Hot Corrosion Of Alloy Research Articles

Hot Corrosion Behavior of a Powder Metallurgy Superalloy Under Gas Containing Chloride Salts

Hot-corrosion behavior of a powder metallurgy superalloy (Alloy 1) under gas containing chloride salts at 700 °C, 750 °C and 800 °C were investigated via weight-gain measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe micro analysis (EPMA). The hot-corrosion behavior of a similar alloy (Alloy 2) at 800 °C and the same conditions were also carried out for comparison. The experimental results showed that the average mass gain of Alloy 1 increases as the temperature elevates. The corrosion kinetics followed linear power law at 700 °C, 750 °C and 800 °C. The corrosion layers obtained after 100 h of hot corrosion were mainly composed of Cr2O3, TiO2, Al2O3, NiO and NiCr2O4. The cross-sectional morphologies and corresponding elemental maps indicated that the chloride salts penetrated into the corrosion product and caused it to produce many cavities and cracks. According to these results, the hot corrosion of Alloy 1 under gas containing chloride salts is confirmed to be an accelerated oxidation process due to the damage in integrity of the oxide film caused by continuous corrosion with chloride salts. Compared to Alloy 2, the increased Co and Al content in Alloy 1 with better hot corrosion resistance at 800 °C promoted the rapid formation of continuous Cr2O3 and Al2O3 protective films on the alloy surface in which Co inhibited internal oxidation of Al through the third element effect.

Stability of oxide-sulfate mixtures and implications for deposit-induced degradation of advanced alloys and coatings

Turbine engines ingest atmospheric aerosols that include multi-cation oxide-sulfate mixtures, which deposit onto surfaces within the engine. These deposits are often implicated in hot corrosion of alloys and bond coats and can accelerate thermomechanical failure of ceramic coatings through reaction and infiltration mechanisms. Understanding the intrinsic stability of oxide-sulfate deposits, including the tendency for sulfate decomposition, is important to identify efficient testing protocols. This work addresses that need by integrating experiments and computational thermodynamics models to systematically analyze the effect of deposit composition and temperature on the sulfate decomposition and melting behavior of five model mixed-anion deposits. Mass loss analysis and energy dispersive x-ray spectroscopy show that sulfate decomposition occurs much faster in the mixtures compared to pure calcium sulfate. The thermodynamic computations show that reaction pathways forming ternary and quaternary silicates accelerate sulfate decomposition faster than those forming binary reaction products. The results also show that multi-cation mixtures can suppress the evaporation of sodium and potassium sulfates, retaining the sulfate-bearing liquid to higher temperatures. The results can be used to differentiate the behavior of sulfate-, sulfate-oxide-, and oxide-based deposits, and to guide the discovery of new high temperature alloys and coating materials.

Hot corrosion of alloy 617 OCC in simulated coal ash environment encountered in USC power plants

Abstract Hot corrosion of Alloy 617 OCC was studied in a simulated coal ash environment at 700 °C. A synthetic mixture of sulphates of alkali metals Na and K and oxides of Al, Si and Fe was used to simulate the coal ash environment developing in advanced thermal power plants. Studies extended up to 5000 h of exposure time, comprising of 10 cycles each of 500 h duration. Progress of the corrosion process was monitored using XRD and SEM-EDAX analysis of surface and transverse sections. An incubation period of about 1500 h was observed, characterised by formation of a single protective layer of chromium oxide, with little or no corrosion damage occurring. Longer exposure times saw formation of a second layer, comprising of oxides of Co, Ni, Fe etc, bringing in significant corrosion. Aluminum and Ti were found to be internally oxidised. The manuscript elaborates on these findings.

Hot Corrosion of Alloy 617 OCC in Simulated USC Power Plant Environment

Alloy 617 OCC, a variant of INCONEL 617 with optimised chemical composition, has been produced in India for manufacture of superheater and reheater tubing in boilers operating in advanced ultrasupercritical (A-USC) power plants. The tubing encounters intense hot corrosion conditions during service. The present study deals with hot corrosion behaviour of 617 OCC in A-USC environment. The environment occurring in A-USC plants was simulated in the laboratory by exposing the material coated with a mixture of salts at 700°C in a flowing gas mixture. For use in A-USC boiler technology, the metal loss due to fireside corrosion of the material should be less than 2 mm in 200,000 hours. The loss obtained in the present study was nearly 5 times this value. The corrosion processes were studied using SEM/EDAX, XRD and thermogravimetry. The degradation mechanisms coming into play, disqualifying the material for use in A-USC plants, would be discussed.

Study of hot corrosion of alloy 800 using linear polarisation resistance and weight loss measurement

A study of the hot corrosion of alloy 800 at 700 and 900°C in 80Na2SO4+20V2O5 (wt-%) has been carried out. The techniques included linear polarisation resistance (LPR) and weight loss measurement during 10 days of experiments, supported by X-ray measurements and electron microscopy. The degradation of the alloy was discussed in terms of the dissolution of the external Cr2O3 layer with subsequent oxidation and sulfidation of the alloy. Both LPR and weight loss results showed that the corrosion rate is slightly higher at 900 than at 700°C. At the beginning of the experiments, the corrosion rates obtained using the two techniques increase at both temperatures, but decrease later on until they reach steady values. The measured corrosion rates were within the same order of magnitude for both techniques, but the rates obtained with the electrochemical technique were lower than those obtained using weight loss at 700°C, while at 900°C, the opposite was observed. This is because at 700°C the step that controls the corrosion process is the diffusion of species in the alloy or in the melt, whereas at 900°C, the step that controls the reaction is either charge transfer or there is a mixed control. It is evident that electrochemical techniques are powerful tools for monitoring on line hot corrosion processes when the corrosion rate is controlled by an electrochemical reaction, but may induce some errors when the molten salt changes from an ionic to an electronic conductor.

Thermodynamics of metal-gas-liquid reactions

The hot corrosion of alloys comprises a complex series of interacting processes. Despite the difficulties involved, the thermodynamic analysis of corrosion ought to take into account the multicomponent nature of the alloy, the constitution of the gas, and the microenvironments present in the corrosion layer. Most importantly the metallic, oxide, sulphide, and salt phases ought to be treated as solutions (mixtures). The methods for assessment and calculations involving solution phases are demonstrated by reference to the salt system NaCl-NaOH-Na2CrO4-Na2S04 and its equilibrium with Cr2O3 as the protect!ve oxide layer and the gaseous combustion products. The data requirements assa.ciated with these developments and their possible application to kinetic analysis are discussed.