C268 Journal of The Electrochemical Society, 161 (5) C268-C275 (2014)0013-4651/2014/161(5)/C268/8/$31.00 © The Electrochemical Society

Aerosol Salt Particle Deposition on Metals Exposed to MarineEnvironments: A Study Related to Marine Atmospheric CorrosionShengxi Li and L. H. Hihara∗,z

Hawaii Corrosion Laboratory, Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu,Hawaii 96822, USA

The characterization and corrosive effect of marine salt particles and seawater droplets that naturally deposited onto carbon steeland pure zinc that were exposed to marine test sites were studied. Most of the salt particles had diameters ranging from approx-imately 2–10 μm, and varied in their composition ranging from almost pure NaCl or KCl to mixtures of NaCl, KCl, CaCl2,and MgCl2. The differences in composition may depend on whether the seawater droplets dehydrate, crystallize, and fragmentwhile airborne, or if they deposit as liquid droplets prior to crystallizing. The deposition of salt particles also decreased asthe wind velocity decreased. The initial stage of marine atmospheric corrosion of the carbon steel and pure zinc induced bythe salt deposition was also investigated. On the carbon steel substrate, NaCl-containing seawater droplets smaller than ap-proximately 30 μm could not initiate corrosion even after 30 minutes of exposure; whereas, droplets larger than approximately30 μm induced corrosion within approximately 30 minutes with lepidocrocite as the corrosion product. Corrosion initiated on thepure zinc samples under droplets of all sizes within 30 minutes. On the zinc samples, simonkollite formed in the central anodicregion; whereas, hydrozincite formed in the peripheral cathodic region.© 2014 The Electrochemical Society. [DOI: 10.1149/2.071405jes] All rights reserved.

Manuscript submitted January 6, 2014; revised manuscript received March 13, 2014. Published April 5, 2014.

Atmospheric corrosion has been extensively studied ever sinceW.H.J. Vernon’s work in the 1920s. Its complexity is based on nu-merous influential parameters, including both meteorological and pol-lution parameters. Among these variables are the airborne sea saltsin coastal regions (marine aerosols), which are generated either inthe open sea or the surf zone.1–3 Marine aerosols consisting of wetaerosols, partially wet aerosols, and non-equilibrium aerosols depend-ing on the atmosphere humidity,4 are carried by the wind and can reachoffshore structures and greatly accelerate the corrosion of metals. Thesizes of the three types of aerosols and the resultant dry aerosols wereestimated by Cole et al.4 The amount of marine aerosols present ina specific marine atmosphere, known as airborne salinity, is usuallymeasured in both atmospheric and corrosion studies. For corrosionstudies, correlating marine salinity and metallic corrosion is a majorfocus. By compiling worldwide research on atmospheric corrosionthat was conducted in the last 40 years, Morcillo et al.5 reported thatthe metallic corrosion rate increases fastest with respect to chloride de-position when its values are between 100 and 400 mg Cl−/m2/day, andslower when the chloride deposition is less than 100 mg Cl−/m2/dayor more than 400 mg Cl−/m2/day.

Studies have also focused on the behavior of marine aerosols,including their production, transport and deposition.1,6,7 Cole et al.1

proposed a comprehensive model that requires the input of significantamounts of data; whereas, Meira et al.7 generated a simplified modelthat depends only on aerosol characteristics and has wind velocityand distance from the sea as independent variables. Airborne salinitygenerally increases with increasing wind velocity, and the effect be-comes very significant when a critical velocity is exceeded. A rangeof critical wind velocity values (3–7.1 m/s) has been reported in theliterature, but a consensus has not been reached.8–10 Meira et al.11 rec-ommended 3 m/s as the critical wind velocity for a significant increasein airborne salinity.

Marine aerosol particles are usually classified by size into twoclasses:12 coarse particles with an equivalent aerodynamic diameterlarger than 2 μm and small particles with a diameter smaller than2 μm. Ambler et al.,13 however, classified the aerosol particles as“falling particles” with a diameter larger than 10 μm and “buoyantparticles” with a diameter smaller than 10 μm.13 They also stated thatthe corrosion of metallic surfaces is only caused by salt particles andsaline droplets with sizes larger than 10 μm, or the falling type.

