Growth behavior of lanthanum conversion coating on hot-dip galvanized steel

    Growth behavior of lanthanum conversion coating on hot-dip galvanized steel Shuang-hong Zhang, Gang Kong, Jin-tang Lu, Chun-shan Che, Lingyan Liu PII: DOI: Reference:

S0257-8972(14)00916-5 doi: 10.1016/j.surfcoat.2014.10.017 SCT 19809

To appear in:

Surface & Coatings Technology

Received date: Accepted date:

30 October 2013 7 October 2014

Please cite this article as: Shuang-hong Zhang, Gang Kong, Jin-tang Lu, Chun-shan Che, Ling-yan Liu, Growth behavior of lanthanum conversion coating on hot-dip galvanized steel, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.10.017

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ACCEPTED MANUSCRIPT MANUSCRIPT Journal

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Surface and Coatings Technology SURFCOAT-D-13-02314

Title of Paper:

Growth behavior of lanthanum conversion coating on hot-dip

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Paper Number:

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galvanized steel

Full Mailing Address:

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*Corresponding Author: Chun-shan Che

School of Material Science and Engineering

South China University of Technology

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Wushan Road

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510640-Guangzhou

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China

Telephone: +86-20-85511540 Fax: +86-20-22236119 E-mail: [email protected]

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ACCEPTED MANUSCRIPT Growth behavior of lanthanum conversion coating on hot-dip galvanized steel

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Shuang-hong Zhang, Gang Kong, Jin-tang Lu, Chun-shan Che*, Ling-yan Liu

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School of Material Science and Engineering, South China University of Technology, Wu shan Road, 510640-Guangzhou, China

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Abstract:

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La conversion coatings in various growth stages were obtained by immersing hot-dip galvanized (HDG) sheets in the lanthanum nitrate solution for different times from 10s to 240min. A growth process model of La conversion coating on hot-dip galvanized steel was

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suggested. It is shown that the conversion coating grows more quickly by the zinc grain boundaries, where the cracks occur first and develop gradually on the whole surface of the

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conversion coating. When the immersion time is increased (more than 60 min), some rod-like

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precipitations containing more Zn and less La are deposited on the conversion coating. Potentiodynamic

polarization

and

electrochemical

impedance

spectroscopy

(EIS)

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measurement results reveal that, with increasing immersion time, the protective property of the La conversion coatings is gradually increased in the early growth stage and then decreased when the cracks develop. Keywords: Hot-dip galvanized steel; lanthanum conversion coating; conversion coating growth; corrosion resistance

*Corresponding author. Tel.: +86-20-85511540; fax: +86-20-22236119. E-mail address: [email protected] (Chun-shan Che). 2

ACCEPTED MANUSCRIPT 1 Introduction Chromates were widely used in the surface treatment industry to improve the corrosion

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protection of many metallic substrates such as steel, galvanized steel, aluminum and

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magnesium alloys. However, hexavalent chromium presents very high toxicity and has bad environmental impact [1]. For this reason, an intense research effort is being undertaken to

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replace chromates by more environmentally friendly compounds. Recent efforts have

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focussed on the use of rare-earth salts as adhesion promoters and corrosion inhibitors [2-7]. As examples, cerium salts for corrosion inhibition on Zinc or Zinc coatings have been investigated by a few authors [8-11] The results suggest that cerium salts lead to increased

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corrosion resistance. In addition, yttrium and lanthanum have been shown to increase corrosion resistance [12, 13]. Montemor et al. [14] investigated rare earth coatings deposited

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on galvanized steel surfaces by simple immersion of the substrate in rare earth (cerium,

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yttrium and lanthanum) nitrate solutions, and the results suggest that they lead to increased corrosion resistance when applied to galvanized steel. Compared with that of cerium and

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yttrium, lanthanum seems to be more effective. Concerning the inhibition mechanism, it is well known that the main action mechanism of rare earth is to induce precipitation of oxide/hydroxide of rare earth on the cathodic areas of the metal, due to local alkalinisation during oxygen reduction [8, 10, 14]. Several researches investigated use of rare earth salt for corrosion inhibitor on different metals and alloys and proposed a cathodic mechanism to explain the formation of the rare earth conversion coating [9, 10, 15]. The conversion coating acts as a physical barrier to protect the metal from corrosion. The protective value of such coatings depends on continuity and thickness of coating. According 3

ACCEPTED MANUSCRIPT to [4, 13, 16-18], the cracks always appeared in the conversion coatings with increasing immersion time, although the thickness of rare earth conversion coatings was developed.

