Steam oxidation resistance of Ni-aluminide/Fe-aluminide duplex coatings formed on creep resistant ferritic steels by low temperature pack cementation process

Steam oxidation resistance and thermal stability were studied at 650 °C for a coating with an outer Ni₂Al₃ layer and an inner Fe₂Al₅ layer formed on P92 steel surface. The parabolic rate law of oxidation was obeyed only in less than 2000 h with positive deviations occurring at longer oxidation times. The outer layer of the coating was transformed to NiAl during oxidation, but it remained stable once it was formed. The mechanisms for the enhanced thermal stability were discussed and a simple approach to enhancing the lifetime of the coating was proposed.     

Steam oxidation resistance and thermal stability of chromium aluminide/chromium hybrid coating on alloy steels formed at low temperatures

This study investigated the feasibility of forming a hybrid coating with a structure consisting of a top Cr-aluminide layer and an inner Cr layer on alloy steels by using a two step process: electro-Cr plating and then pack aluminising at low temperatures. The oxidation resistance of the coating so formed was tested in ultrasupercritical steam of 650 °C and 30 MPa. The factors affecting the oxidation kinetics of the coating were studied by comparing its oxidation behaviour with that of the pack Fe-aluminide coating tested in the same ultrasupercritical steam. The thermal stability of the coating at 650 °C was investigated by a series of isothermal annealing experiments in argon atmosphere. It was demonstrated that the outer Cr-aluminide layer of the coating can improve the steam oxidation resistance of the steel substrate. The inner Cr layer can function as an effective barrier preventing the outward diffusion of Fe from the steel substrate; it can also act as a buffer zone, substantially reducing the rate of the inward Al diffusion process.     

Improvements in corrosion resistance,mechanical properties,and weldability of duplex austenitic/ferritic steels

The corrosion behaviour, toughness and weldability of duplex stainless steel can be improved by controlling the composition and austenite: ferrite ratio in the base metal and in the weld deposits. Addition of nitrogen is beneficial to ductility and restistance to pitting corrosion; the latter is also improved by increased chromium contents which stabilize the passive film. This effect is further enhanced by the addition of molybdenum which, however, should not exceed sigma phase at high temperatures. Copper in amounts up to 1.5% improves the resistance to marine environments; however, a certain upper limit should not be exceeded because of adverse effects on hot ductility. In view of the function of nickel concerning the austenite: ferrite ratio (which should be about 50:50) the nickel content should be appropriately selected and should be higher in the filter metal.     

Simulation and experimental approach to CVD-FBR aluminide coatings on ferritic steels under steam oxidation

The ferritic steels used to produce structural components for steam turbines are susceptible to strong corrosion and creep damage due to the extreme working conditions pushed to increase the process efficiency and to reduce pollutants release. The response of aluminide coatings on the P-92 ferritic steel, deposited by CVD-FBR, during oxidation in a simulated steam environment was studied. The analyses were performed at 650 °C in order to simulate the working conditions of a steam turbine, and 800 °C in order to produce a critical accelerated oxidation test.The Thermo-Calc software was used to predict the different solid phases that could be generated during the oxidation process, in both, coated and uncoated samples. In order to validate the thermodynamic results, the oxides scales produced during steam tests were characterized by different techniques such as XRD, SEM and EDS. The preliminary results obtained are discussed in the present work.     

The initiation of selective oxidation of a ferritic stainless steel at low temperatures and oxygen pressures

Oxidation of a ferritic stainless steel of type Fe-18Cr-2Mo has been performed in the temperature range 285–495°C and oxygen partial pressure range 10^?9-10^?8 torr. The chemical composition of thin oxide layers formed has been analysed by means of Auger Electron Spectroscopy and interpreted in terms of available chromium, iron and oxygen at the solid/gas interface. The selective oxidation of chromium is considered by different probabilities for the oxidation of available chromium and iron respectively. At oxide thicknesses below 100 Å the supplies of chromium and iron are ruled by diffusion in the steel matrix. The results have been used to predict the chemical compositions of two subsequently growing oxide layers provided the thickness of the first oxide layer is below 50 Å.     