NaCl is very abundant among marine aerosols and thus plays a cru-cial role in marine atmospheric corrosion of metals. The NaCl particle-

∗Electrochemical Society Active Member.zE-mail: hihara@hawaii.edu

induced atmospheric corrosion has been studied extensively for var-ious metals. Several salt deposition methods have been employed toprepare the NaCl particle-loaded specimens before exposure to highhumidity. One commonly used approach involves spraying ethanol ormethanol/water mixture saturated with NaCl onto metal surfaces andfollowed by drying in air at room temperature.14–23 Another way ofsalt particle deposition is by manually depositing grounded NaCl par-ticles on metal surfaces with the assistance of optical microscope.24–28

Other innovative methods to deposit NaCl particles include the useof humidifier29, inkjet printer,30–32 or a thermophoretic depositionmethod.33

Despite the enormous amount of studies on NaCl particle-inducedcorrosion, few investigations have focused on the naturally depositedairborne marine salt particles on metal surfaces (especially carbonsteel), including their characterization and effects on the initial stagesof metallic corrosion.4,34 The importance of studying the depositionof natural salt particles lies in the determination of the sizes andtypes of the salt particles. The sizes of the salt particles are importantto correctly simulate the initiation and propagation of atmosphericcorrosion in the laboratory, as the initiation and propagation mech-anisms can be dependent on the salt-particle size. As compared tothe atmospherically-deposited salt particles (dry) determined by Coleet al.,4 many studies24–28 have actually used very coarse NaCl par-ticles that may be more relevant to splash-spray zones as comparedto the smaller airborne salt particles that are found in natural marineatmospheres.34 The types of the salt particles are also important sincedifferent types of salt have different critical relative humidity (RH),which affects the wetting of the sample surfaces. For example, regionswith Mg-rich salt can be wet at RH levels as low as 35% due to thecritical RH of MgCl2 while those with Na-rich salts require RH levelsnear the critical RH of NaCl or approximately 75% for deliquescence.Notice that short-time exposure experiments are required for the de-termination of the sizes and types of the salt particles. If the exposureperiod is too long, the salt will be incorporated into the corrosionproducts making their initial distribution impossible to characterize.

The goal of the present work is to identify the various types, sizeand distribution of naturally-formed salt particles that are deposited oncarbon steel and pure zinc in marine atmospheric environments. Theinitial stage of corrosion caused by the deposited seawater droplets isalso discussed for steel and zinc. The corrosion products were identi-fied by a combination of energy dispersive X-ray analyzes (EDXA),Raman spectroscopy, and Fourier transform infrared spectroscopy(FTIR). As compared with a previous study,4 much shorter exposurewas conducted in this study to avoid extensive corrosion under theseawater droplets, which hinders accurate estimation of the initialseawater droplet size as corrosion propagates over a larger area. In

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Journal of The Electrochemical Society, 161 (5) C268-C275 (2014) C269

addition, this study focuses on the corrosion initiation on carbon steelsubstrates as compared to those on zinc and galvanized steel.4

Experimental

Sample preparation.— Specimens with dimensions of 10 mm× 10 mm were cut from 1018 steel and pure zinc (99.9%) sheet(3 mm thick) using a low-speed diamond saw and were then mountedin epoxy resin (Buehler Epoxicure). The samples were subsequentlyground with 180, 400, and 1200-grit SiC grinding paper; polished with9.0, 3.0, and 1.0 μm METADI SUPREME polycrystalline diamondsuspensions (BUEHLER), and finished with a MASTERPOLISH pol-ishing suspension (0.05 μm alumina + colloidal silica) (BUEHLER).The specimens were then rinsed with ultrapure water (18.0 M� cm)and reagent alcohol and then dried under warm air. Finally, all sampleswere kept in a dry box (1% RH) for 5 days prior to conducting ex-posure experiments to allow for the formation and stabilization of thesurface oxide films. Notice that the oxide films on both carbon steeland zinc are too thin to be detected using the spectroscopic techniquesemployed in the present study.

Sample exposure.— Polished samples (i.e., 1018 steel and purezinc) were exposed to a severe volcanic marine test site (approxi-mately 3 miles from the coastline) at Kilauea (KI) located in theNational Volcano Park, Big Island, Hawaii; and a mild marine testsite on Coconut Island (CI) approximately 25 feet from the coast-line, that is located in Kaneohe Bay, Oahu, Hawaii. Weather data andchloride candle data from the two test sites are given in Table I andTable II, respectively. Notice that the severity of KI test site was mainlydue to the acid rain in the volcanic environment (pH approximatelyequal to 3). However, it is assumed that the acid environment did notaffect the corrosion of metals during the 30-minute exposure sincethe acidification of seawater droplets through the absorption of SO2

only occur to droplets with diameters less than approximately 3 μm,35

which are smaller than the majority of the seawater droplets in thisstudy. The specimens were attached to the exposure racks (angled 30◦

facing the northeast into the trade winds)36 for 30 minutes. The 30-minute exposure is based on an estimation of how many salt particleswere expected to be deposited on a 1 cm2 specimen. The nominalsalt deposition rate for the test sites (e.g., 30 mg/cm2/day for KI testsite and 70 mg/cm2/day for CI test site) were used to calculate theaverage amount of salt particles that would be deposited. Assumingthat the sizes of the salt particles are in the range of 5–10 μm, then thecalculations showed that in just 30 minutes, the salt particle densitywill be approximately 30–250 particles per cm2 at the KI site, and70–600 particles per cm2 at the CI site. After exposure, the sampleswere transported back to the laboratory in Petri dishes in a desicca-tor to prevent the deliquescence of the salt particles, and subsequentcorrosion. Once in the laboratory, the samples were kept in a dry box(1% RH) prior to characterizing with scanning electron microscopy(SEM) equipped with EDXA, Raman spectroscopy, and FTIR.

