Corrosion Science 51 (2009) 1725–1732

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Corrosion Science

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Weatherability of 09CuPCrNi steel in a tropical marine environment

Yuantai Ma, Ying Li *, Fuhui WangState Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Science, Wencui Road 62, Shenyang 110016, China

a r t i c l e i n f o

Article history:Received 28 February 2009Accepted 28 April 2009Available online 9 May 2009

Keywords:A. SteelB. IR spectroscopyC. RustB. EPMA

0010-938X/$ - see front matter Crown Copyright � 2doi:10.1016/j.corsci.2009.04.024

* Corresponding author. Tel.: +86 24 2392 5323; faE-mail address: liying@imr.ac.cn (Y. Li).

a b s t r a c t

The stability of weathering steel (09CuPCrNi) exposed at tropical marine environments for varied periodswas investigated. The synergistic effect of exposure environment and alloy elements on weatherabilitywas also studied. The classical weight loss method was used to evaluate weatherability, while the ironrust layers formed on weathering steel under different conditions were analyzed by using Infrared spec-troscopy (IR), electron probe of mass analysis (EPMA) and electrochemical techniques. The results showthat under low chloride deposition, the enrichment of Cr plays an important role on improving weather-ability; conversely chloride ion is primary factor to whittle weatherability in high chloride deposition.

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1. Introduction

Weathering steels are widely employed in several aspects ofconstruction such as bridges or towers because of their perfectanti-corrosion performance, i.e., a protective layer can develop onthe surface of weathering steel exposed to a suitable environment.The effectiveness of the protective layer depends on several factorsincluding environmental conditions [1–3], contamination [4–6], al-loy elements [7–8], as well as characteristics of corrosion products[9–12]. Corrosion products doped by alloy elements have been re-ported to play a significant role on prohibiting the corrosion pro-cess of steel [13,14]. The elements in the weathering steel canpromote formation of a protective layer and doping effect intothe rust layer favors development of a more compact layer [15,16].

Different series of alloy elements in weathering steel demon-strate diverse roles in improving anti-corrosion performance andvarious explanations have given for this. Misawa et al. [13] ob-served that the existence of Cu and P in the steel can promote Fe(II)complex transformation to amorphous d-FeOOH, which forms acompact rust layer with enhanced corrosion resistance. Cr inlow-alloy steel can favor the formation of crack free, uniform rustlayer and help to produce uniform amorphous ferric oxyhydroxideand improve weatherability of steel [15]. In a series of reports[14,17], Yamashita and coworkers clarified the effect of Cr inenhancing the corrosion resistance of weathering steel and ex-plained its mechanism. Cr accumulates only in the inner rust layer,leading to the formation of a densely compact rust layer with supe-rior ability for protection of the weathering steel against the atmo-spheric corrosives. In addition, Cr3+ in the rust layer is coordinated

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with O2� and positioned in the double chains of vacant sites withinthe FeO3(OH)3 octahedra network in the goethite crystal, leading toimproved protective performance. The Nishimura group [18,19]assessed the influence of Co, Ni, Al and Si on the anti-corrosionproperties of weathering steel by wet–dry cycle test and observedthat all the four alloy elements remarkably advance the corrosionperformance, though by different mechanisms. Co was incorpo-rated mainly into FeOOH and Ni was involved in the formation ofa spinel oxide in rust while Ni helped to convert spinel into a denseand fine structure [18]. Al was present in the complex oxides asAl3+ and Si was identified at an intermediate state as Si2+ in thecomplex oxides of the inner layer; the two elements favorednano-scale complex iron oxides formation in the lower layer ofiron rust, which suppressed the corrosion process [19].

It is widely recognized that alloying elements such as Cr, Ni, Cuand Si improve the performance of weathering steel against atmo-spheric corrosion. Nevertheless, when such weathering steels areexposed to real adverse environment, they still suffer serious cor-rosion despite the presence of the alloying elements [20]. Thus,the anti-corrosion performance of weathering steels depends onboth the effect of alloying elements and the exposure environment.The particular environment in which the presence of alloying ele-ments exerts a significant influence on corrosion performance isnot well understood. It is also not clear whether a synergistic rela-tionship exists between the effects of exposure environment andalloying elements. Detailed insights into such synergistic effectswill enable development of new type of weathering steel with im-proved ability to resist corrosion extremely corrosive environ-ments. In the present study, we investigate the effect of analloying element (Cr) on the corrosion performance of weatheringsteel (09CuPCrNi) exposed to a marine site with high Cl� ioncontent.

