Letter 3778

Problems in Nickel Chrome plating on Zinc Diecast Parts 

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We plate a lot of Nickel Chrome on Mazak parts which go into locks, and wonder if there is any remedy to the problem of copper and zinc regularly dissolving in the Nickel Solution leading to black deposits in the LCD areas.

Why the chrome bath also produces burning on the HCD areas is another mystery to us. Our Supervisors swear that its on account of the same contaminants that are playing havoc in the Nickel Baths and decant both the Nickel and Chrome on a daily basis to remove any fallen Zinc parts but although this needs to be done , I would welcome any assistance to both these problems.

Khozema Vahanwala Saify Ind   Bangalore, Karnataka, India

Hi Khozema.

Yes the zink and copper displays on the low current areas. Plate the bath with a dummy sheets and use a low current for copper 0,1-0,3 amp per dmž and for zink 0,2-0,5 amp per dmž. You must also inspect the bath if the part dropped from the rack. About the chrome solution, following reasons will make burning on the HCD areas. High temperatur, Low Cro3, High ratio between Cro3 and sulfate acid, the current turn on to fast.

Regard
Anders

Anders Sundman     surface finishing engineer Sweden

Hi Khozema. Anders is correct that the Cu and Zn will deposit in the low areas and cause them to be dull. Getting the parts out that fall in the tank is imperative. They dissolve fast. A regular schedule of low current dummying will keep those metals below the level they start to interfere. We also send samples to a lab to have the levels checked. It only takes a few ppm of either metal in the bath before it starts to show. They do have additives that will complex these metals temporarily but they are just harder to remove later.

Jim Conner
- Salisbury, MD

Hi Khozema,

I too agree with both Andrea & Jim, but we do plate these contaminant out by adding GW Additive 42, though the deposit is little dull but its not black. Also try keeping pH on the higher side. You can avoid decanting by just placing a mesh tray coated with PP at the bottom of the tank, this way you'll just have to raise the basket and take out the parts if any left in the tank.

Nippinder Singh
-New Delhi-India

Dear Khozema...all the above suggestions are very good. We run the pH at 4.2-4.5 and check at least hourly. Makes for smaller additions to adjust the pH. The temperature we found of the nickel in this application was 138-142 degrees F. Decorative chrome should be 108-115 degrees F--I try to keep it 108-110 degrees F.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan

What Mr. Singh has mentioned is a particular proprietery additive to be used. Most suppliers of nickel brightener chemistry have both general use formulations and those specifically for zinc alloys. The brightener chemistry in the zinc alloy formulations generally has lower levelling than the other. However, in the general brightener catagory most all manufacturers have a purifier (or similar product) to help overcome the effects of metallic contamination. This will enable consistant plating and reduce the need for low current electrolysis (dummying). Some formulations actually improve the effect of "

Gene Packman
- Syosset, NY

Dear Khozema

We also experience the same chemical burning on our Mazak 3 die castings we have tried many different ways to plate this very difficult material but however thick we lay down the copper the acid nickel still gets through and attacks the mazak leaving the black marks Jig plating is a solution but the cost is prohibitive compared to barrel plating we are investigating other finishes than nickel but so far have had no success but we will let you know if we find one

We also experience discolouration of the bright zinc , we believe that this is due to contamination of either zinc or magnesium in the nickel bath this can only be removed by electro cleaning the bath which then goes back to a bright nickel finish

If we could find a reliable UK plater we would be over the moon

But we haven't found one yet

We wish you good luck in you solution to the problem

Best Regards

Stuart Sharples
- Corby Northamptonshire U.K.

Thanks everyone for the advice. Mr Stuart, despite all the common difficulties, we still consider ourselves as reliable platers ! As labour is cheaper in India, we are yet Jigging and not Barrel plating. Ball pen tips made of Mazak are also being successfully Barrel plated only with one hour of heavy copper undercoat using a potassium cyanide based copper plating.

regards,

Khozema Vahanwala Saify Ind  Bangalore, Karnataka, India

Plating of Mazak will always end up with zinc and copper impurities in the nickel solution. It is true that there are comercial additives (ussualy named purifiers)that give a temporary relief as they create a complex with the zinc metal in the nickel solution. It turns the black low current density into dull nickel. But remember, these additives are tempopary and help for a day or two only. Dummy plating is necessary every few days when plating Mazak. Try to add hydrogen peroxide prior to the dummy plating. Regarding the chrome solution, well, the copper and zinc can cause throwing power problems but NOT burning at high current densities. If you have burning problems, check the parameters mentioned by Anders like, chrome/sulfate ratio, tempreture, current density, contacts etc.

Sara Michaeli     chemical process supplier Israel

Dear Sara M,

When we dummy the bath we generally reduce the Ph to about 3 before dummying although we have never tried adding Hydrogen Peroxide. How much do you recommend ?

Thanks in advance,

Khozema Vahanwala Saify Ind  Bangalore, Karnataka, India