Table I. Weather data at the two test sites.

Test Site Wind Speed (m/s) Relative Humidity (%)

KI 4.964 84.50CI 2.249 78.23

Table II. Typical monthly chloride candle data at the two test sites(unit: mg/m2/day). Wet Candle method was used for determinationof sea-salt components.

Test Site Cl S Na K Mg Ca

KI 31.56 63.24 39.89 1.20 5.22 2.98CI 70.93 11.80 49.18 1.50 6.22 4.04

SEM and EDXA characterization.— A Hitachi S-3400N SEMequipped with an Oxford Instruments energy dispersive X-ray an-alyzer system was used to characterize the retrieved samples. Quanti-tative elemental analysis (fixed-point) was conducted on both the sea-salt particles and the corroded regions of the specimens. Elementalmapping was also conducted on the corroded regions. The specimenswere analyzed without any conductive coatings (e.g., C or Au).

Raman and FTIR characterization.— A Nicolet Almega XR dis-persive Raman Spectrometer (Thermo Scientific Corp.) equipped witha Peltier-cold charge-coupled device (CCD) detector was used for theexperiments. An objective with magnification of 50× with estimatedspatial resolutions of 1.6 μm were used. The instrument was operatedwith laser sources of a green Nd:YAG laser with 532 nm wavelengthexcitation and an infrared diode laser with 780 nm wavelength exci-tation. The laser power was always kept low at approximately 1 mWto avoid sample degradation by laser heating. The accumulation timewas 120 seconds.

FTIR experiments were performed using a Thermo Electron Nico-let Nexus 760 instrument integrated with a continuum microscope. Ablank background spectrum was collected prior to collecting a spec-trum of the sample. An area of 100 μm×100 μm was used for FTIRacquisition. A minimum of 120 scans (1 s/scan) of the specimen wasemployed for each spectrum. The resolution was 2 cm−1.

Results and Discussion

Natural salt particle deposition on 1018 steel.– Kilauea volcanic-marine test site.— Many salt particles were found on the 1018 steelsamples that were exposed to the KI test site for 30 minutes. The RHduring sample exposure was 84.50% (higher than the critical RH forNaCl of ≈ 75%), and therefore, the salt particles were likely depositedas seawater droplets rather than salt crystals. Since the samples weresubsequently transported back to the laboratory in a dessicator, thedroplets likely dehydrated during transportation. Hence, upon inspec-tion with SEM, large centralized salt particles were surrounded bycircular patterns (containing submicron salt residuals) which likelyrepresented the area that had been wet by seawater droplets. Assum-ing that the seawater droplets on metal surface are hemispherical, thesizes of the residual patterns equal to the sizes of the seawater droplets.By counting all of the salt particle regions found on a 1 cm2 sampleand measuring the diameters of the residual patterns (from the SEMimages), a histogram for the size distribution of the droplet diameterswas generated (Fig. 1). The histogram shows that most of the dropletdiameters ranged from 10–30 μm, which rendered dehydrated saltparticles with diameters ranging from approximately 2–10 μm. Thisrelationship between the sizes of the seawater droplets and those ofthe dry sea-salt particles is similar to that reported by Cole et al. forwet aerosol particles.4

Figure 1. Seawater droplet residual areal size distribution histogram for steelexposed at the KI test site. (out of 39 seawater droplet residual areas)

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C270 Journal of The Electrochemical Society, 161 (5) C268-C275 (2014)

Figure 2. (a)-(e) Cubic NaCl particles found on 1018 steel samples that wereexposed to the KI test site for 30 minutes; (f) EDXA spectra of the salt particlesin (e).

Single cubic salt particles were predominant among the deposits(Fig. 2a-2e). Smaller salt particles (D < 1 μm) were found either nearthe larger primary salt particles (Fig. 2a) or along the peripheries ofthe residual regions (Fig. 2b-2e). The large central and satellite saltparticles (Fig. 2e) consisted of only NaCl, as indicated by the EDXAspectrum (Fig. 2f1). The smaller salt particles were either NaCl (asmarked by black arrows in Fig. 2e) or those containing S and Ca(marked by a white arrow in Fig. 2e). Two other major elements in seasalts (i.e., K and Mg) were not detected in any of the salt particles sur-rounding the large particles in Fig. 2 using EDXA, possibly becausetheir content in relatively small seawater droplets (D < 20 μm) resultsin low surface concentration that is below the EDXA detection limit(≈ 0.1%). However, MgCl2 and KCl particles were found aroundlarger salt particles resulting from seawater droplets with diametersgreater than approximately 30 μm (discussed in the following sec-tions).

