CO2 High Temperature Corrosion and Its Prevention of Chromia Forming Fe-base Alloys

High temperature corrosion of chromia forming Fe-base alloys by CO_2 produces not only oxidation but also carburisation. The corrosion kinetics in CO_2-rich gas is found to be increased compared with that in air or oxygen. As a result, higher alloy chromium levels are required to achieve protective chromia formation in CO_2. Corrosion reaction mechanisms in CO_2 are examined and the internal carburisation of alloys in low carbon activity CO_2 gases are analysed based on the variation of p_(O_2) at the interface of oxide and metal. Carbon penetration through chromia oxide scale has been revealed by atom probe tomography. The strategies to resist CO_2 corrosion are reviewed by alloying of Si and/or Mn, forming additional diffusion barrier layers, and by adding sulphur to modify oxide grain boundaries to reduce carbon diffusion along the grain boundaries.     

Metal dusting behaviour of several nickel‐ and cobalt‐base alloys in CO?H2?H2O atmosphere

The metal dusting behaviour of total 11 nickel‐ and cobalt‐base alloys at 680 °C in a gas of 68%CO? 31%H₂? 1%H₂O (a_C = 19.0, = 5.4 × 10^?25 atm) was investigated. All samples were electropolished and reacted in a thermal cycling apparatus. On the basis of their reaction kinetics, these alloys can be classified into three groups: the first, with rapid carbon uptake and significant metal wastage, consists of alloys of relatively high iron content (AC 66, 800H and NS‐163); the second, with intermediate rates, consists of some Co‐base alloys (HAYNES 188, HAYNES 25 and ULTIMET) and the third, with very low reaction rates, consists of nickel‐base alloys with high chromium levels (601, HAYNES HR 160, 230, G‐35 and EN 105). An external chromia scale protected group 3 alloys from carburization and dusting. However, this protective scale was damaged and not rehealed for group 1 and group 2 alloys, allowing carbon attack. In all cases, coke deposited on the surface with two typical morphologies: filaments and graphite particle clusters. Subsurface spinel formation in high iron‐content alloys led to rapid dusting due to the significant volume expansion. Alloy carbon permeability was calculated from a simple law of mixtures, and shown to correlate reasonably well with initial dusting rate except for one cobalt‐base alloy in which iron spinel formation was significant.     

Evaluation of localized corrosion phenomena with electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ANA)

The use of electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA) for non-destructive evaluation of corrosion processes is illustrated for three model systems. EIS can be used to detect and monitor localized corrosion of Al alloys and determine pit growth laws which can be used for lifetime prediction purposes. Electrochemical potential and current noise data can be analyzed in the time and the frequency domain. A comparison of noise data obtained for Pt and an Al 2009/SiC metal matrix composite (MMC) exposed to 0.5 N NaCl has shown that the use of potential noise data alone can lead to erroneous conclusions concerning corrosion kinetics and mechanisms. The electrochemical noise data have been evaluated using power spectral density (PSD) plots in an attempt to obtain mechanistic information. The system Fe/NaCl has been used to determine the relationship between the polarization resistance R_p obtained from EIS data and the noise resistance R_n determined by statistical analysis of potential and current noise data. Potential and current noise can be recorded simultaneously allowing construction of noise spectra from which the spectral noise resistance R can be obtained as the limit for zero frequency. Good agreement between R_P, R_n and R has been observed for iron exposed to NaCl solutions of different corrosivity. For polymer coated steel exposed to 0.5 N NaCl for five months analysis of EIS data allows to draw conclusions concerning the degree of disbonding of the coating and the decrease of the coating resistivity with exposure time. R_n and R obtained from electrochemical noise data for an alkyd coating on cold rolled steel agree with each other and show the same time dependence as R_p and the pore resistance R_po determined from EIS data, but are significantly lower than R_p and R_po. The relationships of derived noise parameters such as R_n and R to coating properties and to the remaining lifetime of a polymer coating are not clear at present.     

