grit to use before alodine?

[coppercone2]

I alodined some aluminum braces I made, and the coating has a different darkness in some places.. there are a few spots on the metal that are coated ,but with a very thin film, wheras most of the metal is a more bronze color

I sanded it down to 1000 grit, used ultrasonic with simple green for ~20 minutes, rinsed well, used alumiprep 33 for 5 minutes per side (using small amount in a tupperware while also swapping it with a soft towel), then rinsed again in hot water and did the same process with alodine 1201.

Am I over sanding the piece, maybe some spot got burnished very flat ? Maybe its because I dry sanded, instead of wet sanded (prefer to avoid wet sanding)

I don't think I will bother doing a microscope inspection but it looks 100% coated,but with different film thicknesses .The bore holes coated fine.

Just a transformer brace I wanted to pretty up/experiment on.. probobly will not paint this

When I did my rinses, I used carbon filtered water. But I am thinking its not a issue with dirt.

[ETITsynthesizer]

are you %100 sure it is the same material with a different surface texture? if that is the case then you just need to sand it all down with the same grit everywhere. the finest grit will be a polish. when everything is polished it should look identical. the polish is finer than the wavelength of light. if you see that you have sanded through the plating you can either stop or sand it all off to have matching brass all over.

[coppercone2]

it just looked like a few spots were really thinly coated, so basically the thickness of the gel film on the part was thin in comparison to the rest of the bracket

Does it coat less thin when its really well polished? before I did the alodine process, it looked pretty uniform

Maybe I will try again with a more detailed inspection. Can you 'expose' something in aluminum, like a patch of different grain structure or whatever? It's one of those flat bars from home depot. I thought it was like maybe 'nodules' of something that does not attract it so well. Like maybe it was not mixed that well when it was molten?

[duak]

At one place I worked, we had a local plating shop handle our alodining and anodizing as we had such small volumes.    Occasionally, something wouldn't go right and alodined parts would come back looking as you described.  Talking to the plating shop's manager, it came down to cleaning and chemical surface prep.  Sometimes one of the baths would be contaminated and they'd have to re-do the part.

Have you seen this?: https://www.finishing.com/65/59.shtml

[coppercone2]

Hmm, perhaps I should have used isopropyl alcohol to degrease it after the long ultrasonic simple green bath. Maybe I contaminated it with whatever is in simple green despite rinsing.

I think I cleaned it with alcohol first, then sanded it, then used the simple green ultrasonic, then rinsed, then used alumiprep, rinsed (pain in the ass at this point), then used the chromate. I was not sure if I should wipe it down again after it was in the ultrasonic for so long. Maybe just spritz it?

I try to minimize use of the aluminum cleaner, I basically put the minimum amount in a shallow narrow container that closely conformed around the part and rocked it back and forth on each side for a while, making sure it was coated.

Maybe it works better if you have a gigantic pool of the stuff, but I have seen decent results on youtube with people literally just spraying airplanes with it to coat (god help the hangar drain). The alumiprep should do a good job, it has a trace of HF in it along with phosphoric acid. I thought simple green was OK because it had some kind of citrate in it, and the alumiprep is basically HF laiden soap as far as I can tell, so it should have cleaned that off fine. ?

I did NOT however scower it when I was coating it. I have seen some people say you need to rub it on, I just swished it. Same method in a similar container.

That link sure likes talking about scotch brite, I will try it, however I am kind of disappointed with the durability of it.

For those interested in WHY alodine is used instead of just anodizing everything like a rabid weasel ( I kind of wondered why the hell they do it in the first place, it can't be THAT cheap in the air): basically it seems that if you anodize an aluminum part that has been specified in an application where it has a finite life due to fatigue failure, anodizing messes with this and causes weird cracks to develop and causes bad failures, which is why they cannot seem to anodize certain parts of airplanes (but I think its a mix, the spec comes down to FEA). I don't know if they are early failures or hidden failures (technicians can actually look at the structure of a plane and determine if its good to fly based on analyzing the cracks, just because there is a crack does not necessarily mean its not air worthy, to my surprise, so anodizing can cause it to look good despite being shit potentially I think).