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Similar phenomena have been identified on the chromium（Ⅲ）conversion coating [19] and

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molybdate conversion coating [20, 21]; the corrosion resistance of the conversion coating decreases because of the local cracking of the coating. In the last decade, most research

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activities have focussed on the modification of rare earth conversion coating with organic and

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inorganic inhibitors [16, 22-24]. However, the cracks still always appeared in the conversion coatings.

In this work the generation and development of the cracks in La salt conversion coating

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on hot dip galvanized coating with the growth of the conversion coating, and the effect of these factors on the protective property of the La salt conversion coating were investigated.

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2. Experimental

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Hot dip galvanized (HDG) steel sheet samples were obtained by the following process: the Q235 cold rolled steel sheets of 40mm×50mm×0.8mm were successively degreased, pickled,

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fluxed, dried and then dipped into a molten zinc bath (prepared by zinc ingot composed of 99.995% Zn) at 450°C for 1 min, withdrawn slowly and quenched in water immediately. The thickness of the galvanized coating measured by a STH-1 thickness gauge was about 50μm. The chemical conversion treatment solution was aqueous solution comprised of 20g/L La(NO3)3_6H2O and 10mL/L 30% (v/v) H2O2. The HDG samples were immersed in the treatment solution at 70°C for different times from 10s to 240min and then dried in air. Although overlong immersion times are not realistic in practice, more information about the conversion coating growth can be obtained. The surface morphology of the coatings was observed by scanning electron microscopy 4

ACCEPTED MANUSCRIPT (SEM, PHILIPS; Model: XL-30-FEG), atomic force microscope (AFM, JEOL; Model: SPI3800N). The chemical composition was analyzed by energy dispersive spectroscopy (EDS,

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OXFORD, Model: Inca300), X-ray Photoelectron Spectroscopy (XPS, XSAM800, Kratos,

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UK), Auger electron spectroscopy (AES, PHI-550 ESCA/SAM, Perkin-Elmer, USA). Auger depth profile was obtained using an Ar ion beam accelerated at 2kV, with a sputtered area of

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1mm×1mm and a sputtering rate of 10nm/min. The sputtering rate was calibrated by a 100

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nm thick Ta2O5 film.

The electrochemical experiments were performed by using a CHI640B electrochemical measurement workstation. The electrochemical cell consisted of a working electrode with

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1cm2 exposure area of the sample, a saturated calomel electrode (SCE) as the reference electrode and a platinum foil as the counter electrode. The scan rate for potentiodynamic

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linear polarization(LPR) was 1mv/s, and the measuring frequency for electrochemical

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impendence spectra (EIS) ranged from 100kHz down to 0.01 Hz. Measurements were taken after the sample was immersed in 5 wt.% NaCl solution at ambient temperature for 10 min

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and the corrosion potential had been stabilized. The EIS data were evaluated using the Zview(version 2.1C) software. 3 Results 3.1 Microstructure and chemical composition of the La conversion coatings The micromorphologies of the lanthanum salt conversion coatings prepared in the solutions with different immersion times (10s-30min) are shown in Fig.1. The SEM observation shows that the sample treated for 10 seconds does not obviously change compared with the untreated sample; the slightly concave zinc grain boundaries appear obviously on the surface, and some wrinkles can also be observed because of zinc reflux (Fig. 1(a)). After treatment for 1 min, the 5

ACCEPTED MANUSCRIPT coating gradually covered the whole surface, which resulted in less clear grain boundaries. Simultaneously, there was local accumulation in the vicinity of the grain boundaries (Fig.

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1(b)). As the immersion time extends to 10 minutes, the accumulation is gathered into a mass

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near the grain boundary, and the local coating in the vicinity of the accumulation is cracked and the zinc grain boundaries became more misty (Fig. 1(c)). By increasing the immersion

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time to 30 minutes, the local cracks increased, and a few small cracks had already appeared

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on the flat area (Fig. 1(d)).

The AFM observation for topography of the flat area of the lanthanum salt conversion coatings with different treatment time (10s, 1min, 10min) shows that the surface of coatings

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was uneven, similar to a dune shape. With increasing treatment time, the root-mean-square roughness (rms roughness) became higher; the rms roughness of three samples was 16.93,

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86.62 and 147.67nm respectively.