Corrosion of high temprature alloys in the primary circuit helium of high temperature gas cooled reactors. – Part I: Theoretical background

The interaction of Ni and Fe-Ni base alloys with the reactive impurities H₂O, CO, H₂ and CH₄ in simulated cooling gas of the primary circuit of the High Temperature Gas Cooled Reactor (HTGR) causes corrosion effects that can significantly influence the mechanical properties. Apart from the formation of surface scales (oxides, carbides or mixed oxides/carbides), structural changes of the alloys are observed; depending on gas composition, gas supply rate and test temperature, carburization or decarburization can occur. In this report it is shown that an interpretation of the basic corrosion effects is possible on the basis of a modified stability diagram for chromium provided that - the kinetics of elementary gas metal reactions are incorporated in the expressions for carbon activity and oxygen partial pressure of the atmosphere and - the gradients of the potentials across the surface scales are taken into account. The interpretation allows the derivation of the corrosion behaviour of NiCr-base alloys in different HTGR helium compositions and enables the limits for the formation of protective chromia surface scales to be given. The influence of alloying elements other than chromium can be explained qualitatively. The results can be transferred to other reactive gas mixtures, which are characterized by an oxygen partial pressure near to the dissociation pressure of the scale forming oxides.     

Corrosion behavior of Fe-20Cr and Ni-20Cr alloys in Ar-H2O-HBr gas mixtures at 1000 K

Discontinuous mass-change measurements and corrosion-product analyses were made for Fe-20Cr and Ni-20Cr alloys after exposing them to Ar-H ₂ O-HBr gas mixtures at 1000 K for 24 hours. Predominantly chromia scales formed on both alloys. Upon cooling, the scales remained adherent to the Fe-20Cr alloy but spalled extensively from Ni-20Cr samples. After tests in HBr-rich gas mixtures, bromine-rich corrosion products were found underneath chromia scales on both alloys while nickel evaporation was observed from Ni-20Cr samples. Preoxidation of the Ni-20Cr alloy prior to exposure to Ar-H ₂ O-HBr gas mixtures increased chromia scale adherence but did not prevent nickel loss from the alloy. Chromia scales formed on the Fe-20Cr alloy were more protective due to the absence of iron oxides in the scale. Large NiO crystals formed over the Ni-20Cr alloy decreased chromia-scale adherence and increased nickel loss from the alloy due to the low stability of NiO in HBr-containing gas mixtures.     

Determination of the polarization resistance from impedance measurements

Four different methods for determining the polarization resistance R from impedance data are discussed. These methods are suitable for online corrosion monitoring. Their use is illustrated for iron in tapwater and in neutral, aerated Na₂SO₄ containing various inhibitors. R-values obtained with the CIRFIT-method are compared with R which is obtained from a linear sweep through E_corr. The integration method has the advantage of computation speed.     

Corrosion of Alloy Haynes 230 in High Temperature Supercritical Carbon Dioxide with Oxygen Impurity Additions

The corrosion of Ni-based alloy Haynes 230 in supercritical carbon dioxide at temperatures of 650 and 750 °C at a pressure of 20 MPa was investigated. In high-purity research grade CO₂, the corrosion performance of the alloy was excellent with a thin, uniform, protective chromium-rich oxide layer forming on the surface. Introduction of 10 and 100 ppm O₂ impurity in the CO₂ environment noticeably enhanced oxidation with evidence of oxide spallation and nodule formation. In these oxygen impurity added tests, increased oxidation led to subsurface voids due to the more rapid outward diffusion of chromium as well as intergranular alumina and chromia. The oxygen concentration at the inlet and the outlet of the autoclave was measured and used to support the results of characterization of the surface oxide to develop a more holistic understanding of the role of oxygen impurity on the corrosion process. In all cases, there some carbon was observed, which manifested as slightly higher concentration of chromium–carbide phase at the grain boundaries compared to the unexposed alloy.     

Evidence for Chromium Evaporation Influencing the Oxidation of 304L: The Effect of Temperature and Flow Rate

The influence of temperature and flow rate on the oxidation of 304L steel in O₂/H₂O mixtures was investigated. Polished samples were isothermally exposed to dry O₂ and O₂+40% H₂O at 500–800°C at 0.02–13 cm/sec flow velocity, for 168 hr. The samples were analyzed by gravimetry, XRD, ESEM/EDX, and AES depth profiling. The oxidation of 304L in water vapor/oxygen mixtures at 500–800°C is strongly influenced by chromium evaporation. The loss of chromium tends to convert the protective chromia-rich oxide initially formed into a poorly protective, iron-rich oxide. The rate of oxidation depends on flow rate; high flow rates result in an early breakdown of the protective oxide. The most rapid breakdown of the protective oxide occurs at the highest temperature (800°C) and the highest gas flow (4000 ml/min=13 cm/sec). The oxide formed close to grain boundaries in the metal is more protective, while other parts, grain surfaces suffer breakaway corrosion. The protective oxide consists of a Cr-rich 50–200-nm thick M₂O₃ film, while the parts experiencing breakaway corrosion form a 10–30-m thick Fe-rich M₂O₃/M₃O₄ scale. The results show that chromium evaporation is a key process affecting the oxidation resistance of chromia formers and marginal chromia formers in O₂/H₂O mixtures.     

