Response of surface mechanical properties to electrochemical dissolution determined by in situ nanoindentation technique

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Abstract

An in situ nanoindentation technique was used to study the mechanical property degradation in the surface layer induced by electrochemical dissolution. The load–depth curves of indentation were measured using pure iron immersed in electrolyte. The experimental result showed that the anodic currents reduced both hardness and elastic modulus of the material surface layer, and the degradation effect declined with increasing distance from the surface. The electrochemical dissolution-induced degradation was explained by the generation of non-equilibrium vacancies in the metal surface layer during dissolution process.

Introduction

In industries, many failures of engineering components are related to the synergistic effect between mechanical and electrochemical factors. The examples include erosion–corrosion, stress corrosion crack and corrosion fatigue [1], [2], [3], [4]. Theoretically, failure process or property degradation of materials is often associated to irreversible thermodynamic processes. In the corrosion-related processes, at least two irreversible fluxes are involved, the electrochemical corrosion and the plastic deformation. In line with non-equilibrium thermodynamics, the irreversible processes will interact with each other [5], [6], [7]. Experimental observations have indicated that deformation can promote corrosion and the corrosion, in turn, can degrade the mechanical properties of materials [7], [8], [9], [10]. The corrosion-induced degradation of mechanical properties of metallic materials has been demonstrated by the accelerated creep rate and the hardness degradation resulting from anodic dissolution [8], [9], [10], [11], [12], [13].

Since corrosion occurs only on material surface, the corrosion-induced degradation of mechanical properties is expected to occur within a very thin surface layer [12], [13]. To further understand the response of mechanical properties to corrosion, a technique for mechanical property evaluation in micro or nanometric scale is needed. Nanoindentation provides a good approach for this intention.

Conventionally, the nanoindentation tests were performed in air [14], [15]. Recently Seo and Chiba [16], [17], [18] developed an in situ nanoindentation technique to study the mechanical properties of a passive film during electrochemical process. This method combines the nanoindentation technique and electrochemical measurement, thereby being able to determine the instantaneous response of surface mechanical properties to electrochemical dissolution.

In the present work, the in situ nanoindentation technique was used to investigate the influence of electrochemical dissolution process on the mechanical properties of the material surface layer.

Section snippets

Experimental methods

Test material was commercial pure iron (wt%: C, 0.003; Si, 0.01; Mn, 0.12; P, 0.006; Cr, 0.01; B, 0.009; Al, 0.32 and Fe balance). Specimens were in the shape of disks with 16 mm in diameter and 3.5 mm in height. Before tests, the specimens were mechanically ground with SiC abrasive paper to grit 600 and then polished with diamond paste of 6 μm and alumina particles of 0.05 μm.

The electrolyte employed for experiments was 0.01 M Na2SO4 solution and 0.1 M H2SO4 solution. The in situ indentation tests

Results and discussion

Fig. 2(a) shows the load–depth curves measured in air with the peak loads in the range of 200–2000 μN. With the analysis method mentioned above, the hardness data can be determined, as presented in Fig. 2(b). It is found that the hardness tended to decrease with the indentation depth. Similar results have also been reported for single crystal copper and silver [21], [22], and this phenomenon has been known as indentation size effect [23], [24].

Before carrying out in situ nanoindentation tests,

Conclusions

  • (1)

    In situ nanoindentation technique is a useful tool to study the surface mechanical property degradation caused by corrosion.

  • (2)

    The presence of anodic dissolution reduces the hardness and elastic modulus of iron in the surface layer. This effect declines with increasing distance from the surface.

  • (3)

    The hardness degradation resulting from anodic dissolution could be attributed to the generation of vacancies in the metal surface layer.

  • (4)

    Cathodic current from the reduction of oxygen does not influence the

Acknowledgement

This work was supported by Natural Sciences and Engineering Research Council of Canada.

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