Process for producing oxide coatings on aluminum and aluminum alloys

Claim 1

A PROCESS FOR COATING ALMINUM AND ALUMINUM ALLOY METAL ARTICLES WITH A HARD AND TOUGH COATING OF OXIDE OF ALUMINIUM, WHICH COMPRISES FORMING AN INITIAL OXIDE COATING ON THE ARTICLE BY PASSING AN ELECTRIC CURRENT THROUGH AN ELECTROLYTIC CELL CONTAINING AN ELECTROLYTE MAINTAINED AT A TEMPERATURE OF BETWEEN ABOUT 40 AND ABOUT 65*F., WITH SAID ARTICLE FORMING THE ANODE, SAID ELECTROLYTE COMPRISING A WATER SOLUTION OF AN ELECTRO-ANODIZING ACID, AND THE PROCEDURE FOR FORMING SAID INITIAL OXIDE COATING INCLUDING INCREASING THE VOLTAGE APPLIED ACROSS SAID CELL DURING THE FORMATION OF SAID INITIAL COATING AT A RATE WHICH DOES NOT CAUSE THE CURRENT DENSITY OF SAID CELL TO INCREASE, FORMING AN ADDITIONAL OXIDE COATING BY CONTINUING TO ELECTROLYTICALLY OXIDE SAID ARTICLE IN AN ELECTROLYTE OF THE AFOREMENTIONED COMPOSITION MAINTAINED AT A TEMPERATURE LESS THAN ABOUT 40* F. BUT NOT LESS THAN 0*F., THE PROCEDURE FOR FORMING SAID ADDITIONAL COATING INCLUDING INCREASING THE VOLTAGE APPLIED COATING AT A RATE WHICH RESULTS FORMATION OF SAID ADDITIONAL COATING AT A RATE WHICH RESULTS IN MAINTAINING CURRENT DENSITIES HAVING VALUES NOT SUBSTANTIALLY IN EXCESS OF THAT OBTAINED AT THE END OF THE INITIAL COATING PROCEDURE.