The influence of nanocrystalline structure and processing route on corrosion of stainless steel: A review

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Corrosion Science xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Review

The inﬂuence of nanocrystalline structure and processing route on corrosion of stainless steel: A review R.K. Gupta a,b,⇑, N. Birbilis c a

Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA Institute for Frontier Materials, Deakin University, VIC 3216, Australia c Department of Materials Engineering, Monash University, VIC 3800, Australia b

a r t i c l e

i n f o

Article history: Received 22 January 2014 Accepted 27 November 2014 Available online xxxx Keywords: A. Stainless steel B. Polarisation C. Passivity C. Pitting corrosion

a b s t r a c t Nanocrystalline materials with a grain size <100 nm have attracted signiﬁcant attention over the past two decades. Various attempts have been made to prepare nanocrystalline stainless steel using various routes, along with the study of attendant corrosion properties. A nanocrystalline structure imparts an improvement in mechanical properties, coupled with distinct corrosion behaviour, not always leading to better corrosion resistance. This paper reviews the relevant works to date which have studied corrosion behaviour of nanocrystalline stainless steels, relating the performance to processing, along with attention given to mechanistic aspects which dictate corrosion of nanocrystalline stainless steel. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Stainless steel (SS), owing to its passivation ability and resistance to environmental degradation, has found wide applications ranging from kitchenware to critical components in nuclear reactors [1–6]. Many types of SS have been developed to meet various industrial demands, with the main alloying elements being chromium and nickel. Stainless steels are divided in four main groups based on their microstructure which is composition and processing dependent: ferritic, austenitic, duplex, and martensitic [4–6]. Corrosion behaviour of the various types of SS is well documented, along with the underlying mechanisms as discussed in the related monographs and journal literature [7–13]. Although stainless steels possess a higher density in comparison to the light metals (Al, Mg, Ti), their speciﬁc strength can be very high, which is in addition to high stiffness, fracture toughness, and excellent corrosion resistance [14–16]. Therefore, SS with improved strength and corrosion resistance could be next generation of metallic materials (due to high speciﬁc strength and durability) for the aerospace and several niche industries. Various strategies, e.g., thermochemical processing, nitriding, carburizing, alloying, etc. have been proved to be effective means of enhancing the surface as well as bulk strength [4,6,17–21]. Grain reﬁnement ⇑ Corresponding author at: Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA. Tel.: +1 330 972 7839; fax: +1 330 972 5856. E-mail address: [email protected] (R.K. Gupta).

has great inﬂuence on the mechanical strength and corrosion behaviour of SS [6,22–25]. Initial investigations have shown that grain reﬁnement can improve both the corrosion and mechanical properties; however, grain reﬁnement was limited to a few microns [6,21–32]. The ﬁnding that grain reﬁnement to <100 nm induces distinctive properties [33–37] stimulated research in developing SS with a nanocrystalline structure. Various nanocrystalline SS have been prepared in recent years via various processing routes and their properties have been investigated. A nanocrystalline structure in SS has been reported to impart signiﬁcantly higher oxidation resistance (owing to greater Cr diffusion and ease of formation of compact Cr-oxide layer) [38–43] and mechanical properties than [44– 49] conventional coarse grain counterparts of the same chemical composition. The unique properties of nanocrystalline materials are associated with a very ﬁne grain size and a large number of structural defects, i.e., grain boundaries and triple points [33–35,50,51]. Such a high fraction of structural defects in nanocrystalline materials can lead to a signiﬁcant increase in stored energy which may increase reactivity. This phenomenon is expected to have a dual effect on corrosion behaviour which depends upon material/environment system as reviewed in [52,53]. In passivating electrolytes (i.e., stainless steel in many aqueous environments) a nanocrystalline structure has been reported to lead to an improvement in corrosion resistance, whilst in depassivating electrolytes a decrease in corrosion resistance is reported [52,53].

http://dx.doi.org/10.1016/j.corsci.2014.11.041 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: R.K. Gupta, N. Birbilis, The inﬂuence of nanocrystalline structure and processing route on corrosion of stainless steel: A review, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.11.041

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Diffusivity of alloying/impurity elements in nanocrystalline materials are reported to be signiﬁcantly higher than that in conventional coarse grained materials due to considerably higher volume fraction of grain boundaries [54–58]. Detailed discussion of the diffusion processes in the nanocrystalline material is reported elsewhere [59–62]. The increase in the diffusivities of solute/impurity atoms in nanocrystalline alloys (i.e., P in Ni-P and Co-P alloys [63–67], and Cr in SS [53,68]) was reported to inﬂuence corrosion behaviour. However, recently it was proposed that the diffusion coefﬁcient of Cr in SS at room temperature, even in nanocrystalline alloys, was too low to cause any signiﬁcant inﬂuence on corrosion behaviour of SS [48,62]. These contradicting views related to the inﬂuence of diffusion on corrosion at room temperature require further investigations. The unique structure of nanocrystalline alloys, which is concomitant with increased free energy, alloying element diffusivity and chemical homogeneity, is expected to induce signiﬁcantly distinct electrochemical/corrosion properties [62]. Initially, it was believed that nanocrystalline materials (including SS), owing to a high surface energy, may have displayed inferior corrosion resistance [69]. However, in recent years, the corrosion resistance of nanocrystalline SS produced by various routes has been investigated and the reported data presents a range of differing corrosion behaviour. Inconsistency in the reported data may be attributed to be a result of the various processing routes and processing parameters used to fabricate a nanocrystalline structure; in addition to the inﬂuence of the various electrolytes in which testing has been executed. Processing route used to fabricate nanocrystalline alloys will not only inﬂuence the grain size, but also several other metallurgical parameters, i.e., chemical homogeneity, dislocation density, phase transformations (i.e., resolutionising), and morphology of inclusions (size range and distribution), etc. The inﬂuence of all these parameters on corrosion behaviour is as important as grain size. The aim of the present review is to investigate works performed on the corrosion of nanocrystalline SS, as fabricated by various processing routes, leading to a timely discussion of the critical factors inﬂuencing the resultant corrosion as presently understood.

2. Passivation and pitting corrosion of stainless steels The corrosion resistance of SS results from the presence of a thin ‘‘passive ﬁlm’’ upon the metal surface that is typically 1– 3 nm thick [11–13]. The phenomenon of formation of such a protective surface ﬁlm is called passivation. Various theories of passivation and passive ﬁlm growth kinetics have been proposed. One of the early models, known as the High Field Model (HFM) assumed that the rate determining step for ﬁlm growth was the transfer of cations between adjacent lattice sites within the passive ﬁlm. This model assumed that driving force for the transport of cations in the passive ﬁlm was an electric ﬁeld [70]. The Mott-Cabrera model was later proposed, which assumed that the rate-limiting step for the passive ﬁlm growth is the emission of metal cations from the metal into ﬁlm at the metal/passive ﬁlm interface [71,72]. The Mott-Cabrara model [71,72] was later modiﬁed by Fehlner and Mort [73] who proposed that the rate-limiting step in passive ﬁlm growth was the emission of anions from the environment into the passive ﬁlm at the passive ﬁlm/environment interface. Macdonald and co-workers proposed a comprehensive model of the passive ﬁlm growth [74,75] which has been evolving since the late 1970s. This model, known as point defect model (PDM), accounts for the cation and anion mobility, vacancy mobility, interaction among cations, anions and vacancies, and reactions occurring at the passive ﬁlm/electrolyte and passive ﬁlm/metal interfaces. The PDM has been extended to account for the break-

