Structural factor in Zn alloy electrodeposit corrosion

Structural factor in Zn alloy electrodeposit corrosion

Applied Surface Science 153 Ž1999. 53–64 www.elsevier.nlrlocaterapsusc Structural factor in Zn alloy electrodeposit corrosion R. Ramanauskas ) Inst...

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Applied Surface Science 153 Ž1999. 53–64 www.elsevier.nlrlocaterapsusc

Structural factor in Zn alloy electrodeposit corrosion R. Ramanauskas

)

Institute of Chemistry, Vilnius 2600, Lithuania Department of Applied Physics, CINVESTAV IPN, 97310 Merida, Mexico Received 15 April 1999; accepted 1 August 1999

Abstract AFM, XRD, XPS analyses were performed to characterize Zn, Zn–Co, Zn–Fe and Zn–Ni electrodeposit topography, texture, lattice imperfections, as well superficial corrosion product film composition and properties. The difference in the coatings’ corrosion resistance manifested when a passivating oxide film formed on the metal surface. Higher values of diffraction line integral breadth for Zn–Co and Zn–Ni alloys were detected, which implied a greater number of lattice imperfections for the samples. Besides, a higher surface activity for oxide film formation for the same surfaces was observed. It was suggested that the metal structural parameters Žlattice imperfection and texture. were responsible for the oxide films’ properties, which determined the coatings’ corrosion resistance. An amorphous structure and a higher amount of hydrated Zn oxide in the corrosion product film reduced Zn–Co and Zn–Ni corrosion rates when compared to that of Zn and Zn–Fe. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Zn electrodeposits; Structural parameters; Corrosion; Oxide films

1. Introduction The focus of electrochemical surface science is the correlation of electrode surface structure with electrochemical reactivity. However, insufficient attention to this problem has been centred in the area of metal corrosion. Metal dissolution is known to occur mainly at surface active sites where atoms are weakly bonded to the crystal surface. The number of such active sites and hence, their relative importance in relation to the corrosion reaction rate, is dependent upon the surface crystallography Žnanometer level. and surface topography Ž10–100 nm level. w1,2x. Crystal structural features can affect metal corrosion )

Corresponding author. Tel.: q370-2-610067; fax: q370-2617-018; e-mail: [email protected]

behaviour in two quite distinct ways w3x. The first one may be considered to happen when the corrosion process occurs under conditions of active Žfilm-free. dissolution and the rate-controlling step takes place at the metal surface. The next one is related to the presence of a thin passive, or thick protective film on the surface. The crystal structure is expected to affect thickness, composition, porosity, adhesion, strength and solubility of these surface films, and hence, the corrosion behaviour of the filmed metal surface. Electrodeposited Zn alloys with Fe group metals significantly extend the steel corrosion protection period with respect to conventional Zn coatings. Certain differences between Zn and Zn alloy electrodeposit corrosion behaviour in natural conditions and in some aqueous solutions were reported in our recent studies w4–6x. In spite of various investiga-

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 3 3 4 - 7

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R. Ramanauskasr Applied Surface Science 153 (1999) 53–64

tions in the field of Zn alloy corrosion w7–12x, the nature of the inhibition phenomenon has not been fully understood. In order to establish the chemical compositionr structure–reactivity relationship for metals in aggressive environments it is essential to characterize the surface from the chemical and structural points of view. Thus, surface topography, texture, crystal lattice imperfections, superficial film composition and properties are required to be both examined and evaluated. In general, the use of a number of techniques is necessary to obtain a complete understanding of the corrosion process. X-ray diffraction ŽXRD. with grazing incidence geometry enables information to be obtained, on the coatings’ texture and lattice imperfection both for the bulk phase and from the superficial layer w4x. As atomic force microscopy ŽAFM. can image the insulating layers, it is therefore appropriate for corrosion studies w13–16x. X-ray photoelectronspectroscopy ŽXPS. is suitable for superficial layer composition determination. All these mentioned techniques were applied in this work. The present investigation was aimed to find evidence that there is a fundamental link between electrodeposited Zn and Zn alloy compositionrstructure parameters, their surface activity and their corrosion behaviour in certain environments.

