Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal Corrosion fatigue failure of steam turbine moving blades: A case study Marko Katinić ⁎ , Dražan Kozak, Ivan Gelo, Darko Damjanović Mechanical Engineering Faculty in Slavonski Brod, Trg Ivane Brlić Mažuranić 2, Slavonski Brod 35000, Croatia ARTICLE INFO Keywords: Corrosion fatigue Steam turbine Corrosion pits Blade Failure ABSTRACT This paper describes the failure of a rotor of a condensing industrial steam turbine installed in a fertilizer production plant. There was a fracture of the two adjacent rotor blades in their roots. An analysis of the failure cause was carried out, which included the analysis of the fracture location on the steam flow path, the corrosion deposit analysis and the modal analysis of the rotor blades. Modal analysis was performed using the finite element method. It was concluded that the cause of the blades failure was a phenomenon known as corrosion fatigue. To extend the useful fatigue life of rotor blades in the future, the entire turbine stage of broken blades was redesigned. The modified design of the turbine stage has increased the fatigue safety factor by about 50% com- pared to the original design. 1. Introduction The failure of the rotor blade is a common problem of a steam turbine and it has been identified as one of the leading cause of unplanned outages for steam turbine. The blade fracture during turbine operation results in safety risk because it can lead to cat- astrophic turbine failure. It is well known that catastrophic failure of the steam turbine, large or small, can cause serious injury or even death. Usually it results in the total loss of the turbine, which causes the plant shut down for an extended period. In this case, the plant has large operational revenue losses. The turbine repair cost is very high, and repair time is rather long. Thus, the reliability of rotor blades is very important for safe and successful operation of a steam turbine. The flow path of steam turbines has several types of potential damage such as corrosion and erosion, which cause deterioration of turbine performance and strength reduction of internal turbine elements (blades, rotor discs, etc.). Localised corrosion is perhaps the most dangerous type of corrosion in combination with mechanical load, either cyclic or monotonic. This type of corrosion takes place at a region where the steam during its expansion crosses the saturation line. This region where condensation begins is known as phase transition zone (PTZ) or Wilson zone. Fig. 1 shows schematically the steam expansion line in PTZ and the possible causes of me- chanical damage to turbine elements. As can be seen there are several possible mechanisms of damage: stress corrosion cracking (SCC), pitting, corrosion fatigue (CF) and erosion in moisture zones [1]. Corrosion fatigue is the mechanical degradation of a material under the action of a synergistic combination of cyclic loading and corrosion. This is one of the main reasons for the premature failure of engineering structures and components. As shown in Fig. 2, the fatigue endurance limit for 13Cr stainless steel in aqueous solution of NaCl is drastically lower compared to air condition. The fatigue endurance limit is lower for solutions with higher NaCl content. For this reason, special attention should be paid to the design of turbine rotor blades in the PTZ and the control of the chemical impurities, especially chlorides, in the steam. https://doi.org/10.1016/j.engfailanal.2019.08.002 Received 13 January 2019; Received in revised form 20 July 2019; Accepted 4 August 2019 ⁎ Corresponding author. E-mail address: mkatinic@sfsb.hr (M. Katinić). Engineering Failure Analysis 106 (2019) 104136 Available online 13 August 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved. T