Study On The High Temperature Corrosion Performance Of Titanium And Aluminium Coatings On Boiler Superheaters In Waste-To-Energy Plants

As a clean and efficient means of waste treatment,waste-to-energy incineration plays an increasingly important role in the future development of China.As a mixed waste,the combustion of waste releases a small amount of SO2,sulphide and a large amount of alkali metal chloride and water vapour,which can cause serious high temperature chloride corrosion of the water-cooled wall tubes and superheater tubes in the boiler.The high temperature corrosion of the metal heated surfaces limits the efficiency of waste treatment and power generation to a large extent.In addition,titanium and aluminium coatings have all shown better behaviour in high temperature corrosion resistance studies.In this paper,we conduct a study on the high temperature sulphur and chloride corrosion protection of waste-to-energy systems,using TiAl coatings prepared by encapsulated aluminization and Ti2AlC coatings prepared by magnetron sputtering to protect the base metal,respectively,and carry out comprehensive tests and analyses on the microscopic morphology,tissue composition and high temperature corrosion resistance of the coatings,and specifically study the effects of sulphur and chloride composition,SO2 and water vapour on high temperature corrosion.It is useful to have an understanding of corrosion in boilers.The details of the study are as follows:The aluminium-based coating will form a dense corrosion-resistant aluminium oxide film on the surface during high-temperature corrosion,which can retard deposited salt corrosion very well.In this study,a titanium aluminium coating was prepared by high temperature aluminisation using Ti6Al4V as the base material.This study focused on high temperature corrosion experiments by applying different ratios of NaCl/Na2SO4 deposition salt mixtures to its surface.The experimental results showed that the coating exhibited the most severe degree of corrosion erosion under the 100%NaCl deposition salt,and the corrosion weight gain as well as the longitudinal erosion depth increased with the increase of NaCl content in the salt mixture.At 600℃,when the NaCl salt content reaches 1:2,the corrosion of the coating increases significantly,as sodium sulphate and sodium chloride form eutectic salts,lowering the melting point and triggering the initial electrochemical corrosion,breaking the initial Al2O3 protective layer and significantly increasing the corrosion rate.100%Na2SO4 in the deposition salt,the coating surface only undergoes trace oxidation because The decomposition rate of the sulphate at 600℃ is too slow and the concentration of decomposed sodium oxide is extremely low,so it dissolves the oxide film to a very small extent.And when there is SO2 in the atmosphere,through the Hargreaves mechanism it leads to lead to the sulphation of NaCl,which inhibits the chlorination and thus reduces the corrosion of NaCl.And H2O vapour is involved in the sulphation of NaCl,reducing the water vapour content at the coating/deposited salt interface and slowing down the corrosion rate to some extent.In the case of TP347H,the combination of the resulting sulphate layer and Fe2O3 prevented the inward migration of chlorine to the corrosion front to some extent.Over time,the un-replenished corrosive material(NaCl)is consumed during the corrosion process.The formation of the less corrosive Na2SO4,the depletion of chlorine at the metal/deposit interface and the continuous flow of the flue gas composition therefore reduces the corrosiveness of the deposited salt.Ti2AlC coatings were prepared by magnetron sputtering and NaCl:Na2SO4=1:2 was selected as the deposition salt in simulated waste incineration high temperature corrosion experiments.The experimental results show that TiC is separated by a pure elemental Al layer,and the chloride affinity of Ti is greatly reduced due to the Ti atoms in Ti2AlC being bonded to carbon atoms through strong covalent and ionic bonds,thus allowing preferential oxidative corrosion of Al.Al first diffuses outwards towards the coating surface and forms an Al2O3 layer,which effectively prevents corrosion by water vapour and improves the corrosion resistance of Ti2AlC.As Al diffuses outwards it leads to the formation of an aluminium depletion layer at the interface and the transformation of Ti2AlC into TiC,a phase change that leads to severe volume shrinkage and gradual coating failure.-This phase change leads to severe volume shrinkage and gradual failure of the coating.