Wear 263 (2007) 258–264 Case study Slurry erosion of thermal spray coatings and stainless steels for hydraulic machinery J.F. Santa, J.C. Baena, A. Toro ∗ Tribology and Surfaces Group, National University of Colombia, Medell´ ın, Colombia Received 2 September 2006; received in revised form 30 November 2006; accepted 3 December 2006 Available online 23 May 2007 Abstract The slurry erosion of two coatings applied by oxy fuel powder (OFP) and wire arc spraying (WAS) processes onto sand-blasted AISI 304 steel was studied, and the results were compared to those obtained with AISI 431 and ASTM A743 grade CA6NM stainless steels, which are commonly used for hydraulic turbines and accessories. The adherence of the coatings to the substrate was measured according to ASTM C 633 standard, while the microstructure and worn surfaces were characterized by optical and scanning electron microscopy. Slurry erosion tests were carried out in a modified centrifugal pump, in which the samples were placed conveniently to ensure grazing incidence of the particles. The slurry was composed of distilled water and quartz sand particles with an average diameter between 212 and 300 m (AFS 50/70) and the solids content was 10 wt% in all the tests. The mean impact velocity of the slurry was 5.5 m/s and the erosion resistance was determined from the volume loss results. The coated surfaces showed higher erosion resistance than the uncoated stainless steels, with the lower volume losses measured for the E-C 29123 deposit. SEM analysis of the worn surfaces revealed intense plastic deformation in both coated and bare stainless steels, with little evidence of brittle fracture in the microstructure. The measured adhesive strength of the coatings was considered acceptable for the processes employed. © 2007 Elsevier B.V. All rights reserved. Keywords: Slurry erosion; Microstructure; Thermal spray coatings; Hydraulic turbines 1. Introduction Stainless steels are widely used in hydroelectric power plants due to their good corrosion properties and acceptable resistance to solid particle erosion, since many components are in con- tact with aqueous solutions containing hard particles that impact against the surface causing significant material loss (slurry ero- sion condition). The magnitude of the damage caused is a consequence of the amount, type and size of solid particles in the flow, together with the mechanical properties of the sur- faces, physical–chemical properties of the water and operating conditions [1,2]. Slurry erosion problems are particularly important during rainy seasons due to the increase in the number of solid par- ticles impacting the surfaces, especially in systems where an exhaustive filtration process is not possible. This is the case of the Francis turbines installed in a hydroelectric power plant in northwestern Colombia, where intense erosive wear has led to ∗ Corresponding author. Tel.: +57 4 425 5339; fax: +57 4 425 5339. E-mail address: aotoro@unal.edu.co (A. Toro). changes in surface texture and loss of adjustment between the liners and the spiral case, as can be seen in Fig. 1. The angle of incidence of the particles is extremely impor- tant to determine the main wear mechanism acting on the surface of the components submitted to erosion. It is well known that micro-cutting prevails for low impact angles whereas for angles close to 90 ◦ the dominant effects are low-cycle fatigue and accumulation of plastic deformation up to a critical value that promotes material detaching [3,4]. In addition, corrosive attack and boundary layer effects develop when the particles are carried by a liquid, configuring a much more intricate situation that is affected by the rheological properties of the carrying fluid such as its density and viscosity [5,6]. A cost-effective way to improve the slurry erosion resis- tance of the components is the application of thermally sprayed coatings [7,8]. The term thermal spray describes a family of pro- cesses that use chemical or electrical energy to melt (or soften) and accelerate particles of a material which is then deposited on a surface [9]. The coatings may have a good erosion resistance depending on the chemical and mechanical properties of the material deposited, the surface preparation prior to application and the deposition conditions [7–9]. 0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.12.061