Letter 27032

EN pits  

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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

"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 Palm International, Inc. - LaVergne, TN

Ed. note: It is our sad duty to alert the readers to the passing of Ron Duncan on Dec. 15, 2006. For those who would like to know more about him, a brief obituary opened Update No. 13 of our Metal Finishing Industry Update Podcast.

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

Kevin Keating
medical device - New York

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 preferrentially deposit on edges and locally stop plating.

Ron Duncan Palm International, Inc. - LaVergne, TN

Ed. note: It is our sad duty to alert the readers to the passing of Ron Duncan on Dec. 15, 2006. For those who would like to know more about him, a brief obituary opened Update No. 13 of our Metal Finishing Industry Update Podcast.

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 Don Baudrand, Consultant   Poulsbo, Washington

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, CA

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