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XPS analyses were carried out on the lanthanum salt conversion coating with 1 min treatment. It shows that the coating is composed of Zn、O、La mainly. The signal for Zn2p

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XPS ionization reveals the presence of Zn2+. The presence of La3+ is confirmed by four peaks (La3d5/2、La3d3/2 and two satellite peaks), and the satellite peak that appears at 836.9eV 和 853.9eV due to the presence of La2O3. The presence of peak for O1s XPS ionization at 531.5eV、529.4eV and 533.4eV was mainly due to the presence of La(OH)3/Zn(OH)2 、 La2O3/ZnO and H2O respectively. Therefore, the lanthanum salt conversion coating might consist of La(OH)3/La2O3 and Zn(OH)2/ZnO. These results are confirmed by a previous report [22]. Fig. 2 shows the AES composition depth profile curve of the lanthanum salt conversion coating with 1 min treatment. The profile shows that the La and O contents are higher and Zn 6

ACCEPTED MANUSCRIPT content is lower on the surface of the coating. With increasing sputtering time, the La and O contents decrease gradually, and Zn content increases gradually along the depth direction of

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the coating. According to the rate and the time of argon ion sputtering, the thickness of the

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film is about 300nm.

As the immersion time extends to 60 minutes, the local cracks on the accumulation

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increased continuously, and the crack had already propagated onto the flat area, and some

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rod-like precipitations were attached to the coating surface, as shown in Fig. 3(a). With increasing treatment time, the attachment was gathered into a mass and the accumulation disappeared(vi). Moreover, a large number of cracks occupied the flat area of the coating (Fig.

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3(b)). Increasing the immersion time to 240 minutes, the rod-like attachment deposited on the whole surface of the coating ( Fig. 3(c)).

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EDS was used to analyze the chemical composition (at. %) of the coatings with different

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treatment time (Fig. 1 & Fig. 3); the results are shown in Table 1. Notably, by increasing treatment time the La content of the flat surface of the coating measured by EDS is increased

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from 0.77 at.% to 7.63 at.%. However, it is significantly lower than that by AES (see Fig. 2). This is because the signals of AES and EDS originate from different thickness beneath the surface; the former is several nm and the latter is about 1μm. As mentioned above, the thickness of the conversion coating with 1 min treatment is about 300 nm. It is confirmed that the EDS signals should be originated from both the conversion coating and a part of Zn coating beneath the conversion coating. Therefore, the higher La content corresponds to the thicker conversion coating; the increase of La content from 0.77 at.% to 7.63 at.% means that the conversion coating has thickened and grown. Similarly, the thickness of the Ce film on Zn coating increases with increasing time of immersion [8]. 7

ACCEPTED MANUSCRIPT For the same sample, La content on the accumulation area is higher than that on the flat one, which indicates that the conversion coating on accumulation area is thicker and grows

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more quickly.

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Moreover, La content of the rod-like attachment is lower than that of the flat area on an identical sample.

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3.2 Electrochemical behavior

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The potentiodynamic polarization curves of the samples treated with different times (10s-180min) and the untreated HDG sample are shown in Fig. 4. Comparing to the untreated HDG sample, the La conversion coating led to inhibit the cathodic reaction of the

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electrochemical corrosion, and the cathodic polarization curves moved towards the direction where corrosion current decreased. The fitting results obtained from potentiodynamic

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polarization curves in Fig. 4 were listed in Table 2. Table 2 shows the calculated values of

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corrosion potential Ecorr，the polarization resistance Rp and corrosion current density icorr. As can be seen, compared with the untreated HDG sample, in the samples with La conversion

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coating the Rp is increased and the icorr is decreased clearly. With increasing immersion time, Rp is increased and icorr is decreased for 10s-10min pretreatment. By contrast, Rp decreased and icorr increased for more than 10 min pretreatment. The La conversion coating with 10 min treatment presents the lowest corrosion current density icorr and the highest protection property. Compared with the untreated HDG sample，the Ecorr of La conversion coating decreases. The decrease of Ecorr may be attributed to the effects of the La coating on the cathodic reaction [16]. The EIS spectra of the untreated HDG sample and the samples treated for different times 8

ACCEPTED MANUSCRIPT (10s-180min) are shown in Fig. 5, the three different equivalent electrical circuits used to fit the EIS spectra are shown in Fig. 6，and the fitting results are shown in Table 3. In the circuits

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Rs is solution resistance, Rct and Cdl represent the charge transfer resistance and the double

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layer capacity of HDG substrate/coating interface respectively, Rf and CPEf represent the coating resistance and constant phase element delegated coating capacity of the conversion

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coatings.

