Impedance Monitoring of Carbon Steel Cavitation Erosion under the Inﬂuence of Corrosive Factors J. Ryl z and K. Darowicki Department of Electrochemistry, Corrosion and Materials Engineering, Gdansk University of Technology, 80-952 Gdańsk, Poland The degradation of carbon mild steel under cavitation erosion–corrosion exposure was studied by means of a 20 kHz ultrasonic device with a piezoelectric inducer. Possibilities of estimation of material failure by current measurement techniques are presented and discussed. The effect of cavitation exposure of the mild steel has been investigated in situ with the dynamic electrochemical impedance spectroscopy technique. Estimation of the surface degradation rate is enabled by an adequate equivalent circuit selection. A dynamic impedance method was used to verify the impedance parameters online during the measurement. These measurements are assisted by estimation of the weight loss function commonly used for the evaluation of erosion–corrosion resistance of materials. © 2008 The Electrochemical Society. DOI: 10.1149/1.2840619 All rights reserved. Manuscript submitted October 1, 2007; revised manuscript received January 11, 2008. Available electronically March 3, 2008. Cavitation erosion is a very common effect in a turbulent ﬂow environment, such as ship propellers, hydrofoils, turbines, pumps, and heat exchangers. Erosion degradation of installation in direct contact with imploding cavitation bubbles is in most cases very undesirable, resulting in frequent breakdowns and shortening their operation time. 1 It is a complex phenomenon involving the interac- tions of hydrodynamic, metallurgic, mechanical, and chemical fac- tors and should be considered as a unique type of material damage in its own right. 2 Some research has been carried out to compare cavitation erosion resistance with other mechanical properties: hard- ness, strain energy, ultimate resilience, or fatigue resistance; 2,3 how- ever, no direct connections were assigned between the effect of cavi- tation and other mechanical treatments. Cavitation is mainly mechanical in its nature, although this type of damage is more severe when cavitation erosion and electrochemi- cal corrosion act synergistically. Wood et al. 4,5 pointed out some examples for synergistic interaction between corrosion and erosion: removal of the corrosion product ﬁlm from the surface, exposing bare reactive metal; local acidiﬁcation at erosion sites, accelerating corrosion rates and prohibiting ﬁlm formation; lowering the fatigue strength of metal by corrosion; removal of work-hardened surfaces by corrosion processes which expose the underlying base material to erosion mechanisms, etc. The overall damage arising from erosion and corrosion, including the interaction between them, is termed cavitation erosion–corrosion. Many researchers focused on the de- termination of this synergistic effect for different metals and alloys. Kwok et al. 6 observed that, for some investigated alloys, the fractional weight loss due to pure corrosion during cavitation expo- sure in 3.5% NaCl solution ranged up to 12%, while that due to the synergistic effect between erosion and corrosion ranged as high as 66%. Wood and Fry 7 concluded that the synergism occurring in the presence of mild corrosion was responsible for 50% of the overall mass loss rate. It was pointed out by Vyas and Hannson 8 that, for some metals, the existence of passive ﬁlms may accelerate or decelerate corro- sion, depending on the intensity of the cavitation and the metallur- gical state of the stainless steel specimens. A highly adhesive, resis- tant, and quickly repassivating layer is important for the reduction of corrosion induced by cavitation erosion. Despite the inﬂuence of corrosion and the synergistic corrosion/ erosion factor on cavitation, there have been only a few electro- chemical methods used to study the cavitation phenomenon up to now: Kwok et al. 6 investigated the free corrosion potential of a sample under cyclic cavitation exposure; Engelberg and Yahalom 9 constructed polarization curves of steel specimens eroded in an ul- trasonic vibratory cavitation device. The issue of constant monitoring of construction in order to pre- vent them from cavitation erosion–corrosion failure, as well as the determination of the proper time of replacement of eroded parts, is crucial. This problem is especially important in the case of pumps, turbines, or heat exchangers where direct, visual determination of erosion–corrosion damage is very hard to obtain, if not impossible. Currently, there is no unequivocal information in the literature on this subject. The aim of this research is the determination of the cavitation inﬂuence on the change of the electrochemical parameters of the system, and veriﬁcation of the dynamic electrochemical impedance spectroscopy DEIS technique for the effective monitoring of cavi- tation erosion–corrosion effects. DEIS can be used to evaluate the kinetics of the cavitation degradation process as well as provide quantitative and qualitative information on the corrosion factors in- ﬂuencing erosion–corrosion. Obtained results were compared with the mean depth of penetration rate MDPR function, which is a standard method for the determination of cavitation erosion resis- tance. Experimental Cavitation was induced in vibratory facilities, according to the ASTM G32 Standard. 10 The nature of the material damage was be- lieved to be similar to that obtained in hydrodynamic facilities. Most experiments in laboratory conditions are performed on vibratory ap- paratus, because of low erosion rates achieved in hydrodynamic cavitation tests. A vibratory facility gives the possibility of electro- chemical measurement, together with the analysis of the inﬂuence of chemical constitution, temperature, and pH. Vibratory apparatus is standardized and the most commonly used technique for cavitation generation in laboratory conditions. A schematic graph of the system used for cavitation erosion degradation is presented in Fig. 1. Measurements were carried out on mild steel. The chemical con- stitution is listed in Table I. Prior to each experiment, the samples were mechanically polished with 500–1600 gradation abrasive pa- per. Samples were mounted 1 mm below the horn of the piezoelec- tric transducer. The surface area exposed for cavitation erosion deg- radation was 1.22 cm 2 . Electrochemical investigations were carried out in a three- electrode system, with the measured sample as the working elec- trode, Ag/AgCl as the reference electrode, and platinum mesh as the counter electrode. All potentials in this article refer to a Ag/AgCl electrode. The solution prepared for the investigations was 3% NaCl. Despite the electrodes, every part immersed in the solution was an insulator to electric current. The cavitation excitation had a cyclic character, with an on/off period of 30 and 90 s, respectively. The measurement setup consisted of the electrochemical mea- surement software Gamry International, an ELPAN EG20 linear sweep generator, and a National Instruments PCI 6120 card that generated the perturbation signal, registered the voltage perturba- z E-mail: jacekr@chem.pg.gda.pl Journal of The Electrochemical Society, 155 4 P44-P49 2008 0013-4651/2008/1554/P44/6/$23.00 © The Electrochemical Society P44 Downloaded 24 Sep 2009 to 128.100.199.222. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp