Evaluation of cavitation erosion–corrosion degradation of mild steel by means of dynamic impedance spectroscopy in galvanostatic mode
Research highlights
► Galvanostatic DEIS allow investigation of cavitation erosion–corrosion. ► Evolution of CPE corresponds to changes of surface roughness. ► Rct drop precedes mechanical degradation and allow corrosion resistance monitoring.
Introduction
Degradation due to cavitation is a complex phenomenon, that involves the joint interaction of mechanical and chemical factors in a hydrodynamic environment [1], [2] and which should be considered as a unique type of material damage. In corrosive media, cavitation erosion degradation is connected with simultaneous corrosion of the material. In such situations, total degradation may exceed the sum of corrosion and erosion impact due to the presence of the synergistic effect of both factors [3]. Cavitation erosion has the effect of mechanically stripping off the corrosion film by the cavitation bubble collapse. Once the film has been removed, fresh reactive corrosion sites are generated depending on the re-passivation rate and the integrity of the film. Other factors also influence corrosion-enhanced erosion: increase in mass transport caused by the high turbulence of the solution or decrease in the fatigue strength by corrosion effects. Erosion-enhanced corrosion takes place in the case of local corrosion at grain boundaries or removal of the hardened surface to expose the underlying base metal.
In general, the nature of the interaction is determined by a number of factors like passivity of the metal surface, adherence of the corrosion products, metallurgical state, diffusion of dissolved oxygen, presence of aggressive ions and the intensity of cavitation [2], [4], [5], [6]. For instance, in 3% w/w sodium chloride solution, the fractional weight loss of cast iron due to pure corrosion ranges from 1% to 10%, while weight loss due to corrosion-induced erosion can range as high as 90% [7]. For copper and its alloys in seawater, synergism ranging to 50% of the overall material loss was determined in the presence of mild corrosion [8].
In erosion processes, it is clear that material removal occurs by mechanical means as a consequence of cavity collapse [2]. This can be enhanced by chemical and electrochemical factors. Few techniques have been used to study corrosion under cavitation conditions, but its mechanism is not yet fully understood. The basic tool for the evaluation of cavitation erosion resistance as well as the synergistic influence of corrosion on the degradation rate is a function of weight loss [3], [9], [10], combined with the corrosion potential and polarisation curves. Engelberg and Yahalom [11] obtained polarization curves of steel specimens eroded in an ultrasonic vibratory cavitation device. Kwok et al. [5] investigated synergistic effect of cavitation erosion and corrosion of various engineering alloys in 3.5% w/w NaCl solution. In their studies, exposure of most investigated materials to cavitation shifts the free corrosion potential towards more negative values; however, in the case of mild steel, cavitation exposure induced a shift of free corrosion potential by approx. 150 mV in the positive direction. Authors couple this effect with the corrosion film or product detachment and increase in mass transport under the generation of ultrasound. This positive shift of potential in the case of mild steels might be a reason of its low corrosion resistance.
Luo et al. and Jiang et al. [6], [11] carried out Electrochemical Impedance Spectroscopy (EIS) measurements of low-alloy steel in different solutions to elucidate the corrosion mechanism under cavitation conditions. The free corrosion potential was shifted to higher positive values and the registered data showed the diminished impedance values during cavitation exposure. However, within the time required to obtain a single impedance spectrum, corrosion resistance of material and its surface roughness changed significantly, which may lead to sample polarisation. Thus, the traditional approach of EIS is limited in such systems.
Precise determination of the dynamics of changes to electrochemical parameters under cavitation exposure plays a key role to understand the synergistic mechanism and minimisation of degradation caused by corrosive factors. Dynamic Electrochemical Impedance Spectroscopy (DEIS) proposed by Darowicki [13] offers an alternative approach. Some of its theoretical background can be found later in the manuscript. DEIS overcomes the problem of non-stationary conditions [14], [15], [16]. Ryl and Darowicki [16] have carried out DEIS electrochemical monitoring of cavitation erosion of carbon steel under the influence of corrosive factors. It was proven that cavitation erosion–corrosion has an influence on the impedance parameters, which depends on the excitation energy of the ultrasonic wave. Evaluation of the qualitative information of corrosion factors influencing the erosion–corrosion were presented and discussed.
Under the influence of erosion–corrosion, the free corrosion potential is shifted. Forcing the constant values of the potential during the experiment leads to the generation of polarisation currents, which do not occur in real conditions. This situation took place in the previous work of the authors [17], where the sample potential was fixed in the quiescence state. Measurements in the galvanostatic mode with the resultant current equal to zero can help examine the behaviour of a freely corroding sample under cavitation influence without any polarisation influence. The concept of modifications of EIS measurements in the galvanostatic mode is not novel and can be found in the literature [18], [19]. This paper presents possible applications of DEIS measurements in a three-electrode system in the galvanostatic mode as a tool to monitor the cavitation erosion–corrosion process.
Section snippets
Dynamic Electrochemical Impedance Spectroscopy
The EIS technique relies on three cardinal conditions: linearity, causality and stationarity, which need to be fulfilled in order to achieve valuable experimental data. The latter condition is the most difficult to achieve because the majority of the electrochemical processes are naturally non-stationary. In classical EIS, the investigated systems are excited step by step with perturbation signals of different frequencies. The principal disadvantage of such an approach is the length of
Experimental
Cavitation was induced with a vibratory facility, which takes high-frequency oscillations of the piezoelectric transducer, to generate pressure fluctuations with its tip immersed in the solution. These fluctuations are sufficient for generation and further implosion of cavitation bubbles. The piezoelectric transducer was vibrating at a frequency of 20 kHz with a peak-to-peak amplitude equal to 30 μm. Vibratory apparatus is standardised according to ASTM G32 [27] and is the most commonly used
Results and discussion
In Fig. 3a, Fig. 3b, one can see changes to the potential, which result from cavitation exposure of the investigated mild steel sample in the galvanostatic mode under zero current conditions. A sudden shift of the potential of around 200 mV to more positive values is a typical behaviour under the influence of the vibratory cavitation generator, although its value depends on the investigated material and solution [2], [4], [5], [17]. Such shifts are recurrent with each cycle and are most likely
Conclusions
The Dynamic Electrochemical Impedance Spectroscopy technique in the galvanostatic mode was used as a tool for observing the changes of electrochemical parameters while submitting mild steel samples to cavitation erosion. Obtaining instantaneous impedance spectra allowed the determination of changes to the electric circuit parameters of the system under cyclic cavitation exposure.
CPE parameters: Q and n changes were bounded mostly to the mechanical erosion of material surface, which leads to an
Acknowledgements
This research was supported by a grant financed by the Polish Ministry of Education and Science, N N507 4476 33. This research was supported by the European Union within the European Social Fund within the framework of the project “InnoDoktorant – Scholarships for PhD students, 1st edition”.
References (35)
- et al.
Tribol. Int.
(2002) - et al.
Mat. Sci. Eng.
(2000) - ASTM G119-93, ASTM, Philadelphia,...
- et al.
International Metals Reviews
(1986) - et al.
Mat. Sci. Eng.
(2000) - et al.
Corrosion
(2003) - et al.
Mat. Sci. Eng. A
(2003) - et al.
Tribol. Int.
(1991) - et al.
J. Fluids Eng.
(1989) - et al.
Corros. Sci.
(1972)
Corros. Sci.
J. Electroanal. Chem.
Electrochim. Acta
J. Electroanal. Chem.
Electrochim. Acta
J. Electrochem. Soc.
Corrosion
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