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Alternative Coating Sealing Process for Passivating Al Mg Substrates

IP.com Disclosure Number: IPCOM000052042D
Original Publication Date: 1981-Apr-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Eldridge, JM: AUTHOR [+3]

Abstract

The stability of thin film magnetic disks in corrosive environments is of considerable interest. Usually, such a disk consists of a multilayer coating on an Al-Mg substrate. The outer layer is a hard protective metal, such as Rh, which is applied over a thin film of magnetic material. Beneath these layers is a several mu m thick Haynes metal layer which is polished to produce a smooth surface and simultaneously provide a barrier layer between the atmosphere and the Al-Mg substrate. In tests, such a structure has not withstood the corrosive ambient. This is believed to be due to the presence of defects in the various layers which permit the ambient to come in direct contact with the substrate. Galvanic corrosion occurs, and the resulting products cause blisters to form.

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Alternative Coating Sealing Process for Passivating Al Mg Substrates

The stability of thin film magnetic disks in corrosive environments is of considerable interest. Usually, such a disk consists of a multilayer coating on an Al-Mg substrate. The outer layer is a hard protective metal, such as Rh, which is applied over a thin film of magnetic material. Beneath these layers is a several mu m thick Haynes metal layer which is polished to produce a smooth surface and simultaneously provide a barrier layer between the atmosphere and the Al- Mg substrate. In tests, such a structure has not withstood the corrosive ambient. This is believed to be due to the presence of defects in the various layers which permit the ambient to come in direct contact with the substrate. Galvanic corrosion occurs, and the resulting products cause blisters to form.

In an earlier article [*], it was shown that corrosion on aluminum alloy disks can be prevented with a multi-step but inexpensive process which introduces a protective layer between the aluminum alloy and the Haynes metal. The first step involves the anodization of a clean, smooth disk in a solution of boric and sulfuric acids. The oxide produced depends on the anodization conditions. Typically, it is a mildly porous layer on the order of 12 to 20 mu m thick with a 1 mu m fluctuation. The residual pores or channels are then sealed using an electrolytic deposition of ionic silicates (water solution of sodium silicate).

The quality of the coating has been assessed using a test described in the above-cited publication in which the rate of field-assisted penetration of Cl through the layer in a 0.1MkCl solution is monitored. This is useful because Cl is the most prevalant catalyst in both aqueous and atmospheric corrosion of Al.

Described here is an alternative process for the passivation of Al-Mg substrates which is as effective as that described in the above-c...