2. Experimental Zn, Zn–Co Ž0.6%., Zn–Fe Ž0.4%. and Zn–Ni Ž12%. coatings of 10 mm thickness were electrodeposited on low-carbon steel samples which had been previously polished mechanically to a bright mirror finish using 0.3 mm alumina powder. Alkaline cyanide-free plating solutions contained ZnO Ž10 gly1 ., NaOH Ž100 gly1 ., organic additives and the ions of alloying elements. The plating bath detail compositions and operating conditions have been previously given w4–6x. All coating deposition technologies have industrial applications. AFM studies were carried out with an AFM AutoProbe CP ŽPark Scientific Instruments. at atmospheric pressure and room temperature in contact mode. Images were obtained with the same microlever; thus the obtained root-mean-squared rough-

ness Ž R rms . parameters can be compared for different samples. Surface analysis was performed in a combined AESrXPS system ŽPerkin-Elmer 560., including a double-pass cylindrical mirror analyzer and a differentially pumped raster ion gun, with the base pressure ; 5 = 10y9 Torr. In XPS the X-ray source was operated using Al K a Ž1486.6 eV. radiation. Binding energy calibration was based on C 1s at 284.6 eV. Surface cleaning during measurements was carried out by Arq sputtering with the beam energy of 4 keV and a beam current of 0.36 mA cmy2 . An X-ray diffractometer with a grazing incident geometry ŽSiemens, model D 5000., Cu monochro˚ Ž35 mA and 40 kV. matic radiation l s 1.5406 A with a 0.018 step and 4.8 s per step counter time was used to determine the coatings’ texture and to identify the composition of the corrosion product films. The X-ray incidence angle was 58 in composition studies and it was chosen until the highest intensity of the diffraction maxima of the corrosion products was obtained and when the peaks corresponding to the substrate ŽFe. appeared. For the coating texture studies a 108 X-ray beam inclination was applied. The quantitative method for determining the degree of crystallographic texture w4,17x was used. All electrochemical measurements were made using a standard three-electrode system with a Pt counter electrode, a saturated calomel reference electrode and a GAMRY CMSr100 corrosion measuring system. Atmospheric field tests were carried out under the conditions of the humid tropical climate of Yucatan, Mexico. The samples were exposed in marine and urban test sites. More detailed test-site characterization and monitoring procedures are given elsewhere w6x. 3. Results 3.1. Zn electrodeposit characterisation 3.1.1. AFM studies AFM images of investigated coatings are presented in Fig. 1a–d. After electrodeposition Zn and low alloyed Zn coating surfaces appeared finely grained with pyramidal-shaped crystals. For Zn and Zn–Co samples ŽFig. 1a–b. the crystal diameter

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Fig. 1. AFM images of Zn and Zn alloy electrodeposits: Ža. Zn, Žb. Zn–Co, Žc. Zn–Fe, Žd. Zn–Ni.

ranged between 25 and 75 nm, whilst for Zn–Fe ŽFig. 1c. it ranged from 50 to 100 nm. Furthermore, all these coatings were bright, compared to Zn–Ni, which were dull in appearance. The latter samples possessed a nodular fine-grained morphology ŽFig. 1d., with the grain size of the order 1–2 mm. The terrace-stepped structure was observed for Zn–Ni sample. In addition, its surface was rougher. The root-mean-squared roughness Ž R rms . of the latter sample was 78 nm, while for the other investigated coatings this parameter varied between 7 and 16 nm. 3.1.2. XRD studies The coatings’ texture was determined from XRD studies. XRD patterns of Zn and Zn alloy samples are shown in Fig. 2. Pure Zn and low alloyed coatings with Co and Fe electrocrystallize with a distorted form of hexagonal close packing and furthermore, investigated ones possessed a similar texture. According to the calculated relative texture coefficient ŽRTC. values ŽTable 1. from 82% to 85%

of these sample crystallites were oriented parallel to the Ž201., Ž202. and Ž203. planes. The main difference in their texture was that Zn–Co coatings possessed a higher amount of crystallites with a lower

Fig. 2. XRD patterns of Zn and Zn alloy electrodeposits.

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Table 1 Relative texture coefficient ŽRTC. and integral breadth Ž b . of diffraction lines of Zn and low alloyed Zn coatings Žhexagonal close packing. Plane Ž hkl .

Ž100. Ž110. Ž200. Ž201. Ž202. Ž203.