In addition to the cubic NaCl particles, individual irregularlyshaped NaCl particles were also found on 1018 steel samples that wereexposed to the KI test site for 30 minutes (Fig. 3). Similar to previousresults (Fig. 2), the primary NaCl particles (D ≈ 10 μm) generallycrystallized in the central region of the original seawater droplets,while smaller salt particles (D < 1 μm) formed surrounding the largesalt particles (Fig. 3a-3c). Notice that some of the large, irregularlyshaped salt particles also contained a trace of Mg (Fig. 3c), whichmight be one reason for their irregular shape. Most of the smaller saltparticles were NaCl particles (as indicated by black arrows in Fig. 3band 3c), while others contained Ca and S (as marked by white arrowsin Fig. 3b and 3c). Unlike the salt particles shown in Figs. 2, 3a and3b, some small particles in Fig. 3c also contained Mg and K (seeFig. 3d2), indicating the existence of Mg2+ and K+ in seawaterdroplets. It may be that Mg2+ and K+ were found because of thehigher content of Mg and K in the larger original droplet (D ≈ 30 μm)that resulted in the salt particles shown in Fig. 3c compared to the

Figure 3. (a)-(c) Irregularly shaped NaCl particles found on 1018 steel sam-ples that were exposed to the KI test site for 30 minutes; (d) EDXA spectra ofthe salt particles in (c).

Figure 4. (a) A cluster of salt particles found on a 1018 steel sample that wasexposed to the KI test site for 30 minutes; (b) EDXA spectra of (b1) a cubicsalt particle and (b2) a round salt particle in (a).

droplets (D ≈ 10 μm) that formed the salt particles shown in Figs. 2,3a and 3b.

Clusters of salt particles were also found on 1018 steel samplesthat were exposed to the KI test site for 30 minutes (Fig. 4). Mostof the salt particles in this cluster were cubic NaCl particles (Fig.4b1) measuring 1–5 μm, while others were non-cubic salt particlescontaining all of the major sea-salt components, i.e., Cl, S, Na, Mg,K and Ca (Fig. 4b2 and Table III).

In addition to NaCl particles, individual KCl particles were found.A relatively large particle (D ≈ 4 μm) (Fig. 5a) showed cubic structure,while smaller particles (D < 1 μm) (Fig. 5b) were spherical. Giventhe concentration of K in seawater (≈ 320 ppm), the size of theKCl particles that form from seawater droplets could be estimated.For a KCl particle with size of 4 μm (Fig. 5a), a seawater dropletwith diameter of 92 μm is required. Hence, the 4-μm KCl particle,likely crystallized while airborne and separated from the other sea-salt constituents. For smaller seawater droplets (10–30 μm) that aredominant in the present study, the KCl particles that could be formedwould have diameters ranging from only 0.4 to 1.2 μm. Notice thatthe submicron-sized salt particles were not counted in Figs. 1 and 8.

Table III. Chemical composition of the round salt particle inFig. 4a (indicated by an arrow).

Element C O Na Mg Al S Cl K Ca Fe

Weight % 8.58 22.86 8.37 1.21 0.33 9.9 5.42 1.23 7.78 34.32Atomic % 18.46 36.91 9.4 1.28 0.31 7.98 3.95 0.81 5.02 15.88

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Journal of The Electrochemical Society, 161 (5) C268-C275 (2014) C271

Figure 5. (a) and (b) KCl particles found on 1018 steel samples that wereexposed to the KI test site for 30 minutes; (c) a CaCO3 particle found on a1018 steel sample; (d) EDXA spectra of the particles in (a)-(c).

Finally, CaCO3 particles (i.e., Fig. 5c), which could originate fromsand or coral particles, were also identified. It is unlikely that theCaCO3 particles precipitated from seawater droplets because dropletresiduals were not observed around the particles. The sand or otherdust particles could also be important in atmospheric corrosion ofmetals as they could alter the surrounding pH, or facilitate dew forma-tion (through capillary condensation) on metal surface and acceleratecorrosion.37 This type of particles, however, was not the focus of thisstudy; hence, only saline-based salt particles were counted in Fig. 1.

The above results indicate that in most cases, NaCl separated fromother sea-salt components, likely during the dehydration process ofthe seawater droplets. Because the RH during sample exposure in theKI test site was 84.50%, the salt particles likely originally deposited asdroplets. When the RH was reduced (in the desiccator), various typesof salts crystallized from the droplets. Sodium chloride crystals werethe most commonly found (Figs. 2–4) because NaCl is the primaryconstituent of seawater (Table II). When NaCl crystallizes, it leavesbehind other ions from which other salts can crystallize. This dehydra-tion process may occur on the specimen substrate or while the dropletsare airborne during periods of low humidity. An indication of airbornedehydration was the individual KCl crystals that were found (Fig. 5a)without coexisting with other sea-salt components. Other salt speciessuch as sodium sulfate tended to coexist with other sea-salt elements(Figs. 2e and 3c) and may not have precipitated as distinct crystals.Individual MgCl2 and CaCl2 particles were also not found, whichmight be due to their low critical RH values (≈ 30–35%), causingthem not to crystallize and to encompass other sea-salt components(e.g., KCl and Na2SO4). The findings indicate that the deposition ofairborne salt particles in marine environments could also be speciesspecific leading to different local wetting and different local corrosionrates based on the salt species.