down of the passive ﬁlm and inﬂuence of alloying elements on passive ﬁlm [76–79]. Various theories of passivity and fundamentals of SS corrosion have been covered in key review articles and monographs [10–13,74–77,80–86]. It is now well documented that the addition of Cr to Fe leads to a dramatic improvement in the corrosion resistance, owing to the replacement of the surface (passive) ﬁlm from the native Fe-oxide to a stable Cr-oxide [56,87,88]. Whilst it is appreciated that a critical Cr content is essential to impart this phenomenon, understanding the enhanced corrosion resistance caused by the Cr derived passivation, and the precise role of Cr, took several decades. Further, it is generally reported that with the an increase in Cr content, the pH region of stable passivity enlarges, passive current density decreases, re-passivation potential shifts to lower (more negative) potentials, and pitting resistance increases significantly [87–90]. Corrosion behaviour of SS is attributed to depend upon the characteristics of the passive ﬁlm which is largely inﬂuenced by Cr content of the alloy. It is generally believed that selective dissolution of Fe and oxidation of Cr leads to the formation of Cr rich passive layer [91–98]. Characteristics of the passive ﬁlm and its relationship with the alloy composition have been investigated widely [99–104]. It was suggested that a Cr content >50% in passive ﬁlm was required for stable passivity [105]. The Cr content of the passive ﬁlm has been demonstrated to increases with increase in Cr content of the alloy [105–107]. An X-ray photo electron spectroscopy study of a series of Fe–Cr alloys indicated that the Cr content in the passive ﬁlm formed in 0.5 M H2SO4 increased abruptly when Cr content in the alloy was above 13% [106,107]. These reports suggested requirement of the critical Cr content in SS to cause passivity, which was later explained on the basis of percolation model [108–112]. Passive ﬁlm was proposed to be composed of an outer and inner layer [97,99–104,113]. Cr3+ was found to accumulate in the inner layer of passive ﬁlm [99,100,106] which was suggested to be pure oxide whereas outer layer was reported to be composed of hydroxides [97,99–104,113]. Chemical composition of the passive ﬁlm was found to depend upon the environment as well as applied potential [99–103]. It was shown that Fe2+, Fe3+ and Cr3+ were present at lower polarisation potentials, Fe3+ content was reported to increase with increase in potential [97,99,100,102]. The electronic properties of passive ﬁlms developed over variety of materials are investigated in the past a few decades [114,115]. It is shown that the passive ﬁlm behaves like a highly doped semiconductor and characteristics of the passive ﬁlm depend upon alloying additions and Cr content of the SS [115–120]. Increasing Cr content of the SS alters the electronic properties of the passive ﬁlm in a manner that improves stability of passive ﬁlm [115,116]. Details of electronics structure of passive ﬁlm and its dependency on composition can be found in [114–119,121–125]. The passive ﬁlm on SS is susceptible to breakdown as a result of aggressive ions (i.e. halides), pH changes, and temperature [126,127]. Local breakdown of the passive ﬁlm can lead to the localized corrosion known as pitting. Pitting can be characterised in three stages [128–130]: (1) Pit initiation or nucleation caused by the breakdown of the passive ﬁlm. Nucleation events are followed by repassivation. (2) Metastable pitting where pit growth stops on the verge of stability. Metastable pits grow for a ﬁnite time and size before repassivation. Metastable pitting is proposed to be a measure of pitting susceptibility [130–136]. (3) Pit growth where pits grow as long as the pit interior can maintain the sufﬁciently aggressive electrolyte such that repassivation is prevented.

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All the three stages of pitting are important and have attracted signiﬁcant research attention. Passive ﬁlm breakdown is complex and various theories for the passive ﬁlm breakdown have been proposed. A competitive ion adsorption theory [137] suggests that both O2 and C1 anions can be adsorbed onto metal surfaces. In conditions where C1 adsorption is favoured over O2 adsorption, breakdown of passivity occurs. Hoar and Jacob [138] proposed a complex ion formation theory which suggested that Cl ions are adsorbed around a cation in the passive ﬁlm surface which forms a complex. This complex is highly soluble and leads to thinning of the passive ﬁlm locally and further formation of complex with Cl. The process of formation of a high energy complex and its dissolution leads to ﬁlm breakdown. So-called ion penetration theory suggested that Cl incorporated into and migrates through the passive ﬁlm and upon reaching the metal/passive ﬁlm interface results in the ﬁlm breakdown [137]. Hoar [139] and Sato [140] suggested that the passive ﬁlm always contain a ‘‘ﬁlm pressure’’. Passive ﬁlm breakdown occurs when ﬁlm pressure exceeds a critical value. Macdonald and co-workers extended the point defect model for passive ﬁlm growth [74] to explain passive ﬁlm breakdown [76]. It was proposed that metal vacancies are created at metal/passive ﬁlm interface as a consequences of diffusion of metal cations from passive ﬁlm/metal interface to passive ﬁlm/electrolyte interface where metal vacancies submerge into the metal. Metal vacancies are suggested to pile up at metal/passive ﬁlm interface when the rate of cation diffusion exceeds the rate of submergence of metal vacancies. The passive ﬁlm collapses locally when metal vacancy concentration attains a critical value. These mechanisms proposes a plausible explanation of the passive ﬁlm breakdown. Metastable pitting behaviour of various SS in various electrolytes has been widely investigated [130–136]. Williams et al. [131,132] proposed that pitting rate is proportional to the total number of metastable pitting events. A similar view has been supported by various authors for SS [129,141] as well as for Al and its alloys [142–145]. Since metastable pitting events are large in number and therefore provides a statistically reliable information about pitting corrosion. Punckt et al. [146] performed real-time microscopic in situ visualisation of pitting events on stainless steel in 0.05 M NaCl and suggested that onset of the pitting corrosion is a cooperative critical phenomenon resulting from interactions among metastable pits. Once a stable pit is formed, its propagation is caused because of the development of a highly concentrated solution condition within the growing pit [147]. The composition and microstructure of SS has a strong inﬂuence on the pitting corrosion. The Cr content of SS has been demonstrated to play a dominant role in pitting corrosion [90,148]. The pitting potential of Fe–Cr alloys as measured in 0.1 M NaCl increased with Cr content [90]. A dramatic increase in pitting potential was reported when Cr content increased the critical 13% value needed to create a stainless character [90]. Such an increase in pitting potential was attributed to a sudden increase in the Cr content of the passive ﬁlm. This view is supported by Asami et al. [106] who investigated the Cr content of the passive ﬁlm developed upon series of Fe–Cr alloys in 0.5 M H2SO4 at 500 mVSCE. Cr content of the passive ﬁlm, as represented by Cr/ (Fe + Cr), increased by 6 times when Cr content in the alloy was increased from 13% to 15% [106]. These reports suggests that pitting corrosion of stainless steel has a dependence on the chemical composition of passive ﬁlm which was found to depend upon Cr content of the alloy. It was proposed that in binary Fe–Cr alloys, pit initiation and propagation depends upon formation of Fe clusters [112]. Presence of Fe clusters leads to the local dissolution and depending upon size of cluster, repassivation may or may not occur. When Fe clusters are large enough, pits form and propagate. The size of Fe

3

clusters depends upon Cr content of the alloy. Ryan et al. [149] investigated pitting corrosion of various Fe–Cr binary alloys in HCl and proposed that pitting does not occur when Cr content of binary alloys was >16%. However, Fe cluster formation occurred in alloys having a Cr content below a so-called critical value. The presence of MnS particles and sulphur (S) content of the SS play critical role in pitting corrosion of SS. Most of the pitting events occur around MnS particles, particularly in environments with a low Cl concentration [150–152]. Various probable mechanisms for pitting due to MnS are discussed in the literature. The formation of sulphates, elemental sulphur, and thiosulphate were proposed to be probably reasons for pitting due to MnS particles [153–155]. These models were criticized by Ryan et al. [156] who proposed the formation of a Cr depleted zone around the MnS particles and pit initiation in Cr depleted zone. Focused ion beam/secondary ion mass spectroscopy (FIB/SIMS) analysis provided experimental evidence of such Cr depleted zone around the MnS [156]. A Cr depleted zone was questioned by some researchers [157] and was supported by others [158]. Alloying elements play crucial role in pitting corrosion of stainless steel [121,129,159–161]. Ni is reported to impart a moderate improvement in the pitting resistance [90,162]. Addition of small amounts of Mo [163–167], N [168–170], W [171], V [171], and Cu [119,172] are reported to impart signiﬁcant improvement in pitting resistance of SS. Combined inﬂuence of these alloying elements is provided by pitting resistance equivalent number (PREN) [12,161,173–176]. The most common formulae to calculate PREN is [12]:

