Surface treatment of magnesium and its alloys

Abstract

Methods of polishing and / or brightening surfaces of magnesium or magnesium alloy are disclosed. Polishing and / or brightening methods suitable to both high and low aluminium content magnesium articles are disclosed. In each of the methods, the surface is polished and then passivated. Using the disclosed methods it is possible to control aspects of the appearance of the surface to thereby, for example, obtain a bright and shiny surface to a magnesium or magnesium alloy article which is both stable and corrosion resistant. It is also possible to provide a predetermined texture to the surface using optional steps.

Description

TECHNICAL FIELD [0001] The present invention relates to surface treatment of magnesium and its alloys and in particular, though not solely, this invention relates to providing a bright shiny and / or textured surface finish to magnesium or magnesium alloy. BACKGROUND ART [0002] Magnesium metal, either pure or in an alloy form, is a highly reactive metal. In its usual fabricated forms the surface appearance is relatively dull and obscured by surface oxidation products or other compounds. Accordingly, magnesium metal does not usually manifest a bright, shiny, metallic finish except when the article has been machined. Although a dull surface may not always be disadvantageous, it is often desirable to provide a bright or polished surface to magnesium for reasons of aesthetics or utility. Owing to the reactivity of magnesium, post-treatments are generally necessary to passivate the surface, and conventionally the desired surface finish is lost during this step. [0003] Conventional treatmen...

AI Technical Summary

Benefits of technology

[0022] Preferably, said chemical polish and / or electrochemical polish removes surface layers and / or reduces microscopic high points from the surface.

[0036] Preferably, said inorganic material coating or sealer is substantially transparent and / or substantially provides corrosion protection and / or at least some protection from mechanically induced damage.

Problems solved by technology

In its usual fabricated forms the surface appearance is relatively dull and obscured by surface oxidation products or other compounds.

Owing to the reactivity of magnesium, post-treatments are generally necessary to passivate the surface, and conventionally the desired surface finish is lost during this step.

Mostly these processes neither brighten nor polish the metal.

While in many applications this may be a desirable finish in itself, this is not regarded as surface polishing or brightening.

However, these techniques are not applicable to magnesium alloys due to their different chemistry.

Although, many brightening methods have been known for a long time, little work has been made to accommodate magnesium and its alloys.

Two other problems exist that make the processing of magnesium and magnesium alloys difficult.

Firstly, many commercial alloy articles (especially die cast articles) exhibit significant amounts of segregation both of the surface and in the interior of the article.

This together with commonly encountered casting defects produces a highly inhomogeneous surface which cannot be improved by conventional means.

Secondly, as magnesium is a highly reactive metal that forms a non-continuous oxide layer, for a viable bright surface finish to be attained the surface must be treated in a manner to prevent the oxide layer forming after surface brightening or polishing treatments.

Although the resulting plated article has a shiny, metallic appearance, this is due to the plated metal and could not be regarded as a system for brightening or polishing magnesium or magnesium alloy articles.

There are also profound disadvantages in plating other metals onto magnesium or magnesium alloy substrates including for example galvanic corrosion problems in corrosive environments, expense, processing difficulties in a plating process and high reject rates from most processes.

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