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Corrosion behavior and composition analysis of chromate passive film on electroless Ni-P coating
Applied Surface Science 256 (2010) 4089–4094
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Corrosion behavior and composition analysis of chromate passive film on electroless Ni-P coating Songlin Mu, Ning Li *, Deyu Li, Liying Xu Department of applied chemistry, Harbin Institute of technology, Harbin 150001, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 November 2009 Accepted 27 January 2010 Available online 4 February 2010
A passive film was formed on electroless Ni-P coating (ENPC) in a bath of K2Cr2O7 30 g/l. XPS and electrochemical methods were employed to analyze its chemical compositions and corrosion behaviors. The potentiodynamic polarization tests indicated the corrosion current of the passivated sample was 1/ 30 that of as-plated ENPC. The XPS analysis illustrated the film comprised Cr, Ni and O. The film thickness was evaluated to be a few nanometers according to the sputtering rate of Ar+ ion. High-resolution XPS spectra suggested that the detected Cr in film was in the form of trivalent compounds, Cr2O3 and Cr(OH)3. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Passive film Electroless Ni-P coating Corrosion behaviors XPS analysis
1. Introduction Electroplating and electroless deposition are two common processes to coat metallic layers on many engineering components. In comparison, an electroless process is more preferred in industry than electroplating because the former can deposit a homogeneous coating on components with very complicated geometries. Electroless method includes electroless Ag [1–3], Au, Pt [4], Cu [5] and electroless Ni-P coating (ENPC) [6,7]. In these methods, electrodes Ni-P coating (ENPC) receives lots of attention due to its shiny appearance, high hardness, uniform thickness, as well as very good wear and corrosion resistance [8–14]. The ENPC has been widely used in electronic industry involving electromagnetic shielding, computer storage, microelectronics such as computer hard disk, wafer, printed circuit board and so on [15,16]. In spite of its excellent properties, the ENPC is apt to be oxidized in air, which makes the surface lose its brightness, even discolor. Furthermore, Ni oxidization will seriously decrease the solderability of the coating, leading to a fatal defect to some electronic components. In the industrial field, many factories avoid this problem by passivation treatments with chromate (VI). For example, the OM Group Inc. (OMG) of America applied a treatment bath: K2Cr2O7 30 g/l (without adjusting pH, original pH = 3.7–3.8). This passivation treatment does not bring any change in the appearance; moreover, it can improve corrosion resistance. More importantly, the solderability of the coating will not be obviously
* Corresponding author. Tel.: +86 451 86413721; fax: +86 451 86221048. E-mail addresses: [email protected] (S. Mu), [email protected] (N. Li). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.089
affected by this treatment. Since the surface of ENPC will turn black or dark in strong oxidizing acid, therefore HNO3 solution can be used as a simplified method to evaluate the corrosion resistance of the passive film on ENPC [17]. Researches concerning the formation and anticorrosion mechanisms of chromate (VI) passive films on Zn, Al, stainless steel etc. have been extensively conducted, and several models have been proposed [18–20]. The forming process generally contains two steps: firstly, the dissolution of metallic matrices increases the concentration of metallic ions and decreases the concentration of H+ in the solution layer very close to the surface of matrices. Secondly, with the concentration of the metallic ions and pH value increasing, metal ions can not stay stably in solution and consequently form insoluble hydroxides which precipitate on matrices. The chromate passive films generally involve trivalent and hexavalent species. Mccreery et al. suggested that these two kinds of species were in the form of a sol-gel, in which Cr(III) species formed the matrix in which Cr(VI) species were trapped [21]. The presence of hexavalent Cr was considered as the main reason that makes the chromate passive films possess a ‘‘selfhealing’’ effect, and was also taken as the main factor that can explain the excellent anticorrosion property of chromate (VI) passive films [22–24]. Although the chromate (VI) treated Ni-P components can meet the industrial demands for solderability and discoloration, the usage of chromate is being progressively restricted due to its toxicity to the environment. From the viewpoint of sustainable development and environmental safety, it is necessary to develop a Cr-free technique. But firstly, we should understand the mechanism of chromate (VI) treatment for Ni-P coating. So far, to the best
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of our knowledge, little work was reported on Cr-free or chromate (III) or even chromate (VI) methods for ENPC. This work aims at studying the composition and the corrosion properties of the passive film on ENPC. 2. Experimental details 2.1. Preparation of Ni-P coating From a commercially operated medium-phosphorus-content nickel bath supplied by OMG, Ni-P coating about 14 mm thick was deposited on a substrate of mild steel sheets. Prior to deposition, the sheets were first mechanically polished with 1000 grade of SiC paper and then degreased by immersion in 10% NaOH at room temperature for 5 min, followed by deionized (DI) water rinse and immersion in acetone. Then, the sheets were activated in 5% HCl solution for 10 s followed by rinsing in DI water. After these procedures, the electroless plating was immediately carried out at 88 2 8C for 40 min with pH of 4.75 (adjusting at room temperature about 20–25 8C). 2.2. Passivation treatment The plated sheets were firstly rinsed with DI water, then passivated immediately in the passivation bath at 60 8C for 10 min. The bath only contains 30 g/l K2Cr2O7, without adjusting pH value. After passivation, the samples were rinsed twice in DI water and finally dried in oven at 120 8C for 20 min. 2.3. Acid exposure test 50 vol.% HNO3 was taken as a testing agent to perform the acid exposure test. The time when HNO3 was dropped on surface of sample to when the surface turned black/dark was recorded to evaluate the corrosion resistance of the as-plated and passivated coatings. For each sample, the acid exposure test will be conducted at least three times at different spots. 