PD ISO/TR 16689:2012

Anodizing of aluminium and its alloys. Experimental research on possible alternative sealing quality test methods to replace the phosphoric acid/chromic acid immersion test. Evaluation of correlations. | Status: Confirmed, Current

1 Scope This Technical Report contains data from an evaluation of candidates to replace the chromic/phosphoric acid solution (CPA) test for the quality of sealing of anodic oxidation coatings on aluminium. Following a review by Qualanod (see Working Group report in Annex A), it was agreed with Sapa Technology that the candidate tests for evaluation would be as follows:

• — acetic acid/sodium acetate solution (AASA) test as described in ISO 2932[10], a method used in the 1970s;
• — sulfuric acid solution (SA) test as described by Manhart and Cochran[9].

The evaluation consists of a comparison of the candidates with the CPA (EN 12373-6[3]), dye absorption (EN 12373-4[12]) and admittance tests (EN 12373-5[13]) using four different sealing methods:

• — hot-water sealing;
• — cold sealing;
• — medium-temperature (midtemp) sealing using a nickel-containing solution;
• — midtemp sealing using a nickel-free solution.

An immersion test based on the CPA test, but without the inclusion of chromic acid, was excluded due to the similarity with the SA test. The scope of the work to develop a new phosphoric acid method was considered too comprehensive for this project. In general, the sealed coating (pores filled by hydration) loses mass and thickness linearly with dissolution time. Different sealing methods (or sealing conditions of time, temperature, pH, composition of sealing solution) result in different pore-filling material with differences in resistance to acid dissolution. When considering replacing the CPA test with an alternative acid dissolution test, there are some criteria for a new test. If possible, the response to the test should be similar for different sealing methods, i.e. it should be possible to use the same standard even if the sealing method is different. There should be a significant difference in the mass loss for a good and a bad sealing.