Q. We are chromating impacted parts made from 7075 T6. We are noticing that some of the parts form a “fuzz” where they are machined. This area is white powder and when removed leaves a black spot on the surface of the tube that will not chromate.
The issue is sporadic and can occur at any stage of chromating, but is primarily seen immediately upon removal from the soak cleaner. We have tried four proprietary cleaners and still have some of the same issues. We have also determined there are no stray currents. Could this be the blisters that you have written about? R.D.
A. You mentioned that the parts are impacted. Do you do the impacting yourself or receive the parts already impact formed for further processing as you mentioned (machining, cleaning and chromating)?
The reason I ask is that I had an experience once with a cleaning process trying to remove lubricant from a part prior to brazing. The part was an impact-formed aluminum piece that was done by an outside company. The part had some machining done on it, then was solvent cleaned. After brazing, we found a black residue in some of the crevices.
We thought we were doing an effective job cleaning and we were not having problems with any other parts. On closer examination with a scanning electron microscope (SEM), we found that the surface of the dark residue and the small crevice had what appeared to be a defined crystal structure on the surface. The crystal structure appeared to be that of a phosphate coating.
Zinc phosphate with a stearate (metallic soap) coating is a common lubricating practice on a part that is receiving a severe forming operating. The crystal structure we were finding on the surface was consistent with that. Additional elemental SEM analysis showed the primary components of the crystal structure to be zinc and phosphorus, confirming the theory that we were dealing with a zinc phosphate coating on the surface.
This discovery necessitated that we change the cleaning process to include a caustic etch followed by a nitric acid desmut. This was needed because the zinc phosphate on the surface would not be removed by a simple degreasing process, either solvent or aqueous-based. The preparation process now required that we remove a small amount of base metal with the sodium hydroxide solution (ambient, about 4–8 oz/gal or 30–60 g/L). The caustic etch then produces a degree of smut on the surface that is common with alkaline etching. That is the remnant of the alloying elements that are not dissolved in the highly alkaline sodium hydroxide tank that is dissolving the aluminum. The nitric acid desmut is then used (typically ambient and about 50% by volume) to remove the remaining alloying elements.
If you have an outside or university lab available to you, it may prove to be money well spent if they can identify the residue on your surface with an SEM (visually first) followed by electron dispersive spectroscopy (EDS) to examine the composition of the residue. A very rough estimate for a one-time analysis like this would be about $1,000. There are some excellent high-magnification SEM photographs in the Electroplating Engineering Handbook, edited by Larry Durney, which should provide a valuable reference point in discerning the structure of the residue.