2.4. Electrochemical corrosion tests Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were carried out by CHI660B electrochemical workstation (Shanghai Chenhua instrument company, Shanghai, China). The sample was loaded in a conventional three electrodes glass cell with a sheet of platinum (about 1.0 cm2) as counter electrode, a saturated calomel electrode (SCE, +0.242 V versus Standard Hydrogen Electrode) as reference electrode connected to the cell by a Luggin capillary, which can minimize the solution resistance, and the sample with an exposed area of 0.785 cm2 as working electrode. The measurement was performed in electrolyte of 3.5 wt.% NaCl (0.6 M, pH = 6.8). To minimize the damage to passive film during EIS testing, the AC excitation amplitude of potential was set at 5.0 mV. The EIS test was conducted prior to the potentiodynamic polarization test, namely both the EIS and potentiodynamic polarization tests were conducted with the same area as working electrode, which could avoid the error caused by different testing areas. It was believed that the corrosion resistance of electroless Ni-P coatings was seriously affected by not only the P content, but also the porosity. Hence, porosity test was conducted by a check solution, which contains 10 g/l K3[Fe(CN)6] and 20 g/l NaCl. A piece of filter paper soaked in the solution was pasted on the sample. After 10 min, the paper was taken from the sample and blue spots on the paper were counted to evaluate the porosity. In this test, no blue spots were observed, which indicated there was no pores on the sample. This result was believable because the deposition rate
of the EN plating bath supplied by OMG was 22–24 mm/h, depositing for 40 min could make the coating thick enough such that no through pore could be found in coating. 2.4.1. Electrochemical impedance spectroscopy Before the EIS test, the sample was mounted in the three electrodes glass cell and the working electrode area was immersed in electrolyte for about 15 min to obtain a steady open circuit potential (OCP). The impedance data were collected in the frequency range from 100 kHz to 0.01 Hz with excitation amplitude of 5 mV under open circuit conditions. After the experiments, the impedance data were plotted as Bode plots. 2.4.2. Potentiodynamic polarization After EIS measurements, the system was settled for 5 min to attain its stable OCP again, then the potentiodynamic polarization measurements were performed by CHI660B electrochemical workstation from 250 mV to +250 mV (with respect to the OCP) at a scan rate of 1 mV/s. The data were recorded as Tafel plots. The corrosion potential (Ecorr), the corrosion current density (icorr) and polarization resistance (Rp) were deduced from the Tafel plot. The corrosion current was obtained using the Stern–Geary equation. 2.5. X-ray photoelectron spectroscopy analysis The passive film was analyzed by XPS, which was carried out with a PHI 5700 ESCA System equipped with a dual anode X-ray source (Mg/Al) and a spherical capacitor analyser (SCA). The X-ray source was operated at 15 kV and 400 W. The electrons emitted from the sample were detected at an angle of 458 with respect to the sample surface. Overview spectra were recorded in the range of 0–1350 eV with constant analyzer pass energy of 187.85 eV. The surface was sputtered using a 3000 eV Ar+ ion beam over an area of 4 mm2 4 mm2 to analyze the change of elemental contents with the depth of passive film. The overview spectra were recorded after the passivated surface was sputtered by Ar+ ion for 0, 1 and 3 min, respectively. It should be noted that before sputtering treatment, the sputter rate of Ar+ to the surface was calibrated with SiO2 as 2 nm/min. The intensities of the Cr 2p, O 1 s and P 2p photoelectron lines were recorded separately with constant analyzer pass energy of 29.35 eV. To exclude any effects on the values of binding energies (BE) due to charging of the sample during the XPS analysis, all data were corrected according to the standard BE of C 1 s. In this work, our testing values were adjusted by a linear shift so that the BE of C 1 s is 284.8 eV [25]. To determine the possible forms of Cr in the passive film, the XPS data for Cr and O were fitted by an XPS analysis software – XPSpeak. 3. Results and discussion 3.1. Acid exposure test In the acid exposure test, the blackening time for as-plated Ni-P coating is 3–7 s; while for the chromate-treated surface, the time is generally more than 300 s. It is easy to conclude from this result that the passivated coating possesses an obvious benefit over the untreated one when exposed under strong oxidizing condition. 3.2. Electrochemical tests 3.2.1. Potentiodynamic polarization Fig. 1 shows the potentiodynamic polarization curves of Ni-P coating and passivated Ni-P coating in 3.5% NaCl solution. From the curves, it can be clearly seen that the passivated sample has a lower icorr than the unpassivated coating. Unlike the Ecorr of Cr6+-treated Zn moving towards positive direction [26], the Ecorr of passivated
S. Mu et al. / Applied Surface Science 256 (2010) 4089–4094
Fig. 1. Potentiodynamic polarization curves for Ni-P coating and passivated Ni-P coating in 3.5% NaCl solution.
Table 1 Corrosion potential (Ecorr), corrosion current (icorr) and polarization resistance (Rp) for Ni-P coating and the passivated Ni-P coating. Type of surface
Ecorr (V vs SCE)
icorr (mA/cm2)
Rp( 104 Vcm2)
Ni-P coating Passivated Ni-P coating
0.34 0.55
5.89 0.22
1.02 30.81
SCE: saturated calomel electrode.
Ni-P coating shifts about 200 mV towards the negative direction compared to the as-plated coating. From the Tafel polarization curves, icorr, Ecorr and the according Rp were calculated and listed in Table 1. The icorr for Ni-P coating is about 6.0 mAcm2, while only 0.22 mAcm2 was determined for passivated one. This result indicates that the passive film dramatically lowered the icorr, and accordingly slowed down the corrosion rate. The polarization resistance of the passivated coating is 307890 Vcm2, about 30 times higher than that of the as-plated Ni-P coating. 3.2.2. Electrochemical impedance spectrum studies Fig. 2 shows the Bode plot for as-plated coating and passivated coating obtained in 3.5% NaCl with 5 mV perturbation amplitude against their respective OCP. The curves for the as-plated and passivated coating exhibit a single inflection point, but the whole curve for passivated one is above that of Ni-P coating, which indicates that the passive film on Ni-P coating brings a higher jZj in comparison with the as-plated coating. This higher jZj means that
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Fig. 3. Overview X-ray photoelectron spectroscopy spectra for the passivated Ni-P coating sputtered by Ar+ for 0 min (a), 1 min (b), and 3 min (c).
