F. Pedraza

Aluminizing and chromizing bed treatment by CVD in a fluidized bed reactor on austenitic stainless steels

The deposition of Al and Cr on steel coupons by chemical vapor deposition in fluidized bed reactors (CVD-FBR) has been studied. Before performing the experiments, thermochemical calculations were run using the free-energy minimization method afforded by the HSC Chemistry software. Metal sub-halide chemistry was used to produce the gas precursors of deposition. With this method we have achieved 10 μm thick Al coatings at low deposition temperatures and short deposition times. However, we have not yet succeeded in the deposition of Cr and further work is needed.

High temperature corrosion protection of austenitic AISI 304 stainless steel by Si, Mo and Ce ion implantation

Abstract The influence of implanted silicon, molybdenum and cerium on the oxidation behaviour of a 18Cr8Ni stainless steel was studied at 1173 K up to 144 h in air under isothermal conditions in order to verify the enhanced selective oxidation of chromium by these elements. The implanted surface and the corrosion products formed were characterized by means of AES, SEM, EDS and XRD. Implanted depth profiles were calculated by TRIM96 computational code. The implanted silicon and cerium markedly improved protection against oxidation of the alloy by enhanced chromium transport while molybdenum gave rise to an accelerated oxidation due to the formation of volatile MoO 3 species. Further, ion implantation was found to play a beneficial effect against decarburization of the alloy.

Kinetic studies of Cr and Al deposition using CVD-FBR on different metallic substrates

Abstract Thermochemical calculations for the deposition of aluminium and chromium using the metal–subhalide chemistry have been performed by the HSC chemistry software. Before depositing, the flow regimes that lead to a fluidized bed without particle elution have been measured at room and deposition temperatures. Aluminium diffusion coatings have been obtained on austenitic AISI 304 stainless steel and IN-100 Ni-base alloy; the deposition being much faster on the steel substrates. However, the chromium diffusion coatings are only achieved on the IN-100 Ni-base superalloy since in the steel, volatilization of Fe occurs at the first stages of deposition.

Comparison of the High-Temperature Oxidation of Uncoated and CVD-FBR Aluminized AISI-304 Stainless Steel

Aluminide coatings were obtained by means of the CVD–FBR technique at 525°C for 1.5 hr under a reactive-gas mixture composed of 10 vol.% H2+1 vol.% HCl, the rest being Ar as the fluidizing inert gas. Subsequent heat treatment at temperatures to 900°C was conducted to enhance interdiffusion of the components. As a result, substoichiometric Ni–Al phases were found to form. Uncoated and coated plus heat-treated specimens were then subjected to 950°C oxidation for up to about 200 hr under isothermal conditions. The coated plus heat-treated specimens had much lower oxidation rates than the uncoated ones because of the formation of protective alumina scales. Loss of protective behavior occurred only by spalling of the alumina scales upon cooling from the oxidation temperature. The higher oxidation rates of uncoated specimens have been attributed to nodule formation and minor subsequent spalling of the oxide scale.

Effect of fluidized bed CVD aluminide coatings on the cyclic oxidation of austenitic AISI 304 stainless steel

Abstract Austenitic AISI 304 stainless steel finds a vast variety of applications due to its good performance in different environments and its relatively low price. However, its high temperature operation limit is restricted to approximately 950°C due to the formation of volatile CrO3 under oxygen environments. Thus, the application of aluminide coatings may increase the upper temperature LIMIT of this steel. Chemical vapour deposition in fluidised bed reactors (CVD-FBR) has been used to coat AISI 304 steel at temperatures of 525°C for 1.5 h followed by a heat treatment up to 900°C. Cyclic oxidation experiments of both coated and uncoated specimens have been conducted under atmospheric pressure air at 950°C. The results are very promising for the application of the CVD-FBR process as a surface modification technology since the behaviour of the coated specimens is much better than that of the uncoated ones.

