Chromate-free ceramic coating compositions for hot corrosion protection of superalloy substrates

A multilayer coating system for hot corrosion protection of a substrate, comprising:

• a substrate;
• a basecoat on said substrate, said basecoat comprising:
• a first ceramic matrix formed from a slurry comprising a first hexavalent chromium-free, aluminum-phosphate based binder, wherein said first hexavalent chromium-free, aluminum-phosphate based binder has a molar ratio of Al:PO4 higher than about 1:3;
• and
• metallic oxide particles embedded into the first ceramic matrix;
• a topcoat, comprising:
• a second ceramic matrix formed from a slurry comprising a second hexavalent chromium-free, aluminum-phosphate based binder, wherein said second hexavalent chromium-free, aluminum-phosphate based binder has a molar ratio of Al:PO4 higher than about 1:3.

A method of preparing a multilayer coating system for hot corrosion protection of a substrate, comprising:

• preparing surface of a metal substrate;
• applying a basecoat slurry, said basecoat slurry comprising a first binder comprising an aluminum phosphate based aqueous solution having a molar ratio of Al:PO4 higher than about 1:3, the first binder characterized by an absence of hexavalent chromium, and further wherein metal oxide particles are incorporated into the first binder;
• curing the basecoat slurry to form a basecoat,
• applying a topcoat slurry onto the basecoat, the topcoat slurry comprising a second binder comprising an aluminum phosphate based aqueous solution having a molar ratio of Al:PO4 higher than about 1:3, the second binder characterized by an absence of hexavalent chromium;
• and
• curing the topcoat slurry to form a topcoat.