Zn Ni alloy coatings pulse-plated on magnesium alloy

Size: px
Start display at page:

Download "Zn Ni alloy coatings pulse-plated on magnesium alloy"

Transcription

1 Surface & Coatings Technology 191 (2005) Zn Ni alloy coatings pulse-plated on magnesium alloy Y.F. Jiang*, C.Q. Zhai, L.F. Liu, Y.P. Zhu, W.J. Ding National Engineering Research Center of Light Alloys Net Forming, School of Materials Science and Engineering, Shanghai Jiaotong University, 1954 Huashan Road, Shanghai , PR China Received 17 December 2003; accepted in revised form 26 March 2004 Abstract Zn Ni alloy coatings are plated on AZ91 magnesium alloy using a pulse potential after an appropriate pretreatment. The current density, frequency, t on /t off and time have effects on thickness, morphology, Ni content, microhardness and anticorrosion of Zn Ni coatings. Corrosion can reach above 200 h in salt spray test according to ASTM B117; the corrosion behavior of coatings is interfacial corrosion of particles and results in particle degradation as observed by scanning electron microscopy. Weight loss method and polarization curves are also used to investigate anticorrosion properties. Bonding strength is above 14.8 MPa in adhesive tensile tests. D 2004 Elsevier B.V. All rights reserved. Keywords: Zn Ni alloy coating; Magnesium alloy; Pulse-plating; Corrosion behavior 1. Introduction Surface treatments for improving the corrosion resistance of magnesium and magnesium alloys are of great importance in many applications. Anode oxidation, micro-arc oxidation or electroless nickel coatings are applied to magnesium alloys for anticorrosion protection at present. However, these methods seem to be inefficient to meet the requirements of anticorrosion properties and conductivity in such industrial applications as automotive, aerospace and marine industries [1,2]. Zn Ni coatings can be formed on the surfaces when pretreatment like zinc immersion or electroless nickel coating is carried out on magnesium alloys. Such coatings are promising to improve the corrosion resistance of magnesium alloys, because Zn Ni alloys have very good anticorrosion properties when the content of Ni is wt.% [3,4] due to the presence of intermetallic phase, viz g-zn 21 Ni 5 [5]. On the other hand, pulse-electrodeposition process has an advantage over continuous process, i.e. pulse current electrodeposition induces a higher rate of grain nucleation and results in a more refined grain structure, which benefits the deposit properties of coatings [6]. Pulseelectrodeposition processes are likely able to form Zn * Corresponding author. Tel.: address: sjtujyf001@sohu.com (Y.F. Jiang). Ni coatings with high corrosion resistance since the composition and structure play important roles in anticorrosion properties of coatings. In this article, Zn Ni coatings are deposited on AZ91 magnesium alloy by a pulse potential process after zinc immersion and Zn Cu alloy plating. A Zn layer and a Zn Cu alloy layer under the Zn Ni coating are applied to improve the adhesion and electrode potential of the surface coating to AZ91 substrate. The mechanisms of the good anticorrosion property of the Zn Ni coatings are also discussed. 2. Experimental AZ91 has a nominal composition as Al 9%, Zn 1% and Mg balance as substrate. Before deposition, substrate surfaces were processed in a standard industrial way: polishing with alumina sand paper, alkaline degreasing, chemical pickling, activation, zinc immersion and Zn Cu alloy plating. A power supplier with rectangular pulse potential was used. And electrodeposition of Zn Ni was carried out in a bath with two anode electrodes that were used to ensure a homogeneous electrical field. The average current density, i m, is defined as Eq. (1): i m ¼ i c t on ðt on þ t off Þ ð1þ /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.surfcoat