PREN ¼ %Cr þ 3:3ð%MoÞ þ 1:65ð%WÞ16ð%NÞ ðall compositions are in weight%Þ A major drawback of using PREN is that it ignores the inﬂuence of microstructural features and assumes that all the alloying elements are present in the matrix as solid solution. Carbon present in the SS forms carbides and therefore the amount of alloying elements present in the solid solution decreases with an increase in C content [12,175]. Various mechanisms for inﬂuence of these alloying elements on the pitting resistance are proposed in the literature and reviewed [129,148]. However, most of the proposed explanations are probable theories and a precise mechanistic understanding is not yet developed. A detailed discussion on the pitting behaviour of SS could be found in [76,112,126,141,147,177–182]. For the purposes of this paper, it is important to understand that Cr content, alloying elements and impurities, chemical homogeneity, morphology and distribution of secondary phases (i.e., MnS, inclusions), and compactness and homogeneity of the passive ﬁlm are the main parameters inﬂuencing the pitting corrosion of SS in given environmental conditions [126,156,157,183–185]. These parameters are inﬂuenced by nanocrystalline structure and processing route. Therefore pitting corrosion of SS is expected to be altered due to the nanocrystalline structure as well as processing route used to fabricate nanocrystalline SS. In summary, classical theories of corrosion/passivation/pitting for conventional coarse grained SS must be considered in the light of novel properties of nanocrystalline materials in order to understand the inﬂuence of nanocrystalline structure and processing route on corrosion performance of SS. 3. Inﬂuence of nanocrystalline structure and processing route on corrosion of stainless steel Investigating corrosion behaviour of nanocrystalline SS and underlying mechanisms has attracted considerable interest. Nanocrystalline SS used to study corrosion behaviour were prepared

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from several processing routes and were tested in various environments, which are expected to have a signiﬁcant impact on corrosion performance. It should be noted that processing techniques used to produce a nanocrystalline structure not only reﬁne grain size but also impart signiﬁcant changes in other metallurgical parameters which inﬂuence corrosion behaviour. For instance, sputtering and high-energy ball milling are known to cause extended solid solubilities and a homogenous microstructure. Segregation and inclusion formation may not occur in alloys produced by these techniques. Some of the techniques, e.g., equal channel angular pressing (ECAP) and rolling may reﬁne inclusions present in the parent material. The reported corrosion behaviour of SS also reﬂects the inﬂuence of processing methods and parameters employed. This may be one reasons for the reported inconsistency in corrosion behaviour of nanocrystalline SS. For example, most nanocrystalline SS produced by surface mechanical attrition treatment (SMAT) has inferior corrosion resistance that could be due to the dominating inﬂuence of defects generated during processing, or cross contamination by metals of differing nobility [186]. In order to develop a full understanding of the nanocrystalline structure, a detailed investigation of the nanocrystalline SS produced by various methods is required. Therefore a review of corrosion of nanocrystalline SS in this section is divided according to the processing routes being employed to fabricate nanocrystalline structure. 3.1. Nanocrystalline stainless steel produced by sputtering Sputtering is a vacuum evaporation process that physically removes portions of a coating material (called the target), and deposits a thin and ﬁrmly bonded ﬁlm onto an adjacent surface, the substrate [187–191]. The process occurs by bombarding the surface of the sputtering target with gaseous ions under high voltage acceleration. As these ions collide with the target, atoms or occasionally entire molecules of the target material are ejected and propelled against the substrate, where they form a very tight bond. The resulting coating is held ﬁrmly to the surface by mechanical forces [187–189]. This technique is known to produce amorphous materials, super saturated solid solutions, and nanocrystalline materials. Sputtered materials are reported to be more homogenous and exhibit extended solid solubilities [187–189] that can inﬂuence corrosion behaviour. Inturi and Szklarska-Smialowska [192] were ﬁrst to systematically investigate the effect of nanocrystalline structure on corrosion of SS. Corrosion behaviour of a conventional coarse grained 304 type SS (grain size of 30 lm) was compared with that of its nanocrystalline counterpart (grain size of 25 nm as shown in Fig. 1) in 0.3 wt.% NaCl. Corrosion resistance of the nanocrystalline SS was superior to that of coarse grained material, in spite of the same chemical composition and lower ratio of the Cr to that of Fe in the air born oxide on nanocrystalline SS. Breakdown potential of the nanocrystalline SS was reported to be approximately 850 mV nobler than that of coarse grained SS. The improved pitting resistance of the nanocrystalline SS was attributed to the high microstructural homogeneity and extremely reﬁned grain size [192]. However, the role of MnS and other inclusions was not considered. Schneider et al. [193] reported that inﬂuence of nanocrystalline structure on the corrosion performance of a Fe–10Cr alloy in 0.1 M Na2SO4 was largely dependent upon pH. Nanocrystalline Fe– 10Cr alloy showed improved pitting corrosion resistance for pH > 4, which was attributed to the increase in selective dissolution of Fe and therefore Cr enrichment of the passive ﬁlm [193]. Meng et al. [188] compared the corrosion behaviour of nanocrystalline Fe–10Cr alloy (grain size of 20–30 nm) with that of conventional coarse grained Fe–10Cr alloy (grain size 1000 lm) in 0.05 M H2SO4 + 0.25 M Na2SO4 and 0.05 M H2SO4 + 0.5 M NaCl. Corrosion

Fig. 1. Bright ﬁeld transmission electron microscope (TEM) image of sputter deposited nanocrystalline stainless steel. A solid ring shaped selected area deﬂation pattern due to nanocrystalline structure is presented in the inset [192].

current density of nanocrystalline Fe–10Cr alloy was reported to be greater than that of its coarse grained counterpart which was attributed to the higher stored energy (caused by greater defects) in nanocrystalline alloy. Passivation behaviour of nanocrystalline alloys was improved and was attributed to the faster diffusion of Cr to form passive ﬁlm. Mott–Schottky analysis showed lower donor density in the passive ﬁlm developed on nanocrystalline alloy. Similarly, Ye et al. [194] reported improvement in pitting resistance of nanocrystalline 309 SS in acidic NaCl due to the formation of a more compact and stable passive ﬁlm with lower donor density [194]. Inhibition of MnS formation in sputter deposited nanocrystalline coatings has been posited, but not experimentally studied. The inﬂuence of nanocrystalline structure on pitting resistance of a new type of austenitic SS was investigated in 3.5 wt.% NaCl where nanocrystalline structure exhibited signiﬁcantly improved pitting resistance [195,196]. Electrochemical noise analysis in combination with in-situ atomic force microscopy under anodic potential control conditions indicated that mechanism of pit initiation and growth in nanocrystalline SS was signiﬁcantly different than that of its coarse grained counterpart. Nanocrystalline structure exhibited a fast metastable pit initiation as well as re-passivation [195,196]. In a later study, this behaviour was attributed to the change in chemical structure (presence of elemental Mn and S) of SS surface due to nanocrystalline structure [197]. Presence of elemental Mn and S in nanocrystalline SS lead to higher metastable pitting rate but lowered the probability of transition from metastable to stable pit. On the other hand, in conventional coarse grained materials, these elements combined to form deleterious MnS which although decreased metastable pitting rate (due to decrease in number of electrochemical heterogeneities), probability of metastable pits transforming to stable pits was increased. One criticism of the work related to metastable pitting of nanocrystalline SS is that some critical experimental parameters (data acquisition rate, applied potential with respect to pitting potential, etc.) were not reported clearly in these studies. It becomes important in light of recent work [143,144] related to metastable pitting which shows the importance of applied potential. Greater Cr enrichment of the passive ﬁlm developed upon nanocrystalline structure was speculated to be reason behind the improved corrosion resistance of sputtered SS. It was indeed conﬁrmed later in year 2012 [68], where X-ray photo electron spectroscopy (XPS) studies revealed signiﬁcantly greater Cr content of the passive ﬁlm development upon nanocrystalline SS in 0.05 M H2SO4 + 0.2 M NaCl. Formation of more compact passive ﬁlm,

