Elsevier

Wear

Volumes 464–465, 15 January 2021, 203526
Wear

Damage mechanisms in cavitation erosion of nitrogen-containing austenitic steels in 3.5% NaCl solution

https://doi.org/10.1016/j.wear.2020.203526Get rights and content

Highlights

  • Changes in microstructure are shown by confocal microscopy after cavitation.

  • No indicators for corrosive damage such as pitting corrosion were found.

  • Nitrogen steels show different capabilities to absorb same cavitation impact damage.

  • Effect of salt content in water on cavitation bubble formation may reduce mass loss.

Abstract

Interruptions of the passive layer of stainless steels by cavitation erosion expose the bare metal surface to the environment and can lead to cavitation-erosion-corrosion damage and synergistic effects. However, the probability for pitting corrosion is decreased during cavitation exposure of stainless steels in chloride solutions because mechanical passive film removal shifts corrosion potentials to lower cathodic values. In this study, the impact of 3.5 wt% NaCl in water on mass loss and damage features of two austenitic stainless N-containing steels is investigated to amend the understanding of cavitation erosion of passivating steels. Ultrasonic cavitation tests were carried out on steels 316LVM and CNMo0.95 in distilled water and 3.5% NaCl solution. Exposed surfaces were characterized qualitatively by light- and electron-microscopy and quantitatively by confocal microscopy. Damage mechanisms vary between the two steels but not with NaCl content in the solution. 316LVM also displayed the same mass loss in both solutions. CNMo0.95 possesses twice the strength as 316LVM, resulting in lower intensities of ductile damage mechanisms and slower damage progression. Mass loss of CNMo0.95 was lower in 3.5% NaCl solution compared to distilled water, which was primarily assigned to the effect of the salt content in the water on cavitation bubble formation.

Introduction

The term cavitation refers to the formation of vapour or gas bubbles in a liquid due to a rapid reduction in local pressure [1]. An immediate rise of the pressure can lead to a violent collapse of these cavities. Such a collapse of a bubble close to an adjacent surface of a solid material can generate shock waves and a high-velocity liquid jet (microjet) which both can lead to plastic deformation and erosive wear, well-known as cavitation erosion (CE). CE damage is the most obviously noticeable damage type by cavitation. The accumulation of the mechanical shock damages become apparent as fatigue cracks on the surface of the material leading to material loss which commonly occurs in hydrodynamic systems, e.g. ship propellers, hydraulic turbines and pumps [2].

A further material loss during cavitation can arise from electrochemical processes which is referred to cavitation-erosion-corrosion (CEC). The electrochemical removal of material due to corrosion is typically avoided by the formation of a thin protective passive layer on top of the exposed surface of passive metals. The shock impacts originating from the collapsing cavitation bubbles can break this passive layer exposing the bare metal surface to the corrosive environment. Such passive metals are prone to local corrosion, e.g. pitting and intergranular corrosion. The interaction between mechanical and electrochemical damage can evoke synergy effects resulting in a higher total damage compared to both damage mechanisms acting separately. However, this synergy effect is not yet fully understood and can vary for different materials. Vyas and Hansson show that during cavitation exposure of a solution annealed stainless steel (equivalent to AISI 304 SS) in 3.5 wt% sodium chloride (NaCl) solution the probability of corrosive pitting is decreased by an increased pitting potential leading to a reduction in total damage and not an increase compared to the quiescent condition. An increase of the passivation tendency due to the creation of a more oxidizable environment over the metal surface by cavitation could be a reason for this observation [3].

The resistance against CE and CEC attack depends on many factors including material composition and the microstructure which enhances the complexity in identifying resistant materials. Corrosion resistant ductile metals with a high work hardening rate could be appropriate for applications in which CEC attack dominates.

Interstitially dissolved Nitrogen (N) in high alloyed austenitic steels leads to an improvement in mechanical properties, e.g. yield strength and tensile strength which both can be further increased by work hardening. With increasing N-content fatigue resistance and hardness increases and wear resistance improves [4]. The addition of N leads to a stabilization of the austenite phase γ, whereby the tendency of deformation induced ε und α′ martensite formation at low temperatures is reduced. Nickel, which also functions as an austenite stabilizer, but reduces the N-solubility, can be largely replaced by manganese (Mn), which increases the N-solubility [5]. Austenitic steels containing N can be divided into three classes: Cr–Ni steels with an N-content of 0.05–0.1 wt%, austenitic steels with an N-content of 0.1–0.4 wt%, e.g. with Cr–Ni–Mn–N alloy system and high nitrogen steels (HNS) with an N-content > 0.4 wt% with the main alloy systems Cr–Mn(–Ni)–N and Cr–Ni–Mo–Mn–N. An increased N-content in solid solution in austenitic steels improves the local corrosion resistance, especially against pitting and crevice corrosion [6]. The pitting corrosion resistance of 316 L with N-contents up to 0.22 wt% was shown to be improved with cold working from 0% to 20% [7].

