Method of forming a hydrophilic coating on an aluminum surface

DETAILED DESCRIPTION OF THE INVENTION It has been found that the best results for a hydrophilic coating are obtained when the treatment is applied to a roughened aluminum surface.
Accordingly, the first step of the process of the invention involves the etching of the aluminum surface to provide this desirable rough surface.
By choosing the etchants carefully, the amount of metal lost in the etching step may be controlled so that it does not occur at an undesirable rate.
This is to be contrasted with the inevitable large degree of etching that occurs when an aluminum surface is treated according to the prior art process using MVB coating procedures; in that case the degree of etch cannot be controlled without affecting the formed coating.
The etching step also acts to clean the aluminum surface preparatory to the coating steps, and so ensure a clean, deoxidized surface.
The etchant used in the process of the invention may be any convenient etchant which is suitable for use with aluminum.
While the etchant may be either acidic or alkaline, it is highly preferred to employ an alkaline etchant.
Such alkaline etchants for aluminum are well known in the art.
Preferred alkaline etchants include the alkali metal hydroxides, and the particularly preferred etchant is sodium hydroxide.
In the present invention it is usually most convenient to employ the etchant in aqueous solution, and this solution may also contain conventional inhibitors and bath extenders, if desired.
A particularly preferred etchant for use in this invention is an aqueous solution containing from about 2 to about 20 g/l sodium hydroxide.
By way of example, two etchant compositions which may be used with advantage in the method of the invention are:
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Etchant A
sodium hydroxide 2.
95 g
gluconic acid 0.
088 g
water q.
s.
for 1 liter
Etchant B
sodium hydroxide 14.
76 g
gluconic acid 0.
44 g
water q.
s.
for 1 liter
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Gluconic acids in the etchant solution acts as a sludge modifier, that is it softens the scale deposits on the surfaces of the treating tank walls and heating coils employed in the treatment tank