Anodizing vs. Electroless Nickel (EN) Plating of Aluminum products

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby

(note: this book is two volumes)

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"Electroless Plating"
by Mallory & Hajdu

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A. You are using the hardest alloy of aluminum, including heat treat status, of all of the alloys to anodize or plate.

A forging is better than a cast part to work on, but not as good as a wrought part that is machined.

EN is good, has excellent wear, is highly resistant to some chemicals, looks great, goes on very uniformly and has a fair pit bridging capability.

This should be put on with a thickness of 0.0015 thousandths per side for optimum coverage, wear and chemical resistance.

It fingerprints quite badly, it will not cover all pits, generally costs more than HA and has many formulations of P content that have plusses and minuses. I would certainly specify an alkaline strike EN under the regular EN and over a proper zincate.

Some EN shops are better than others just as some HA shops are better than others. If this is not an "in house" coating, I would start with trying three other HA shops with small orders. Some will process samples to show you how good they are. (free)

A. We have processes to hard anodize or EN plate aluminium. Both processes work fine. We have been hard anodizing for 20 years with no problems. If your pitting is a cleaning issue, you will have pitting with EN also.

One of the many benefits of EN plating is that it does not reduce the fatigue life of the base metal, and repeated processing doesn't destroy the part as opposed to anodizing.

Nickel-boron EN has slightly less wear resistance than hard anodize, and high phos EN has slightly less than that.

To coat 0.0025" of high phos EN would take us about 10 hours.

A. Mr. Plahuta,

I recently published a paper that performs a feasibility study on electroless nickel and anodizing. It may be found in the Sur/Fin 2002 Proceedings for the Light Metals Finishing Committee. This can be found at the American Electroplaters and Surface Finisher's Society website, www.nasf.org

Best of Luck,

A. Hello Jeff. Anodizing of aluminum converts some of the aluminum on the surface of the item into an adherent aluminum oxide film which is hard, abrasion resistant, corrosion resistant, dyeable, and non-conductive. Electroless nickel plating puts a layer of nickel-phosphorous onto the surface which is hard, abrasion resistant, corrosion resistant, shiny & metallic looking, and conductive.

Electroless nickel is generally more resistant to chemical exposure, but we probably can't say too much more without knowing the proposed application. Sort of like asking whether driving or air travel is better without saying whether you want to cross oceans or cross town.

A. Hi, Brett, it still depends on what that "something" is. So let's say that if that something is something that spins around rapidly under water, driven by a motor, and has forces exerted on it like having to transit propulsive forces to a water craft, and occasionally running in mud or sand for a moment it should definitely not be nickel plated or electroless nickel plated because those are barrier layer coatings which, if perforated, cathodically accelerate the corrosion of aluminum.

A. Type III on most alloys is dark grey to charcoal, rather than clear. There is a pseudo hardcoat some people call type II-1/2 that would be dyeable, but I don't think it would exhibit the brilliant colors you might want and I don't happen to know of anyone doing splash anodizing on it.

Perhaps rigid control of type II splash anodizing is the thing you want. For the reasons previously stated I don't think electroless nickel is a good idea.

A. Hands down it's hardcoat with PTFE infusion / sealing depending on what your specific processor is calling it. EN with PTFE is far softer and much more prone to corrosion; the PTFE particle leaves corrosion paths to the basis metal unless a substantial under layer, typically high phosphorous EN of 1 mil or more is put down first. However, hardcoat forms a surface of aluminum oxide which is harder than hard chrome - it's a ceramic rather than a metal. EN w/ PTFE is semi-hard nickel matrix with soft PTFE surrounding it - not very resistant to abrasion either.

A. I agree with Mr. Stevenson but if you really want a very long lasting and trouble-free mold, maybe you should reconsider aluminum as the base material. No matter how good the coating is, it will fail at the weakest point due to the abrasive nature of your aggregates and the extreme pressures exerted, suddenly exposing the substrate to a highly accelerated wear or subsequent damage which may be very expensive to repair.

Thank you Milt and Guillermo for the responses. Guillermo- we chose aluminum for its heat transfer characteristics as well as raw material costs. Because we are not injection molding or using any other process that exhibits high wear on the tool, we felt it was a suitable alternative to other options such as invar. Although, if you have a specific material in mind, I would love to hear it.

A. At 500 °F you will have a thermal expansion problem. For the anodized aluminum the underlying aluminum alloy will expand 5 times the expansion of the anodic film. At 0.002" this could be a problem. Hopefully an EN expert will comment on the expansion of EN over aluminum. Also in the case of heated 7000's alloys, the zinc will migrate and leave tiny pits. In the case of forgings, if the anodic film penetrates the thickness of the work hardening, the resultant stresses will cause the oxide to flake off.

A. Hi Vitaly. Can the bobbin be anodized, perhaps with teflon "infusion". Or perhaps the bobbin just be anodized, and the steel coated with teflon infused electroless nickel?

Regards,

A. Vitaly, for aluminum and steel tool parts here which are frequently assembled and disassembled (alignment/tight fits), we have tried both hard anodize with Teflon and EN with PTFE to AMS2454. Our experience has been that the EN w/PTFE far outlasts the hard anodization.

The EN coating appears to maintain the low friction and low surface energy properties we want even after some wear, and additionally it does not wear as quickly.

The hard anodize with Teflon appears to have a pure Teflon layer on the hard anodize surface which is easily scratched and worn, and then the surface that is exposed is no different in behavior than hard anodize without Teflon.

For parts which must maintain no magnetism, specify a high phosphorous content greater than 11.2%, which also improves wear properties. Plus my opinion is that the EN w/PTFE parts look excellent, a nice low gloss gray.

A. Hi Freddy. People would like to help but it is very difficult with almost no information. We don't even know whether the finish is anodizing or electroless nickel plating; we have no pictures of the parts, and have to hope that your concept of a pit is the same as ours; we don't know what the 'test' is that they are failing (an hour in distilled water or 5 years?), nor whether it's garden variety DI vs. highly corrosive super purity water.

If I must take one single guess it would probably have to be that the parts are electroless nickel plated and it is not thick enough, such that at corners and edges, which are most susceptible to both roughness and thinness, porosity is indeed causing galvanic corrosion of the aluminum because the nickel is much more noble. More facts please.

Regards,