Hard Anodizing

[jamerson]

Just wondering if it wood be possible to achieve a hard coat thickness.
would it work if i chilled the bath to 32 degrees at 25 amps per square foot for 2 hours or so.
im not sure just a expirement.

[sswee]

I'm only familiar with it by what I've read. Acid concentration is suppose to be different and the temp is suppose to be maintained pretty close.
SS

[neilfj]

Sswee...hope you don't mind the hit-and-run...

Jamerson..we are going to have to get into some definitions;
First, Hard Coat, Hard-Cote, Mil-Spec are NOT necessarily Type-III anodizing.

Hard Coat (Hard-Cote) are marketing terms that mean absolutely nothing, and have no specifications attached to them. Anyone using these terms to sell something is either purposely trying to confuse the buyer, lying or just doesn't understand what they are selling.

Mil-Spec anodizing has a variety of meanings, and unless the exact specification, paragraph and subsection are quoted, it has no meaning either. The Mil-Spec for anodizing includes detailed specifications for a variety of Type-I, Type-II, Type-III processes, as well as a number of other factors such as chemical composition of the electrolyte. So, saying Mil-Spec doesn't really say anything other than the coating applies to one of the many specifications listed in the manual.

That being said, I'm going to try to answer your question..I'm going to assume you are asking about Type-III anodizing as commonly understood.

Obtaining Type-III thickness is the easy part. Type-III thickness is defined as any thickness greater than 1mil (25.4 microns). To achieve this using the LCD process, just use the 720 rule and anodize your parts for the amount of time required to obtain a thickness of over 1 mil. Most places that improperly say that their product is Hard Coat or Mil-Spec anodized only anodize only anodize to a thickness of 26 - 30 microns (1.01 - 1.2 mils). This is not true Type-III, but is infact just a thick Type-II coating (Type-II.5)

To get true Type-III anodizing, besides the thickness requirement previously mentioned, it also has to contain a minimum cell density. The anodizing cells are much smaller in Type-III when compared to the Type-II cells. This means that there are more cells per sq/cm in Type-III. Since the cells are smaller, more can fit within a given area.

An easy way to tell, which is almost always a good rule of thumb, is that if the anodizing has been dyed, it is Type-II. Type-III is difficult to dye, as the cells are smaller and the dye doesn't penetrate these smaller cells. If the color is gray, ranging from med to dark gray, it is probably a real Type-III coating. Type-III anodizing has a greyish color to it. The thicker the coating, the darker the gray is, until it reaches almost black. This is why they call it Hard Coat or Mil-Spec, rather than using the more precise Type-III classification. If you've ever seen Calphalon Hard Anodized cookware, you'll see that it is a dark grayish-black color..true Type-III andozing. If the color is anything other than gray or grayish black, it isn't Type-III.

Manufacturers know that most people can't tell the difference. The only way to determine if a coating is truely Type-III is to be aware of the color limitations of Type-III or to chemically dissolve the aluminum, leaving only the aluminum oxide, which is then weighed to determine the density. A rather expensive, time consuming and destructive process.

Now, if you expect to dye the anodizing, stick with the Type-II coating, but grow it as thick as you want. I've been able to grow anodized layers over 3 mils thick (77 microns). The only downside using the LCD process is that the time required. Fibergeek was also able to obtain thicknesses of 70-72 microns using LCD. If you were to ask me what the maximum thickness you could grow, I'd recommend you stay in the 2.0/2.5 mil range. If you are feeling brave, you could go thicker, but there is a disadvantage. As the coating gets thicker, it is more likely to be susceptible to cell damage. As the cells get longer, impacts to the surface can cause the cells to snap, producing a white residue. The thicker the coating, the easier it is to damage the cells. This is especially true on part edges. Since anodizing only grows at 90 degree angles to the aluminum surface, it is easy to strike the edge and break off the anodized cells.

