Aggressive Atmospheres Research Articles

Oxidation resistance improvement of arc-evaporated TiN hard coatings by silicon addition

Ti–Si–N coatings were deposited on M2 steel by arc evaporation using a Ti–Si composite target in an industrial reactor. The films structure before and after heat treatment at 700 °C was characterised by XRD. In addition, two types of quantitative experiments were performed in thermobalance: oxidation rate was deduced from isothermal thermogravimetric analyses at 800 °C, while the temperature of oxidation beginning ( Tc) was measured in dynamic mode. Tc was then calculated by a mathematical approximation based on the non-linear least square. The results were compared to those obtained using TiN and SiN x standards. Depending on the deposition conditions, ternary films have been deposited with an atomic ratio Si/Ti of 0.10 and 0.15. The hardness of the films was close to 40 GPa. Only the TiN phase was detected by XRD. The mean crystal size was estimated to be in the 6–8 nm range, which suggested the nanocomposite nature of the coatings. After air oxidation at 700 °C, it was found that this crystal size was not affected by the thermal treatment, indicating a good thermal stability of the structure. Moreover, incorporation of silicon into TiN-based coatings led to a drastic decrease of their oxidation rate, together with a shift of 200 °C of Tc. The high resistance of oxidation of Ti–Si–N films at elevated temperature is attributable to the network of refractory SiN x , which acted as a diffusion barrier for oxygen and insulated TiN nanograins from the aggressive atmosphere.

Stainless steel rebar: the choice of service life

Use of stainless steel rebar for concrete reinforcement is increasing worldwide. The enhanced properties of stainless steel prove to be a viable solution when life cycle costs of the structures are taken into account. Austenitic and duplex stainless steels are used as rebar because they present an excellent combination of corrosion resistance in concrete, high strength and good ductility. Stainless steel reinforcements are compared with conventional carbon steel reinforcements to point out the specificities and advantages of the stainless steel solution. The influence of the fabrication method (hot or cold rolling) on microstructure, strength and ductility is also discussed. The resistance to pitting corrosion of various stainless steels has been evaluated in the laboratory by electrochemical tests. This helps to select the most suitable grade according to the environment in which the concrete structure is located and the risks of exposure to aggressive atmospheres and attack during its life. The studied grades are: 1.4301, 1.4311, 1.4404, 1.4429, 1.4362 and 1.4462. The tests were carried out at pH between 8 and 12 in chloride-containing solutions to simulate the concrete pore liquid in a seawater environment or penetration by road de-icing salts. A classification of the selected austenitic and duplex grades was created.

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

AbstractIt is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr2O3 with some FeCr2O4 at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.

Determination of Corrosion Layers and Protective Coatings on Steels and Alloys Used in Simulated Service Environment of Modern Power Plants

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper, the following examples are discussed. (i) The oxidation behavior of 9% Cr steels in simulated combustion gases: The effects of O2 and H2O content on the oxidation behavior of 9% Cr steels in the temperature range 600-800°C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. In contrast, that in the presence of water vapor, after an incubation period, the scale became nonprotective as a result of a change in the oxidation mechanism. (ii) The development of NiCrAlY alloys for corrosion-resistant coatings and thermal barrier coatings of gas turbine components: The increase of component surface temperature in modern gas turbines leads to an enhanced oxidation attack of the blade coating. Considerable efforts have been made in the improvement of the temperature properties of MCrAlY coatings by the additions of minor elements, such as yttrium, silicon, and titanium. The experimental results show the positive, but different influence of the oxidation behavior of the MCrAlY coatings by the addition of these minor elements. (iii) The development of lightweight intermetallics of TiAl-basis: TiAl-based intermetallics are promising materials for future turbine components because of the combination of high-temperature strength and low density. These alloys, however, possess poor oxidation resistance at temperatures above 700°C. The experimental results showed that the oxidation behavior of TiAl-based intermetallics can be strongly improved by minor additions of 1-2at.% silver. (iv) The oxide-dispersion-strengthened (ODS) alloys provide excellent creep resistance up to much higher temperatures than can be achieved with conventional wrought or cast alloys in combination with suitable high-temperature oxidation/corrosion resistance. The growth mechanisms of protective chromia and alumina scales were examined by a two-stage oxidation method with O18 tracer. The distribution of the oxygen isotopes in the oxide scale was determined by secondary ion-mass spectroscopy and SNMS. The results show the positive influence of a Y2O3 dispersion on the oxidation resistance of the ODS alloys and its effect on growth mechanisms.