the passivated coating has a better anticorrosion property in corrosion medium. In the curves of phase angle versus log f, the asplated coating shows a single phase angle maximum, close to 908, indicating a pure capacitive behavior. This means that the process involves only a single time constant. A similar conclusion was drawn by Liu et al. [17] and Balaraju et al. [27]. The curve for passivated coating shows a single but wider peak, angle maximum almost 908, but the shape of the curve is different from the asplated coating. It can be seen two inflexion points appear on the curve at about 10 and 300 Hz, which indicates this process might involves two time constants. However, the short distance between the two time constants on log f-axis makes the two peaks overlapped and look like one broadened peak. 3.3. X-ray photoelectron spectroscopy studies 3.3.1. Chemical composition and thickness of the passive film Fig. 3 shows the overview XPS spectra of the passivated Ni-P coating sputtered by Ar+ ion for 0, 1 and 3 min, respectively. In the 0-min sputtered spectrum Fig. 3a, it can be seen that the outermost surface contains C, O, Ni, Cr and Cl. Their contents were calculated by the software attached with the XPS testing device and listed in Table 2. A substantial amount of C is detected on surface, about 40 at.%. Perhaps the chromate conversion coatings could easily adsorb C, and the samples were in the ambient atmosphere before XPS analysis. The same phenomenon was mentioned by Berger et al. [24]. Since no chemical agent containing Cl was used in passivation bath, it can be inferred that Cl is a contaminant. Although the C is a contamination, it is useful for the analysis of XPS spectrum. In many literatures [24,25,28], the C1 s was usually used as a standard to calibrate the XPS spectrum. To exclude the disturbance of contaminant elements C and Cl, the testing sample was sputtered by Ar+ for 1 min and then the XPS spectrum was recorded Fig. 3b. From the spectrum, it can easily be seen that the C and Cl were removed from the surface by sputtering. Only O, Ni and Cr were detected, their contents were calculated and also tabulated in Table 2. To evaluate the Table 2 Elemental contents on the surface of passivated coating sputtered by Ar+. Sputtering time
Fig. 2. Bode plots for Ni-P coating and the passivated Ni-P coating.
0 min 1 min 3 min
Elemental content (at.%) C
O
Ni
Cr
Cl
P
40.1 – –
43.1 59.4 40.4
1.0 5.6 27.5
14.1 35.0 26.6
1.7 – –
– – 5.5
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Fig. 4. X-ray photoelectron spectroscopy spectra of element P detected on the surface sputtered by Ar+ for 3 min. Fig. 5. X-ray photoelectron spectroscopy spectrum for element Ni detected on passivated Ni-P coating.
change of elemental contents with the depth of passive film, the testing sample was sputtered for another 2 min (considering the former 1 min, totally sputtered for 3 min). Four elements were detected on the surface: O, Ni, Cr and P. The appearance of P is an important sign, which means the passive film was sputtered to its bottom and the P in Ni-P coating was detected. For confirming the chemical state of P, its high-resolution XPS spectrum was analyzed. As can be seen in Fig. 4, the peak at 130.0 eV is attributed to P 2p. By analyzing its binding energy, it can be concluded that the detected P should be elementary substance, indicating it comes from Ni-P coating. In a previous work reported by Li et al. [25], the peak of P 2p in Ni-P coating was observed at the same binding energy. By the appearance of P on the 3 min-sputtered surface, it can be naturally concluded that the passive film was sputtered to its bottom by Ar+ sputtering for 3 min. Taking into account the sputtering rate of Ar+ to the testing surface is about 2 nm/min, the thickness of passive film was 6 nm or so. Thus, it is reasonable that the elements and their according concentrations on 1 min-sputtered interface was taken as the main composition of the passive film. 3.3.2. Discussion on chemical state of Cr, Ni and O To determine the chemical state of the elements in passive film, the high-resolution spectra of Cr 2p, Ni 2p and O 1 s for 1 minsputtered interface were analyzed by XPS analysis software–XPSpeak 4.1. As can be seen in Fig. 5, the peak at the BE of 852.3 eV is attributed to Ni 2p3/2, while that at 869.6 eV is attributed to Ni 2p1/2. By the peak position, it can be easily concluded that the detected Ni element is in the form of metallic Ni [29]. This may indicate that the passive film is too thin to cover Ni-P coating very well. Generally, the Cr in chromate (VI) passive films on Zn [24], Mg [22] and Al [30] involve hexavalence and trivalence. There is a broadly accepted model to explain the excellent corrosion resistance of chromate (VI) conversion film. In this model, the insoluble trivalent Cr compounds (Cr2O3 and/or Cr(OH)3) form an extremely protective film, while the soluble hexavalent Cr compounds (CrO42 and/or Cr2O72) are filled in the tiny gaps among the insoluble compounds. It has been suggested that there is a kind of covalent Cr(III)-O-Cr(VI) bond between the soluble and insoluble Cr species [31]. When scratched or damaged, the film can release soluble chromate to react with the fresh metal exposed because of damage, and build new passive film again on the damaged area [21–23,30,32]. This process is called ‘‘self-healing’’ effect on the chromate (VI) passive film. According to the literatures [33–36], the binding energies for Cr(VI) species in XPS spectra are generally located at 578-
580 eV, while the BE for Cr(III) is smaller than 577.5 eV. In our experiment on chromate (VI) Ni-P coating, the maximum of Cr peak (Fig. 6) appears at 577.2 eV, which means that although treated in Cr6+-containing bath, there is no Cr(VI) species contained in passive film. Two component curves were used to fit the Cr 2p curve to evaluate the possible existence forms of trivalent Cr compounds. As shown in Fig. 7a, the fitted peak 1 at binding energy of 576.5 eV should originate from the Cr-O bond in Cr2O3 (labeled as Cr(a)). The fitted peak 2 at 577.2 eV is attributed to Cr(OH)3 (labeled as Cr(b)) [34,37]. In order to further verify the rationality of fitting Cr as Cr2O3 and Cr(OH)3, the high-resolution spectra of O 1 s were also analyzed. The O 1 s line is asymmetrical, implying that more than one oxygen species exists in the passive film. Taking into account the detected elements are Ni, Cr and O, and Ni is in the form of metallic state, the possible states of O in film are Cr2O3 and Cr(OH)3. The O 1 s were decomposed with these two species, which were symbolized by O(a) (for Cr2O3) and O(b) (for Cr(OH)3), see in Fig. 7b. The concerning fitting parameters (peak area and full width at half maximum, FWHM) were summarized in Table 3. The quantities of Cr in Cr2O3 and Cr(OH)3, O in Cr2O3 and Cr(OH)3) were respectively calculated by their peak areas. About 87.0 at.% of the total Cr is in the form of Cr2O3, while Cr(OH)3 accounts for the rest 13.0 at.%. As to the O element, 76.6 at.% of the total O exists in Cr2O3 and 23.4 at.% is in Cr(OH)3. Based on the elemental contents on the 1 min-sputtered interface in Table 2,
Fig. 6. High-resolution X-ray photoelectron spectroscopy spectra of Cr 2p for Cr6+treated Ni-P coating.
