alodine questions

[Chrisfis]

I recently started alodining parts so any advice would be apreciated. My process now is as follows:

1. clean parts with diswasher.

2. acid dip 3-4 min.

3. spray with hot water.

4. soak parts in room temp water 3-4 min

5. dip parts in alodine for 3.5-6 min

6. let parts air dry 30 sec.

7. dip in room temp water for 10-15 sec.

8. air dry


I'm having mixed results some problems i'm having are:

Parts have white streaking from holes.

light coloring.

a few parts don't plate usually in the middle of the rack.

staining around holes and bottom of part. this is better when blotting is done before total drying.


Any help/advice would be great.

Thanks,

Chris

 

[kenvlach]

You are not really developing a process (most Alodines, e.g., 1200, have been around for 50 years), but merely troubleshooting, so this question belongs in the
Paint/coatings engineering Forum
forum332

What Al alloy & what is pre-existing surface condition?
Through or blind holes? Threaded?
Which Alodine solution? pH & concentration, too.
What acid & concentration?

Re: Spraying with hot water and soaking in water 3-4 mins. Why? This just gives the deoxidized surface more time to re-passivate. Better to dip rinse up & down several times quickly, spray rinse out the holes, rinse in DI water, then Alodine.

"white streaking from holes" indicates inadequate draining & rinsing. At each step, lift & tilt the rack with parts to drain holes, then submerge & tilt to fill holes. Especially, do so in the Alodine to get the holes filled with the solution.

"let parts air dry 30 sec." Usually lift from the Alodine, tilt a few directions to drain holes, then rinse w/o drying. Up & down rinsing, tilting at different angles, to drain & rinse holes.

 

[Chrisfis]

Sorry for the lack of info pretty new to this. 6061-t6 and 2024-t3 for mat. mostly thru holes. The alodine is 600 series. The alodine strength is at it's max. I haven't been able to find a ph tester that goes under 4 but it is under 4. What do you meen by DI water?

Thanks,

Chris

 

[kenvlach]

For 2024 & 6061, a good deoxidizer is 20-50 vol% nitric acid, spray rinsed & then dip rinsed in de-ionized (DI) water. Spray rinse holes with presssurized DI water. Immerse in the Alodine ASAP after shedding excess water, but never allow to dry.
Tapwater contaminants can precipitate on the aluminum surface and interfere with the Alodine coating. Any idea of your water's TDS?

Get Technical Process Bulletin for Alodine 600 here:

http://www.aerospace.henkel.com/Images/Datasheet_PDF/hst_alodine600.pdf

pH range should be 1.5 to 2.0; aim for 1.7-1.8.
Too low pH usually gives no coating, too high gives powdery.

 

[Chrisfis]

The water is well water (Hard water). Any idea where to get a ph test to go that low? What is added to incread ph? I recently increased the temp to 70deg for the alodine and parts have come out lighter. Would the temp increase have anything to do with it? Should I be aciding the parts longer I heard of people doing it for 15 min for non 2024 parts? I beleive it's a phosforic acid dip.

Thanks,
Chris

 

[orenda1168]

Chrisfis:

Any pH tester will cover the range of 1 - 14!


Orenda

 

[kenvlach]

As per the Alodine 600 Bulletin, temp. should be 70-90^oF.

The longer you etch (more than needed to remove surface oxide), the worse the results. On a machined surface, just degrease & 30 seconds acid deox. Maybe even skip (unless the aluminum still has heat treat scale) -- the Alodine will just take longer.

Don't like the phosphoric acid (what concentration & temperature??), it is difficult to rinse & cause phosphate contamination of the Alodine. Phosphate on the aluminum surface interferes with Alodining; it also uses up the acidity of the Alodine solution which has to dissolve it. Suggest only using 5% phosphoric & warm water rinsing to help get it off. Nitric acid is far better and is needed to lower the pH of the Alodine.

If you don't want to buy a pH tester (& pH 4 & pH 7 calibration solutions & DI water), there is pH paper available for range 1-2.5 (a bit crude, only indicates pH's of 1, 1.5, 2, 2.5):

http://www.mcmaster.com

P/N 1434T14

 

[Chrisfis]

Thanks,

The acid is 2 to 1 and is hard to get off. The hot stray does help but I was told that the parts should be rinsed in cool water for the best results. The main problem I'm having is parts turn out to light. 2024 doesn't seem to color at all but if you get the part wet it shows up gold then fades back to purple/silver color when dry. From what I've heard in practice most people just add more alodine to solution to get better color.

Chris

 

[kenvlach]

Re "The acid is 2 to 1 and is hard to get off."
Is this 2 parts by volume phosphoric (75 or 85 wwt%?) to 1 part water??? -- more appropriate for rusty steel; it will leave a phosphate film.

Suggest cleaning the 2024 with a ScotchBrite pad & DI or distilled water (grocery stores sell), then Alodine.
Or, as with out-of-the way places on airplanes that can't be dipped, clean the surface with ScotchBrite pad & DI water, then apply the Alodine while rubbing with ScotchBrite pad, leaving the surface wet, dripping on more Alodine as needed, until the color takes.
Of course, Alodine has hex. Cr & acids, so wear appropriate PPE & use plastic dropcloths.

 

[Chrisfis]

Last night I plated some parts and they came out the best I've ever seem. The only changes made were the temp of alodine droped from 70 to 60. I did add enough alodine to bring it up 1 point. I also did a bucket test (with fresh alodine and water) and found some material that will not mix with the rest. It appears to be the color of mustard and very fine grade but will not mix. The alodine I have expires the first of this year could it just be old? (The Acid is 2 parts water 1 part acid. The cold water spray seemed to fix the white streaks.)

Thanks,
Chris