Sol-gel coatings on carbon steel: Electrochemical evaluation

Degradation of carbon steel has always been a concern. The use of coatings is especially recommended in aggressive atmospheres at moderate temperatures. Ceramic films can be used to improve the resistance against high temperature oxidation and corrosion of metals. Amid the different options, a sol-gel process provides a low cost, simple and non-hazardous method for processing ceramic coating with controllable composition and microstructure. This work evaluates the electrochemical behaviour of carbon steel coated by sol-gel method. Hybrid organic–inorganic silica sol-gel coatings were obtained by dip coating of planar samples in an organically modified silica sol made from hydrolysis and polycondensation of tetra-orthosilicate (TEOS) and methyltriethoxisilane (MTES) by acidic catalysis. Coatings free of defects were obtained at a sintering temperature of 400 °C. The coated samples were inspected by optical and electron microscopy and coating thickness was measured by using a Talystep surface roughness tester. Electrochemical evaluation was made by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. A comparison of the corrosion resistance of the coated metal with the uncoated one is presented. The measurements show the improvement of the corrosion resistance of the coated carbon steel.

Which tool to distinguish transient alumina from alpha alumina in thermally grown alumina scales?

Alumina scales constitute excellent protective barriers when they form on alumina-forming steels. If they keep tightly adherent to the underlying substrate, they isolate it from the surrounding aggressive atmosphere at high temperature. The protectiveness of the alumina scale is highly dependant upon its growth mechanism. The nucleation and transformation of transient alumina (mainly γ-Al2O3 and θ-Al2O3) is known to play an important role on alumina scale formation. It is therefore fundamental to characterise these transient alumina especially during the early stages of the oxidation process. The morphology of the transient alumina was observed by scanning electron microscopy (SEM), their crystallographic phases determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectrometry (XPS) analyses were performed on reference samples and then compared to the alloys oxidised 5 and 30 minutes at 850, 900 and 950°C.

High spatial resolution X-ray microdiffraction applied to biomaterial studies and archeometry

The high spatial resolution X-ray microdiffraction by using X-ray optics can provide unique information on regions with very high gradients in physical quantities, as in the case of interfaces. Among the several available X-ray optics for synchrotron radiation producing high intensity micron and sub-micron beams, the X-ray waveguide (WG) can provide the smallest X-ray beam in one direction. Moreover, its applicability has been widened by an improved set-up installed at ID13 beamline at ESRF, where a new undulator is combined with an horizontally focusing mirror. In this work, we show different applications of waveguide-based microdiffraction, the first two regard biological problems and in particular the structural analysis of newly formed bone in ceramic scaffolds. The second application regards archeometry and in particular the sulphatation process and the thin gypsum crust formation on the surface of carbonate rocks (travertine, marbles), due to the exposure of the monuments at aggressive atmospheres. In the three cases, the local structural information derived thanks to the high spatial resolution demonstrates the power of the microdiffraction technique based on WG, and the possibility to apply this new methododogy in different scientific fields.

The Use of SIMS, SEM, EPMA, LRS and X-Ray Diffraction Measurements for the Examination of Corrosive Layers and Protective Coatings on Steels and Alloys in Advanced Power Stations

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper the following examples are discussed:

Evaluation of the physical and electrochemical properties of adobe reinforced and of its component materials