S. Mu et al. / Applied Surface Science 256 (2010) 4089–4094
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Fig. 7. Peak fitting for X-ray photoelectron spectroscopy spectra: (a) Cr 2p; (b) O 1 s.
Table 3 Summary of fitting for Cr 2p, O 1 s and Ni 2p detected in passive film. Fitting species
BE (eV)
FWHM
Peak area
Content (at.%)
Cr(a) (in Cr2O3) Cr(b) (in Cr(OH)3) O(a) (in Cr2O3) O(b) (in Cr(OH)3) Ni
576.5 577.2 530.9 532.4 –
3.0 2.9 2.1 1.7 –
32841.3 4899.7 17595.5 5366.1 –
30.5 4.5 45.5 13.9 5.6
In our passivation system, the possible reactants for anodic reaction are Ni and P. The XPS results showed that no element P was detected in the passive film, suggesting two possibilities: the P in Ni-P coating does not participate in the passivation reaction; the P takes part in the reaction, but the products of P dissolve in the bath and do not precipitate in the passive film.
BE: binding energies.
the percentage of Cr with different chemical states was calculated and also listed in Table 3. From the ratio of Cr(a) to O(a) (30.5/ 45.5), Cr(b) to O(b) (4.5/13.9), it is clear that the results of curvefitting are satisfactory. The above results prove again that there are no hexavalent Cr compounds existing in the passive film. The present results are far different from those researches on passivation on Zn, Al (alloy) and Sn (alloy) coating [21,22,34,38]. On the chromate (VI)-treated surfaces of these metallic coating, both trivalent and hexavalent Cr were involved. In the light of above analysis, the elements detected on the passivated coating involves Cr, Ni and O. As can be seen in Table 3, on the 1 min-sputtered interface, the Cr atoms in Cr2O3 and Cr(OH)3 account respectively for 30.5 at.% and 4.5 at.% of the total detected atoms. Unlike the thickness of chromate (VI) passive films on Zn or Al, usually more than tens of nanometers [18,26,33,34], on Ni-P coating the passive film obtained in Cr6+-containing bath is only 6 nm or so based on the sputtering rate of Ar+ to the tested surface. But this very thin film owns excellent anticorrosion properties according to the HNO3 exposure test and electrochemical tests. It should be noted here that this passive film is difficult to thicken. For the chromate (VI) passive film on Zn, two factors can be used to explain its high corrosion resistance: the ‘‘self-healing’’ effect of Cr6+ and the thickness of the film. As to the chromate (VI) passive film on Ni-P coating, although the passive film is thin and no Cr6+ contained in film, it still possesses excellent corrosion properties. This perhaps can be explained by its dense structure and relatively simple composition, main Cr, O and small part of Ni. Obviously, the passivation reactions can occur when Ni-P coating immersed in K2Cr2O7 solution, and based on the chemical state of Cr species in passive film, it can be naturally deduced a cathodic reaction on the surface of Ni-P coating: Cr2 O7 2 þ 14Hþ þ 6e ! 2Cr3þ þ 7H2 O According to the previous research conducted by Lu et al. [39], an anodic reaction will take place when Ni-based alloys were immersed in passivation bath: Ni ! Ni2þ þ 2e
Whether will element P participate passivation reactions or not? The detailed studies will be conducted and reported in our following research. 4. Conclusions The Ecorr of the chromate-treated coating shifted about 200 mV towards negative direction in comparison with that of ENPC. The Icorr of the treated sample decreased to 1/30 of the untreated coating. This indicates that the corrosion resistance of the coating was improved dramatically by passivation treatment. XPS analysis illustrates that the passive film was mainly made up of Cr, Ni and O. The thickness of the film was about 6 nm. High-resolution XPS and fitting analysis indicates that no hexavalent Cr could be detected in passive film, Cr was in the form of Cr2O3 (87.0 at.% of total Cr) and Cr(OH)3 (13.0 at.%). Acknowledgements This research was financed by the Suzhou branch of OMG Inc. of America. Professor Ming-ren Sun and Mr. Yang are gratefully acknowledged for sample testing and helpful discussion. References [1] J.H. Moon, K.H. Kim, H.W. Choi, S.W. Lee, S.J. Park, Ultramicroscopy 108 (2008) 1307–1310. [2] H. Zhao, J.Z. Cui, Surf. Coat. Tech 201 (2007) 4512–4517. [3] T.X. Liang, W.L. Guo, Y.H. Yan, C.H. Tang, Intern. J. Adhes. Adhes. 28 (2008) 55– 58. [4] D.J. Dı´az, T.L. Williamson, X.Y. Guo, A. Sood, P.W. Bohn, Thin Solid Films 514 (2006) 120–126. [5] J.H. Byeon, H.S. Yoon, K.Y. Yoon, S.K. Ryu, J. Hwang, Surf. Coat. Technol 202 (2008) 3571–3578. [6] C.D. Gu, J.S. Lian, G.Y. Li, L.Y. Niu, Z.H. Jiang, Surf. Coat. Technol 197 (2005) 61–67. [7] Y.D. He, H.F. Fu, X.G. Li, W. Gao, Scrip. Mater 58 (2008) 504–507. [8] M. Palaniappa, S.K. Seshadri, Wear 265 (2008) 735–740. [9] M. Crobu, A. Scorciapino, B. Elsener, A. Rossi, Electrochim. Acta 53 (2008) 3364– 3370. [10] C.K. Lee, Surf. Coat. Tech 202 (2008) 4868–4874. [11] D.D.N. Singh, R. Ghosh, Surf. Coat. Technol 201 (2006) 90–101. [12] Y.W. Song, D.Y. Shan, E.H. Han, Electrochim. Acta 53 (2008) 2135–2143. [13] G.J. Lu, G. Zangari, Electrochim. Acta 47 (2002) 2969–2979. [14] G.F. Cui, N. Li, D.Y. Li, Surf. Coat. Technol 200 (2006) 6808–6814. [15] P. Snugovsky, P. Arrowsmith, M. Romansky, J. Electron. Mater 30 (2001) 1262–1270.