2Q

39.06 70.05 83.76 86.57 94.82 109.16

Zn

Zn–Co

Zn–Fe

RTC Ž%.

b Ždeg.

RTC Ž%.

b Ždeg.

RTC Ž%.

b Ždeg.

2.2 3.9 17 17.9 24.1 26.4

0.34 0.79 0.39 0.70 0.70 1.05

1.6 13 28.2 12.9 23.9 16.7

0.37 0.89 0.49 0.76 0.88 1.25

2.8 5.7 7.4 18.2 27.4 29.4

0.37 0.80 0.41 0.75 0.79 1.10

index Ž110. orientation Ž13%., meanwhile for Zn and Zn–Fe it varied only between 3% and 5%. Zn–Ni coatings consisted of a g-Zn 21 Ni 5 phase, which had a body-centred cubic crystal system. Approximately 43% of these sample crystallites were orientated parallel to Ž110. and 18% of them parallel to Ž100. planes ŽTable 2.. The integral breadth of diffraction lines was measured ŽTables 1 and 2.. The introduction of alloying metal in the Zn matrix increased the integral breadth of all diffraction lines for low alloyed Zn–Co and Zn–Fe samples. Furthermore, from the previously mentioned coatings, Zn–Co alloy had the highest values of this parameter. As Zn–Ni possess a different crystal lattice symmetry it is inappropriate to compare directly this parameter for all investigated coatings; nevertheless, it should be mentioned that for Zn–Ni samples the measured values for integral breadth were even higher when compared to Zn–Co coatings.

shows the complete spectra of Zn and Zn alloy coatings. Zn 2p 3r2 , O 1s, C 1s, N 1s, Co 2p 3r2 , Fe 2p 3r2 and Ni 2p 3r2 peaks were analysed. The calculated atomic concentrations are listed in Table 3. Oxygen and carbon appeared as impurity elements for all investigated Zn electrodeposits. The carbon represented only inclusions from organic compounds and had been detected in slightly higher amounts Ž0.1% and 0.2 at.%. for Zn and Zn–Co samples; oxygen might also arise from the former, as well as from the oxide phase inclusions. The lowest amount of oxygen Ž0.6 at.%. was detected for Zn–Ni coatings. Nitrogen was found as well for the samples with the highest carbon content, supporting the idea that these organic compounds form a part of the nonmetallic inclusions. However, the amount was not high and reached approximately 2% for low alloyed deposits, while Zn–Ni contained only ; 0.7% of nonmetallic phase.

3.1.3. XPS studies XPS analysis was carried out to determine the composition of investigated electrodeposits. Fig. 3

Table 2 Relative texture coefficient ŽRTC. and integral breadth Ž b . of diffraction lines of Zn–Ni coatings Žbody-centred cubic packing. Plane Ž hkl .

2Q

RTC Ž%.

b Ždeg.

Ž330. Ž600. Ž444. Ž552. Ž741. Ž660.

42.85 62.26 73.26 78.69 89.36 94.26

25.4 18.6 10.3 16.5 10.8 14.4

0.91 1.27 1.99 0.73 1.10 1.07 Fig. 3. XPS spectra of Zn and Zn alloy electrodeposits.

R. Ramanauskasr Applied Surface Science 153 (1999) 53–64 Table 3 Composition Žat.%. of Zn and Zn alloy coatings ŽXPS data. Sample Zn 2p 3r 2 O 1s C 1s N 1s Co 2p 3r2 Fe 2p 3r2 Ni 2p 3r2 Zn Zn–Co Zn–Fe Zn–Ni