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2. Experimental procedure

2.1. Exposure tests

For this investigation, the weathering steel specimen (CortenA;Table 1) was cut into coupons of dimensions 100 mm � 45 mm �5 mm. The coupons were wet-polished down to 800 grade emerypaper, cleaned ultrasonically in acetone rinsed with distilled water,dried, weighed and stored in a moisture-free desiccator prior touse.

Atmospheric corrosion tests were undertaken at two differentexposure stations in Wanning city (marine site: 110�050 easternlongitude and 18�580 northern latitude), i.e., one exposure stationwas positioned 25 m (SITE 1) and another station was positioned375 m (SITE 2) from the water front. The metal test panels werepositioned at 30� to the horizontal, with skyward surface directedtoward the sea to emphasize the marine fog effect. The climaticcharacteristics and the main environmental parameters measuredare listed in Table 2. Chloride concentration in the test environ-ment was determined monthly using the ‘‘wet candle” technique,according to ISO standard 9225 [21,22].

The test duration was 24 months (May 2005–May 2007). Fivereplicate metal samples were retrieved from the different locationsat 3, 6, 12 and 24 months. Three of the retrieved samples were usedfor weight loss experiments; another was used for electrochemicalmeasurements; while the fifth was used to characterize the mor-phology of the rust layers formed and the steel surface throughSEM-EDAX using Philip XL30FEG. Corrosion products on the spec-imen surfaces were removed chemically by immersion in a specificsolution (500 ml HCl + 500 ml distilled water + 3.5 g hexamethy-lenetetramine) that was vigorously stirred for �10 min at 25 �C.After removal of corrosion products, the specimens were rinsedwith distilled water, dried in warm air and then weighed to deter-mine their mass loss.

2.2. Rust analysis

The study of atmospheric corrosion products on the steel cou-pons was performed by FTIR. The outer layer was prepared usingthe corrosion products which flaked off the steel substrate; innerlayer was scraped from the surface of steel. The KBr technique[23] was used to prepare the rust layers for IR analysis. The IRabsorption spectra were generated in a Magna-IR 560 infraredspectrophotometer. The surface and cross-section of the corrosionproducts were characterized by SEM, the alloy elements were de-tected by using EPMA.

2.3. Electrochemical measurements

Polarization measurements were performed with a PARSTAT2273 potentiostat/galvanostat at a 0.333 mV/s scan rate in air sat-urated 0.1 M/L Na2SO4 in a conventional glass cell at 25 ± 2 �C,using a saturated calomel electrode as reference and a Pt counterelectrode. The scan range was �250 mV (SCE) relative to the opencircuit potential to 800 mV (SCE). The whole steel sample wasmanually sectioned into coupons of dimensions 45 mm�10 mm � 5 mm, which served as the working electrode. The work-ing electrode surface was covered with a mixture of olefin and ro-sin to leave an exposed area of 2–3 cm2.

Table 1Chemical compositions of weathering steel studied (wt.%).

Steel C S P Mn Si Cr Ni Cu Al

CortenA 0.09 0.005 0.081 0.35 0.30 0.48 0.27 0.28 0.37

The electrochemical impedance spectroscopy (EIS) measure-ments were carried out over the frequency range 100 kHz to10 mHz with a 5 mV amplitude signal at open circuit potential. Be-fore all the electrochemical experiments were performed, the spec-imens were kept in the solution 1 h in order to attain a stable opencircuit potential. All experiments were run in triplicate.