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For the HDG sample and the samples treated for 10s, 1min and 60min, the EIS spectra could be described by equivalent electrical circuit presented in Fig. 6(a). In the case of HDG, the Rf can be ascribed to the formation of the corrosion products consisting of zinc

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hydroxychlorides and zinc oxides on the zinc coating in chloride environment, which suppress the process of charge transfer [25].

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For the samples treated for 10min and 30min, the cracks occurred on local accumulation

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in the vicinity of the grain boundaries and the electrolyte solution penetrated into the cracks of the coating, the coating resistance Rf was decreased and the reaction at the bottom of the

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cracks was accelerated. Simultaneously, the diffusion and the supplement of oxygen through the narrow cracks was difficult, which cause the oxygen concentration difference at the bottom of the cracks, the diffusion field and the Warburg impedance W associated to the oxygen diffusion process appeared. The equivalent electrical circuit and the fitted curve for 30 min treatment were shown in Fig. 6(b). For the sample treated for 60min, the local cracks on the accumulation increased and widened continuously, and more cracks had already developed onto the flat area, it means the more and the wider paths of diffusion, the effect of oxygen diffusion was decreased markedly, the Warburg impedance may be considered to disappear. 9

ACCEPTED MANUSCRIPT For the sample treated for 180min, there was a layer consisting of many rod-like precipitations on the surface as shown in Fig. 3. Therefore, Rf1 and CPEf1 were added in the

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equivalent circuit as shown in Fig. 6(c), the value of Rf1 was far less than that of Rf, it is

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corresponds to the loose, porous layer penetrated by electrolyte solution. It is shown in Table 3 that the total impedance value of the treated samples is higher than

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that of the untreated one, and the total impedance are increased with increasing time until the

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treatment time is up to 10 min. Contrarily, while the treatment time exceeds 10 min, this is decreased with increasing time, and corresponds to the crack development. The sample of 10min-treatment has the best protective performance.

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4 Discussion

The growth mechanism of rare earth conversion coating on the surface of zinc layer is the

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cathode nucleation [5-7, 11]. There are a large number of micro cathodes and micro anodes on

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the surface of zinc coating immersed in weak acidic rare earth salt solution. The following

Anodic reaction ：Zn-2e→Zn2+

（1）

Cathode reaction：2H2O+O2+4e→4OH-

（2）

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electrode reaction may occur:

The result of the cathode reaction is that an alkalescent environment is provided and La(OH)3 and Zn(OH)2 are formed on the coating surface: Zn2++2OH-→Zn(OH)2

（3）

La3++3OH-→La(OH)3

（4）

Due to the solubility conduct of La(OH)3 (2×10-19) is much lower than that of Zn(OH)2 (1×10-17). Therefore, La(OH)3 preferred to precipitate on the zinc coating and La content is higher than Zn content on the top layer of the conversion coating. 10

ACCEPTED MANUSCRIPT There are many defects on the surface of metallic material, including some reactive intermetallics. It was found that the amount of Ce detected on the magnesium-rich

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intermetallics of the surface of the Al alloy treated with Ce salt solution is larger than that on

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the matrix, which indicates that the region on intermetallic is very reactive and constitutes sites where a more intense cerium precipitation can occur [3,26].

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For hot dip galvanized coating, the free zinc layer on the surface is formed by the

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solidification of the molten zinc. There are many lattice defects and impurities near the zinc grain boundary, where the reactivity is higher than that inside the grain and the zinc anode is dissolved more quickly. At the same time, the cathodic process is also accelerated because

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reaction (1) and (2) are a pair of conjugated reaction and the charge balance must be maintained. As a result, the conversion coating near the zinc grain boundary grows more

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quickly.

