F.J. Bolívar

Coating Design Using Computational Codes for Steam Oxidation Conditions

Thermodynamical calculations were performed using Thermo-Calc codes to obtained equilibrium diagrams of the heterogeneous system (steel P92, Fe-30Al-6Cr coating (wt. %))|(oxide compounds)|(O2 + H2O gas mixture). Calculations were made for 650 o C, constant water vapour pressure, equal 10 -1 atm, and variable oxygen partial pressure from 1⋅10 -20 to 1⋅10 -1 atm. From obtained results indicate that in the steam gas environments the highest volatile compounds form Mo, W and Cr oxides. Formation of volatile products depends on the oxygen pressure in the O2 + H2O gas mixture. There is change of the principle volatile species in the Fe-Cr system, when at constant water vapour pressure, the oxygen partial pressure decrease. In such conditions there is a decrease of CrO2(OH)2 vapour pressure, which is accompanied by an increase of Fe(OH)2 vapour pressure. At higher oxygen partial pressure vapour pressure of CrO2(OH)2 exceeds that of the most volatile aluminium species, Al(OH)3, by many orders of magnitude.

Study of Oxyhydroxides Formation on P92 Steel on Al-CVD coating in Contact with Ar+20% H2O at 650°C by TG-Mass Spectrometry

Volatile species released during the initial stages of oxidation of a P92 ferritic steel, with and without an aluminized coating, at 650°C in Ar+20%H2O for 150h were studied. TG-MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam. From those results, the role of the different coating element could be established and optimized for the coating durability. An oxidation mechanism based on the TG-MS results is given. The morphology/composition and structure of the oxidized samples were also studied using SEM/EDS and XRD techniques.

Aluminización de aceros ferriticos-martensiticos (hcm-12a) mediante cvd-fbr

The Al-coating are well-known for its good resistance against oxidation and sulfidization due to the formation of a protective layer of alumina. The Chemical Vapour Deposition by Fluidized Bed Reactor (CVD-FBR), amongst its advantages, it should be pointed out the high transferences of mass and heat obtained at low temperatures of operation. In addition, it has the advantage of working at atmospheric pressure. The deposition of Al coating on the ferriticmartensitic steel (HCM-12A) has been developed by means of CVD-FBR. The initial parameters of the process were determined by the thermodynamic simulation of the process with the help of the computer program Thermocalc software .

Behaviour of Aluminide Diffusion Coatings on HCM12-A Steel by Al/Ce and Al/La CVD-FBR in Oxidation.

The steam oxidation behaviour at 800°C of aluminized HCM12A ferritic-martensitic steel has been studied. The aluminization process used was CVD in fluidized bed reactor (CVD-FBR), using a reactive bed modified with Ce or La particles. The obtained coatings were mainly composed of (Fe,Cr)2Al5 intermetallic phase. Long term oxidation (1000h) behaviour of the coated HCM12A was studied in 100%H2O atmosphere. By the application of the protective coating, the ferritic-martensitic steel oxidation rate is reduced considerably because of the alternately formation of Al2O3 and Cr2O3 + (Fe,Mn)3O4 protective scales on the substrate surface due to the diffusion processes that take place during the exposure at high temperatures in combination with the aggressive environment.

The Influence of CVD-FBR Coatings on HCM12 Corrosion Behaviour under Molten Salt Conditions

The influence of Al and Al-Si coatings on the corrosion behaviour of HCM12 in molten KCl-ZnCl2 mixture at 650°C in air has been characterized by electrochemical impedance spectroscopy (EIS). Al and Al/Si protective coatings were developed by chemical vapour deposition in fluidized bed reactor (CVD-FBR) at moderate temperature to respect to mechanical properties of substrate. Scanning electron microscopy (SEM) was used to analyse the damage on the HCM12 electrode surface. Al-Si coating was found to be more resistant to the molten chlorides attack than Al coating; and both coatings increased the corrosion resistance of HCM12 in these conditions. The surface composition has been determined by X-ray diffraction (XRD).

Evaluation of Si Coating on Ferritic Steels by CVD-FBR Technology in Steam Oxidation

The development of new power generation plants firing fossil fuel is aiming at achieving higher thermal efficiencies of the energy conversion process. The major factors affecting the efficiency of the conventional steam power plants are the temperature and, to a lesser extent, the pressure of the steam entering the turbine. The increased operating temperature and pressure require new materials that have major oxidation resistance. Due to this problem, in the last years numerous studies have been conducted in order to develop new coatings to enhance the resistance of steels with chromium contents between 9 and 12% wt against steam oxidation in order to allow operation of steam turbines at 650 0C. In this study, Si protective coatings were deposited by CVD-FBR on ferritic steel P-91. These type of coatings have shown to be protective at 650 0C under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. Morphology and composition of coatings were characterized by different techniques, such as scanning electron microscopy (SEM), electron probe microanalysis, and X-ray diffraction (XRD). The results show a substantial increase of steam oxidation protection afforded by Si coating by CVD-FBR process.

Study of the Role of Small Additions of Zr in the CVD- FBR Aluminization Process

In this work, iron aluminide coatings were developed by Chemical Vapor Deposition in Fluidized Bed Reactor (CVD-FBR) on ferritic-martensitic steels. Small additions of zirconium powder were introduced in the fluidized bed; as a consequence, the obtained coatings are thicker than that without zirconium additions. When Zr powders are added in the fluidized bed, the deposition atmosphere drastically changes, leading to increase the deposition rate. Thermodynamic calculations were carried out to simulate the modifications in the CVD atmosphere in the Al/Zr deposition system in comparison to the single aluminization. In order to optimize the conditions of the deposition, parameters such as temperature and concentration of zirconium introduced into the bed were evaluated and compared with the results obtained for the single aluminum deposition.

Evaluation of Al-Hf Coating on Ferritic Steels by CVD-FBR Technology in Steam Oxidation

The steels with chromium contents between 9 and 12%wt are used for power plants with advanced steam conditions. These steels possess good creep properties similar to the 9% Cr steels as well as good creep and good oxidation resistance at temperatures between 500-600°C. In the last years efforts have been made to develop coatings for protection against oxidation in order to allow operation of steam turbines at 650°C. In this study, Al-Hf protective coatings were deposited by CVD-FBR on the ferritic steel HCM-12A followed by a diffusion heat treatment, and were shown to be protective at 650°C under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. The morphology and composition of the coatings were characterized by techniques, including scanning electron microscopy (SEM), electron probe microanalysis, and Xray diffraction (XRD). The results showed a substantial increase of steam oxidation protection afforded by Al-Hf coating deposited by the CVD-FBR process.

Al-Mn CVD-FBR Protective for Steam Corrosion Applications

Ferritic steels are usually used in boiler or supercritical steam turbines which operate at temperatures between 600-650°C under pressure. Protective coatings are often applied in order to increase their oxidation resistance and protect them against degradation. In this study new Al-Mn protective coatings were deposited by CVD-FBR on P92 ferritic steel. The initial process parameters were optimized by thermodynamic calculations using Thermo-Calc software. Then, those parameters were used in the experimental procedure to obtain Al-Mn coatings at low temperature and atmospheric pressure. Co-deposition was achieved at moderate temperatures in order to maintain the substrates` mechanical properties. The coatings` microstructure and phase constitution was characterized. Fe-Al intermetallic coatings containing Cr and Mn were obtained. The phase constitution is discussed with reference to the Fe-Al-Mn ternary phase diagram. The effect of diffusion heat treatment on the phase transformations as well as the steam oxidation resistance of these coatings at 650°C and 800°C was investigated.