Abstract
In this study, we investigated the galvanic corrosion performance of an Aluminum–Boron Nitride (Al–BN) abradable seal coating system (with a Ni5Al bond layer and a 0Cr17Ni4Cu4Nb substrate) in chloride solution by electrochemical methods. The results indicated a three-stage process occurred during the anodic dissolution of the coupled coating system, consisting of a spontaneous pitting stage I under charge transfer control with a decreasing rate, a corrosion developing stage II under mass transfer control with an increasing rate, and a final steady stage III. Precipitation of Al(OH)3 restricts the oxygen transport process to the cathode and induces localized acidification of the occluded pores of the Al–BN layer, which was the mechanism that could explain the changes of corrosion performance during the three immersion stages of Al–BN coating system. The study suggests that galvanic corrosion of the porous multi-layer Al–BN abradable coating system is mostly influenced by its corrosion product deposition.
Highlights
A primary design requirement for improving aero-engine efficiency is to minimize the clearance gap between blade tip and shroud, reducing “over-tip leakage” to keep the engine pressure, and reduce specific fuel consumption (SFC) [1]
Abradable seal coating wears preferentially to the blade, and the gas path clearance is maintained at the smallest scale without blade wear, so that to improve the efficiency of the engine by reducing SFC [3,4,5,6]
Zhang et al [14,16] investigated corrosion behavior of two kinds of abradable seal-coating systems, containing a Ti3Al/BN and NiTi/BN abradable layer respectively; the results indicated that galvanic corrosion played a significant role in the deterioration for abradable materials
Summary
A primary design requirement for improving aero-engine efficiency is to minimize the clearance gap between blade tip and shroud, reducing “over-tip leakage” to keep the engine pressure, and reduce specific fuel consumption (SFC) [1]. Abradable seal materials often operate in highly demanding environments, and the seal coatings are designed to have excellent environmental resistance (i.e., abrasion resistance, corrosion and oxidation resistance and particles erosion resistance). To meet all these requirements, abradable materials are normally composed of a metallic matrix and a non-metallic phase as solid lubricant with a controlled amount of porosity. A typical abradable material currently used in the compressor section of a gas turbine is the air plasma applied Al–BN coating, with an Al metal matrix and BN self-lubricating phase, and usually with a plasma sprayed NiAl as bond layer [8,9,10]
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