3. Results

3.1. Corrosion rate of weathering steel

The corrosion rate of weathering steel in different sites was cal-culated by using the equation in Ref. [24]

Vn ¼dn � dn�1

tn � tn�1ð1Þ

where V is the corrosion rate (lm/year), d is the thickness loss (lm), tis the exposure time (month), n is the period of sampling (when n = 1,means the sample is exposed for 3 months, in turn n = 2, 3, 4, 5, 6,means the sample is exposed for 6, 9, 12, 18, 24 months). Fig. 1 showsthe variation of corrosion rate of weathering steel in both test sites. Inmarine environment, the trend of corrosion rate exhibits similarbehavior regardless of exposure site. Corrosion rate gradually in-creased during the initial periods of exposure till about 6 months ofexposure and then slowly diminished as exposure time prolonged.The obvious difference was that weathering steel corroded at a rela-tive high rate (500–600 lm/year) in SITE 1, while the corrosion ratein SITE 2 was approximately 50 lm/year. This implies that the corro-sion performance of weathering steel was significantly diminished inSITE 1 (close to the water front) compared to SITE 2.

3.2. Composition and structure of rust layer

Rust layer composition was analyzed by IR. Misawa et al. [15]have taken the following peaks as the key absorption bands:890 cm�1 for a-FeOOH, 1020 cm�1 for c-FeOOH, 580 cm�1 forFe3O4 and 3380 cm�1 for amorphous ferric oxyhydroxide. Muller[25] also gave 690 and 450 cm�1 as well as 840 cm�1 for the infraredspectra of b-FeOOH. In SITE 1, the main corrosion products were c-FeOOH, a-FeOOH, ferrihydrite and amorphous oxyhydroxide in ini-tial stage of exposure, however, the b-FeOOH existed in the innerlayer during the periods from the sixth month of exposure. Withthe exposure time prolonged, the rust gradually evolved into adual-layer structure; the outer layer was composed of c-FeOOH,a-FeOOH, Fe3O4 and amorphous oxyhydroxide, while the innerlayer was only composed of b-FeOOH, c-Fe2O3 and amorphous oxy-hydroxide in 12th month of exposure. At the 24th month of expo-sure, the main corrosion products of inner layer were c-FeOOH, a-FeOOH, c-Fe2O3 and amorphous oxyhydroxide. In SITE 2, the rustlayer formed on the surface of weathering steel presented singlelayer structure and the composition of rust was invariable and com-posed of c-FeOOH, a-FeOOH and amorphous oxyhydroxide.

The morphologies of the cross-section of inner layer formed inSITE 1 are shown in Fig. 2. The inner layer is dense and crack freein initial stages of exposure and then changes to a loose and porousstructure in sixth month of exposure, subsequently the thicknessdecreases and it changes to a dense structure again. EDAX semi-quantitative analysis reveals that chloride is enriched in the innerlayer in sixth month and Cr is slightly enriched in inner layer at theend of exposure (24 months), as can be seen in Fig. 3. In SITE 2, therust layer is also dense and crack free during the entire exposureperiod while the thickness gradually increases (Fig. 4); and Cr isslightly enriched in rust layer in a similar manner to SITE 1 accord-ing to EPMA results. The remarkable difference is that rust layerpresents double-layer structure in SITE 1and a single layer struc-ture in SITE 2.

Table 2Environmental conditions of natural exposure sites.

Test site Average temperature (�C) Average relative humidity (%) SO2 (mg 100 cm�2/day) Wetness time (h/year) Sunshine (h/year)

Wanning 24.7 87% 0.06 6723 2123

Fig. 1. Corrosion rate of weathering steel in different sites as a function of theexposure time.

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3.3. Electrochemical measurements

To determine the protective characteristics of the rust layerformed during various exposure periods, polarization and EIS mea-surements were performed on the rusted samples, as in previouslyreported [26–34]. Fig. 5 shows results of polarization measure-ments for weathering steel after the exposure tests. In SITE 2, theanodic current decreases markedly as exposure time extends,meanwhile the cathodic current increases compared to the castsample. Nevertheless, the cathodic current of rusted steel gradually

Fig. 2. The cross-sectional morphologies of inner layers in SITE 1

decreases with extended exposure time. It seems that the exis-tence of rust prohibits the progress of the anodic reaction and atthe same time restrains the cathodic reaction. As to polarizationbehavior of rusted steel in SITE 1, the trend of anodic current issimilar to those of rusted steel in SITE 2, although reversion reversephenomenon appears in sixth month of exposure. The anodic cur-rent suddenly rises and then decreases with subsequent exposure.Compared to cathodic current of cast sample in the aqueous solu-tion, the cathodic current of rusted steel increases exclusively forweathering steel exposed for 12 and 24 months.