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Corrosion behavior and composition analysis of chromate passive film on electroless Ni-P coating Songlin Mu, Ning Li *, Deyu Li, Liying Xu Department of applied chemistry, Harbin Institute of technology, Harbin 150001, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 November 2009 Accepted 27 January 2010 Available online 4 February 2010
A passive film was formed on electroless Ni-P coating (ENPC) in a bath of K2Cr2O7 30 g/l. XPS and electrochemical methods were employed to analyze its chemical compositions and corrosion behaviors. The potentiodynamic polarization tests indicated the corrosion current of the passivated sample was 1/ 30 that of as-plated ENPC. The XPS analysis illustrated the film comprised Cr, Ni and O. The film thickness was evaluated to be a few nanometers according to the sputtering rate of Ar+ ion. High-resolution XPS spectra suggested that the detected Cr in film was in the form of trivalent compounds, Cr2O3 and Cr(OH)3. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Passive film Electroless Ni-P coating Corrosion behaviors XPS analysis
1. Introduction Electroplating and electroless deposition are two common processes to coat metallic layers on many engineering components. In comparison, an electroless process is more preferred in industry than electroplating because the former can deposit a homogeneous coating on components with very complicated geometries. Electroless method includes electroless Ag [1–3], Au, Pt [4], Cu [5] and electroless Ni-P coating (ENPC) [6,7]. In these methods, electrodes Ni-P coating (ENPC) receives lots of attention due to its shiny appearance, high hardness, uniform thickness, as well as very good wear and corrosion resistance [8–14]. The ENPC has been widely used in electronic industry involving electromagnetic shielding, computer storage, microelectronics such as computer hard disk, wafer, printed circuit board and so on [15,16]. In spite of its excellent properties, the ENPC is apt to be oxidized in air, which makes the surface lose its brightness, even discolor. Furthermore, Ni oxidization will seriously decrease the solderability of the coating, leading to a fatal defect to some electronic components. In the industrial field, many factories avoid this problem by passivation treatments with chromate (VI). For example, the OM Group Inc. (OMG) of America applied a treatment bath: K2Cr2O7 30 g/l (without adjusting pH, original pH = 3.7–3.8). This passivation treatment does not bring any change in the appearance; moreover, it can improve corrosion resistance. More importantly, the solderability of the coating will not be obviously
* Corresponding author. Tel.: +86 451 86413721; fax: +86 451 86221048. E-mail addresses: [email protected] (S. Mu), [email protected] (N. Li). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.089
affected by this treatment. Since the surface of ENPC will turn black or dark in strong oxidizing acid, therefore HNO3 solution can be used as a simplified method to evaluate the corrosion resistance of the passive film on ENPC [17]. Researches concerning the formation and anticorrosion mechanisms of chromate (VI) passive films on Zn, Al, stainless steel etc. have been extensively conducted, and several models have been proposed [18–20]. The forming process generally contains two steps: firstly, the dissolution of metallic matrices increases the concentration of metallic ions and decreases the concentration of H+ in the solution layer very close to the surface of matrices. Secondly, with the concentration of the metallic ions and pH value increasing, metal ions can not stay stably in solution and consequently form insoluble hydroxides which precipitate on matrices. The chromate passive films generally involve trivalent and hexavalent species. Mccreery et al. suggested that these two kinds of species were in the form of a sol-gel, in which Cr(III) species formed the matrix in which Cr(VI) species were trapped [21]. The presence of hexavalent Cr was considered as the main reason that makes the chromate passive films possess a ‘‘selfhealing’’ effect, and was also taken as the main factor that can explain the excellent anticorrosion property of chromate (VI) passive films [22–24]. Although the chromate (VI) treated Ni-P components can meet the industrial demands for solderability and discoloration, the usage of chromate is being progressively restricted due to its toxicity to the environment. From the viewpoint of sustainable development and environmental safety, it is necessary to develop a Cr-free technique. But firstly, we should understand the mechanism of chromate (VI) treatment for Ni-P coating. So far, to the best
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of our knowledge, little work was reported on Cr-free or chromate (III) or even chromate (VI) methods for ENPC. This work aims at studying the composition and the corrosion properties of the passive film on ENPC. 2. Experimental details 2.1. Preparation of Ni-P coating From a commercially operated medium-phosphorus-content nickel bath supplied by OMG, Ni-P coating about 14 mm thick was deposited on a substrate of mild steel sheets. Prior to deposition, the sheets were first mechanically polished with 1000 grade of SiC paper and then degreased by immersion in 10% NaOH at room temperature for 5 min, followed by deionized (DI) water rinse and immersion in acetone. Then, the sheets were activated in 5% HCl solution for 10 s followed by rinsing in DI water. After these procedures, the electroless plating was immediately carried out at 88 2 8C for 40 min with pH of 4.75 (adjusting at room temperature about 20–25 8C). 2.2. Passivation treatment The plated sheets were firstly rinsed with DI water, then passivated immediately in the passivation bath at 60 8C for 10 min. The bath only contains 30 g/l K2Cr2O7, without adjusting pH value. After passivation, the samples were rinsed twice in DI water and finally dried in oven at 120 8C for 20 min. 2.3. Acid exposure test 50 vol.% HNO3 was taken as a testing agent to perform the acid exposure test. The time when HNO3 was dropped on surface of sample to when the surface turned black/dark was recorded to evaluate the corrosion resistance of the as-plated and passivated coatings. For each sample, the acid exposure test will be conducted at least three times at different spots. 