Problems with anodizing 356 castings!

[evnetics]

Hey folks. I've been doing lots of searches on here the last few days and have picked up some good info, but it has all been tangential so far.

We made a big bad motor controller that is housed in a CNC milled aluminum enclosure/heatink. The prototypes we milled out of solid billets of 6061-T6 but for production we wanted to have the rough shape cast out of 356 alloy to cut down both on waste and milling time (otherwise we are chucking out 80# of aluminum to make a 12# part).

So far, so good, until we take the parts to get anodized. They leave the shop all gleaming and with the surface roughness you'd expect from a final finishing pass, but are are coming back significantly rougher (120 grit?) and with loads of tiny white spots unless we stick with clear.

My understanding is that the increase in roughness is from using a caustic (alkaline) etch which dissolves aluminum but not silicon, but as for the spots we aren't sure if the problem is that the raised bits of silicon are the problem, or pin hole porosity or what.

From what I have gathered - mainly from searching the forums here - it appears a slightly different anodizing process needs to be used for 356 aluminum. The process I have come up with (and would greatly appreciate help filling in the particulars) is:

1. degrease/clean
<Spray rinse>
2. mild acid etch (instead of the typical lye solution)
<Spray rinse>
3. deox/desmut (with a fluoride salt/strong acid - nitric?)
<Spray rinse>
4. Normal sulfuric acid anodizing (4.5-6ASF)
<Spray rinse/quick dip in baking soda solution/spray rinse>
5. Dye
6. Seal (preferably a low temperature sealer - nickel acetate?)

One anodizing shop said I needed to have the castings "impregnated" which, frankly, sounds like a real nightmare. Another said that bead blasting the exterior would take care of our problems.

Also, I have used the typical lye etching bath to strip anodizing, but that clearly is a no-no with castings. Suggestions on that would be appreciated, too, because a machined enclosure has a lot of "added value" in it at that point and it really sucks to have to chuck into the recycling bin because the anodizing didn't go well.

Thanks!

[spankey666]

i found this for you....
you cannot colour 356 succesfully

356 ALUMINUM
QQ-A-601 Composition 3 as is sometimes called, describes the T6 heat treated version of this popular alloy.


(Primary or Secondary). 356 aluminum is the industry standard and is considered aircraft quality. Alloy 356 is easy to cast and requires only a minimal amount of skill to gate and riser, which means very consistent castings for the customer. Aluminum 356 will age in one year to full properties. If your castings are going to be turned or milled in less than a year, then you will probably want us to heat treat your parts. Heat treating will automatically bring your parts to full properties. The process takes only a short time. It is important to consider the issue of heat treating, because Aluminum 356 is very soft from the outset. For those not familiar with machining aluminum 356 which has not cured the proper amount of time, you can anecdotally compare it to machining a cube of butter. Heat treating, however, adds a small amount of lead-time to your casting job, as we send the parts out to get this process done. Also, as a second consideration with heat treating, there can be a problem with warpage. Once warped, straightening a part can be difficult. If you need the properties of alloy 356 and do not want to deal with the potential warpage from heat treating then consider alloy 713 Alloy 356 will not color anodize,

but this will

[evnetics]

That's part of the problem I'm having - when I search the internet for info on anodizing 356 I get conflicting information, but the majority seem to think that it anodizes "good". Few places even mention the 7xx series, but many say the 5xx aluminum/magnesium alloys anodize the best but don't have such good casting properties and this is apparently a moderately challening part to cast (has lots of wavy fins in it).

I would feel better about insisting on a different casting alloy if I knew exactly why 356 doesn't dye well. In the meantime, I am planning on setting up an ad hoc anodizing line to see if I can't do better than these supposed pros.

[evnetics]

Well, I talked with R.H. Probert today about our situation and he assured me that 356 can be anodized and dyed just fine. He suggested a little bit different process than what I proposed, but agreed that a strong alkali etch was bad news. Unfortunately, the chemicals required are really nasty. Here's the process he suggested for successfully anodizing cast 356 aluminum:

1. Degrease with a mildly alkaline, non-silicated degreaser.
2. spray or dip rinse
3. Deox/Desmut w/ 100% nitric acid + 2lbs/gal. of ammonium bifluoride - keep in this nasty, vile solution until the casting starts to turn frothy white (20-30 seconds)
4. spray rinse, or dip rinse in two separate containers of water quickly
5. Anodize as usual with 15-16% sulfuric acid at 70F and 12ASF
6. spray or dip rinse
7. dip in 5% nitric acid (he acknowledged that the "old timers" used to use a weak solution of sodium bicarbonate - baking soda - though).
8. spray or dip rinse
9. dye
10. seal