Directions for future research in high-temperature corrosion

The future requirements for high-temperature corrosion research have been reviewed in light of current knowledge and the changing attitudes toward research ethos and industrial priorities. Medium-term requirements are relatively clear but the longer-term future, as always, is obscure. High-temperature corrosion engineering appears well placed for development in the current climate, with appropriate integration into design and systems engineering approaches. High-temperature corrosion science research is extensive worldwide, there is no shortage of interesting work to be done, nor enthusiasts to do it, but it would benefit from several really novel new areas, particularly in a competitive funding scenario which favors technology transfer and industrially orientated research. The authors discuss various areas from their own perspective but also draw attention to recent publications and workshops aimed at obtaining balanced or consensus views about the way ahead.     

Synthesis and high-temperature corrosion resistance of Ti-Zr-Si-N coatings deposited by means of sputtering

ABSTRACT

We present a study of the synthesis and an evaluation of the corrosion resistance of Ti-Zr-Si-N coatings deposited by means of the co-sputtering technique on 316L steel and Ti6Al4V alloy substrates. The thin films were obtained using Ti₅Si₂ and Zr targets with 99.9% purity for the deposition, and a power of 200 W was applied in the DC source and 170?W in the RF source. The corrosion resistance was evaluated via the cyclic oxidation test with consistent cycles of 1 hour of cooling, 1 hour of heating, and 16 minutes of stabilisation for 300 cycles at a temperature of 600°. The coatings were characterised by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and confocal laser microscopy. The coating was effective up to 150 cycles for the case of 316L steel, and up to 50 cycles for the Ti6AlV alloy.     

Interaction between erosion and high-temperature corrosion of metals: The erosion-affected oxidation regime

The combined erosion-oxidation of nickel, cobalt, and the oxides of these metals have been studied at 780°C in air to examine two regimes of interactionnamely, a regime of erosion-enhanced oxidation during which an oxide scale of constant thickness covers metal specimens, and a regime of oxidation-affected erosion that is characterized by a composite surface layer of metal, oxide, and erodent. In the case of cobalt, these two regimes have been documented and the transition from one regime to another described. For the range of conditions examined, only the oxidation-affected erosion regime was observed for nickel due to its lower oxidation rate compared to cobalt.     

Improvement of cyclic high-temperature corrosion resistance of pack-aluminized heat-resistant stainless steels

Modification of pack aluminizing for heat-resistant stainless steels was studied to improve corrosion resistance by controlling the microstructure of the coating layer. The major process parameters examined include the pack powder composition, coating time, and temperature. Depending on the combination of these parameters, the microstructure of the coating layer can be controlled to form either a continuous layer of internal-diffusion barrier (IDB) or an interdiffusion zone (IZ). At the coating-process temperatures, the IDB forms as a mixture of - and -aluminide, whereas the IZ forms as a mixture of -ferrite and -aluminide. But the phase shown in the IZ at room temperature is formed by transformation from the phase during cooling. Even though the hardness of the IDB is higher than the other phases present in the coating layer, the aluminide coating layer with the IDB shows outstanding cyclic high-temperature corrosion resistance. As long as the stable IDB forms, the corrosion resistance increases with the thickness of the aluminide-coating layer.     

Surface destructive mechanism on high-temperature ablation, supersonic-erosion, dreg-adherence and corrosion

The exhaust and flame from a supersonic airborne missile high-energy smoke-born engine (SAMHSE) may lead to high-temperature ablation, supersonic-erosion, dreg-adherence (HTASED) and corrosion on the launcher slide track, causing serious problems to the operation and decreasing the lifetime of the launcher. Therefore, it is imperative to study the destructive mechanism so as to guarantee the smooth operation and increase the lifetime of military equipments. Accordingly, HTASED and corrosion were systematically observed and analyzed with the emphasis placed on the mechanism investigations making use of a series evaluation tests, typical missile engine simulation tests, national military standard methods, scanning electron microscopy and electrochemical corrosion tests. It is found that the thermal impact of high-temperature flame and supersonic erosion of corrosive melting particle jet of the SAMHSE lead to surface defects of micro-cracks, denudation and corrosive residue. Some defects reach to metal base becoming to "corrosive channels". Repetitive HTASED may cause ablation-adhesion fatigue stress, which enhances the surface corrosion and destruction. HTASED and corrosion are related to the type of a SAMHSE fuel and experience of the launcher. Surface destruction is related to synergistic effects of the HTASED. The ablated and failed Al or steel surface is liable to electrochemical corrosion characterized by pitting in humid and salt-spray environment.     

