JASON

Topic: Rissing sulphate levels

 Hello. My sulphate levels in two of my three baths are getting high. We try to maintain an approx 2.5 gram per liter to a 250 gram per liter between the chrome and sulphuric. Our sulphuric levels keep going up slowly month by month. I guess what I am looking for is, what are some contributing facture of rising levels of sulphate?

DustinGebhardt

Re: Rissing sulphate levels

You must be dragging in the sulfate from somewhere.  Take a sample of the bath before the chrome and test it for sulfate.  I've seen this problem before when the rinses between nickel plating and chrome plating were insufficient and  sulfate was being dragged into the chrome.  We fixed this by creating a chrome pre-dip bath before the chrome bath.  This was a stagnant bath with ~4opg of chromic acid and a maintained level of sulfate that was 1/2 of the concentration in the actual chrome bath.  The level of the sulfate would rise over time, and once it reached the same concentration of the chrome bath, the pre-dip was dumped and remade. 


On a side note, we also electrified this bath, whcih greatly helped with maintaining an active surface before chrome plating.  This greatly reduced the amount of false burn we produced.

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

Ira Donovan

Re: Rissing sulphate levels

The most common sources are incoming water (city or DI) and drag-in.  Depending on your location, the city water provided at the tap may have levels of sulfates that will build over time in your tank.  DI water should be clean, however there are those cases where people regenerate their resins with sulfuric acid.  Due to insufficient backwashing, there can be an initial slug of dilute sulfuric in the make-up water.

Finally, drag-in is commonly due to poor rinsing after a sulfuric acid pickle.  Another potential source could be entrapped pickle solution in blind holes or recesses that is not being effectively rinsed.  Along the same lines, if your racks or fixtures have peeling plastisol or other coatings, pickle solution can become entrapped in those locations as well.

I hope this helps.

Ira Donovan, M.S.F.

Burns & McDonnell Engineering. 

Ira Donovan's Website

JASON

Re: Rissing sulphate levels

Thanks for the reply Dustin. We only do hard chrome, no nickel. I was told that the only way to increase your sulphate levels was to add sulphuic acid. We have not added any for months. I will be changing my bath in the new year do the high levels of other contaminants. Can high evels of iron, copper and trivalent be the culprite? I am currently adding barium carbonate to try and nutrilize the sulphate. I am open to any suggestions. We also do alot of inside dia. plating in the two problem tanks.

JASON

Re: Rissing sulphate levels

The most logical explanation for the rise is, tap water. We use our local water to rinse parts and replenish lost solution levels. We have our own lab and I will get them to test the sulphate levels in our tap water.

Jason

Ira Donovan

Re: Rissing sulphate levels

Jason,

The incoming city water is a definite concern for you.  The incoming levels will concentrate as you continue to add back water for evaporation.

As for your contaminants, I do not think those will contribute to any sulfate issues.  Are you using a porous pot or dummy cell to control your contaminants and trivalent chrome?  This is common practice for most hard chrome operations to extend bath life.

Regards,

Ira Donovan, M.S.F.

Burns & McDonnell Engineering.

Ira Donovan's Website

DustinGebhardt

Re: Rissing sulphate levels

I second the recommendation for the porous pot.  When run correctly, they greatly extend the life of a chrome bath by filtering out contaminants and converting trichrome back to hexchrome.  


As far as reducing the buildup of the contaminants in the first place, I would check your rinsing practices.  Poor rinsing, especially with blind parts or parts that can trap solution, is a major contributor to contamination.

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

JASON

Re: Rissing sulphate levels

We do not use porous pot. What is the typical bath life if using porous pot? We are mostly a recon shop, so small defects are usually not a concern do to overplating and finish grinding to a certain size. A tipical bath life for use is 6 to 7 years. I seems if we keep our ratio in check, we do o.k. This is now the problem.

Jason

Rindy Smith

Re: Rissing sulphate levels

I think you may have forgotten another couple of sources for sulfate.  It is my experience with Hard Chrome platers that cleaning  is done in the tank by reverse current before applying direct current.  If the cylinders were processed with a sulfur bearing oil used as a coolant then that can be a potential source and contaminate in the plating tank.  The Chromic acid you purchase also contains small amounts of sulfates.  Please ask your chrome supplier for a lot analysis.  A fluoride bearing catalyst may also be the culprit if the souce of fluoride is from a inorganic sulfate.