98.0 97.3 97.8 88.4

1.8 1.7 1.7 0.6

0.1 0.2 0.05 0.07

0.05 0.2 – –

– 0.6 – –

– – 0.4 –

– – – 10.9

In addition, the presence of Na was also detected in the investigated coatings by SIMS and direct current plasma emission spectroscopy w18x, however, its concentration varied from 0.05 to 0.12 wt.%. 3.2. Corrosion behaÕiour 3.2.1. Aqueous solutions The active Žfilm-free. metal dissolution takes place in 1 M ŽNH 4 . 2 SO4 solution w19x. Meanwhile, Zn corrosion in near neutral, uncomplexing solutions occurs with oxiderhydroxide film formation. There is evidence w20x that in unbuffered Cly media this film is somewhat porous, and therefore, not of passiy soluvating type, while in HCOy 3 containing Cl tion, the oxide film is supposed to be more compact, adherent and less soluble, thus exhibiting a passivating character w21x. Thus, anodic polarization curves were obtained for Zn and Zn alloy specimens in aerated 1 M ŽNH 4 . 2 SO4 ŽpH 6.0., 0.9 M NaCl, 0.6 M NaCl q 0.2 M NaHCO 3 ŽpH 6.8. and 0.1 M NaOH ŽpH 13.5. solutions. A significant difference was not found in the shape of anodic curves for various investigated coatings, thus polarization curves for Zn sample only are presented in Fig. 4. The corrosion current densities i corr were determined from Tafel plot extrapolation. They were obtained from a computer fit to the data and are listed in Table 4. The highest i corr values wŽ5.5–5.8. = 10y5 A cmy2 x were detected in ŽNH 4 . 2 SO4 solution and in addition, they were very similar for different coatings. Meanwhile, the lowest wŽ4.5–11.2. = 10y6 y A cmy2 x were obtained in HCOy 3 containing Cl solution. The most significant differences between i corr for various investigated coatings were observed under the latter conditions. The active to passive transition for all investigated Zn coating systems occurs in the y1.4 to y1.3 V

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ŽSCE. potential range in a NaOH solution. In addition, Zn alloys and especially Zn–Ni and Zn–Co, possessed lower passive current densities Ž i pass . compared to those of pure Zn, and Zn–Fe ŽTable 4.. 3.2.2. Atmospheric conditions The variations of Zn coatings’ atmospheric corrosion in marine and urban test sites are shown in Fig. 5. The samples’ weight losses after corrosion product removal are plotted as a function of exposure time. It is evident that electrodeposit corrosion behaviour depends on the type of atmosphere. In the marine test site, which possessed a higher Cly ion concentration, corrosion rates of all investigated coating systems are higher than in the urban atmosphere. It can be observed that coatings with respect to their resistance in both test-site conditions can be divided into two groups. The lower corrosion rates are shown

Fig. 4. Anodic polarization curves of Zn electrode in: Ž1. 1 M ŽNH 4 . 2 SO4 , Ž2. 0.9 M NaCl, Ž3. 0.6 M NaClq0.2 M NaHCO 3 solutions, potential sweep rate 0.1 mV sy1 Ža., and 0.1 M NaOH solution, potential sweep rate 1 mV sy1 Žb..

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Table 4 Electrochemical parameters for Zn and Zn alloy corrosion in naturally aerated aqueous solutions Solution pH

1 M ŽNH 4 . 2 SO4

0.9 M NaCl

0.6 M NaCl q 0.2 M NaHCO 3

0.1 M NaOH

Parameter

Ecorr ŽV. ŽSCE. ba ŽmV decy1 . i corr ŽA cmy2 . Ecorr ŽV. ŽSCE. ba ŽmV decy1 . i corr ŽA cmy2 . Ecorr ŽV. ŽSCE. ba ŽmV decy1 . i corr ŽA cmy2 . i pass ŽA cmy2 .

Coating Zn

Zn–Co

Zn–Fe

Zn–Ni

y1.137 36.5 5.5 = 10y5 y1.051 20.3 1.2 = 10y5 y1.116 39.2 1.0 = 10y5 1.3 = 10y4

y1.116 37.5 5.5 = 10y5 y1.024 18.6 8.7 = 10y6 y1.099 40.1 5.5 = 10y6 8.0 = 10y5

y1.109 38.9 5.8 = 10y5 y1.045 25.3 2.6 = 10y5 y1.102 39.5 1.9 = 10y5 1.1 = 10y4

y1.157 56.6 5.6 = 10y5 y1.018 20.3 8.5 = 10y6 y1.006 54.5 4.5 = 10y6 5.1 = 10y5

by Zn–Ni and Zn–Co electrodeposits, while Zn and Zn–Fe have higher ones. The specimen exposure time was limited up to the first sign of base metal ŽFe. rust appearance. This type of damage, which

corresponded approximately to 30 g my2 of specimen weight loss, was observed in marine atmosphere on Zn and Zn–Fe coatings after 15 months, while for Zn–Co more than 24 months and Zn–Ni after 48 months of exposure. 3.3. Corrosion products

Fig. 5. Zn and Zn alloy electrodeposit atmospheric corrosion Žweight loss. at different test-site conditions.

