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Unexplained pH shift in nickel sulfamate bath

October 20, 2009

We are a reel to reel job shop plater and have several lines running text book nickel sulfamate formulated baths. CD may range from 50-200 ASF and cells may see as high as 200 amps @ 8-9 volts. Normal pH range 3.4-4.0. Ni metal 65-80 gpl. S-Round Ni anodes are used in bagged Ti anode baskets. Baskets are fully packed w/S-Rounds.

Recently we purchased "brand new" Ti anode baskets from a new vendor. After only several hours we observed the pH of the bath, which normally rises, had dropped to 2.4. Plating rate did not seem to be affected and amperage/voltage appeared normal. A bluish sludge was observed forming between the anode bags and the baskets. After several days running, the pH had not risen above 2.4. The deposit continues to remain & tests acceptably.

1 gram of the sludge was dissolved in 500 mls DI and titrated for Ni and boric w/0.058 oz/gal Ni and 0.12 oz/gal boric results. pH of this sludge/solution was 2.40.
10 grams of the sludge was dissolved in 500 mls DI and titrated for Ni and boric w/0.12 oz/gal Ni and 0.76 oz/gal boric results. pH of this sludge/solution was 1.40
The new Ti baskets were removed from the tank and inspected and after only 36 hours operation had already blown holes in the Ti mesh and dissolved thru parts of the Ti straps.

We are now in the process of replacing all the "new" Ti baskets with baskets purchased from the former vendor, whose baskets typically lasted for years. The only variable in this equation are the new baskets. What occurred?

Normally low anode level in the baskets can lead to overvoltage and damage to the baskets so we try hard to avoid that condition. But in this case the anode levels were high from the beginning. We hope we have not destroyed our Ni bath with some form of dissolved Ti. Did not think we could dissolve Ti w/o nitric/hydrofluoric acid?

Greatly appreciate any comments.

Earl Miller
process engineer - So. Plainfield, New Jersey, USA

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First of two simultaneous responses --

The anodes were passivated enough that the SD anodes did not have good contact. This is a serious problem because there is decomposition of the sulfamate producing a sufite compound that causes high compressive stress in the deposit along with brittle deposits that are also harder and less ductile. The same happens when non soluble anodes are used.

Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]
           and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])
October 22, 2009

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Second of two simultaneous responses --

I will bet my cup of coffee that it is the S rounds at fault. I am a believer in High pressure washing and a quick dip in 20% HCl every time they are removed from the tank and allowed to dry.
If the anodes are not corroding properly, the anodes will at that high a voltage.

James Watts
- Navarre, Florida
October 22, 2009

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Surely if the anode is passivating, the pH will increase, not decrease. From what you say the pH stabilised at about 2.4 and was constant after a few days of running. This suggests to me that the pH has decreased because of an acid contamination, possibly when you prepared your new anode baskets. Did you thoroughly rinse the anodes (basket, bags and SD nickel) before putting them into the tank?

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
October 27, 2009

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First of two simultaneous responses --

Trevor,
Insoluble anodes (like platinized Ti) do lower the pH out of the normal operating range. It also lowers the tensile internal stress to excessively compressive. Girl scouts honor.

My Boo boo on the previous post. If the nickel is passivated, then the anode BASKETS will fairly rapidly erode at the high voltage mentioned.

James Watts
- Navarre, Florida
October 27, 2009

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Second of two simultaneous responses --

We are a hard chrome and nickel plating company and use a sulfamate nickel bath strictly for heavy build-up salvage work. Please expand on the theory of the Ti anode baskets being passive. In the past, my company has experienced a bluish sludge develop at the solution level on the anode bags as well as the bags themselves turning a light tint of yellow (S rounds). The pH didn't change significantly in either direction, but we experienced severe HCD pitting on heavy deposits. Our woods strike was not effected as the anode bags did not discolor (S rounds).
What is the proper conditioning for new Ti anode baskets, as we have never done this?
Could our pitting issue have been caused by poor anode activation or passivation of the Ti baskets?

Thank you-

Jeff Hanaway
Job Shop - Meadville, Pennsylvania, United States
October 28, 2009

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I used to process a new anode basket thru an electrolytic caustic clean and a acid etch before use as some had a very dark blue/black coating on them. In other shops where I had a nitric/HF acid tank, I would use that for the acid etch which works a lot faster. On a rare occasion, I would grit blast the hangers to clean them up.
I never saw a blue scum but frequently had a green build up on the bags/anode hangers.
The yellow on the bags is a combination of old age and organics. Bags need to be changed periodically or the solution can not get thru them adequately.
S-rounds is a poor to bad anode material for a true Woods nickel strike. I strongly favor a rolled depolarized nickel and electrolytic nickel chips as a second choice. The sulfur from the S-rounds will make the tank stink and you will gradually have problems that will result in replacing the entire solution. NOT cheap.
HCD pitting is more likely caused by poor solution agitation and/or shelf roughness from trash in the tank.
Also, check your antipit level.How are you currently checking it? If you are using a rather high HCD, you might need a localized fluid or air agitation.

James Watts
- Navarre, Florida
October 29, 2009

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Hello,

definitely your are forcing passive anode baskets. Prior of first use it is recommended to activate them in an sulfuric acid solution with the help of current (Ti-anode basket anodically switched).

Regards,

Dominik Michalek
- Sydney, Australia
November 1, 2009

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Thanks to all who responded to our question. The good news is that the situation has not reoccurred since we returned to the original basket manufacturer. Though we still do not fully understand the reason for the dissolving Ti Baskets. Our basket vendor has provided us with a simple pre-use treatment. We're ordering new baskets this week and presume they will operate fine.
Regards, earl miller

Earl Miller
- South Plainfield, New Jersey
March 17, 2010