Many studies show that the CE resistance can be improved by an increased N-content, e.g. a study by Qiao et al. in which an HNS (0.66 wt% N) exhibits a higher CE-resistance than a CrMnN steel with a lower N-content (0.3 wt% N). The higher work hardening rate of the HNS and the thick work hardening layer, formed by CE stress, may be a reason for the higher CE-resistance of the HNS. The dominant deformation mechanism in an HNS is planar slip since the low stacking fault energy (SFE) of the HNS inhibits the recombination of partial dislocations. Dislocations can be activated at low stresses due to the low Peierls potential in fcc metals and when the critical stress for the activation of dislocation is reached, the number of dislocations increases fast leading to a high slip band density, which increases the hardness [8]. The SFE influences the slip behaviour of dislocations, twinning and also the deformation induced γ → ε transformation, which occurs at a sufficiently low SFE [9]. The combinatorial interstitial alloying of Carbon (C) and N in CrMnCN steels leads, compared to a standard-austenitic steel like AISI 304, also to an improved CE-resistance, which is attributed to a lower SFE [10]. Bregliozzi et al. show that the CE-resistance of a N-containing steel (0.3 wt% N) can be increased by a reduction in grain size since plastic flow can be hindered by material pile-ups at grain boundaries [11]. Increasing the N-content in the surface layers of the bulk material by high-temperature gas nitriding (HTGN) leads also to an improvement of the CE-resistance. The incubation phase (IP), i.e. the time in which no material loss occurs, was 4.6 times longer and the erosion rate was 8.6 times smaller for a nitrided AISI 304 L steel compared to a non-nitrided AISI 304 L steel under CE stress. Σ3 grain boundaries (twins) were attacked twice as frequently as high-angle grain boundaries [12]. The susceptibility of twins, due to the high misorientation, was also observed in other studies [13,14]. CE-resistance is also influenced by grain orientations since they are characterized by a different number of slip systems and hence exhibit differences in plastic flow. The (001)-plane of an HNS (0.9 wt% N) was found to be most susceptible for CE-stress [13]. Niederhofer et al. show as well that, in case of a CrMnCN steel, the (001)-oriented grains are more prone to CE-stress than (111)- and (101)-planes [14]. It was observed that solution treatment of a high-nitrogen nickel-free stainless steel (S–HNSS) at 1150 °C and quenching in water leads to a decrease in hardness due to a bigger grain size and dissolved Cr2N precipitates, but also to a decrease in cumulative mass loss during CE experiments of up to 50% due to enhanced elastic properties compared to non-solution treated hot forged samples (HNSS) [15]. Interfaces between Cr2N precipitates were shown to be preferable sites for CE damage [15,16].

So far there are a few studies in which the CEC-resistance of N-containing alloys were investigated. Potentiodynamic tests in a study by Qiao et al. show that under cavitation conditions the corrosion current density of a nickel-free HNS (0.66 wt% N) in 3% NaCl solution is ≈100 times larger than in quiescent condition. The corrosion current density of the HNS under cavitation exposure was approximately three times larger compared to a CrMnN steel (0.3 wt% N), thus the general corrosion resistance of the HNS was lower compared to the CrMnN steel. Nevertheless, the pitting potential was shown to be higher for the HNS. For both steels, the free corrosion potential was shown to be shifted in cathodic direction. The cumulative (cum.) mass loss of the HNS and CrMnN steel after 8 h cavitation stress testing in 3% NaCl solution was 1.16 and 1.04 times higher, respectively, compared to tests performed in distilled (dist.) water. For both HNS and CrMnN steel the mechanical CE-stress dominates the whole damage process and the effect of the synergy between erosion and corrosion was low [8]. After 8 h cavitation exposure in ≈3% NaCl solution, a nickel-free HNS (0.66 wt% N) exhibited a 1.17 times larger mass loss compared to cavitation in dist. water. The synergistic effect in the total cumulative mass loss was evaluated to be 14.78% in ≈3% NaCl solution under cavitation exposure and the mechanical effects were dominant [16]. In another study by Kwok et al. the austenitic stainless steels 316 L and 304 show under cavitation conditions an active shift of the free corrosion potential. A difference in mass loss of the materials between cavitation in dist. water and 3.5% NaCl solution was not observed and the reason may be due to both the combination of stirring of the electrolyte by cavitation and the simultaneous breaking of the passive layer that disturb the local environment for pitting. Thus, the dominant damage mechanisms under cavitation in a solution with the presence of chloride ions tend to be mechanical and the synergy effect by corrosion-erosion was found to be negligibly low for stainless steels [17]. Nitrided surfaces (0.65 wt% N, 0.8 wt% N and 1.15 wt% N) of duplex steels by HTGN show an austenitic structure and exhibit a higher CE-resistance after cavitation exposure in sea water than duplex steels without treatment by HTGN. The HTGN treated duplex steels show a 14–20 times longer IP and 7.6–23.3 times lower mass loss rates compared to the non-nitrided duplex steels [18]. Corrosion on the other hand gained in significance, and the synergy effect of a duplex steel (0.02 wt% N) after CE tests in 3.5% NaCl solution was evaluated to be 36.5% [19].