3.3.1. Alkaline solution Only adherent and nonporous corrosion product films were investigated. During the anodic polarization of Zn in strongly alkaline solutions, oxiderhydroxide film forms on the metal surface w22x. The amount of the potentiostatically formed passive layer, which is related to film thickness, was determined from the coulometric measurements in NaCl solution. The obtained cathodic polarization curves are presented in Fig. 6. Oxide phase reduction peak was integrated after the base line was created, and the charge Ž Qc . consumed for this process was evaluated. After 2 h of oxidation thinner layers were achieved on Zn–Ni and Zn–Co surfaces Ž450 to 600 ˚ ., while on Zn and Zn–Fe coatings its thickness A ˚ ranged between 900 and 1300 A. Oxide film composition was determined from XPS measurements and it appeared that this parameter also depended on the coatings type. Zn 2p 3r2 , O 1s and C 1s core level spectra were investigated. O 1s peaks of oxidized Zn sample XPS spectra ŽFig. 7. had a shoulder at the higher values of the binding energy. The deconvoluted O 1s peak showed that the resolved peaks were located at 530 eV ŽO1. and at

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Table 5 Composition of the oxide layers Žat.%. on Zn and Zn alloy surfaces Žanod.U , formed by anodic oxidation in 0.1 M NaOH solution; atmosph.UU , formed during atmospheric corrosion. Sample

Layer character

O1

O2

Zn 2p 3r 2

C 1s

Zn

anod.U atmosph.UU anod. atmosph. anod. atmosph. anod. atmosph.

27.8 26.2 26.0 9.7 29.3 17.1 21.1 8.3

8.7 6.2 10.6 30.6 8.4 9.2 17.3 33.4

45.2 55.5 48.9 46.9 45.6 60.5 47.0 46.9

18.3 12.1 14.2 12.8 14.9 13.2 13.7 11.4

Zn–Co Zn–Fe Zn–Ni

Fig. 6. Cathodic reduction curves of anodically formed Ž2 h, 0.1 M NaOH solution, Ea 0.4 V, SCE. oxide films on Zn and Zn alloy surfaces in 0.9 M NaCl solution, potential scan rate 2 mV sy1 . Ž1. Zn, Ž2. Zn–Co, Ž3. Zn–Fe, Ž4. Zn–Ni.

532 eV ŽO2., respectively. The lower energy peak ŽO1. has been reported to correspond to O–Zn bonding, while the higher energy peak ŽO2. might be assigned to OH andror H 2 O species w23,24x. The latter fact indicates the possible presence of a hydrated oxide. The calculated oxide film composition is listed in Table 5. O1 values were detected to be higher than O2 for all coatings. However, it should be assumed that oxide films on more resistant coatings possessed a higher amount of hydrated oxide. If

Fig. 7. O 1s spectra with the two resolved O bonding components for anodically formed oxide films on Zn and Zn alloy surfaces.

O2 values for Zn and Zn–Fe samples varied between 8 and 9 at.%, for Zn–Co it was close to 11%; meanwhile for Zn–Ni it exceeded 17%. Impedance measurements with the same oxide electrodes w5x indicated a nonstoichiometric and highly disordered character of these passive films. A poor crystallinity of investigated oxide layers especially on Zn–Ni and Zn–Co surfaces might be considered to occur as well, as it was impossible to detect oxide phase by the XRD technique, even when low incidence angles were applied w25x. 3.3.2. Atmospheric conditions Corrosion products after different sample exposure periods at marine and urban test sites have been characterized by XRD and XPS techniques. The diffractograms of coatings exposed for a 6-month period at a marine station are presented in Fig. 8. A semiquantitative analysis was carried out, and the integrated intensities of the diffraction peaks were calculated. The obtained data are listed in Table 6. These values represent the amount of each phase which are present on the sample surface. The long-term corrosion product film consists of two layers: an inner oxide layer and an outer layer of basic salts w26x. Sodium chlorhydrysulfate ŽNaZn 4 ClŽOH. 6 SO4 P 6H 2 O. and zinc hydroxychloride ŽZn 5 ŽOH. 8 Cl 2 P H 2 O. were determined to be the main constituent compounds of the outer corrosion film ŽXRD data., formed at both test sites on all of the investigated coatings. The integrated intensities of the diffraction peaks of detected compounds and of the base metal ŽZn., indicated that the corrosion product films on the coatings with higher corrosion

