The protective nature of passivation films on zinc: surface charge

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Corrosion Science 46 (2004) 2337–2354

The protective nature of passivation filmson zinc: wetting and surface energy

Tim H. Muster *, Aaron K. Neufeld, Ivan S. Cole

Corrosion Science and Surface Design Team, CSIRO Manufacturing and Infrastructure

Technology, Graham Road, P.O. Box 56, Highett, Victoria 3190, Australia

Received 18 November 2003; accepted 13 January 2004

Available online 5 March 2004

Abstract

The influence of passivation film surface chemistry on the atmospheric corrosion of zinc

was investigated. Crystalline oxide compounds replicating natural zinc corrosion products

were deposited from aqueous solution, and grown as coatings on rolled zinc plates. The

energetic properties of the oxide surfaces were characterised by water contact angle deter-

mination and inverse gas chromatography. Water contact angles were determined by liquid

penetration and dynamic Wilhelmy techniques, and a good correlation was observed between

the two methods. In comparison with wetting approaches, inverse gas chromatography was

found to overestimate values for the dispersive contribution to the solid surface energy. Zinc

hydroxychlorides possessed the lowest contact angles whilst naturally formed zinc oxide–

hydroxide coatings were the most hydrophobic. The ability of zinc oxidation products to

retain moisture was investigated by correlating wetting data with the criteria for droplet

retention on planar surfaces. The response of the various oxides to rain washing events and

water adsorption processes was discussed. Comparison of the corrosion rates of thin liquid

films versus droplets was explored experimentally by adjustment of droplet contact angles

through the addition of a non-ionic surfactant. Mass gain data indicated that corrosion rates

decreased with decreasing contact angle, suggesting that hydrophobic surfaces are not nec-

essarily advantageous for improving corrosion performance.

Crown Copyright � 2004 Published by Elsevier Ltd. All rights reserved.

Keywords: Zinc; Passive films; Oxidation products; Contact angle; Surface energy; Runoff

* Corresponding author. Tel.: +61-3-92526293; fax: +61-3-92526253.

E-mail address: [email protected] (T.H. Muster).

0010-938X/$ - see front matter Crown Copyright � 2004 Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2004.01.001

mail to: [email protected]

2338 T.H. Muster et al. / Corrosion Science 46 (2004) 2337–2354

1. Introduction

A vast depth of knowledge has been reported concerning the atmospheric corro-

sion of common engineering metals [1,2]. Understanding the corrosion performance

of zinc is particularly relevant due to its widespread usage as a protective coating on

steel. The corrosion resistance of zinc (and metals in general) is strongly related to theprotective nature of surface oxides [3], and therefore, a thorough understanding of the

characteristics of surface oxides is important. The wetting properties of passive films

are expected to play a number of roles in determining atmospheric corrosion rates,

and may influence the following: electrolyte film thicknesses [4], the time-of-wetness

(TOW) [5], the efficiency of impurity adsorption [6,7], the rate of runoff and the

likelihood of moisture retention [8].

The ability of a surface to wet is largely controlled by the interfacial energy and to

some extent by roughnesses and porosities [7]. Surfaces are classified into two broadcategories, those that are hydrophilic and those that are hydrophobic. Hydrophilic

surfaces possess a high surface energy and permit the spreading of aqueous solutions

such that the three-phase contact angle [9] approaches zero. Hydrophobic surfaces

are low-energy and create high water contact angles approaching or greater than 90�,and therefore droplets tend to ‘‘bead’’ on the surface. Also, the roughness of surfaces

and the porosity of oxidation products influence droplet spreading, runoff phe-

nomena, salt distribution, and the retention of moisture in pores.

In terms of mass-transport processes, the spreading behaviour of an electrolyte(i.e. whether droplets or thin films are formed) will directly influence both the solid–

liquid contact area and the liquid–vapour interfacial area. The solid–liquid contact

area has a direct influence on the rates of oxide and metal dissolution, and controls

the access of oxygen and water to cathodic sites [10]. The liquid–vapour inter-

face area controls the extent of oxygen/gas diffusion and controls bulk evapora-

tion rates. Also, the chemical and physical nature of a surface will change with time,

and therefore wetting phenomena is thought to play a rather dynamic role in

influencing metal corrosion. For example, work by Cole et al. [11] has shown that theenergy of a zinc surface increases upon its short-term exposure to outdoor environ-

ments.

A supporting study to the present article summarized in the common oxide

compositions (zinc oxide, zinc hydroxychloride, zinc hydroxycarbonate and zinc

hydroxysulphate) that form naturally at the zinc–environment interface, and inves-

tigated the effect of oxide surface charge on corrosion protection [12]. This article

aims to elucidate differences in the wetting properties of naturally formed oxides, and

emphasise the role of wetting and surface properties in controlling atmosphericcorrosion rates. More specifically, the wetting and energetic properties of the syn-

thetic oxides are characterised by contact angle and surface energy determinations.

