Protection of internal and external surfaces of gas turbine airfoils

A method for preparing a coated gas turbine airfoil, comprising the steps of

• providing a gas turbine airfoil made of a base metal and having an internal cooling passage therein defined by an internal airfoil surface, and an external airfoil surface;
• forming a diffusion aluminide protective layer at the internal airfoil surface of the internal cooling passage, the step of forming including the step of taking care that there is substantially no aluminum deposited on the external airfoil surface of the gas turbine airfoil during the step of forming;
• and
• depositing an overlay protective coating on the external airfoil surface of the gas turbine airfoil, there being substantially no diffusion aluminide between the overlay protective coating and the external airfoil surface,
• operating the airfoil in a gas turbine engine;
• removing the airfoil from the gas turbine engine;
• and
• repairing the airfoil, the step of repairing including the steps of
• removing the overlay protective coating from the external airfoil surface of the airfoil, and
• depositing a second overlay protective coating on the external airfoil surface of the gas turbine airfoil, there being substantially no diffusion aluminide layer between the second overlay protective coating and the external airfoil surface of the airfoil.

A coated gas turbine airfoil, comprising:

• a gas turbine airfoil formed of a base metal and having internal cooling passage therein and an external airfoil surface;
• a diffusion aluminide protective layer at a surface of the internal cooling passage;
• and
• an overlay protective coating on the external airfoil surface of the gas turbine airfoil, there being substantially no aluminide layer and no diffusion aluminide between the overlay protective coating and the external airfoil surface of the airfoil.

A method for preparing a coated gas turbine airfoil, comprising the steps of

• providing a gas turbine airfoil formed of a base metal having an internal cooling passage therein defined by an internal airfoil surface, the internal cooling passage extending to an opening in an external surface of the gas turbine airfoil;
• thereafter
• depositing a β-phase NiAl intermetallic overlay protective coating on the external airfoil surface of the gas turbine airfoil, there being substantially no aluminum layer between the overlay protective coating and the external airfoil surface;
• and
• forming a diffusion aluminide protective layer at the internal airfoil surface of the internal cooling passage.