Combining 3D Scanning with Standard Microscopy Methods for Assessment of Pitting Corrosion in Turbine Blades

Abstract:

Pitting corrosion is a known process in metals. Pitting, observed in turbine blades, is caused by corrosion mechanisms in operation environment that may contain aggressive chemical compounds. Environment, material composition and operating conditions determine the corrosion process causing a cavity formation and their critical growth. Crack initiation can be a following mechanism. Thus the critical cavity size is the important parameter and its precise estimation can avoid a turbine blade failure. Cavity shape as well as corrosion products inside cavities should be taken into account. 3D scanning technique performed during turbine shutdowns is shown to be a suitable method for measurement of critical cavity size. In this study, the accuracy of 3D technique was confirmed by standard sample preparation methods followed by microscopy focused on corrosion products observation. Cross-sectioning of blades was performed close to the cavities, sections were grinded to show the maximum cavity dimensions and finally polished. Cavity size was measured in few steps by Light Optical Microscopy during grinding to find the maximum dimensions and compare to 3D measurements. Local corrosion in cavities was evaluated by Scanning Electron Microscopy using Backscattered Electrons and Energy Dispersive X-ray Spectroscopy. Non-uniform Fe, Cr, Si rich oxides with dimensions from tens up to hundreds of micrometres were analysed in cavities. No significant effect on oxide removing was observed after using sandblasting technology except possible effect in larger pits. Cracks and pores were detected close to the vicinity of trailing edge of blade.

• Periodical: Solid State Phenomena (Volume 270)
• Pages: 168-173
• DOI: https://doi.org/10.4028/www.scientific.net/SSP.270.168
• Online since: November 2017
• Authors: Petra Bublíková*, Vlastimil Habrcetl, Pavel Mareš, Roman Janura, Alena Kobzová, Zuzana Vávrovcová
• Keywords: 3D Scanning, EDS, Pitting Corrosion, SEM

References

[1] J. Qiu: Stainless Steels: Why they resist corrosion and why they fail. Corrosion special topical papers. School of Material Engineering. Nanyang Technological University. Singapore. Available from http: /www. corrosionclinic. com/corrosion_resources/stainless_steels_why_how_p1. htm.

[2] The titanium solution To Turbine Blades Reliability. EPRI journal, vol 10. No 9: November 1985, Palo Alto.

[3] J. Patera, P. Zahrádka: Corrosion pitting monitoring on low pressure turbine blades, 12th International Conference on Non Destructive Evaluation in Relation to Structural Integrity for Nuclear and Pressurized Components, October 4-6, 2016 Dubrovnik.

[4] G. Williams, H.N. McMurray: Pitting Corrosion of Steam Turbine Blading Steels: The influence of Chromium Content, Temperature and Chloride Ion Concentration. Corrosion, vol. 62, no. 3: March (2006).

DOI: 10.5006/1.3278269

[5] J. Kasl. el al: Turbines – methodology development for turbine blades control with using of information from EPRI" – objectives and current progress of the M1 - M4 and V1 stages, workshop from results and benefits of project VaV, 27th February, Pilsen.

[6] P. Bublíková, R. Janura: Monitoring of corrosion pits: Metallographic analysis of corrosion pits. Int. reports, PL 1137, 1203. 2015, 2016. Husinec – Řež.