From information gained from the surface energy studies, the response of each oxide

type to rain events, water adsorption, water runoff and evaporation is discussed, and

their influence on corrosion rates implied.

T.H. Muster et al. / Corrosion Science 46 (2004) 2337–2354 2339

2. Experimental

2.1. Substrate and particle preparation

Rolled zinc plates (Zintane 83, Union Miniere, France) were cleaned using a

commercial surfactant (Neutracon90e, Decon).Wetting experiments were carried outon both cleaned and synthetic-oxide coated, rolled zinc plates, and also on a series of

synthesised oxide particles. The details for oxide preparations are reported elsewhere

[12]. We make special mention of the sample heterogeneity of the synthetically pas-

sivated plates. XRD revealed that both zinc hydroxychloride and zinc hydroxysul-

phate films also possessed minor phases of zinc hydroxycarbonate material, whilst zinc

hydroxycarbonate films showed traces of zinc oxide and some sodium carbonate [12].

2.2. Washburn liquid penetration contact angles

Liquid penetration contact angles of synthesised oxide particles were determined

using the approach of Washburn [13], where the rate of liquid travel through a

particle bed is governed by the Poiseuille and Laplace equations:

h2

t¼ reffclv cos hp

2gð1Þ

where h is the height of liquid at time t, reff is the effective radius of pores, g is theliquid viscosity, clv is the liquid surface tension, and hp is the contact angle. The watercontact angle can be obtained from the ratio of liquid penetration rates of water and

a perfectly wetting liquid (cyclohexane), thus avoiding the need for the direct

determination of reff [14–16]. For consistent packing of particle beds, a known weightof each oxide was placed into glass wool plugged glass capillaries (2 mm internal

diameter) and compacted to a constant volume fraction. Packed capillaries were

stored at 40% relative humidity overnight prior to analysis.

2.3. Dynamic Wilhelmy contact angles

The Wilhelmy technique measures the mass (force) of a solid object when sub-

merged and then withdrawn from a liquid [17]. The total force, F , acting on the solidat any time is the sum of capillary and bouyancy forces. At the point where the

sample and liquid first contact, the bouyancy force is negligible, and extrapolation by

linear fit over the range of submerging and withdrawing data allows the calculationof the advancing, hA, and receding, hR, contact angles, respectively:

cos hA=R ¼ Fcappclv

ð2Þ

where Fcap is the capillary force and p is the wet perimeter. Advancing and recedingtraces were performed on plates with oxide films using a number of probe liquids:


water, hexane, cyclohexane, formamide and diiodomethane. The perimeter was

measured physically using a micrometer. Studies were also undertaken to estimate

the wet perimeter using the approach of Pepin et al. [18], who based their calcula-

tions on the theory of Fowkes [19]. Pepin and co-workers determined the capillary

forces (F1 and F2) for two apolar wetting liquids (i.e. 1-bromonaphthalene and di-iodomethane). The wet perimeter is solved using Eq. (3).

p ¼F2a�

F1clv2clv1

clv2ð1� aÞ where a ¼ clv2clv1

� �0:5ð3Þ

2.4. Inverse gas chromatography (IGC)

A Hewlett Packard 5890 Series II gas chromatograph was used to obtain the net

retention volumes, VN , for solvent vapour flows through columns filled with syntheticzinc hydroxide–oxide, zinc hydroxychloride and zinc hydroxycarbonate particulates.

Stainless steel chromatographic columns with an internal diameter of 2 mm and

length of approximately 200 mm were cleaned using 10% HNO3 for 1 h, rinsed with

distilled water and oven dried prior to use. The oxide particulates were sieved to a

size 150 lm < x < 300 lm and packed into the columns and compressed by gentlytapping. Glass wool was used to block the ends of the column. To ensure the oxides

were free of moisture prior to characterisation, each system was conditioned by

passing helium gas through the column for 24 h. Experiments were carried out at

elevated temperatures (40–150 �C) to achieve workable retention times.The alkane series from pentane (C5) to decane (C10) were used to determine the

dispersive component (cd) of the solid surface energy from the slope of a plot of

RT ln VN against aðcdl Þ0:5, which is based upon Eq. (4) [19–21].

RT ln VN ¼ 2Naðcds Þ0:5

� �� ðcdl Þ

0:5 þ c ð4Þ

where R is the gas constant, T is the absolute temperature, N is Avogadro’s number,a is the cross-sectional surface area of a probe molecule, c is a constant, and cds and cdlare the dispersive components of surface energy of the solid and the probe,

respectively. The specific, or polar, component of the free energy of adsorption

(DGsp) was evaluated by comparing the retention volume of both acidic (benzene, diand tri-chloromethane) and basic (tetrahydrofuran) probes with the retention time of

the alkane probes (Eq. (5)).