Cole et al.4,38 also studied salt particles that crystallize on metalsurfaces but from laboratory-sprayed saturated ASTM seawater andnaturally deposited seawater droplets (on zinc). They found that theprimary crystal was cubic NaCl, while the fine crystals surroundingthe main crystal contained either Na, Mg and Cl or (less frequently)Ca and S. Our natural salt particle deposition experiments showedsimilar results but with some differences. We found that most of thesmall salt particles that surrounded the major NaCl particles wereprimarily pure NaCl instead of the Mg-containing particles observedby Cole et al. Only a few salt particles contained S and Ca, whilesalt particles containing Mg were scarce. In addition, individual KClparticles, both large cubic and fine round particles, were identified in

this study, but not in Cole’s work. Finally, salt particles (D ≈ 5 μm)that contained various types of salts resulting from the evaporation oflarger seawater droplets (D ≈ 30 μm) were also found (Fig. 4). Thisfinding indicated that larger mixed salt particles (D ≈ 5 μm) can formfrom larger seawater droplets (D > 30 μm) that have relatively largeamounts of other sea-salt components besides NaCl. The differencesbetween Cole’s work and this investigation could be caused by 1) avariation in the seawater composition (i.e., ASTM vs. natural), 2) avariation in the rate of evaporation, which can affect the nucleationand growth of salt crystals, and 3) evaporation-hydration cycles andsalt particle fragmentation that could take place while natural saltparticles are airborne.

Some important observations were made regarding the lack ofcorrosion initiation near Cl-containing particles that originated fromoriginal droplets < 30 μm (Figs. 2–5). In these cases, the salt particlesand clusters were generally smaller than 10 μm in diameter and crys-tallized from seawater droplets < 30 μm in diameter, as indicated bythe residual features left on the substrate. This result generally agreeswell with our laboratory study on 1018 steel,29,39 in which corrosiondid not initiate from small NaCl droplets (D < 45 μm) that formed bythe deliquescence of pre-deposited NaCl particles even after 6 hoursof exposure, but did initiate under droplets larger than approximately45 μm after only 5 minutes of exposure. The droplet-size effect on thecorrosion of carbon steel is important especially for the modeling ofsalt particle-induced marine atmospheric corrosion since corrosion isnot only affected by the salt loading but also by the salt-particle size.The fact that approximately 85% (counts) of the seawater dropletsare smaller than 30 μm (Fig. 1) and do not initiate corrosion on steelindicates that only a fraction of salt deposition determined by con-ventional methods (i.e., wet chloride candle) contributes to the initialstage of marine atmospheric corrosion. This effect may extend to sev-eral wet/dry cycles if the coalescing of the small seawater dropletsdoes not occur.

Corrosion was found to occur under larger seawater droplets (D >30 μm) (Fig. 6a-6c). Corrosion products (Fe and O) were detected overthe entire original seawater droplet area (Fig. 6d). Sea salts, mostlyNaCl but also others, i.e., MgCl2 (in the center), were found to coexistwith the corrosion products. The existence of MgCl2 in the major saltparticle (Fig. 6d1) corroborates the previous discussion concludingthat MgCl2 is more likely to be found in deposits from larger seawaterdroplets. Notice that MgCl2 also exists in the satellite salt particles(D < 2 μm) surrounding the major deposits (Figs. 3d2 and 6d1).The satellite salt particles are similar to the small Mg-containing saltparticles (D = 1–3 μm) reported by Cole et al. on zinc and galvanizedsteel.4

Previous studies on corrosion under NaCl droplets24,29 have re-vealed that Na+ and Cl− ions migrate to cathodic and anodic sites,respectively, during corrosion. Our EDXA elemental mapping overthe corroded regions also showed ion migration during the 30-minuteexposure period. Elemental maps of Cl and Na (Fig. 6e) show someregions in the center with Cl and no Na (marked by ovals) and othersalong the perimeter with Na and no Cl (marked by circles), indicat-ing the migration of Na+ and Cl− ions. Quantitative analysis throughline-scan reconstruction (line (f) in Fig. 6c) from the mapping datashows a higher atomic concentration of Cl than Na in the center ofthe original seawater droplet area, while a higher concentration Nawas found along the periphery (Fig. 6f), confirming that some Na andCl ions migrated to the perimeter and the center of the corroded area,respectively, under the initially deposited seawater droplet.

