Electroforms are sticking to cathode/mandrels

Q. Hello at all,
We're using potassium dichromate solution for doing a passivation layer on copper or nickel. After nickel plating on that, the parts can divided without any problems.

Are there any alternative solutions to substitute the chrome-VI ?

Thanks and all the best,
Frank

Frank Kirschner
- Berlin Gemany
May 3, 2023

---

---

⇩ Closely related postings, oldest first ⇩

---

---

Q. Hello,
I am a graduate student at Laval University working on my research project which is pure iron electroforming. I use type 316 stainless steel as cathode, Armco iron as the anode, and a ferrous chloride-calcium chloride solution as electrolyte.

The cathode surface is polished with SiC abrasive paper 1000 grit and immersed in Na dichromate solution (for passivation) before the deposition process. Since the process is electroforming, we do not want any adherence between deposit and substrate, however in most cases the deposit adheres to the cathode and is not removable.
Would you please advise me how to solve this problem?

Thank you for your attention,

Maryam Moravej
Laval University - Quebec, Qc, Canada
2006

Q. Hello,

Thanks for your advice. I was trying all methods you have suggested which were unfortunately not successful.
I increased the passivation time of 2N sodium dichromate solution and also tried to passivate the substrate with potassium permanganate, sodium metabisulphite and 30% acid nitric solutions. I also did an electropolishing on stainless steel after the mechanical polish to have a perfect surface finish before passivation which did not work either. I have also tried the thermal shock without any results due to the little difference in the thermal expansion coefficients of iron and SS.
The substrate which is used for the iron electroforming is a cubic piece of SS 316 mounted in resin and therefore, the exposed surface is planar without angles.

Could you please suggest me other materials for the substrate? I am not sure if I can use aluminium or copper alloys because of the highly corrosive bath (FeCl2-CaCl2 and HCl with pH=1 at 90 °C). Do you think that nickel would be appropriate? or can I use a conductive polymer?

Thank you,

Maryam Moravej
Laval University - Quebec, Qc, Canada

---

---

Dammit, Jim, we're printers, not chemists! /Dr. McCoy voice

Q. We are a printing company that has developed a holographic printing division over the years. After a recent visit from the state Dept. of Health and Env., we are looking at alternatives for potassium dichromate that we use in the production of holographic printing plates. We use the dichromate to 'passivate' nickel. Since I have looked at the archives on this and other sites, I realize that passivate means different things in different situations, so I will describe our process.

1) starting with a holographic image on resin on glass
2) we paint the resin image with silver
3) after cleaning and preparing the silver, we grow nickel on silver electrolytically to 5-10 mm in a nickel sulfamate bath; this becomes the electroforming master
4) [Step of interest] we 'passivate' the nickel surface by spraying it with potassium dichromate-- this is done at room temperature, and the dichromate is left on for a minute or less. The reason for this step is so that the duplicate image(next step) will peel off the master.
5) rinse dichromate off with D.I. water
6) grow a mirror image of the master electrolytically in same nickel sulfamate bath to about 3-6 mm. This becomes the printing plate that is used to impress the holographic image onto end stage plastic or foil.

I hope any suggestions could note how much change is required from the current procedure, as we are change averse, because, like I said, we are not chemists, and when things are going well we would prefer not to change anything (but in this day and age, getting rid of hexavalent chromium looks like a good idea).

George Parker
Commercial Printer - Topeka, Kansas, USA
March 11, 2011

---

---

Q. Greetings all,

Recently I was hired as a chemical engineer at a company that uses nickel sulfamate electroforming to produce optical tooling. I've been using this site and the past letters to really gain a lot of knowledge, so thank you all for being such great sources.

A want has arisen to investigate replacing our current method of post-forming passivation with potassium dichromate. This is obviously a health-related issue, so we're looking into any options that would be safer but still allow the nickel electroforms to be removed from the mandrel without too much difficulty.

I've been referred to electrochemical passivation as a possibility, but I'm trying to gather as many possibilities as I can before starting comparisons and trials for our specific needs. Are there any specific alternatives people have previously used with success?

Thank you all in advance for your suggestions.

Eric Janosko
Electroforming chemical engineer - Rochester, New York, USA
May 1, 2012

Thanks Trevor. The mandrels we typically use are nickel as well, or at least the "functional" side I should say.

I had heard of the sodium metabisulfite and I have begun working with it, though it did "color" one of my test parts. Because we're working with optics, the cosmetics are extremely important, so this is a bit concerning. I am in the process of restarting my testing due to some tooling issues that I believe were giving me false positives, so I will try it again just in case.

I also have plans of trying citric acid as well as the egg albumin that have been mentioned in the past. The permanganate is new information to me, so I will have to look into that as well.

Thank you for your information. Should I come up with something interesting, I will be sure to share.

Eric Janosko [returning]
- Rochester, New York, USA

Trevor,

Q. That's exactly where the metabisulfite discoloration appeared. Unfortunately, that is a show-stopper due to the resulting electroforms being used for either solar applications or reflective product manufacturing. A "cleaning" dip afterwards may be a possibility, but I'm hoping to find a drop-in replacement and not have to change our current system too severely.

I'm working on an egg albumin recipe now while I make a few more "disposable" mandrels for testing.

We currently use potassium dichromate for the process, which of course works great, but hex chrome replacement is the whole catalyst for this experiment. I know that trivalent chromium is a possibility as well, but with the jury still out on on whether the hexavalent state reappears or not, I'm trying to avoid that route.

These replacement attempts aren't exactly required as we haven't had any issues at all, we're just trying to make it a little safer around here if possible. So if nothing can match the dichromate for our operation, then we'll likely stick with it for the time.