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Fig. 8. XRD patterns of Zn and Zn alloy electrodeposits after 6 months of exposure at marine test site conditions.

resistance ŽZn–Ni and Zn–Co. contained a lower number of compounds and were also thinner. However, some significant differences in the composition of the outer layer of corrosion product film, which might have caused variation in their corrosion rates, were not detected. Many Zn corrosion products formed under atmospheric conditions were found to be crystalline. Nevertheless, for initial exposure periods, when zinc oxide and carbonate were considered to be the main Zn surface reactions with the environment products, this was not always the case. The presence of ZnO in the corrosion films which were formed under atmo-

Fig. 9. XPS spectra of Zn and Zn alloy electrodeposits after 7 days exposure at marine test site conditions and 0.5 h surface sputtering.

spheric conditions was detected by the appearance of a wide peak at 36.2 2 u ŽFig. 8., only for Zn and Zn–Fe samples. However, it can be assumed that all the samples contained this compound in the corrosion films, but the amorphous nature made it difficult to detect. XPS analysis was carried out to determine the corrosion film composition and to obtain information about the amorphous oxide phase. Therefore, the coatings at initial stages of corrosion were examined. XPS spectra of samples after 7 days’ exposure at the marine test site are presented in Fig. 9.

Table 6 Integrated intensities of diffraction maxima of the principal compounds, detected on Zn and Zn alloy samples after exposure at marine test site conditions Phasea

Zn 7 days exposure

N A M S T H ZnO Zn gZn 21 Ni 5 a

23.7 16.8 12.3 89.3 – 9.3 12.8 1241.7 –

Zn–Co

Zn–Fe

Zn–Ni

6 months exposure

7 days exposure

6 months exposure

7 days exposure

6 months exposure

7 days exposure

6 months exposure

361.8 13.3 18.8 162.1 – 44.4 66.8 387.8 –

173.9 – – 34.1 – 16.4 – 1336.7 –

28.9 5.7 47.8 13.4 – – – 479.8 –

306.6 – – 87.6 – 31.6 13.8 851.3 –

459.1 25.3 5.1 143.9 – 83.1 41.7 422.4 –

61.9 – – 111.1 – – – – 2459.2

56.3 – – 67.0 34.1 – – – 886.7

N: NaZn 4 ClŽOH. 6 SO4 P 6H 2 0; A: ZnSO4 P 3ZnŽOH. 2 P 5H 2 O; M: Zn 4 Cl 2 ŽOH.4 SO4 P 5H 2 O; S: Zn 5 ŽOH. 8 Cl 2 P H 2 O; T: NiŽSO4 . 0.3 P ŽOH.1.4 ; H: Zn 5 ŽCO 3 . 2 ŽOH. 6 .

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Zn–Co and Zn–Ni coatings contained a large amount of hydrated Zn oxide, as the O2rO1 ratio for the more resistant coatings varied in the range 3–4, whilst it was 0.2–0.5 for Zn and Zn–Fe. 3.4. Surface actiÕity

Fig. 10. O 1s spectra with the two resolved O bonding components for inner oxide layer of corrosion products on Zn and Zn alloy surfaces.

In order to obtain information about the inner layer oxide phase composition, the sample surface was sputtered with Arq ions. Zn 2p 3r2 , O 1s and C 1s core level spectra were investigated, when after surface treatment the Cl 2p 3r2 peak was undetectable and when the C 1s peak became symmetrical, with its maximum at 284.5 eV. The C 1s peak shape has been chosen as one of the criteria to reach oxide inner film, because C 1s peak was detected to be composed of two parts ŽFig. 9. for the outer layer of the corrosion products. The first one at lower values of the binding energy 284.5 eV ŽC1. corresponds to carbon, while that at 290.2 eV ŽC2. may be assigned to carbonate w27x. Another important condition was not to enter the metal phase and in doing so XPS data of coatings unexposed to corrosion damage w18x were taken into account. The O 1s peak resulting from the inner oxide layer Žspectra are presented in Fig. 10. was detected to have a shoulder at higher values of binding energy. In order to obtain detailed information on layer composition, curve fitting procedures were applied. The calculated oxide film composition is listed in Table 5. It could be assumed that atmospherically formed oxide layers on