Electrochemical impedance spectroscopy (EIS) measurementsof the rusted steel were carried out in 0.1 mol/L Na2SO4 solution.Fig. 6 presents the Nyquist plots of the EIS data. An equivalent cir-cuit is proposed for modeling the impedance data in Fig. 7. Fig. 8shows the variation of RR (resistance of rust) with exposure time.It can be seen that the value of RR in SITE 1 is higher than that ofSITE 2, but the corrosion rate of steel in SITE 2 is lower than thatof SITE 1 according to weigh loss test. It means that the value ofRR conflicts with the weigh test results, that is the RR cannot reflectthe protective property of rust layer in a special environment con-taining high chloride ion. Now, we will focus on discussing thisinconsistency between the variation of RR and corrosion rate interms of alloy element and environment parameter subsequently.

4. Discussion

4.1. Analysis of the weatherability of weathering steel

Weatherability is a measure of the anti-corrosion performanceof weathering steel, which means that protective rust layer formed

: (a) 3 months; (b) 6 months; (c) 12 months; (d) 24 months.

Fig. 3. Elemental analysis for inner rust layer: (a) elements distribution of point Ain Fig. 2a; (b) elements distribution of point B in Fig. 2d.

Fig. 5. The polarization curves of rusted steel exposed at two different sites as afunction of the exposure time: (a) weathering steel exposed in SITE 1; (b)weathering steel exposed in SITE 2.

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on the surface of steel prohibits the progress of the corrosion reac-tion and lowers the corrosion rate. Weathering steel presents per-fect weatherability if the corrosion rate is minimal. In SITE 1, theaverage corrosion rate during every period of exposure is higherthan that in SITE 2, indicating reduced weatherability of weather-ing steel exposed at SITE 1 Weatherability is closely associatedwith corrosion products, alloy elements and exposure environmentas well as pollutants. These factors could act synergistically toinfluence the corrosion process and determine weatherability.

Fig. 4. The cross-sectional morphologies of rust layers in SITE 2: (a) 3 months; (b) 6 months; (c) 12 months; (d) 24 months.

Fig. 6. Nyquist diagrams for weathering steel after various exposure periods inmarine site. (a) SITE 1; (b) SITE 2.

Fig. 8. The variation of resistances of rust layer in two sites.

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4.2. Electrochemical analysis

The electrochemical nature of the corrosion process allows eval-uation of the protective properties of different rust layers via theirdifferent capacities to block the charge transfer as should occurduring the wetting cycles in the atmosphere. The degree of weath-erability can be inferred from Fig. 5a and b. The formation of rustlayers on the surface of the steel suppresses the anodic currentand the reduction of b-FeOOH causes to the increase of cathodiccurrent [35]. Vera et al. [36] investigated the protectiveness ofthe rust layers formed under different conditions in terms of ano-dic and cathodic polarization behavior and suggested that the vari-ations in the anodic limiting current confirm the tendencyobserved from weight loss. In SITE 2, the anodic limiting current

Fig. 7. Equivalent circuit for rusted steel in an electrolyte solution. Routerlayer, Couterlayer: tcapacitance of inner layer. Q is CPE (constant phase element) parameter. Rct, charge tran

decreases as exposure time is prolonged, indicating the high bar-rier effect of rust to the anodic reaction, as can be seen in Fig. 9.It is reasonably presumed that rust thickness and compactnessgradually augment, being consistent with weight loss and SEMobservation. In contrast to the cathodic curve of bare steel in aque-ous solution, the cathodic curves of weathering steel with variousrust layers proves that the increase of cathodic current is attributedto the reduction of the rust. Interestingly, the cathodic current ofrusted weathering steel decreased as the exposure time increased,indicating suppression of the cathodic reaction. It has been re-ported that, in a chloride-free environment, the c-FeOOH phaseacts as the oxidation agent that accelerates corrosion [37]. Further-more, reduction of the c-FeOOH phase other than oxygen was thepredominating effect in the cathodic reaction when thick rust lay-ers formed on the surface of steel [12]. In SITE 2, the corrosionproducts were mainly composed of c-FeOOH and a-FeOOH as wellas amorphous oxyhydroxide, among which a-FeOOH and amor-phous oxyhydroxide are non active in the corrosion process[14,38]. Actually, the cathodic reaction is the reduction of c-FeOOHphase, which is gradually inhibited because of the formation of afine a-FeOOH phase and enrichment of Cr in the rust/steel inter-face [14]. The simultaneous suppressed effect of anodic and catho-dic reaction is conducive for elevation of the open circuit potentialwith prolonged exposure time, as can be seen in Fig. 10. Accordingto the Pourbaix criterion [39] this effect is consistent with therust’s high protectiveness, indicating that weathering steel in SITE2 presents better weatherability.

