High-temperature corrosion protection of radiation ducting

PO ZIL, I. P. Bardin TsNIICHERMET. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 39–41, February, 1990.     

High-temperature sulfide corrosion of cobalt-chromium alloys

The kinetics and mechanism of high-temperature sulfidation of cobaltchromium alloys were investigated. It has been found that the sulfidation rate of low-chromium alloys, up to 2% Cr, is higher than that of pure cobalt. The one-layer homogeneous scale on these alloys is a solid solution of chromium sulfide in cobaltous sulfide. In the case of alloys containing more chromium, up to 43%, sulfidation rate decreases gradually with chromium content, the scale consisting of two layers. The growth mechanism of scales as well as the influence of chromium concentration on sulfidation rate is discussed.This work was carried out under contract No. 03.10 coordinated by the Institute of Physical Chemistry, Polish Academy of Sciences.     

High-temperature sulfur corrosion of iron-chromium alloys

The kinetics and the mechanism of sulfurization of Fe-Cr alloys containing from 0.35 to 74 at. % Cr were studied in the temperature range 700–1000°C. The sulfurization was conducted in sulfur vapor at atmospheric pressure. The reaction rate was determined by the continuous gravimetric method. The scale composition was studied by X-ray and electron microprobe methods. The contribution of the reactants to the process of material transport through the scale was examined with both the marker method and the autoradiographic method using the radioactive isotope³⁵S. It has been found that, irrespective of alloy composition and temperature, the sulfurization follows a parabolic rate law. On the dilute alloys (up to about 2 at. % Cr) the scales formed are monophase consisting of Fe_1–xS only. In the range 2–40 at. % Cr the scale is a heterophase mixture of Fe_1–xS and the mixed spinel FeFe_2–xCr_xS₄. On the chromium-rich alloys the scale is monophase and is built of a solid solution of FeS in Cr₂S₃. The scale growth on the alloys under examination proceeds without the participation of the inward diffusion of sulfur. The mechanism of the scale formation has been proposed.     

High-temperature corrosion of dilute chromium-lanthanum alloys

The kinetics of oxidation in air of chromium-lanthanum alloys have been investigated in the temperature range 1100–1400°C. The positive effect of lanthanum on the heat resistance of chromium was established and is explained as a result of the formation of a barrier oxide film consisting of Cr₂O₃ and LaCrO₃. The dispersed particles of lanthanum chromite distributed on grain boundaries form diffusion barriers that change the oxidation law from parabolic to logarithmic. An empirical equation which quantitatively describes masstransport processes with decreasing effective diffusion area and simultaneous sublimation of scales is proposed.     

The stability of localized corrosion

Chloride pitting of iron group metals at noble potentials proceeds in the polishing state dissolution, provided that metal chloride in the pit solution is maintained above a critical concentration. It ceases to progress by pit repassivation if the pit is smaller than a critical size, or transforms into the active state pitting if the pit size is greater. The boundary potential between the polishing state and the active state pitting may be represented by the passivation-depassivation potential in the pit solution of the critical chloride concentration. Crevice corrosion is characterized by the crevice protection potential, at which the hydrogen ion concentration in the crevice solution is equivalent to pH_pd—the passivation-depassivation pH of the crevice metal. It continues to corrode at more noble potentials than the protection potential, where the crevice solution is more acidic than pH_pd, but is inhibited in the less acidic crevice solution at less noble potentials.     

High-temperature corrosion in mixed gas environments

Scaling reactions between pure metals and multiple oxidant gases are reviewed briefly. It is recognized that elemental oxidant activities are usually so low that the actual reactant species are heteronuclear molecules such as SO₂, CO₂, etc. The formation of duplex, sulfide-oxide scales on iron and manganese, even when sulfide is unstable with respect to oxide, is attributed to direct reaction with SO₂. The persistence of the metastable sulfide is due to its preservation by the rapidly growing scale. The reaction of pure chromium with a number of mixed gases is also discussed. The continued formation of carbides and nitrides beneath an external Cr₂O₃ scale layer indicates that the latter material is permeable to gas species. Interaction among different gas species is observed, and is attributed to selective adsorption on internal surfaces within the chromium oxide. New work on the reaction of alloys with mixed gases is reported. Several austenitic heat-resistant alloys were exposed at 1000°C to gases containing one, two or all of the oxidants carbon, sulfur and oxygen. Gases containing two or more oxidants produced multiple zones of internal precipitation. The precipitates were chromium-rich oxides, sulfides and carbides arranged in order of thermodynamic stability: oxides beneath the external scale, carbides deepest within the alloys and sulfides in an intermediate zone overlapping the oxide zone. Each precipitate zone widened according to parabolic kinetics. This finding confirms the as yet untested prediction made by J. L. Meijering in 1971. However, the rate at which a particular zone grows changes according to presence of other oxidants. Interactions between the oxidants can be large and reaction rates are currently not predictable.     