It is known40,41 that any geometrical factor that renders a higherconcentration of oxygen at one part of a steel surface and a lowerconcentration (or zero) at another will result in the former becomingthe cathode and the latter the anode of the corrosion cell, thus leading tolocalized corrosion. In the case of natural seawater droplets depositedonto a steel surface, if we assume that mass transfer occurs onlyby diffusion, it is apparent that the diffusion of oxygen from theatmosphere to the metal surface will occur most rapidly through thethin layer of solution along the periphery of the droplet and mostslowly at the center of the droplet. Therefore, the metal substrate

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C272 Journal of The Electrochemical Society, 161 (5) C268-C275 (2014)

Figure 6. (a)-(c) Corroded regions found on 1018 steel samples that wereexposed to the KI test site for 30 minutes; (d) EDXA spectra of the corrosionproducts and salt particles in (c); (e) elemental maps of (e1) Cl and (e2) Nafrom the corroded area in (c). The circle regions show the existence of Naonly, while the oval regions show the existence of Cl only; (f) comparison ofelements Cl and Na along the dashed line shown in (c). The data in (f) wereobtained by performing line-scan reconstruction from the EDXA elementalmapping data. (color online)

under the periphery of the droplet becomes the cathode and that underthe center of the droplet becomes the anode of the differential aerationcell.41

The active dissolution of steel in the central anode generates Fe2+

(equation 1) that reacts with water (hydrolysis) to form H+ (equation2). To preserve charge neutrality, Cl− migrates to the anode. Similarly,Na+ migrates to the peripheral cathode that is rich in OH− generatedby O2 reduction (equation 3). The Fe(OH)2 is not stable in oxygenatedaqueous media and will further react to form other iron oxides or oxy-hydroxides.42–44

Fe → Fe2+ + 2e− [1]

Fe2+ + 2H2O → Fe(OH)2 + 2H+ [2]

O2 + 2H2O + 4e− → 4OH− [3]

Figure 7. Salt particles found on 1018 steel samples that were exposed to theCI test site for 30 minutes.

Coconut Island moderate marine test site.— In contrast to the KItest site, less salt particles and corroded areas were found on 1018steel samples that were exposed to the CI test site. Possible reasonswere the absence of shore-breaking waves, and the relatively lowwind velocity which was approximately only 2.2 m/s (during sampleexposure) that is below the critical value for salt-particle generationfrom ocean waves.11 Notice that the relatively high monthly-averagedchloride deposition rate at this test site (Table II) was likely causedby higher wind velocity during other periods. A few salt particlesthat were collected (Fig. 7) showed that the larger salt particles werecomposed of mainly NaCl and sometimes contained S, K, Mg and Caas well. The fine salt crystals that scattered over the initial seawaterdroplet areas were composed of NaCl. Again, noticeable corrosionwas not detected over the entire initial seawater droplet areas, sincethe droplet diameters were typically less than 30 μm.

Raman and FTIR identification.– Carbon steel.— The corrodedregions on carbon steel samples exposed in KI test site for 30 minutesshowed Raman signal mainly of lepidocrocite with peaks at 248, 311,376, and 527 cm−1 (Fig. 11a).43–45 This result agrees well with pre-vious laboratory study43 which indicated that lepidocrocite is alwaysthe first rust phase formed in NaCl particle-induced marine atmo-spheric corrosion on steel. The Raman spectrum (Fig. 11b) from anirregular-shaped salt particle similar to those shown in Fig. 3 showsa strong peak of sulfate ion (SO4

2−) at 983 cm−1.44 Notice that theRaman spectra obtained from the cubic salt particles similar to thoseshown in Fig. 2 do not show any Raman signal, and were identifiedas NaCl particles through SEM/EDXA. However, some of the smallsalt particles surrounding the major NaCl particles (Fig. 2) showedRaman peaks of SO4

2− at 983 cm−1, a result corroborating the EDXAanalysis in Figs. 2 and 3.

FTIR analysis of the exposed carbon steel sample also showedthat the corrosion products were mainly lepidocrocite (Fig. 12a) withmajor peaks at 750, 883, 1020, and 1140 cm−1.46 The irregular-shapedsalt particles have typical FTIR spectra given in Fig. 12b, which isbelieved to be corresponded to sulfates.47 The peak at 1624 cm−1 mighthave come from water molecules contained in the salt particle.46

The mechanism of the airborne NaCl particles-induced atmo-spheric corrosion of steel is believed to be the same as that pre-viously reported on laboratory simulated studies.43 Green rust andlepidocrocite are the initial corrosion products that form on steel un-der NaCl droplets, both natural airborne and laboratory simulated.The reason why green rust was not detected is probably due to theirunstable nature and possible transformation to other types of rust.In addition, the green rust may have been underneath outer corrosionproducts and encrusted salt particles since it only forms in the confinedanodic region.

Natural salt-particle deposition on pure zinc.– Kilauea volcanic-marine test site.— Because the surface oxide film on the pure zincsample is not protective, all of the seawater droplets that depositedonto it were able to initiate corrosion. A histogram (Fig. 8) of thecorroded region (on a 1 cm2 zinc sample) size distribution shows thatmost of the corroded regions or the original droplet areas measured

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Journal of The Electrochemical Society, 161 (5) C268-C275 (2014) C273

Figure 8. Seawater droplet residual areal size distribution histogram for zincexposed to the KI test site. (out of 33 seawater droplet residual areas)

10–30 μm, a result similar to that observed for the steel sample, exceptthat corrosion on zinc also initiated from the smaller droplets.