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3.2. Nanocrystalline stainless steel produced by cold rolling Wang et al. [198] compared the corrosion properties of bulk nanocrystalline 304 SS (produced by severe rolling technique) with that of the conventional coarse grained 304 SS in HCl. Nanocrystalline structure showed signiﬁcantly improved pitting resistance which was attributed to the formation of a compact passive ﬁlm due to stronger oxygen adsorption, larger work function, and weaker Cl adsorption [198]. Pitting resistance of nanocrystalline 316L SS produced by cold rolling followed by annealing was found to be superior in 3.5 wt.% NaCl [199]. Improvement in pitting resistance was speculated to be the Cr enrichment of surface layer due to faster diffusion of Cr through grain boundaries. To investigate the mechanism of improved pitting resistance caused by nanocrystalline structure, Pan el al. [200] compared the passive ﬁlm growth mechanisms in: (1) deep rolled bulk nanocrystalline SS (average grain size 100 nm), (2) magnetron sputtered nanocrystalline SS (average grain size 50 nm), and (3) conventional rolled coarse grained SS (average grain size 100 lm) by electrochemical measurements and in situ AFM observation in 0.05 M H2SO4 + 0.2 M NaCl solution. The growth of the passive ﬁlm on all three materials was three-dimensional and the growth rates were in decreasing order; nanocrystalline thin ﬁlm (sputtered) > bulk nanocrystalline (deep rolled) > coarse grained 304 SS. The growth mechanism of passive ﬁlm formation in coarse grained SS was termed ‘‘progressive’’ whereas it was termed ‘‘instantaneous’’ for nanocrystalline SS. The instantaneous mechanism was attributed to the faster nucleation of the passive ﬁlm posited to be caused by a nanocrystalline structure. The grain size of the nanocrystalline SS produced by sputtering is much ﬁner, whilst sputtering process is expected to result in a more uniform microstructure and lack of MnS formation. Deep rolling can also alter the morphology of inclusions and texture in addition to grain size. The inﬂuence of microstructural features on corrosion behaviour of stainless steel is expected to be important, and the difference in corrosion behaviour is a combined result of grain size and these microstructural changes. 3.3. Nanocrystalline stainless steel produced by high-energy ball milling followed by consolidation High-energy ball milling, also known as mechanical alloying (MA) is a solid-state powder processing technique involving repeated welding, fracture and rewelding of powder-particles in a high-energy ball mill [201,202]. This technique was originally developed to produce oxide-dispersion strengthened (ODS) nickel and iron-base superalloys for applications in aerospace industry [203]. Later, MA has shown to be capable of synthesizing a variety of equilibrium and non-equilibrium phases including nanocrystalline and amorphous materials, and recently it became the most versatile and economical process for synthesis of nanocrystalline materials, due to its simplicity, low cost, and ability to be scaled

up for large production. Synthesis of nanocrystalline metals and alloys, using high-energy ball milling and related phenomena, have recently been reviewed [201,202,204,205]. Gupta et al. prepared nanocrystalline Fe–Cr alloys using high energy ball milling and successfully consolidated them using a novel approach [45,206]. Nanocrystalline and coarse grained alloys thus prepared were used to investigate inﬂuence of nanocrystalline structure on corrosion performance in H2SO4 with and without Cl [48,62,207]. Corrosion behaviour of nanocrystalline Fe–Cr alloys (52 nm) was found to be superior to that of their coarse grained (1.5 lm) counterparts. Nanocrystalline structure was reported to decrease passive current density, increase breakdown potential (in presence of chloride ions), and decrease passivation potential and critical current density (Fig. 2). This marked improvement in corrosion behaviour was attributed to the greater Cr enrichment of the passive ﬁlm which was conﬁrmed by X-ray photo electron spectroscopy and secondary ion mass spectrometry (Fig. 3). Gupta et al. suggested that Cr enrichment of passive ﬁlm by diffusion of Cr from the bulk to the passive ﬁlm/metal interface at room temperature (as reported in literature) is less likely. An alternative mechanism for Cr enrichment: accelerated selective dissolution of Fe and oxidation of Cr due to nanocrystalline structure was proposed. Other possible mechanisms (formation of homogenous and compact passive ﬁlm with improved stability) for enhanced corrosion resistance caused by nanocrystalline structure were also discussed. Most of the investigations comparing corrosion behaviour of nanocrystalline and coarse grained SS were carried out on commercial alloys where presence of inclusions, impurities, segregation, etc. plays crucial role and observed corrosion behaviour was a combined effect of all these parameters. Therefore effect of grain size could not be investigated exclusively. Investigation carried out by Gupta et al. [48,62,207] was a discrete effort to understand the role of nanocrystalline structure where they chose a simple binary system to keep the level of impurities same in coarse grained and nanocrystalline material. Both coarse grained (1.5 lm grain size) and nanocrystalline specimen were prepared by the same processing route which resulted in two identical samples with only difference in the grain sizes. However, the temperature used to sinter coarse grained Fe–Cr alloys (840 °C) was higher than that for nanocrystalline alloys (600 °C), and therefore possibilities of formation additional phases cannot be disregarded. Detailed investigation of microstructure would help in developing further insight into mechanism of corrosion of nanocrystalline SS. 1200 1000 800

Potential (mVSCE)

higher ratio of Cr oxide to Fe oxide in passive ﬁlm, less incorporation of Cl in passive ﬁlm and outstanding formation ability of the passive ﬁlm were additional factors attributed to the increased corrosion resistance of nanocrystalline SS [68]. All the studies of nanocrystalline SS as produced by sputtering reported an improvement in pitting resistance [192,195–197], which was attributed to the reﬁned grain size by most the authors, whilst none of the studies cited herein reported the inﬂuence of sputtering on microstructural features, i.e., texture, segregation, inclusions, twinning, dislocations, etc. – which may have an attendant role on corrosion of SS. Future studies dedicated to the chemical and microstructural analysis in conjunction with corrosion behaviour of sputter deposited nanocrystalline SS will help in developing further mechanistic understanding.

600 400 200 0 -200

nc Fe20Cr cg Fe20Cr

-400 -600 10-3

10-2

10-1

i

100

(mA/cm2)

Fig. 2. Anodic polarisation curves of nanocrystalline (nc) and coarse grained (cg) Fe20Cr in 0.05 M H2SO4 solution showing a signiﬁcant decrease in passive current density, critical current density and passivation potential due to the nanocrystalline structure [48].

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Fig. 3. Typical XPS survey scan spectrum of the nanocrystalline (nc) and coarse grained (cg) Fe–20Cr alloys after potentiostatic polarisation at 300 mV/30 min in 0.05 M H2SO4 solution [62].

3.4. Nanocrystalline stainless steel produced by equal channel angular pressing (ECAP) ECAP imparts severe plastic deformation (SPD) which results in signiﬁcant grain reﬁnement [208,209]. One advantage of this technique is that the cross section of the ECAP extruded billet remains unchanged, and therefore a high level of strain can be imparted without reducing cross section [208,209]. ECAP has been widely used for the preparation of ultraﬁne grained light metals [210– 212], whilst the use of ECAP in producing nanocrystalline SS is relatively new [213,214]. Nanocrystalline SS (80–120 nm grain size) produced by ECAP showed and improved corrosion behaviour (as represented by lower corrosion current density and nobler corrosion potential in 0.5 M H2SO4) in comparison to its conventional coarse grained counterpart [215]. XPS characterisation performed on air born ﬁlm showed no obvious difference in Cr content or thickness due to nanocrystalline structure. Improvement in corrosion resistance of nanocrystalline stainless steel was attributed to the improved compactness of the passive ﬁlm, not to the chemical composition. However, it should be noted that the mechanism of formation of airborne ﬁlm and therefore its composition is expected to be different than passive ﬁlm developed in aqueous electrolyte. In addition to grain reﬁnement, ECAP is expected to inﬂuence twinning, texture, and perhaps reﬁnement of intermetallics (i.e., MnS). The inﬂuence of these parameters on corrosion of SS remains to be investigated. 3.5. Nanocrystalline stainless steel produced by cavitation Nanocrystalline surface layers of 316L with various grain sizes (varying from 91 nm to 167 lm) were prepared by cavitation followed by low-temperature annealing and their corrosion performance was compared in 0.9 wt.% NaCl via potentiodynamic polarisation tests [216]. The nanocrystalline structure was reported lower susceptibility to pitting corrosion and increase repassivation power as indicated by shift in the pitting and protection potentials in more noble directions, and lower corrosion current density. Increased electron work function (EWF) and Cr enrichment of passive ﬁlm, and more uniform passive ﬁlm were speculated to be responsible for increased corrosion resistance. 3.6. Nanocrystalline stainless steel produced by surface mechanical attrition treatment (SMAT) Surface mechanical attrition treatment (SMAT) is a relatively new technique where dynamic plastic deformation of surface is