Eric Janosko [returning]
- Rochester, New York, USA

Brown streaks on nickel electroforms from permanganate passivation

Q. Hi Everyone,

Recently I have been doing a lot of work developing a potassium permanganate passivation for our nickel electroforms to replace our use of dichromate. In general, it has worked very well. However, we occasionally get some brown streaks across the tools, which considering they are optical pieces is somewhat of a problem. I am able to treat the tools to remove the stains, but I'd rather try and attack this problem at the source. Does anyone have any idea what could be causing these stains?

Eric Janosko [returning]
CEF, chemical engineer - Rochester, New York
November 4, 2013

A. Harry is right. I have tried using permanganate as a dichromate substitute and generally been successful, although it is not as good as dichromate. The brown streaking is due to reduced permanganate to a manganese oxide or hydroxide. I would not like to say which oxidation state the manganese is, but it will be the lower ones. I suggest that the passivation time is too long, so try reducing it. Also, it is possible that the permanganate solution is old and has a lot of lower state oxides building up in it, so try using a fresh bath. I have also found that using an electrolytic permanganate bath works well at room temperature and you can get away with very short passivation times - perhaps as low as 15 seconds. An alternative passivation solution is 200 g/l sodium metabisulphite - this works on nickel and stainless steel and (I believe) copper mandrels. It gives a very dark film that is excellent for nickel separation. The immersion time should be about 2-5 minutes at room temperature. When it is spent, you can use it the reduce Cr(VI) to Cr(III)! Trevor Crichton R&D practical scientist Chesham, Bucks, UK Harry, Thank you for your response. That actually is my prevailing theory that the manganese dioxide is the culprit. The pH sits around 4-5 most of the time, and I've seen the changes already by fluctuating the pH. Since the short time that I sent in this letter, I actually stumbled across a patent that has provided some interesting results for me: https://www.google.com/patents/US4268487 So far, in my initial testing, this has been working well. Still a long way to go yet, but it has definitely stopped the staining on the tool. Eric Janosko [returning] - Rochester, New York

Q. Trevor,

The solution is quite fresh. The sodium metabisulfite route doesn't work for us due to our high-precision optics, I tried it before the permanganate.

Currently our passivation time is 1 minute, though we haven't tried anything less yet. So far so good with the sodium silicate solution though.

Eric Janosko [returning]
- Rochester, New York
November 13, 2013

---

---

Q. We are electroforming copper using a non-cyanide alkaline bath onto 304L stainless mandrels. We had previously used an orthophosphate bath, and the stainless surface was electrocleaned and then passivated cathodically with a sodium phosphate bath. We never had any problem with adhesion to the SS using the orthophosphate bath, although we did sometimes run into trouble removing the electroform from the mandrel.

After switching to the non cyanide alkaline, we had big problems with adhesion. I was told that a Woods nickel strike would be required to get decent adhesion, so the mandrels were struck at ~15ASF for about one minute in a standard Woods bath. They were then rinsed thoroughly and cleaned. After the plating, we have found it impossible to remove the electroform from the mandrel. I have read of different release agents, including dichromate and egg albumin, but I was wondering if maybe there was a different process for activating the stainless that did not offer such a degree of adhesion.

Thank you.

Chris Smith
Tech - Tucson, Arizona USA
February 26, 2014

A. Hi Chris, Even if you have zero adhesion, it doesn't follow that the electroform should part from mandrel during plating. I'm not surprised that after the nickel strike the electroform cannot be separated. The strike is intended to give very good adhesion. My thinking is that the new electrolyte gives a deposit of high internal stress and bends off the mandrel. If it adheres strongly to the mandrel and electroform will continue to adhere when you attempt separation. I can see your aim is to get just enough adhesion to hold the electroform in position but not to prevent separation. The methods you propose are worth trying and I would add a couple of other suggestions: 1) Clean and nickel strike as at present but then flash plate with copper followed by passivation of the copper using benzotriazole [on eBay or Amazon] dissolved in sodium hydroxide. 2) Omit the nickel strike but activate the stainless steel with hydrochloric acid just to get a little adhesion. If stress is the problem I an concerned about the viability of the process. A prime feature of an electroforming bath is low internal stress of the deposit either tensile or compressive. For my interest, would you confirm that you previously used orthophosphate and not pyrophosphate. Harry Harry Parkes - Birmingham, UK As a follow up we had great success using a chromium based passivant. Part was polished to 1 micron, cathodically cleaned in an alkaline bath, and passivated in 2% chromic acid. After a thorough rinse it was struck in a low copper concentration pyrophosphate bath for about 30 seconds, then rinsed and transferred to the non-cyanide alkaline bath. The electroform was able to be removed by hand from the mandrel. Chris Smith [returning] - Tucson, Arizona USA

---

---

Staining on Nickel Mandrels

Q. Hi,

I am working at a nickel electroforming factory and we are having an issue with repeated use of our mandrels.

We use mandrels made of Nickel and after about 15-20 cycles, the mandrels begin to develop a staining (either brownish in color or white in color).

Our process for cleaning the mandrels involves anodic electroclean, sulfamic acid dip and sodium dichromate coating with DI water rinsing after each process.

Anyone have any experience with this that could point us in the correct direction for things we should be looking into? This appears to be some type of oxidation but we cannot identify where it is coming from or what is causing it. We want to increase the number of uses before replacing a mandrel so this has been a troublesome issue for us.

Thanks in advance for your help.

Brett Clemins
September 18, 2014
R&D Engineer - Bismarck, North Dakota

----
Readers: We have several other threads on similar subjects, including

---