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topic 50211

Sealing of anodised aluminium and testing of sealed layer

October 6, 2008

we are getting the round pipes anodised and sealed and we never had the consistent quality of sealing on the layer, can we have the optimum solution formulation for sealing and also the check the sealing layer quality.

BALDEV singh
process engineer - Gujarat, India

October 9, 2008


Depends on which sealing you require and what the anodising is.

The easiest to do is hot water seal. Use distilled or demineralised water with a conductivity below 150 microsiemens and a pH of about 6 (5.5-6.5 is not uncommon). Seal for 10-30 minutes at 96 °C minimum, preferably boiling.

Dichromate seal can be done in anything from a ppm solution of sodium or potassium dichromate up to a 10% solution. The higher the concentration the greener the parts will look (dependent on the alloy, the type of anodising and the process time). Use a 96 °C minimum solution again and again it is preferred if it is boiling.

To check whether the part is successfully sealed one of the most common tests for hot water sealed parts is the dye test which basically checks whether the pores have been fully hydrated and a full barrier has been produced.
Another test is the salt spray test in accordance with ASTM B117 [link by ed. to spec at TechStreet] . A properly sealed anodised coat should easily withstand a salt spray duration of 336 hours and a dichromate sealed sulphuric acid anodised part should withstand 500 hours without a problem.

Brian Terry
Aerospace - Yeovil, Somerset, UK

October 12, 2008

Dear Mr Brian Terry

Thanks a lot for you answer on the issue , can you please let me know the sealing quality of the clear anodised film by nickel acetate and sodium fluoride solution method at temp 25 °C. Also how the dye is prepared for testing of sealing film.

Baldev Singh
- Vadodara, Gujarat

First of two simultaneous responses -- October 13, 2008

Adding on - the blue dye test which is being cited is fully detailed in ASTM B136-84 [link by ed. to spec at TechStreet] and is termed a "seal integrity" test. It isn't suggested for chromic anodized parts nor those sealed in dichromate. Also, "cold" seals such as sodium fluoride are not fully sealed for 24-hours following sealing. Testing immediately after anodizing will likely indicate a failure; try them the following day. Good luck....

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

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Second of two simultaneous responses -- October 14, 2008

Dear Brian,
My query is on your '96 °C., preferably boiling' recommendation.
I am given to understand that 70-75 °C is the correct temp for Ni-Acetate Sealing (15-20 min immersion). Anything higher and the differential expansion of the Al-Hydrate layer and the base Aluminium kicks in to possibly destroy the hard anodised layer!
I have read about it somewhere on this site and the author is Mr.R.H.Probert "Aluminium How T".
Please clear my doubts.
Ravi Rao

Ravi Rao
- Belgaum, India

October 16, 2008


My comments were specifically aimed at chromic acid or sulphuric acid anodising. The chromic acid anodising has a thickness of about 2-5 microns, the sulphuric about 8-20 microns, dependent on the conditions of anodising.

It is not uncommon to hot water or dichromate seal hard anodising but there is a consequent loss in hardness and an improvement in fatigue performance. In my experience I have yet to see hard anodising lost from a surface due to differential expansion of the oxide to the base. Maybe this is because we are softening the anodising or maybe it is because we are not in the solution long enough to cause such differential expansion to affect the coating, that I don't know and maybe someone can enlighten us on that.


I concur with Milt on the sealing. I have little experience with low temperature seals but my understanding is that you have to let them work their magic over a period of time for them to be truly effective. The sealing test is also described by BS EN 12373-6. I don't know if it is exactly the same as the ASTM, I haven't carried out any gap analysis to find out.

Brian Terry
Aerospace - Yeovil, Somerset, UK

May 3, 2009

Dear Mr. Baldev,
What is the process Type I or II. Sealing solution's pH and bath analysis maintained properly? What is the sealing temperature? The quality of DM Water, Bath analysis,Temperature and pH of Sealing solution will effect the quality of sealing. You can check the sealing test also on components and optimize the process.

T.Chandra Sekhar Reddy
plating shop employee - Mumbai, Maharashtra, India

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