3.4.1. Aqueous solutions A different surface activity of bare Zn and Zn alloy coatings for some electrochemical reactions were detected. The amount of oxide film which formed on the coating surface during immersion Žcorrosion. in NaCl q NaHCO 3 solution was determined from the cathodic polarization measurements. Q c values were estimated in a similar manner as for anodically formed oxide films. Fig. 11 shows the variations of Qc for different sample immersion times. It is evident that coatings which had lower i corr values ŽZn–Ni and Zn–Co. also possessed thinner corrosion product films. However, it is of great interest that during the initial corrosion period Žapproximately 15 min. Q c values were higher for Zn–Co and Zn–Ni electrodes wŽ0.6–0.7. = 10y5 C cmy2 x, compared to Zn and Zn–Fe ones wŽ0.1–0.14. = 10y5 C cmy2 x. Such performance indicated a different surface activity of bare coating surfaces for oxide film formation; furthermore, Zn–Co and Zn–Ni surfaces were more active.

Fig. 11. Charge Ž Qc . consumed for oxide film reduction on Zn and Zn alloy surfaces after immersion in 0.6 M NaClq0.2 M NaHCO 3 solution.

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Table 7 Zn and Zn alloy coating corrosion film masses Žg my2 . during initial exposure periods in marine test-site conditions Exposure time Žmonths.

Coating Zn Zn–Co

Zn–Fe

Zn–Ni

0.5 1 3

5.9 12.2 18.4

6.3 15.6 19.9

9.1 12 13.8

8.4 11.3 14.8

3.4.2. Atmospheric corrosion Similar results which indicated a different coating surface activity was observed and for outdoor exposed samples. Zn atmospheric corrosion film mass Ždifference between exposed sample mass and its mass without corrosion products., which were approximately related to the corrosion film thickness, are presented in Table 7. The coatings with the higher corrosion resistance, starting from the third month of exposure at marine test-site conditions, were covered with a smaller amount of corrosion products. Corrosion film mass for Zn–Ni and Zn–Co were in the order 13–15 g my2 , whilst for Zn and Zn–Fe it ranged between 18 and 20 g my2 . However, during the initial exposure period Žup to 0.5 month. corrosion product masses were higher for the more resistant coatings, e.g., the values for Zn–Ni and Zn–Co were 8–9 g my2 and ; 6 g my2 for Zn and Zn–Fe.

4. Discussion In a neutral environment, which contains complexing agents, oxide does not form on the Zn surface and the oxygen reduction is diffusion controlled w28x. The influence of metal structural features on its corrosion rate might be expected to be minimal under such conditions. The determined corrosion rates of investigated coatings in ŽNH 4 . 2 SO4 solution confirmed this statement. The obtained i corr values in this environment were the highest and in addition, were very similar for all Zn coatings. The differences between Zn and Zn alloy coating corrosion behaviour appeared only under the conditions when a passivating corrosion product film formed on the sample surface ŽNaCl q NaHCO 3 solution, outdoor exposure.. It is evident that the investigated

coatings, according to their corrosion behaviour under such conditions, can be divided into two groups. Zn–Ni and Zn–Co coatings have lower corrosion rates, whilst Zn and Zn–Fe samples higher ones. A roughness factor describes the initial surface irregularity. Rough surfaces expose more weakly bonded sites and exhibit higher dissolution rates. However, surface topography Žroughness of 10–100 nm scale. was not of principal importance for the Zn coating corrosion process. Zn–Ni samples possessed the highest surface roughness Ž R rms values. of all the investigated coatings, and at the same time were the most resistant ones. Zn–Co and Zn coatings consisted of similar pyramidal-shaped crystals and possessed similar R rms values; however, they exhibited a different corrosion resistance. Nonmetallic inclusions can support specific chemical events that can accelerate the local metal dissolution. For this reason, this factor is of significant importance to the metal corrosion process. From the literature data w29x, Zn hydroxide inclusions can account up to 18% of the total Zn coating mass, thus the detailed composition analysis of investigated coatings has been done. However, it was detected ŽXPS data. that the difference in the amount of nonmetallic inclusions for various Zn coatings was not significant, thus the variances in their corrosion behaviour could not be associated with this factor. The surface crystallography effect on the sample corrosion appeared to be more important. The assumption that Zn polycrystal grains of different crystallographic orientations might corrode at different rates has been proposed after Zn single-crystal corrosion studies w19,21,30x. Metal atom local coordination is related with planar packing densities, meanwhile the activation energy for dissolution was suggested to increase as the packing density increases w19,31,32x. High index planes are more active for dissolution due to the lower binding energy of the surface atoms. The higher presence of crystallites with low-index plane orientations in Zn–Ni and Zn– Co coatings might be one of the reasons for their higher corrosion resistance. Furthermore, it has been shown w33x that the growth of Zn oxide film on Zn surfaces is dependent upon the index of specific plane. The basal plane is observed to form a thin oxide that has been found to be quite protective whereas the other planes form thicker films, which

