Impact of shot peening on corrosion performance of AZ31 magnesium alloy coated by PEO: Comparison with conventional surface pre-treatments

https://doi.org/10.1016/j.surfcoat.2022.128773Get rights and content

Highlights

  • Shot peening (SP) was studied as a pre-treatment step for PEO alongside conventional pre-treatments.

  • Degradation mechanisms of PEO coatings depend on the type of pre-treatment method.

  • Beneficial effect of SP on mid-term corrosion resistance of PEO coating was obtained.

Abstract

This paper deals with the evaluation of the influence of selected pre-treatments, namely shot peening, polishing and grinding, used prior to plasma electrolytic oxidation (PEO) on the corrosion characteristics of AZ31 magnesium alloy in solution of 0.1 M NaCl. Additionally, selected combinations of pre-treatment and PEO coating were optimized in terms of the PEO preparation time. Corrosion characteristics of prepared surfaces were determined by the electrochemical impedance spectroscopy (EIS) during 168 h of exposure at the laboratory temperature followed by the equivalent circuit analysis of measured Nyquist diagrams. Localised corrosion events of selected combinations of pre-treatment and PEO coating were observed by local electrochemical impedance spectroscopy (LEIM) in 1 mM NaCl supplied by the potentiodynamic polarization tests. The obtained results showed that despite the negative impact of shot peening on the electrochemical reactivity of AZ31 alloy, this technique significantly enhanced corrosion stability of the subsequently formed PEO coating.

Introduction

Magnesium and its alloys suffer from low corrosion resistance which diminishes their usage across different disciplines including automotive, aircraft, marine and biomedical. On the other hand, this group of materials offers a variety of significant benefits convenient in terms of ecological aspects such as low weight, high specific strength, biocompatibility or the possibility to be used as a sacrificial anode at the end of their lifetime. It is believed that low corrosion stability is caused by a mixture of Mg natural properties, namely negative standard potential (−2.36 V vs. SHE), presence of impurities responsible for galvanic coupling and quasi protective passive surface film consisting of MgO or/and Mg(OH)2 providing protection only in alkaline environments [1], [2], [3], [4], [5]. Insufficient corrosion resistance is usually overcome by modification of surface characteristics of Mg alloys by surface treatment, e.g., cathodic protection, paints, or coatings. The latter offers a whole range of techniques and procedures for successful control of magnesium corrosion rate, including PVD, fluoride-based coatings, organic coatings and least but not last, plasma electrolytic oxidation (PEO) [6], [7], [8], [9], [10], [11], [12]. PEO represents a novel, affordable, and easy to perform technique for the fabrication of coatings on lightweight alloys from a variety of eco-friendly electrolytes (mostly based on silicates, phosphates, or aluminates) using an external electrical power source in DC or AC regime. Coatings prepared by PEO offer increased mechanical properties represented by improved hardness, Young modulus or wear resistance. On the other hand, PEO coating would negatively influence the mechanical response under cyclic loads. These coatings provide improved corrosion resistance for Mg alloys despite their porosity; porosity is regarded as a negative feature since the corrosive species are able to penetrate through these tunnels towards Mg substrate, limiting the protection provided by PEO coatings [13], [14], [15], [16], [17], [18], [19], [20]. Generally, surface pre-treatment prior to the applied coatings plays a significant role in terms of the final electrochemical and mechanical properties; grinding, polishing and chemical etching are the most common pre-processing treatments applied in this case [21], [22], [23]. Another technique considered as a potentially suitable surface pre-treatment can be shot peening (SP). This method causes plastic deformation of the surface leading to enhanced surface hardness, induced compressive residual stresses and even surface grain refinement if the high energy process is applied. Important parameter of SP is its intensity expressed as the Almen intensity. Almen intensity measurement is a standardized process for evaluation of the kinetic energy transferred by the shot stream to material. Shot peening specifications refer to this energy as “intensity at saturation”. The measurement of peening intensity is accomplished by determining its effect on standard test strips. By this test is not determined the absolute kinetic energy, but it is a comparison method, which was widely accepted in production engineering [24]. The compressive residual stresses introduced by SP are well-known to greatly enhance the fatigue lifetime of cyclically loaded parts [25]. This treatment has been also applied to modulate the corrosion behavior of biodegradable materials like pure iron and Mg alloys [26], [27], [28]. Shot peening has been found to have adverse effects on corrosion resistance; while this is a positive feature for application of pure iron in biomedical field leading to improve its extremely slow biodegradation rate, it's effect on magnesium alloys is not as appreciated; this is because Mg alloys are already characterized with a high degradation rate leading to premature failure. The reason of decreased corrosion resistance after SP is explained by the enhanced surface reactivity of treated material which in the case of magnesium alloys leads to promoted corrosion degradation that is unacceptable in many applications [26], [29], [30], [31], [32], [33], [34].