It's very interesting if you are in the habit of making brackets using aluminum, hammer and bolt cutters. They sometimes crack when you bend them, but they are usually still pretty good because chances are its super over built for small uses)


For those interested, the plating was about 98% uniform if you go by surface area plated vs the defect rate, which is pretty good, but it stands out. In the wrong light those areas look unplated but you can see the film if you do a good job with lighting.

[duak]

Well, alcohols aren't particularly good degreasers - Acetone is better and stuff like paint thinner (mostly hexane) better yet but only go so far.  Oils form monolayers on most surfaces and are quite tenacious.  Hydrocarbon based solvents dissolve and carry away most of the oil but there is usually a little bit left that is concentrated as the solvent evaporates and leaves an oil monolayer.  Alkali based solutions like sodium hydroxide (lye) do a good job but also attack the aluminum.  Acidic solutions can also do a good job but if they are oil based may leave something behind.  I like Simple Green but I don't know if it leaves a residue.  I think it might because it doesn't work as well as Windex for cleaning eyeglasses.  BTW, methanol and, I believe, other alcohols can attack aluminum.

When I was with the then small company mentioned above I was taught to clean a metal surface with a powdered cleanser like Comet and a brush.  When the oil monolayer was gone, the rinse water spread out into a thin sheet.  If there was any oil present the water would coalesce into streams or droplets.  For something like a heat sink that was going to have something epoxy bonded to it, this isn't too difficult to do.  For larger stuff, sodium carbonate (washing powder) in hot water works well enough.  Note that Na2CO3 is mildly basic and can discolor aluminum if left in too long.  If you have a dishwasher you've probably noticed how dry or degreased (squeaky clean) the objects feel vs those handwashed with a liquid detergent.  Tri-sodium phosphate is also a good degreaser but I haven't used it on aluminum for critical cleaning.

I don't spritz anything off without being followed by a water rinse, a shake and a blast of clean, oil-free compressed air.  If I can, I air dry it with warm air from a heater or heat gun.  If I'm cleaning my eyeglasses I just use the warm exhaust air from a desktop computer.  My fingers are a good source of oils so I make sure my hands are clean and I don't let any water off my hands get on the item.  That being said, I don't think the alodine process is that sensitive to oil monolayers (unless they are silicone) and the prep should deal with them.

Hope this gives you some ideas,

Cheers,

[ETITsynthesizer]

regarding contamination in the plating process, yes, I did this for work. it is a big problem. when silicone gets into a plating tank it permanently contaminates the polypropylene tank. imagine the cost of such a mistake. the plated parts in the semiconductor industry are worth pennies. they make the money back on the machines after 5 years. replacing a part of a machine because of contamination sets off your ROI accounting by an entire month. the same as if you just took a month off work and didn't contaminate the tank.

[coppercone2]

how do they detect that?

Does it diffuse btw, like I thought to make small custom sized plating tanks for parts, like to make a series of them, because I hate working in tupperware shapes. The easy way to do it is to use silicone sealant. So long it cures properly (say 2 weeks), is it some kind of diffusion threat? Nothing too big, but Just.. heavy built good quality containers. For my PCB exposure trays, I made them with epoxy and then used flowable silicone (car window) sealant for the seam to fill any gaps and leave a bead on the corner (I angle the tank in a V groove, pour it in with a syringe, let it cure, then rotate the tank to get all the sides. I wait till it stops flowing before I rotate the tank). But thats just a narrow tray to put exposure chemicals in (I like them because I make them heavy and solid, so they clean well, and I don't risk bumping it gently to spill chemicals, since I make them really heavy). 

BTW I am not the type of person to leave things in a tank, I usually drain them. Unless its the 3d printer because that shit sucks to work with completely (alien goo).

[ETITsynthesizer]

it all depends on your process. if you have defects, I would trace that back to the silicone as most likely. here is a video to give you an idea of how plating tanks are made.

[coppercone2]

that song rocks