Overview
ISO/TR 16689:2012 evaluates candidate methods to replace the traditional phosphoric acid / chromic acid (CPA) immersion test used to assess the sealing quality of anodic oxidation coatings on aluminium and its alloys. The Technical Report compiles experimental data (Sapa Technology) and literature review of acid-dissolution and electrical/dye-based methods to determine which tests correlate with CPA across different sealing processes.
Key topics
- Scope and purpose
- Comparison of candidate immersion tests with the CPA reference test for sealing quality.
- Focus on immersion-type tests that behave similarly to CPA and practical production use.
- Test methods evaluated
- CPA (chromic/phosphoric acid solution) - established referee test (EN 12373‑6); solution typically 2% chromic acid + 5% phosphoric acid at 37.8 °C for 15 min; surface-specific mass-loss measurement.
- AASA (acetic acid / sodium acetate) - historic ISO 2932 method used in the 1970s; boiling immersion for ~15 min.
- Sulfuric acid (SA) immersion test - method described by Manhart and Cochran; different temperature/time conditions.
- Kape (acidified sulfite) test and other literature tests are discussed.
- Non-immersion methods: dye absorption and admittance tests (EN 12373‑4 and EN 12373‑5) were included in comparisons.
- Sealing processes assessed
- Hot-water sealing
- Cold/double-step sealing
- Medium-temperature (mid‑temp) sealing with nickel-containing and nickel-free chemistries
- Main findings
- The evaluated acid-immersion candidates did not consistently correlate with CPA across different sealing methods.
- The admittance test distinguished sealing quality for all methods but measures a bulk coating property, whereas CPA is surface‑specific.
- Environmental/regulatory concerns surround CPA because it contains hexavalent chromium (Cr(VI)), recognized as hazardous.
Applications and users
ISO/TR 16689:2012 is useful for:
- Anodizing plants and process engineers evaluating or validating sealing processes
- Quality control and testing laboratories that perform sealing‑quality assessments
- OEMs, fabricators and specifiers (architects, facade engineers) who rely on sealing acceptance criteria
- Standards committees and regulatory bodies seeking alternatives to Cr(VI)-containing tests
- R&D teams developing new non‑chromium sealing chemistries or test methods
Practical value: the report helps users understand the limitations of alternative immersion tests, the comparative behavior of different sealing methods, and the need for a test that predicts superficial cosmetic degradation (staining, blooming, iridescence) rather than only bulk protection.
Related standards
- EN 12373‑6 (CPA mass-loss test)
- EN 12373‑4 (dye absorption)
- EN 12373‑5 (admittance)
- ISO 3210, ISO 2932 (historical references)
- ISO 7599, Qualanod and ASTM references discussed in the report
Keywords: anodizing of aluminium, sealing quality, CPA test, chromic acid, phosphoric acid, AASA test, sulfuric acid test, admittance test, ISO/TR 16689:2012, anodic oxidation coatings, alternative sealing tests.