The role of alloying elements in commercial alloys for corrosion resistance in oxidizing‐chloridizing atmospheres. Part II: Experimental investigations

In an extensive study the role of the alloying elements in commercial alloys for corrosion resistance was studied in air without and with 0.1 and 2 vol.% Cl₂, respectively. In the first part of this paper [1] the thermodynamic fundamentals were discussed on the basis of the new concept of the quasi‐stability diagrams. The second part which is presented here reports the results from investigations at 650, 800 and 1000°C and testing times up to 1000 hrs where 14 commercial alloys were tested with regard to their corrosion behavior. The materials were selected so that the role of the alloying elements Mo, C, Si, Al, N, Fe, Ni and Cr would be evident from the results. The exposure tests were followed by extensive microstructural analyses of the corrosion scales and the metal subsurface zones so that type, mechanism and extent of corrosion could be characterized in great detail. At the end a ranking was possible of the different materials and with regard to the detrimental or beneficial role of the different alloying elements. The present results thus provide a much deeper insight into materials resistance in oxidizing‐chloridizing environments at high temperatures.     

In situ corrosion testing of various nickel alloys at Måbjerg waste incineration plant

The majority of waste in Denmark is disposed via waste to energy (WTE) incineration plants which are fabricated from carbon steel. However, due to the increasing corrosiveness of waste over the years, more corrosion resistant alloys are required. In Denmark, Inconel 625 (UNSN06625) is the weld overlay material currently being used to give improved corrosion resistance. In order to assess the use of alternative nickel alloys, test panels have been manufactured and inserted into Måbjerg waste incineration plant. Inconel 625 as a 50% weld overlay, two layered weld overlay and as a spiral weld overlay was exposed. Other nickel materials exposed were weld overlay Alloy 686, Alloy 50 and Sumitomo Super 625 coextruded tube. Exposure has been undertaken from 2003 to 2009 in the first pass and 2005–2009 in the second pass, and sections have been removed and investigated during this period. The composition of the deposits from the exposed waterwall panels was also analysed each time sections were removed. This paper will compare the various nickel alloys in the two areas and assess the results of the long‐term testing project.     

Long‐term exposure of austenitic steels and nickel‐based alloys in lignite‐biomass cofiring

The aim of this study was to assess the long‐term impact that the addition of biomass provokes on superheater materials exposed to fireside corrosion environments. Alloys covering a broad range of commercially available materials were investigated. Their corrosion kinetics under different corrosive deposits and atmospheres was evaluated, and their corrosion products analyzed to deepen understanding of the underlying corrosion mechanisms. Therefore, three nickel‐based alloys and three austenitic steels containing 20–24 wt.% Cr were tested at 650°C for 7,000 hr. The long‐term exposure shows new mechanistic aspects of Type II hot corrosion that were revealed by accelerated material depletion. The formation of Ni–NiS eutectic and the formation of a Cr depleted zone close to the substrate corrosion product interface are indicative of the breakaway occurrence. Differences in the corrosion behavior are related to the balance of Ni, Mo, Co, and Cr and can serve as the material selection argument. The evaluation concluded with the finding that alloys presenting Mo and Ni might be preferentially used in fireside corrosion in the presence of biomass, whereas the use of austenitic steels suffer less corrosion if no biomass is present in the corrosive atmosphere.     

Chromate replacement in coatings for corrosion protection of aerospace aluminium alloys

Mixed rare earth organophosphates have been investigated as potential corrosion inhibitors for AA2024‐T3 with the aim of replacing chromate‐based technologies. Cerium diphenyl phosphate (Ce(dpp)₃) and mischmetal diphenyl phosphate (Mm(dpp)₃) were added to epoxy coatings applied to AA2024‐T3 panels and they were effective in reducing the amount and rate of filiform corrosion in high humidity conditions. Ce(dpp)₃ was the most effective and characterisation of the coating formulations showed approximately a factor of 5 reduction in both the number of corrosion filaments initiated as well as the length of these. Mm(dpp)₃ appeared to reduce the corrosion growth rate by a factor of 2 although it was the more effective inhibitor in solution studies. Spectroscopic characterisation of the coatings indicated that the cerium based inhibitor may disrupt network formation in the epoxy thus resulting in a coating that absorbed more water and allowed greater solubilisation of the corrosion inhibiting compound.     

