A job shop electroplating facility was asked to plate magnesium alloy parts, which they had never done before.
Frank AltmayerThey currently plate onto aluminum and steel and had read numerous articles about zincate dip but could not find a formulation. To answer their question, I needed to go back more than 40 years.
In a paper presented at the 48th Annual Convention of the American Electroplaters Society, H.K. DeLong of the Technical Service and Development Laboratory at Dow Metal Products Co. detailed the preparation of magnesium for plating. The following is a summary of Dr. Delong’s paper:
There are three methods of obtaining electroplated deposits on magnesium alloys:
- Electroplating over an immersion zinc coating.
- Direct electroplating with nickel.
- Plating with electroless nickel.
Of these methods, plating over a zinc immersion pretreatment offers the best corrosion resistance. In any of the methods used for electroplating magnesium, clean parts are essential, and appropriate methods of removing the soils present must be employed.
Electroplating Over an Immersion Zinc Coating
The zinc immersion method of pre-plate treatment consists of the following steps:
- Surface conditioning.
- Zinc immersion.
- Copper strike.
Surface Conditioning: These include the ferric nitrate pickle, which consists of dipping the parts in a solution of chromic acid (24 oz/gal), ferric nitrate (85 oz/gal), and potassium fluoride (15 oz/gal); the phosphoric acid pickle, which consists of dipping the parts in a 90% solution of phosphoric acid (85%); and the acetic nitrate pickle, which consists of pickling the parts in a solution of glacial acetic acid (25.5 fl. oz/gal) and sodium nitrate (6.67 oz/gal). In cases where little dimensional change is permissible, an aqueous solution of chromic acid (24 oz/gal) can be used.
Activation: For activating the magnesium parts before immersion zinc is applied, Mr. DeLong recommended a water solution containing 10% ammonium acid fluoride and 20% phosphoric acid, operated at room temperature. Parts should be immersed for 15 seconds to 2 minutes.
Zinc Immersion: The zinc coating process consists of chemical reduction from a pyrophosphate bath, dissolving surface oxides, and depositing a thin, adherent zinc coating.
Copper Strike: Immediately after rinsing the immersion zinc-coated parts, copper striking for six minutes minimum is required before further plating. The periodic reverse is helpful at this stage because it helps to produce bright, smooth deposits and good adhesion.
Direct Plating Magnesium
Dr. DeLong also described a process for plating nickel over magnesium without a zincate. The magnesium was activated in a chromic-nitric acid dip followed by a rinse, hydrofluoric acid etch, rinse, and immediate transfer to the nickel plating tank.
Dr. Delong indicated that the corrosion resistance of this plated part would be inferior to one treated with a zincate and copper strike.
Plating Magnesium with Electroless Nickel
Dr. DeLong did not detail the procedure used, but we can assume the same preparation method as described for nickel electroplating would work here as well.
By now, you are probably saying, “Here we go again!” Dr. DeLong did not provide a formulation for the zincate used on magnesium. Fortunately, my good friend, Dr. Jack Dini, wrote a paper titled “Plating on Some Difficult to Plate Metals and Alloys” for an AES Symposium held in October 1980. In his paper, he discusses how the use of an acid pickle ahead of zincate improves adhesion, and he provides a zincate formulation. Jack compared the adhesive strength of three preparation cycles:
- Clean, zincate, copper strike, nickel plate.
- Clean, immerse in 100 g/L ethylenediamine tetra/methyl phosphonic acid at pH 6.0–7.3, zincate, copper strike, nickel plate.
- Clean, immerse in 5% nitric acid, zincate, copper strike, and nickel plate.
Processes 2 and 3 yielded similar adhesive strength (20,700 psi vs. 21,000 psi, ring shear), while process 1 was significantly lower (16,400 psi).
The zincate Jack used was:
- 5 oz/gal Zinc Pyrophosphate (Zn2P2O7⋅7H2O) + 27 oz/gal sodium pyrophosphate (Na4P2O7 ⋅ 10H2O) + 2 oz/gal potassium fluoride (KF ⋅ 2H2O)
The immersion time was 2 minutes. The temperature of the zincate was 160°F, and the pH was controlled to 10.0. Jack recommended that the copper strike be 2–5 microns (0.00008”-0.0002”) thick.
Thanks to Dr. Dini and Dr. DeLong, you should be able to develop a process for your own application.
Photo courtesy of Arlington Plating
Frank Altmayer is a Master Surface Finisher and an AESF Fellow who is the technical education director of the AESF Foundation and NASF. He owned Scientific Control Laboratories from 1986 to 2007 and has over 50 years of experience in metal finishing. He was the recipient of the AESF Past Presidents Award, NAMF Award of Special Recognition, AESF Leadership Award, AESF Fellowship Award, Chicago Branch AESF Geldzahler Service Award, and NASF Award of Special Recognition.
