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Scripta Materialia 63 (2010) 1201–1204

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Revealing the relationship between grain size and corrosionrate of metals

K.D. Ralston, N. Birbilis⇑ and C.H.J. Davies

ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering,

Monash University, Victoria 3800, Australia

Received 29 July 2010; revised 18 August 2010; accepted 18 August 2010Available online 24 August 2010

A definitive understanding of how grain size affects the corrosion rate and passivity of different metallic alloys has not been devel-oped to date. A review of the pertinent literature, combined with the authors’ works, reveals that a relationship exists between cor-rosion rate and grain size. This phenomenon correlates with total grain boundary length and reveals an important fundamentalrelationship that can be exploited for material durability and design.� 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Corrosion; Grain boundaries; Grain refining; Hall–Petch effect; Severe plastic deformation

Grain boundaries have distinct properties rela-tive to bulk material in terms of atomic coordination,reactivity and diffusion rates. Consequently, it is notunreasonable to expect surfaces with relatively highgrain boundary densities to exhibit different electro-chemical behaviour (namely corrosion rates) than coar-ser grained surfaces with lower grain boundary densities.The Hall–Petch relation reveals that yield strength is in-versely proportional to grain size [1,2]. However, ananalogous relationship between grain size and corrosionrate has yet to be determined, and is the prime focus ofthis work. There have been a number of studies on dif-ferent materials that involve grain size variation andcorrosion performance, but, as detailed in a recent re-view, there is little consensus as to an all-encompassingeffect applicable across different materials or even withinsimilar alloy classes (readers are directed towards the re-view by Ralston and Birbilis [3] for more details and acurrent summary of the state of the field). This lack ofconsensus is not surprising for at least three reasons,the first two of which do not relate to scientific aspects.First, authors have not previously presented corrosionrates as a function of grain size raised to a selection ofpowers. Secondly, most works to date have only studiedcoarse and fine grained alloys (and not a spectrum ofgrain sizes), making any general relationship difficult

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⇑Corresponding author. Tel.: +61 03 9905 4919; fax: +61 03 9954940; e-mail: nick.birbilis@monash.edu

to extract. Lastly, studies of grain size effects on corro-sion are inherently difficult because any processing andor alloy additions used to achieve grain refinementmay impart physical or chemical changes to the materialin addition to the intentional grain size modification.Such secondary consequences from thermo-mechanicalprocessing include the development of texture, internalstresses and segregation of alloying additions to grainboundaries, each of which may have an impact on cor-rosion response, in addition to the role of the test elec-trolyte or environment.

Of studies that that considered grain size when inter-preting corrosion, many have suggested that as grainsize decreases corrosion rate decreases (i.e. corrosionresistance improves). Improved resistance generally isattributed to an ability of high grain boundary densitysurfaces to passivate more readily or to the physicalbreakdown of second phase intermetallic particles[4–7], but such claims are rarely substantiated withinthe said papers. Such studies have deployed differentprocessing paths to achieve grain-refined microstruc-tures, which include rolling operations and severe plasticdeformation (SPD) techniques, such as surface mechan-ical attrition (SMAT) [8,9] and equal channel angularpressing (ECAP) [10,11]. The ECAP and SMAT pro-cesses have been noted to both increase [5,12–18] anddecrease [19–21] corrosion resistance. A better represen-tation of the overall impact of grain size upon corrosionfor all known relevant references is given in Figure 1.The corrosion rate data presented in this figure were

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Figure 2. The relationship between corrosion rate and grain size forlow to passive current densities. Data were obtained from: Birbiliset al. [15]; op’t Hoog et al. [16]; Liu and Duh [25]; Ghosh et al. [26];Ralston et al. [22].

Figure 1. The effect of grain size on corrosion rate (expressed asA cm�2) for different materials and environments. Data were obtainedfrom: Afshari [24] for Fe in 10 wt.% NaOH; Liu and Duh [25] forNi50.5Al49.5 in 0.9 wt.% NaCl; Ghosh et al. [26] for Ni–Cu in deaerated3 wt.% NaCl; Mishra and Balasubramaniam [27] for Ni in 1 M H2SO4;Song et al. [20] for Mg (99.9%) in 3.5 wt.% NaCl; Mahmoud [28] forAA6063 in 1 M HCl; Hamu et al. [29] for AZ31 Mg in 3.5% NaClsaturated with Mg(OH)2 at pH 10.5; Lin et al. [30] for alloy 600 inboiling ferritic sulfate (31.25 g l�1)–50% H2SO4; op’t Hoog et al. [16]for Mg (99.9%) in 0.1 M NaCl; Ralston et al. [22] for Al (99.999%) in0.1 M NaCl; Birbilis et al. [15] for Mg (99.9%) in 0.1 M NaCl.

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originally determined using a variety of methods, includ-ing Tafel extrapolation on potentio-dynamic polarisa-tion curves, mass loss and linear polarisations; withinthis manuscript icorr values have been standardised tobe given as A cm–2. This figure only includes data whereat least three different grain sizes existed (as studies withonly one “coarse” and one “fine” grained specimen arenot subsequently amenable to linearisation of the data).Figure 1 reveals that the literature data spans several or-ders of magnitude of corrosion rate, necessitating (onlyfor this representation) a log scale on the y-axis. Thisrepresentation is important, because it shows that thedata in the literature can be separated into two classes,including a range of material–environment combina-tions that show low to passive corrosion rates(610 lA cm�2) and those which display corrosion ratesof several tens to hundreds of lA cm�2. This differenti-ation is also significant because the specific effect thatgrain size has on corrosion varies depending on whethera material exhibits active corrosion or low to passive -dissolution rates. If the data with corrosion rates610 lA cm�2 are isolated, then a relation becomesapparent when such data are plotted as corrosion rateversus grain size�0.5, as shown in Figure 2 (the signifi-cance of the reciprocal square root dependence isdiscussed further below).