Potential high temperature corrosion problems due to co‐firing of biomass and fossil fuels

Over the past few years, considerable high temperature corrosion problems have been encountered when firing biomass in power plants due to the high content of potassium chloride in the deposits. Therefore, to combat chloride corrosion problems co‐firing of biomass with a fossil fuel has been undertaken. This results in potassium chloride being converted to potassium sulphate in the combustion chamber and it is sulphate rich deposits that are deposited on the vulnerable metallic surfaces such as high temperature superheaters. Although this removes the problem of chloride corrosion, other corrosion mechanisms appear such as sulphidation and hot corrosion due to sulphate deposits. At Studstrup power plant Unit 4, based on trials with exposure times of 3000 h using 0–20% straw co‐firing with coal, the plant now runs with a fuel mix of 10% straw + coal. Based on results from a 3 years exposure in this environment, the internal sulphidation is much more significant than that revealed in the demonstration project. Avedøre 2 main boiler is fuelled with wood pellets + heavy fuel oil + gas. Some reaction products resulting from the presence of vanadium compounds in the heavy oil were detected, i.e. iron vanadates. However, the most significant corrosion attack was sulphidation attack at the grain boundaries of 18‐8 steel after 3 years exposure. The corrosion mechanisms and corrosion rates are compared with biomass firing and coal firing. Potential corrosion problems due to co‐firing biomass and fossil fuels are discussed.     

Identification of degradation mechanisms in coatings for supercritical steam applications

The development and qualification of coatings for materials used in modern steam power plants stems from the increased demand for higher efficiency, and hence higher operating temperatures. Within the EU funded project ‘SUPERCOAT’, several coatings, both overlay and diffusion type, were investigated. Seven different coatings are presented in this work. They included two commercially available HVOF coatings (Ni–20Cr and Ni–50Cr), an aluminium‐based slurry coating (IPCOTE), together with two further variations of this slurry coating containing sputter‐coated inter‐layers. An overlay slurry coating consisting of silica particles embedded in a matrix of alumina and chromia was also examined. The final coating to be investigated was a pack‐aluminised sample of P92. All the coating systems examined showed superior oxidation resistance compared to the 9%Cr steel substrate (P91 or P92) in extended exposures to a steam environment at 650 °C. However, in service component lifetime will be limited by degradation of the coating, therefore it is essential that the mechanisms controlling this behaviour are understood. This paper reviews several degradation mechanisms that have been observed during long‐term exposure of these coatings. The mechanisms that have been observed include depletion of active alloying elements, diffusion of aluminium into the substrate from the coating, formation of Kirkendall porosity and mechanical failure of the coatings. Examples of each of these mechanisms will be presented. Possible processing routes to avoid these degradation mechanisms will also be discussed.     

High temperature corrosion mechanisms and effect of alloying elements for materials used in waste incineration environment

Corrosion products on two typical materials, SA213-T12 steel and alloy 625 exposed to the actual combustion gas, were analyzed in addition to laboratory tests for penetration of corrosive matter. It has been clarified that corrosion products of oxides containing a little chlorides and sulphides show lamellar structures and that at the alloy-scale interface, chlorination, sulphidation, and oxidation occur under a low P_O2-high P_Cl2 condition. The formation of scale structures and the effect of corrosion-resistant alloying elements can be explained according to the stability tendencies of metals, chlorides, and oxides in the M-Cl-O equilibrium diagrams. The severity of corrosion environments at the interface is influenced by the penetration extent of corrosive matters through deposits and scales, and the protective effects of oxide films derived from alloying elements play an important role in preventing the corrosion. On the other hand, it has been shown that thermal fluctuation characterized in this kind of environment makes the lamellar scale structures and sometimes breaks and peels off the scale, and thus accelerates the corrosion. On the basis of the above mentioned knowledge, a new corrosion model is presented.     

Corrosion of furnace wall materials in waste-wood fired power plant

Abstract

Corrosion tests were performed with four different materials exposed at the furnace wall in a power boiler burning recycled wood, with the aim of evaluating coatings to reduce the corrosion. The nickel base Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT had the lowest corrosion rates followed by the stainless steel 310S. The low alloy steel 16Mo3, from which the walls are constructed, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. Thermodynamic modelling showed that chlorine gas exists at extremely low levels under the prevailing conditions and the hydrated form is thermodynamically favoured.     

