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When is Hard Anodize really Hard Anodize – Questions to ask your finisher.

Scott Turner — Vice President Business Development
As a 32 year veteran of the anodizing and plating industry. I have been active in trade organizations, and I am a past President of the International Hard Anodizing Association (IHAA). Throughout these 32 years, I have worked closely with our customers, our suppliers, and our facilities to find creative solutions to our customer’s challenging metal finishing projects. Over these years, I have always been surprised by the general lack of understanding and confusion in the marketplace around Hard Anodizing.

The original specifications for Type III Hard Anodizing were established in the UK and US in the 1940’s; primarily for military applications where the hard anodize allowed lighter weight aluminum to be used in aircraft and naval applications to reduce weight, while retaining wear resistance, or corrosion resistance required for the fit and function of the assembly.

In the USA, the Mil A-8625 specification was developed to insure the military received the quality of Hard Anodize which was specified.

Aluminum offers many advantages compared to steel, brass, bronze, copper and other materials due to its high strength to weight ratio, ease of machinability, forming and extruding. Low cost, custom shapes can be produced from aluminum which have excellent strength, and with the addition of anodize films, make the component superior to many other material choices. Both functionally, and cosmetically, anodize creates value for aluminum parts producers.

Military and Industrial Hard Anodize is produced using high current density DC electricity, usually 24 ASF (amps per sq ft) or more, and a refrigerated electrolyte, typically Sulfuric Acid, at 32F. The cold temperature is necessary to provide resistance of the applied Hard Anodize oxide layer, to resist the solvent action of the electrolyte. This low temperature also raises the voltage of the anodizing circuit, and reduces pore size in the porous structure of the Hard Anodize films.

The parameters of this engineered process mean that the anodizing facility uses large amounts of electricity to grow the oxide coatings, and large electric refrigeration plants to cool the electrolyte. This is due to the properties of the anodic coating. As the oxide forms on the aluminum surface, it is non-conductive. The electricity is “forced” thru this nonconductive layer by successively higher voltages in the circuit as the film grows. The aluminum parts in the tank in effect, become a heating element in the hard anodizing tank. At the end of a 50 minute cycle, we apply over 60V DC , at 8000 Amps, to a relatively small surface area about 300 sq ft. The interface temperature between the oxide and the electrolyte can be in the 100’s of degrees, yet the aluminum part remains cool and unaffected.

If anything goes wrong, this high amperage, high voltage circuit can result in damage or burning due to improper bath conditions, tooling choices, or part design. Add to this, the 100’s of variations in alloy (material conductivity) and heat treat (grain structure and precipitates), and the anodizing facility needs to have tight controls over their processes to get repetitive successful results from their coating lines.

Some less than ethical companies have attempted to confuse the market place with claims that they can produce hard oxide coatings, on the same lines which produce architectural or cosmetic anodize parts, using the similar parameters as Type II decorative anodize, i.e. 12-15 ASF, and 60F. Coatings which mimic Hard Anodize can be created in these conditions with the use of expensive chemical additives, but the performance in critical wear applications is never to the same standards as the 32F Hard Anodize.

The capital costs of a Hard Anodize line are much higher than a Type II anodize line. 2S-3X, once the electrical system, refrigeration, and tooling requirements are defined.

There are anodizing companies that purport to supply the same quality. But when subjected to objective wear testing, such as SUGA wear testing, there are no substitutes for traditional 32F Hard Anodize. The “pseudo hard” oxide layers created at 60F-70F show lower (softer) Vickers results, increased Suga wear during testing, and inferior service life cycles. There are a few specific applications, where a mid temperature Hard Anodize has preferable attributes, but it is mainly cosmetic or high voltage resistance applications without wear requirements.

Is Hardness the same as wear? Harness is rarely a surrogate of wear resistance. With Hard Anodize, we talk about comparable wear.
When tool steel is tested with Rockwell C test equipment, and an indentation is made on the steel substrate material, we get a value which is very high (C65-70)

With Hard Anodize, the substrate is aluminum covered with .0017”-.0023” of Hard Anodize Oxide film over the aluminum layer. This is similar to a thin sheet of glass bonded to a thick rubber layer. If you press on the glass hard enough, the glass layer shatters, and the underlying rubber indents easily. This is what happens with a Rockwell C test on hard anodize.

The Anodic Hard Oxide layer, when tested without the aluminum layer in play, will show a Vickers Hardness of 350 – 425, yet it will wear very comparably to Hardened Steel with a Rockwell C 65-70. The Hard Anodize part will test very low on the Rockwell C scale due to the substrate indentation of the aluminum

The Vickers test measures the hardness of the oxide film from a transverse cut thru the oxide layer, and tested from the side, just in the oxide layer. If you want more information on our testing capabilities, contact your Pioneer representative.

The moral to this story, is you get what you pay for. There are no cheap substitutes for a high quality Hard Anodize oxide layer. As a specifier, if you do your work, you will get the performance you expect from your hard anodize products. Don’t let your commercial team make the mistake of choosing on price, without validating performance. You may be comparing Apples and Oranges.

As the Nation’s Largest Metal Finisher, Pioneer has extensive experience in producing Hard Anodize coating solutions. Our customer list includes some of the largest companies in the country. Let us help you in designing and specifying your Hard Anodize project to insure your company, and your customers get the full value from your investment in Hard Anodize coatings.