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molybdate passivation treatment for hot-dip galvanized steel sheet
Corrosion Science 52 (2010) 3385–3393
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A two-step roll coating phosphate/molybdate passivation treatment for hot-dip galvanized steel sheet Cheng-Yang Tsai a, Jen-Shou Liu a, Pei-Li Chen b, Chao-Sung Lin a,* a b
Department of Materials Science and Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei 106, Taiwan New Materials R&D Department, China Steel Corporation, 1, Chung Kang Road, Hsiao Kang, Kaohsiung 802, Taiwan
a r t i c l e
i n f o
Article history: Received 10 April 2010 Accepted 18 June 2010 Available online 30 June 2010 Keywords: A. Steel A. Zinc B. EIS B. XPS B. Polarization C. Passive films
a b s t r a c t A two-step roll coating passivation treatment employing phosphate followed by molybdate solutions has been performed on hot-dip galvanized (GI) steel sheet. The phosphate coating was primarily porous, amorphous Zn phosphate. A second step coating treatment resulted in more hemispherical Zn phosphate particles and the incorporation of molybdate ions and molybdenum oxide into the existing phosphate coating, giving rise to an improved corrosion resistance. The coating reaction during the second step roll coating treatment and the corrosion protection afforded by the second step molybdate treatment are discussed, with emphases on the comparison with the coating formed via immersion. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Zinc coating has been extensively employed to enhance the corrosion resistance of steel via cathodic protection. To further enhance the corrosion performance of Zn-coated steel, hexavalent chromium-based passivation treatment is generally adopted. However, hexavalent Cr is highly toxic and its use has been strictly restricted according to international regulations such as RoHS and WEEE. Non-chromate passivation treatments have, therefore, received an ever-increasing attention, including phosphate [1–9], cerium-based [10–14], silica-based [15,16], tungstate-based [17], and molybdate-based treatments [18–30]. Phosphating is one of the most widely used passivation treatments and generally conducted in acid aqueous phosphate solution containing nitrate (NO3 ) as accelerator to promote the oxidation and dissolution of Zn, and metal cations to refine the size of Zn phosphate crystals [4]. Finer Zn phosphate crystals facilitate the coverage of phosphate coating, which, in turn, improves the corrosion resistance of the phosphate coating [6–9,31,32]. Nevertheless, the phosphate conversion coating formation via the growth and coalescence of the Zn phosphate crystals inevitably leaves pores, which are detrimental to the corrosion resistance of the coating. Post-treatments to seal the pores of the phosphate coating are, thus, employed to improve the corrosion resistance, such as silicate and molybdate sealing treatments [6,7]. * Corresponding author. Tel.: +886 2 33665240; fax: +886 2 23634562. E-mail address: [email protected] (C.-S. Lin). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.06.020
Like hexavalent Cr ions, molybdate anions are an effective corrosion inhibitor for Zn [33–37]. However, molybdate anions are less toxic than hexavalent Cr ions [22]. Molybdate passivation treatment is, thus, considered as one of the possible alternatives to chromate treatment [18–30] and a review article has been reported recently by Walker and Wilcox [18]. Since introduced in the mid1980s, the passivation of Zn in the molybdate solution was generally conducted cathodically [19]. Later on, a simple immersion technique was developed by Wilcox and Gabe [20] who demonstrated that the solution containing 5–30 g L1 sodium molybdate at pH 5 rendered a molybdate conversion coating on Zn with satisfactory performance. Magalhaes et al. [28] investigated the molybdate conversion coating on 99.9% pure Zn and electrogalvanized steel (EG) via an immersion coating process. They found that the morphology of the molybdate coating was strongly dependent on the acid (nitric, sulfuric, or phosphoric acids) used and the pH of the solution. The presence of phosphate ions resulted in a thin, compact molybdate coating, which had a better corrosion resistance than those formed in the molybdate baths containing nitrate or sulfate ions. Treacy et al. [26] explored the corrosion behavior of molybdate-based conversion coatings exposed to neutral salt fog. The coating displayed a superior corrosion resistance after 21 h of salt fog test. This is because Mo(V) or Mo(VI) species in the coating were reduced to Mo(IV) or Mo(III) species, which were further incorporated in the corrosion product layer acting as a barrier to prevent further corrosion attack. Recently, metal cations, such as Al3+, and additives, such as silicate and silane, have been introduced to the molybdate conversion solution for Zn-coated steels [27,30]. The conversion coating thus formed
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generally exhibited a composite nature and an improved corrosion resistance. Phosphating and molybdate conversion coating treatments for Zn-coated steel using a simple immersion technique are abundant in the literature. In contrast, the passivation treatment of steel sheets using a roll coating process is still scarce in the literature. Roll coating is a process commonly used to coat a flat substrate with a protective coating as the substrate passes through a pair of rollers. This technique is extensively used in the coil coating industry such as the coating process for steel coil [38]. Industrial steel coil roll coating process generally employs a baking procedure to facilitate the formation of the coating. During baking, the concentration of the various ions in the liquid film on the steel coil increases as water is evaporated. This is different from the coating formed by an immersion process, during which a diffusion layer depleted of the reacting species forms on the surface of the growing conversion coating. This study investigated a two-step roll coating passivation treatment on hot-dip galvanized (GI) steel sheet, i.e., phosphating followed by a molybdate treatment. The detailed microstructure and composition of the conversion coating was characterized after each step of treatment. The corrosion resistance was evaluated by potentiodynamic polarization measurement, electrochemical impedance spectroscopy (EIS) and salt spray test. Based on the microstructure and properties of the coating, the microstructural evolution and corrosion resistance mechanism afforded by the phosphate/molybdate passive coating on GI steel sheet are proposed and discussed. 2. Experimental 2.1. Passivation treatment The material used for this study is commercial GI steel sheets. The thickness of the Zn coating is around 10 lm. The plate with a size of 150 100 0.8 mm was degreased in an alkaline solvent, cleaned, and rinsed with deionized water, and finally dried in a stream of hot air. The passivation treatment was conducted by a two-step roll coating process: the first step using a phosphate solution and the second step using a molybdate solution. The phosphate solution contained 2.4 g L1 zinc oxide (ZnO), 30 g L1 sodium nitrate (NaNO3), 22 ml L1 phosphoric acid (H3PO4), and 5 g L1 magnesium nitrate (Mg(NO3)26H2O), and the pH of the solution was adjusted to 2.5 by the addition of sodium hydroxide (NaOH). The molybdate solution consisted of 30 g L1 sodium molybdate (Na2MoO42H2O) and the pH of the solution was adjusted to 4.5 via the addition of H3PO4. The phosphate solution was applied onto the GI steel sheet using a bar coater. The plate was, then, baked in 200 °C air for 12 s and cooled to ambient temperature in room temperature air. Subsequently, the molybdate treatment was likewise conducted on the phosphated sample. A one-step roll coating process using the phosphate solution only was also studied to elucidate the effect of the second step passivation using the molybdate solution. 2.2. Microstructural characterization The surface morphology of the coating was investigated using a JEOL JXA-8200 scanning electron microscopy (SEM) at backscattered electron (BSE) mode. Cross-sectional transmission electron microscope (TEM) specimen was prepared using a combined mechanical and ion-beam thinning technique or a focused ion beam instrument (FEI Nova-200 NanoLab Dual Beam). The TEM specimen was examined using a TEM (FEI Tecnai F20 G2) at 200 kV. The composition of the coating was measured by energy dispersive spectrometry (EDS) in TEM and the crystallinity was characterized by