High-temperature corrosion of nickel in SO2

The reaction of nickel with SO₂ has been studied in the temperature range 650–1100°C at SO₂ pressures from 10 to 760 Torr. Reaction kinetics have been studied by thermogravimetry; the reacted specimens have been characterized by means of metallography, scanning electron microscopy, and electron microprobe analysis. The reaction involves oxidation and sulfidation except at sufficiently high temperatures and low pressures of SO₂ (e.g., 1000°C and 10 Torr SO₂) where only formation of NiO takes place. Approximately linear reaction kinetics are observed between 650 and 900°C. Reaction mechanisms are discussed, and the relative importance of oxide formation and sulfidation is interpreted in terms of the thermodynamics of the Ni-O-S system.     

High-temperature corrosion performance of plasma-sprayed CrNiMoSiB coatings

This study examined the effects of application parameters for plasma spraying and CCh- laser glazing of two types of chromium-base coatings. Coatings were deposited by low-pressure and atmospheric plasma spraying. The high-temperature corrosion resistance of Cr-Ni-2.5Mo-lSi-0.5B (55 and 58% Cr) coatings was evaluated with respect to structural and compositional changes both in the as- sprayed condition and after CCb- laser glazing. Coatings that were deposited by atmospheric plasma spraying and subsequently laser glazed showed excellent resistance to oxidation and sulfate- vanadate attack at 900 °C due to the formation of a protective chromia film and a high silica concentration on the top layers of the oxide.     

High-temperature corrosion: Issues in alloy selection

This article examines the modes of high-temperature corrosion that are often responsible for equipment failures in a variety of industries, including aerospace and gas turbines; heat treating; mineral and metallurgical processing; chemical processing; refining and petrochemical processing; ceramic, electronic, and glass manufacturing; automotive; pulp and paper; waste incineration; and power generation and energy conversion. Corrosion data related to each corrosion mode are reviewed to provide readers with a brief materials selection guide.     

High-temperature corrosion of NiCrAl alloy in oxygen-chlorine mixtures

In order to decrease the attack of chlorine in O₂? Cl₂? N₂ mixtures to NiCr alloys at high temperatures, the corrosion of a preoxidized Ni 20 Cr 4 Al alloy was investigated in N₂? O₂ mixtures with 5 to 150 torr O₂ and 20 torr Cl₂ at 800°. For the generation of a protective layer of Al₂O₃ both the untreated and Al evaporated alloy sample was preoxidized in a H₂? 20 torr H₂O atmosphere at 900° C. As was found, the catastrophic corrosion by Cl₂ in a N₂? Cl₂ gas mixture with 20 torr Cl₂ could not be prevented by an Al₂O₃ layer. However, in the simultaneous presence of an oxygen content in the corroding gas mixture not lower than 10 torr, the attack of chlorine could be drastically decreased. The presence of 50 torr Cl₂ postulates an oxygen pressure which is technically unrealistic. Since the formed oxide layer spalled off during the cooling period, the Ni 20 Cr 4 Al alloy cannot be used as material for cyclic oxidation. To what extent ThO₂ particles dispersed in the alloy can prevent a spalling effect of the oxid film is at present an open question.     

