Bare spots issue with hot dip gavanizing of heat treated fasteners

[Drewman030]

I work with a fastener manufacturer who has our hot dip galvanized product sent to an outside processor. Our processor has had issues with bare spots (see attached photo) and is having issues with finding the cause. I'm not particularly well versed in HDG defects and was looking for some help in areas of the operation to look for a cause.

Their process consists of a Degreasing tank, water rinse, 10-12% HCl, water rinse, flux, hot dip galvanizing, and quenching.

Thank you in advance for your help!!!

 

[CoryPad]

Have the bare spots been analyzed by a lab? Are there oxides present? Nonmetallic substances? The HCl concentration may be too low to fully remove embedded oxides and/or forming lubricants (liquid phase and solid phase). There should be a drying operation after the flux to insure the parts will have good zinc adhesion.

 

[Drewman030]

We have not sent samples out to an outside lab as of yet. We are seeing if we it's an easy fix first.

I had felt that there was a possiblity that we weren't cleaning well the processor insists that they have been in control of cleaner concentrations but we have seen their operations and question how well this is being done.

I'll suggest running a trial to ensure there isn't wet flux going into the pot while we are having the bare spots analyzed. THANKS!!!!

 

[unclesyd]

I think you are on the right track as you will have to eliminate the surface preparation problems before you can look at other possibilities.

Are your bath's proprietary bath or homemade?

Check the rinse step after the degrease bath. Dragout from the alkaline bath will quickly kill the acid bath. Additive like a rinse aid are extremely important in an alkaline bath.

The acid bath deserves attention to the little details.
There should be wetting agent and rinse aid incorporated in the bath. The iron concentration needs to be controlled.
One thing that often happens is that the acid bath will be out of spec and the operator will just increase the time and or temperature to clean the part. This will sometimes cause a residue of "smut" which can be black or almost invisible that will prevent the adherence of zinc.

It's been a long time but we used a mixture of HCl and H3PO4 for our acid bath and stearic acid as the flux in our galvanizing lines. .

 

[Drewman030]

unclesyd,

I've thought about what carryover from their degreaser (which is proprietary) but in general they do a pretty good job of rinsing. They control the iron and HCl concententration in the acid bath and I've never seen smut on the parts when visiting the operation. (I've actually only seen smut when dealing with steel in H2SO4 acid baths.)

I don't think that they are using a rinse aid or wetting product in the bath. What are the advantages of each and what types of products do you suggest?

 

[unclesyd]

These sites, TruChem and Harry Miller are just two of the suppliers of additives for pickling and cleaning lines. Not really recommending these products I mention these two as I've used both companies products will very good results. Both companies have been good about responding to information.
I would contact each with you problem. Make sure you have the exact details of your process handy.

The first URL is a good explanation of a rinse aid and second is the complete line of products.

http://www.truchem.com/doc/798.pdf

http://www.truchem.com/reference.htm

Activol 1590-CP is just one of several additives for your bath. It has a good wetting agent as well as other ingredients that enhance the cleaning ability of the base acid.

http://www.harrymillercorp.com/product_activol-acid-inhibitors.htm

Just remember that no amount additives can eliminate the need for continuous quality control and good operators.

 

[Drewman030]

Thanks for taking the time for this issue all!!! I'll look into these products in more detail and see what our processor thinks!!!