Alodining

Charlie Wagner

Originally published October 1996

Disclaimer: The information presented below is believed to be accurate. However, no warranty of any kind accompanies this information.

What is alodining?

Alodining is the chemical application of a protective chromate conversion coating on aluminum.

Why would anyone want to alodine aluminum?

  1. It provides good corrosion protection. It even protects when scratched. Alodined 2024 aluminum withstands salt spray 150-600 hours before forming white corrosion. Untreated 2024 corrodes in less than 24 hours.
  2. It provides an excellent electrically conductive surface. This helps to provide good electrical bonding in an airframe.
  3. Paint sticks to it extremely well. In some cases, it can substitute for primer.
Advantages of alodining compared to other coatings such as primer or anodizing:
  1. Adds no measurable weight.
  2. Does not alter the dimensions of parts (does not make holes smaller).
  3. Requires essentially no cleanup after application. Encourages treatment of all small parts as they are fabricated and installed.
  4. Requires no electricity or skill to apply. (This is why I use it !)
Disadvantages of alodining:
  1. For the best possible job, requires two large tanks capable of immersing each part. (Anodizing also requires similar large tanks.)
  2. Should not be applied if the tank temperature is below 70°F (21°C).
  3. The alodined surface is not as durable as anodizing or a good paint.
  4. Assembled parts cannot be alodined. (Neither can they be anodized.)
  5. Disposal of spent tank contents can be a problem.

How much alodine solution is required to treat aluminum?

Aircraft Spruce, in their catalog, claims a gallon of alodine solution will treat about 400 square feet of aluminum. The company I bought mine from claimed 80 square feet. I estimate that my RV-6A has between 1600 and 2600 square feet of aluminum surface. (This is the total surface area of all aluminum parts in the whole airplane.) Somewhere between 5 and 30 gallons would be needed.

How large are my tanks?

They measure 5 feet long by 1 foot wide and are filled to a depth of about 2 1/2 feet. This translates to about 94 gallons, or enough to do at least 3 aluminum airplanes like mine. I built the wooden tank structures around custom-made liners of 0.100" black polyethylene. These tanks are big enough to immerse the overwhelming majority of the RV-6A parts, including many skins.

If I were building new tanks today, how large would I make them?

I would make them 5 1/2 feet long by 1 foot wide and about 3 1/2 feet deep. I would fill them to the 3 foot level, or about 125 gallons. They would then accept about half of the parts the current tanks cannot hold. Had I ordered them that size in the first place, the price would probably have been the same.

How much did the tank liners cost, and where did I get them?

The liners cost $250 each plus tax (custom made to my dimensions). I ordered them from a company called Multi Lining and Coating in Bakersfield. Their phone numbers are 800-966-0650 and 805-589-0650. I don't remember the address.

Isn't the large quantity of alodine expensive? Aircraft Spruce gets $17.50 per gallon.

Aircraft Spruce charges a lot for water. I purchased a 10 lb can of Iridite 14-2 powder which, when mixed with water, makes about 128 gallons. This 10 lb can cost $140 in January of 1994. This translates to about $1.09 per gallon. The other tank holds a preconditioning acid solution. Fifteen gallons of the concentrated acid called Isoprep 184, enough for about 100 gallons of solution, cost $112.50. Naturally, tax and freight are extra. I avoided freight charges by picking up the acid and alodine in person at the company plant in Los Angeles. At the time the company was Allied Kelite division of Witco Chemical Corp. Since then, the division was sold to MacDermid, Inc. I have not done business with them. I do not know anything about the current product line, current prices, or even if the Los Angeles plant is still in existence. MacDermid' s phone number is 800-325-4158. The number is probably in Connecticut.

What is the exact process used to alodine a part?

  1. Wipe the part with acetone to get all the ink, glue, and other stuff off.
  2. If the part is bright Alclad rub with very fine Scotchbrite type pads to take the shine off. If the part is not clad (such as an extruded angle). take off the oxide layer with aggressive use of Scotchbrite type products such as polishing wheels or discs. If the part is non-Alclad weathered 2024-T3 with a dark colored surface (yes, Vans included a few parts like that in my kit), much work is needed to clean it down to the bright aluminum. Alclad parts that have been formed into bulkheads and ribs and have a dull finish do not need to be rubbed with Scotchbrite. Freshly machined parts also do not need any Scotchbrite.
  3. Wash in Dawn dishwashing detergent and rinse thoroughly. (Do not attempt to substitute a different detergent. All my attempts at using a substitute have met with dismal failure.) After this washing, the part must support an unbroken film of water. Areas that the water film will not cover indicate grease. The part must be washed again.
  4. Immerse in the acid tank for 3 minutes.
  5. Dip rinse in the acid rinse barrel.
  6. Immerse in the alodine tank for 3 minutes. Move the part around a little every 30 seconds or so.
  7. Dip rinse in the alodine rinse barrel.
  8. To avoid drip marks, blow dry the part. If drip marks do not bother you, you can let the part air dry. Avoid rough handling of the part for 12 to 24 hours. The fresh alodine film is easily damaged, but will harden significantly over time. If you are careful, the part can be riveted in immediately.

This process sounds horrible. Isn't painting a lot easier?

Well, with a few small parts, I can start at step 1 and do everything up to step 9 in 15 to 20 minutes. I could never set up to paint parts and clean up in that time.

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Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 10 July 1997