Assessment of Acid Immersion Tests for the Sealing of Anodized Aluminium

SummaryAnodic films have been formed on aluminium in sulphuric acid, sealed under various conditions and subjected to the acidified sulphite solution (with nitric acid predip) or phosphoric/chromic acid tests. Films produced have been examined by transmission electron microscopy, and gravimetric and admittance measurements have been made. The regions of the sealed film that contribute to the protective value of the coating during the immersion treatments have been identified, and the manner in which this structure is modified by different sealing conditions has been studied. It has been found that testing- solution temperature and pH are important parameters in determining the differential rates of attack of unsealed, partially sealed and well sealed films. Also, the acid anions present in the testing medium are important in determining the manner of attack and the reactions occurring. Thus the nitric acid predip is relatively insensitive to degrees of poor sealing quality. For closer to optimum sealing conditions, the phosphoric/chromic acid test attacks the coating relatively uniformly, while the nitric acid predip undermines and rapidly removes the surface layer of sealing product (smut). The acid sulphite immersion is less sensitive to sealing quality, and precipitation onto the film surface may occur during this treatment. Whereas the suitability of these tests for the assessment of the quality of finishes produced using established anodizing and hydrothermal sealing procedures is accepted, it has been shown that they may not be appropriate to evaluate modified sealing methods. This indicates that if new “energy saving” sealing methods are being contemplated, great care must be exercised in assessing claims not supported by appropriate atmospheric exposure performance data.