DGsp ¼ RT lnVNacid=baseVNref

� �ð5Þ

The specific interaction enthalpy, DH sp, was determined by plotting DGsp as afunction of absolute temperature and extrapolating back to T ¼ 0 K, at whichDH sp ¼ DGsp.Hamieh et al. [22] recently demonstrated that the value of a is not known with

good accuracy and presented values calculated using various models. For this study,

a values for temperatures higher than 100 �C were obtained from the two-dimen-


sional modelling of van der Waals contributions, whilst an average of values ob-

tained using Kiselev [22] and Cylindrical models was used at lower temperatures [20].

2.5. Droplet spreading and runoff

The runoff of single water droplets from rolled zinc plates with synthetic oxide

films was determined as a function of drop size and plate inclination. The samples

were mounted to a tilting beam assembly carrying a digital protractor (PM Pro 360).

Ten to fifty microliters droplets of water (18 MX resistivity) were placed onto theoxide coated zinc samples and the angle of runoff recorded.

2.6. Corrosion rates under droplets of varying surface tension

Droplets of NaCl-containing aqueous solutions with masses in the range of 15–

1000 mg were placed onto the surface of pre-weighed cleaned rolled zinc sections.

The droplet mass was recorded in grams to five significant figures. Corrosion pro-

cesses were allowed to take place under ambient conditions for 16 h, after which the

sections were dried at 70 �C and reweighed for mass gain. Two aqueous solutionscontaining 1 M NaCl were used, one with the addition of 0.1% Tween80� non-ionicsurfactant.

3. Results and discussion

3.1. Contact angle determination

Table 1 shows the estimated water contact angles for synthesised oxide particles

and synthetic passivation films on zinc plates. General trends in the ranking of oxide

hydrophobicity showed good agreement, with zinc hydroxychlorides being the most

hydrophilic and zinc oxide the most hydrophobic. Contact angles for zinc hy-

droxysulphate and zinc hydroxycarbonate appear to be of a similar magnitude. Forsimilar compounds the Wilhelmy technique generally resulted in higher contact

angles, an observation that may be attributed to the inability of the liquid pene-

tration technique to evaluate contact angles greater than 90�. Cole et al. [23] reported

Table 1

Average contact angle data for synthetic zinc oxidation products

Oxide Particle

(liquid penetration)

Plate

(Wilhelmy technique)

hp (�) hA (�) hR (�)

Zinc oxide–hydroxide 88.6 107.8 71.0

Zinc hydroxychloride 76.7 72.8 54.8

Zinc hydroxycarbonate 86.2 89.1 47.2

Zinc hydroxysulphate 86.5 82.9 55.4


sessile drop water contact angles on clean galvanised steel to be in the range of 65–

85�, and after exposure to various sites, contact angles ranged from 16� to 80�.Considering that sessile drop contact angle measurements generally provide an

intermediate angle between hA and hR, the magnitude of the current data appears tobe in good agreement with that generated by Cole et al. There is some conjecture that

clean metal surfaces should be perfectly wet by water [24]. However, several authors[25,26] suggest that for various metals (i.e. gold, copper, nickel and steel) that the

contact angle can vary from anywhere between 0� and 90�. Certainly, zinc, whichrapidly forms an oxide, appears to possess a finite water contact angle.

The rate of liquid flow through glass capillaries packed with synthesised oxide

particles was consistent with Poiseuille capillary flow theory for both water and

cyclohexane, with the height squared showing a linear relationship with time (Fig. 1).

The estimated effective radius of capillaries ranged from reff ¼ 0:14 to 0.62 lm forzinc hydroxysulphate, which indicates an assumed spherical particle radii ofapproximately 1–2.8 lm. Reproducibility in the rate of liquid penetration was within±5% for repeat runs of the same liquid through particle beds prepared on conse-

cutive days. This provides confidence that observed differences in the rates of pen-

etration reveal clear information concerning the ranking of particle surface energy

properties.

Fig. 2 shows selected raw wetting traces obtained from Wilhelmy balance

experiments. Repeat determinations of dynamic contact angles were found to vary

by less than 3% when altering the wetting rates in the range of 20–150 lms�1. Allpresented data was obtained using a wetting rate of 100 lms�1. The information

Fig. 1. Selected data points showing the liquid penetration height squared (h2) as a function of time forwater (filled symbols) and cyclohexane (open symbols) through 3 mm diameter glass capillaries packed

with synthetic oxide particles. (� �) Zinc oxide–hydroxide, (r }) Zinc hydroxychloride, (N M) zinc

hydroxycarbonate, (j �) zinc hydroxysulphate.