In the study performed by Peral et al. [35], the authors have provided shot peening at room temperature up to 360 °C in order to evaluate the effect of surface treatment on the mechanical and corrosion integrity of AZ31 alloy. The results showed that increased temperature of SP had a negative effect on grain refinement and mechanical properties. Moreover, corrosion resistance of AZ31 measured in Ringer solution was worsened in terms of corrosion current density by SP performed at room temperature; this negative influence was even biased by the increased temperature of the SP process. The reason for this behavior was found in increased surface roughness caused by multiple impacts with high energy during the SP treatment [35]. On the other hand, Wu et al. proposed positive effect of high-energy SP on electrochemical characteristics of WE43 magnesium alloy in 0.9 % NaCl. The authors stated that besides increased mechanical properties, improved corrosion stability was reached due to the presence of compressive residual stresses leading to the creation of dense passivation film of Mg(OH)2. These statements were supported by potentiodynamic polarization tests indicating shift of the potential towards positive values and lower current density. Performed salt spray tests confirmed the abovementioned positive effect of SP by lower measured corrosion rates compared to untreated WE43 [36].

Studies dealing with SP used as a pre-treatment for PEO or other coatings are very rare to be found, especially on magnesium alloys. SP has been used as a pre-treatment prior to DCPD (dicalcium phosphate dihydrate) coating on AZ31 Mg alloy in [37]. The results of the performed experiments showed that although mechanical characteristics were improved, SP tended to decrease the size of DCPD coating units with increasing Almen intensity and increased the number of nucleation sites due to the enhanced surface reactivity. However, potentiodynamic tests and EIS measurements revealed that a combination of SP and DCPD could accelerate the corrosion rate of AZ31 alloys as a result of an insufficiently covered surface by DCPD units [37]. SP has been also used as a pre-treatment for PEO coating prepared from mixed silicate and phosphate electrolyte on 2024 Al alloy. The results of EIS analysis proved that duplex treatment of SP and PEO could not reach the corrosion resistance of the PEO coating alone. The authors explained this behavior by the undesirable presence of iron transported from the steel shots to the substrate as well as its possible occurrence within the PEO coating leading to an increased corrosion degradation [38].

There is an evident lack of knowledge about the effect of duplex treatment by SP and PEO technique on magnesium alloys. A combination of these procedures can bring favourable mechanical properties in terms of improved substrate surface hardness provided by SP and improved corrosion resistance given by PEO treatment, both needed properties to widespread application of magnesium alloys in transportation area represented by harsh operating conditions with superposed impact of corrosive species within environment and cyclic mechanical loading. Therefore, this paper is aimed at new knowledge about the effect of SP in comparison with other pre-treatment techniques on corrosion stability of the PEO coatings in chloride-containing solution with optimization of PEO processing time involved as well in order to provide the highest corrosion protection. The design of performed experiments is directly linked to our previously published results in [39].

Section snippets

Materials and methods

AZ31 Mg alloy was used as a material for experimental procedures. The alloy was fabricated by continuous casting and subsequently heat-treated for 16 h at the temperature of 420 °C. The chemical composition of the AZ31 measured by spectrometer QUANT'X EDXRF is given in Table 1.

Characterization of original treated surfaces prior to PEO treatment

Fig. 2 shows microstructure and corresponding cross section morphologies of treated surfaces. Microstructure of the cast AZ31 (Fig. 2a) consists of polyhedral grains of solid solution of Al, Zn and other alloying elements in magnesium [37]. Surfaces after grinding and shot peeing (Fig. 2b,d) are considerably more deformed compared to the polished one (Fig. 2c) with even-like appearance. However, most notable deformation is observed on the shot peened surface accompanied with the presence of

Discussion

One of the factors that was studied in terms of the durability of the PEO coatings was the effect of surface pre-treatment. Samples with PEO on the ground surface were partly included in the discussion for a better understanding of the electrochemical behavior. Based on the SEM images of the PEO coating prepared on the polished surface (Fig. 4) and corresponding roughness measurements it is clear that created coating is less uniform and porous compared to the layer formed on the ground surface

Conclusions

Based on the results of the performed experiments, the following concluding remarks can be stated:

  • The plasma electrolytic oxidation (PEO) coatings prepared on the shot peened and polished surfaces provided significantly improved corrosion resistance compared to the counterparts without PEO coating.

  • Coatings prepared on the polished surface reached lower Rsum compared to the coatings fabricated on ground surfaces due to the effect of higher surface roughness and a lower compactness of the coating.

CRediT authorship contribution statement

Daniel Kajánek: Conceptualization, Methodology, Investigation, Writing - original draft, review. Filip Pastorek: Methodology, Investigation, Validation, Formal analysis, Funding acquisition. Branislav Hadzima: Conceptualization, Investigation, Validation, Supervision, Funding acquisition, Sara Bagherifard: Methodology, Investigation, Validation, Writing - original draft, review. Michal Jambor: Investigation, Validation, Formal analysis. Pavol Belány: Investigation, Formal analysis. Peter

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The research was financially supported by the Science Grant Agency of the Slovak Republic through project No. 1/0117/21 and No. 1/0153/21 and Operational Program Integrated Infrastructure 2014–2020 of the projects: Innovative Solutions for Propulsion, Power and Safety Components of Transport Vehicles, code ITMS 313011V334 and Intelligent Operating and Processing Systems for UAVs, code ITMS 313011V422, both co-financed by the European Regional Development Fund. The authors would like to thank

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