Hi Bevel, use a nut and bolt with washers to span over the ends, or if your not bothered about the inside getting anodized, use Dave's idea.

Regards Nick.

I have always used Davids method.

Sure. Personally I have never done just the outside. Either it hasn't mattered or I have wanted everything coated. It's quite doable to seal off the inside, but it's more work and adds challenges like buoyancy.

Bit late for this one but when anidising or plating something like that I use bulldog or market stall clips . Fit the handles inside the pipe and let them expand to grip the pipe . Fasten the power supply to the clip and away you go. Simple and effective.

I would be a little hesitant with putting any foreign metal into the anodizing bath, even if it wasn't connected to the power. This is another place where a 3D printer might come in handy. IIRC ABS can handle weak sulfuric acid fairly well.

I have never had any problems getting a full coat on the inside of the tube, but then again I have never measured the coating thickness. So I wouldn't be surprised if the internal coating ends up thinner. After all the current density will be far less on the inside. At first that doesn't matter much as the coating increases the resistance, distributing the current to any low resistance areas. But after a while the resistance stops increasing, at least in my setup it quickly stabilizes to a constant current at 20-25V.

And while a constant current setup is the best I have gotten very good results by simply using 12V constant voltage.

It actually does both. It's an electrolytic oxidation, forming an porous oxide coating. IIRC the coating thickness for the typical type II anodizing is less than 30um (1mil) where 1/3 to 1/2 will be buildup.

Yes and yes

I calculate based on 6mA per square cm which is near as dammit the same as 6A / square foot. I then use a constant current power supply to maintain this. For a protective coating, I find that 1 hour is enough, or 2 hours if the anodising will by dyed. (I think that this is the same basis as the '720' rule with 13µm or 25µm thickness respectively).

I have a simple spreadsheet that I use to calculate simple parts as a combination of blocks, cylinders and discs, but it the part is drawn in CAD, I can get the surface area reported from it.

Note that I quite often require >20 volts to maintain the required current. (This is with sodium bisulphate rather than sulphuric acid, but I don't think that should make any difference – the chemistry is the same.). A warmer bath needs less volts, but I've had rough looking coatings if the bath warms up too much. I don't bother with agitation, but then I'm not chasing any sort of specification.

I clean the parts using fairy liquid and Scotchbrite under running tap water. I avoid them drying out between final cleaning and anodising by keeping them submerged in tap water. They have a quick dip in dilute sodium hydroxide (2% solution for 1-2 minutes – long enough for them to be covered in fine, white bubbles), then a rinse with DI water from a spray bottle, and into the anodising bath. They get another spray rinse when they come out, and I seal in almost boiling tap water for ~20 minutes.

I have been pleasantly surprised by the predictability and consistency of the results. It's a lovely way to tidy up aluminium parts.