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A NEW TYPE OF high temperature oxidation and hot corrosion resistant overlay coatings were obtained by a diffusion aluminizing process with or without the addition of chrome, tantalum and rare earth elements on-to electroplated and/or chemical plated Co and/or Coalloy based platings. The diffusion process and the properties of the duplex coatings will not be influenced by the superalloy substrates and will be easily overhaul. Degradation studies show that these coatings had a high resistance to oxidation and hot corrosion. Co and its alloy platings become the basic part of the diffusion coatings and also positioned at the interface to serve as an effective diffusion barrier to prevent further diffusion of coating elements. Co barrier can be used as an effective barrier instead of precious and rare element barriers in lower cost.