Doug B

Topic: Need help with Chromate color variation, HELP!

I'm not a "plating guy".  I'm a mechanical engineer.  I'm having some steel frames CAD plated with a Chromate conversion coating overtop. 

I'm desperate for an answer, and found this website, searching for help online.

The first sample I got back from the vendor was a nice gold color, all over, like a gold bar from Fort Knox.

Since then, the housings I get back have variation in color across the housing.  There are several holes in the housing, and around the holes I get the nice gold color... but in the field between the holes the color gradually shifts to a gold-pink color.

If I wet my finger and rub the gold-pink color, it turns to a nice matte gold until the water dries, then it goes back to gold-pink.

I don't feel comfortable shipping these housings to the customer until I know that the chromate is good.

I'm asking my plating vendor to fix the problem, but he's struggling with an answer, and I need to assemble these parts and ship them to my customer.

We are magnetic-particle inspecting the housings prior to plating.  The vendor is bead blasting the housings, baking them at about 120F, then re-bead blasting and cleaning prior to plating. 

Help!  What can I tell my vendor to try in order to get consistant color in the chromate, and ensure the chromate is good?

I need help ASAP. Please email me below if you have an ideas....

Doug Brindle

douglas.a.brindle@saic.com

DustinGebhardt

Re: Need help with Chromate color variation, HELP!

Hi Doug.  Let's see if we can help.


#1) How do you know that the chromate is bad?  Have you performance tested the questionable parts?  Or do you have a visual quality criteria for a solid, uniform color?

#2) Typically, an iridescent chromate coating is inferior to a solid gold coating.  There are many exceptions to the rule, however.  Based upon your description of the holes being gold but the field being iridescent leads me to believe that they total amount of chrome is higher near the holes than the field.  This can happen for many reasons, so let's list them here:

a) plating thickness.  You will tend to have more plating thickness around edges, like the holes. (this is probably not your solution)

b) solution uniformity.  I doubt that the chromate solution has such drastic swings in concentration, so I'll throw this idea away.

c) chromate thickness.  The area around the holes has more solution "access" or movement.  The layer may then be thicker as it forms more quickly.  See what happens if your plater goes with a longer cycle in the chromate.

d) chromate concentration.  The concentration of the chromate may be low, leading to an iridescent color.  This would tend to be uniform across the part, however. 


Ultimately, chromate color is determined by the actual quantity of chrome on top of the plated deposit.  you can have a thick coating with a low concentration or a thin coating with lots of chrome included in the layer.  Also remember that there are 2 competing reactions taking place during the conversion step.  The chromate is consuming the plated Cd and forming the conversion layer.  And the acidity in the bath is slowly removing the conversion layer as it forms.  As the chrome solution concentration goes up, the faster the layer tends to form.  As the ph goes down, the faster you eat away at your newly formed conversion layer.  Strangely, as you add chromate to the bath, the pH tends to drop.

I would recommend that you increase the chromate concetration and keep the same immersion time, or increase the immersion time with the current bath.

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

Doug B

Re: Need help with Chromate color variation, HELP!

Thanks for the information.  I'm not specifically saying the Chromate I have is "bad", but it's not what I'm used to seeing.  I'm used to seeing the continuous gold color all the way across my housings.  The iridescent pink/gold in the fields is not what I'm used to seeing.

These housings are for use by the Navy, where corrosion control offered by the Chromate is important to me. 

What's also a little 'strange', is that I can wet my finger, rub it across the housing and the pink/gold color will change to a nice gold color, until the water evaporates, then it changes back to the pink/gold color.

My question basically is.... do the housings I have with the iridescent coloring meet the requirements of SAE-AMS-QQ-P-416, Type II, Class Optional ?

 

 

DustinGebhardt

Re: Need help with Chromate color variation, HELP!

Without the requirements of the spec, I can't be sure.


Regarding the change of color when wet, recall that chromate conversion coatings are "gelatinous".  As you rehydrate/dehydrate them, their properties for reflection and refraction can change.  This is very common, especially on "lighter" chromates, like the clear/blues and other iridescent coatings. 

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