The search of solutions to the habitacional crisis that exists in Latin America has favored to the use of the soil-cement-sisal, adobe reinforced like alternative material of building, in such sense, prevailing to determine the vulnerability of a building of this type, is for that reason that, the characterization of the physical, mechanical and electrochemical properties of the materials that composes it, helps to relate causal the external ones of deterioration to the internal ones. In this particular case it was studied, the permeability, the porosity, the capillaiy absorption, the corrosion potentials (referring to Cu/CuSO 4 ) and the corrosion rates of adobe reinforced, of its components and their interfaces. In the methodological aspect, permeabilimeter of modified Figg. was used to determine the permeability to the water The porosity and capillary absorption were determined following methods traditional, the corrosion potentials were determined using multimeter and the corrosion rates was made by means of the equipment Gecor 6. Two groups was tried, a first group of the component materials: adobe, mortar for stucco and mortar of reinforcement. The adobe with 5% of cement, mortars for stucco: MFA with 12% of cement and MFB with 16% of cement and internal mortar of reinforcement: MRD , of relation water/cement 0,50. A second group of the composed materials, conformed by 3 specimens test of each one of both types of wall of adobe in where all the individual components are combined: mortar of stucco, adobe and internal mortar: (MFA-Adobe-MRD) and (MFB-adobe- MRD) . The coefficients of permeability to the water of the component materials oscillate between 5.2.10 -5 for adobe up to 9.71.10 -9 for the internal mortar of reinforcement MRD (a/c=0.50), in mortars for stucco (MFA and MFB) this in the 10 -7 order For the case of the composed materials oscillates between 9,38.10 -8 for (MFA-Adobe-MRD) until 3.28.10 -8 for (MBA-adobe-MRD). The specimens test tubes were exposed to the natural environment during 200 days, being there been dew with water previous to the measures, the corrosion potentials and the corrosion rate were moderate weekly during the days of exhibition. These results in durability terms indicate that the greater benefit in this type of systems and with the smaller cost is obtained improving the quality of the mortar that is used like stucco. Also one concluded according to the values of Porosity and Capillary Absorption the greater than to 19% and 2.23.10 -4 respectively, that the analyzed walls of adobe reinforced are not adapted for aggressive atmospheres if it is taken like reference because these properties are very below the established standards.

Environmental Protection of Titanium Alloys in Centrifugal Compressors at 500°C in Saline Atmosphere

The use of the titanium alloy Ti-6246 (Ti–6Al–2Sn–4Zr–6Mo, wt-%) for gas turbine compressors allows an increase in working temperature and stress level. Under severe service conditions, the material experiences combined high temperature and high mechanical stress and, in saline atmospheres, stress corrosion cracking (SCC) can occur, leading to catastrophic mechanical failure. The present study was performed to evaluate the potential of several surface treatments to protect Ti-6246 alloy, after salt deposit, from hot salt SCC at temperatures ≤500°C and 500 MPa static mechanical stress conditions. Shot peening, thermal oxidation and metal–ceramic coatings were investigated. Experimental results confirm the existence of brittle stress corrosion phenomena marked by a low residual elongation of test samples and the presence of oxides on the fracture surfaces. Both shot peening and metal–ceramic coatings increase the hot salt SCC resistance of the alloy. Times to rupture were improved by a factor of 3 for shot peening and by a factor of 10 for metal–ceramic coatings. Inversely, the time to rupture of preoxidised alloys has been halved compared with uncoated alloys. As well as these interesting quantitative results, structural studies of metal–ceramic coatings showed that they are mechanically and chemically compatible with the titanium alloy substructure and should work under severe thermomechanical stresses and aggressive atmospheres. SE/504

Metal-ceramic interfaces: wetting and joining processes

Metallic alloys and ceramic materials are employed in aggressive and hostile environments, ranging from aerospace to energy production, from offshore to biological applications. Today, production requires materials able to survive for a long time at high temperatures, in highly aggressive atmospheres, both from the chemical and the mechanical points of view. No single material can offer these characteristics, so that "composite" structures (composites, multilayer materials, metal-ceramic joints) are designed and tested under extreme conditions. In this paper are presented the basic principles underlying joining technologies, which can be seen also as a particular process pertaining to solidification phenomena, a short discussion of the thermodynamic background of wetting processes, recent developments related to non-reactive and reactive wetting, the influence of trace chemical elements (in the solid, liquid and gaseous phases), and some specific aspects of brazing and transient liquid phase joining processes.