High-temperature corrosion of manganese in pure SO2

The corrosion of manganese in 1 atm of pure SO₂ has been studied from 700 to 900°C. The reaction shows two approximately parabolic stages, the first being faster; the reaction products are MnO and MnS. In the early moments of the reaction a fine mixture of oxide and sulfide is formed directly at the scale surface, which is not in equilibrium with the gas phase due to rate control by the surface reaction. This process ends quickly due to the lowering of the Mn activity at the scale surface, and then an outer region of MnO surmounting a dispersion of coarse MnS particles in the oxide develops. The rate control shifts to solid-state diffusion, and the scale surface equilibrates with the gas. The sulfide is produced in this stage in the interior of the scale as a consequence of sulfur penetration by two mechanisms whose relative importance is discussed. The reaction produces a protective scale, and the rate is close to that for oxidation of Mn in 1 atm O₂ at the same temperature.     

A new perspective on measuring the corrosion rate of localized corrosion

Many corrosion phenomena are nonuniform, which means that anodic and cathodic locations are spatially separated. An example is macrocell corrosion of steel in concrete. Under these conditions, determining the corrosion rate from polarization resistance measurements and using the Stern–Geary equation is fundamentally not possible. We present a novel theoretical approach for the interpretation of galvanostatic pulse measurements, to make them applicable as a method for corrosion rate measurements in situations of localized corrosion. Experiments show that it is important to consider that (a) only a fraction of the applied current flows through the anode of the macrocell, and (b) this current is not constant over time. We propose an approach to quantify and consider these two effects, based on information generally accessible in condition assessment of concrete structures. Our results show that galvanostatic pulse measurements are a robust method to determine the corrosion current. With the traditional empirical approach, the measurement error was generally below factor 3, and occasionally up to factor 10. With the novel approach, this error could be reduced to a factor of maximum 2 in all cases.     

The localized corrosion of Cr-Ni stainless steel

The kinetics of localized corrosion of Cr-Ni stainless steel is characterized by the quadratic time dependence of the total current density, by the linear increase in the number of pits and by the decrease of the current density in the pits. The pits have a shape of a rotary ellipsoid. The Tafel slope of the metal dissolution in the pits is 0.50 V. The dissolution rate is the highest on the pit bottom and the lowest at the mouth of the pit. The different rate of metal dissolution is caused by the different concentration of chloride ions over the pit surface.     

High-temperature corrosion of low Cr-Fe alloys in sulfur vapor

The sulfidation behavior of low Cr-Fe alloys was studied over the range of 700–900°C under 1 atm of pure sulfur vapor. Sulfidation of alloys with 3.7 and 7.4 wt.% Cr does not follow the parabolic rate law at 750–800° C, although for other alloy compositions and temperatures it obeys that law. Sulfide scales consisted of a three-layer structure. The outer layer was FeS, but the inner and intermediate layers contained FeS, FeCr₂S₄, and Cr₃S₄, their relative amounts and morphologies depending on the corrosion temperature and alloy composition. Increasing Cr contents resulted in a change in the morphology of FeCr₂S₄ from dispersed particles to a thick layer. Ultimately, FeCr₂S₄ was displaced by Cr₃S₄. The formation of FeCr₂S₄ and the scale growth mechanism are discussed also.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

High-temperature inhibitor of steel corrosion in sulfuric acid solutions

A new high-temperature (up to 140°C) inhibitor of steel corrosion in sulfuric acid, IFKhAN-92, that yields Z = 99% at a content of at least 1 wt % was developed. To enhance the protective effect of IFKhAN-92 at an elevated temperature, KI additions may be used. IFKhAN-92 was shown to be effective in hindering electrode reactions on steel. The high efficiency of this catalyst in H₂SO₄ solutions is most likely conditioned by the specific adsorption of IFKhAN-92 molecules on a metal surface.     

High-temperature corrosion of Sn-Bi-Zn-Ga alloys as heat transfer fluid

The new heat transfer alloy is highly reactive at high temperatures,and the corrosion of the container mate-rial determines the service life of the heat transfe...     

An ellipsometric approach to localized corrosion processes

Qualitative uses of ellipsometry, that can provide valuable insights into localized corrosion processes are reviewed. Without attempting to determine optical constants or film thickness with any accuracy, interesting data can be obtained, particularly correlations of passive film growth rates, as a function of localized corrosion factors, with the velocity of stress corrosion cracking or the rates of pit nucleation. Several applications of qualitative ellipsometry are described, relating to pitting and crevice corrosion, and stress corrosion cracking of carbon steels, stainless steels and titanium alloys.