A New Approach to Characterization of Gas Turbine Components Affected by Pitting Corrosion
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TECHNICAL ARTICLE—PEER-REVIEWED
A New Approach to Characterization of Gas Turbine Components Affected by Pitting Corrosion M. Jasiczek • J. Kaczorowski • E. Kosieniak M. Innocenti
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Submitted: 19 September 2011 / in revised form: 17 January 2012 / Published online: 8 February 2012 ASM International 2012
Abstract Among gas turbine components such as compressor blades made of precipitation-hardened martensitic stainless steel, a phenomenon known as pitting corrosion can occur on the surfaces of components during exposure to a marine environment. This can lead to surface degradation followed by crack initiation and further propagation with high-cycle fatigue as the main mode of damage. In order to obtain detailed statistical characterization of pitting severity, which may enable obtaining preventive models of the phenomena, a new study on pit size analysis and pitting inspection technique is proposed. In this investigation, pit depth values are considered as the main factor behind this type of failures. This article presents the results of pit depth assessment and other pitting characteristics with the use of confocal microscopy as a non-destructive technique. The results obtained show that confocal microscopy can be successfully used as a non-destructive tool for the evaluation of pitting severity. Keywords Pitting Corrosion Gas turbine Axial compressor Confocal microscopy
Introduction Pitting corrosion, also known as pitting, is a localized form of corrosion occurring in various components across the M. Jasiczek (&) E. Kosieniak Warsaw Institute of Aviation, Warsaw, Poland e-mail: [email protected] J. Kaczorowski General Electric Company Polska Sp. z o.o., Warsaw, Poland M. Innocenti General Electric Nouvo Pignone S.r.l., Florence, Italy
gas turbine industry, such as axial compressor blades or centrifugal compressor impellers [1, 2]. As a result of this type of corrosion, the metal surface suffers damage from the formation of small cavities, which correspond to the loss of material due to chemical reactions taking place during contact with a corrosive medium. The failure of any component due to the effects of pitting corrosion can occur by various means. The change in shape of the component’s surface due to pit development leads to an increase in the local stresses near the defect, with the pits acting as effective stress concentrators [3, 4]. When defect sizes reach a specific value, the resulting increase in stresses in the vicinity can initiate cracks and propagate them by a fatigue failure mode [4]. Gas turbines operating in a marine environment could be exposed to various corrosive media containing elements such as chlorine or sulfur. During their operation, inadequate filtering systems or lack of proper maintenance of the inlet air can contribute to the occurrence of pitting corrosion and, as a consequence, can lead to failure of gas turbine compressor blades. An example of such a failure in which pitting corrosion led to liberation of an airfoil is shown in Fig. 1. During the failure inve
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