Black Chromate

[hdway]

New to the Caswell Electro Plating system, got my equipment set up about 3 weeks ago and tried some test plates. Did 5-1.5" square plates with copy cad then did two with Yellow chromate and two with Black chromate, they all came out very acceptable.
Today I got set up to do a bunch of bolts and screws with copy cad and black chromate using a plastic plating basket I made with copper wire laced through the holes in the plastic on the bottom and 1" up the sides to provide continuity. Plated them with zinc for 20 minutes and then rinsed, 3 second dip in 5% muratic rinsed and into black chromate solution, they would not turn black just black haze and streaks. Backed right up bead blasted, degrease, rinse, pickel, zinc plate, rinse, pickel, rinse and black chromate. This time they just got a yellow tint to them, tried redoing just keeps comming out the same.
Any idea's

[zachhh]

i had the same problem, the first day i used black chromate on copy cad it turned out great. the the solution turned cloudy and eventually went from black chromate to a bronze color. dose anyone know what is wrong with the chromate solution? could it be acid contamination from the pickle before the dip in the chromate?

[hdway]

Mine is very cloudy also.

[seanc]

You've got different problems here:

a plastic plating basket I made with copper wire laced through the holes in the plastic on the bottom and 1" up the sides to provide continuity.

Your basket design won't work. The parts are in what's called "anode shadow". Most of the current is going to your coopper wire laced up the sides, and very little gets to the parts, so they're not plating properly.

I've tried the basket, and other methods. All have been failures for one reason or another.

Read the stick post: Best way to prep & plate nuts & bolts

3 second dip in 5% muratic rinsed and into black chromate solution

Use only sulphuric acid for black chromate parts.

the solution turned cloudy and eventually went from black chromate to a bronze color

Black chromate is extremely sensitive to contamination. Anything you put into the chromate solution must be rinsed absolutely clean.

Muriatic acid drag-out is bad for it. Black chromate is sulphuric acid based.

If you drag ANY electrolyte solution into the black chromate, the chromate bonds to the free zinc ions. Once bonded, those chromate molecules are used up forever.

If you see any milky streaks developing in the chromate when you immerse the parts, then they are not rinsed well enough. The more often you get milky streaks, the less effective the chromate becomes.

If the entire chromate solution has turned "milky coffee" color, it is shot! If your parts won't get beyond the "bronze" color, the chromate is all used up.

Sean

[hdway]

Sean
I have a 25 amp rectifier and think that I got the plating on the parts OK but maybe not, after the muratic pickle mainly used to clean the cad plate grey on the parts I rinsed with distilled water sprayed liberly over the basket of parts but may still of had muratic on them I did get a start on black cnromate but was poor. Eventally I abandoned the basket and tried one bolt it has so much zinc on it now that the nut won't screw on still can not get anything but a yellow tint, if the black chromate gets contaminated so esily how can you get any amount of items chromated with out replacing the bath. What about the particals of white in the bottom of the chromate are they suppose to br there? When my chromate settled overnight it cleared up. Will the sulphuric acid affect the bath as much or is it compatable with the sulphur base?
Have aranged to get some nitric acid and sulphuric should have it tonight the problem is the quantity I would have to buy, the only nitric supplier has minimum 75.00 sale and can only find sulphuric at NAPA in 20 litre don't want to store it in my shop as you always get some fumes leaching out and it is -35 C here so it freezes in my machine shed. As far as the rinses I don't like the distilled water spray would rater give it a quick rinse with the softened water in my sink and then spray with distilled.
Appreciate any help you can give as I have bolts to get chromated so I can get my car back together and it takes me two weeks by the time I get supplies from Caswell, will be ordering there blue chromate for my cad finish after looking at your pictures.

[seanc]

Originally Posted by hdway

if the black chromate gets contaminated so esily how can you get any amount of items chromated with out replacing the bath

Ya gotta realize that all chromates are "consumables". ie, is has a life span. The active ingredients eventually get used up, and parts won't chromate properly any more. Any extraneous contamination that uses up the active ingredients just shortens the life.

I'm no chemist, but I think I have a handle on this aspect. Chromates have 2 main ingedients:

1. chromium - which imparts the corrosion resistance
2. a "colorant" - some chemical which imparts the desired color (if any)

In the case of black chromate, the colorant is a silver salt (contains either silver and/or silver oxide). When silver tarnises, it turns black, and that's what gives our parts the black color. There's only so much silver in the chromate solution to begin with. As you use it, the remaining silver decreases, until there's not enough any more.

Any contamination that attaches to the free silver, essentially uses it up, and it precipitates out to the bottom of the tank. The zinc electrolyte has billions (maybe more) free zinc molecules which attach readily to the silver. Even small amounts of it will severely use up the free silver. 1cc is enough to completely waste a gallon of black chromate (don't ask how I know).

What about the particals of white in the bottom of the chromate are they suppose to br there?

No. That's the precipitated silver/zinc.

When my chromate settled overnight it cleared up.

Yes, it will clear up, but it's still missing the silver that's precipitated out, so doesn't work properly anymore

Will the sulphuric acid affect the bath as much or is it compatable with the sulphur base?

I really don't know how detrimental "other" acids are, but since the black chromate is sulphuric based, at least it's not an "un-natural" ingredient. I suspect that's why black chromate manufacturers recommend only sulphuric acid for bright dips.

Too much of any acid is going to upset the chemical balance, so best to have parts thoroughly rinsed.

As far as the rinses I don't like the distilled water spray would rater give it a quick rinse with the softened water in my sink and then spray with distilled

That should work fine. I use running tap water for #1 rinse, then spray down w/distilled.

So how long will the black chromate last? It's not possible to say. It just depends on how many square inches of parts you chromate, and how long you leave them in for (and, how much gets used up by contamination).

When I first started to use my black chromate, parts would turn deep black w/in 15-20 seconds. By the time I had the mishap that ruined the chromate, it was taking 30-40 seconds to get deep black. So it was clear to me the chromate was depleting. By this time I had done about 350-500 sq-inches. YMMV.

After the mishap, this is what parts come out like, even after 5 minutes in the chromate!



Sean

[hdway]

I get the point on the chromate depleteing the silver and the sedement but what I found was when I first mixed up the bath it did not appear to mix very good and I have only done the 2 small test plates and then tried to do my basket of bolts that only partially blackened I did rinse the basket with distilled water but due to a lack of force from the spray maybe there was still some muratic left from the dip in it.

I think Caswell better improve there instructions my version 9 manual that I just got with there kit does not contain instructions on the black chromate
which when I called tech support on the instructions they said they would have them in the next revision of the manual.
The tech I spoke to told me just to mix it as the black chromate container stated and dip the parts in it, I then ask him to email or fax the instructions to me and was told they hadn't been writen yet. I think that Caswell should replace the product at no charge seeing it was anounced as being new in 2006 as I found out on this forum. The instructions that they issue should also contain some of this information like using sulfuric acid as the muratic is not compatable. Also want to go into the blue chromate after seeing the bolts that you did to prevent the water marking and white rust, what I am plating is for use on my show car and needs to maintain the Cadium and Black oxide appearence on bolts and hardware this is why I bought the kit so I could get the black finish that will last, the oxide will get rust on it just from trailer travel and humidity.
Thanks for all the input.