Formation of corrosion pockets in FeNiCrAl at high temperatures investigated by 3D FIB-SEM tomography

A recently published study of high temperature nitridation of iron chromium aluminum alloys (FeCrAl) at 900°C in N₂–H₂ has redundantly shown the formation of locally confined corrosion pockets reaching several microns into the alloy. These nitrided pockets form underneath chromia islands laterally surrounded by the otherwise protective alumina scale. Chromia renders a nitrogen-permeable defect under the given conditions and the presence of aluminum in the alloy. In light of these findings on FeCrAl, a focused ion beam–scanning electron microscope tomography study has been undertaken on an equally nitrided FeNiCrAl sample to characterize its nitridation corrosion features chemically and morphologically. The alloy is strengthened by a high number of chromium carbide precipitates, which are also preferential chromia formation sites. Besides the confirmation of the complete encapsulation of the corrosion pocket from the alloy by a closed and dense aluminum nitride rim, very large voids have been found in the said pockets. Furthermore, metallic particles comprising nickel and iron are deposited on top of the outer oxide scale above such void regions.     

Evaluation of corrosion inhibitors for cooling water systems operating at high concentration cycles

The present work aimed at evaluating AISI 1020 carbon steel corrosion resistance of a 6:4:1:1 (MoO/HEDP/PO/Zn²⁺) inhibitor mixture present in a solution which simulates an industrial cooling water system operating at high concentration cycles (1050 ppm Cl⁻ and 450 ppm Ca²⁺). High concentration cycles are desirable, because system purge and treated water consumption are decreased. On the other hand, a high number of concentration cycles can increase the concentration of salts and dissolved impurities, causing corrosion, incrustations, and deposits inside the pipes, heat exchangers, and cooling towers. Thus, the chloride (Cl⁻) and calcium (Ca²⁺) ions aggressiveness was studied on the proposed inhibiting mixture, at the temperatures of 40 and 60 °C, through electrochemical techniques like open circuit potential measurements, anodic and cathodic polarization, and weight loss. The results showed that the inhibitor mixture conferred adequate protection to carbon steel in low concentrations, even in high aggressive media.     

Effect of Si on the Corrosion behaviour of Fe-B and Fe-Ni-B glassy alloys

The effect of substituting Si for B on the electrochemical behaviour of the glassy alloy families Fe-B and Fe-Ni-B has been investigated in solutions with pH ranging from 4 up to 8.4. The influence of the aggressive ions (Cl^? and SOu) has also been studied. AES and XPS have been used to further characterize the alloys in order to interpret their behaviour in the various solutions.     

Carburization of high temperature PM-materials

Carburization of powder metallurgically processed materials for high temperature use is interesting for several reasons: Production of carbide powders e.g. for hard metals, formation of carbide layers on bulk materials to improve corrosion and/or wear resistance and degradation of material properties like strength and ductility by metal-carbon reactions. Molybdenum, TZM (Mo-0.5Ti-0.07Zr-0.05C) and tungsten are finding wide application as construction material in high temperature furnaces which are operated under high vacuum or inert/protective gas conditions. If grafite is used in the same system reactions of the refractory metal components with carbon containing species have to be considered. Therefore a variety of examinations was performed on the carburization characteristics of molybdenum and tungsten (and alloys), especially at temperatures above 1200°C where carburization rates become technically relevant. High temperature oxidation resistant alloys like steels or Ni- and Co-based superalloys can withstand severe carburization as long as their surfaces are covered with tight, protective chromia scales. In case of porous or cracked scales or conditions where chromia is not stable anymore a front of carburization proceeds through the materials – frequently along the grain boundaries. The PM-materials Ducropur (pure chromium) and PM 2000 (Fe-19Cr-5.5Al-0.5Ti-0.5Y₂O₃) show distinctly lower carburization rates: Ducropur forms tight chromium carbide layers, whereas PM 2000 is nearly unaffected up to 1100°C because of its tight and stable alumina scale.     

