Sealing of Anodized Coatings

Because all of the anodic processes produce porous AI2O3 coatings, it is often desirable to seal the coating to close these pores and to eliminate the path between the aluminum and the environment. Sealing involves immersing the coating in hot water this hydrates the AI2O3 

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Tests for quality of sealing of anodic coatings have become internationally standardised. They include dye spot tests with prior acid treatment of the surface (ISO 2143 1981 and BS 6161 Part 5 1982), measurement of admittance or impedance (ISO 2931 1983 and BS 6161 Part 6 1984), or measurement of weight loss after acid immersion (ISO 3210 1983 and BS 6161 Part 3 1984, and ISO 2932 1981 and BS 6161 Part 4 1981). Of these the chromic-phosphoric acid immersion test (ISO 3210) has become the generally accepted reference test. 

Corrosion Kinetics and Applications of Electrochemistry 141 Sealing of Anodic Coatings... 

The method described by ASTM B 457, Test Method for Measurement of Impedance of Anodic Coatings on Aluminum, is used to study the seal performance of anodized aluminum 141,1421. This method uses a 1-V rms 1-kHz signal source from an impedance bridge to determine the sealed anodized aluminum impedance. The test area is again defined with a portable cell, and a platinum or stainless steel auxiliary electrode is typically used. The sample is immersed in 3.5 % NaCl (see ASTM B 457). 

ASTM B 680 Standard test method for seal quality of anodic coatings on aluminium by acid dissolution Measurement of the thickness of anodic coatings... 

B 4S7 Test Method for Measurement of Impedance of Anodic Coatings on Aluminum B 680 Test Method for Seal... 

As a final step, anodized parts must be sealed to ensure corrosion resistance of the anodic coating. Sealing involves plugging the anodic pores completely so contaminants caimot reach the base metal. A variety of sealing methods are used by anodizers (see Sealants). 

Dichromates and chromic acid are used as sealers or after-dips to improve the corrosion resistance of various coatings on metals. Eor example, phosphate coatings on galvani2ed iron or steel as well as sulfuric acid anodic coatings on aluminum can be sealed by hexavalent chromium baths. 

Electrically produced anodic coatings from chromic, sulfuric, or oxalic acid electrolytes are more dense and less porous. Their corrosion resistance is improved by hot-waler sealing, which is even more effective with the inclusion of dichromale. 

Anodizing is an electrolytic method involving aluminum alloy as the anode, which is converted to an aluminum oxide layer of 5-30 pm thickness. Anodic coatings obtained in sulfuric acid, when properly sealed, are particularly effective against discoloration. Anodic coatings are used in the case of alloys used in architecture and as well as paint bases. 

Corrosion can be brought about as a result of water ingress or the coupling of anodic metals and carbon fiber composites in the absence of a barrier coating [29], If the adhesive layer is improperly coated or sealed, it absorbs moisture and may corrode. The core material in honeycomb sandwich materials is prineipally subject to bondline corrosion [29]. Coupling a cathodic material such as carbon with an anodic metal can cause galv anic corrosion if a protective barrier is not present. 

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