Black and streaked deposits of EN coating

A. It is very difficult to give you an answer since only the vendor of your chemistry knows what's in it. I can only say that nitric is a venom and kills the catalytic reaction in most acid formulations, so it is not likely to be the cause. It may be something else such as a slight imbalance of the make-up chemistry that improves over time with replenishment and work. Have you tried preparing a new bath without leaching the tank, or vice-versa to see what happens?

A. Nitric acid indeed could be the cause of your problem. The addition of sulfamic acid will mitigate small amounts of nitric acid. But the best solution is to use sulfuric acid/hydrogen peroxide to strip the tanks thus eliminating nitrates altogether.

A. After stripping a tank with nitric acid (or with sulfuric/peroxide), drain as much of the acid as possible, then fill the tank with water, add enough ammonia [on eBay or Amazon] or potassium carbonate to raise the pH to at least seven, and recirculate the solution for at least 15 minutes. Then drain the neutralizer water solution, and thoroughly rinse the tank (preferably by filling it) with DI water. You can purchase nitrate test papers to check that the rinse water is free of nitrates.

A. Rivon,

You do not say if the "1st to 3rd" runs after stripping out the tank are a new bath make-up -- a critical piece of information. If you put back into service a bath that was working fine before strip-out, it would make sense that you have a contamination problem, either from the Nitric or from your pumps or the holding tank.

If it is a new make-up, you have a problem with the EN formulation. Not that it is "bad", just that it does not fit your situation. What is your bath loading? In the 10 HR run time, How many metal turns do you use? Are you titrating and adding back to the solution frequently enough to NEVER add back more than 10% of the bath make-up ? If you are under-loading the bath I would NEVER exceed 5% addition. Are you maintaining the PH in the recommended range? Do you have a roughness or tank plate-out condition? Have you tried reducing your agitation on the first 3 runs?

I would suggest that you are using a bath that is high in stabilizer and or brightener for your bath loading. Mostly in the make-up Chemistry. Which could be why you are having success after 3 runs.

Awaiting more detail,

A. Rivon,

I was thinking more along the lines of a EN chemistry problem, and I still believe that, I have never seen a small amount of nitrates cause a major problem. If you have gross contamination it will shut the bath down. So now I have more questions. Could you better define what you mean by "run" as in the first to third "runs"? What TYPE of EN are you using? Phosphorous content? Is it one of the CAD and LEAD free formulations? What size is your bath? Are you using air agitation or solution sparging? If you are not having a tank plate-out condition until after 4 turns, why are you passivating with nitric before this? What material are you plating,and what is your required EN thickness? What is your tank material? Are your properly leaching your filters before use?

The bath loading may be a little low, have you tried increasing the load size?

#4 above concerns me, It says to me that the problem may be low stabilizer in the add back chemistry, and high stabilizer in the make-up chemistry, my first analysis, but if this was the main problem, when you passivated the tank and returned the used chemistry, it should have worked like before you passivated the tank!

As I await your response, here may be some quick solutions.

Avoid nitric all together by using Liners in your tank, use them for a few turns and dispose of them. Or, pump out your tank more frequently and manually clean the tank walls- if you do this often enough you may never need the nitric.(If this is truly the bad guy)

Re-plumb your tank, install clean outs in the lowest points of your plumbing so you KNOW you will properly rinse after passivation.

You will find the answer, Steve.

A. Hi Pivon,

Try to use ammonia with water to pump at last step in your EN tank and the filtered, before you fill the tank with EN solution. Often black deposit can come from water rinses with sulfuric contamination, but this happens after EN plating and rinses.

Regards,

A. Hello Norm,
I have touched up parts that had blemishes in EN before with success. I used a wet scotch brite pad and "blended" the finish around the defect, cleaned, re-activated and went back into the EN. The parts in your photos however look all black. This could be the photo color quality though. I am not sure if my method would help you, and would depend on the severity of the defect. The finish may have to be stripped. There are Ni strippers available on the market if you can't scrap the parts. If you do a lot of EN, it's a good idea to have a strip tank in place. Let the supplier of the stripper know the type of EN and that brass is the base metal. Be careful with internet home brew products and having to mix strong acids. A Ni strip supplier can furnish the product to you ready to use. Good Luck to you.

A. Blue Magic polish will work. You can get at any auto store.

The black discoloration was pretty much on 100% of the surface. I had it in my head to remove it the way it went on, by immersion in chemistry. I tried several different brands of C-L-R and they seem to be all the same chemistry. They may work for some things, but not this. I finally removed the black with a very fine ScotchBrite pad, working under water. The parts are now usable and cosmetically pretty.

Blue metal polish would have been my next step. Thanks.

BTW, I had to have my DI water ion exchange resin tanks changed twice more, a week apart. Typically, they've been good for two or three months. The service tech tells me that it may be due to road salt runoff. We'll see how long this set of tanks last.

Thanks to all,
Norm