Posts by DustinGebhardt

It sounds like you are having some fun cutting your proverbial teeth.  :-) 

I wish I could get back to my hometown, but I can't.  Stay classy, San Diego!

If you are seeing nickel delamination, most likely the parts are losing their connection while in the nickel bath.  If it is the whole barrel, I would check the barrel contacts.  If it is only a few parts within the barrel, I would either add more load weight, investigate new danglers, or slow down the barrel rotation. 

If you want to make your deposit more uniform, you can reduce the amount of nickel metal in the bath.  The higher the nickel metal, the more efficient the bath is a high current densities, and the easier it is for the bath to build up larger thicknesses.  Increasing load weight and/or rotation speed can also help increase uniformity.  You can also try adding some brighteners, which on the small scale, can help reduce the buildup of metal in the high current density areas and preferentially deposit in the low current density areas.  Although brighteners can also make the deposit more deposed toward passivation, wh …

While I don't have the actual spec, doing a Google search came up with this spec:

GMNA GM4350M - Painted Part Performance Requirements

I would assume that this spec would cover your question.  If the bumper is plastic, this line may apply:

"For painted plastic parts only, the combinations of substrate, surface
preparation and applied paints shall use that which are shown in GM
Approved Paint on Parts Systems (APOPS) database. When APOPS testing is
performed each test must be completed on a minimum of three (3)
samples, each obtained from separate parts."

So you may be able to go to the guy that did the touch up and ask to see his proof that his paint, equipment, and pretreatment are all APOPS certified.

I'm assuming that the heat spots you are referring to are due to the heat generated during the extrusion process?

Do you have a picture of the problem?

Have you tried annealing the part after extrusion?

What alloy is this?

What is your anodizing process?

I did a quick Google search and I keep seeing recommendations to increase the cooling on the runout table.

For the utmost in paint adhesion, you typically want 2 things:

1) A clean surface.  Usually a solvent degreaser or auqueous cleaner is used to remove any dirt and oils.

2) A micro/macroscopically rough surface for maximum paint adhesion.  Many painters use media blasting to get this surface.  There are also some other means to get this, but they typically don't apply to nickel substrates.

With nickel (and most other metals) you also have to deal with oxidation.  Media blasting can remove the majority of the oxidation.  You can also try acid dip the part.  There are additives that you can use with the acid to help remove oxides.  In a best case scenario, you can also try running reverse current through the acid bath, which drastically eliminates the oxide.

Most likely, this is due to unequal current density on the parts.  I would expect that the darker parts are closer to the cathodes?  I suspect that the portion of the part that is closest to the cathode is getting more current, which is building up a thicker anodize film (oxide layer). 

Are you dying the parts after anodizing them?  Dying the oxide film can be tricky, but usually you can increase the time in the anodize bath to help smooth out any inconsistencies.  

If you are not dying the parts, I would greatly suggest that you try to make the part equal-distant from the cathodes or use auxiliary cathodes.

More info might help us.  What type of anodizing is this?  Type I, II, or III?  What is your process?  Seal type?

You can also try a continuous external dummy. 

How do you know you are at the limit for chromic acid?  How are you analyzing for chromic acid?  Is this a bath that is supplied by an outside vendor?  If so, have you contacted them?

In my experience, you make up a chromate seal bath at 5% using sodium chromate or potassium chromate.  The pH is then adjusted using either NaOH/KOH or acetic acid (glacial) to a range of 5.5-6.0.  If you use NaOH, be sure to use rayon grade or lab grade (1.0N generally works for small adjustments).  Temp should be around 200 or more.  

If I recall correctly, we ended up using a bunch of the acetic acid to control the pH while the bath was not in use.  We had 2 x 4 gallon cases of the stuff on hand for a 80gal tank.

Phosphating is generally considered the best pretreatment for powdercoating.  It provides the mechanical bonding of abrasive blasting with cleaning superior to solvents.

I've looked into this sort of thing in the past.  About the only I could come up with was an Autotitrator with an autosampler that took samples from the bath.  The titrator would then communicate with a PLC or some other sort of relay logic to trigger a dosing pump.  The process would repeat at a certain time interval.  Unfortunately, the cost was very prohibitive in my case.  I estimated the complete setup to cost somewhere close to US$100,000.00. 

And that was just for 1 parameter.  Plus, you still have to calibrate the unit every so often.

I typical zinc/chromate coating can be tested for corrosion resistance in a neutral salt spray chamber.  You can also use one of the accelerated tests, too, but much more common is the neutral.  The first sign of failure will be "white rust" caused by the chromate failure.  A low grade chromate should last 12 to 24 hours.  High-end chromates and/or chromates with topcoats can go over 120 hours to white rust.  The next failure mode is red rust.  The time to red rust is a function of the zinc thickness.  The more zinc, the longer the part should last before significant red rust.  Hundreds of hours is not uncommon.

To perform the test, you will need a salt spray chamber and all the ancillary equipment.  There might be a local shop that already has one in your area if you don't want to purchase a unit.