the selected area electron diffraction (SAED) technique. Finally, X-ray photoelectron spectroscopy (XPS) was performed using a PHI Quantera SXM Scanning X-ray Microscope with an Al Ka monochromated source. All spectra were corrected using the signal of C1s peak at 284.5 eV. 2.3. Corrosion performance evaluation The corrosion resistance of the coating was evaluated using potentiodynamic polarization, EIS, and salt spray test. The polarization curve measurement and EIS were performed via a Model 263A Potentiostat/Galvanostat and FRD100 Frequency Response Detector (EG&G Instruments) and conducted in 5 wt.% sodium chloride (NaCl) solution using a conventional three-electrode cell. The specimen under study with an exposure area of 1 cm2, a platinum plate of 16 cm2 and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrode, respectively. All electrochemical measurements were measured after a steady open circuit potential (OCP) was reached. The polarization curve was measured potentiodynamically by sweeping the potential in the positive direction at a scan rate of 1 mV/s and the sweep range was from an initial potential of 0.2 V to a final potential of 0.2 V (both are relative to OCP). EIS was recorded at OCP with a sinusoidal perturbation of 10 mV amplitude over the frequency range from 100 kHz to 0.01 Hz. The neutral salt spray test was performed according to the ASTM B 117-03 [39] standard using a chamber (Model SST-B, Ten Billion Co., Taiwan) with 5 wt.% NaCl. The specimens were placed perpendicularly with an incline angle of 30° and were continuously sprayed for 24 h in a neutral salt spray test chamber. The corrosion resistance of the sample was evaluated according to the percentage of the corroded area. 3. Results 3.1. Surface morphology of the coating Before passivation treatment, the GI steel sheet displayed a bright contrast under SEM/BSE mode (not shown here). Fig. 1a shows the overall surface morphology of the GI steel sheet after the first step roll coating using the phosphate solution. Several island-like spots showing a dark contrast were visible in addition to the bright-contrast areas that are characteristic of the GI steel sheet before passivation treatment. Herein, these island-like spots were denoted as the phosphate particles. Magnified observations revealed the presence of flower-like phosphate particles (marked as the arrow in Fig. 1c) and round phosphate particles (marked as the double arrows). Fig. 1b shows the overall surface morphology of the GI steel sheet after a complete two-step roll coating treatment. The population density of the phosphate/molybdate particles markedly increased after the second step roll coating using the molybdate solution, in particular the spherical particles. Moreover, the average diameter of the spherical particles slightly increased after the second step treatment, i.e., 2.2 vs. 3.4 lm. Magnified observations identified the coexistence of spherical, flower-like particles, and gray-contrast films (marked as the arrow in Fig. 1d). EDS analyses revealed that both the regions with and without particles contained Zn, O, P, and traces of Mg and Mo species (data not shown). Because less O and more Zn were detected in the region without particles, the coating on the region without particles was apparently thinner than that on the region with particles. 3.2. Cross-sectional TEM characterization Fig. 2 show the cross-sectional TEM characterization of the GI steel sheet after the first step roll coating treatment. A continuous,
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Fig. 1. Surface morphology of the GI steel sheet after (a) the first step roll coating treatment and (b) a complete two-step roll coating treatment. (c and d) are the magnified views of (a and b), respectively.
porous phosphate coating was observed on top of the Zn substrate. Nevertheless, the thickness of the phosphate coating varied from place to place. The coating of approximately 1.5 lm thick (Fig. 2a) corresponded to the flower-like particles shown in Fig. 1c; meanwhile, the coating of 0.2 lm thick (Fig. 2b) was the round particles. Moreover, a thin passive film adjacent to the flower-like particle was observed and intimately contacted the Zn substrate, as shown in Fig. 2c. The thickness of this thin passive film ranged from 10 to 60 nm. Insets in Fig. 2a and b are the SAED patterns of the flower-like and round particles, respectively. Both SAED patterns consisted of diffused halos, indicating that these particles were amorphous. EDS analyses revealed that the phosphate coating contained Zn, O, P, and Mg species (Table 1). The flower-like and round particles contained significant amounts of O, P, and Zn, indicating they were primarily the Zn phosphate. In contrast, the thin passive film was mainly composed of O and Zn, suggesting the presence of Zn oxide/hydroxide. The cross-sectional TEM of the coating prepared by the twostep roll coating treatment is shown in Fig. 3. The spherical particles shown in the SEM micrograph (Fig. 1d) were clearly seen in TEM. When observed cross-sectionally in TEM, these particles displayed a hemispherical shape and a porous structure (Fig. 3a). The average diameter of the hemispherical particles was around 1 lm. In addition to the hemispherical particles, the Zn substrate was completely covered with a continuous, porous passive layer with a thickness ranging from 10 to 250 nm. The relatively-thick passive layer (Fig. 3b) was apparently correlated to the gray-contrast areas observed in SEM (Fig. 1d). Meanwhile, the relatively-thin passive layer was primarily observed around the hemispherical particles,
as shown in Fig. 3c. Moreover, the relatively-thin passive layer was more compact than the hemispherical particle and the relatively-thick passive layer. The SAED patterns inset in Fig. 3a and b revealed that both the hemispherical particles and the relativelythick passive layer had poor crystallinity. Table 2 shows the EDS results of the distinct features shown in Fig. 3. Both the hemispherical particle and the relatively-thick passive layer were composed of Zn, O, P, and traces of Mg and Mo species. As a result, they were primarily the Zn phosphate. The relatively-thin passive layer also contained Zn, O, P, and trace of Mo species. Nevertheless, the relatively-thin passive layer had more Mo and less P than the hemispherical particle and the relatively-thick passive layer. 3.3. XPS analysis Fig. 4a and b shows the XPS depth profile of the GI steel sheet after one-step and two-step roll coating treatments, respectively. Both the coatings were mainly composed of O, Zn, and P species, in which the Zn content increased and the O content decreased along the depth of the coating. In contrast, the content of P, Mg, and Mo displayed a monotonous decrease along the depth of the coating. High-resolution spectrums of Zn, P, and Mo of the coating prepared by the two-step roll coating treatment are shown in Fig. 5a–c, respectively. The spectrum of Zn 2P can be identified as ZnO/Zn(OH)2 and Zn3(PO4)2 [40]. The P 2p spectrum was likely to result from PO4 3 and HPO4 2 [41]. Finally, the Mo 3d spectrum included three peaks, in which two peaks corresponded to molybdate ion, MoO4 2 , and one peak was assigned as Mo dioxide, MoO2 [42–46].