R. Ramanauskasr Applied Surface Science 153 (1999) 53–64

are less protective. Thus, Zn coating texture could have a dual effect on its corrosion rate. The coatings, which possessed a low-index plane texture, might be more stable due to a higher metal atom coordination and because of the fact that oxide films on such surfaces were more resistant. The corrosion process is essentially a surface phenomenon; thus it might be strongly related to crystalline perfection, e.g., highly stepped metal surfaces, since the presence of dislocations makes the steps indestructible. It is reasonable, therefore, to argue that the lattice distortions must be important in the corrosion process. X-ray diffraction line broadening is recognised to be caused by crystallite size and lattice strains w34–36x. In general, the grain size of Zn electrodeposits lies in the range 0.1–10 mm, for which X-ray diffraction is quite insensitive to its variations w37,38x, so the observed line broadening will be affected mostly by lattice imperfections. The highest values of diffraction line integral breadth among Zn and low alloyed coatings were detected for Zn–Co samples. This permitted the assumption that the latter coatings possessed a more disordered lattice compared to Zn and Zn–Fe. The decrease in the crystalline perfection affects the surface reactivity and usually increases it. Consequently, this factor might be the reason why bare Zn–Co surface was more active for oxide film formation. For Zn–Ni coatings the values of the integral breadth of diffraction lines were even greater than those of Zn–Co; however, this alloy possesses different crystal lattice symmetry, thus it is unsuitable to compare directly this parameter with those of other coatings. Even so, the surface activity for oxide formation during initial corrosion process stages was also higher for the Zn–Ni sample compared with Zn and Zn–Fe. Thus, it should be assumed that Zn–Ni alloy possessed a more disordered lattice and hence its bare surface was more active. A higher surface activity of certain Zn alloys, and hence the metal structure, might be the precursor for oxide film formation, which contained a large amount of Zn hydroxide and exhibited poor crystallinity. The amorphous structure and lower electron conductivity of hydrated Zn oxide compared to Zn oxide w10x made such alloys more stable in corrosion environments where passive films formed on the metal surface.

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The complexity of real systems made it difficult to evaluate individually and correctly the relationship of surface chemical composition and structure to the reactivity in aqueous solutions. The role of alloyed elements in terms of passivity promoters Želements that enhance passivity., or dissolution moderators, or blockers Želements that slow down the metal dissolution rate. is recognized as well w39x. However, the obtained results on Zn coating structure and corrosion behaviour indicated that metal structure parameters such as texture and lattice perfection may be of principal importance while determining electrodeposit corrosion rates.

5. Conclusions The differences in the corrosion behaviour between Zn and Zn alloy coatings manifest under the conditions when passivating corrosion product films form on the metal surface. A higher lattice imperfection of Zn–Co and Zn–Ni electrodeposits makes its bare surfaces more active for oxide layer formation and influences the oxide film properties. An amorphous structure and a higher amount of hydrated Zn oxide in corrosion product films makes Zn–Co and Zn–Ni coatings more resistant compared to Zn and Zn–Fe ones. The higher presence of crystallites with low-index plane orientations may be an additional reason for higher Zn–Co and Zn–Ni corrosion stability.

Acknowledgements The author wishes to acknowledge P. Quintana, I.A. Oliva, and P. Bartolo-Perez for experimental support and valuable discussions and CONACYT, Mexico Žproject No. 2207P-A. for financial support in conducting various phases of this investigation.

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