Surface treatment method for aluminum or an aluminum alloy and treating fluid used therefor

#This invention relates to a method for the surface treatment of aluminum or an aluminum alloy. More particularly, it relates to a method for the surface treatment of aluminum or an aluminum alloy wherein a film is formed thereon by a chemical reaction, without supplying energy (e.g., electricity) externally.

#The conventionally employed alumite process is a method for forming a hard film of aluminum oxide on the surface of aluminum by oxidizing aluminum anodically in an acidic bath. However, this method has the disadvantage of involving a high cost because it requires equipment for electric supply and the rate of film formation is slow.

#Now, there has been developed a technique for forming a film on the surface of aluminum by heating an aqueous solution containing magnesium silicofluoride and ammonium silicofluoride to a temperature of 70 to 100° C. and soaking aluminum in this aqueous solution (see Japanese Patent Provisional Publication No. 11-193478/'99). This technique makes it possible to simplify the equipment and reduce treating costs and also to form a film having excellent sliding properties on the surface of aluminum.

#Embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings in which:

#FIG. 1 is a graph showing the relationship between the pH of the treating fluid and the treated area of aluminum in one embodiment of the surface treatment method of the present invention;

#FIG. 2 is a graph showing the relationship between the thickness of the film and the treated area of aluminum in one embodiment of the surface treatment method of the present invention;

#The treating fluid used in the present invention contains ammonium silicofluoride [(NH4)2SiF6] and another aluminum-free fluorine compound. The other aluminum-free fluorine compound may be any fluorine-containing compound, except ammonium silicofluoride and aluminum compounds. Specific examples thereof include silicofluorides such as magnesium silicofluoride (MgSiF6·6H2O), zinc silicofluoride (ZnSiF6·6H2O), potassium silicofluoride (K2SiF6), sodium silicofluoride (Na2SiF6) and manganese silicofluoride (MnSiF6·6H2O); borofluorides; zirconium fluorides; and titanium fluorides. Among these fluorine compounds, silicofluorides are preferably used and, in particular, magnesium silicofluoride and the like are preferably used. The use of this treating fluid makes it possible to form a film having excellent sliding properties, corrosion resistance and like properties on the surface of aluminum.

#As to the compounding proportions of ammonium silicofluoride and the aforesaid other aluminum-free fluorine compound, ammonium silicofluoride is preferably used in an amount of 0.05 to 15 parts by weight, more preferably 0.1 to 10 parts by weight, per 100 parts by weight of water; and the fluorine compound is preferably used in an amount of 0.1 to 20 parts by weight, more preferably 0.2 to 15 parts by weight, per 100 parts by weight of water. If the amount of ammonium silicofluoride is less than 0.05 part by weight, or if the amount of the fluorine compound is less than 0.1 part by weight, the reaction will undesirably be retarded to prolong the treating time. On the other hand, if the amount of ammonium silicofluoride is greater than 15 parts by weight, or if the amount of the fluorine compound is greater than 20 parts by weight, it will be undesirable because of difficulty in dissolving them.

#Moreover, in addition to the aforesaid ammonium silicofluoride and the like, the treating fluid used in the present invention contains an aluminum-containing substance or powdered aluminum having the same effect as aluminum dissolution occurring immediately after bath preparation. Specific examples of the aluminum-containing substance include aluminum fluoride (AlF3), aluminum hydroxide [Al(OH)3], aluminum silicate [Al2(SiO3)3] and magnesium aluminate metasilicate [Mg(OAlSiO3)2]. Among these aluminum-containing substances, aluminum fluoride or the like is preferably used. It is undesirable to use a compound releasing an ion other than those possessed by the above-illustrated substances, because the properties of the formed film may be different from the expected ones. Although these aluminum-containing substances and powdered aluminum do not dissolve at a pH of the order of 2, they can readily dissolve in the treating fluid of the present invention because of the addition of the aforesaid fluorine compounds.