Coating 10
Coating 10 : Corrosion: Metallic overlay and conversion coatings are used to impart oxidation and hot-corrosion resistance to alloy substrates. Ceramic TBCs are expected to exhibit corrosion resistance and to protect the substrate as well. Corrosion behavior is determined by a variety of static and dynamic tests in environments selected to simulate expected, pertinent aspects of turbine-operating environments. Static tests in furnaces with stagnant or low gas flows provide information on thermodynamic stability and reaction kinetics. Data usually consist of weight changes with time, corrosive penetration determination by metallographic examination, phase-change identification, and corrosive product formation. Dynamic testing is usually conducted in burner rigs operating at atmospheric pressure and gas velocities of less than 100 feet per second. High-velocity and elevated-pressure burner rigs that more closely simulate turbine conditions are used less frequently. Specimen analysis of burner-rig samples usually consists of metallographic examination and compositional analysis of corrosion products. Standards for the conduct of these tests, the specimen types, and the interpretation of data are not available. Nonetheless, significant amounts of research have allowed the comparison of data among the different tests. Recognition of the value of understanding the relationships among the test methodologies for hot corrosion is reflected in the early publication of ASTM STP-421 that provides focused coverage of the tests used in the 1960s (ASTM, 1967). Similar comparisons of more recently developed tests have not been published. The evaluation of corrosion and thermal- shock resistance of TBCs, particularly plasma-sprayed coatings, has been the subject of considerable research. Engine manufacturers and the NASA Lewis Research Center have used burner rigs of various designs for this purpose, and research at the latter institution has highlighted the necessity of well-designed experiments (Miller et al. , 1993)
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