High-pressure jet-assisted cooling: a new possibility for near net shape turning of decarburized steel

Ultra-high-pressure cooling (UHPC) in turning operations is an effective method for achieving higher productivity. Previous research has demonstrated that the introduction of a high-pressure fluid jet into the gap between the tool and chip interface can control chip form and breakage. The present work shows the effect of UHPC in the turning of near net shape (NNS) decarburized parts. The workpiece material properties are strongly influenced by the loss of carbon atoms to a depth of up to 1 mm, due to the aggressive atmosphere during forming (decarburization). The extremely soft material makes chips difficult to control. Consequently, productivity decreases since the machine must be frequently stopped in order to manually remove the chips from the working area. The results show the influence of UHPC on chip form, surface topography and tool life when turning decarburized parts close to NNS. An interesting observation was that the combination of small cutting depth (near net shape) and soft material (decarburization) allowed for the presence of built-up edge formation, even at a high cutting speed.

Surface chemical reactions of aluminosilicate composites at extreme atmospheres using electron spectroscopy for chemical analysis

Aluminosilicates are important materials and are mostly applied to high temperature structural engineering. However, they are susceptible to degradation in aggressive and extreme atmospheres. The chemical degradation and subsequent failure of aluminosilicates can be severe in both reducing atmospheres and high temperatures in the presence of water vapor. The potential use of electron spectroscopy for chemical analysis to monitor such effects on aluminosilicate composites in a simulated land-based gas turbine environment is presented in this study. The chemical changes caused by the environment can be observed as systematic binding energy shifts in the O(1s) spectrum as a function of varying Si/Al ratio. The chemical changes are further correlated to thermodynamic calculations at selected temperatures and partial pressures.

Atmospheric corrosion of bare and anodized aluminium in a wide range of environmental conditions. Part I: Visual observations and gravimetric results

This study compares the behaviour of aluminium in the bare condition and when protected with anodic films of approximately 7, 17 and 28 μm thickness for 42 months of exposure in 11 atmospheres of very different aggressivities, with salinity values ranging from between 2.1 and 684 mg Cl − m −2 d −1. The anodic films, obtained in a sulfuric acid bath and sealed for 60 min in boiling deionized water, were characterised before and after different exposure times by means of: visual inspection; observation with a magnifying glass; and using classic gravimetric techniques. Optical and electron microscopy were occasionally used with some specimens that showed symptoms of localised corrosion. Aluminium behaves as a passive material in atmospheres of low salinity but exhibits pitting corrosion after 1 year at chloride pollution levels of ≥50 mg Cl − m −2 d −1 and after 2 years at levels of ≥10 mg Cl − m −2 d −1. Anodising and sealing prevents the risk of pitting corrosion even in the most aggressive atmospheres, except in the case of the coatings of the lowest thickness.

Zero etch clear — a new modular clear coat system with excellent scratch/mar performance

The aggravated demands regarding the etch/mar resistance of clear coats due to aggressive atmosphere pollution has led to new clear coat systems with extraordinary etch performance. One of the most promising key-chemistries is based on the epoxy/acid reaction. In contrast to the introduced clear coats on this chemistry basis, this new clear coat system has changed the former drawbacks, storage stability and scratch resistance to advantageous properties. Key ingredients are an epoxyfunctional acrylic resin and an acidic polyester resin. By incorporation of latent acidic groups into the epoxy acrylate and by branching of the resin during synthesis a high cross-linking density even at lower baking temperatures is realised. Although a high reactive system is obtained the material shows excellent storage stability. The scratch resistance is attributed to the structure of the tailor-made high-functional, highly branched acidic polyester. Furthermore, there is an extraordinary broad application field: waterborne, solventborne, high solids and powder clear formulations have been developed. The superior etch performance of the waterborne as well as the solventborne epoxy/acid clear coats is proven in laboratory etch tests and especially in outdoor weathering tests in aggressive atmosphere, e.g. at Jacksonville.

Joining Technology in Metal-Ceramic Systems

Metallic alloys and ceramic materials are employed in aggressive and hostile environments, ranging from aerospace to energy production, from offshore to biological applications. Today, production requires materials able to survive for a long time at high temperatures, in highly aggressive atmospheres, both from the chemical and the mechanical points of view. No single material can offer these characteristics, so that “composite” structures (composites, multilayer materials, metal-ceramic joints) are designed and tested under extreme conditions. In this paper are presented the basic principles underlying joining technologies, a short discussion of the thermodynamic background of wetting processes, recent developments related to non-reactive and reactive wetting, the influence of trace chemical elements (in the solid, liquid and gaseous phases), and some specific aspects of diffusion bonding, brazing and transient liquid phase joining processes.

