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Improved Corrosion Protection of Anodized Aluminum by Electrolytic Sealing Technique

IP.com Disclosure Number: IPCOM000051447D
Original Publication Date: 1981-Jan-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 43K

Publishing Venue

IBM

Related People

Homola, AM: AUTHOR [+3]

Abstract

An aluminum substrate for a magnetic recording disk is sealed against corrosion by the deposition thereon of a layer of polymerized silica.

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Improved Corrosion Protection of Anodized Aluminum by Electrolytic Sealing Technique

An aluminum substrate for a magnetic recording disk is sealed against corrosion by the deposition thereon of a layer of polymerized silica.

Prevention of galvanic corrosion in aluminum is usually based on total exclusion of the transporting media from the metal surface. This is especially important in thin film disk technology where the manifestations of even extremely small corrosion sites has a detrimental effect on the quality of the disks.

Currently, coatings of aluminum substrates consist of four layers. The outermost layer, which comes into primary contact with the environment, is often a protective layer of hard metal, i.e. rhodium and chromium. Such coating is applied over a thin film layer of magnetic material, which in turn is applied over a relatively thick layer of Haynes metal. To improve adhesion and corrosion resistance, the Haynes metal can be sputtered on a pretreated aluminum substrate. The pretreatment may be of two distinct kinds: chemical conversion coatings and/or anodic oxide coatings. Both coatings have been used as surface pretreatment prior to the application of the above-mentioned layers. Chemica conversion coatings for aluminum are usually phosphate- and/or chromate- based. The aluminum or aluminum alloy to be treated is dipped into an acid solution of soluble phosphates and/or chromate salts. On the other hand, anodic coatings on aluminum are usually applied by anodic polarization in any of a variety of electrolytes. These films may be either non-porous, if formed in electrolytes such as borates or boric acid solutions, or porous, if formed in sulfuric or chromic acid solutions. The former types of electrolytes give limited thickness of less than 1 micron, while the latter provide coatings up to several mils thickness.

In order to obtain a relatively thick layer (20 microns) of oxide with relatively low porosity, a combination of sulfuric acid and boric acid is often used. The aluminum oxide layer formed in the sulfuric/ boric acid combination contains, in fact, a large number of micropores of about two hundred angstroms.

In a corrosive environment, these pores may act as conductive channels connecting two dissimilar metals or alloys. Such a mixture of different metals constitutes a short-circuited electric cell in which the most anodic metal suffers from accelerated corrosion. The prevention of galvanic corrosion of aluminum alloys is of paramount importance in the thin film technology, and it is also of importance in magnetic media composed of particulates.

Depending on the oxide layer structure, one of two sealing procedures can be adopted. The first procedure allows for sealing of micropores 100-300 angstroms in diameter, such as those formed in a sulfuric/ boric acid combination bath.

In more aggressive anodization baths, such as 20% sulfuric...