Fig. 2. Wilhelmy traces showing mass change as a function of immersion distance for zinc plates with

oxide coatings.


available from the Wilhelmy trace can be summarised by the following [17]: (1)

displacement of the trace in the positive force direction indicates a decreased contact

angle, (2) the distance between the advancing (lower) and receding (higher) traces is ameasure of the contact angle hysteresis, (3) the slope of the trace is characteristic of

the bouyancy force and is directly related to the perimeter of the solid, and (4)

fluctuations in the data are due to physical heterogeneities of the sample. Advancing

contact angles increased as zinc hydroxychloride < zinc hydroxysulphate < zinc hy-

droxycarbonate < zinc oxide. Receding contact angles were 25–40� lower than thecorresponding advancing angles, including the ‘‘as-received’’ zinc oxide–hydroxide

sample. Roughening of the plate surface occurred during the application of syn-

thesised zinc oxidation compounds, as was evidenced by increased fluctuations in theadvancing wetting traces when compared to the ‘‘as-received’’ zinc oxide–hydroxide

plates.

The determination of accurate dynamic contact angles is dependent upon the

estimation of the wet perimeter [27]. Three approaches of obtaining wet perimeter

were considered in the present study. Firstly, the plate perimeters were measured

using a micrometer. Secondly, plates were immersed into liquids (cyclohexane and

hexane) that were assumed to perfectly wet the solid, and the perimeter calculated as

p ¼ F =clv. Thirdly, the use of two non-polar liquids using the Pepin et al. [18] ap-proach. Table 2 provides the wet perimeter details for the first two approaches. Wet

perimeters determined using both cyclohexane and hexane were in good agreement

and showed that the porosities and roughnesses of the oxide coated plates increased

by less than 7% compared to the micrometer measurements. Of the synthesised oxide

coatings, if would appear that zinc hydroxychloride and zinc hydroxysulphate

compounds have a higher roughness and/or porosity than zinc oxide–hydroxide and

zinc hydroxycarbonate coatings. Mention should be made that the third approach to

Table 2

Perimeter measurements and wet perimeter estimates

Physical perimeter

(mm)

‘‘Perfect wetting’’

perimeter (mm)

‘‘Perfect wetting’’:

physical

Zinc oxide–hydroxide 41.34 41.22± 0.13 0.997

Zinc hydroxychloride 41.28 43.87± 0.25 1.063

Zinc hydroxycarbonate 41.10 40.86± 0.43 0.994

Zinc hydroxysulphate 41.24 43.65± 0.10 1.058


wet perimeter determination appears to be problematic, returning unrealistic andnegative values.

3.2. Inverse gas chromatography (IGC)

The flow of non-polar alkane vapours through particle beds of zinc hydroxy-

chloride and zinc hydroxycarbonate allowed the estimation of cdsDGsp and DH sp

values (Table 3). However, long retention times for the transport of non-polar

hydrocarbon vapours through the zinc hydroxide–oxide column provided unusable

and inconclusive data. Even at 100 �C, data was collected for C5–C7 hydrocarbons,whilst C8 vapour was not detected after 75 min. Also, the decomposition of zinc

hydroxide, which occurs at around 120–130 �C, restricted the use of higher experi-mental temperatures. For zinc hydroxycarbonate and zinc hydroxychloride particlesexposed to alkane vapour over a range of temperatures, plots of RT ln VN againstaðcdl Þ

0:5provided a linear correlation with a squared regression ðR2Þ > 0:997. The

value of cds at 100 �C for zinc hydroxychloride was 43 mNm�1, which was signifi-

cantly lower than the value of 91 mNm�1 for zinc hydroxycarbonate at the same

temperature. The high estimated value of cds (>90 mNm�1) from IGC would suggest

perfect wetting by water [28]. However, the wetting results (Table 1) show that this

clearly does not occur.

The calculated values of DGsp are associated with the polar contributions to thesolid surface energy, and the more negative its value, the stronger the acid–base

interactions between the adsorbent solid and the adsorbate. Zinc hydroxychloride

showed strong acid characteristics as the specific interactions between the stationary

solid phase and the acidic probes (benzene) were low, and were effectively zero for

Table 3

Surface energy and interaction energy data obtained from inverse gas chromatography

Oxide cds (100 �C)(mNm�1)

�DGsp (100 �C) (kJmol�1) �DH sp (kJmol�1)

C6H6 CHCl3 CH2Cl2 THF C6H6CH2Cl2 THF

Zinc hydroxychloride 43 0.6 0 – Very

high

12.1± 2 –

Zinc hydroxycarbonate 91 2.1 – 0.3 13.6 5.1± 2 1.6


the case of CHCl3. The basic probe, THF, had a strong specific interaction with the

acidic moieties on the solid phase and did not diffuse through the column (data not

shown) at any temperature conducted. Zinc hydroxycarbonate showed slightly in-

creased interactions with acidic probe molecules when compared to zinc hydroxy-

chloride, indicating that it possesses more basic characteristics. In contrast to zinc

hydroxychloride, interactions between THF and zinc hydroxycarbonate were mea-surable, indicating a lesser degree of surface acidity.

