Water Vapor Corrosion Research Articles

Corrosion and recession of mullite in water vapor environment

In the development of environmental barrier coating system for silicon nitride ceramic, a corrosion behavior of mullite was examined under static state water vapor environment at elevated temperatures. Results showed that a small amount of sodium, which was present in mullite bulk as an impurity, accelerated the water vapor corrosion of silica component and decomposition of mullite into alumina phase. To improve the selective corrosion of the boundary phase in silicate compounds, a mullite/Lu 2Si 2O 7 eutectic system without boundary glassy phase was developed. The eutectic composition was Al 2O 3:SiO 2:Lu 2O 3 = 27.3:54.6:18.1 in molar ratio and the eutectic temperature was estimated to be ∼1500–1520 °C. No weight loss was measured after the static state water vapor corrosion test at 1300 °C for 100 h. However, the recession of mullite phase in the eutectic oxide occurred after exposure to high speed steam jet environment at 1200 °C.

Corrosion and recession behavior of zircon in water vapor environment at high temperature

A corrosion test in static state water vapor environment and a recession test in high velocity steam jet environment for zircon bulk were performed at 1300 °C. The trace of the water vapor corrosion could be recognized on the grain surfaces and at the grain boundaries for the sample after the static state corrosion test. Sand ripple like morphology was generated on the grain surfaces and etch pits with less than 0.1 μm size were formed at the grain boundaries. A porous structure was formed on the bulk surface of the sample after the steam jet test. A glassy phase enriched with silica could be recognized on the surface of the sample after the test. Cracks were induced on the bulk surface during the test. The zircon phase at the bulk surface decomposed into monoclinic zirconia phase and the fraction of silica component at the bulk surface decreased by the steam jet test. The monoclinic zirconia phase was observed to re-generate and grow on the bulk surface.

Development of EBC for Silicon Nitride

Various coating methods of EBC layer for silicon nitride were discussed. High density EBC layer was successfully coated by different techniques such as sputtering, sol-gel and reaction sintering methods. Water vapor corrosion and recession mechanisms of Lu2Si2O7 which is a potential material for EBC were discussed. The problems in the development of EBC revealed by corrosion tests were summarized. The most important problem addressed here was the corrosion of silica at grain boundary. Due to corrosion of silica at the boundary, formation of porous surface is inevitable, then the silicon nitride substrate gets easily oxidized and/or corroded by water vapor. To resolve this issue, we propose a new EBC material without boundary silica and the corrosion mechanism of this improved EBC material is discussed.

Water vapor corrosion behavior of lutetium silicates at high temperature

The corrosion behavior of hot-pressed lutetium disilicate (Lu 2Si 2O 7) bulk sample in static state water vapor environment was examined at 1300 °C. The apparent corrosion rate was estimated as 1.427 × 10 −6 g/cm 2 h. The X-ray diffraction pattern showed a trace of Lu 2O 3 phase on the sample surface after corrosion test. SEM observation on the surface of the sample after corrosion test revealed that the surface morphology was not homogeneous but showing three different types of features. During high-temperature water vapor corrosion, lutetia-rich component first melted, and then removed from the grain surface by water vapor attack. Later, the excess boundary silica phase was removed by water vapor corrosion. These underlying corrosion mechanisms of bulk lutetium disilicate sample in static state water vapor environment were discussed in detail.

Corrosion behavior of Al2O3in static state water vapor environment at high temperature

Water vapor corrosion test at 1500 ◦C for single crystal and polycrystalline Al2O3 was performed in static state water vapor environment. In both cases, there was an increase in weight. The lattice parameter for polycrystalline bulk decreased by the corrosion test. The grain surface was corroded by water vapor and terrace fields were generated. No anisotropic corrosion behavior was observed. Alumina is a hopeful material for high temperature application due to its high stability at elevated temperatures [1]. Because the oxygen diffusion in alumina at high temperatures is low enough [2], it is considered that alumina phase is a promising material for oxidation protection layer for metal or non-oxide ceramics in high temperature applications. Non-oxide ceramics, such as silicon nitride and silicon carbide, oxidize at high temperatures and form a silica layer on their surface. It is known that the silica phase easily corrodes in water vapor environment at high temperatures [3, 4]. Recently, Yuri and Hisamatsu reported the recession rate for pure Al2O3 at high temperature and high water vapor partial pressure in high velocity gas flow [5]. In that report, the recession rate of Al2O3 is high enough when compared to that of silicon nitride, i.e., Al2O3 is easily corroded in actual gas turbine environment. A detailed recession rate expression for polycrystalline Al2O3 in actual combustion gas flow was derived previously [5]. However, Yue et al. reported that no weight change was observed by the water vapor corrosion test in static state water vapor environment at 1700–2000 ◦C [6]. On the other hand, Tai et al. suggested that the weight of Al2O3 phase increased in water vapor environment at high temperatures, even if the boundary phase was corroded and removed [7]. In that report, the weight is decreased up to 1500 ◦C and is increased above 1600 ◦C in Ar-H2O static state environment [7]. More detailed consideration of water vapor corrosion mechanism has not been established yet. Al2O3 (99.99% purity, High Purity Chemicals Co. Ltd.) powder was used as the starting material for preparing single crystalline and polycrystalline Al2O3. The high temperature water vapor corrosion test was performed for single crystal and polycrystalline Al2O3 using a Corrosion Testing Machine (Japan Ultra-high Temperature Materials Research Center). The sample was placed on a high purity Al2O3 plate and then heated under the following conditions; temperature: 1500 ◦C, time: 100 hrs, gas flow: 30 wt% water (air:H2O = 70:30 (wt %)), gas flow rate: 175 ml/min that correspond to gas velocity 4.6 × 10−4 m/s. To exclude the water vapor corrosion occurring at low temperatures, the corrosive gas was introduced when the temperature reached 1500 ◦C and the gas flow was stopped after the 100 hrs testing period. The density of the sintered polycrystalline Al2O3 bulk was 3.581 g/cm3 that is 90.0% of ideal value. No remarkable changes in external view for polycrystalline bulk were observed after the corrosion test. The weights of the samples (single crystalline and polycrystalline Al2O3) were increased during the corrosion test. The weight gain for single crystal and

