Anodic coatings seals

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. 

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. 

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. 

Galvanizing helps prevent corrosion in two ways. 0 The zinc coating seals the iron from air and water by forming a barrier of zinc oxide that repels water and oxygen. 0 If the zinc coating breaks, the zinc acts as a sacrificial anode. 0 Metal objects that are left outside are often galvanized to prevent rust and corrosion caused by the elements. 

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). 

Anodized coatings are produced by an electrolyte process. The surface of the aluminum is immersed in an acidic bath and made the anode, and a direct current is applied. Oxidation of the surface produces a hard porous film of aluminum oxide that is bound as tenaciously to the alloy as the natural oxide film but is much thicker. Most anodized coatings are produced using either sulfuric acid, chromic acid, or phosphoric acid baths. Other solutions can also be used (e.g., boric acid, oxalic acid). The porous oxide film is sealed by immersion in boiling water, which imparts impermeability to the film. The porous film may be dyed to impart color before sealing. 

Fig. 11. After plasma etching for 200 RF hours. Boron carbide coated anodized aluminum sealed with HL126 sealant provides the best corrosion in all configuration.

Anodized coatings are typically 2 to 25 pm thick, and consist of a thin nonporous barrier layer next to the metal with a porous outer layer that can be sealed by hydrothermal treatment in steam or hot water for several minutes. The resultant oxide is also nonconductive. This particular property of the anodic oxide is useful in the production of electrolytic capacitors using a special bath of boric and/or tartaric acids. 

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

A number of trade names have been associated with anodized coatings. Often dyes are applied on anodized coatings before the sealing process. The drying materials are generally hydroxide of copper, cobalt and aluminum. 

An anodized Mg alloy is much more corrosion resistant than an nn-anodized one. A commercially anodized Mg alloy without any post-treatments or sealing can survive in standard salt spray (ASTM B117) for about 400-600 hours. Figure 16.11 presents the corrosion performance of a recently developed anodized coating (Song, 2004), which shows that the anodized AZ91D survives in 5 wt% NaCl environments for one month and no obvious corrosion damage can be visualized on the surfaces. 

In principle, sealing is a process for species from a sealing solution to deposit on the anodized coating, particularly in the defective sites (such... 

H2SO4 anodic coatings 23 xm (0.9 mil) thick, sealed in boiling water on test panels 100 X 150 mm (4X6 in.) cut from sheet and extrusions ... 

---