Field exposure of FeCrAl model alloys in a waste‐fired boiler at 600°C: The influence of Cr and Si on the corrosion behaviour

The aim of this study was to examine the performance of FeCrAl model alloys in a waste‐fired boiler and investigate the influence of chromium and silicon content on the corrosion behaviour. The investigation was executed by utilising an air‐cooled probe, giving a material temperature of 600°C throughout a 672 hr exposure. The material loss measurements were performed by utilizing an ultrasonic thickness gauge in combination with scanning electron microscopy analysis. It was found that increasing the chromium content significantly reduced the overall material loss of the FeCrAl model alloys but further accelerated the corrosion attack on the windward side. Simultaneously, the increased chromium content caused embrittlement of the material. Minor additions of silicon drastically reduced the material loss of the FeCrAl model alloys, whereas the sample ring with no silicon present was completely deteriorated. The trends observed in this field study correlated well with what has been observed in previous laboratory studies. A state‐of‐the‐art alloy in the present environment, Inconel 625, was simultaneously exposed and showed similar performance to the silicon‐containing FeCrAl model alloys with ≥10 wt% Cr.     

Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells

A novel conversion coating process has been developed to meet the stability requirements of stainless steel hardware in the demanding MCFC fuel cell environments. The process applies a perovskite-based coating by exploiting spontaneous oxidizing reactions of the metallic surface with La₂O₃ in eutectic alkali carbonate mixtures. By using well controlled synthesis procedures, conversion coating layers covering the entire metallic surface with a uniform and compact structure could be obtained. The as-formed coatings with a surface morphology of agglomerated crystallite particles consisted of a thin (<5 μm) LaFeO₃ perovskite layer grown over a thicker (>5 μm) LiFeO₂-rich layer. Test coupons of 316L stainless steel with the perovskite conversion coating were analyzed for corrosion protection and interfacial resistivity properties. It was found that the conversion coating is highly conductive while showing excellent long-term corrosion stability in simulated MCFC environments. These results suggested that perovskite coatings formed by molten salt conversion reactions could be particularly attractive to confer optimal protection and electrical continuity to MCFC current collectors.     

Oxidation and corrosion of silicon nitride ceramics with different sintering additives at 1200 and 1500 °C in air, water vapour, SO2 and HCl environments - A comparative study

The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of silicon nitride based ceramics was investigated. Comparative tests were conducted at 1200 and 1500 °C in air, in water vapour, and with the highly corrosive gases HCl and SO₂. Si₃N₄ was prepared with MgO, Al₂O₃, Y₂O₃ and Al₂O₃ + Y₂O₃ sintering additives. Hot pressed discs were tested for a total time of up to 128 h. The electrically conductive ceramic composites Si₃N₄ + TiN and Si₃N₄ + MoSi₂ were also tested under the same conditions. The effects that the different corrosion environments have on the different ceramics are presented. SEM studies of the oxidised ceramics show the direct transformation of Si₃N₄ grains into SiO₂ through a reaction interface layer.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

Effect of preoxidation and Ti or Hf additions on the corrosion behavior of Fe-23Cr-5Al alloys in Ar-10SO2 at 1200 K

An Fe-23Cr-5Al alloy and those containing 0.21% Ti or 0.12% Hf were cyclically corroded in flowing Ar-10SO₂ gas under atmospheric pressure and in a temperature range varying from room temperature to 1200 K. The corrosion kinetics were assessed by gravimetry, while morphological examinations were carried out using x-ray diffractometry, scanning electron microscopy, and microanalysis. Similar corrosion tests and examinations of the corrosion products were performed for the specimens preoxidized at 1200 K for 20 ksec in pure oxygen under atmospheric pressure. The main corrosion product under all the experimental conditions was -alumina, growing mainly outward in the form of thin needles. Many voids formed beneath the adherent scales. The degree of outward growth decreased by the additives. Partial spallation and formation of scales repeated on the base alloy, resulting in continuous mass losses. The addition of Ti prevented scale spallation, resulting in gradual mass gains; whereas the addition of Hf resulted in poorly adherent scales, under which grooving at alloy grain boundaries and void formation inside the grains occurred. Preoxidation resulted in the formation of adherent scales on all the alloys; however, with increasing corrosion cycles, the outward growth of the oxide became significant. Penetration of sulfur to the substrate under the adherent scale took place.