Effect of yttrium and erbium ion implantation on the oxidation behaviour of the AISI 304 austenitic steel

Abstract The beneficial effect of the addition of yttrium and erbium by ion implantation on the oxidation behaviour of AISI 304 stainless steel at 1173 K has been investigated. Isothermal oxidation tests have been conducted for up to 500 h. The effect of ‘rare earth elements’ (REE) have been studied previously in order to enhance the oxidation behaviour of different alloys. The results show that yttrium and erbium have similar effects, reducing the rate of continuing scale growth and inhibiting scale failure processes. It is concluded in this study, that both reactive elements inhibit the growth of the poorly protective and adherent oxides rich in iron and chromium, which help the spalled behaviour, together with a smaller oxide grain size.

Growth of oxide scales upon isothermal oxidation of CVD-FBR aluminide coated stainless steel

Abstract The performance of different alloys exposed at high temperature environments depends upon their mechanical resistance as well as their corrosion/oxidation properties. When the mechanical requirements are not critical, austenitic stainless steels may play a role in substituting the more expensive Ni and Co base alloys. However, at temperatures close to 950°C, the chromia scale usually grown to protect the alloy may be further oxidised into CrO 3 , which is a volatile oxide and thus, the naked material may undergo a catastrophic oxidation. Fe–Cr–Al alloys have been shown to be oxidation resistant at high temperatures. This relies on the formation of alumina scales to protect the alloy, having a chromium reservoir so as to reduce the aluminium amount needed to maintain the oxide scale. In this work, aluminide coatings were deposited by means of the CVD-FBR technique on AISI 304 substrates, at 525°C for 1.5 h. A subsequent heat treatment up to 900°C was applied to the coated specimens to enhance interdiffusion of the species, which led to substoichiometric NiAl phases. Uncoated as well as coated plus heat-treated specimens have been oxidised at 950°C, up to 200 h, under continuous and discontinuous isothermal conditions. The results indicate that aluminide coatings provide a much higher beneficial effect under continuous oxidation than in discontinuous tests, the latter undergoing breakaway oxidation. Growth morphologies and compositions will show an iron enrichment during the latter oxidation stages, which will be responsible for oxidation resistance failure in the discontinuous tests.

Effects of yttrium and erbium ion implantation on the AISI 304 stainless steel passive layer

In this work, the response of AISI 304 stainless steel after having been implanted with 1017 ions/cm2 doses of Y and Er at an acceleration potential of 150 keV is examined. Computational estimates of the implanted profile have been performed using the PROFILE code. The nature of the outermost surface of the steel, i.e. the passive layer and onwards is studied by means of Auger Electron Spectroscopy as well as X-Ray Photoelectron Spectroscopy techniques, supported by thermodynamics calculations. It is shown that the incorporation of both reactive elements in the steel surface brings about different changes in the nature of their respective passive layers. Both implanted ions induce oxidation of the base material to a certain extent, probably due to the implantation process itself giving rise to Cr2O3 and FeO and/or Fe·OH species, together with different RE oxides as well as Y(OH)3.

Soft X-ray absorption spectroscopy study of the effects of Si, Ce, and Mo ion implantation on the passive layer of AISI 304 stainless steel

Abstract The chemical modifications introduced in the passive layer of AISI 304 stainless steel after Si, Ce, and Mo ion implantation were investigated and compared with non-implanted steel by soft X-ray absorption spectroscopy. The influence of ion implantation on the passive properties was evaluated by measuring soft X-ray absorption spectra at the Cr, Fe, Ni, Mn and Si 2p in addition to oxygen 1s thresholds. All ion implanted samples show a relative Cr-enrichment at the surface as compared with non-implanted samples. Fe 2p as well as O 1s spectral changes reveal chemical differences in the passive layer as a function of the element ion-implanted.

Adhesion properties of aluminide coatings deposited via CVD in fluidised bed reactors (CVD-FBR) on AISI 304 stainless steel

Aluminide coatings are widely used to protect a vast variety of metals and alloys for high temperature applications. Under certain environments, these are not only designed to improve high temperature corrosion, but also erosion. Therefore, the mechanical properties (such as toughness) of these protective coatings obtained by CVD-FBR are of prime importance taking into account the advantages of this coating technique. In this paper, preliminary studies of mechanical characterisation of aluminide coatings obtained on AISI 304 stainless steel by CVD-FBR deposited at 525°C and different processing times are presented. The effect of a subsequent thermal treatment of the aluminide coatings has also been investigated. From the results, the relationship between coating process, heat treatment, and toughness is established.