Improving Acid Zinc Coverage and Brightness

Q. We plate our parts in a mixed acid-zinc rack bath, containing both ammonium and potassium chloride. How do we improve our zinc deposit’s clarity, brightness and thickness, especially in the low current density (LCD) areas of our parts without sacrificing throw power and without plating for excessively long periods of time?

A. To ensure that your high current density (HCD) areas are not receiving too much metal and their LCD areas are receiving a sufficient amount, it is important to lower the zinc metal concentration of the bath from 4.5 ounces per gallon (opg) to around 3.5 opg. Lower zinc metal concentrations—ranging between 3.2 to 3.8 opg—improve both throw and cover power.

Because the terms “throw” and “coverage” are used interchangeably in our industry, I feel obligated to provide a measure of clarification. Covering power is a bath’s ability to initiate plating in the recesses of a part, such as deep within the inner dimensions (ID) of tubular components, while throwing power deals with deposit uniformity. When the HCD area is close in thickness to the LCD area, the bath is displaying “good throw.” Alkaline-non-cyanide-zinc (ANCZ) baths have exceptional throw power, whereas acid zinc baths generally do not. However, acid zinc can achieve exceptional cover power, often exceeding that of their ANCZ counterparts.

Keeping the zinc metal on the higher end will alleviate HCD burning, which is the formation of zinc-hydroxide nodulation that mimics the appearance of metal in contact with a burning blowtorch. Ammonium chloride is often incorporated as an outstanding pH buffer and is able to adequately lessen the intensity of any burn that may want to manifest. A good carrier (proprietary additive) will also act to spread the current across the current-density spectrum, assuaging any inclination toward a potential burn. All of this affords us the opportunity to operate on the lower end of the zinc metal spectrum and reap the benefits of doing so.

We had a similar issue with a customer. Although the standard recommended ratio of total chloride to zinc metal is around 4.5-to-1, we decided to run with a zinc metal figure of about 3.5 opg and a total chloride figure at around 21 opg, which is closer to a 6-to-1 ratio, made possible by using ammonium chloride and the particular carrier we selected for this operation. An average mixed bath tends to run the ammonium chloride concentration at 3 to 6 opg, but we have found that the 6 to 9 opg range seems to help demonstrably with buffering, deposit clarity, throw and coverage.

To avoid large upswings in zinc metal concentration, it is important to avoid high anode current densities (ACD) and low pHs, as these process parameters may cause an excessive degree of anode dissolution. Anything beyond 25 ASF is not recommended. High ACDs will also increase the brightener consumption, which may decrease LCD brightness, thus defeating one of our main objectives. Occasional weekly dilutions of the zinc bath or running at a high pH may be necessary to maintain a proper metal concentration.

One key general property of the chloride ion is its conductivity, which enhances LCD deposition due to its ability to assist with ionic movement. Too much chloride can lower a bath’s cloud point, causing LCD dullness and HCD burning, so care must be taken when making salt additions. Cloud point is the point at which a bath’s most insoluble ingredients precipitate out of solution.

Proper brightener concentration, as would be expected, is fundamental to ensuring LCD area brightness and coverage. Organic byproducts often exhibit most prominently in LCD areas in the form of dullness, so proper filtration via cellulose powder and carbon is integral. Overdoses of brightener can actually cause reduced thickness or even skip plating, as can high iron (>75 ppm) and low current densities. To ensure that the brightener is solubilized and stabilized, a proper amount of carrier is needed. pH in the 5.0 to 5.4 range can improve LCD brightness, but if too low, it increases the potential for additive oil out from solution as well as an increase in the dissolution of anodes and metal growth. To ensure that the brighteners are not consumed by excessive heat, the bath should be properly cooled; however some heat (80 to 90°F) will help improve deposit distribution (throw) as will good agitation, which also helps with LCD coverage and clarity.

As is plainly evident in this solution, there are an abundant number of options available for the acid-zinc plater to improve LCD coverage, brightness and throw. Our experience has shown that the two biggest indicators for enhancing these three quality measurements are keeping the zinc metal low and the total chloride figure high.

Adam Blakely is a technical service representative for MacDermid Enthone. For more information, visit macdermidenthone.com.