induced by repeated impacts with high-velocity balls. Due to the severe plastic deformation, the coarse grained structure at the surface is reﬁned into the nanostructure without changing the overall chemical compositions [217–219]. The SMAT has been successfully applied in many metallic material systems including SS and various surface properties after SMAT have been investigated [219– 222]. In addition to grain reﬁnement, SMAT has been reported to impart signiﬁcant surface defects, inclusions from processing media, reﬁnement of inclusions present in parent material, and residual stresses, which all inﬂuences corrosion behaviour [12,186,222–225]. Corrosion performance (general corrosion as well as pitting corrosion) of nanocrystalline layer of 304 SS produced by sandblasting was found to be inferior to that of its coarse grained counter parts in 3.5 wt.% NaCl [223,224]. Annealing (350 °C/1 h) of the sandblasted surface, which retained the nanocrystalline structure (grain size of 20 nm), was reported to remove the surface imperfections and therefore an improvement in corrosion behaviour was reported. Scratch tests performed at various applied potentials, showed that passivation abilities of nanocrystalline (annealed) surface were superior to that of coarse grained 304 SS. Electron work function (EWF) of passive layer developed on nanocrystalline surface (annealed) was found to be greater than that on coarse grained SS. Improved corrosion behaviour of nanocrystalline (annealed) SS was attributed to the improved adhesion of passive ﬁlm and the faster diffusion of Cr. Hao et al. [225] compared corrosion resistance of nanocrystalline 316 stainless steel (produced by SMAT) in 0.1 M NaCl and reported signiﬁcant decrease in pitting resistance (as demonstrated by pitting potential and critical temperature for pitting), which was attributed to the formation of cracks during SMAT. Annealing was reported to improve corrosion resistance of nanocrystalline surface. Most recently, stress corrosion cracking susceptibility of nanocrystalline SS produced by SMAT [226,227] was found to higher than that of its coarse grained counterpart and this behaviour was attributed to the defects induced by SMAT process. Corrosion behaviour of coarse grained 1Cr18Ni9Ti SS was compared with that of nanocrystalline SS (produced via shot pinning, grain size of 145 nm) in 3.5 wt.% NaCl [228]. Nanocrystalline structure showed improved pitting resistance as shown by signiﬁcantly higher pitting potential and decrease in number of pits. Toppo et al. [229] used a novel thermo-mechanical surface treatment approach, involving conventional shot blasting followed by laser surface heating to engineer microstructural modiﬁcation in type 304 austenitic SS. Thermo-mechanical surface treatment resulted in the formation of ﬁne recrystallized grains with some straininduced martensite on the modiﬁed surface, and a signiﬁcant improvement in its resistance against uniform as well as pitting corrosion in deaerated acidiﬁed 0.5 M NaCl. Grain reﬁnement and dispersion of alumina inclusions on the modiﬁed surface were considered to be the key factors responsible for improvement in corrosion resistance. It was shown that the processing parameters used for SMAT have large inﬂuence on the corrosion properties of resulting surface [230,231]. Corrosion resistance was not only inﬂuenced by the extent of grain reﬁnement but also by the micro strain and defects induced during SMAT [230]. With increasing SMAT time, simultaneous decrease in grain size and increase in surface defects (e.g., cracks) were reported. Resulting surface properties (including corrosion) can be controlled by choosing right processing parameters, i.e., diameter of balls and SMAT time. Inferior corrosion resistance of nanocrystalline surfaces prepared by SMAT as reported in a few reports could be because of the deleterious inﬂuence of surface defects surpassed beneﬁcial effect of grain reﬁnement on corrosion performance. However, a verity of materials in a wide range of SMAT conditions must be investigated to reach any general con-

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clusion. Moreover, the inﬂuence of SMAT on phase transformation, inclusions from processing media, dislocation density, and micro cracks need to be characterised and their inﬂuence on corrosion behaviour remains to be investigated.

low disk shaped. Such pit geometry offers easier repassivation. Lager and semi-elliptic pits covered with ﬁnely perforated covers (Lacy covers) were observed in coarse grained SS. Perforated covers act as diffusion barrier and help in maintaining aggressive electrolyte inside the pits, leading to pit stabilisation [9,197,232]. Most of the investigations attributed the nanocrystalline structure alone as the primary reason for distinct corrosion behaviour, which seems a reasonable ﬁrst order interpretations, and in many cases is supported by the studies reviewed. For example, highenergy ball milled nanocrystalline and coarse grained materials had very similar chemical composition and microstructure except grain size – in essence allowing the investigation of grain size exclusively [48,62]. However, in case of commercial alloys where signiﬁcant amount of C, S and impurities are present; inﬂuence of processing parameters on metallurgical factors other than grain size becomes very important, and need future research attention.

3.7. General discussion Corrosion resistance of nanocrystalline SS produced by various processing routes is summarised in Table 1. Above discussion and Table 1 show that nanocrystalline structure as prepared by various processing routes could lead to a signiﬁcant improvement in corrosion performance of SS. Tables 2 and 3 summarises reported effects of nanocrystalline structure on the passivation behaviour and pitting resistance respectively. The observed difference as reported in the literature is largely dependent on the processing routes. Some of the studies showed that nanocrystalline SS prepared by SMAT had inferior pitting corrosion resistance, which was attributed to the micro cracks and impurities introduced by processing. It was shown that by choosing appropriate SMAT parameters and post-SMAT treatments (i.e., low temperature annealing), corrosion resistance can be increased. Corrosion current density of nanocrystalline SS in most of the cases was either close to that of coarse grained SS or higher which was attributed to increased surface energy. Critical current and passive current densities were decreased signiﬁcantly and passivation potential became less noble, indicating improved passivation abilities due to nanocrystalline structure. An example is given in Fig. 2 which shows a clear inﬂuence of nanocrystalline structure on passivation of a Fe20Cr alloy in acidic media. Critical current density, passivation potential, and passive current densities decreases signiﬁcantly. Detailed mechanisms of the inﬂuence of nanocrystalline structure on corrosion performance of SS are discussed in the following section. In chloride containing media, pitting potential of nanocrystalline stainless steel was found signiﬁcantly nobler than that of microcrystalline stainless steel. Pit morphology in nanocrystalline and conventional coarse grained SS was reported to be signiﬁcantly different (Fig. 4). Pits in nanocrystalline SS were smaller and shal-

4. Mechanisms for the inﬂuence of nanocrystalline structure on corrosion of stainless steel Published literature related to the corrosion of nanocrystalline SS as produced by various processing routes has been reviewed in the previous section. Most of the investigations showed that the corrosion performance, in particular passivation abilities and pitting resistance improved signiﬁcantly due to nanocrystalline structure. Recent studies, based on X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and electrochemical impedance spectroscopy (EIS) have helped in developing mechanistic understanding. In this section, mechanistic aspects of inﬂuence of nanocrystalline structure on the corrosion behaviour of SS are discussed. 4.1. Cr enrichment of the passive ﬁlm Most of the early investigations on corrosion behaviour of nanocrystalline SS speculated that the greater Cr enrichment of the passive ﬁlm developed upon nanocrystalline SS was responsible for enhanced corrosion performance. However, experimental

Table 1 Summary of reported corrosion behaviour (derived from potentiodynamic polarisation curves) of nanocrystalline stainless steel (produced by various processing routes). In this table corrosion parameters (Ecorr: corrosion potential, Ep: passivation potential, Eb: breakdown potential, icrit: critical current density and ip: passive current density, icorr: corrosion current density) obtained from various papers are presented. Superscript ‘‘nc’’ denotes nanocrystalline and ‘‘cg’’ denotes coarse grained materials. Material

Electrolyte

Preparation technique

Grain size (nm)

cg inc corr/icorr

cg inc crit/icrit

cg inc p /ip

cg Enc p Ep (mV)

cg Enc corr Ecorr (mV)

cg Enc b Eb (mV)

Reference

SS304 Fe10Cr

0.3 wt.% NaCl 0.05 M H2SO4 + 0.25 M Na2SO4 0.1 N H2SO4 (pH < 1) 0.1 N Na2SO4 (pH > 4) 0.05 M H2SO4 + 0.25 M Na2SO4 0.05 M H2SO4 + 0.5 M NaCl 3.5 wt.% NaCl 3.5 wt.% NaCl 0.05 M H2SO4 + 0.5 M NaCl 3.5 wt.% NaCl 3.5 wt.% NaCl 3.5 wt.% NaCl 1 M H2SO4

