Clear Zinc Plating Corrosion issue...Need some education

J
#1
Customer received these parts and said they looked fine. 6 weeks later they rejected them back for corrosion showing up.

The attached picture shows an example of the corrosion.
The black spots might be from anti-splatter used at welding. The white appears to be from something leaching out from between the bar and the plate.

There are 14 of these flanges welded at various distances and grouping along a 17 inch shaft. The plates can be as close as .25 to each other.

Our plating suppliers all seem to be of the opinion that we cannot get away from this.

Any ideas on how to avoid or minimize this?
This is an issue that we seem to struggle with on a number of our parts.

Thanks
James
 

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bobdoering

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#2
The white is the initial corrosion step of the plating, followed by red rust. These areas are clearly the low current density areas, which will always have the lowest level of plating. If you check plating thickness elsewhere on the part (which is typical) these will be lower - maybe far lower.) Are they rack or barrel plated? Do you have the option of using a sealant? In some cases where it has been difficult to stop corrosion, a clear sealant has been used to extend corrosion resistance. This technique was very popular at the introduction of tri-valent zinc chromates, when they struggled to meet stringent corrosion requirements.

It has been a long time since I have dealt with this, so I am no expert at this point - but this may provide some lead-in to valuable discussion.
 
J
#3
The white is the initial corrosion step of the plating, followed by red rust. These areas are clearly the low current density areas, which will always have the lowest level of plating. If you check plating thickness elsewhere on the part (which is typical) these will be lower - maybe far lower.) Are they rack or barrel plated? Do you have the option of using a sealant? In some cases where it has been difficult to stop corrosion, a clear sealant has been used to extend corrosion resistance. This technique was very popular at the introduction of tri-valent zinc chromates, when they struggled to meet stringent corrosion requirements.

It has been a long time since I have dealt with this, so I am no expert at this point - but this may provide some lead-in to valuable discussion.
These are rack plated.
We may be able to use a sealant. We are in discussion with the customer now on this. My concern is that we are dealing with something that is under the plate welded onto the shaft and that a sealant will not really have an effect on that since it is already "inside" the plating so to speak.
This is part of what I am trying to understand.

Thanks

James

James
 

bobdoering

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#4
If the plating is not adequate because of welding, is there a need for better cleaning, pickling or passivation prior to plating?
 
J
#5
If the plating is not adequate because of welding, is there a need for better cleaning, pickling or passivation prior to plating?
I don't know that the problem is particular to welding. Rather it seems to be the case where two component parts meet and there is a narrow, slip fit, void between them. The feedback we are getting is that no matter what is done, there will always be a high risk of contamination (acid, cleaner, etc) remaining in that space and that eventually it will begin to leech out.
We are looking at requiring a more extensive cleaning and physically blowing out these areas to see if that helps.

I do think the issue has to be in the cleaning because I noted that parts with a very shiny finish had far fewer problems than those that had more of a matt looking finish.

Just as a side note, I've only recently been charged with supplier development and had this sort of thing dropped on me. I am not a plating person so this is all basically new to me.
Thanks for your patience.

James
 
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