, cascading rinses and closed-loop systems should be watched extra closely.
2. Incorrect pH in rinses or dragout recovery nickel tanks. Again, past experience will dictate what the proper pH of these should be. The pH of rinses should normally be 4.0-8.0 for best results, while the nickel dragout should be approximately the same pH as the nickel solution. In cases where water-recovery systems are used, frequent pH adjustments may be required.
3. Long delay times between nickel and chromium tanks. The period of permissible delay depends pretty much upon the activity of the nickel surface. It will be in inverse ratio to the brightener or organic levels in the nickel deposit. Simply stated, the greater the organic level in the nickel deposit, the less time one may delay before the chromium plating step.
There are cases in which freshly nickel- plated nickel parts have been left to dry for several hours and then successfully chromium plated with little or no activation. This is however, a rather rare occasion, and the type and condition of the chromium bath and contamination of the surrounding air will have a direct influence.
In general, long times in either rinse or air will require subsequent cathodic activation, either acid or alkaline, prior to chromium plating.6 Permissible periods of delay between nickel and chromium steps also will depend upon temperature, pH and the contamination levels of the water rinses. All should be checked and adjusted in trying to solve a problem.
4. Chromic acid pre-dips. Usually these should contain 2-15 g/liter (1/4 -3 oz/gal) of chromic acid or the proprietary material used for chromium plating. Best activation occurs in this concentration range. How these pre-dips work has not been fully documented and it is open for conjecture. It can be theorized that in all probability, the chromic acid serves to remove organic material or oxidation products from the nickel surface. Chromic acid also develops a light, uniform, passivating film on the nickel. This film resists the formation of heavier films and is easily removed in a chromium plating tank.
It should be stated that at this point, an educated guess is as good as it gets. No matter what, small amounts of catalyst are usually necessary, probably to have a cathode surface film that is not too much different than the one normally preset at the very beginning of the chromium plating step. Although there is no universal agreement upon definite concentration, such conditions from no sulfate to very high sulfate in the chromium pre-dip have been experienced and documented as sources of chromium plating problems.
5. Electrolytic activating solutions. There are many proprietary cathodic alkaline activators on the market today. They, as well as cathodic acid activators, perform their function by removing interfering organic or oxide films from the surface of the nickel.
Activators must be periodically replaced. Otherwise, accumulated impurities may be deposited on the nickel surface. These impurities interfere with the subsequent chromium plating and cause not only false burns, but many other undesirable conditions, and rejects as well. Again, experience and judgment are required.
Unequivocally, one could say that unfortunately, there is no standard activating procedure, since compositions of nickel deposits vary widely and surface conditions are different in character. In turn, chromium receptivity is a variable, too. Here we might mention that freshly plated nickel usually differs from mechanically satin finished nickel for chromium acceptance. As a result, satin nickel may require changes in the activating procedures. Activation for replating of nickel over nickel is more demanding than if the parts were to go directly into chromium, and so on.
Clearly the correct sequence is the one that provides acceptable work. This sequence can only be arrived at by experimentation, close observation, good judgment and sometimes an educated guess.
The concern for the right sequence can be minimized by proper operation of the bright nickel. The age-old adage “if you want good chromium deposits, you must start with good nickel,” is certainly true.
6. Stray currents. On hoist lines, uninsulated hoist hooks or carrier bars can provide an electrical path to ground. This stray current can lead to rejects of many varieties.
Stray currents also can exist in tanks that are not electrified but have some conductive salts or acid present in solution. They are presented in detail in references.2-5
To determine if stray current is present on a hoist line, first turn off the rectifier, if the tank is electrified. The bar is raised with the racks just off the saddle and an ammeter inserted between the bar and the saddle (an ammeter measuring from 1-10 A should be sufficient). The test is then duplicated with an empty bar. A continuity test with an ohmmeter between the bar and cable hook also should be performed for confirmation. The plant electrician should be consulted.