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Fig. 2. Cross-sectional TEM micrographs of the GI steel sheet after the first step roll coating treatment showing the existence of three distinct regions: (a and b) correspond, respectively, to the flower-like particle and round particle observed on the SEM micrographs, and (c) a thin passive film intimately contacting the Zn substrate. Note the Pt layer and C film are deposited prior to the TEM sample preparation using FIB, whereas the M-Bond epoxy is used to sandwich two treated GI steel sheets for the TEM sample preparation using a combined mechanical and ion-beam thinning technique.
Table 1 TEM/EDS results of the distinct regions of the coating prepared by the one-step roll coating using the phosphate solution. Distinct regions
Flower-like particle Round particle Thin passive film
Atomic percentage Zn
O
P
Mg
Totals
16.0 17.5 31.1
67.0 65.0 64.5
14.4 16.1 4.2
2.6 1.4 0.2
100.0 100.0 100.0
3.4. Polarization curves Fig. 6 shows the polarization curves of the various GI samples in 5 wt.% NaCl. Compared with the as-received GI steel sheet, both the GI steel sheets after one-step and two-step roll coating treatments had polarization curves shifting to lower current densities and nobler potentials. The coating formed after the two-step roll coating treatment displayed an apparent passive region and had a nobler corrosion potential (994 mV(SCE) vs. 1017 mV(SCE)) and lower corrosion current density (1.19 lA/cm2 vs. 1.45 lA/cm2) than that formed after the one-step roll coating treatment. This signifies that a subsequent molybdate passivation treatment markedly improves the corrosion resistance of the phosphated GI steel sheet.
3.5. EIS analysis Fig. 7a and b shows the Nyquist and Bode plots of the as-received GI steel sheet, and the GI steel sheets after one-step and two-step roll coating treatments, respectively. The Nyquist plot of GI steel sheet comprised one high frequency (HF) capacitive loop, which is related to the double layer and charge transfer on the Zn substrate, and one low frequency (LF) inductive loop, which corresponds to the dissolution of the Zn substrate [6,7,29,47–50]. In addition to the HF capacitive loop and LF inductive loop, a capacitive loop was observed for the GI steel sheet after the onestep roll coating treatment and is generally related to the diffusion phenomena in the phosphate coating [6,7]. The phosphate coating formed during the first step roll coating treatment apparently enhanced the polarization resistance of the GI steel sheet. However, for the GI steel sheet after the two-step roll coating treatment, the LF inductive loop disappeared and a HF capacitive loop related to the phosphate/molybdate coating followed by a diffusion tail in the LF region was observed, signifying that the corrosion is diffusion-controlled [49,50]. The LF (0.01 Hz) impedance of Bode plots (Fig. 7b) is generally recognized as the polarization resistance of the sample [6,51]. The 0.01-Hz impedance of the as-received GI steel sheet and those
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Fig. 3. Cross-sectional TEM micrographs of the GI steel sheet after a complete two-step roll coating treatment showing (a) hemispherical particle, (b) relatively-thick passive layer, and (c) relatively-thin passive layer.
Table 2 TEM/EDS results of the distinct regions of the coating prepared by a complete twostep roll coating treatment. Distinct regions
Hemispherical particle Thick passive layer Thin passive layer
ing treatment was less than 5%. Consequently, the salt spray test results are in good consistence with the polarization curve and EIS measurements.
Atomic percentage Zn
O
P
Mg
Mo
Totals
4. Discussion
18.0 18.1 28.8
66.4 64.4 61.3
13.6 15.8 7.3
1.6 1.5 0.0
0.4 0.2 2.6
100.0 100.0 100.0
4.1. Formation of the phosphate/molybdate passive coating during roll coating
after one-step and two-step roll coating treatments was 0.296, 1.142, and 5.153 kX cm2, respectively. Consequently, the second step treatment using the molybdate solution markedly improved the corrosion resistance of the phosphate coating on GI steel sheet. 3.6. Salt spray tests The visual aspect of the various GI samples after 24 h of salt spray test is shown in Fig. 8. The as-received GI steel sheet and the GI steel sheet after the one-step roll coating treatment were nearly completely covered with white rusts. In contrast, the white rust area fraction of the GI steel sheet after the two-step roll coat-
The conversion coating formation during immersion is well recognized to proceed with the dissolution of the metal substrate, followed by the hydrogen evolution and the reduction of the reacting ions in the solution. In a simple phosphate solution, the Zn ions dissolved from the Zn substrate can either react with the phosphate ions, which result from the dissociation of the dihydrogen phosphate ions due to pH rise, to form Zn phosphate particles or with hydroxyl anions to form Zn hydroxides [4,32]. In a molybdate solution, in addition to the hydrogen evolution, MoO4 2 can be reduced to MoO2 in accompany with the oxidation of the Zn substrate, as shown:
MoO4 2 þ 4Hþ þ 2e ! MoO2 þ 2H2 O
ð1Þ
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Fig. 4. XPS depth profile of the GI steel sheet after (a) one-step and (b) two-step roll coating treatments.
As a result, the molybdate conversion coating on Zn is mainly composed of Zn oxide/hydroxide, Mo (IV) oxide, and incorporated Mo(VI) species. The formation of the phosphate conversion coating apparently relies on sufficient Zn2+, OH, and PO4 3 concentrations built up on the interface between the Zn substrate and the solution. During immersion, parts of the Zn2+, OH, and PO4 3 ions react to form Zn hydroxide and phosphate, while parts of them diffuse away from the interface. The conversion coating thus formed mainly consists of crystalline Zn phosphate particles [4,6,7]. In contrast, the concentration of Zn2+, OH, and PO4 3 ions in the liquid film applied by a roll coating process increase during baking. The resulting conversion coating comprises a thin Zn hydroxide layer and Zn phosphate particles with poor crystallinity, as shown in Fig. 2. This poor crystallinity can result from the supersaturated reacting ions, mainly Zn2+, OH, and PO4 3 , built up during the baking process. Surface SEM and cross-sectional TEM observations revealed that the thickness of the coating prepared by the one-step roll coating treatment is non-uniform. Because the precipitation of Zn phosphates, the major constituent of the passive coating, relies on the dissolution of Zn and the rise in solution pH, the presence of non-uniform phosphate layer is due to local enhanced oxidation and dissolution of the Zn substrate. This enhance dissolution is likely associated with the defects and non-uniformity of the air-
Fig. 5. High-resolution spectrums of (a) Zn, (b) P, and (c) Mo of the coating prepared by the two-step roll coating treatment.
formed oxide layer on the hot-dip Zn coating. This is consistent with the results made by Tegehall and Vannerberg who demonstrated that the nucleation rate of Zn phosphate on cold-rolled steel was higher at an oxide layer with many weak points [52].