DC sputtering Magnetron sputtering

25 20–30

– 2.4

– 0.16

– 0.8

– 89

– 61

850 0

[192] [188]

Magnetron sputtering Magnetron sputtering Magnetron sputtering

40 40 <50

Similar Similar –

cg – inc p > ip Similar cg nc cg 2.5 inc p < ip Ep < Ep cg cg inc Enc crit < icrit – p < Ep

Similar – –

– – –

[193] [193] [194]

Magnetron sputtering

<50

Similar

–

–

–

700

[194]

Magnetron sputtering Magnetron sputtering Magnetron sputtering

20 50 50

– – –

– –

Similar – Similar – 100 –

100 50 –

900 700 650

[195,196] [197] [68]

Sandblasting Sandblasting + annealing High energy shot pitting Shot blasting + laser treatment Multi-pass cold rolling Deep rolling

20 20 18

– – –

– – –

cg inc p > ip – cg inc p < ip – cg inc < i p p –

150 150 –

150 0 600

[223,224] [223,224] [228] [229]

40 100

0.075

–

cg inc p < ip – cg inc p < ip –

36 mV cg Enc corr > Ecorr

500 400

[199] [200]

– –

0.25 0.17

0.55 0.37

0 0

0 119

[48] [48]

Fe10Cr Fe10Cr 309SS 309SS Austenitic SS 304SS 304SS 304SS 304SS 1Cr18Ni9Ti 304SS 316L SS 304SS Fe20Cr Fe20Cr

3.5% NaCl 0.05 M H2SO4 + 0.2 M NaCl 0.05 M H2SO4 0.05 M H2SO4 + 0.5 M NaCl

High-energy ball milling 52 High-energy ball milling 52

–

180 190

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Table 2 Summary of reported effect of nanocrystalline structure on the passivation behaviour of stainless steel. Material

Electrolyte

Passivation ability

Comments

Reference

Fe10Cr/magnetron sputtering

0.05 M H2SO4 + 0.25 M Na2SO4 0.1 N H2SO4 (pH < 1) 0.1 N Na2SO4 (pH > 4) 0.05 M H2SO4 + 0.5 M NaCl 3.5 wt.% NaCl

Improved

Lower critical current density and passivation potential. Higher repassivation potential. Lower done density and increased Cr content caused by diffusion

[188]

Decreased

Due to higher microcracks, inclusions from processing media, and residual stresses

[193]

Improved

Due to faster selective dissolution of Fe and therefore Cr enrichment

[193]

Improved

Homogenous, compact and stable passive ﬁlm

[194]

Improved

[223,224]

Fe10Cr/magnetron sputtering Fe10Cr/magnetron sputtering 309SS/magnetron sputtering 304SS/sand blasting and annealing 1Cr18Ni9Ti/high energy shot pitting 316L SS/multi-pass cold rolling 304SS/deep rolling

Fe20Cr/high-energy ball milling Fe20Cr/high-energy ball milling

3.5 wt.% NaCl

Improved

Improved diffusion of Cr, higher EWF of passive ﬁlm, Purportedly improved bonding between metal and interface No discussion

3.5% NaCl

Improved

Faster diffusion of Cr

[199]

0.05 M H2SO4 + 0.2 M NaCl 0.05 M H2SO4

Improved

Instantaneous nucleation and faster three dimensional growth of passive ﬁlm was suggested

[200]

Improved

[48]

0.05 M H2SO4 + 0.5 M NaCl

Improved

Cr enrichment of passive ﬁlm through a purportedly more rapid selective dissolution of Fe and oxidation of Cr, formation of homogenous and compact passive ﬁlm, improved oxygen adsorption Cr enrichment of passive ﬁlm through a purportedly more rapid selective dissolution of Fe and oxidation of Cr, formation of homogenous and compact passive ﬁlm, improved oxygen adsorption

[228]

[48]

Table 3 Summary of the reported effects of nanocrystalline structure on the pitting corrosion of stainless steel. Material

Electrolyte

Pitting Comments resistance

Reference

SS304/DC sputtering Fe10Cr/magnetron sputtering Austenitic SS/ magnetron sputtering 304SS/magnetron sputtering 304SS/magnetron sputtering 304SS/sand blasting and annealing 316L SS/multi-pass cold rolling 304SS/deep rolling Fe20Cr/highenergy ball milling

0.3 wt.% NaCl

Improved Attributed to nanocrystalline grain size and homogeneity of sputtered surface

[192]

0.05 M H2SO4 + 0.25 M Na2SO4 3.5 wt.% NaCl

Improved Attributed to lower done density and increased Cr content caused by faster diffusion

[188]

Improved Increased passivation rate of metastable pits. Improved formation and continuous growth of passive ﬁlm

[195,196]

3.5 wt.% NaCl

Improved Suppression of combination of Mn and S and therefore decrease in deleterious MnS

[197]

0.05 M H2SO4 + 0.5 M NaCl

[68]

3.5 wt.% NaCl

Improved Decreased diffusivity of defects in passive ﬁlms, decreased adsorption of Cl on the surface, higher Cr content in the passive ﬁlm caused by diffusion of Cr Improved Improved diffusion, higher EWF of passive ﬁlm, better bonding between metal and interface

3.5% NaCl

Improved Faster diffusion of Cr

[199]

0.05 M H2SO4 + 0.2 M NaCl 0.05 M H2SO4 + 0.5 M NaCl

Improved Instantaneous nucleation and faster three dimensional growth of passive ﬁlm Improved Cr enrichment of passive ﬁlm through faster selective dissolution of Fe and oxidation of Cr, formation of homogenous and compact passive ﬁlm, improved oxygen adsorption

[200] [48]

validation of greater Cr enrichment the passive ﬁlm was presented most recently [48,62,68,207]. Gupta et al. compared the Cr content of the passive ﬁlm developed upon nanocrystalline and coarse grained FeCr alloys in acidic medium (with and without chloride ions) using SIMS and XPS and reported that the passive ﬁlm in nanocrystalline alloys had signiﬁcantly higher Cr content [48,62,207]. Sputtering time required to reach to the base composition was similar in the two alloys, indicating similar thickness of passive ﬁlm developed upon nanocrystalline and coarse grained FeCr alloys. XPS scans as obtained after passivation at potential in the middle of passive region (Fig. 3), shows higher Cr content of the passive ﬁlm developed upon nanocrystalline alloy. Similarly, greater Cr content in the passive ﬁlm developed upon nanocrystalline 304 SS was reported by Pan et al. [68]. It was reported that in spite of higher Cr content, the oxidation states of Cr and Fe present

[223,224]

in the passive ﬁlms were similar [62]. Passive ﬁlm was composed of both oxides and hydroxides of iron and chromium. Additional peaks relating to metallic Fe and Cr were observed, which were attributed to the presence of thin passive ﬁlm and therefore signals from the base metal contributed in the analysis of passive ﬁlm [62]. Greater Cr enrichment caused by a nanocrystalline structure has been speculated due to higher fraction of grain boundaries and other structural defects, which are known to provide faster diffusion paths for Cr [68]. However, it was suggested that diffusion coefﬁcient of Cr at room temperature, even in nanocrystalline alloys was too low to cause any signiﬁcant inﬂuence on Cr content of passive layer through diffusion of Cr from the bulk to the passive ﬁlm [48,62]. This view was also supported by two previous studies where Cr content of the air born oxide ﬁlms on nanocrystalline and coarse grained SS was found similar. Gupta et al. [48,62] suggested

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4.3. Microstructural homogeneity

Fig. 4. SEM micrographs showing typical pit morphology in (a) coarse grained (b) nanocrystalline stainless steel [197].