The value of DH sp describes the energy of interaction between the probe molecule

and the solid phase independently of temperature-based entropic effects. The esti-

mates of DH sp provided in Table 3 suggest that the interaction energy between the

acidic probes (benzene/CH2Cl2) and zinc hydroxychloride exceeds that for the zinc

hydroxycarbonate. This trend is in contrast to that observed for DGsp values ob-tained with the same acidic probes, an observation that is attributed to the high

entropic dependence of adsorption onto zinc hydroxychloride surfaces (see Fig. 3).Fig. 3 also shows that the free energy of interaction between zinc hydroxycarbonate

and THF probe molecules has a positive slope, suggesting an increase in entropy

with temperature. All other interactions possessed negative slopes and show a de-

crease in entropy with temperature.

3.3. Surface energy determinations

Tables 4 and 5 present the estimates of surface energy values calculated fromcontact angle using a number of approaches as outlined in Appendix A. Whilst

considerable variations were obtained in the estimated surface energy values, each

Fig. 3. Plot of the specific free energy of various polar probe molecules adsorbing onto synthetic zinc

oxide particles: (�) benzene adsorption onto zinc hydroxychloride, (�) benzene adsorption onto zinchydroxycarbonate, (}) CH2Cl2 adsorption onto zinc hydroxycarbonate, and (M) THF adsorption ontozinc hydroxycarbonate.

Table 4

Surface energy data derived from (1) the equation of state approach of Li, Kwok and Neumann [28,45], (2)

acid–base approach of Van Oss et al. [30], and (3) the reciprocal mean approach of Wu [29]

Oxide Particle csv (mNm�1) Plate csv (mNm

�1)

1 2 3 1 2 3

Zinc oxide–hydroxide 30.1 20.5 43.2 18.4 18.8 53.3

Zinc hydroxychloride 37.6 22.7 45.3 40.0 28.2 31.1

Zinc hydroxycarbonate 31.6 17.2 38.8 29.8 17.0 34.2

Zinc hydroxysulphate 31.4 17.0 37.6 33.7 22.9 35.8

Table 5

Surface energy data derived from acid–base theory approach of Van Oss et al. [30]

System Particle Plate

cLVWsv

(mNm�1)

cþs(mNm�1)

c�s(mNm�1)

cLVWsv

(mNm�1)

cþs(mNm�1)

c�s(mNm�1)

Zinc oxide–hydroxide 15.34 0.80 8.45 16.32 0.40 3.67

Zinc hydroxychloride 15.89 0.58 20.01 21.94 0.51 18.84

Zinc hydroxycarbonate 13.32 0.17 22.71 13.36 0.28 11.92

Zinc hydroxysulphate 13.12 0.25 14.74 19.39 0.24 12.81


calculation method was able to rank the surface properties of the studied systems

into a similar but not identical order. In addition, a good correlation was observed

between calculations made for particulate and plate systems. It would appear from

this study that the surface energy of common zinc surfaces found in the environmentare in the range of 20–40 mNm�1. These values are in agreement with a previous

study by Cole et al. [23].

Given that the zinc oxide–hydroxide system was found to exhibit the highest

water contact angles (Table 1), it would appear that estimations using Wu’s ap-

proach [29] overestimate the surface energy for this particular system. It was noted

that the specific components of surface energy calculated using this approach sig-

nificantly outweighed dispersive contributions, a result that is not probable. For

example, the dispersive component of the surface energy for zinc hydroxycarbonatewas just 0.4 mNm�1 of the total predicted energy of 38.4 mNm�1. Therefore, we

suggest that the information provided in Table 4 be used with caution.

A breakdown of the surface energy components calculated using the Van Oss

et al. [30] approach is provided in Table 5. Lifshitz–van der Waals dispersive energies

were in the range of 13–22 mNm�1, values significantly lower than those obtained

using IGC. A series of authors have published data comparing the non-polar

component to the surface energy estimated using IGC and wetting techniques [31–

33]. In each case the value of cd is higher for IGC, a result that is attributed to thecontrasting mechanisms of adsorption [34]. Wetting experiments are based on the

interactions between a surface and a bulk liquid, whilst IGC measures the interaction

between infinitely diluted gas molecules and surface. It follows that gas adsorption

has the ability to target ‘‘high-energy’’ sites whereas wetting experiments provide an


average of the interactions over a macroscopic area. It follows that the particularly

high value of cd estimated for zinc hydroxycarbonate, may be a result of the abilityof non-polar probe molecule targeting.