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The conversion coating nucleates and grows continuously, and quickly covers the surface of the zinc coating. Simultaneously, some accumulations appear in the vicinity of the grain

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boundary, where the conversion coating might be thicker than that on other area. As mentioned previously, there is more La on the accumulation. Besides, the atomic number of La is much greater than that of Zn; the accumulations observed in SEM are especially bright (Fig. 1). When the thickness of the conversion coating increases to a certain extent, the cracks first occur on the accumulations, and then develop onto the flat area inside the grain, and some rod-like precipitations are deposited on the conversion coating surface. Immersed for a long time, many more rod-like precipitations cover the entire conversion coating and crack seriously. A growth process model of La conversion coating on hot-dip galvanized steel is illustrated in Fig.7. 11

ACCEPTED MANUSCRIPT The reason why the rod-like precipitations accumulated on the conversion coating contain less La for long time immersion is unclear. A hypothesis is suggested as follows: in case the

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above reaction (1) and (2) occur simply together with reaction (3), the Zn2+ produced from the

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anode reaction (1) may be entirely compensated by consumed Zn2+ for Zn(OH)2 formation, whereas, in case the reaction (1) and (2) occur together with reaction (3) and (4), the Zn2+

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produced must be partially left behind and accumulated on the interface between solution and

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conversion coating. For the sample immersed for a long time, the result may be that the local concentration of Zn2+ is enhanced markedly and the rod-like precipitations with less La are deposited on the local zone of the conversion coating.

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5 Conclusions

As regards immersed hot-dip galvanized sheets in La salt solution, conversion coating

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composed of oxide/hydroxide of La and oxide/hydroxide of Zn are formed on the zinc coating.

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In the early stage of the conversion coating growth, with increasing treatment time the protective property of the conversion coating is increased. The conversion coating grows

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more quickly in the vicinity of the zinc grain boundary, where the reactivity of zinc is higher than that inside the grain. The local conversion coating in the vicinity of the grain boundary is cracked first, and then cracks develop gradually and spread finally over the entire surface of the conversion coating, which results in decreasing the protective property. By immersing for a long time, loose rod-like precipitation is deposited on the surface of the conversion coating.

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ACCEPTED MANUSCRIPT References [1] C. Desai, K. Jain, D. Madamwar, Bioresour. Technol. 99 (2008) 6059–6069.

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[2] S. Bernal, F.J. Botana, J. J. Calvino, M. Marcos, J. A. Pérez-Omil, H. Vidal, J. Alloys

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Compd. 225 (1995) 638–641.

[3] L. Paussa, F. Andreatta, N.C. Rosero Navarro, A. Durán, L. Fedrizzi, Electrochim. Acta.

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70 (2012) 25–33.

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Technol. 213 (2010) 105–116.

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[7] M. Machkova, E. A. Matter, S. Kozhukharov, V. Kozhukharov, Corros. Sci. 69 (2013)

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396-405.

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[9] K. Aramakik. Corros. Sci. 43 (2001) 2201–2215. [10] M.A. Arenas, J. J.de Damborenea. Surf. Coat. Technol. 187 (2004) 320–325. [11] C. Motte, N. Maury, M. G. Olivier, J. P. Petitjean, J. F. Willem, Surf. Coat. Technol. 200 (2005) 2366–2375. [12] M. Bethencourt, F. J. Botana, J. J. Calvino, M. Marcos, M.A. RodrÍguez-Chacón, Corros. Sci. 40 (1998)1803–1819. [13] M. Tran, D. Mohammedi, C. Fiaud, E.M.M. Sutter, Corros. Sci. 48 (2006) 4257–4273. [14] M. F. Montemor, A. M. Sim es, M. G. S. Ferreira, Prog. Org. Coat. 44 (2002) 111–120. [15] F. Andreatta, M. E. Druart, A. Lanzutti, M. Lekka, D. Cossement, M. G. Olivier, L. 13

ACCEPTED MANUSCRIPT Fedrizzi, Corros. Sci. 65 (2012) 376–386. [16] K. Gang, L.Y. Liu, J. T. Lu, C. S. Che, Z. Zheng, Corros. Sci. 53 (2011) 1621–1626.

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[24] D. C. Chen, J. F. Wu, Y. Q. Liang, S. L. Ye, W. F. Li, Transactions of Nonferrous Metals Society of China 21 (2011) 1905–1910. [25] B. L. Lin, J. T. Lu, G. Kong, Surf. Coat. Technol. 202 (2008) 1836–1841. [26] L. Paussa, F. Andreatta, N. C. Rosero, A. Duran, L. Fedrizzi, Electrochim. Acta. 70 (2012) 25–33.

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ACCEPTED MANUSCRIPT Tables Table 1 Chemical composition of various areas in Fig. 1 and Fig. 3 (EDS, at.%).

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Table 2 Results of potentiodynamic polarization obtained by fitting the polarization curves.