In SITE 1, the suppression of anodic reaction still exits, but thereverse phenomenon appears in the sixth month of exposure; thatis the anodic limiting current suddenly augments and then de-creases again with increase of exposure time. It has been reported

he resistance and capacitance of outer layer. Rinnerlayer, Cinnerlayer: the resistance andsfer resistance. ZW, Warburg diffusion impedance.

Fig. 9. Anodic limiting current of rusted steel in two sites as a function of theexposure time.

Fig. 10. Open circuit potential of rusted steel in two sites as a function of theexposure time.

Fig. 11. The distribution of alloy element in the rust layer formed in two sites: (a) inner l2.

1730 Y. Ma et al. / Corrosion Science 51 (2009) 1725–1732

that Fe3O4 acts as a good electronic conductor in aqueous solution[40]. Antony [41] reported that c-FeOOH and b-FeOOH were ableto exhibit high reduction reactivity, in the order c-FeOOH < b-FeO-OH. When these two phases coexist, b-FeOOH plays the primaryrole in accelerating the corrosion rate. The appearance of Fe3O4

in the outer layer and b-FeOOH in the inner layer jointly promotethe anodic and cathodic reactions compared to rusted steel ex-posed for 3 months. Subsequently, the anodic limiting currentgradually diminishes because of augmentation of the thicknessand compactness. As to the cathodic process, the reduction of b-FeOOH can cause the increase of cathodic current within12 months of exposure, relative to the cathodic process of the baresteel as also observed in Ref. [35]. As exposure time prolonged, b-FeOOH phase was transformed to other phases and the cathodicreaction became inhibited.

4.3. Influence of Cr on the weatherability

The anodic limiting current can be used to test the protective-ness of rust layers [36]. Comparing the anodic limiting current ofrusted weathering steel between SITE 1 and SITE 2, it is observedthat the variation of anodic limiting current faithfully reflects dis-crepancy of weatherability of weathering steel exposed for 6 and12 months. Samples exposed for 3 and 24 months showed contrarybehavior. This paradoxical behavior is ascribed to the effect of Crenriched in the rust/steel interface. It is widely recognized thatCr is a powerful element to promote the anti-corrosion perfor-mance of weathering steel [14–15,42]. Yamashita et al. [14] re-ported that protective rust layer formed on the weathering steelneeded long-term field exposure. Cr enrichment in the inner rustlayer leads to thinning of corrosion product particles and promo-tion of compactness. In Fig. 11, EPMA results show that only Cr isslightly enriched in rust layer formed at 24 months’ exposure, be-fore this exposure time Cr presents little effect to the anti-corro-sion performance [14]. In SITE 1, the reduction of iron rust isinhibited because of the existence of Cr [43]; meanwhile, theenrichment of Cr in the rust promotes compactness and prohibitsthe anodic reaction. The two effects should impart good corrosionperformance on weathering steel in SITE 1. Unfortunately however,weathering steel suffered serious corrosion in SITE 1 according to

ayer exposure for 24 months in SITE 1; (b) rust layer exposure for 24 months in SITE

Fig. 12. The amount of chloride ion deposition during the period of retrievingrusted steel.

Y. Ma et al. / Corrosion Science 51 (2009) 1725–1732 1731

Fig. 1. On the other hand, the enrichment of Cr in the rust layer andthe influence on the anodic and cathodic reaction actually makesweathering steel in SITE 2 exhibit better weatherability accordingto Fig. 1. It is reasonably assumed that the ability of Cr to enhancethe weatherability requires suitable exposure environment,whereas aggressive environments such as the marine site withhigh chloride ion can whittle the positive effect of Cr.