4. Discussion

4.1. Forms of moisture on surfaces

Using the information obtained from wetting studies, we now consider the

influence of zinc passivation film surface chemistry on the formation of moisture

layers under atmospheric conditions. Aqueous environments on exterior metal sur-

faces are generally established by either rain or through water adsorption processes.

We concentrate firstly on the deposition of moisture by rain. Where light rain is

concerned, the spreading behaviour of individual droplets will be determined by the

surface energy, roughness and porosity. That is, if the energy of the surface is high,

droplets impacting on the surface will tend to spread and form a thin continuous filmof moisture. In contrast, light rain on a low-energy hydrophobic surfaces will yield

discrete droplets with a finite contact angle. For this reason hydrophobic surfaces

tend to favour more isolated forms of corrosion damage whereas hydrophilic sur-

faces favour general corrosion mechanisms.

The ability of a surface to retain droplets is not solely dependent upon the

hydrophobicity of a surface, rather it is the difference between the advancing and

receding contact angles that provides the resistance against a droplet sliding down an

inclined surface [8]. At an infinitely low wetting rate the critical angle, ac, leading todroplet sliding is given as:

qgV sin ac ¼ wclvðcos hR � cos hAÞ ð6Þ

where q is the liquid density, g is the acceleration due to gravity, V is the dropletvolume, w is the width of the droplet, clv is the liquid surface tension and hA and hRare the advancing and receding contact angles, respectively.

As rain becomes heavier, droplets impacting a surface tend to coalesce as repre-

sented in Fig. 4. Once the droplet size reaches a critical size, gravitational forces

overcome the adhesional forces of the droplet to the zinc and runoff occurs (seecontact area dependence of Eq. (6)). When rain events reach a critical flux, forced

wetting of a surface occurs and a sheet of moisture will cover the surface. However,

when the wetting event halts on a hydrophobic surface, the thin film ruptures and the

liquid attempts to recede to form disconnected droplets. The ability of a thin film to

avoid rupturing is controlled by the disjoining pressure and the ability of the water to

spread over the surface [35]. The dewetting of surfaces by the growth of dry regions

has been described by Kheshgi and Scriven [36]. As will become clear later, rain is

not highly detrimental to the corrosion performance of zinc as it performs animportant function in washing salt contaminants from the surface. It has been

suggested that the pH of the rainwater and the total solids content of the air appear

to have more significance than the time-of-wetness [37].

Fig. 4. The spray wetting of contaminated zinc surfaces with electrolyte: (a) impacting droplets were

approximately 500 lm in size and exhibited an equilibrium contact angle in the order of 65� (sessile drop),(b) the continued spraying led to droplet growth and the eventually combining of adjacent droplets.


Since values of hA and hR are available from dynamic wetting experiments, theability of zinc surfaces with synthetic oxide films to control droplet runoff was

investigated with respect to Eq. (6). A plot of sin ac versus cos hR � cos hA (Fig. 5)shows a positive correlation between the angle of runoff and the hysteresis in the

dynamic wetting traces. The data in Fig. 5 also provides an appreciation of the in-

creased energy required for the removal of small droplets from a surface.

Given the data on the oxide surface properties, some hypotheses can be madeconcerning the ability of the studied zinc oxidation species to retain moisture on

surfaces after rain events. Zinc with naturally forming oxide–hydroxide surface films

are relatively hydrophobic and for rolled zinc, exhibit a large contact angle hyster-

esis. Therefore, moisture on the surface exists as individual droplets that are not

easily removed. Zinc hydroxycarbonate films appear to behave similarly to zinc

oxide–hydroxide films in that droplets adhere to the surface remarkably well (see

Fig. 5). However, the contact angle of the zinc hydroxycarbonate is lower than the

Fig. 5. Correlation between the dynamic contact angle of oxide coated zinc plates and the tilt-angle of zinc

plates required for visible movement of 30 and 50 ll droplets.


native oxide and wets somewhat easier (Table 1). The similarity in the physical and

wet perimeter determined using a perfectly wetting liquid seem to suggest that the

hydroxycarbonate layers deposited on the surface of rolled zinc have a low porosity.

This result would correspond with the many reports of zinc hydroxycarbonates

protecting zinc by forming a low-porosity insoluble film [38]. Zinc hydroxychloride

coatings appear to wet easily, have significant porosity and allow droplets to runoff

easily. Surfaces consisting of zinc hydroxychloride will wash easily based upon self-cleaning theory [39], allowing the removal of any soluble salts in the process. The

properties of zinc hydroxysulphate surfaces appear to fall between the extremes of

zinc hydroxycarbonate and zinc hydroxychloride surfaces. The energetic properties

of zinc hydroxysulphates resemble those of the hydroxycarbonates but appear to be

more porous and allow droplets to runoff easier.

