Salt spray test for anodized aluminum

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. Hi cousin Prakkash. In short, your parts or panels failed the salt spray test :-(
The many reasons they may have is the subject of dozens of long highly detailed threads here :-)

The only way to start is either to retain an anodizing consultant or to investigate with full particulars. Are you testing parts or test panels, what spec are you anodizing to and to what thickness, how many hours of salt spray, what is the alloy, what is the cleaning, etching, de-smutting, anodizing, dyeing, and sealing process, etc?

Luck & Regards,

A. Hi Ajeet. A better way to answer your question may be to say that you should be anodizing to some specification, and that specification will tell you how many salt spray hours are required. For example if you are anodizing to MIL-A-8625 it requires 336 hours.

Luck & Regards,

A. Leonardo,

MIL-A-8625 / MIL-PRF-8625 [on DLA] specifications refer to ASTM B117 for the salt spray requirements. The test is specified at 336 hrs with the 3"x10" test specimens. This is a very standard testing application.

Best of Luck.

A. Anodizing and its thickness is only part of the equation. The largest determinant in corrosion resistance of an anodic coating, regardless of electrolyte type, thickness, etc. is the SEAL. While HEXAVALENT CHROMIUM historically has performed best for this purpose, the EU is legislating this practice out of existence! Hot DI or nickel acetate seals can meet the standard requirement of 336 hours of salt spray BUT the quality of the seal, temperature and time are CRITICAL.

A. Hello Sir

It all depends on the cast-alloy you're dealing with. High silicon content will result in thin anodizing layer, and poor coloring.

A. Hi Peter E.
In addition to what Peter M. advised, anodizing probably is more gravel-resistant than powder coating. But anodizing (and most inorganic coatings) will not pass a 1000-hour salt spray test designed for organic coatings. One third of that, 336 hours, is a fairly standard test for anodized coatings on test coupons. This is not a statement that anodizing is less corrosion resistant, but about the fact that salt spray test results are not a prediction of real life beyond their use as an early warning that a process is getting out of control. Good luck!

Regards,

A. "Aluminum How-To" lists the following impurity levels in ppm's: Phosphates 5, Silicates 10, Heavy Metal (Cu,Zn,Fe) 10, sulphates 100. Chlorides 50, Fluorides 5. Look out for galvanic voltage between titanium and stainless steel tank.
Keep pH as high as possible but under 6.0 where the nickel flocks out. Zero on Calcium and Magnesium. Understand that the seal is the weakest solution in your line THEREFORE it is the easiest to contaminate. One ounce of junk in the anodizing tank is 1/22 contamination. One ounce of junk in the seal is 1/1 contamination. Anything that makes the pore large interferes with complete sealing: high anodizing temp, high anodizing acid, low anodizing current density.

A. Cal

1] Too low of nickel (acetate) content
2] Too low of temperature
3] Too short of immersion time

I am sure there are more.

A. Hi Nitin. We appended your inquiry to a thread which notes that 336 hours is a widely accepted standard.

However, industries like architecture often have specific standards. For example, "architectural anodizing" is usually 0.007" thick rather than the lesser thicknesses often used in other applications, and some sort of inorganic dyeing (two-step, integral color, or overdyeing) rather than the common organic dyeing is usually required. Whether architectural standards would apply to RV exterior panels, or whether there are other standards expected in your industry, sorry, I don't know.

Regards,

A. Hi Karthick. To me the abbreviation 'TCP' means Navair's 'Trivalent Chromium Process' as a replacement for MIL-DTL-5541 [on DLA] Chromate Conversion Coating. I don't know quite what TCP means to you ... some proprietary sealing process for anodized aluminum based on trivalent chromate? Sorry, not being familiar with it, I don't know whether you should expect 336 hours of salt spray resistance or not.

I am not highly experienced in salt spray testing, but why are you using 2024-T3 panels if the parts are 6061-T6? It is a surprise to me that 2024-T3 panels can survive 336 hours while 6061-T6 parts last only 48 hours, especially when you anodized the two different alloys simultaneously -- but it does seem like something is defective in your 6061-T6 material. Are you sure the panels stayed with your work pieces through cleaning, etching, de-smutting, anodizing, and sealing?

Regards,