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Fig. 6. Polarization curves of the various GI samples in 5 wt.% NaCl.
Fig. 8. The pictures of the various samples after 24 h of salt spray test: (a) the untreated GI steel sheet, and the GI steel sheet after (b) one-step and (c) two-step roll coating treatments.
Fig. 7. EIS analysis of the various GI samples in 5 wt.% NaCl: (a) Nyquist and (b) Bode plots.
The population density of spherical particles increased significantly after the second step roll coating treatment, as illustrated by comparing Fig. 1b to a. The phosphate coating formed after
the first step roll coating treatment was apparently attacked by the molybdate solution of pH 4.5 during second step roll coating, giving rise to extensive coating reactions. This is markedly different from that processed by immersion. Lin et al. employed a molybdate solution to seal the pores of the phosphate coating on GI steel sheets via immersion and found that the morphology of the Zn phosphate coating hardly changed after the immersion treatment in the molybdate solution [6]. The Mo 3d XPS spectrum revealed the presence of MoO2 in the coating formed after the two-step roll coating treatment, indicating that MoO4 2 can be reduced to MoO2 in accompany with the oxidation of the Zn substrate, as shown by Reaction (1). The Mo 3d XPS spectrum further shows that the ratio of MoO4 2 to the total Mo species in the coating is larger than that of MoO2 to the total Mo species. Since the GI steel sheet became passivated by a complete phosphate coating layer after the first step roll coating treatment, the Zn substrate was less attacked during the second step roll coating treatment. This less attack on the Zn substrate determines the extent of Reaction (1). Nevertheless, the formation of spherical phosphate particles occurs predominantly at the weak points of the phosphate coating, which is highly porous as shown in Fig. 2. Meanwhile, the porous nature of the phosphate coating favors the absorption of MoO4 2 , HPO4 2 , and PO4 3 anions. As a result, the second step roll coating treatment, on one hand, results in the incorporation of MoO4 2 and MoO2 to the existing passive coating. On the other hand, this process tends to identify the weak
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points of the phosphate coating and repair them via the formation of MoO4 2 /MoO2 containing spherical particles. 4.2. Corrosion resistance mechanism of the phosphate/molybdate passive coating on GI steel sheet via a complete two-step roll coating treatment Although the GI steel sheet after the first step roll coating treatment displays a lower corrosion current density, nobler corrosion potential, and higher polarization resistance than the as-received GI steel sheet, the porous phosphate coating provides little protection on the Zn coating during 24 h of the salt spray test. Meanwhile, an LF inductive loop corresponding to the dissolution of the Zn coating is still present for the GI steel sheet after the first step roll coating treatment. This inferior performance can be due to the defects associated with the phosphate coating, specifically the numerous nano pores within the coating (Fig. 2) [53–56]. The second step roll coating treatment using the molybdate solution following the first step treatment using the phosphate solution markedly improves the corrosion resistance of the GI steel sheet, as shown by a passive region on the polarization curve, an obvious diffusion tail in the LF region on the Nyquist plot, and a reduced corrosion damage after 24 h of the salt spray test. The passive region can be related to the presence of MoO4 2 and PO4 3 anions incorporated in the phosphate/molybdate coating. Molybdate ions are generally recognized as an effective corrosion inhibitor. Phosphate ions in the micro-arc anodized coating on an AZ91D magnesium alloy have been shown to display a self-healing capability during salt spray test [57]. Another benefit from the second step roll coating treatment is that the defects of the phosphate coating can be identified, attacked, and then repair during the coating process, as manifested by a marked increase in the population density of the spherical Zn phosphate particles. It was noted that the thickness of the passive coating changed slightly during the second step roll coating treatment. As a result, the absence of the LF inductive loop and the presence of the diffusion tail in the LF region are likely to result from the repair of the defects of the phosphate coating via a subsequent molybdate passivation treatment. Because only small amounts of Mo species were incorporated into the phosphate/molybdate coating, the repair of the defects in the phosphate coating should play an important role in improving the corrosion resistance of the phosphate/molybdate passive coating on the GI steel sheet. Again, this improved corrosion resistance afforded by the second step molybdate passivation via roll coating is different from that by the molybdate sealing via immersion. That is, the immersion in the molybdate solution results in the molybdate conversion coating formation on the pores in between the Zn phosphate particles, which, in turn, seals the pores present after immersion in the phosphate solution [6]. This sealing itself improves the corrosion resistance of the phosphate coating on Zn, while has little effect on the composition and microstructure of the Zn phosphate particles. The nano pores in the phosphate coating apparently act as the reservoir for the absorption of MoO4 2 and PO4 3 anions during the second step roll coating using the molybdate solution. This effective absorption enhances the corrosion resistance of the phosphate coating. To test this enhancement, the GI steel sheet without the phosphate coating was subjected directly to the roll coating treatment using the molybdate solution. The corroded area fraction of the molybdate-coated GI steel sheet was approximately 50% after 24 h of the salt spray test (data not shown). Consequently, the molybdate coating formed via the roll coating process itself cannot provide sufficient corrosion protection for GI steel sheets. Accordingly, the two-step (phosphate/molybdate) roll coating process provides a synergistic effect on the corrosion protection for GI steel sheets.
5. Conclusions The GI steel sheet has been passivated by a roll coating process using a phosphate solution followed by a molybdate solution. The conclusions are included as the following: 1. The coating formed after a roll coating treatment using the Zn phosphate solution is mainly composed of Zn phosphate containing trace of Mg. The coating is porous, amorphous, and relatively non-uniform in thickness. 2. A second step coating treatment using the molybdate solution results in an obvious increase in the population density of the spherical particles. Consequently, the coating reaction occurring during the second step coating treatment proceeds with a local attack and dissolution of the existing phosphate coating, and a subsequent precipitation of spherical particles. Meanwhile, MoO4 2 anions and MoO2 are incorporated into the porous phosphate coating. 3. The GI steel sheet after the two-step roll coating treatment demonstrates a better corrosion resistance than that after the one-step roll coating treatment. The passive region on the polarization curve is likely due to the presence of MoO4 2 and PO4 3 anions in the phosphate/molybdate coating. The absorbed MoO4 2 and PO4 3 anions inhibit the attack of chloride ions during the salt spray test. Moreover, the weak points of the phosphate coating have been identified and repaired during the second step roll coating treatment. The phosphate/molybdate passive coating with less defects effectively prevents the GI from corrosion after 24 h of the salt spray test. Moreover, a LF diffusion tail is present and the LF inductive loop is absent for the phosphate/molybdate passive coating.
Acknowledgments This study was financially supported by China Steel Corporation, Taiwan, and National Science Council of Taiwan under Grant No. 982221E002056MY3.