2.4. Electrochemical corrosion tests Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were carried out by CHI660B electrochemical workstation (Shanghai Chenhua instrument company, Shanghai, China). The sample was loaded in a conventional three electrodes glass cell with a sheet of platinum (about 1.0 cm2) as counter electrode, a saturated calomel electrode (SCE, +0.242 V versus Standard Hydrogen Electrode) as reference electrode connected to the cell by a Luggin capillary, which can minimize the solution resistance, and the sample with an exposed area of 0.785 cm2 as working electrode. The measurement was performed in electrolyte of 3.5 wt.% NaCl (0.6 M, pH = 6.8). To minimize the damage to passive film during EIS testing, the AC excitation amplitude of potential was set at 5.0 mV. The EIS test was conducted prior to the potentiodynamic polarization test, namely both the EIS and potentiodynamic polarization tests were conducted with the same area as working electrode, which could avoid the error caused by different testing areas. It was believed that the corrosion resistance of electroless Ni-P coatings was seriously affected by not only the P content, but also the porosity. Hence, porosity test was conducted by a check solution, which contains 10 g/l K3[Fe(CN)6] and 20 g/l NaCl. A piece of filter paper soaked in the solution was pasted on the sample. After 10 min, the paper was taken from the sample and blue spots on the paper were counted to evaluate the porosity. In this test, no blue spots were observed, which indicated there was no pores on the sample. This result was believable because the deposition rate
of the EN plating bath supplied by OMG was 22–24 mm/h, depositing for 40 min could make the coating thick enough such that no through pore could be found in coating. 2.4.1. Electrochemical impedance spectroscopy Before the EIS test, the sample was mounted in the three electrodes glass cell and the working electrode area was immersed in electrolyte for about 15 min to obtain a steady open circuit potential (OCP). The impedance data were collected in the frequency range from 100 kHz to 0.01 Hz with excitation amplitude of 5 mV under open circuit conditions. After the experiments, the impedance data were plotted as Bode plots. 2.4.2. Potentiodynamic polarization After EIS measurements, the system was settled for 5 min to attain its stable OCP again, then the potentiodynamic polarization measurements were performed by CHI660B electrochemical workstation from 250 mV to +250 mV (with respect to the OCP) at a scan rate of 1 mV/s. The data were recorded as Tafel plots. The corrosion potential (Ecorr), the corrosion current density (icorr) and polarization resistance (Rp) were deduced from the Tafel plot. The corrosion current was obtained using the Stern–Geary equation. 2.5. X-ray photoelectron spectroscopy analysis The passive film was analyzed by XPS, which was carried out with a PHI 5700 ESCA System equipped with a dual anode X-ray source (Mg/Al) and a spherical capacitor analyser (SCA). The X-ray source was operated at 15 kV and 400 W. The electrons emitted from the sample were detected at an angle of 458 with respect to the sample surface. Overview spectra were recorded in the range of 0–1350 eV with constant analyzer pass energy of 187.85 eV. The surface was sputtered using a 3000 eV Ar+ ion beam over an area of 4 mm2 4 mm2 to analyze the change of elemental contents with the depth of passive film. The overview spectra were recorded after the passivated surface was sputtered by Ar+ ion for 0, 1 and 3 min, respectively. It should be noted that before sputtering treatment, the sputter rate of Ar+ to the surface was calibrated with SiO2 as 2 nm/min. The intensities of the Cr 2p, O 1 s and P 2p photoelectron lines were recorded separately with constant analyzer pass energy of 29.35 eV. To exclude any effects on the values of binding energies (BE) due to charging of the sample during the XPS analysis, all data were corrected according to the standard BE of C 1 s. In this work, our testing values were adjusted by a linear shift so that the BE of C 1 s is 284.8 eV [25]. To determine the possible forms of Cr in the passive film, the XPS data for Cr and O were fitted by an XPS analysis software – XPSpeak. 3. Results and discussion 3.1. Acid exposure test In the acid exposure test, the blackening time for as-plated Ni-P coating is 3–7 s; while for the chromate-treated surface, the time is generally more than 300 s. It is easy to conclude from this result that the passivated coating possesses an obvious benefit over the untreated one when exposed under strong oxidizing condition. 3.2. Electrochemical tests 3.2.1. Potentiodynamic polarization Fig. 1 shows the potentiodynamic polarization curves of Ni-P coating and passivated Ni-P coating in 3.5% NaCl solution. From the curves, it can be clearly seen that the passivated sample has a lower icorr than the unpassivated coating. Unlike the Ecorr of Cr6+-treated Zn moving towards positive direction [26], the Ecorr of passivated
S. Mu et al. / Applied Surface Science 256 (2010) 4089–4094
Fig. 1. Potentiodynamic polarization curves for Ni-P coating and passivated Ni-P coating in 3.5% NaCl solution.
Table 1 Corrosion potential (Ecorr), corrosion current (icorr) and polarization resistance (Rp) for Ni-P coating and the passivated Ni-P coating. Type of surface
Ecorr (V vs SCE)
icorr (mA/cm2)
Rp( 104 Vcm2)
Ni-P coating Passivated Ni-P coating
0.34 0.55
5.89 0.22
1.02 30.81
SCE: saturated calomel electrode.
Ni-P coating shifts about 200 mV towards the negative direction compared to the as-plated coating. From the Tafel polarization curves, icorr, Ecorr and the according Rp were calculated and listed in Table 1. The icorr for Ni-P coating is about 6.0 mAcm2, while only 0.22 mAcm2 was determined for passivated one. This result indicates that the passive film dramatically lowered the icorr, and accordingly slowed down the corrosion rate. The polarization resistance of the passivated coating is 307890 Vcm2, about 30 times higher than that of the as-plated Ni-P coating. 3.2.2. Electrochemical impedance spectrum studies Fig. 2 shows the Bode plot for as-plated coating and passivated coating obtained in 3.5% NaCl with 5 mV perturbation amplitude against their respective OCP. The curves for the as-plated and passivated coating exhibit a single inflection point, but the whole curve for passivated one is above that of Ni-P coating, which indicates that the passive film on Ni-P coating brings a higher jZj in comparison with the as-plated coating. This higher jZj means that
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Fig. 3. Overview X-ray photoelectron spectroscopy spectra for the passivated Ni-P coating sputtered by Ar+ for 0 min (a), 1 min (b), and 3 min (c).
