This blog is for entertainment purposes only, and is not meant to teach you how to build anything. The author is not responsible for any accident, injury, or loss that occurs as a result of reading this blog. Read this blog at your own risk.

Wednesday, January 14, 2015

Anodizing - part 1

Setting up shop

One of the collateral tasks of building any airplane, including a composite one, is making sure that metal parts are protected from corrosion. Although aluminum is not as susceptible to corrosion as chromoly steel is, it will still eventually succumb to this oxidizing plague, especially if the plane is kept in a coastal location.
I have previously used Alodine to protect my aluminum parts, and I have liked the results, but while effective, it has had a few issues that have forced me to search for an alternative.

Chemically etching the main gear fwd attachment point (the gasses were atrocious).

Same part in the Alodining bath. 

Among the things I dislike are its high price, difficulty of acquisition, toxic fumes, dangerous nature of the chemicals, short shelf life, difficulty of disposal, weakness of the alodined layer, and the occasional mixed results.

Anodizing can also be done at home, but has some advantages over Alodining. Chemicals are cheaper, easier to find (battery acid), fumes are slightly less toxic (basically hydrogen and a few other unpleasant gasses), shelf life is longer, acid is very diluted (15% H2SO4) which is more easily disposed of, finish is much tougher, and wide variety of colors to choose from.

A quick look into titanium and aluminum anodizing

Anodized parts in different colors

Finally getting a small return on the cost of College tuition, I was briefed by my daughter (chemical engineer and PHD student/researcher) on the damages sulphuric acid can inflict on bare skin, and how one of her acquaintances had to take a semester off from school waiting for his flesh to grow back, after an accidental splash. 

The main drawback to anodizing is the more complicated setup, and procedure. 

Anodizing requires the parts to be perfectly clean, so a heated degreaser bath at 130℉ (55℃) is used. 

Aluminum degreaser

A heated de-smut bath, at 70℉ to 110℉ (21℃ to 43℃), eats the non aluminum metals off the surface, presenting a purer aluminum exterior to be anodized. 

Aluminum De-oxidizer/De-smut

The anodizing takes place in a bath of 70℉ (21℃) 15% diluted sulfuric acid, using a constant current power supply connected to a lead (Pb) or aluminum plate on one side (cathode), and the parts to be anodized on the other (anode).

Lead plate hanging off titanium wires

3 gallons (12 liters) of battery acid

Anodizing creates a non-conductive porous surface, that readily accept color from a heated dye (140℃ or 60℃). 

Golden orange and electric blue anodizing dyes

The surface can be subsequently sealed using boiling water, or a 202℉ to 210℉ (94℃ to 99℃) sealer solution (for better results), to lock in the color and corrosion protection.

Anodizing sealant 

The results are so dependent on cleanliness of the parts, and adherence to the steps listed in the instructions, that even a  single fingerprint will show up in the finish.

With anodizing the surface of the part actually grows slightly, perhaps by 0.002” (0.05 mm), and hardens, making it more scratch proof, though marginally weaker.

Armed with all these facts, I decided to go for it, and started looking for some sturdy 5 gallons (19 liters) buckets that I could use with heated acids. The ones I settled on are 30% thicker than the usual ones for sale at Home Depot or Lowes (0.090” versus 0.070”).

High Density PolyEthylene is resistant to many different solvents  

HDPE plastic can withstand pretty warm temperatures (120°C/ 248°F for short periods, 110°C /230°F continuously)

I also decided to purchase Gamma Seal water proof spin-on lids in order to safely store the chemicals when not in use.

Gamma Seal lids 

When the buckets arrived, I filled up one ½ gallon at a time, and recorded the water levels on the outside. Afterward, I recorded the distances from the bottom of the bucket to all the water levels.

Master bucket graduated in ½ gallons, and inches from bottom.

This would become my "master bucket", and I used the distances to mark the rest of the buckets.

Marking another bucket with ½ gallon graduations, based on distances from bottom.

I bought three 1500 watts immersion heaters from China that will be used to warm up the different solutions (I wish they had come with matching nuts).

1500W immersion heaters

Multiple PID temperature controllers will regulate the heating elements immersed in the various baths, keeping the solutions at the desired temperatures, or at least that is the idea. 

I ordered this one on eBay as a proof of concept, and will purchase more once I get it wired up, and get it working correctly.

Temperature controller

Because I didn’t want to poke any holes in my expensive colorful waterproof lids, I decided to use the ones that came standard with the buckets as “working lids” in which to drill holes as needed. Besides, I was going to toss them anyway.

Roughing out a 1.6" (4 cm) hole

Heater screwed into the plastic lid

Lid supporting the immersion heater

To prevent touching any of the parts after the cleaning stage, and to avoid getting near the various acids, I made a hanger for the parts out of leftover steel I had laying around from the computer stand project. It looks like a fishing pole for parts, with titanium wires.

I love having a welder standing by. Here I am welding a handle to the parts holder.

The “fishing pole” is not able to get too close to the lead plate, thanks to the two vertical tabs. This way the parts should remain well clear of the cathode, thus reducing  the possibility of short circuits.

This is as far as the part holder is able to go toward the lead plate 

The parts hanging in the empty bucket are scraps I am using for reference, though they will get anodized during initial testing.

Later, I decided to weld another retaining tab, recycling more scrap steel, to keep the parts hanger from being able to slide backward.

Using a welder this way is like working with a glue gun for metal

Multiple screws will allow flexibility in arranging parts

The little tab under the handle hooks the rim of the bucket, and prevents backward movement of the parts holder.

Using this hanger, I should be able to go from bucket to bucket, rinsing in between (in a water bucket), without ever touching, or otherwise disturbing the parts.

At least, that is the main idea. 

I plan on keeping a detail log of all the variables involved with each anodizing session, for the purpose of improving quality.

Anodizing bucket array

It remains to be seen how the implementation will work, but I am hopeful for good results in time, as I experiment, and tweak my anodizing recipe. 

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