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A two-step roll coating phosphate/molybdate passivation treatment for hot-dip galvanized steel sheet Cheng-Yang Tsai a, Jen-Shou Liu a, Pei-Li Chen b, Chao-Sung Lin a,* a b
Department of Materials Science and Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei 106, Taiwan New Materials R&D Department, China Steel Corporation, 1, Chung Kang Road, Hsiao Kang, Kaohsiung 802, Taiwan
a r t i c l e
i n f o
Article history: Received 10 April 2010 Accepted 18 June 2010 Available online 30 June 2010 Keywords: A. Steel A. Zinc B. EIS B. XPS B. Polarization C. Passive films
a b s t r a c t A two-step roll coating passivation treatment employing phosphate followed by molybdate solutions has been performed on hot-dip galvanized (GI) steel sheet. The phosphate coating was primarily porous, amorphous Zn phosphate. A second step coating treatment resulted in more hemispherical Zn phosphate particles and the incorporation of molybdate ions and molybdenum oxide into the existing phosphate coating, giving rise to an improved corrosion resistance. The coating reaction during the second step roll coating treatment and the corrosion protection afforded by the second step molybdate treatment are discussed, with emphases on the comparison with the coating formed via immersion. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Zinc coating has been extensively employed to enhance the corrosion resistance of steel via cathodic protection. To further enhance the corrosion performance of Zn-coated steel, hexavalent chromium-based passivation treatment is generally adopted. However, hexavalent Cr is highly toxic and its use has been strictly restricted according to international regulations such as RoHS and WEEE. Non-chromate passivation treatments have, therefore, received an ever-increasing attention, including phosphate [1–9], cerium-based [10–14], silica-based [15,16], tungstate-based [17], and molybdate-based treatments [18–30]. Phosphating is one of the most widely used passivation treatments and generally conducted in acid aqueous phosphate solution containing nitrate (NO3 ) as accelerator to promote the oxidation and dissolution of Zn, and metal cations to refine the size of Zn phosphate crystals [4]. Finer Zn phosphate crystals facilitate the coverage of phosphate coating, which, in turn, improves the corrosion resistance of the phosphate coating [6–9,31,32]. Nevertheless, the phosphate conversion coating formation via the growth and coalescence of the Zn phosphate crystals inevitably leaves pores, which are detrimental to the corrosion resistance of the coating. Post-treatments to seal the pores of the phosphate coating are, thus, employed to improve the corrosion resistance, such as silicate and molybdate sealing treatments [6,7]. * Corresponding author. Tel.: +886 2 33665240; fax: +886 2 23634562. E-mail address: [email protected] (C.-S. Lin). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.06.020
Like hexavalent Cr ions, molybdate anions are an effective corrosion inhibitor for Zn [33–37]. However, molybdate anions are less toxic than hexavalent Cr ions [22]. Molybdate passivation treatment is, thus, considered as one of the possible alternatives to chromate treatment [18–30] and a review article has been reported recently by Walker and Wilcox [18]. Since introduced in the mid1980s, the passivation of Zn in the molybdate solution was generally conducted cathodically [19]. Later on, a simple immersion technique was developed by Wilcox and Gabe [20] who demonstrated that the solution containing 5–30 g L1 sodium molybdate at pH 5 rendered a molybdate conversion coating on Zn with satisfactory performance. Magalhaes et al. [28] investigated the molybdate conversion coating on 99.9% pure Zn and electrogalvanized steel (EG) via an immersion coating process. They found that the morphology of the molybdate coating was strongly dependent on the acid (nitric, sulfuric, or phosphoric acids) used and the pH of the solution. The presence of phosphate ions resulted in a thin, compact molybdate coating, which had a better corrosion resistance than those formed in the molybdate baths containing nitrate or sulfate ions. Treacy et al. [26] explored the corrosion behavior of molybdate-based conversion coatings exposed to neutral salt fog. The coating displayed a superior corrosion resistance after 21 h of salt fog test. This is because Mo(V) or Mo(VI) species in the coating were reduced to Mo(IV) or Mo(III) species, which were further incorporated in the corrosion product layer acting as a barrier to prevent further corrosion attack. Recently, metal cations, such as Al3+, and additives, such as silicate and silane, have been introduced to the molybdate conversion solution for Zn-coated steels [27,30]. The conversion coating thus formed
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generally exhibited a composite nature and an improved corrosion resistance. Phosphating and molybdate conversion coating treatments for Zn-coated steel using a simple immersion technique are abundant in the literature. In contrast, the passivation treatment of steel sheets using a roll coating process is still scarce in the literature. Roll coating is a process commonly used to coat a flat substrate with a protective coating as the substrate passes through a pair of rollers. This technique is extensively used in the coil coating industry such as the coating process for steel coil [38]. Industrial steel coil roll coating process generally employs a baking procedure to facilitate the formation of the coating. During baking, the concentration of the various ions in the liquid film on the steel coil increases as water is evaporated. This is different from the coating formed by an immersion process, during which a diffusion layer depleted of the reacting species forms on the surface of the growing conversion coating. This study investigated a two-step roll coating passivation treatment on hot-dip galvanized (GI) steel sheet, i.e., phosphating followed by a molybdate treatment. The detailed microstructure and composition of the conversion coating was characterized after each step of treatment. The corrosion resistance was evaluated by potentiodynamic polarization measurement, electrochemical impedance spectroscopy (EIS) and salt spray test. Based on the microstructure and properties of the coating, the microstructural evolution and corrosion resistance mechanism afforded by the phosphate/molybdate passive coating on GI steel sheet are proposed and discussed. 2. Experimental 2.1. Passivation treatment The material used for this study is commercial GI steel sheets. The thickness of the Zn coating is around 10 lm. The plate with a size of 150 100 0.8 mm was degreased in an alkaline solvent, cleaned, and rinsed with deionized water, and finally dried in a stream of hot air. The passivation treatment was conducted by a two-step roll coating process: the first step using a phosphate solution and the second step using a molybdate solution. The phosphate solution contained 2.4 g L1 zinc oxide (ZnO), 30 g L1 sodium nitrate (NaNO3), 22 ml L1 phosphoric acid (H3PO4), and 5 g L1 magnesium nitrate (Mg(NO3)26H2O), and the pH of the solution was adjusted to 2.5 by the addition of sodium hydroxide (NaOH). The molybdate solution consisted of 30 g L1 sodium molybdate (Na2MoO42H2O) and the pH of the solution was adjusted to 4.5 via the addition of H3PO4. The phosphate solution was applied onto the GI steel sheet using a bar coater. The plate was, then, baked in 200 °C air for 12 s and cooled to ambient temperature in room temperature air. Subsequently, the molybdate treatment was likewise conducted on the phosphated sample. A one-step roll coating process using the phosphate solution only was also studied to elucidate the effect of the second step passivation using the molybdate solution. 2.2. Microstructural characterization The surface morphology of the coating was investigated using a JEOL JXA-8200 scanning electron microscopy (SEM) at backscattered electron (BSE) mode. Cross-sectional transmission electron microscope (TEM) specimen was prepared using a combined mechanical and ion-beam thinning technique or a focused ion beam instrument (FEI Nova-200 NanoLab Dual Beam). The TEM specimen was examined using a TEM (FEI Tecnai F20 G2) at 200 kV. The composition of the coating was measured by energy dispersive spectrometry (EDS) in TEM and the crystallinity was characterized by