Eigenschaften von Elektrolyt-Filmen bei der atmosphärischen Korrosion

Properties of electrolyte films formed through atmospheric corrosion An investigation has been carried out into the composition of the electrolyte films which are formed on non-metallic materials (glass) as well as on metals (Cu, Zn, Fe) in an atmosphere containing SO₂. Fresh as well as pre-corroded specimen were used. It was found that the SO₂ absorbed in the solution is very rapidly oxidized into SO if the electrolyte film contains dissolved particles of the corrosion products. With 1 to 55 ppm SO₂ in the atmosphere, the change in the pH value of the electrolyte is but small and does not vary with the SO₂ partial pressure.     

Carburization of Cr-based ODS alloys in SOFC relevant environments

Chromium based ODS alloys are being discussed as interconnector materials in solid oxide fuel cells (SOFC's). One of the major requirements for the interconnect material is sufficient corrosion resistance in the anode and cathode side gases at the operating temperature of around 950°C. In the present study the corrosion behaviour of a number of chromium based ODS alloys has been investigated in carbon containing simulated anode gas of an SOFC. Under these experimental conditions all alloys studied are prone to carburization. The carbon uptake, which mainly occurs in the early stages of exposure, leads to the formation of a mixed carbide/oxide layer beneath the external chromia scale. The carburization resistance of the alloys depends on the type and concentration of the oxide dispersion but even to a larger extent on its distribution. Therefore the careful control of the alloy manufacturing process is of great significance for obtaining sufficient carburization resistance under SOFC conditions.     

Korrosionsverhalten von Gasturbinenwerkstoffen unter strömenden Heißgasbedingungen

Corrosion behaviour of gas turbine alloys under high velocity burnt fuels The aim of alloy development in the field of nickel based superalloys for flying and land based gas turbines is to enhance significantly the mechanical properties at high temperatures thus leading to a higher temperature capability. The higher temperature capability of the structural elements of gas turbines results in an increased efficiency, a lowered fuel consumption and less emissions. To achieve an increased high temperature capability, however, surface degradation of the material must be adjusted adequately, hence corrosion resistance has to be improved. Additional to the isothermal and cyclic oxidation tests which are performed in stagnant air the oxidation behaviour of alloy 2100 GT and alloy C‐263 was investigated by means of burner‐rig‐experiments under high velocity burnt fuels. In the burner rig test facility the sample is exposed to a hot gas stream of burned natural gas with gas velocities in the range of 60 m/s to 150 m/s. The metal temperature of the sample can be adjusted in the range of 900°C to 1200°C. In the tests described in this paper the gas velocities were chosen to be 60 m/s, 100 m/s and 140 m/s. The test duration was 1 h and 10 h. The test temperature was kept constant at 1000°C. After 1 h of testing both alloys showed mass gain which was significantly higher for alloy C‐263. After 10 h of testing the mass loss of alloy C‐263 was enhanced with increasing gas velocity. Alloy 2100 GT showed only at the highest gas velocity a mass loss. The examinations by means of SEM and light‐optical microscopy of the oxide scale and of the microstructure showed that alloy 2100 GT has a dense adherent alumina scale and suffers no internal oxidation even under burner‐rig‐test conditions. Alloy C‐263 forms a mixed chromia and Cr‐Ti‐mixed oxide scale. The chromia is evaporated with increasing gas velocity, leaving (Cr‐Ti)O₂‐needles on the surface. In the isothermal and cyclic oxidation tests alloy 2100 GT shows an excellent oxidation behaviour up to 1200°C with a corrosion rate of less than 0.1 mm/a. The aluminium content of app. 3 wt.‐% which is remarkably high for a wrought alloy leads to the formation of a thin dense and adherent alumina scale. Alloy C‐263 is a chromia former which is not suitable for temperatures higher than 1000°C.     

Influence of the sulphate reducing bacteria on API‐X70 steel corrosion

The corrosion behaviour of API X70 immersed in a specific medium with a strain of thermophilic sulphate reducing bacteria (SRB) was analysed. Anaerobic corrosion test was carry out for 32 days at 50 °C. During the exposure time, pH, sulphate (SO) and hydrogen sulphide (H₂S) concentration were measured. Corrosion potential, linear polarization resistance and potentiodynamic polarization curve were used in order to get the influence of the SRB in the corrosion phenomenon. Scanning electron microscopy was used to determine corrosion morphology. Results show that the SRB activity influenced the overall corrosion process. The anodic branches in the polarization curves show a passivity feature, whereas, the cathodic branches were not affected. A localized corrosion attack was found.     