Figure 2 illustrates how linear regression (Origin 8�)can be used to determine a relationship between corro-sion rate and grain size. To our knowledge, such a rela-tionship has not been previously reported. Figure 2ashows the results from two studies for magnesium spec-imens that had been grain refined by various SPD pro-cesses, Figure 2b shows results from a Ni–Al and aNi–Cu alloy, and Figure 2c shows results from a system-atic study on refined aluminium. The relationship ob-served in all cases is:

icorr ¼ ðAÞ þ ðBÞgs�0:5 ð1Þ

where the constant A is likely to be a function of theenvironment (since, conceivably, the same materialmay have a different corrosion rate in different electro-lytes for a given grain size) and B represents a materialconstant, which will differ according to composition orimpurity level. In Birbilis et al. [15], op’t Hoog et al.[16,17] and Ralston et al. [22] the authors were diligent

Figure 4. The relationship between corrosion rate and grain size forhigh current densities. Data were obtained from: Mahmoud [28]; Linet al. [30]; Song et al. [20].

K. D. Ralston et al. / Scripta Materialia 63 (2010) 1201–1204 1203

about using material from the same ingot as the testmaterial.

Such a linear correlation is not only interesting tonote, but also an important mechanistic revelation. Inexaminations of systems where low rates of corrosionoccur or systems that exhibit some level of passivity(where by definition an oxide is present on the metal sur-face) corrosion rates decreased with grain size reduction.This, therefore, allows us to link this finding to emergingwork in the literature, including our own, whereby grainrefinement is posited to allow different rates of oxidegrowth. This is in line with pure grain refinement beingprincipally confined to controlling the rate of anodicreactions and having little role in altering the rate atwhich cathodic reactions can be sustained (presumablybecause cathodic reactions rely on electronic more thanionic conduction, with increased grain boundary densitybeing important for the latter). This is in agreement withthe detailed experiments of Lee and White [23], who ele-gantly revealed that small increases in oxide film ionconduction are associated with grain boundaries whencomparing polycrystalline Al with single crystals, whichis wholly in line with the observations herein.

The presentation of a relationship such as that pro-posed by Eq. (1) would, therefore, require rationalisa-tion of the �0.5 dependence. Based on a hypothesis ofgrain boundary conduction and reactivity, icorr shouldvary with grain boundary length. A general expressionfor grain boundary length (gbl) can be presented as:

gbl ¼ ðCÞ þ ðDÞgs�a; 0:5 6 a 6 1 ð2Þ

where the constant C is an area term and D is a scaleterm. For the ideal case of perfectly equiaxed grains(phenomenologically shown in Fig. 3a) a = 1. However,simulation of random grain structures (with a distrib-uted variation in shape and size) and subsequent evalu-ation of skeleton length (via Fovea Pro) yields adispersed relationship closer to a = 0.5. Such a grainstructure is seen in Figure 3b. Hence, in the more realis-tic case of the metals tested in Figure 2, a approaches0.5, giving Eq. (1) the corresponding dependence.

If grain boundary density dictates the oxide film con-duction rate on surfaces with low to passive corrosionrates, then fine grain structures are expected to be more

Figure 3. Simulated grain structures illustrating an (a) ideal and (b) random

corrosion resistant. In the absence of an oxide film, suchas when dissolution rates are >10 lA cm�2, increasedgrain boundary densities will likely enhance overall sur-face reactivity and, consequently, the corrosion rate.This is indeed seen, as reported in Figure 4. The appar-ent discrepancy between the results of Song et al. [20](Fig. 4) and those of op’t Hoog et al. [16] and Birbiliset al. [15] (Fig. 2a) for compositionally similar Mg islikely due to electrolyte potency; Song et al. tested in3.5 wt.% NaCl (�0.6 M) while op’t Hoog et al. and Bir-bilis et al. used 0.1 M NaCl.

In terms of corrosion resistance, the reported phe-nomena can be considered analogous to the bounds ofthe classical Hall–Petch relationship (albeit for differentmechanistic reasons). Whereas Hall–Petch is confined tothe yield behaviour of a material, the utility of grainrefinement for reduction in corrosion rates as describedherein is confined to materials when an oxide/passivityexists.

Specific details regarding the processing and testingwhich relates to the results herein are given in the respec-tive papers, however, it is apparent that grain sizealteration offers the engineer latitude in tailoringmicrostructures for enhanced corrosion resistance or

structure with a values of approximately 1 and 0.5, respectively.

1204 K. D. Ralston et al. / Scripta Materialia 63 (2010) 1201–1204

designing materials with the same bulk chemistry tohave varied corrosion rates, and is thence clear thatthe same bulk material can have different corrosion ratesowing to non-chemical structural changes alone. Thisrevelation has important ramifications for alloy designand functional materials.

Financial support from the Australian ResearchCouncil (Centre of Excellence for Design in LightMetals) and Victorian State Government for the estab-lishment of the Victorian Facility for Light MetalsSurface Technology are gratefully acknowledged.

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