Porosity Reduction in New Thin Films of Ceramic Coatings on Stainless Steel by Annealing at Reduced Pressure
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POROSITY of ceramics coatings is a beneficial property in some applications as catalysis where, according to the method of synthesis, it can vary from micro-porosity to macro-porosity, determining the specific catalyst for a chemical reaction. However, when the aim is to employ these coatings as a protective method against corrosion of metal substrates, forming a physical barrier between the electrolyte and the metal to be protected, the presence of porosity is detrimental, because it will accelerate the rate of interaction of the electrolyte with the substrate.[1,2] The corrosion behavior of physical vapor deposition (PVD) ceramic coated in aqueous solutions is an unresolved problem in many cases.[3,4] A surface pore constitutes a channel in the
G.I. CUBILLOS and E. ROMERO are with the Group of Materials and Chemical Processes, Department of Chemistry, Universidad Nacional de Colombia, Av.Cra. 30 No. 45-03, Bogota´, Colombia. Contact e-mail: [email protected] M. BETHENCOURT is with the Department of Materials Science, Metallurgy Engineering and Inorganic Chemistry, International Campus of Excellence of the Sea (CEI-MAR), University of Cadiz, Avda. Repu´blica Saharaui, 11510 Puerto Real, Spain. J.E. ALFONSO is with the Group of Material and Surface Science, Department of Physics, Universidad Nacional de Colombia, Bogota´ AA-14490, Colombia. E. RODRI´GUEZ-CASTELLO´N is with the Department of Inorganic Chemistry, Crystallography and Mineralogy (Unit Associated to ICP-CSIC), Faculty of Sciences, University of Malaga, Campus de Teatinos, 29071 Ma´laga, Spain. Manuscript submitted February 22, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
bulk material that conducts the electrolyte to the substrate and facilitates active substrate-electrolyte interaction. The presence of pores quickly deteriorates the coatings, providing direct paths for corrosive electrolytes to reach the coating/substrate interface, where localized galvanic corrosion can be initiated due to the corrosion potential difference between the coating and the metal.[5–7] The intermediate point in the application of porous coatings constitutes the field of implants, which on the one hand aims at a porous surface that favors cell growth[3] and on the other an implanted part with a high resistance to corrosion in biological fluids.[4,5,8] Achieving a waterproofing of the surface of a ceramic material deposited on a metal, taking advantage of the chemical inertia of the ceramic to increase the resistance to corrosion of the metal, would be very advantageous. If we try to increase the corrosion resistance of a stainless steel by sintering the ceramic deposited on it, we encounter another disadvantage: the sintering processes are carried out at high temperature, and stainless steels, depending on their chemical composition, can be sensitized by precipitation of the chromium carbides at the grain boundaries.[6] This triggers a loss of its corrosion resistance properties by decreasing the concentration of chromium in the matrix of the steel. The objectiv
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