In addition to rain events, aqueous films can be formed by water adsorption

processes, which can occur through a variety of mechanisms [40]. Firstly, there is

specifically adsorbed water on the surface, which can remain even at low relativehumidity but will only consist of a few moisture layers. The amount of uptake for

adsorption is directly related to (i) the surface area of the material, (ii) the energetics

of the interaction between the surface and water molecules, and (iii) the relative

humidity. Vernon [41] found that the relative humidity in the presence of sulphur

dioxide showed marked increases in corrosion rate of zinc, firstly above 60%RH, and

then at >90%RH. He referred to these as primary and secondary critical humidities.

Secondly, condensation of moisture onto a metal surface occurs when the air tem-

perature drops below the dew point or the when the temperature of a metal is belowthat of the air temperature at high humidity [37]. The formation of dew is a common

occurrence and is enhanced by, the presence of surface porosity, increased surface

roughness, surfaces that dissipate heat easily (i.e. thin sheet), and the presence of

impurities that act as nucleation sites for condensation. Clean zinc surfaces have


been reported to result in the formation of evenly distributed dew droplets [42]. The

third mechanism for water adsorption discussed is thought to be the most important.

Highly soluble salts such as MgCl2 and NaCl deliquesce above a critical relative

humidity. That is, they absorb moisture from the atmosphere and dissolve into that

moisture, eventually forming an electrolyte droplet. Due to the ability of chloride

ions to breakdown passive films, it is the latter method of water uptake that providesthe major threat to the onset of corrosion. Considering that adhesion theories [7]

predict that high-energy surfaces have a higher affinity to adsorb and retain par-

ticulate materials, the hydrophobicity of passive films will almost certainly influence

real corrosion rates. Therefore, a more hydrophilic surface such as zinc hydroxy-

chloride may be more susceptible to adsorbing hydroscopic salts than a more

hydrophobic surface such as zinc oxide. This assumption is yet to be evaluated.

4.2. The influence of droplet spreading on corrosion rate

It is commonly thought that hydrophobic surfaces are advantageous for

increasing corrosion resistance, a view that seems to originate from the protective

nature of chromate conversion coatings, which are inherently hydrophobic [43–45].The main advantages being that low-energy hydrophobic surfaces reduce the contact

area between the electrolyte and the metal surface, and that pores are less prone to

aqueous penetration. However, where conversion coatings are required in the ab-

sence of an organic topcoat, highly hydrophilic surfaces may offer advantages in

terms of dispersing salt and increasing evaporation rates. More hydrophobic oxide

surfaces such as stainless steel and chromate coatings have a habit of corroding more

severally in specific locations, and forming colonies of pits around areas of primary

attack [46,47]. To the author’s knowledge there have been no comprehensive studiescomparing the aggressiveness of the corrosive environments created in droplets of

varying contact angle in the 10–500 ll range. Here we explored the possibility ofusing non-ionic surfactant additions to decrease the droplet contact angle and

therefore allow the influence of wetting on corrosion rate to be explored. Fig. 6

shows the corrosion severity, represented as a mass gain, as a function of both

droplet size and non-ionic surfactant addition. The data suggests that for droplet

volumes greater than approximately 70 ll, more corrosion occurred under thedroplets with no added surfactant (i.e. a surface with poorer wetting properties).Therefore, whilst an increased corrosion rate may result from a greater oxygen

concentration in the thin film (increased thermodynamic driving force for the elec-

trochemical reactions [37]), under the present conditions, the increased evaporation

rate of the thin film was able to reduce the overall corrosion rate. This is not to say

that this trend will hold for variations in metal substrate, droplet chemistry, or va-

pour conditions. Other studies have shown that pitting corrosion is enhanced by thin

films when compared to the bulk [46], an observation attributed to increased con-

centrations of oxygen and chloride ions. Another related study by Gilbert andHadden [48] reported that for a constant total volume of electrolyte, the corrosion

rate of zinc under high humidity conditions increased as the size of the water droplets

decreased. Further studies are required to better understand the dependence of

Fig. 6. Corrosion severity (mass gain) dependence upon contact angle: (�) droplets of 1 M NaCl with no

added surfactant placed on rolled zinc, (�) droplets of 1 M NaCl with a lowered contact angle due to the

addition of 0.1% Tween80� non-ionic surfactant.


corrosion rates on wetting properties, and the way in which surface energy affects the

corrosion mechanisms.

5. Conclusions

The ability of metals to wet has been shown to vary with the chemistry of its

surface oxide. Contact angle determinations using liquid penetration and Wilhelmybalance techniques were able to systematically rank the oxides in order of their

wetting properties. Naturally forming zinc oxide–hydroxide films appear to be more

hydrophobic than zinc hydroxycarbonates and zinc hydroxysulphates. Zinc hy-

droxychloride oxidation products appear to give zinc the most hydrophilic nature.