Extensive corrosion occurred under most of the seawater dropletsthat deposited onto pure zinc samples after 30 minutes of exposure inthe KI test site (Fig. 9), with corrosion morphologies similar to thosereported by Cole et al.35 The EDXA spectrum (Fig. 9f) of a corrodedarea (in Fig. 9e) shows the presence of corrosion products (Zn and O)together with sea salts, i.e., Cl, S, Na, and Mg. Corrosion also propa-gated out of the relatively large seawater droplet areas (D ≈ 30 μm)on zinc substrates (Fig. 9a, 9b and 9e), possibly due to the secondarydroplet spreading24 that occurred during corrosion. Small seawaterdroplets (D < 10 μm) caused corrosion only under the droplets andnot in the surrounding areas (Fig. 9c and 9d). In addition, the seawaterdroplets that attached to the sample surface at different times may

Figure 9. (a)-(e) Corroded regions caused by seawater droplets on pure zincsamples that were exposed to the KI test site for 30 minutes; (f) EDXA spectrumof the corroded area in (e).

Figure 10. (a)-(c) Sea-salt particles found on pure zinc samples that wereexposed to the KI test site for 30 minutes; (d) EDXA spectra of the salt particleand corroded area in (c).

have caused corrosion to different extents. A droplet deposited onto asample early during the 30-minute exposure period might have causedmore corrosion, while a droplet that deposited onto the zinc surfacenear the end of the 30-minute exposure is likely to have resulted in lesscorrosion (Fig. 9c and 9d). Sometimes, NaCl particles crystallized ontop of the corrosion products (Fig. 9d) instead of mixing with them.

Individual sea-salt particles, mainly NaCl (Fig. 10d1), were alsofound on pure zinc samples in regions with very little corrosion(Fig. 10d2) exposed to the KI test site (Fig. 10a-10c), indicating thatthe salt deposition occurred late in the 30-minute exposure. Unlike thecase for steel, however, corrosion can occur on zinc even under smallseawater droplets (D < 30 μm) due to the lack of passive surfacelayers.

Coconut Island moderate marine test site.— Neither corroded ar-eas caused by seawater droplets nor sea-salt particles were foundon the pure zinc samples that were exposed to the CI test site for30 minutes, which was likely due to relatively low wind velocities dur-ing the sample exposure and small specimen size, reducing chancesfor salt deposition.

Raman and FTIR identification.—Zinc.— Two typical Ramanspectra were obtained from the corrosion products on zinc samplesexposed in KI test site for 30 minutes (Fig. 13). Most of the cor-roded regions under NaCl salt droplets showed Raman spectra of

Figure 11. Raman spectra from (a) a corroded region and (b) an irregular-shaped salt particle on a carbon steel sample exposed in KI test site for30 minutes.

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C274 Journal of The Electrochemical Society, 161 (5) C268-C275 (2014)

Figure 12. FTIR spectra from (a) a corroded region and (b) an irregular-shaped salt particle on a carbon steel sample exposed in KI test site for 30minutes.

Figure 13. Raman spectra from different locations on a zinc sample exposedin KI test site for 30 minutes.

Figure 14. FTIR spectra from different locations on a zinc sample exposed inKI test site for 30 minutes.

simonkollite (Zn5(OH)8Cl2 · H2O) (Fig. 13a), a typical corrosion prod-uct that forms in Cl-containing environments. The two peaks at 260and 393 cm−1 are characteristic peaks of simonkollite.48–52 Notice thatthe strong peak at 3460 cm−1 is characteristic of the OH− vibrationsthat appear at high frequencies.48,49 In some regions in the periphery ofthe larger circular corroded regions (e.g., Fig. 9a and 9b), other typesof corrosion products were detected and were identified as zinc oxide(ZnO) and hydrozincite (Zn5(OH)6(CO)2) (Fig. 13b). The peak at 568cm−1 is attributed to zinc oxide,48,50–53 while the peaks at 386, 738,1071, 1356, and 1554 cm−1 are corresponded to hydrozincite.49,51,52,54

Peaks in the range of 2850–2950 cm−1 are possibly due to the O–Hstretching in hydrozincite.52 Notice that the broad peak at approxi-mately 3450 cm−1 is due to OH− vibrations in H2O.