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Table 3 Fitted parameters for HDG and La conversion coating using the equivalent circuits in

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Fig. 6.

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ACCEPTED MANUSCRIPT Table 1 Element

Analyzed area

Zn

O

La

10s

86.60

12.62

0.77

Fig.1(a)

Flat area

1min

72.40

26.96

0.91

Fig.1(b)

Flat area

10min

32.04

63.56

4.41

Fig.1(c)

Flat area

30min

32.63

62.54

4.83

Fig.1(d)

Flat area

60min

35.63

60.02

5.34

Fig.3(a)

Flat area

180min

42.46

53.91

6.63

Fig.3(b)

Flat area

240min

23.15

69.22

7.63

Fig.3(c)

Grain boundary

1min

13.43

72.97

13.60

Fig.1(b)

Grain boundary

10min

14.15

73.40

12.46

Fig.1(c)

30min

11.04

73.18

15.79

Fig.1(d)

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Grain boundary

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Flat area

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Treatment time

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Character

60min

23.89

73.65

2.46

Fig.3(a)

Rod-like particle

180min

32.44

65.77

1.80

Fig.3(b)

Rod-like particle

240min

28.59

67.73

3.69

Fig.3(c)

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Rod-like particle

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Table 2 Ecorr vs. Rp/kΩ·cm2

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Sample

-1061

Sample treated for10s

-1032

Sample treated for 1min

-1029

Sample treated for 30min

9.951

3.314

6.741

-1028

10.569

2.265

-1026

4.290

4.204

-1024

3.643

4.659

3.289

7.223

ED

Sample treated for 60min

-1022

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Sample treated for 180min

13.00

1.482

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Sample treated for 10min

0.583

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HDG

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SCE(mv)

icorr/μA·cm-2

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ACCEPTED MANUSCRIPT

CPEf1

CPEf Y0/×10 / /kΩ·cm

-1

-2

Ω ·cm ·s

-n

n

2

kΩ·cm -1

-2

Ω ·cm ·s

--

--

--

0.493

32.11

10s

--

--

--

0.505

16.89

1min

--

--

--

1.401

10min

--

--

--

0.225

30min

--

--

--

0.121

60min

--

--

--

180min

0.410

1.224

0.650

0.747

2

Cdl/

W-R

mF·cm

W-T

W-P

-2

/kΩ·cm2 mF·cm-2

0.615

2.042

--

--

--

0.753

0.954

4.332

--

--

--

19.72

0.760

1.504

2.042

--

--

--

13.56

0.688

4.108

0.184

1.368

52.72

0.562

12.43

0.596

3.195

0.605

1.754

391.02

0.388

1.420

22.33

0.765

2.980

1.627

--

--

--

0.560

28.34

0.723

1.480

3.682

--

--

--

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HDG

-n

D

/Ω·cm

n

TE

Y0/×10 / 2

Ws

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Sample

Rct/

-5

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Rf

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Rf1 -5

CR I

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Table 3

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ACCEPTED MANUSCRIPT Figure(s) Fig. 1 SEM images of La conversion coatings with (a) 10s, (b) 1min, (c) 10min, (d) 30min

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treatment.

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Fig. 2 AES composition depth profile curves of La conversion coating with 1 min treatment. Fig. 3 SEM images obtained on La conversion coatings with (a) 60min, (b) 180min, (c)

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240min treatment.

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Fig. 4 Potentiodynamic polarization curves for the untreated HDG sample and the samples with La conversion coating treated for different times in 5%NaCl solution. Fig. 5 Nyquist diagrams of the untreated HDG sample and the samples treated for 10s -180

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Fig. 6 Nyquist diagram, equivalent circuit and the fitted curves for the sample treated for (a)

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10s, (b) 30 min and (c) 180 min.

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Fig. 7 Schematic illustration of growth process of La conversion coating with increasing

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(d) Fig.1

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(a)

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Fig.2

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(a)

(c)

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Fig.3

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(b)

Fig.4

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Fig.5

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Fig.6

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Fig.7

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ACCEPTED MANUSCRIPT Journal: Surface and Coatings Technology Manuscript Title: Growth behavior of lanthanum conversion coating on hot-dip galvanized

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steel

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Highlights

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1. A growth process model of the La conversion coating was suggested. 2. The La conversion coating on Zn grain boundary grows more quickly.

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3. EIS spectra for La coatings in various growth stages were investigated.

Sincerely,

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Shuang-hong Zhang

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