the selected area electron diffraction (SAED) technique. Finally, X-ray photoelectron spectroscopy (XPS) was performed using a PHI Quantera SXM Scanning X-ray Microscope with an Al Ka monochromated source. All spectra were corrected using the signal of C1s peak at 284.5 eV. 2.3. Corrosion performance evaluation The corrosion resistance of the coating was evaluated using potentiodynamic polarization, EIS, and salt spray test. The polarization curve measurement and EIS were performed via a Model 263A Potentiostat/Galvanostat and FRD100 Frequency Response Detector (EG&G Instruments) and conducted in 5 wt.% sodium chloride (NaCl) solution using a conventional three-electrode cell. The specimen under study with an exposure area of 1 cm2, a platinum plate of 16 cm2 and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrode, respectively. All electrochemical measurements were measured after a steady open circuit potential (OCP) was reached. The polarization curve was measured potentiodynamically by sweeping the potential in the positive direction at a scan rate of 1 mV/s and the sweep range was from an initial potential of 0.2 V to a final potential of 0.2 V (both are relative to OCP). EIS was recorded at OCP with a sinusoidal perturbation of 10 mV amplitude over the frequency range from 100 kHz to 0.01 Hz. The neutral salt spray test was performed according to the ASTM B 117-03 [39] standard using a chamber (Model SST-B, Ten Billion Co., Taiwan) with 5 wt.% NaCl. The specimens were placed perpendicularly with an incline angle of 30° and were continuously sprayed for 24 h in a neutral salt spray test chamber. The corrosion resistance of the sample was evaluated according to the percentage of the corroded area. 3. Results 3.1. Surface morphology of the coating Before passivation treatment, the GI steel sheet displayed a bright contrast under SEM/BSE mode (not shown here). Fig. 1a shows the overall surface morphology of the GI steel sheet after the first step roll coating using the phosphate solution. Several island-like spots showing a dark contrast were visible in addition to the bright-contrast areas that are characteristic of the GI steel sheet before passivation treatment. Herein, these island-like spots were denoted as the phosphate particles. Magnified observations revealed the presence of flower-like phosphate particles (marked as the arrow in Fig. 1c) and round phosphate particles (marked as the double arrows). Fig. 1b shows the overall surface morphology of the GI steel sheet after a complete two-step roll coating treatment. The population density of the phosphate/molybdate particles markedly increased after the second step roll coating using the molybdate solution, in particular the spherical particles. Moreover, the average diameter of the spherical particles slightly increased after the second step treatment, i.e., 2.2 vs. 3.4 lm. Magnified observations identified the coexistence of spherical, flower-like particles, and gray-contrast films (marked as the arrow in Fig. 1d). EDS analyses revealed that both the regions with and without particles contained Zn, O, P, and traces of Mg and Mo species (data not shown). Because less O and more Zn were detected in the region without particles, the coating on the region without particles was apparently thinner than that on the region with particles. 3.2. Cross-sectional TEM characterization Fig. 2 show the cross-sectional TEM characterization of the GI steel sheet after the first step roll coating treatment. A continuous,
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Fig. 1. Surface morphology of the GI steel sheet after (a) the first step roll coating treatment and (b) a complete two-step roll coating treatment. (c and d) are the magnified views of (a and b), respectively.
porous phosphate coating was observed on top of the Zn substrate. Nevertheless, the thickness of the phosphate coating varied from place to place. The coating of approximately 1.5 lm thick (Fig. 2a) corresponded to the flower-like particles shown in Fig. 1c; meanwhile, the coating of 0.2 lm thick (Fig. 2b) was the round particles. Moreover, a thin passive film adjacent to the flower-like particle was observed and intimately contacted the Zn substrate, as shown in Fig. 2c. The thickness of this thin passive film ranged from 10 to 60 nm. Insets in Fig. 2a and b are the SAED patterns of the flower-like and round particles, respectively. Both SAED patterns consisted of diffused halos, indicating that these particles were amorphous. EDS analyses revealed that the phosphate coating contained Zn, O, P, and Mg species (Table 1). The flower-like and round particles contained significant amounts of O, P, and Zn, indicating they were primarily the Zn phosphate. In contrast, the thin passive film was mainly composed of O and Zn, suggesting the presence of Zn oxide/hydroxide. The cross-sectional TEM of the coating prepared by the twostep roll coating treatment is shown in Fig. 3. The spherical particles shown in the SEM micrograph (Fig. 1d) were clearly seen in TEM. When observed cross-sectionally in TEM, these particles displayed a hemispherical shape and a porous structure (Fig. 3a). The average diameter of the hemispherical particles was around 1 lm. In addition to the hemispherical particles, the Zn substrate was completely covered with a continuous, porous passive layer with a thickness ranging from 10 to 250 nm. The relatively-thick passive layer (Fig. 3b) was apparently correlated to the gray-contrast areas observed in SEM (Fig. 1d). Meanwhile, the relatively-thin passive layer was primarily observed around the hemispherical particles,
as shown in Fig. 3c. Moreover, the relatively-thin passive layer was more compact than the hemispherical particle and the relatively-thick passive layer. The SAED patterns inset in Fig. 3a and b revealed that both the hemispherical particles and the relativelythick passive layer had poor crystallinity. Table 2 shows the EDS results of the distinct features shown in Fig. 3. Both the hemispherical particle and the relatively-thick passive layer were composed of Zn, O, P, and traces of Mg and Mo species. As a result, they were primarily the Zn phosphate. The relatively-thin passive layer also contained Zn, O, P, and trace of Mo species. Nevertheless, the relatively-thin passive layer had more Mo and less P than the hemispherical particle and the relatively-thick passive layer. 3.3. XPS analysis Fig. 4a and b shows the XPS depth profile of the GI steel sheet after one-step and two-step roll coating treatments, respectively. Both the coatings were mainly composed of O, Zn, and P species, in which the Zn content increased and the O content decreased along the depth of the coating. In contrast, the content of P, Mg, and Mo displayed a monotonous decrease along the depth of the coating. High-resolution spectrums of Zn, P, and Mo of the coating prepared by the two-step roll coating treatment are shown in Fig. 5a–c, respectively. The spectrum of Zn 2P can be identified as ZnO/Zn(OH)2 and Zn3(PO4)2 [40]. The P 2p spectrum was likely to result from PO4 3 and HPO4 2 [41]. Finally, the Mo 3d spectrum included three peaks, in which two peaks corresponded to molybdate ion, MoO4 2 , and one peak was assigned as Mo dioxide, MoO2 [42–46].