Development of Oxide‐Based EBC for Silicon Nitride

In search of a candidate EBC material for silicon nitride ceramics, water vapor corrosion test was performed for a series of monolithic materials such as Al2O3, TiO2, ZrO2, HfO2, Al2TiO5, Al6Si2O13, Lu4Hf3O12, Ln2Si2O7 (Ln = Yb and Er) and mullite/Lu2Si2O7 eutectic bulk at above 1300°C. Based on the high‐temperature water vapor corrosion test results and the resultant microstructures, the mechanisms of water vapor corrosion of these materials were discussed in this paper. In this present study, ZrO2 and HfO2 showed excellent resistance to water vapor corrosion at high temperatures. Among low‐thermal‐expansion materials, specifically silicate‐based materials, Lu2Si2O7 showed better results in the aspect of weight loss rate during the corrosion test, which also desirably has a coefficient of thermal expansion very close to that of silicon nitride. To verify the recession mechanism of Lu2Si2O7, a recession test was performed for this bulk in high‐velocity combustion gas flow at 1300‐1500°C. This material showed excellent recession resistance compared to Al2O3 and Si3N4. Different coating techniques, such as sputtering, sol‐gel, and reaction sintering, were employed to coat a high‐density Lu2Si2O7 EBC layer on a silicon nitride substrate. However, in polycrystalline Lu2Si2O7 phase, the grain boundary silica was prone to water vapor corrosion. It was observed that the corrosion resistance of a Lu2Si2O7/mullite eutectic bulk, without boundary silica, was more excellent than that of polycrystalline Lu2Si2O7 bulk.

Water vapor corrosion of mullite containing small amount of sodium

The corrosion test for hot-pressed mullite bulk was performed in static state water vapor environment at higher temperatures such as 1300 and 1500 °C. After corrosion test, the samples exhibited weight gains with a rate of 6.60 × 10 −6 and 8.42 × 10 −5 g/cm 2 h at 1300 and 1500 °C, respectively. Microstructural and phase analyses revealed, corundum phase formed on mullite bulk surface and a large amount of pores were formed near the bulk surface. Crystalline corundum was directly grown from silica rich sodium containing aluminosilicate phase.

Sol–gel synthesis and coating of nanocrystalline Lu 2Si 2O 7 on Si 3N 4 substrate

In this present work, we report sol–gel synthesis of nanocrystalline α-Lu 2Si 2O 7, a candidate rare-earth disilicate phase being used recently as environmental barrier coating (EBC) layer for Si 3N 4 ceramics in high temperature water vapor corrosion environment. The method consists of the reaction of Lu(NO 3) 3 and TEOS in the adequate molar ratio to form a complex gel composed by the two cations, Si and Lu, which yields the stoichiometric Lu 2Si 2O 7 after calcining. Thermal history and phase identification of the synthesized phase were examined using TG–DTA and XRD analyses, respectively. High dense coating layer is obtained by pressureless sintering at relatively low temperature.

Anisotropic Behavior of Water Vapor Corrosion of Rutile TiO<SUB>2</SUB> at High Temperature

Water vapor corrosion behavior of rutile TiO 2 was examined at 1773 K in 30 mass% H 2 O environment. The corrosion rate for this sample was 2.2 × 10 - 4 kg.m - 2 .h under this experimental conditions. Anisotropic corrosion behavior was recognized in the planes that are parallel to c-axis Many rectangular shaped etch-pits were generated at the grain surfaces. The corrosion proceeded from the side wall of the etch-pits and then, terrace fields were grown on the grain surface. Ridge-shaped microstructure was observed on the side wall of the terrace owing to the anisotropic corrosion.

