A hydrophilic coating can be optionally corrosion resistant and/or microbial resistant for a substrate such as a heat exchanger. The coating is provided by a zeolite layer that can be formed from a synthesis solution comprising a structure directing agent, a base, a silicon source, an aluminum source, and a solvent. In one preferred embodiment, the synthesis solution comprises tetrapropylammonium hydroxide, sodium hydroxide, aluminum oxide, tetraethylorthosilicate, and water. The layer is characterized by a zeolite MFI structure and by a composition having the formula of M.sub.n/m [Al.sub.n Si.sub.(96-n) O.sub.192 ], or [Al.sub.n Si.sub.(96-n) O.sub.192 ].4[(CH.sub.3 CH.sub.2 CH.sub.2).sub.4 N--OH] wherein M is a metal ion of valence m.sup.+ (e.g., Na.sup.+) and 27>n>=0. After formation of the coating, the organic structure directing agent can be left intact inside the zeolite coating to make the coating corrosion resistant. Alternatively, and after removal of the organic structure directing agent, a biocidal metal ion can be incorporated into the coating by an ion exchange process to render the coating microbial resistant. A hydrophilic coating that is also corrosion resistant and microbial resistant can be made by a zeolite coating with two sub-layers--the bottom sub-layer being corrosion resistant and the top sub-layer being microbial resistant.
CROSS REFERENCE TO RELATED APPLICATION
This is a divisional application of U.S. application Ser. No. 09/535,000 filed Mar. 23, 2000, now U.S. Pat. No. 6,500,490.
A composition of matter including a substrate of a metal that is susceptible to corrosion in a high pH alkaline solution, a corrosion-resistant base layer disposed on the surface of the substrate, the base layer having a pure or high silica zeolite having a silicon:aluminum atomic ratio of at least about 100, a middle mixed zeolite layer disposed on the surface of the base layer, and a top hydrophilic layer disposed on the surface of the middle layer, the top layer having a high aluminum zeolite having a silicon:aluminum atomic ratio of less than 5. The middle mixed zeolite layer includes a zeolite having a silicon:aluminum atomic ratio range that is between the silicon:aluminum ratio of the pure or high silica zeolite of the base layer and of the high aluminum zeolite of the top layer.