Electrochemical heterogeneities, i.e., segregation, inclusions, precipitates, and intermetallics are present in the conventional SS, which are deleterious to overall corrosion performance and are major cause of pitting corrosion. Processing techniques used to produce nanocrystalline SS are known to develop a more homogenous microstructure and many of the investigations, particularly sputtered deposited and high-energy ball milled alloys, suggested that removal of segregation and formation of more uniform microstructure to be the main reason of improved corrosion performance of nanocrystalline SS [192,194]. According to the percolation theory, passivation is achieved only when a certain number of Cr atoms surround Fe atoms [108–112]. In conventional stainless steel, segregation of Fe atoms decreases the percolation tendency which increases pitting susceptibility. However, in nanocrystalline alloys, atoms are more uniformly distributed and tendency of segregation is less which leads to more percolation and more uniform passive layer causing improved passivation abilities and higher pitting resistance [108–112]. The size and chemical composition of the inclusions/precipitates in SS has a signiﬁcant effect on the pitting resistance. Inclusions of size below a critical limit are known not to initiate pit irrespective to the chemical composition. The critical size was reported to largely depend upon the chemical composition of the inclusion. It was reported that MnS particles <300 nm as present in type 304 SS do not cause pitting corrosion when exposed to 0.6 M NaCl [233]. Nanocrystallization is reported to remove and/ or reﬁne the MnS particles present in the commercial stainless steel which is expected to improve over all corrosion performance [194]. The morphology of MnS seems highly dependent on the processing route, whilst the sputtering process was posited to inhibit formation of MnS. High-energy ball milling of commercial SS was not performed and hence whether it is a technique which removes MnS is unknown. Pan et al. [197] reported the existence of elemental Mn and S in nanocrystalline SS as prepared by DC magnetron sputtering, whereas Mn and S combined to form MnS in conventional SS. Elemental Mn and S were reported to be less deleterious in comparison to MnS particles and therefore corrosion performance of nanocrystalline SS was found to be improved. 4.4. Increased surface activity

that greater Cr enrichment of passive ﬁlm caused by nanocrystalline structure seems possible to explain based on selective dissolution of Fe and oxidation of Cr during corrosion process. Nanocrystalline structure increases reactivity, which accelerates the selective dissolution of Fe and oxidation of Cr to Cr-oxide or hydroxide [48,62]. This process leads to the development of a passive ﬁlm with higher Cr content. 4.2. Electronic structure of passive ﬁlm The point defect model (PDM) as proposed by Macdonald and coworkers [74–76] suggests that density and diffusivity of point defects (e.g., oxygen vacancies and metal cation vacancies) play important role in passivation and pitting. The growth and breakdown of the passive ﬁlm involves generation and annihilation of these point defects which is inﬂuenced by the substrate material. Thus, the key parameters in determining the passivity and pitting are density and the diffusivity of the defects in the passive ﬁlm. These parameters can be determined by Mott–Schottky analysis. Most of the early investigations reported that donor density of the passive ﬁlm formed on nanocrystalline SS was signiﬁcantly less than that of coarse grained SS which indicated improved passivation behaviour and pitting resistance of nanocrystalline alloys [188].

Increased surface activity of nanocrystalline SS leads to the acceleration of reactions causing the passivation, i.e., dissolution of Fe and oxidation of Cr. Therefore, passivation potential of nanocrystalline SS was reported to be less noble than that of conventional coarse grained SS of same chemical composition [48,62,207,234]. Some of the studies showed that electron work function (EWF) is a measure of activity and reported that passive ﬁlm developed upon nanocrystalline alloys had higher EFW, indicating them to be less reactive and more protective [223]. Some of the studies showed than nanocrystalline structure lowers EFW of the alloy, leading to the higher reaction rate and also easier formation of passive ﬁlm [69,198]. Attempts were made to link EFW of alloy with the adhesive strength of the passive ﬁlm and reported that higher reactivity of nanocrystalline surfaces leads to enhanced adhesion strength between passive ﬁlm and the alloy surface due to the increase in the electron activity at grain boundaries and possible pegging of the passive ﬁlm into the grain boundaries [69,235]. 4.5. Formation of compact passive ﬁlm Nucleation and growth is the main mechanism of passive ﬁlm formation. The nucleation of oxide is favoured at the high energy sites, i.e., surface defects in the form of dislocations, grain bound-

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aries, triple points, impurities, etc. Since nanocrystalline materials are composed of the large fraction of surface defects therefore they offer a large fraction of closely spaced nucleation sites. The presence of closely spaced nucleation sites reduces the lateral distance necessary for the lateral growth of a uniform oxide layer to cover the entire surface. Change in the nucleation and growth mechanism due to nanocrystalline structure and therefore enhanced passivation abilities of 304 SS is reported in literature [200]. Compactness of the passive ﬁlm was investigated by monitoring the passive current as a function of time as it was shown that current versus time plot usually follows the following relationship [68,194,236]

I ¼ 10ðAþk log tÞ

ð1Þ

where k represents the slope of double-log plot for the potentiostatic polarisation. k = 1 indicates the formation of compact, highly protective passive ﬁlm, whilst k = 0.5 indicates the presence of porous ﬁlm growing as a result of a dissolution and precipitation processes. Based on above analysis, many of the investigations proposed formation of compact passive ﬁlm on nanocrystalline alloys where k was close to 1 for nanocrystalline SS [68,194]. More recently AFM studies suggested formation of more uniform passive ﬁlm over nanocrystalline surface as large fraction of grain boundaries leads to instantaneous formation of passive ﬁlm [200]. 4.6. Decreased probability of metastable to stable pit formation Pitting corrosion of nanocrystalline stainless steel has been investigated widely and reviewed recently [53]. Morphology of pits in nanocrystalline and microcrystalline materials was very different (Fig. 4). Current transients, a representative of metastable pitting events, obtained under potentiostatic control showed a marked inﬂuence of nanocrystalline structure [53]. Metastable pitting rate and its repassivation probability in nanocrystalline material was signiﬁcantly higher than that in microcrystalline alloy. It was suggested that increased metastable pitting rate was due to: (1) elemental distribution of Mn and S, and (2) higher activity of nanocrystalline surface [53]. Nanocrystalline structure lead to faster repassivation rate and therefore probability of metastable to stable pit transition in nanocrystalline SS were lower. 4.7. Incorporation of elements from processing media Processes used to produce nanocrystalline alloys are known to form extended solid solubilities and formation of metastable phases. Possibilities of incorporation of elements from processing media (e.g., high-energy ball milling media, atmosphere used in high-energy ball milling or sputtering) cannot be overruled. Corrosion of SS is reported to be sensitive to small additions of elements, e.g., N, S, C and therefore incorporation of elements from processing media, even in very small amounts, is expected to have significant impact on corrosion properties. For instance, sputter deposited pure Al thin ﬁlms exhibited high pitting potential which was initially attributed to the grain reﬁnement and improved electrochemical homogeneity [237–240]. However such large inﬂuence on pitting corrosion due to these parameters in pure metals like Al seems less likely. More recently, it has been shown that a signiﬁcant amount of O was incorporated in sputter deposited Al [241]. The incorporation of O to the sputtered Al ﬁlm is attributed to the enhanced pitting resistance [242]. These studies could be very important in explaining improved corrosion resistance of nanocrystalline SS. N, O, or other elements from atmosphere could be introduced into SS during synthesis of nanocrystalline surface and could inﬂuence corrosion properties. Possibilities of incorporation of these elements depend upon processing method. Dedicated