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Fig. 2. Cross-sectional TEM micrographs of the GI steel sheet after the first step roll coating treatment showing the existence of three distinct regions: (a and b) correspond, respectively, to the flower-like particle and round particle observed on the SEM micrographs, and (c) a thin passive film intimately contacting the Zn substrate. Note the Pt layer and C film are deposited prior to the TEM sample preparation using FIB, whereas the M-Bond epoxy is used to sandwich two treated GI steel sheets for the TEM sample preparation using a combined mechanical and ion-beam thinning technique.
Table 1 TEM/EDS results of the distinct regions of the coating prepared by the one-step roll coating using the phosphate solution. Distinct regions
Flower-like particle Round particle Thin passive film
Atomic percentage Zn
O
P
Mg
Totals
16.0 17.5 31.1
67.0 65.0 64.5
14.4 16.1 4.2
2.6 1.4 0.2
100.0 100.0 100.0
3.4. Polarization curves Fig. 6 shows the polarization curves of the various GI samples in 5 wt.% NaCl. Compared with the as-received GI steel sheet, both the GI steel sheets after one-step and two-step roll coating treatments had polarization curves shifting to lower current densities and nobler potentials. The coating formed after the two-step roll coating treatment displayed an apparent passive region and had a nobler corrosion potential (994 mV(SCE) vs. 1017 mV(SCE)) and lower corrosion current density (1.19 lA/cm2 vs. 1.45 lA/cm2) than that formed after the one-step roll coating treatment. This signifies that a subsequent molybdate passivation treatment markedly improves the corrosion resistance of the phosphated GI steel sheet.
3.5. EIS analysis Fig. 7a and b shows the Nyquist and Bode plots of the as-received GI steel sheet, and the GI steel sheets after one-step and two-step roll coating treatments, respectively. The Nyquist plot of GI steel sheet comprised one high frequency (HF) capacitive loop, which is related to the double layer and charge transfer on the Zn substrate, and one low frequency (LF) inductive loop, which corresponds to the dissolution of the Zn substrate [6,7,29,47–50]. In addition to the HF capacitive loop and LF inductive loop, a capacitive loop was observed for the GI steel sheet after the onestep roll coating treatment and is generally related to the diffusion phenomena in the phosphate coating [6,7]. The phosphate coating formed during the first step roll coating treatment apparently enhanced the polarization resistance of the GI steel sheet. However, for the GI steel sheet after the two-step roll coating treatment, the LF inductive loop disappeared and a HF capacitive loop related to the phosphate/molybdate coating followed by a diffusion tail in the LF region was observed, signifying that the corrosion is diffusion-controlled [49,50]. The LF (0.01 Hz) impedance of Bode plots (Fig. 7b) is generally recognized as the polarization resistance of the sample [6,51]. The 0.01-Hz impedance of the as-received GI steel sheet and those
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Fig. 3. Cross-sectional TEM micrographs of the GI steel sheet after a complete two-step roll coating treatment showing (a) hemispherical particle, (b) relatively-thick passive layer, and (c) relatively-thin passive layer.
Table 2 TEM/EDS results of the distinct regions of the coating prepared by a complete twostep roll coating treatment. Distinct regions
Hemispherical particle Thick passive layer Thin passive layer
ing treatment was less than 5%. Consequently, the salt spray test results are in good consistence with the polarization curve and EIS measurements.
Atomic percentage Zn
O
P
Mg
Mo
Totals
4. Discussion
18.0 18.1 28.8
66.4 64.4 61.3
13.6 15.8 7.3
1.6 1.5 0.0
0.4 0.2 2.6
100.0 100.0 100.0
4.1. Formation of the phosphate/molybdate passive coating during roll coating
after one-step and two-step roll coating treatments was 0.296, 1.142, and 5.153 kX cm2, respectively. Consequently, the second step treatment using the molybdate solution markedly improved the corrosion resistance of the phosphate coating on GI steel sheet. 3.6. Salt spray tests The visual aspect of the various GI samples after 24 h of salt spray test is shown in Fig. 8. The as-received GI steel sheet and the GI steel sheet after the one-step roll coating treatment were nearly completely covered with white rusts. In contrast, the white rust area fraction of the GI steel sheet after the two-step roll coat-
The conversion coating formation during immersion is well recognized to proceed with the dissolution of the metal substrate, followed by the hydrogen evolution and the reduction of the reacting ions in the solution. In a simple phosphate solution, the Zn ions dissolved from the Zn substrate can either react with the phosphate ions, which result from the dissociation of the dihydrogen phosphate ions due to pH rise, to form Zn phosphate particles or with hydroxyl anions to form Zn hydroxides [4,32]. In a molybdate solution, in addition to the hydrogen evolution, MoO4 2 can be reduced to MoO2 in accompany with the oxidation of the Zn substrate, as shown:
MoO4 2 þ 4Hþ þ 2e ! MoO2 þ 2H2 O
ð1Þ
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Fig. 4. XPS depth profile of the GI steel sheet after (a) one-step and (b) two-step roll coating treatments.
As a result, the molybdate conversion coating on Zn is mainly composed of Zn oxide/hydroxide, Mo (IV) oxide, and incorporated Mo(VI) species. The formation of the phosphate conversion coating apparently relies on sufficient Zn2+, OH, and PO4 3 concentrations built up on the interface between the Zn substrate and the solution. During immersion, parts of the Zn2+, OH, and PO4 3 ions react to form Zn hydroxide and phosphate, while parts of them diffuse away from the interface. The conversion coating thus formed mainly consists of crystalline Zn phosphate particles [4,6,7]. In contrast, the concentration of Zn2+, OH, and PO4 3 ions in the liquid film applied by a roll coating process increase during baking. The resulting conversion coating comprises a thin Zn hydroxide layer and Zn phosphate particles with poor crystallinity, as shown in Fig. 2. This poor crystallinity can result from the supersaturated reacting ions, mainly Zn2+, OH, and PO4 3 , built up during the baking process. Surface SEM and cross-sectional TEM observations revealed that the thickness of the coating prepared by the one-step roll coating treatment is non-uniform. Because the precipitation of Zn phosphates, the major constituent of the passive coating, relies on the dissolution of Zn and the rise in solution pH, the presence of non-uniform phosphate layer is due to local enhanced oxidation and dissolution of the Zn substrate. This enhance dissolution is likely associated with the defects and non-uniformity of the air-
Fig. 5. High-resolution spectrums of (a) Zn, (b) P, and (c) Mo of the coating prepared by the two-step roll coating treatment.
formed oxide layer on the hot-dip Zn coating. This is consistent with the results made by Tegehall and Vannerberg who demonstrated that the nucleation rate of Zn phosphate on cold-rolled steel was higher at an oxide layer with many weak points [52].
