Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation
Reducing Iron Contamination in the Hard Plating Bath
Q. Hi there, I'm a grad student who, as part of a course, has to look into pollution prevention strategies for local company that produces chain saw cutters with a hard chrome finish. They've tried to minimise iron buildup in the plating baths with a porous pot system and an ion exchange system in the past, but in both instances the systems proved to be more costly and troublesome than the benefits they provided. I've been looking into reducing iron contamination from the start (as opposed to removing the iron that ends up in the bath after the fact). The bath currently uses HCR 840 as a chrome source, with HCR 842 as a fluoride catalyst, 10 seconds of reverse etching, and runs at 1800A and 6-12V. What kinds of challenges are faced with switching to HEEF 25 as a chrome source - in your opinion, is this product effective? Also, do you know of any sources that discuss ways to determine optimal reverse etching times - I get the sense that the company I'm working with has stuck by 10 seconds because "it's the way it's always been done", but I'm wondering if iron contamination could be reduced by shortening the R.E. time? Thanks for any tips or suggestions that you can offer!
A. You ask a great question about an age old dilemma. Reverse etching in the plating bath and the resulting iron contamination is an ongoing and significant problem. I'm a proponent of reverse etch for the good mechanical bond, but I too work hard to minimize the affects where ever possible.
Since your question seems to be the shortest possible reverse, the first consideration is a close relationship between the part and a properly designed conforming anode. Immediately you can shorten reverse time in one second intervals with testing to verify adhesion, until the “shortest” possible time is established. The savings could be as much as 5 seconds or more. This would make a positive impact on the amount of iron introduced into the bath over time.
Next, a proper parts warm up with bath agitation prior to application of current can have a positive cleaning affect on metal. There is also a practice in chrome plating known as “cathodizing”, which is the application of forward current at 2 to 2.2 volts for several seconds to a minute. This action creates tiny hydrogen bubbles which have a scrubbing affect on metal.
You can easily prove out these theories by experimenting. 1.) anode to cathode spacing, 2.) warm up time before application of current, and 3.) cathodizing at 2 volts in forward polarity, and 4.) reducing the reverse etch time by one second intervals.
As always, parts must be as clean as possible with no grease, oils, or other contaminants present going in. Common ferrous alloys will behave best, CRES, high heat treated mat’ls and other exotic metals may not tolerate change.
A sensible alternative might be a separate process tank in line before the chrome plating tank, to serve as an independent reverse etch bath. It becomes an additional step in the process, but has substantial benefit. Following this step, parts can go directly into the healthy chrome bath in a forward polarity. No reversing in the plating bath will extend bath life. This plan will only work if you have the space for an additional tank, or the ability to remove the Cr tank and replace with a split or baffled process tank of similar dimensions. The build up of iron in the reverse etch tank can theoretically go on indefinitely. A reverse etch bath would be made up basically the same as the plating bath, approx 30-36 oz/gal. Cr+3 and water, needing no catalyst, a temperature range of 120oF to 130oF. Ventilation of course, required.
Regarding Heef 25, I happen to know that it is an exceptional hard chrome bath, producing high hardness and exceptional uniformity. For the best results, use with well designed anodes and tooling. Cathode efficiency of the Heef 25 bath is in the neighborhood of 22-25%. I can’t think of any challenges to switching solutions except maybe cost. Atotech is a consummate supplier and should be able to assist you with more information about the product.
It’s always refreshing to hear from someone with positive forward thinking ideas and concepts about metal finishing. The best of luck with your project.