the passivated coating has a better anticorrosion property in corrosion medium. In the curves of phase angle versus log f, the asplated coating shows a single phase angle maximum, close to 908, indicating a pure capacitive behavior. This means that the process involves only a single time constant. A similar conclusion was drawn by Liu et al. [17] and Balaraju et al. [27]. The curve for passivated coating shows a single but wider peak, angle maximum almost 908, but the shape of the curve is different from the asplated coating. It can be seen two inflexion points appear on the curve at about 10 and 300 Hz, which indicates this process might involves two time constants. However, the short distance between the two time constants on log f-axis makes the two peaks overlapped and look like one broadened peak. 3.3. X-ray photoelectron spectroscopy studies 3.3.1. Chemical composition and thickness of the passive film Fig. 3 shows the overview XPS spectra of the passivated Ni-P coating sputtered by Ar+ ion for 0, 1 and 3 min, respectively. In the 0-min sputtered spectrum Fig. 3a, it can be seen that the outermost surface contains C, O, Ni, Cr and Cl. Their contents were calculated by the software attached with the XPS testing device and listed in Table 2. A substantial amount of C is detected on surface, about 40 at.%. Perhaps the chromate conversion coatings could easily adsorb C, and the samples were in the ambient atmosphere before XPS analysis. The same phenomenon was mentioned by Berger et al. [24]. Since no chemical agent containing Cl was used in passivation bath, it can be inferred that Cl is a contaminant. Although the C is a contamination, it is useful for the analysis of XPS spectrum. In many literatures [24,25,28], the C1 s was usually used as a standard to calibrate the XPS spectrum. To exclude the disturbance of contaminant elements C and Cl, the testing sample was sputtered by Ar+ for 1 min and then the XPS spectrum was recorded Fig. 3b. From the spectrum, it can easily be seen that the C and Cl were removed from the surface by sputtering. Only O, Ni and Cr were detected, their contents were calculated and also tabulated in Table 2. To evaluate the Table 2 Elemental contents on the surface of passivated coating sputtered by Ar+. Sputtering time
Fig. 2. Bode plots for Ni-P coating and the passivated Ni-P coating.
0 min 1 min 3 min
Elemental content (at.%) C
O
Ni
Cr
Cl
P
40.1 – –
43.1 59.4 40.4
1.0 5.6 27.5
14.1 35.0 26.6
1.7 – –
– – 5.5
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Fig. 4. X-ray photoelectron spectroscopy spectra of element P detected on the surface sputtered by Ar+ for 3 min. Fig. 5. X-ray photoelectron spectroscopy spectrum for element Ni detected on passivated Ni-P coating.
change of elemental contents with the depth of passive film, the testing sample was sputtered for another 2 min (considering the former 1 min, totally sputtered for 3 min). Four elements were detected on the surface: O, Ni, Cr and P. The appearance of P is an important sign, which means the passive film was sputtered to its bottom and the P in Ni-P coating was detected. For confirming the chemical state of P, its high-resolution XPS spectrum was analyzed. As can be seen in Fig. 4, the peak at 130.0 eV is attributed to P 2p. By analyzing its binding energy, it can be concluded that the detected P should be elementary substance, indicating it comes from Ni-P coating. In a previous work reported by Li et al. [25], the peak of P 2p in Ni-P coating was observed at the same binding energy. By the appearance of P on the 3 min-sputtered surface, it can be naturally concluded that the passive film was sputtered to its bottom by Ar+ sputtering for 3 min. Taking into account the sputtering rate of Ar+ to the testing surface is about 2 nm/min, the thickness of passive film was 6 nm or so. Thus, it is reasonable that the elements and their according concentrations on 1 min-sputtered interface was taken as the main composition of the passive film. 3.3.2. Discussion on chemical state of Cr, Ni and O To determine the chemical state of the elements in passive film, the high-resolution spectra of Cr 2p, Ni 2p and O 1 s for 1 minsputtered interface were analyzed by XPS analysis software–XPSpeak 4.1. As can be seen in Fig. 5, the peak at the BE of 852.3 eV is attributed to Ni 2p3/2, while that at 869.6 eV is attributed to Ni 2p1/2. By the peak position, it can be easily concluded that the detected Ni element is in the form of metallic Ni [29]. This may indicate that the passive film is too thin to cover Ni-P coating very well. Generally, the Cr in chromate (VI) passive films on Zn [24], Mg [22] and Al [30] involve hexavalence and trivalence. There is a broadly accepted model to explain the excellent corrosion resistance of chromate (VI) conversion film. In this model, the insoluble trivalent Cr compounds (Cr2O3 and/or Cr(OH)3) form an extremely protective film, while the soluble hexavalent Cr compounds (CrO42 and/or Cr2O72) are filled in the tiny gaps among the insoluble compounds. It has been suggested that there is a kind of covalent Cr(III)-O-Cr(VI) bond between the soluble and insoluble Cr species [31]. When scratched or damaged, the film can release soluble chromate to react with the fresh metal exposed because of damage, and build new passive film again on the damaged area [21–23,30,32]. This process is called ‘‘self-healing’’ effect on the chromate (VI) passive film. According to the literatures [33–36], the binding energies for Cr(VI) species in XPS spectra are generally located at 578-
580 eV, while the BE for Cr(III) is smaller than 577.5 eV. In our experiment on chromate (VI) Ni-P coating, the maximum of Cr peak (Fig. 6) appears at 577.2 eV, which means that although treated in Cr6+-containing bath, there is no Cr(VI) species contained in passive film. Two component curves were used to fit the Cr 2p curve to evaluate the possible existence forms of trivalent Cr compounds. As shown in Fig. 7a, the fitted peak 1 at binding energy of 576.5 eV should originate from the Cr-O bond in Cr2O3 (labeled as Cr(a)). The fitted peak 2 at 577.2 eV is attributed to Cr(OH)3 (labeled as Cr(b)) [34,37]. In order to further verify the rationality of fitting Cr as Cr2O3 and Cr(OH)3, the high-resolution spectra of O 1 s were also analyzed. The O 1 s line is asymmetrical, implying that more than one oxygen species exists in the passive film. Taking into account the detected elements are Ni, Cr and O, and Ni is in the form of metallic state, the possible states of O in film are Cr2O3 and Cr(OH)3. The O 1 s were decomposed with these two species, which were symbolized by O(a) (for Cr2O3) and O(b) (for Cr(OH)3), see in Fig. 7b. The concerning fitting parameters (peak area and full width at half maximum, FWHM) were summarized in Table 3. The quantities of Cr in Cr2O3 and Cr(OH)3, O in Cr2O3 and Cr(OH)3) were respectively calculated by their peak areas. About 87.0 at.% of the total Cr is in the form of Cr2O3, while Cr(OH)3 accounts for the rest 13.0 at.%. As to the O element, 76.6 at.% of the total O exists in Cr2O3 and 23.4 at.% is in Cr(OH)3. Based on the elemental contents on the 1 min-sputtered interface in Table 2,
Fig. 6. High-resolution X-ray photoelectron spectroscopy spectra of Cr 2p for Cr6+treated Ni-P coating.
