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EN pits

Using a high phosph EN, I am getting pits on sharp edges of a substrate. The pits seem to originate at a pin-point on the substrate, and have a scalloped or "terraced" wall. The substrate is a machined copper-nickel alloy. They occur in a fresh solution as well as older, and agitation rate does not appear to help. I use a periodic reverse alkaline cleaner, sulfuric predip, and a woods' strike before EN. The pits are random -sometimes 100% of the parts (which are shaped like a round-head screw), sometimes 10%. They are racked horizontally with the head facing out. It is only the sharp edge of the head that pits. Any ideas?

Kevin Keating
Process Engineer - Rochester, New York
2004

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2004

"Terraced" sides to a pit are oftentimes caused by contaminants being released progressively from a pore in the substrate at the bottom of the pit. Often these pit resemble an oyster shell when viewed from above. You did not say whether the copper-nickel was forged or cast, etc.

Ron Duncan [deceased]
- LaVergne, Tennessee
It is our sad duty to note Ron's passing on Dec. 15, 2006. A brief obituary opens Episode 13 of our Podcast.

Its a Copper-nickel-tin alloy that is extruded then rolled.

Kevin Keating
medical device - New York
2004

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2004

If the substrate is extruded and then rolled, it is unlikely to contain any pores. The only other process that might produce your defects is edge pull back, but it does not usually produce pits. Rather it is usually a planar defect along the edge. Edge pull back is due to the presence of heavy metals like lead that preferentially deposit on edges and locally stop plating.

Ron Duncan [deceased]
- LaVergne, Tennessee
It is our sad duty to note Ron's passing on Dec. 15, 2006. A brief obituary opens Episode 13 of our Podcast.

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I agree with Ron. Edge effect often is the result of high stabilizer, or too much agitation. Suggest reduce the agitation to a very low rate, and check the total stabilizer content. reduce the stabilizer.

Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]
           and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])
2004

Kevin,

We have had extensive issues with pitting in our Mid-Phos EN baths when applying over .001" on >16 finish aluminum. The solution for us was, reduce agitation and maintain lead and cad below 1.5 ppm combined. The newer the solution, the less agitation, the less cad and lead. If you are unsure of these levels, try plating the bath down to 85%, adjust the PH and run some parts. If the pitting goes away, pull a sample and have your supplier check the cad and lead levels.

Good Luck !

Steve Parkhurst
Metal Finishing - San Diego, California
2004