Vanambrose

Topic: Zinc Plating - Blistering

Can anyone give me ideas why parts that are barrel plated in a acid tri-valent zinc bath are blistering.  The part is a 1" cap plug made from C12L14.  The only area that blisters is the top of the cap that is a surface of 125 micro-finish or better.  The part has a knurl on the OD, and an ID Thread.  The blistering appears as little bubbles on the surface with no specific area or pattern other than the plating only blisters on the top of the cap.  Failure rate appears to be somewhere in the area of 30% - 40%.  We worked with the supplier to determine the issue but were not able to come up with an acceptable corrective action response.  Bath concentrations were monitored and within acceptable limits.  Tank temperatures were also within acceptable levels.  Acid levels are not monitored nor are the two rinse tanks after the acid bath.  This was the only one of 7 parts plated on this day that had this issue.  The part has been plated by this source 15 times over the last 3 years with this the only occurrence.  Just wondering where else to look.

DustinGebhardt

Re: Zinc Plating - Blistering

Blistering is typically a sign of poor adhesion, usually due to poor pre-treatment.  If you are not monitoring your acids, you should begin to monitor them.  I realize that in a barrel, it can be difficult to visually see the parts, but it would be a good idea to check the parts right before they go into the plating bath.  Stop the line and remove some of the parts.  Rinse them in the final pre-plate rinse and see if the water sheet breaks.  If the water flows nicely from the surface, the part can be considered clean.  If the water sheet breaks, the part is still dirty.

-Dustin Gebhardt,
CEF
Plating Engineer
Danaher Tool Group
Gastonia, NC

MetalHead

Re: Zinc Plating - Blistering

I agree with Dustin, blistering and associated adhesion issues are almost always a cleaning problem.  We do a lot of  plating both barrel and rack on 12L14 material. Since this is a leaded steel, we have found that very aggresive cleaning results in more problems with the lead on the surface resulting in more adhesion issues. Make sure the parts are oil free after the cleaner, then I would try a reduced amperage electro clean cycle, and a reduced duration in the acid. if possible.  Both of these cycles will tend to draw the lead to the surface, resulting in blister problems.  This is one of the few times, less is more in terms of cleaning.

DustinGebhardt

Re: Zinc Plating - Blistering

I'm not familiar with the alloy, but if indeed this is a leaded alloy, a fluoridated cleaner is highly recommended.  A typical hydrochloric acid will only make things worse, as MetalHead has already said.

-Dustin Gebhardt,
CEF
Plating Engineer
Danaher Tool Group
Gastonia, NC

Vanambrose

Re: Zinc Plating - Blistering

I have heard several references to "Drawing Lead to the Surface".  In this case, their is no unfinished barstock surface anywhere on the part.  All areas of the part are subjected to the acid, yet only the end of the part has the blistering.  I understand the acid etches the part, but isn't the lead already exposed anyway?  Why doesn't the part blister randomly on all surfaces of the part rather than on the end of the part?  Please help me understand.  Thanks! 

DustinGebhardt

Re: Zinc Plating - Blistering

My knowledge of metallurgy is limited, but this is how I understand it to work:


During the forming process, be it forging, drawing, casting, etc, the metal is not 100% homogeneous.  There are areas where the alloy composition may differ slightly from the next area, but overall the alloy should conform to the spec.  It is these areas that cause a metal finisher fits.  One surface of your part may be low lead, while another end may be higher than the norm. 


I'm not sure as to the exact mechanism, but fluoridated acids are a great help with dealing with leaded alloys.  They've saved my butt many times when having to deal with sub-par brass castings and forgings coming from overseas. 

-Dustin Gebhardt,
CEF
Plating Engineer
Danaher Tool Group
Gastonia, NC