Zincating of aluminum: single or double and why?

A. Hi Arvinda. There are proprietary alternatives to zincate, like Bondal (a nickel-zinc immersion process) and Alstan (a stannate immersion process followed by tin plating). I have heard claims that electroless nickel has sometimes been successfully done directly on aluminum. But the basic issue is that aluminum instantly forms an oxide surface, so a replacement process rather than direct electroplating is required.

Luck & Regards,

A. Dear Tom ,

I am generally not a fan of double zincating , having said that I know of shops where it was the answer to ongoing problems , However , in my opinion the best zincate is one that gives an even dark grey color without any splotches or light colored patches , to achieve this I would recommend a dip time of approximately 2 minutes , agitation of the zincate ( not violent but enough to keep it moving ) and most importantly frequent analysis to establish a drag - out figure and hence a solution maintenance schedule .

What you must remember is that the Zinc dissolves in the NiP very quickly and any patches that bare up before plating has initiated will give rise to bad adhesion problems , remember also the rate of dissolution of the Zinc film increases rapidly as the temperature increases .

Have you considered an Electrolytic Strike of say Watts Nickel between Zincate & NiP coating ?

Regards

A. John is right, the zincate layer dissolves in the NiP solution, that's why you should use a special NiP with high PH (8-9) that are specially formulated for EN plating on aluminum. You can use a high PH En solution to plate a flash EN layer and then enter into a regular low PH (4.6-5) for the heavy layer plating.

Beside, I hope you use a cleaner and deoxidizing agent prior to the double zincate.

A. If you have no answer or want to be absolutely sure you can buy the complete AMS2404 text (Aerospace Materials Specification) and check it yourself. It'll cost you less than US$60.

A. With respect to MIL-C-26074E, the direct answer is no. However, to meet the adhesion or salt spray requirements, your EN process may require the pretreatments mentioned for certain substrates. There is no 'Class B;' Classes 1-4 refer to heat treatments, and Grades A, B and C refer to minimum EN thicknesses. Caveat: haven't seen MIL-DTL-26074F yet, a Controlled Distribution Document.

FED-QQN-290 (replaced by AMSQQN290) (Electrodeposited Nickel) does have specific requirements for copper underplating on steel, zinc and high-Zn brasses. This spec. does not mention plating on aluminum.

A. My belief is that "from experience" this gave results with fewer rejects, therefore became practice.

A. Hi Surendra. The short answer is, as Geoff says, because it works better, at least sometimes :-)

This isn't seriously debated anymore (for certain types of aluminum) as it has been repeatedly demonstrated by testing over the years.

The longer answer, which is a bit above my pay grade, is that the zinc reacting with the aluminum, followed by the nitric acid reacting with the zinc deposit, followed by the zinc reacting with this "cleaner" layer of zinc produces greater adhesion. If you do a computerized literature search you will find numerous studies of this phenomena. The MFIS CD-ROM (available from Metal Finishing Information Services / Finishing Publications Ltd., Stevenage, Herts, England) for example lists about a dozen such published articles.

One of the more exhaustive references is David Lashmore's "Immersion Deposit Pretreatments for Electroplating on Aluminium - Literature Review" in Plating & Surface Finishing, April 1978, which references 40 earlier articles. Good luck.

Regards,