Hot corrosion of coated and uncoated single crystal gas turbine materials

摘要

Single crystal superalloys were originally developed for use in the hot gas paths of aero-engines with fuels and air that could both contain low levels of elements that had the potential to produce significant corrosive damage to these materials. However, in the development of power systems with increased efficiencies, single-crystal materials are now being used in industrial gas turbines, which operate with higher levels of potentially corrosive elements. Therefore, the hot corrosion of single crystal materials, both bare and with coatings, is of interest in assessing the viability of their use in combusted gas environments that contain progressively higher levels of potentially damaging elements. The paper summarises work that has been carried out to investigate the hot corrosion of two single crystal alloys (CMSX-4 and SC{sup}2-B), both bare and with Pt-Al coatings. A series of laboratory tests has been carried out using the 'deposit replenishment' technique to investigate the sensitivity of the corrosion damage to single crystal materials and coatings to a range of exposure conditions anticipated for gas turbines using combustion gases that contain high levels of potentially damaging elements. The exposure variables studied during the series of 500 h tests included: 1) Gas composition: the effects of different levels of SO{sub}x, HCl and steam in air; 2) deposition flux: the effects of 4/1 (Na/K){sub}2SO{sub}4 deposits applied with deposition fluxes of 1.5, 5 and 15 μg/cm{sup}2/h. 3) temperature: 700 and 900℃. During the course of the tests, the performance of the materials was monitored using conventional mass-change methods. But the main aim of the work was to develop predictive models for the hot corrosion damage to the materials in terms of metal loss. Such quantitative data on the damage to the materials in terms of metal losses have to be obtained using dimensional metrology: for this study this was carried out using pre-exposure contact measurements and post-exposure measurements of features on polished cross-sections. These measurement methods have allowed distributions of damage data to be determined and permitted different approaches to corrosion modelling to be investigated and their viability to be assessed.