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Fig. 6. Polarization curves of the various GI samples in 5 wt.% NaCl.
Fig. 8. The pictures of the various samples after 24 h of salt spray test: (a) the untreated GI steel sheet, and the GI steel sheet after (b) one-step and (c) two-step roll coating treatments.
Fig. 7. EIS analysis of the various GI samples in 5 wt.% NaCl: (a) Nyquist and (b) Bode plots.
The population density of spherical particles increased significantly after the second step roll coating treatment, as illustrated by comparing Fig. 1b to a. The phosphate coating formed after
the first step roll coating treatment was apparently attacked by the molybdate solution of pH 4.5 during second step roll coating, giving rise to extensive coating reactions. This is markedly different from that processed by immersion. Lin et al. employed a molybdate solution to seal the pores of the phosphate coating on GI steel sheets via immersion and found that the morphology of the Zn phosphate coating hardly changed after the immersion treatment in the molybdate solution [6]. The Mo 3d XPS spectrum revealed the presence of MoO2 in the coating formed after the two-step roll coating treatment, indicating that MoO4 2 can be reduced to MoO2 in accompany with the oxidation of the Zn substrate, as shown by Reaction (1). The Mo 3d XPS spectrum further shows that the ratio of MoO4 2 to the total Mo species in the coating is larger than that of MoO2 to the total Mo species. Since the GI steel sheet became passivated by a complete phosphate coating layer after the first step roll coating treatment, the Zn substrate was less attacked during the second step roll coating treatment. This less attack on the Zn substrate determines the extent of Reaction (1). Nevertheless, the formation of spherical phosphate particles occurs predominantly at the weak points of the phosphate coating, which is highly porous as shown in Fig. 2. Meanwhile, the porous nature of the phosphate coating favors the absorption of MoO4 2 , HPO4 2 , and PO4 3 anions. As a result, the second step roll coating treatment, on one hand, results in the incorporation of MoO4 2 and MoO2 to the existing passive coating. On the other hand, this process tends to identify the weak
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points of the phosphate coating and repair them via the formation of MoO4 2 /MoO2 containing spherical particles. 4.2. Corrosion resistance mechanism of the phosphate/molybdate passive coating on GI steel sheet via a complete two-step roll coating treatment Although the GI steel sheet after the first step roll coating treatment displays a lower corrosion current density, nobler corrosion potential, and higher polarization resistance than the as-received GI steel sheet, the porous phosphate coating provides little protection on the Zn coating during 24 h of the salt spray test. Meanwhile, an LF inductive loop corresponding to the dissolution of the Zn coating is still present for the GI steel sheet after the first step roll coating treatment. This inferior performance can be due to the defects associated with the phosphate coating, specifically the numerous nano pores within the coating (Fig. 2) [53–56]. The second step roll coating treatment using the molybdate solution following the first step treatment using the phosphate solution markedly improves the corrosion resistance of the GI steel sheet, as shown by a passive region on the polarization curve, an obvious diffusion tail in the LF region on the Nyquist plot, and a reduced corrosion damage after 24 h of the salt spray test. The passive region can be related to the presence of MoO4 2 and PO4 3 anions incorporated in the phosphate/molybdate coating. Molybdate ions are generally recognized as an effective corrosion inhibitor. Phosphate ions in the micro-arc anodized coating on an AZ91D magnesium alloy have been shown to display a self-healing capability during salt spray test [57]. Another benefit from the second step roll coating treatment is that the defects of the phosphate coating can be identified, attacked, and then repair during the coating process, as manifested by a marked increase in the population density of the spherical Zn phosphate particles. It was noted that the thickness of the passive coating changed slightly during the second step roll coating treatment. As a result, the absence of the LF inductive loop and the presence of the diffusion tail in the LF region are likely to result from the repair of the defects of the phosphate coating via a subsequent molybdate passivation treatment. Because only small amounts of Mo species were incorporated into the phosphate/molybdate coating, the repair of the defects in the phosphate coating should play an important role in improving the corrosion resistance of the phosphate/molybdate passive coating on the GI steel sheet. Again, this improved corrosion resistance afforded by the second step molybdate passivation via roll coating is different from that by the molybdate sealing via immersion. That is, the immersion in the molybdate solution results in the molybdate conversion coating formation on the pores in between the Zn phosphate particles, which, in turn, seals the pores present after immersion in the phosphate solution [6]. This sealing itself improves the corrosion resistance of the phosphate coating on Zn, while has little effect on the composition and microstructure of the Zn phosphate particles. The nano pores in the phosphate coating apparently act as the reservoir for the absorption of MoO4 2 and PO4 3 anions during the second step roll coating using the molybdate solution. This effective absorption enhances the corrosion resistance of the phosphate coating. To test this enhancement, the GI steel sheet without the phosphate coating was subjected directly to the roll coating treatment using the molybdate solution. The corroded area fraction of the molybdate-coated GI steel sheet was approximately 50% after 24 h of the salt spray test (data not shown). Consequently, the molybdate coating formed via the roll coating process itself cannot provide sufficient corrosion protection for GI steel sheets. Accordingly, the two-step (phosphate/molybdate) roll coating process provides a synergistic effect on the corrosion protection for GI steel sheets.
5. Conclusions The GI steel sheet has been passivated by a roll coating process using a phosphate solution followed by a molybdate solution. The conclusions are included as the following: 1. The coating formed after a roll coating treatment using the Zn phosphate solution is mainly composed of Zn phosphate containing trace of Mg. The coating is porous, amorphous, and relatively non-uniform in thickness. 2. A second step coating treatment using the molybdate solution results in an obvious increase in the population density of the spherical particles. Consequently, the coating reaction occurring during the second step coating treatment proceeds with a local attack and dissolution of the existing phosphate coating, and a subsequent precipitation of spherical particles. Meanwhile, MoO4 2 anions and MoO2 are incorporated into the porous phosphate coating. 3. The GI steel sheet after the two-step roll coating treatment demonstrates a better corrosion resistance than that after the one-step roll coating treatment. The passive region on the polarization curve is likely due to the presence of MoO4 2 and PO4 3 anions in the phosphate/molybdate coating. The absorbed MoO4 2 and PO4 3 anions inhibit the attack of chloride ions during the salt spray test. Moreover, the weak points of the phosphate coating have been identified and repaired during the second step roll coating treatment. The phosphate/molybdate passive coating with less defects effectively prevents the GI from corrosion after 24 h of the salt spray test. Moreover, a LF diffusion tail is present and the LF inductive loop is absent for the phosphate/molybdate passive coating.
Acknowledgments This study was financially supported by China Steel Corporation, Taiwan, and National Science Council of Taiwan under Grant No. 982221E002056MY3.
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