High-temperature water vapor corrosion behavior of Lu 4Hf 3O 12 phase

High-temperature water vapor corrosion behavior of Lu 4Hf 3O 12 was studied at 1500 °C in 30 wt.% H 2O environment for 50 h. The rate of weight loss for this sample during the corrosion test was 1.347×10 −6 g/cm 2 h. On the surface of corrosion-tested specimen, a localized smooth surface was observed in some places and the grain boundary interface was covered by other phase that appears like string shape. The melted phase when measured by EDX method contained 37.2 mol% of Lu 2O 3 that was slightly lower than that of Lu 2O 3 content in Lu 4Hf 3O 12 stoichiometric composition. And the Lu 2O 3 content of the string like melted phase was as higher as 53.7 mol% revealing a Lu-rich composition. The X-ray diffraction analysis of the after corrosion-tested sample showed very small peaks corresponding to melted phase.

High Temperature Water Vapor Corrosion Behavior of Titanium Aluminate (Al2TiO5)

Water vapor corrosion resistance of titanium aluminate (Al 2 TiO 5 ) was examined at 1500°C in 30 wt% H 2 O flow. The weight loss rate of this substance was 8.6×10 -6 g/cm 2 .h during a 100-h test. No phase change occurred during the corrosion test, but anisotropic corrosion behavior was observed. In the X-ray diffraction patterns of the sample surface following the corrosion test, the relative intensities of 230 and 420 peaks were drastically decreased compared with that of the sample prior to corrosion. Geometrical patterns as like to sand ripples or terraced fields were observed on the sample surface after testing corresponding to the anisotropic water vapor corrosion.

A study of ion plating parameters, coating structure, and corrosion protection for aluminum coatings on uranium

A study of ion-plating parameters (primarily deposition rate and substrate bias voltage), coating structure, and the corrosion protection provided by aluminum coatings on uranium is presented. Ion plating at low temperatures yields a variety of aluminum coating structures on uranium. For example, aluminum coatings produced at high deposition rates and low substrate bias voltages are columnar with voids between columns, as expected for high-rate vapor deposition at low temperatures. On the other hand, low deposition rate and high bias voltage produce a modified coating with a dense, noncolumnar structure. These results are not in agreement with other studies that have found no relationship between deposition rate and coating structure in ion plating. This discrepancy is probably due to the high deposition rates used in these studies. An accelerated, water vapor corrosion test indicates that the columnar aluminum coatings provide some corrosion protection despite their porous nature; however, the dense noncolumnar coatings provide significantly greater protection. These results indicate that ion-plated aluminum coatings produced at low deposition rates and high substrate bias voltages creates dense coating structures that are most effective in protecting uranium from corrosion.

Advances in the Development and Description of Tritium Permeation Barriers in High-Temperature Alloys

Recent experimental findings on tritium permeation barriers are described with special emphasis on the interpretation in terms of mechanical behavior. Kinetic measurements of the water vapor corrosion reaction with Incoloy-800 have been performed first by determining hydrogen production and permeation rates on line. Growth laws of the oxide scales have been determined indicating that a visually parabolic phase can be attributed to a scale of enhanced impeding effect against permeation. A certain amount of the hydrogen created by the corrosion reaction permeates spontaneously through the metal at a fraction varying between 1 and 10%.A new quality of oxide layer has been identified that can be characterized by enhanced activation energies for hydrogen permeation of ∼150 kJ/mol as well as a modified pressure dependence proportional p1 in a limited range. Such scales show improved impeding factors ≫ 100. Moreover, the effect of an additional layer on the opposite side of the tube specimen has been studied that shows a different impeding behavior dependent on the direction of the hydrogen/tritium flow. A model has been discussed describing the impeding effect of oxide scales in terms of surface controlled reaction steps rather than bulk diffusion, as has been the usual procedure hitherto. The model proposed offers a qualitative understanding of experimental findings characterizing high-quality layers.Acoustic emission and hydrogen permeation measurements as indicators for cracking have been combined in an attempt to define oxide layer qualities with respect to behavior during temperature cycling. Three alloys with different damage rates have been investigated. The results of such experiments seem to offer the possibility of a quantitative correlation between the intensity of the acoustic emission signal and the loss of impeding effect due to cycling.

Static Fatigue of Glass. I

Water vapor corrosion of a simple soda-lime glass has been studied in regard to its effect on static fatigue of the same glass. A mechanism of dissolution has been proposed in which alkali ion self-diffusion controls the initial steps of water corrosion and leads to breakdown of the glass network. Since experiments show that an expansion of a glass network enhances corrosion rate, it is postulated that asymmetrical contitions of expansion around a surface flaw, brought about by applied stress, could lead to growth of the flaw in a preferential direction to bring about delayed failure.