Letter 27052

Black spot problem in Zinc Iron Yellow Barrel Plating [India] 

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We do have 30 % rejection in Zinc Iron Yellow Barrel Plating. Part plated is Wheel Spoke. The reason for rejection is black spots / streaks. Suggest the possible cause to overcome this defect.

GOPINATHAN NARAYANAN
Engineer - HOSUR, TAMIL NADU, INDIA

A frequent cause of black spots is iron contamination (almost any finely divided metal deposit will appear black). We do have an article on-line about that problem, "Improving Your Chloride Zinc Plating Operation", although it applies to chloride zinc plating rather than zinc-iron plating.

A 30 percent rejection rate is a calamity though, Mr. Narayahan. You need to call in your chemical vendor or a consultant immediately, not fool around on the internet. By the time you get a few promising ideas, the substrates will have been ruined by repeated stripping and replating to the point where progress is nearly impossible.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

I'm very inclined to agree with Ted with regards to Iron contamination. This results in a condition I had in the past called barrel "print-out". To remedy this problem I added a dilute solution of Hydrogen Peroxide ( 0.25mls per litre) of 35% aqueous peroxide. This resulted in the solution turning a dark-brown colour due to the formation of ferric hydroxide which was trapped in the filter cartridge and which cleared after time. To perform a test for Iron: Take EXACTLY 100mls of plating solution in a measuring cylinder add EXACTLY 5 DROPS of 30% aqueous peroxide stir with a glass rod and allow to settle for some time in an area free of vibration. Any iron precipitated as ferric hydroxide will settle to the bottom. Good luck!

Nigel D Gill, B.Sc. MIMF AIEMA MRSC
- Glasgow, Scotland

Dear Mr Gopinath,

It's uncommon to use Zinc-Iron with yellow as as the appearance of the finish is extremely unappealing.

The black streaks after chromating are caused by the presence of iron in the deposit. It is extremely difficult to control this without compromising on the integrity of the zinc-iron plating.

I would look at zinc cobalt instead if I were plating a spoke.

Khozema Vahanwala Automaxx   Bangalore, Karnataka, India

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