The conversion of contact angle data to solid surface energies using various ap-

proaches provided values in the range of 20–40 mNm�1 for the oxidised zinc sur-

faces. Inverse Gas Chromatography provided insights into the acid–base nature of

zinc hydroxychloride and zinc hydroxycarbonate compounds, but appeared tooverestimate the Lifshitz–van der Waals component of the surface energy.

Data from wet perimeter measurements on passivated zinc plates suggested that

zinc hydroxychlorides and zinc hydroxysulphates possess a greater porosity than

zinc oxide–hydroxide and zinc hydroxycarbonate films. Further work will be re-

quired to verify the effects of roughness and porosity on controlling the wetting

properties of zinc surface oxides.

The ability of zinc to retain droplets on its surface was shown to be a function of

the wetting properties of the surface oxide, the angle of inclination, and the drop size.Preliminary experiments have shown that thin moisture films can be less corrosive

than droplets possessing a finite contact angle. A more detailed study is required to

better understand links between droplet form and corrosion rate.


Acknowledgements

Our appreciation is extended to Pon Kao (CSIRO, Manufacturing and Infra-

structure Technology) for his time and assistance with inverse gas chromatography

measurements.

Appendix A. The conversion of contact angle data to surface energy values

Many approaches have been used to convert the information gained from contact

angle determination into estimating the surface energy values of solids. At present,

there are two dominating approaches; those that suggest that the total surface energy

is made up of non-polar (dispersive) and polar (acid–base) components [19,30], andsecondly, those that suggest that the intermolecular characteristics of the liquid play

little part in determining the magnitude of the contact angle, and consequently an

equation of state approach is proposed [28]. The approaches used in this study are

detailed below.

Surface tension components approach 1: Van Oss et al. [30] proposed that the solid

surface energy is a sum of cLVWs and acid–base contributions, cABs , and is related tothe contact angle through Eq. (A.1):

cLVWs cLVWlv

� �0:5 þ cþs c�lv� �0:5 þ c�s cþlv

� �0:5 ¼ 12ðcos hÞ0:5 ðA:1Þ

where cþI and c�i represent the acid and base energy components, respectively.Diiodomethane was used as a probe liquid to determine the Lifshitz–van der

Waals net dispersive component of the solid surface energy, cLVWs using the approach

of Van Oss et al. [30], and is calculated using Eq. (A.2):

cLVWs ¼ cLVWlv

ð1þ cos hadvÞ2

4ðA:2Þ

where cLVWlv is the Lifshitz–van der Waals net component to the diiodomethanesurface tension.

The acid and base contributions to the surface energy were calculated from the

contact angle data of water and formamide. The values used for the Lifshitz–van der

Waals and acid–base components of liquid surface tensions are given in Table 6.

Surface tension components approach 2: An alternative method for estimating the

polar contributions to the solid surface energy was proposed by Wu [29], where the

non-polar, cdi and polar, cpi contributions to the surface energy could be related tothe contact angle via a reciprocal mean approach to describe the work of adhesionbetween two phases:

Wad ¼ 4cds c

dlv

cds þ cdvl

�þ cps c

plv

cps þ cpvl

�¼ cs þ clv � cdlvð1þ cos hÞ ðA:3Þ

which was solved using the contact angle data for two liquids with known non-polar

and polar components to their surface tension (i.e. water and formamide).

Table 6

Surface tension and viscosity data for liquids from Van Oss et al. [30]

Liquid clv(mNm�1)

cLVWlv

(mNm�1)

cABlv(mNm�1)

cþlv(mNm�1)

c�lv(mNm�1)

g (Pa s)

Water 72.8 21.8 51.0 25.5 25.5 0.089

Diiodomethane 50.8 50.8 0 0 0 0.028

Formamide 58.0 39.0 19.0 2.28 39.6 0.0455

1-bromonaphthalene 44.4 44.4 0 0 0 0.0489

Cyclohexane 25.0 25.0 0 0 0 0.0895

Hexane 18.4 18.4 0 0 0 0.0298


Equation of state approach 1: Li and Neumann [49] presented an equation of state

approach to calculate solid surface energies based upon combining a modification of

Berthelot’s rule with experimental data.

cos hY ¼ �1þ 2ffiffiffiffiffifficsvclv

re�bðclv�csvÞ2 ðA:4Þ

where the empirically determined value of b ¼ 0:0001247 (mmN�1)2 and hY isYoung’s contact angle.

Equation of state approach 2: A similar equation to (A.4) was later published by

Kwok and Neumann [28], which yields essentially identical values for solid surface

energy:

cos hY ¼ �1þ 2ffiffiffiffiffifficsvclv

rð1� bðclv � csvÞ

2Þ ðA:5Þ

where the value of b ¼ 0:0001057 (mmN�1)2.

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The protective nature of passivation films on zinc: surface charge

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