Aspects of the anodic behaviour of duplex stainless steels aged for long periods at low temperatures

Duplex stainless steel samples were aged at low temperatures (300 and 400 °C) for 1000 and 2000 h. Their anodic behaviour was studied in a 0.1 N concentration of sulphuric acid, which revealed that the polarisation curves were dependent on the aging temperature and time. Complex anodic current peaks occurred at low and high potentials due to the dual austenite and ferrite microstructure. The aging treatment promoted silicon enrichment of the passivated film, probably over the austenite phase, as indicated by X-ray photoelectron spectroscopy measurements. G phase precipitation and spinodal decomposition was identified by transmission electron microscopy.     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Preparation of aluminide coatings at relatively low temperatures

1 Introduction Protective coatings by pack aluminizing are frequently applied to metals to protect them from high temperature oxidation and hot corrosion attack [1, 2]. Pack aluminizing consists of heating the parts to be coated in a closed or vented pac…     

Evaluating the oxidation resistance of bond coats on thermal barrier coatings

Coatings are being increasingly used as an engineering alternative for advanced projects. Various techniques and processes are available for applying coatings, depending on the specific situation they are intended for. Thermal barrier coatings, known as TBCs, form part of a particular series of metal-ceramic coatings that are traditionally used in the aeronautical industry and are being increasingly applied in the automotive industries and for industrial turbines. One of the main issues with TBCs is degradation owing to the oxidation of the bond coat in high temperatures, resulting in the failure of the coating due to peel off. This study investigates and compares the behaviour of the oxidation of the TBC bond coat when the material used is NiAl alloy; this alloy is commonly used because of its characteristics at high temperatures and to ensure strong adhesion on various substrates. The bond coat was applied on an AISI 1020 Steel substrate using the flame spraying process. In order to carry out isothermal oxidation tests, the furnace used was regulated at a temperature of 1000 °C in static air. The samples were exposed for 24, 48 and 96 h and cooling was carried out in atmospheric air at ambient temperature. The analysis of the thermally grown oxide for each sample was carried out based on the exposure times and the oxide rates were evaluated by measuring the mass gained by the samples with oxidized coatings and using Scanning Electron Microscopy and Optical Microscopy.     

Study of the cyclic oxidation resistance of Al coated ferritic steels with 9 and 12%Cr

Ferritic stainless steels exhibit good oxidation resistance due to formation of a protective chromia scale. However, the protective chromia scale spalled or cracked after the metal substrate, was subjected to repeated thermal cycling oxidation. Aluminide coatings applied by FB-CVD were found to improve the cyclic oxidation resistance of P92 and HCM12 stainless steels. The protective effect depends on the coating properties. The cyclic oxidation resistance of P92 and HCM12 coated with Al was limited by spallation of the alumina scale due to the presence of voids and cavities at the coating-oxide interface. The computer simulation of cyclic oxidation behaviour of uncoated and coated ferritic steels using COSP, DICOSM programs showed the region of the universal cyclic oxidation life map where these substrates were located.     

Influence of welding thermal cycles on microstructure and pitting corrosion resistance of 2304 duplex stainless steels

Different welding thermal cycles from single-pass to triple-pass were performed on two kinds of 2304 duplex stainless steel through Gleebe thermal–mechanical simulator. The corresponding microstructure was observed, while the pitting corrosion resistance was investigated in 1.0 M NaCl by potentiostatic critical pitting temperature (CPT). The results showed that single-pass welding deteriorated microstructure and pitting corrosion resistance significantly. As the welding pass increased, the ferrite content decreased and CPT increased. However, CPT was still lower than that of the base metal. Nitride precipitated at the boundary between ferrite and austenite phase for low-alloyed 2304 after the single-pass welding thermal cycle.