Letter 33011

Problem in Ni sulfamate plating on Al [India] 

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Dear sir

I am going to Ni plating on Al-8% Si (casting).

1- Immersion in 2% HF+ 25% HNO3+H2O.

2- Ni electroless , composition of bath is:
Nickel chloride 30 g/l
Ammonium chloride 50 g/l
Sodium citrate 100 g/l
Sodium hypophosphite 10 g/l
PH 8.5-9
Temp. 91 C
After one hour , color of deposit is gray! Why?

3- After EN , Ni electroplating with sulfamate bath, composition of the bath is:
Nickel sulfamate 200 g/l
Nickel chloride 78 g/l
Boric acid 40 g/l
PH: 3.2-3.6
Temp. 58 C
Current density : 7 A/dm2

During the plating on cathode was formed very bulb! Why?

Please guide me!

Thank you.

Ziba Zorivi
- Roorki, India

8% silicon is not the best alloy to plate onto! In fact it is quite difficult. I do not know what you mean by "bulb" but assume the deposit has blistered. I notice you haven't mentioned a zincate treatment; this is essential in plating aluminium. Personally I do not like electroless nickel directly onto such high alloy aluminiums as there is always the risk of heavy oxides and other non-conducting things on the surface and these are often not conducive to good electroless coating. High silicon alloys need a good etch followed by a chloride-free metal eletrodeposit. This will reduce the chance of choride attacking the aluminium; you should be able to find a bromide based electrolyte from a reputable supplier.

Trevor Crichton R&D practical scientist - UK

First of all you need an alkaline etch prior to the HF with nitric acid.

Your electroless nickel is a home brew which is not stabilised. Why is it base on Nickel chloride?

Why 1 hour in EN? 5-10 minutes will do!

Your Nickel Chloride in the sulphamate is too high! 7 g/l will do. Nickel bromide is better.

Sara Michaeli     chemical process supplier Israel

Dear Sara

Choride bath is better for aluminum alloy (refer to Modern Electroplating)

I used Pb+ as stabilizer.

Rate of deposit is very low, therefore time of plating was 1 hr.

Correction, Chloride in sulfamate bath is 8 g/l.

Thank you

Zorivi
- India

Interesting article in "Plating and Surface Finishing" November 2004 about plating onto aluminium-silicon alloys. It suggests using a cleaner, followed by an etch with HNO3 and NH4HF2. It is then suggests double zincating with an etch between the two. Electroless nickel is then put down, followed by electrolytic nickel. Have a look. I agree with Sara's comments and would like to add that the pH for sulphamate nickel seems a bit low to me, try 3.7-4.5. Also, why use 8g/l nickel chloride in sulphamate? This is courting disaster as the chloride will go for any exposed aluminium. If you are not using sulphur depolarised nickel, then add nickel bromide. If you do have sulphur nickel, in theory you don't need any halogen salts, although you can still use some if you are really unhappy about not having them. The presence of chloride will certainly induce tensile stress, which could be a problem to you.

Trevor Crichton R&D practical scientist - UK

Ziba. I think the article in the Modern Electroplating is wrong. If you use a high chloride nickel solution for plating on casting aluminum alloy. This will happen, the chloride will dissolve some alloying elements in the casting and nickel deposit will begin to build some bridge over the pits, and in the pits will some acid staying left and lift the deposit from the aluminum. This can also happens when you over etch aluminum.

Regards

Anders Sundman     surface finishing engineer Sweden

Dear Sundman

Do you have any articles that confirm your idea?

I do it again, Quality of deposite is good up to 35 Micron.

As I wrote, I used electroless for undercoating (max 2 Micron) and after Ni plating with sulfamate bath .

Thank you

Zorivi
- India

Zorivi,

Not in English but only in Swedish. But I have did studies by my self on this, who I can see at this can happens. To understand this try to read a metallurgical book about aluminium alloy. 2 micron EN deposit is not good enough to withstand acid from the sulfamate solution.

Regards

Anders Sundman
Sweden

I agree with Anders, aluminium in the presence of chloride is NOT a good idea.

Trevor Crichton R&D practical scientist - UK

Dear Sir

What is your suggest?

How do plating on Al-Si?

If I use acid bath (for EN) zincate removed!

Please guide me!

Thank you

Zorivi
- Roorki

Run your electrolytic nickel at about pH 3.8-4.5. If you must use a nickel halogen salt, DO NOT use chloride, but bromide. Also take note of the earlier comments above.

Trevor Crichton R&D practical scientist - UK

Zorivo,

To electroplate or chemical plate on Al-Si if the Si as a alloying element is to high. I will prefer a cleaning in a non etching cleaner at first. Etch the part for a short time in an alkaline solution only to obtain a black film on the part. This film must be removed in a ammonium biflouride acid. Rinse in water. Etch in sulfuric deoxidizer solution up to one minute. Rinse in water. Desmut in a nitric acid for a few seconds. Rinse and go to the zincate up to 30-60 seconds. Use an alkaline EN solution up to 10 micron or a neutral EN solution up to 10 micron. Or you can use a alkali copper solution as a strike. A nickel sulphate is only good enough to plate on Clean aluminum parts.

This is one and must easier method to get a uniform plating on aluminum and this alloys. To really get the best plating deposit you must let us know the other element in the alloy.

Regards,

Anders Sundman
Sweden

Dear Sundman & Crichton

Thanks alot.

My base metal is Al 356.

I have to coating up to 400 Micron.

I think ,use of copper plating is not good.(as striking)

I used HNO3 & HF (9:1).

How can I obtain Ni plating, 400 micron?

Thank you

Zorivi
- India

Zorivi,

The aluminum alloy from the 300 series contains a lots of Mg in the a nitric acid the Mg will be passive like magnesium nitrate and you must use a sulfuric acid to make it active to magnesium sulphate who are more easy to dissolve. I know the pretreatment steps for you are one of the problems.

Regards,

Anders Sundman
Sweden

400 microns is quite thick for electroplating and needs to be treated as if it was electroforming. Otherwise you may get into problems with irregular and nodular growths. I therefore suggest you use electroforming conditions for your deposit.

Trevor Crichton R&D practical scientist - UK

Dear Trevor Crichton

What is your purpose "electroforming conditions "?

For electroforming usually use sulfamate bath, I do too.

Please explain your idea?

Thanks.

Zorivi
- India

Higher filtration rates, better local current density control, more controlled deposition rates. Basically eerything you need for good electroforming. It is not just the solution you use for electroforming, its the way you do it. There are lots of textbooks around that explain in quite some detail about electroforming.

Trevor Crichton R&D practical scientist - UK

Dear Sir,
Chloride is too high and increase the concentration of Nickel Sulphamate to 600g/l. Use 2g/l Nickel chloride and use S Nickel as anode.

Ramesh Modi
- Dahanu Road, Maharashtra

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