4.4. Analysis of environment parameter

A corrosive environment is defined as a wet–dry cyclic environ-ment, among which marine environment represents high corro-siveness with the existence of chloride ion. As an exposure fieldfor atmospheric corrosion, the climate of Wanning district is trop-ical marine with high relative humidity (average 80%), where theaverage temperature is 30 �C and rainfall is extremely abundantwith annual average ranging between 500 and 800 mm. The atmo-spheric corrosion is a wet–dry cycle process with the occurrence ofelectrochemical reaction [44,45] and is influenced by TOW (time ofwet) to some extent [46]. High relative humidity and abundantrainfall as well as long-term sunlight both cause the obviouswet–dry cycle on the surface of steel; besides, the thicker outerrust layer formed during the period of exposure in SITE 1 can pro-hibit the penetration of water [15]. Once wet cycle in process, itwould spend long time resuming dry cycle, indicating the innerrust layer chronically situated the wet condition. It is implied thatthe period of dry cycle diminished and wet cycle prolonged, whichprovided location for incessant electrochemical reaction in the in-ner rust–steel interface, inducing to poor weatherability in SITE 1.

Fig. 12 shows the variation of chloride ion deposition againstexposure time at different sites. High amount of chloride ion depo-sition in SITE 1 promotes the formation of double rust layer, com-posed of loose outer layer and relatively dense inner layer. Chlorideis abundantly enriched in the inner layer, which promotes the for-mation of b-FeOOH [47]. The iron rust formed in SITE 2 with lowamount of chloride ion deposition presents single layer structurewithout the existence of b-FeOOH and chloride is scarcely detectedin the rust layer. It is reported that the formation of b-FeOOH ismain reason for accelerated the corrosion of steel exposed at mar-ine environment [24]. The formation of b-FeOOH in SITE 1 withhigh amount of chloride ion deposition causes the corrosion ofweathering steel to proceed at a relative high rate and weakensthe weatherability, even though Cr is slightly enriched in rust/steelinterface. However, the enrichment of Cr in rust layer lowers thecorrosion rate of weathering steel in SITE 2 and improves weather-ability. It is reasonably inferred that chloride ion and Cr together

influence weatherability; when the amount of chloride ion deposi-tion reaches some level conducive to the formation of b-FeOOH,the effect of Cr is impaired and chloride ion plays a significant roleon weatherability. Nevertheless, the effect of Cr in thinning of therust phase and increasing compactness begins to function, whenthe amount chloride ion deposition can’t promote the formationof b-FeOOH.

5. Conclusions

The weatherability of weathering steel at two different expo-sure sites is discussed and the synergistic effect of environmentand alloy element are clarified. The following conclusions can bemade:

(1) According to weight loss test, weathering steel at two differ-ent sites exhibits various weatherability, which in SITE 1 itpresents poor weatherability and in SITE 2 it possesses bet-ter weatherability. However, the electrochemical measure-ments show the reverse results to weatherability, whichare inconsistent with weight loss test.

(2) The formation of b-FeOOH in SITE 1 with high chloride iondeposition is the main reason for deterioration of weather-ability, where b-FeOOH phase promotes the formation of athick outer rust layer thus enhancing the cathodic current.The existence of outer layer makes the time of wet (TOW)longer in the rust/steel interface, which provides suitablelocation for electrochemical reactions, thereby inducingincessant corrosion and poor weatherability in SITE 1.

(3) EPMA results show the enrichment of Cr in rust layer onlyafter 24 months of exposure in the two sites, but its effecton weatherability is non uniform in the two different sites.The enrichment of Cr scarcely improves weatherability inSITE 1 compared with SITE 2, where the enrichment of Crin rust layer improves weatherability.

(4) The combined effect of chloride ion and Cr influencesweatherability of weathering steel. In the condition of highchloride ion deposition, chloride ion play significant rolerather than the enrichment of Cr in rust layer on weather-ability; conversely to the low chloride ion deposition, theenrichment of Cr remarkably improves weatherability.

Acknowledgements

The investigation is supported by the National Natural ScienceFund of China under the Contract Nos. 50499331-6 and50671113. The authors are also grateful to Dr. E.E. Oguzie for themodification of English.

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