The synergistic erosion-corrosion effects for N-containing steels vary in the aforementioned studies. Generally, aggressive chloride anions found in natural environments can provoke localized pitting corrosion on passive metals [20]. However, during cavitation exposure in 3.5% NaCl solution it was observed that the pitting potential increases [3] and that the open circuit potential (OCP) shifts to lower potentials compared to the quiescent condition. This is due to continuous mechanical passive film removal by cavitation erosion, which mitigates the pitting corrosion mechanism [21]. From these observations it can be assumed that pitting corrosion attack will not occur in stainless steels during cavitation exposure. Thus, localized corrosive damage is expected to be irrelevant in cavitation erosion of N-containing stainless steels, unless the OCP is higher than the pitting potential. The aim of this study is therefore to investigate the CE-resistance of two austenitic stainless N-containing steels (max. 0.1 wt% N and 0.875 wt% N) by means of ultrasonic cavitation tests, based on the standard ASTM G32-10 [22], in dist. water and 3.5% NaCl solution. The two steels further possess different strength and hardness, resulting in different mechanical damage mechanisms. This study shall provide further insights into the impact of a chloride solution on mass loss and damage features, and amend the understanding of cavitation-erosion of passivating steels.

Section snippets

Materials, specimen manufacturing and preparation

In this study the CrNiCN0.1 steel 316LVM (X2CrNiMo18-15-3; 1.4441) with max. 0.1 wt% N and the steel CNMo0.95 (X13CrMnMoN18-14-3; 1.4452) with 0.875 wt% N were tested both in solution annealed (SA) condition.

The stainless steel 316LVM exhibits an excellent corrosion resistance due to a high purity level by a vacuum melting method and is used therefore in biomedical applications, e.g. medical instruments [23]. The austenitic microstructure of the steel 316LVM, which features a comparably low

Erosion due to ultrasonic cavitation in dist. water and 3.5 wt% NaCl solution

The mean cumulative mass loss curves of the steels 316LVM and CNMo0.95, each after cavitation testing in dist. water and 3.5% NaCl solution, are shown in Fig. 3. Each cumulative mass loss curve in Fig. 3 is the calculated mean of the obtained values of three different tested samples. Scatter is marked by error bars for the measurement points in Fig. 3. For several measurement times the deviation between the respective obtained values for CNMo0.95 was too small to be plotted as error bar in the

Discussion

As assumed in the introduction, no corrosion damage after cavitation exposure was found for both steels under the conditions investigated in the present study. On the contrary, signs of a slight reduction of material loss in the presence of NaCl in the fluid were observed.

Conclusions

In this study ultrasonic cavitation tests following ASTM G32 standard in an indirect test set up were carried out on the low nitrogen containing steel 316LVM (max. 0.1 wt% N) and the HNS CNMo0.95 (0.875 wt% N) both in dist. water and 3.5% NaCl solution.

  • 1

    In SEM and confocal microscopy measurements no indicators for corrosive damage such as pitting corrosion were found. The dominating erosion mechanism during cavitation in both solutions is mechanically driven.

  • 2

    Both steels experienced similar

CRediT authorship contribution statement

Mario Paolantonio: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Stefanie Hanke: Conceptualization, Methodology, Validation, Resources, Data curation, Writing - review & editing, Supervision.

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.

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