How to Barrel Plate Sulfamate Nickel
Coventya’s Brad Durkin explains what to do when barrel parts experience poor adhesion, flaking and deposit poor ductility resulting in high rejection rates.
Q. We have a 400-gallon sulfamate nickel tank used to process ferrous-based parts that are rack plated to 18 microns for a high heat exposure application. Every so often we barrel plate smaller ferrous parts to 12 microns in this tank and we encounter problems. We generally have a high yield and overall good results when plating the rack parts, but with the barrel parts, we experience many problems with poor adhesion, flaking and deposit poor ductility resulting in high rejection rates. What can we do?
A. Unfortunately, the “art” and practical experience of plating sulfamate nickel (SN) is getting lost in our industry today. Earlier in my 33-year career, I had the experience of working with many of the pioneers who lived and breathed life into many types of applications for SN plating extending from the 1960s through the early 1990s. In July 1954, at the 41st annual convention of the American Electroplaters’ Society, Richard C. Barrett of the Barrett Chemical Products Co., who was one of the pioneers for this process, presented a good overview of this relatively new process, which helped the technology find a market. Barrett, who ultimately sold the technology, know-how and manufacturing rights in the 1960s to Kelite Co., carried on with marketing the “Barrett Sulfamate Nickel” brand. The brand represented high quality technology that found its way into the USA Space Program and other military or industrial demands including electroforming. It was in the Allied Kelite era of the mid 1970s to 1994 that a lot of application work was completed to continue commercializing the Barrett SN process technology for many high-end applications. Unfortunately today in our industry, SN technology is too often thought of as a commodity, which makes it difficult for suppliers to help applicators solve problems as was traditional in the past.
Specific for SN barrel plating, Barrett and Kelite performed a lot of studies to understand the application. One advantage of an SN deposit is the use of minimal additives in those systems that occlude organics into their deposits. As a result, these deposits have very low porosity and good grain structures that provide unequalled ductility, critical for part forming after plating. Their deposit purity also lends to very high temperature application protection, up to 1,260°C, and their low internal stress provides a range of benefits for many critical applications.
Because of this high purity characteristic, under given conditions during plating, sometimes deposit passivity can be the result. At traditional high current density applications and constant electrical contact, typical of rack plating, there is less susceptibility for passive deposits to occur. In barrel plating, there is higher risk of a passive nature resulting from this high deposit purity because the tumbling action of parts causes interruptions of the flow of current to the surfaces in a given load.
When barrel plating SN is required, the following four conditions and adjustments for the SN chemistry help increase deposit activity and have demonstrated success reducing passive defects occurring in these applications:
- SN chemistries require anode corrosion additives traditionally based on halogens. For rack systems, these additives are in the concentration ranges of 3 g/L but for barrel applications, the increase of the concentration to 21 g/L helps to maintain the deposit surface activity and improve the conductivity of the solution for the low current density plating.
- Increase the operating temperature to 140°F (57°C), which is 20-30°F higher than for rack plating conditions. Some have argued that this creates an increased risk of sulfamate break down to byproducts ammonium and sulfate that are contaminants to the chemistry, but the conditions required for this chemistry conversion to occur are well outside this range.
- Decrease operating pH in the SN plating bath. For normal rack plating applications, the operating pH target is 4.0. For barrel plating, it is important to reduce the pH with a high-quality sulfamic acid or proprietary pH adjustment component to 3.0. This lower operating pH maintains the deposit and solution activity to minimize any potential for deposit passivity from the current flow “make and break contact” during the rotation of the barrel.
- Eliminate the use of a wetting agent in the SN solution. Traditionally for rack plating, higher cathode current densities (1 - 10 A/dm2) are reached where pitting is more likely to occur on the plated surfaces. The function of the wetting agent is to sufficiently reduce the surface tension in the deposition reaction zone to allow for hydrogen gas bubbles to release, and for any particulate, that might need to be removed so it does not co-deposit. The wetting agent, if used in a barrel application, only serves to cause more passivity of the pure nickel deposit. Barrel plating is carried out at a range of less than 1 A/dm2, so the wetting agent becomes a contaminant to the system and is normally not required.
In this case, the best answer to your question ,with respect to chemistry requirements of the SN and because of the variation required, is to consider a new solution makeup or new tank installation for use when barrel processing parts. Trying to utilize a single chemistry with the rack parameters will only cause continued issues plating sulfamate nickel in a barrel application.
Brad Durkin is director of international product management at Coventya, coventya.com.
Originally published in the February 2017 issue.
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