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Corrosion phenomena on nickel base alloys for advanced ultra super critical technologies

pubblicazioni - Presentazione

Corrosion phenomena on nickel base alloys for advanced ultra super critical technologies

Vengono presentati i risultati del monitoraggio della corrosione su leghe a base nickel effettuato con il sistema RSE nella Centrale ENEL di Fusina ,operante in regime di co-combustione di CSS e carbone, simulando l’esercizio con un ciclo a vapore Ultra Super Critico.

Hot corrosion mechanisms, known as deposit-induced corrosion of metals and alloys occurring in a hot oxidizing gaseous environment, is a complex phenomenon due to the presence of high temperature and ash deposit and oxidizing ambient rich in SO3. Literature data on this phenomenon are spread and rather applicable to the relevant materials, combustion environment and operating temperature at the same time. In particular, when Refuse Derived Fuel (RDF) is burnt together with coal, RDF may affect the SO2/SO3 ratio; moreover chlorine and alkali contents in RDF can play a relevant role in hot corrosion attack by different mechanisms.

This leads to increased uncertainties when a steam boiler operating at high temperature has to be designed. At the same time, nickel base alloys are more and more important in the field of Advanced Ultra Super Critical steam generator (AD-USC), due to their high mechanical property also at very high temperature (700°C). For these reasons, a dedicated monitoring system has been used in the Enel’s co-firing power plant located in Fusina (Italy), fed by bituminous coal and RDF, in order to characterize corrosion effects on different materials operated at temperatures typical of the AD-USC steam cycles.

A long-term experimental campaign has been carried out by RSE in collaboration with ENEL, exposing a 0,6 m long probe into the convective pass and three shorter probes at the membrane wall of the boiler. Two different Nickel alloys have been tested: A617B and C263. In case of exposure into the convective pass, the sample was mounted onto a special corrosion probe developed by RSE for monitoring the corrosion of metallic materials exposed into a boiler at very high temperature (flue gas up to 1100°C). The sample was made by three heterogeneous welded joints of the materials C263 and A617B and its temperature has been set to 700°C. Other samples were exposed via a different kind of RSE probe, suitable for monitoring the corrosion at the membrane wall of the boiler.

In this case samples are exposed into the combustion chamber close to the membrane wall (approx 10 cm internal length) of the boiler and their temperatures were set at 650°, 700° and 750°C respectively. During and after exposure, both base materials and welded joints have been analysed in laboratory to evaluate the corrosion damage degree: thickness measurement using Light Optical Microscopy (LOM), Vickers hardness measurements (HV10), semi-quantitative analysis by EDS (Energy Dispersive Spectroscopy) and metallographic characterization by LOM and SEM (Scanning Electron Microscopy) of both sample and deposit have been performed. The results show a good corrosion behaviour of A617B and C263 materials in co-combustion conditions at temperatures typical of AD-USC steam generators

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