studies investigating the possibilities of incorporation of O, N, Ar, etc. from the processing atmosphere could be very useful in explaining change in corrosion/mechanical properties of these alloys. 4.8. Oxygen adsorption According to adsorption theory [85,243,244], the passivity is regarded as adsorption of atoms which retarded the metal dissolution. Increased adsorption of oxygen on nanocrystalline alloys is reported [34,35,198,245,246] and therefore it may be one of the reasons of increased passivation ability of nanocrystalline SS. Wang et al. [198] showed an increase in oxygen adsorption on nanocrystalline SS surface and attributed it to be one of the reasons leading enhanced corrosion resistance of nanocrystalline SS. 5. Prospects and gaps Corrosion behaviour of SS as prepared by a variety of processing routes has been reported. Most of the investigations regarding the corrosion behaviour of nanocrystalline SS have associated grain reﬁnement with improved pitting corrosion resistance. The role of processing route and the associated processing parameters that impart signiﬁcant microstructural changes are not well investigated. Processing routes used for preparation of a nanocrystalline structure are expected to result in homogenous microstructures without any clustering, which is expected to result in improved resistance against pitting corrosion. Synthesis techniques, e.g. sputtering, high-energy ball millings are known to cause extended solid solubilities and therefore incorporation of elements from processing media/atmosphere is plausible. Microstructure and electrochemical characteristics of nanocrystalline SS must be investigated at atomic level to understand the corrosion mechanisms. Alloying SS with low levels of Mo, Mn, S, W, N, Ta, C, Cu, etc. plays an important role in microstructure and corrosion. The review of the role of such elements is beyond the scope of this study, whilst such alloying effects are discussed brieﬂy in section two of this paper and covered in monographs for conventional SS [11,12]. The inﬂuence of these alloying elements on the corrosion behaviour of nanocrystalline stainless steel has not been investigated and needs future research attention. The proposed mechanisms for the Cr enrichment of the passive ﬁlms as presented in the associated works remain somewhat rudimentary. The faster diffusion of Cr leading to higher Cr enrichment of passive ﬁlms is yet to be validated by speciﬁc experiments. The investigations of the diffusivity of Cr from bulk to metal/electrolyte interfaces would help in developing a better understanding. Compositional characterisation of passive ﬁlm developed over nanocrystalline SS in various environmental conditions has attracted only limited attention. Detailed characterisation of the passive ﬁlm (i.e., oxidation states and quantiﬁcation of elements as a function of thickness of the passive ﬁlm) should help in understanding the mechanistic aspects of passivity in nanocrystalline SS. The inﬂuence of nanocrystalline structure on the various stages of pitting corrosion (i.e., pit initiation, metastable pitting, and pit growth) has not been investigated in detail, and therefore the inﬂuence of a nanocrystalline structure on these stages is not known. Perhaps most critically, if nanocrystalline SS are ever considered for in-service use, the repassivation potential as obtained via cyclic potentiodynamic polarisation, is a key parameter that is not reported in literature in the context of nanocrystalline SS. Detailed future studies investigation various stages of pitting corrosion and repassivation mechanism will further help in understanding role of nanocrystalline structure in passivation and pitting of SS.

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One of the biggest challenges limiting the application of nanocrystalline SS is their poor ductility [247–249] and thermal stability [250–252], and therefore lack of processing techniques capable of producing large samples [206]. Some recent studies [45,253– 255], however, have shown that inferior ductility as reported in nanocrystalline materials it is not the inherent property of nanocrystalline structure and could be a manifestation of defects induced during the processing. Consequently, dedicated work is required to explore the possibilities of producing nanocrystalline SS with reasonable ductility whilst retaining a high yield strength. Compaction of nanocrystalline SS powders as produced by highenergy ball milling (powder metallurgy route) seems a viable method to produce large samples; however, high strength and poor thermal stability of nanocrystalline materials are the biggest hurdles. Issues of high temperature stability were investigated and various methods to hinder grain growth were proposed recently [256–259]. Use of small amount of grain growth stabilizer (i.e., Zr addition) can pin down the grain growth and these materials can be processed at higher temperature leading to improved properties [259–261]. Recently various methods, e.g. spark plasma sintering, annealing prior to compaction, in situ consolidation, etc. have been proposed for consolidation of ball milled nanocrystalline powder. Dedicated work, combining modern processing techniques and understanding of properties of nanocrystalline SS should help in developing techniques for production of large samples. Stress corrosion cracking (SCC) susceptibility of ﬁne microstructure generated by various machining [262–264] or intentionally [227] developed nanocrystalline structures was analysed and it was found that a nanocrystalline structure was more susceptible to SCC. However, SCC susceptibility seemed to be induced by the micro cracks generated during processing. Nanocrystalline SS possess a large fraction of defects and therefore diffusivity of hydrogen can be increased, which is expected to inﬂuence hydrogen embrittlement susceptibility of SS. Similarly, intergranular failure (be it corrosion or embrittlement due to potentially enhanced H-levels) of SS may be inﬂuenced by a nanocrystalline structure. However, the inﬂuence of nanocrystalline structure on the crevice corrosion, intergranular corrosion (IGC), and hydrogen embrittlement of SS has not yet been investigated. Any investigation regarding the effect of a nanocrystalline structure (produced by various processing routes) on the SCC, IGC, HE, and crevice corrosion of SS will be of great interest.

6. Potential applications of nanocrystalline stainless steel The nanocrystalline SS can be used to prepare corrosion/wear resistant coatings – the latter reliant on superior mechanical properties from nanocrystalline structure. Many of the reported techniques, i.e., sputtering, pulse electrochemical deposition, and SMAT can be readily used for this purpose. Rolling can be used to produce bulk nanocrystalline or ultraﬁne grained SS with improved mechanical properties, and in the appropriate environment, improved corrosion resistance. Methods for grain reﬁnement such as ECAP are more difﬁcult to upscale, however for particular applications may be suited; whilst extreme deformation imparted via high pressure torsion (HPT) only remains relevant to scientiﬁc studies. Due to the improved diffusivity of elements in nanocrystalline materials, nitriding, carburizing, etc. may be performed at lower temperatures or different operating conditions, and could be used to prepare functionally graded materials. The strength of nanocrystalline SS has been reported to be signiﬁcantly higher than that of conventional coarse grain SS [44–47]. For instance, the yield strength of nanocrystalline 316L SS as produced via SMAT was reported to be 1450 MPa, which is signiﬁ-

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cantly higher than that of conventional 316L SS [44]. Similarly, the yield strength of in situ consolidated nanocrystalline Fe– 20Cr–10Ni (grain size <10 nm) was reported to be 2.6 GPa [45]. Such increased strength leads to very high speciﬁc strength, which is in addition to corrosion resistance and high stiffness, may provide nanocrystalline SS unique property proﬁles. Nanocrystalline SS produced by high-energy ball milling has the potential to replace sintered stainless steel components (prepared by compaction and sintering of SS powder) which are used in the automotive industry [265]. These components usually require excellent high temperature corrosion resistance (as engine parts are exposed to high temperatures), wear resistance, and high strength. Published data related to high temperature oxidation of Fe–Cr alloys shows that nanocrystalline Fe–Cr alloys offer signiﬁcantly improved oxidation resistance at lower Cr content in elevated temperature applications [38–43]. Use of nanocrystalline SS powders are expected to improve the quality of conventional sintered components. A criticism of high-energy ball milling for production of nanocrystalline SS is the associated high cost, which involves production of metal powder prior to high-energy ball milling. However, recent studies have shown that starting material for high-energy ball milling need not to be in powder form. The starting material could be industrial by-products such as swarf/ chips formed during machining [266]. High-energy ball milling of such industrial by-products is attractive; moreover, nanocrystalline powders as produced by high-energy ball milling may also be a starting material for net shape manufacturing of components. Cost associated with the production of nanocrystalline SS could be compensated with the beneﬁts of net shape manufacturing (i.e., reduced material and machining cost) and improved properties of nanocrystalline SS. Nanocrystalline SS exhibited signiﬁcantly improved corrosion resistance at lower Cr content and therefore this may contribute to alloys for use in clean energy generating systems where harsh conditions (i.e., high temperatures in solid oxide fuel cells, corrosive biomass, etc.) exist. Dedicated work is however required to explore the possibilities of economical production of nanocrystalline materials and applications of nanocrystalline SS, in the face of conventional corrosion and heat resistant alloys that can presently be commercially acquired (i.e. Ni alloys).

7. Conclusions This review has covered the body of literature that pertains to the corrosion of nanocrystalline stainless steel. Based on the works reviewed, the general consensus is that corrosion resistance of stainless steel improves due to the nanocrystalline structure for the majority of test electrolytes. It was also determined from a wider analysis, that corrosion resistance was largely dependent upon the processing route used to generate or impart the nanocrystalline structure. Of the studies surveyed, the passive ﬁlm developed over nanocrystalline stainless steel in various electrolytes was purported to be more stable, more compact and contain a lower defect density, whilst also possessing a higher Cr content in the passive ﬁlm (for an equivalent bulk Cr content). Nanocrystalline stainless steels were noted for having a more uniform microstructure with implications in improvement of resistance against pitting corrosion. The associated mechanisms related to the corrosion and instances of improvement in corrosion resistance have been discussed herein. In addition to nanocrystalline structure, processing route had strong inﬂuence on corrosion behaviour of stainless steel. Dedicated future investigation on nanocrystalline stainless steel prepared by various processing routes are required in order to develop further understanding of the inﬂuence of nanocrystalline

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Please cite this article in press as: R.K. Gupta, N. Birbilis, The inﬂuence of nanocrystalline structure and processing route on corrosion of stainless steel: A review, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.11.041

The influence of nanocrystalline structure and processing route on corrosion of stainless steel: A review

The influence of nanocrystalline structure and processing route on corrosion of stainless steel: A review

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