The air oxidation of two-phase Cu-Cr alloys at 700–900°C

The oxidation in air of three two phase Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt. % was studied at 700–900°C. The alloys corroded nearly parabolically, except at 900°C, when the corrosion rates decreased with time more rapidly than predicted by the parabolic rate law. The corrosion rate decreased for higher Cr contents in the alloy under constant temperature and generally increased with temperature for the same alloy composition. The scales were complex and consisted in most cases of an outermost copper oxide layer free from chromium and an inner layer composed of a matrix of copper oxide or of the double oxide Cu₂Cr₂O₄, often containing particles of chromium metal surrounded by chromia and then by the double oxide. Metallic copper was also frequently mixed with chromia. Cr-rich regions tended to form continuous chromia layers at the base of the scale, especially at the highest temperature. No chromium depletion was observed in the alloy.     

Grain size effects on the oxidation of two ternary Cu-Ni-20wt.% Cr alloys at 700-800 °C in 1 atm O2

Two nanocrystalline two-phase Cu-Ni-Cr alloys, both prepared by mechanical alloying and containing about 20 at.% Cr but with different Ni contents (40 and 20 wt.%, respectively), have been oxidized in 1 atm O₂ at 700-800 °C. Their oxidation behavior has been compared with that of two cast alloys of the same composition, already studied previously, to examine the effects of a large reduction of the size of the individual phase grains and particles. The nanophase alloy with 40 wt.% Ni formed a flat external layer of chromia of regular thickness, while the corresponding cast alloy produced a very irregular chromia layer, often protruding deeply into the alloy, only after an initial stage of rather fast corrosion involving also copper and nickel, associated with some degree of internal oxidation. By oxidation at 700 °C the nanophase alloy with 20 wt.% Ni formed an irregular chromia layer associated with low corrosion rates. The corresponding cast alloy formed complex scales containing Cu, Ni and Cr oxides, extending into the alloy in the form of large pegs, even though a very irregular and discontinuous innermost chromia layer was still able to produce low corrosion rates. On the contrary, at 800 °C both alloys formed complex scales containing mixtures of the oxides of the three metal components. However, the scales grown on the cast alloy were much more irregular in thickness and formed large protrusions into the alloy. In spite of this, the corrosion kinetics of the nanophase 20 wt.% Ni alloy at 800 °C were more irregular and, except for an initial stage, less protective than that of the cast alloy with the same composition.     

Inhibierung der Lochbildung und des Lochwachstums bei Aluminium

Inhibition of pit initiation and pit growth on aluminum Under potentiostatic conditions over 40 organic and inorganic compounds have been investigated in order to determine their inhibiting effect on the pitting corrosion of aluminum in 0.01 molar NaC1 solutions. The concentration dependent inhibitor efficiency decreases in the order: Na-oleate, NO ? WO > IO, MoO, BrO, Na-benzoate > CrO > NO, Na-tartrate. SO, ClO and the other organic compounds tested accelerate the pit growth as well as too low concentrations of inhibiting ions. The accelerating effect depends directly on the conductivity of the electrolyte. The inhibitor efficiency depends on the potential and is deteriorated by non inhibiting ions such as ClO or SO. The minimum concentration required to inhibit completely the pit initiation and the pit growth is often not identical. This fact explains partly the inconsistent data found in the literature. The phenomena observed are explained on the basis of the Temkin isotherm.     

Adsorption studies of water on copper,nickel, and iron using the quartz-crystal microbalance technique: Assessment of BET and FHH models of adsorption

In the atmospheric corrosion of copper, nickel, and iron, the adsorption of water affects the corrosion rates. Knowledge of water adsorption and metal oxyhydroxide formation is important in understanding the atmospheric corrosion process. The purposes of the present research were (1) to measure the adsorption of water on metal surfaces as a function of temperature and relative humidity (RH) and (2) to assess Brunauer-Emmett-Teller (BET) model and Frenkel-Halsey-Hill (FHH) model for water adsorption. In the present research, the quartz-crystal microbalance (QCM) technique was used to measure the mass changes of copper, nickel, and iron at 0 to 100% RH and 7–90°C under nitrogen environments. Less water was adsorbed on copper, nickel, and iron which formed oxides than on gold. BET and FHH models could not fit the data points with single functional relationships. ΔH values were calculated using modified BET method and they decreased with temperature.