FTIR characterization of the corrosion products on zinc alsoshowed the presence of simonkollite (Fig. 14a) and hydrozincite

(Fig. 14b). The peaks at 714, 904, 1040, 1610, and 3490 cm−1

(Fig. 14a) are attributed to simonkollite,55–60 while the peak at1140 cm−1 is not assigned. In more detail, the three peaks at 714,904, and 1040 cm−1 are related to O–H deformation vibrations ofthe simonkollite; the peak at 1610 cm−1 is related to deformation vi-brations of O–H bonds in the incorporated water molecules; and thebroad peak at 3490 cm−1 corresponds to stretching vibrations of O–Hbonds.60 In Fig. 14b, the peaks at 707, 735, 827, 890 (shoulder), and943 cm−1 correspond to the –C–H group, while the peak at 1040 cm−1

is attributed to the C–O group.59 The strong peaks at 1380 and1480 cm−1 are related to the asymmetric stretching vibrations of thecarbonate ion.55 The broad peak at 3370 cm−1 is due to O–H stretching.Notice that the IR peaks corresponding to zinc oxide at approximately560 cm−1 is not detected because it is below the instrument cutoff at600 cm−1.

The corrosion of zinc under NaCl droplets occur rapidly due tothe corrosivity of Cl ions on the surface oxides. Uniform corrosionoccurred under small droplets (D < ≈ 30 μm) and simonkollite wasthe major corrosion products. For larger droplets, a second spreadingeffect occurred around the salt droplets,24 indicating a relatively strongcathodic activity caused by the Evans effect.61 The central area servedas anode (equation 4) while the secondary spreading area served ascathode (equation 3).24,35,62

Zn → Zn2+ + 2e− [4]

The Cl− rich anodic area favored the formation of simonkollitefollowing reaction:49

5Zn2+ + 8H2O + 2Cl− → Zn5(OH)8Cl2 + 8H+ [5]

Zinc ions from the anodic area may diffuse to the cathodic areaand react with OH− and CO3

2− and then form the corrosion productby the following reaction:57

5Zn2+ + 6OH− + 2CO32− → Zn5(OH)6(CO3)2 [6]

The OH− originates from the reduction of dissolved O2 at thecathodic site, and the CO3

2− originates from atmospheric CO2.57

Notice that there is not a well-defined critical droplet size forEvans’ effect to occur on zinc under seawater droplets. According tothe results in this study and that from Cole et al., an estimated rangeof this critical droplet size falls in a range of approximately 30–100μm for zinc.

Conclusions

To study natural salt particle deposition and the initial stage ofmarine atmospheric corrosion, sea-salt particles were allowed to nat-urally deposit onto 1018 steel and Zn surfaces for only 30 minutes inmarine test sites.

On 1018 steel samples that were exposed to a severe volcanic-marine (KI) test site, both 1) small, distinct sea-salt particles (D < 5μm) and sea-salt clusters (D < 10 μm) formed by dehydration on topof the steel substrate that did not corrode under relatively small seawa-ter droplets (D < 30 μm), and 2) sea-salt clusters integrated with ironcorrosion products that formed on the steel substrate that did corrodefrom larger seawater droplets (D > 30 μm). Among all the small saltparticles, single cubic NaCl particles (D < 5 μm) were predominant.Fine salt particles (D < 1 μm) containing Ca, S and Mg were foundscattered around the NaCl particles. Individual KCl particles were alsofound separated from other sea-salt components. Corrosion productswere not detected with small sea-salt particles/clusters (D < 5 μm)that originated from small seawater droplets (D < 30 μm), indicatingthe lack of corrosion initiation on steel during the 30-minute exposureperiod. The corrosion that occurred under larger seawater droplets(D > 30 μm) showed the typical characteristics of droplet corrosion,with Cl− and Na+ ions having migrated to the central anode and pe-ripheral cathode, respectively. The corrosion products were identifiedas lepidocrocite. Few sea-salt particles and the subsequent corrodedregions were found on the 1018 steel samples that were exposed to amild-marine (CI) test site due to low wind velocity during the sampleexposure period.

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Journal of The Electrochemical Society, 161 (5) C268-C275 (2014) C275

The separation of salt species during their deposition on carbonsteel affects the wetting of the steel surface and the subsequent cor-rosion. Therefore, the determination of the types and sizes of the nat-urally deposited salt particles on carbon steel provides a foundationfor laboratory studies of sea-salt particle-induced marine atmosphericcorrosion and its modeling. In addition, salt loading determined byconventional methods, i.e., wet chloride candle can over estimate theamount of corrosion-initiation sites during modeling of the initialstage of marine atmospheric corrosion due to the droplet-size effectsince droplets less than approximately 30 μm do not initiate corrosionon carbon steel.

Corrosion occurred on pure zinc samples under seawater dropletsof all sizes due to the lack of a passivating layer. The corrosion productsunder larger droplets were identified as simonkollite in the centralanodic region and hydrozincite in the peripheral cathodic regions. Itis likely that the Zn ions diffused from the anodic site to the cathodicsites forming the hydrozincite.

Acknowledgments

The authors are grateful for the support of the US Army RDECOM-ARDEC for a past project entitled “Pacific Rim Environmental Degra-dation of Materials Research Program” (Contract #: W15QKN-07-C-0002). The authors are particularly grateful to program manager BobZanowicz.

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