M.J. Cristóbal

The influence of yttrium ion implantation on the oxidation behaviour of powder metallurgically produced chromium

The beneficial effect of adding yttrium by ion implantation on the oxidation behaviour of powder metallurgically (PM) produced chromium at 900 °C has been investigated. Comparative studies were made in which yttrium was added as an oxide dispersoid. For further insight into the oxidation process, a two stage oxidation, using oxygen isotope tracers, was performed. The oxide scales formed were analysed by a wide range of analytical techniques, but mostly secondary neutral mass spectroscopy (SNMS). The addition of yttrium changes the oxide scale growth processes. Its presence in the oxide scale, possibly as a segregated phase on the oxide grain boundaries, reduces cation diffusion, causing the oxide growth to be dominated by anion transport. This segregation can cause many, if not all, of the reported beneficial reactive element effects. Ion implantation is shown to be a powerful tool in oxidation studies, and thus for advanced material development.

Galvanic coupling between carbon steel and austenitic stainless steel in alkaline media

The galvanic currents between carbon steel and two types of stainless steels have been studied in solutions of pH close to that of concrete. The situations analysed were passivity and active corrosion induced by chlorides. In all cases the galvanic effects recorded were of minor importance indicating that no significant risk of galvanic corrosion exists when carbon steel and stainless steel are electrically coupled in reinforced concrete structures. Oxygen reduction current is lower for stainless steel than for passive carbon steel. The results have been interpreted by means of EIS in terms of the resistivity of the passive layer formed on steels.

A Study of the Initial Stages of Oxidation of Yttrium Implanted Chromium Using X-Ray Diffraction and Absorption Spectroscopy

Abstract The beneficial effect of the addition of yttrium by ion implantation on the oxidation behaviour of chromium has been studied. Emphasis was placed on the initial stages of oxidation during rapid heating to 900 °C by exposing samples to intermediate temperatures and withdrawing them for analysis. Characterization of the surface layers was carried out with two X-ray techniques at glancing angles. Glancing angle X-ray diffraction (GAXRD) at various angles was used to obtain structural and texture information as a function of depth. X-ray absorption spectroscopy at the yttrium K-edge enabled information to be gained on the local structure around this element. The short-range structure of two yttrium-rich oxides, Y 2 O 3 and YCrO 4 , was observed. The latter is believed to represent the structure around yttrium segregated to the oxide grain boundaries. It is thought that this segregation in the chromia scales can cause many, if not all, of the reported beneficial reactive element effects. The Y 2 O 3 , present as particles, is expected to play only an indirect role by forming a source of yttrium, which slowly segregates to the grain boundaries. The possible effects of the grain boundary segregation of yttrium on the oxidation behaviour of the material and the oxide scale properties are discussed.

High-Temperature Oxidation Studies of Low-Nickel Austenitic Stainless Steel. Part I: Isothermal Oxidation

The oxidation behavior of low-nickel austenitic stainless steel (LNiSS) in air at 873 and 973 K was investigated for 500 hr. The oxide scales formed during the process were examined by a wide range of experimental techniques including SEM/EDS, XRD, and EPMA, in order to determine their influence on kinetics behavior. Kinetics laws were close to parabolic at both temperatures, but the morphology of scales showed important differences with temperature. At 873 K the oxide scale was thinner, with irregular growth, intrusions, and without spallation. It was concluded that slower kinetics and advantageous scale morphology suggest that LNiSS is a suitable material for isothermal oxidation in air at 873 K. At higher temperatures, uniformly thick scales plus iron-rich nodules were observed with different composition regions. The most destructive feature was the formation of Fe-rich nodules, which were vulnerable to spalling during cooling.

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.

Characterisation of the electrochemical behaviour of cerium implanted stainless steels

Passive layers formed on a Ce implanted stainless steel in alkaline media are studied to clarify its beneficial effect on stainless steel corrosion resistance. An important decrease in iron and chromium activity peaks is detected by electrochemical techniques. X-ray photoelectron spectroscopy (XPS) analysis shows passive film thickness decreasing together with changes in chemical composition. Cerium implantation modifies the conductivity properties of the passive film, as it is inferred from EIS measurements.

Electrochemical Impedance Spectroscopy as a Tool for Studying Steel Corrosion Inhibition in Simulated Concrete Environments—Red Mud Used as Rebar Corrosion Inhibitor

Red mud is the main by-product of the Bayer process for alumina extraction from bauxite. Rich in iron, aluminum and silicon oxides, it is believed to have corrosion protection properties. Some are studied in the present paper. The corrosion behavior of steel rebars in chlorine containing solutions and mortars is studied and compared with the same systems containing red mud in suspension (case of solutions) or added as powder to the mix (case of mortars). The results in solution show that red mud is able to maintain steel passivity for more than 90 days in highly chlorinated alkaline solutions while in NaOH and Ca(OH)2 solutions having the same pH pit formation is observable at 25 and 55 days, respectively. Additions of red mud to mortar as powder representing 2 % by weight of cement are able to block chloride depassivation. EIS data allow understanding the protecting mechanism and comparing the behavior in mortar and in solution.

Modifications of the stainless steels passive film induced by cerium implantation

In this work X-ray photoelectron spectroscopy studies and cyclic voltammetry are combined to elucidate the effect of Ce implantation on the formation and evolution of passive layers generated on AISI 430 and AISI 304L steels in alkaline medium. The passive films develop in implanted steels result thinner and less defective than in unimplanted. This result is interpreted in terms of a cerium-induced inhibition of the passive film re-oxidation (formation of iron and chromium oxides). The main difference between the behaviour of the two types of steel (ferritic and austenitic) seems to be related to the enrichment in nickel promoted by cerium in the outermost layer of AISI 304L.

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.

Corrosion Protection of Low-Nickel Austenitic Stainless Steel by Yttrium and Erbium-Ion Implantation Against Isothermal Oxidation

The beneficial effect of ion-implanted yttrium and erbium on the oxidation behavior of low-nickel austenitic stainless steel (LNiSS) at 973 K has been investigated up to 500 hr of oxidation test in air. The resulting oxide scales were examined by a wide range of experimental techniques, including SEM/EDS, XRD, and EPMA. The results indicate that both reactive elements have similar effects. The most significant effects have been to significantly reduce the corrosion rate and to improve the oxide scale adhesion. It is concluded that ion implantation is a powerful tool as surface-modification process introducing reactive elements in the top surface.