Process for producing elevated temperature corrosion resistant metal articles

In a process for providing a coating on a metallic substrate, the coating renderng the substrate resistent to corrosion at elevated temperatures, said process comprising the steps of providing a ductile metallic overlay on the substrate surface, said overlay being of a composition normally resistant to corrosion at elevated temperatures, and applying an outer layer on said overlay, said outer layer being formed of a material more resistant to corrosion at elevated temperatures, the outer layer comprising an encapsulating means thereby preventing penetration of gas, the improvement comprising the steps of thereafter subjecting said coated substrate to a hot isostatic pressing operation by locating the coated substrate in a pressure-tight chamber, and subjecting the coated substrate to a temperature and pressure applied through a gaseous atmosphere sufficient to eliminate porosity, said temperature and pressure application also causing diffusion of substrate ingredients from one direction into the overlay and diffusion of outer layer ingredients from the other direction into the overlay thereby modifying the composition of the coating composed of said overlay and said outer layer.