S. Mu et al. / Applied Surface Science 256 (2010) 4089–4094
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Fig. 7. Peak fitting for X-ray photoelectron spectroscopy spectra: (a) Cr 2p; (b) O 1 s.
Table 3 Summary of fitting for Cr 2p, O 1 s and Ni 2p detected in passive film. Fitting species
BE (eV)
FWHM
Peak area
Content (at.%)
Cr(a) (in Cr2O3) Cr(b) (in Cr(OH)3) O(a) (in Cr2O3) O(b) (in Cr(OH)3) Ni
576.5 577.2 530.9 532.4 –
3.0 2.9 2.1 1.7 –
32841.3 4899.7 17595.5 5366.1 –
30.5 4.5 45.5 13.9 5.6
In our passivation system, the possible reactants for anodic reaction are Ni and P. The XPS results showed that no element P was detected in the passive film, suggesting two possibilities: the P in Ni-P coating does not participate in the passivation reaction; the P takes part in the reaction, but the products of P dissolve in the bath and do not precipitate in the passive film.
BE: binding energies.
the percentage of Cr with different chemical states was calculated and also listed in Table 3. From the ratio of Cr(a) to O(a) (30.5/ 45.5), Cr(b) to O(b) (4.5/13.9), it is clear that the results of curvefitting are satisfactory. The above results prove again that there are no hexavalent Cr compounds existing in the passive film. The present results are far different from those researches on passivation on Zn, Al (alloy) and Sn (alloy) coating [21,22,34,38]. On the chromate (VI)-treated surfaces of these metallic coating, both trivalent and hexavalent Cr were involved. In the light of above analysis, the elements detected on the passivated coating involves Cr, Ni and O. As can be seen in Table 3, on the 1 min-sputtered interface, the Cr atoms in Cr2O3 and Cr(OH)3 account respectively for 30.5 at.% and 4.5 at.% of the total detected atoms. Unlike the thickness of chromate (VI) passive films on Zn or Al, usually more than tens of nanometers [18,26,33,34], on Ni-P coating the passive film obtained in Cr6+-containing bath is only 6 nm or so based on the sputtering rate of Ar+ to the tested surface. But this very thin film owns excellent anticorrosion properties according to the HNO3 exposure test and electrochemical tests. It should be noted here that this passive film is difficult to thicken. For the chromate (VI) passive film on Zn, two factors can be used to explain its high corrosion resistance: the ‘‘self-healing’’ effect of Cr6+ and the thickness of the film. As to the chromate (VI) passive film on Ni-P coating, although the passive film is thin and no Cr6+ contained in film, it still possesses excellent corrosion properties. This perhaps can be explained by its dense structure and relatively simple composition, main Cr, O and small part of Ni. Obviously, the passivation reactions can occur when Ni-P coating immersed in K2Cr2O7 solution, and based on the chemical state of Cr species in passive film, it can be naturally deduced a cathodic reaction on the surface of Ni-P coating: Cr2 O7 2 þ 14Hþ þ 6e ! 2Cr3þ þ 7H2 O According to the previous research conducted by Lu et al. [39], an anodic reaction will take place when Ni-based alloys were immersed in passivation bath: Ni ! Ni2þ þ 2e
Whether will element P participate passivation reactions or not? The detailed studies will be conducted and reported in our following research. 4. Conclusions The Ecorr of the chromate-treated coating shifted about 200 mV towards negative direction in comparison with that of ENPC. The Icorr of the treated sample decreased to 1/30 of the untreated coating. This indicates that the corrosion resistance of the coating was improved dramatically by passivation treatment. XPS analysis illustrates that the passive film was mainly made up of Cr, Ni and O. The thickness of the film was about 6 nm. High-resolution XPS and fitting analysis indicates that no hexavalent Cr could be detected in passive film, Cr was in the form of Cr2O3 (87.0 at.% of total Cr) and Cr(OH)3 (13.0 at.%). Acknowledgements This research was financed by the Suzhou branch of OMG Inc. of America. Professor Ming-ren Sun and Mr. Yang are gratefully acknowledged for sample testing and helpful discussion. References [1] J.H. Moon, K.H. Kim, H.W. Choi, S.W. Lee, S.J. Park, Ultramicroscopy 108 (2008) 1307–1310. [2] H. Zhao, J.Z. Cui, Surf. Coat. Tech 201 (2007) 4512–4517. [3] T.X. Liang, W.L. Guo, Y.H. Yan, C.H. Tang, Intern. J. Adhes. Adhes. 28 (2008) 55– 58. [4] D.J. Dı´az, T.L. Williamson, X.Y. Guo, A. Sood, P.W. Bohn, Thin Solid Films 514 (2006) 120–126. [5] J.H. Byeon, H.S. Yoon, K.Y. Yoon, S.K. Ryu, J. Hwang, Surf. Coat. Technol 202 (2008) 3571–3578. [6] C.D. Gu, J.S. Lian, G.Y. Li, L.Y. Niu, Z.H. Jiang, Surf. Coat. Technol 197 (2005) 61–67. [7] Y.D. He, H.F. Fu, X.G. Li, W. Gao, Scrip. Mater 58 (2008) 504–507. [8] M. Palaniappa, S.K. Seshadri, Wear 265 (2008) 735–740. [9] M. Crobu, A. Scorciapino, B. Elsener, A. Rossi, Electrochim. Acta 53 (2008) 3364– 3370. [10] C.K. Lee, Surf. Coat. Tech 202 (2008) 4868–4874. [11] D.D.N. Singh, R. Ghosh, Surf. Coat. Technol 201 (2006) 90–101. [12] Y.W. Song, D.Y. Shan, E.H. Han, Electrochim. Acta 53 (2008) 2135–2143. [13] G.J. Lu, G. Zangari, Electrochim. Acta 47 (2002) 2969–2979. [14] G.F. Cui, N. Li, D.Y. Li, Surf. Coat. Technol 200 (2006) 6808–6814. [15] P. Snugovsky, P. Arrowsmith, M. Romansky, J. Electron. Mater 30 (2001) 1262–1270.
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