If you absolutely require Type-III, (remember, you will have to live with the gray-blackish color as you probably won't be able to successfully dye it), it can be done.

Change the acid concentration to 2:1 (2 parts water + 1 part battery acid). This is the easy part. Now you have to control the temperature of your electrolyte. Ideally, Type-III requires that the temperature of the electrolyte be maintained between 37-40F for the entire time you are anodizing. You also have to make sure that your electrolyte agitation is aggressive and keeps the fluid moving. Anodizing tends to cause thin boundary layer of heat to build up between the surface of the aluminum and the electrolyte. This boundary layer if not removed by having the electrolyte constantly moving, will actually act as an insulator prohibiting the cold electrolyte from coming in contact with the aluminum surface and you'll end up with a Type-II coating even though your electrolyte temp was in the proper range. I hope that makes sense!

You can anodize at higher currents, probably 18-24 amps/sqft, but be aware that the higher the current you use, the more issues you will have with controlling the total heat produced and the heat boundry layer.


SSWELL; I probably went into more detail than Jamerson wanted, but I figured you would want a more detailed explanation.

Neil

[sswee]

neilfj,
I don't mind at all. Appreciate the info and glad to see you still pop in on occasion.
SS

[jamerson]

Not at all that was perfect !

[RobertHoffman]

Oxalic acid is often used for type III anodizing. I tried it at home once as an experiment and got a nice solid gray/green finish like I usually get from commercial hardcoat treatments. Have no idea what the coating thickness was, but it looked cosmetically good. I didn't try to dye it. Oxalic acid baths do not need refrigeration. Oxalic acid can be bought in the paint department at most home centers as "wood bleach". A white powder. More expensive than sulfuric acid, but not unreasonable. Needs a higher voltage than sulfuric anodizing. I did somewhere around 5% by weight. Don't know what concentration the commercial finishers use.

[RayJ]

I'm new to annodizing. I need to Hard coat(type 3) tuned aluminum exhaust pipes for my rc cars. The pipes are typically 8 inches long, 1.5 inches in diameter. The pipes are approx. .050" thick of work hardened 1100 aluminum.

I have a 30amp 12 volt continuous unregulated power supply. Can someone recommend a rectifier? Or do I need one?

I'm very curious about Oxalic Acid hard annodizing. Some of the articles I've read by the Japanese use a 3% Oxalic acid concentration(no sulfuric acid). They are using 40volts, and 15-20amps per square foot for 40 to 60 mins. They are also able to hard annodize at 70 to 90 degrees F with agitation.

I'm tempted to go this route since it would elimante the need of an expensive cooling system. Any suggestions would be greatly appreciated.

[destroyer125]

I have never heard of doing a type 3 anodizing the way you discribed

[sswee]

Just for my own curiosity, why do you need the hard anodize? Type II anodize grown to the thicker levels is difficult to tell from Type III.

It is preferred and will eliminate many problems if you have a PS that is CC capable.

I've read some on the oxalic acid anodizing but don't know enough to recommend anything on it. Something to consider. Was anything said about the size of tank using that method? The agitation will have to be super aggressive and even with the agitation at an optimum level, the tank size will have to be on the large size to dissipate the heat generated from running that much current through the circuit. From experience with a 20 gallon tank, pushing 7 to 10 amps through the circuit was causing heat problems even with aggressive agitation and cold ambient cooling. One pipe with 73.341" SA will have a set amperage of 7.64A for 15 amps per square foot and a set amperage of 10.19A for 20 amps per square foot. That's only one pipe. I'm not trying to dissuade you on anything but maybe bring to light something not considered.

SS

[RayJ]

Thank-you for the input. The tanks they used and cooling methods wern't described. Perhaps it is much more involved than I thought through there simplistic descriptions.

I prefer Type III because of cell size. The larger celled Type 11 I thought would be more difficult to grow, and not as tough as the Type III. I am new to this and perhaps the equipment I will need to invest in would go beyond me trying to hard anodize on my own.