Electrochemical monitoring of high-temperature molten-salt corrosion

Hot molten-salt corrosion can cause serious metal degradation in boiler plant, incinerators, and furnaces. In this research, electrochemical-impedance and electrochemical-noise techniques have been evaluated for the monitoring of hot-corrosion processes in such plants. Tests have been carried out on Ni-1 % Co and Alloy 800, a commercial material of interest to operators of industrial plants, utilizing a bulk molten-salt environment and also a simulated combustion test, where thin films of molten salt were established on the alloy surfaces. Electrochemical-impedance and electrochemical-noise data were compared with the results of metallographic examination of the test alloys and showed reasonable correlation between the electrochemical data and the actual degradation processes. Current-noise analysis gives valuable information on the initiation stages of hot corrosion, while impedance measurements can detect propagation or rapid corrosion of the base metal. This preliminary work indicated that the electrochemical techniques show considerable promise as instruments for the monitoring of high-temperature corrosion processes.     

Stress analysis for the high-temperature gaseous corrosion of metal tubes

Stress analysis for the high-temperature gaseous corrosion of metal tubes is presented. The mechanical stresses arising from the difference in thermal expansion coefficients and volumes of a metal and its oxide can result in cracking of the oxide scale and in an increase of the oxidation rate. The model interprets the different behavior of oxide scale growths on outer versus inner tube surfaces. The oxidation behavior of tantalum tubes at temperatures of 825 °C and 930 °C is calculated based on this analysis. Results of the calculations are in good agreement with observed experimental data.     

High-temperature corrosion and creep of Ni-Cr-Fe alloys in carburizing and oxidizing environments

The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH₄-H₂ mixtures in the temperature range 900–1100°C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M₇C₃ and M₂₃C₆ are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H₂O-H₂ mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000°C in a CO-H₂O-H₂ atmosphere where Cr₂O₃ is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.     

Hydrogen evolution and corrosion behavior of zirconium tubing in sub- and supercritical water

Hydrogen evolution is inevitable during the oxidation of zirconium in high-temperature water. A fraction of this evolved hydrogen diffuses into the cladding material, and the remaining is carried away by the reactor coolant. In this study, hydrogen evolution and corrosion behavior of zirconium-702 in high-temperature water are investigated using a continuous tubular flow-through reactor. The results show that at a constant pressure of 25 MPa, the evolution of hydrogen gas from an oxidized zirconium reactor surface is approximately 24 times larger at 500°C than at 350°C. At higher temperatures, the zirconium reactor tubing exposed to water shows ballooning, with bending before the rupture near the exit end of the reactor tube, where the concentration of evolved hydrogen is the highest.     

Investigations of the degradation of high-temperature alloys in a potentially oxidizing-chloridizing gas mixture

The degradation of high-temperature alloys in argon-5.5% oxygen-0.96% hydrogen chloride-0.86% sulfur dioxide at 900°C under isothermal and thermal cycling conditions has been investigated. All the alloys showed reasonable resistance under isothermal conditions, although the Al₂O₂ ***-forming material, alloy 214, gave the lowest amount of corrosion, consistent with Al₂O₃ being a more effective barrier than Cr₂O₃ to inward penetration of chlorine or sulfur-containing species from the environment. Significant internal corrosion was observed for some alloys. Degradation of all the alloys was much more severe under thermal cycling conditions because of the failure of the protective scales. In all cases, formation of volatile chlorine-containing compounds was observed. Degradation of the alloys resulted from the penetration of chlorine-containing species through the initially formed oxide scale and formation of chlorides or, possibly, oxychlorides at the alloy-scale interface or in the subjacent alloy. The sulfur dioxide did not play any obvious role in the process.     

Corrosion of a 316L stainless steel in a gaseous environment polluted with HCl: Mechanism

The corrosion of a 316L stainless steel (SS) exposed to a humid gas flow polluted with HCl has been studied. The mixture is carried out in a reactor connected to two gas feedthroughs: one with wet air and one with HCl(g). The corrosion mechanism comprehension is based on several steps. The presence of humid air polluted by HCl involves the creation of a precursor film that can evolve to droplet formation. In contact with this acid and chloride concentrated electrolyte, the 316L SS corrodes producing soluble ferrous chloride. This corrosion product can evolve to the oxide formation, depending of the HCl concentration. For high concentrations, 316L SS corrodes uniformly. However, this phenomenon is accompanied by pits when the HCl concentration is not sufficient or the HCl flow is not continuous. The particularity of the corrosion mechanisms is shown as well as the problems using materials in an HCl-polluted gaseous environment.     

Numerical modeling of high-temperature corrosion processes

Numerical modeling of the diffusional transport associated with high-temperature corrosion processes is reviewed. These corrosion processes include external scale formation and internal subscale formation during oxidation, coating degradation by oxidation and substrate interdiffusion, carburization, sulfidation and nitridation. The studies that are reviewed cover such complexities as concentration-dependent diffusivities, cross-term effects in ternary alloys, and internal precipitation where several compounds of the same element may form (e.g., carbides of Cr) or several compounds exist simultaneously (e.g., carbides containing varying amounts of Ni, Cr, Fe or Mo). In addition, the studies involve a variety of boundary conditions that vary with time and temperature. Finite-difference (F-D) techniques have been applied almost exclusively to model either the solute or corrodant transport in each of these studies. Hence, the paper first reviews the use of F-D techniques to develop solutions to the diffusion equations with various boundary conditions appropriate to high-temperature corrosion processes. The bulk of the paper then reviews various F-D modeling studies of diffusional transport associated with high-temperature corrosion.     