Main features of thermostructural composites for space, aeronautic and industrial applications

Thermostructural Composites are made of inorganic fibers, like carbon or ceramic fibers, associated with an inorganic matrix. So, they are able to sustain high temperatures. Many combinations and even more processing routes may be implemented. Fibers and matrix are usually brittle materials, but the composite may present a much higher toughness and a good strain to failure. Mechanical loads on thermostructural composites generate cracks which propagate under monotonous or alternate loadings. The crack pattern governs most of the composite properties. Furthermore, it allows the penetration of oxidative or corrosive agents, thus affecting the long-term properties. Design of parts must take into account the geometrical singularities, where fiber orientations, fiber ratio and matrix content may vary rapidly. Special tests on technological samples are the more efficient way to ascertain the design in these areas. Thermostructural composites are more and more used in high temperature and aggressive atmospheres, because they are light and damage-tolerant.

Thermochemical Treatments for Protection of Steels in Chemically Aggressive Atmospheres at High Temperatures

Boronizing is a well-known thermochemical treatment process that has found applications in a variety of industries. Boride layers on Fe-alloys present high hardness values (∼ 1900 Hv), good wear properties and are resistant to various aggressive chemical environments. The mechanical behavior of boride layers on Fe-alloys has been well documented, but there seems to exist limited information on the chemical behavior of these coatings. We present in this paper the results of a systematic study of the chemical properties of boride layers, in aggressive environments that include dilute HCl, H3PO4, H2SO4 and HNO3, acid solutions and vapors of molten Al alloy and Zn metals in various temperature ranges. Results indicated that with the exception of HNO3 acid, the presence of boride layers on Fe-alloys, greatly improved their corrosion resistance. These results are discussed in terms of the potential applications they may have in various industrial sectors, including field tests carried out in fossil-fuel fired boilers.

Electrochemical filters for oxygen sensors with improved surface properties

Oxygen sensors based on YSZ (yttria stabilized zirconia) are best known for high temperature measurements. The protection of such oxygen sensors with dense electrochemical filters is an effective way to increase their life time in aggressive industrial atmospheres. This work reports on the performance of sensors protected by composite or single phase mixed conducting filters. The first group includes materials based on GCO (Gd2O3 doped CeO2)+LC (LaCoO3), and on YSZ+LCM (La(Mn,Co)O3). Sensors protected with GCO+LC composites showed the best performance amongst the composite filters already evaluated. Electroded single phase GCO filters were also tested to check on the role of surface reaction kinetics on the overall oxygen transport process across the filter. The fastest sensor responses to a sudden decrease in oxygen partial pressure were obtained with deposited electrodes either on both surfaces of the filter or on the filter cathode side. These preliminary results suggest that the cathodic process is more relevant than the anodic one in determining the filter permeability (and protected sensor performance), which fulfils the logical requirement of electrode free surfaces in contact with the furnace atmosphere.

Effect of surface depletion of lithium on corrosion behaviour of aluminium alloy 8090 in a marine atmosphere

Aluminium-lithium alloy 8090 is of great interest due to its low density and good mechanical properties. The high reactivity of lithium, however, makes it more prone to corrosion than other aluminium alloys, limiting its use in certain fields. In general, there is an intergranular attack with exfoliation in the most severely attacked sections of this alloy in marine and industrial environments. This attack also depends on the metallurgical history of the material, i.e. the part of the ingot from which the specimens are extracted. After one and two years' exposure to a moderately aggressive marine atmosphere, it was found that the materials taken from the centre of the ingot have a more widespread attack than specimens taken from the edges. This might be explained by the differing composition of the specimens according to their origin; more precisely, by the uneven distribution of the lithium, which tends to spread though the matrix of the ingot and form a lithium-impoverished layer several microns deep as a consequence of the series of thermal treatments applied.