Post-test corrosion analysis of high-temperature thermal energy storage capsules

Thermal energy storage capsules have been freeze-thaw cycled in vacuum at 1000 ± 100 K. The capsules were fabricated from Inconel 617^® (Inco Alloys International, Inc.) and contained eutectic fluoride mixtures of sodium, magnesium, lithium, and potassium. Samples that were thermal cycled for 20,000 and 30,000 hr were analyzed for corrosion effects. Radiography indicated neither flaws nor inhomogeneities, and there were no significant microstructural changes in the container alloy. The microstructural damage penetrating the inside surface was as deep as 120 m and that penetrating the outside surface was approximately 150 m. Microprobe results on the containers revealed a concentration gradient of alloying elements. The aluminum concentration was reduced from 1.34% in the original matrix to 0.4% at 10 m from the inside surface, and chromium was reduced from a nominal value of 23% to 10% at the outer surface. The depletion of aluminum and chromium from the outer surface was due to vacuum vaporization at elevated temperature. X-ray diffraction revealed the formation of possible protective films consisting of MgNaF₃ and MgF₂. The measured and theoretically predicted concentrations of aluminum and chromium were in good agreement. It was concluded that the corrosion process is a solid-state diffusion-dominated process and an expected lifetime of 5 to 7 years is a reasonable estimate.     

Investigations of the aqueous corrosion of pretreated magnesium alloys by means of electrochemical noise measurements

An important possibility to improve the corrosion behaviour of magnesium alloys is the application of protective coatings. The quality of such coatings depends mainly on the pretreatment and the exposure conditions after pretreatment. Since magnesium surfaces change much faster under atmospheric conditions than those of almost any other technical material, it is necessary to pay special attention to this particular feature. The activity of acid‐pickled surfaces of the magnesium alloys AZ31 and AZ91 in dependence on the exposure time and the humidity conditions was investigated with electrochemical noise (EN) measurements. In addition to pickling, plasma chemical vapour deposition processes open new possibilities for an economical, as well as ecologically quite safe, pretreatment. The results of EN investigations after acid pickling as well as specific plasma oxidation treatments of the two magnesium alloys after exposure to air with different humidities are presented.     

A method for investigation of hot corrosion by gaseous Na2SO4

A novel in situ exposure method for investigating hot corrosion of aluminde coatings in the absence of an alkali sulphate melt is presented. The samples are exposed to Na₂SO₄(g) at a temperature above the dew point of the gas. Results from 100 and 500 h exposures at 900 °C of Ni-based In792 covered with SIF232 aluminide coating are presented. Already after 100 h of testing, attack characteristic for Type I hot corrosion is observed. The reactions taking place during the testing are discussed and the results are compared with those from the well-known ex situ salt hot corrosion test.     

Modeling pitting corrosion by means of a 3D discrete stochastic model

Pitting corrosion is difficult to detect, predict and design against. Modeling and simulation can help to increase the knowledge on this phenomenon as well as to make predictions on the initiation and progression of it. A cellular automaton based model describing pitting corrosion is developed based on the main mechanisms behind this phenomenon. Further, a sensitivity analysis is performed in order to get a better insight in the model, after which the information gained from this analysis is employed to estimate the model parameters by means of experimental time series for a metal electrode in contact with different chloride concentrations.     

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.     

纯Sn焊料表面高温氧化膜的腐蚀演化行为（英文）

对150℃高温时效条件下纯锡焊料表面氧化膜形貌、组成、厚度及耐蚀性的演化行为进行研究。结果表明,高温时效加速焊料表面原有自然氧化膜层中的Sn(OH)₄向SnO₂转变,同时加速新鲜Sn基体的氧化,从而使纯Sn焊料表面氧化膜厚度和粗糙度随时效时间的延长逐渐增加。然而,表面氧化膜层的耐蚀性随时效时间的延长呈先增强而后减弱的趋势。此外,还对纯Sn焊料表面氧化膜层的成膜机制及膜层演化机制进行讨论。     

Investigation of pitting corrosion of stainless steel by means of acoustic emission and potentiodynamic methods

Measurements of pitting corrosion by potentiodynamic method and also by acoustic emission technique were carried out simultaneously. Cumulative distribution function of pitting corrosion (CDF_P) occurrence was determined based on the results of potentiodynamic measurements. Independently cumulative distribution function of acoustic events (CDF_AE) was found. The correlation analysis of both distributions was also performed. The occurrence of pitting corrosion was checked by metallographic observation. The measurements proved that observable acoustic activity is an effect of hydrogen evolution inside the pits. The random character of acoustic events during polarization, statistical analysis and existence of pits are good confirmation.