2 394 Y.F. Jiang et al. / Surface & Coatings Technology 191 (2005) Fig. 1. Schematic illustration of the pulse potential. where i c is the cathodic current density, t on is the time of the cathodic pulse (on-time) and t off is the time between two continuous pulses (off-time). The pulse electrical process is schematically shown in Fig. 1. Zn Ni coatings were deposited in an alkaline bath with a composition as ZnO 10 g/l, NaOH 150 g/l, NiSO 4 6H 2 O15 g/l, triethanolamine 50 g/l. The process was carried out in a temperature range of jc. The other operating conditions were in Table 1. Corrosion behaviors of the Zn Ni coatings were studied by conducting salt spray tests according to ASTM B117. Samples were exposed to salt spray in a chamber. The corrosion extent was evaluated by calculating the area fraction of the corroded parts, which were occupied by the white rust. A conventional three-electrode cell is used to perform polarization curves in a cell containing 3.5% sodium chloride (NaCl) solutions in room temperature A stainless steel and a saturated calomel electrode (SCE) are used as counter and reference electrode, respectively. Microstructural characterization of corroded surfaces was carried out with optical microscopy (OM) and scanning electron microscopy (SEM). Microhardness testing was conducted with Vickers microhardness instrument. Chemical analyses were performed with energy-dispersive X-ray spectroscopy (EDXS). 3. Results and discussion 3.1. Coating of Zn Ni alloy on AZ91 Before the coating process of Zn Ni, the specimens were processed with a procedure as polishing, chemical cleaning, acid pickling, activation, zinc immersion and Zn Cu plating. A zinc layer, a Zn Cu alloy layer and a Zn Ni alloy layer are formed subsequently on the substrate surface, as shown in Fig. 2. Zinc immersion layer can mainly improve adhesion between the outer layers and substrate. At the same time, it enhances electrode potential of surface on the substrate surface. Coating of a Zn Cu layer is aimed to make use of the small electrode potential difference between Zn Cu layer and Zn Ni layer to protect the substrate. Since Zn Ni and Zn Cu layers have same Zn matrix that is similar to g-brass crystal structure, these layers can be expected to have strong adhesion. Some microvoids can be observed at the interfaces between the Zn Ni and Zn Cu layer, as shown in Fig. 2a. Fig. 2b shows microstructures of a Zn Ni coating. As this figure shows, Zn Ni alloy adhered onto the substrate surfaces as fine particles that have compact arrangement along abrasive traces. Such a compact structure has been suggested as the main reason for the good appearance, high hardness [5]. In an alkaline bath, regardless of the electrodeposition current density, the Zn Ni coating is composed by two phases, i.e. g and h, while g being predominant [7]. The thickness of Zn Ni layers increases with both time and electric current density, as shown in Fig. 3a and b, respectively. On the other hand, the thickness of Zn Ni decreases with the increase of electric current frequency and t on /t off, as shown in Fig. 3c and d, respectively. This can be attributed to the fact that both the nucleation rate of Zn Ni particles and the deposition rate of Zn Ni alloy decrease with the increase of electric current frequency and t on /t off [7]. Fig. 3e shows relationship between the content of Ni and the processing time. Under the process conditions as attached to the figure, the Ni content decreases with the increase of time initially. After a maximum content of about 14.2% at 1.2 ks, it increases again. The content of Ni in Zn Ni coating increases with the increase of both electric frequency and electric current density, as shown in Fig. 3f and g, respectively. When a direct current is used, it has been reported that the Ni content in Zn Ni coating does not vary with the current density [8]. That is, the dependence of Ni content on electric current density is a specific phenomenon of pulse current electrodeposition of Zn Ni coatings. Obviously, it is easier to control the Ni content in Zn Ni coatings to improve corrosion resistance in pulse current process. Table 1 Operating conditions of Zn Ni pulse-plating No. Plating time (s) Frequency (Hz) Current density (A/cm 2 ) t on /t off (%) *

3 Y.F. Jiang et al. / Surface & Coatings Technology 191 (2005) Fig. 2. Zn Ni alloy coatings plated on AZ91 by pulse potential for sample No. 1 in Table 1: (a) cross-section morphology (OM); (b) surface morphology (SEM). When the relationship between the Ni content and t on / t off is studied, it can be found that it increases with t on /t off firstly and then decreases, as shown in Fig. 3h. The maximum content of about 18% can be obtained, when t on /t off is about 30%. The microhardness of Zn Ni coatings also increases with the increase of processing time, electric current density, electric frequency and t on /t off, as shown in Fig. 3i l, respectively. However, there are differences in the rate of increase for different parameters. This may be Fig. 3. Effects of time, current density, frequency and t on /t off on thickness and Ni contents with (a), (e) and (i) sample No. 1; (b), (f) and (j) sample No. 2; (c), (g) and (k) sample No. 3; (d), (h) and (l) No. 4 in Table 1.

4 396 Y.F. Jiang et al. / Surface & Coatings Technology 191 (2005) Fig. 3 (continued). Fig. 4. Corrosion morphology of Zn Ni alloy plated for sample No. 1 in Table 1 on AZ91 after salt spray test: (a) 72 h; (b) 144 h; (c) 200 h; A: uncorroded particle; B: corroded site.

5 Y.F. Jiang et al. / Surface & Coatings Technology 191 (2005) Fig. 5. Dependence of area fraction of corrosion rust on time for plating for sample No. 1 in Table 1 on AZ91. related to the Ni content, the thickness, compactness of coatings and the size of particles in the coatings. This behavior is not easy to explain because there are several factors which contribute to the microhardness of the these materials including (1) solid solution hardening, (2) hardness effects owing different phases and (3) grain size effect. In addition, according to Hall Petch relationship, the grain size is an important variable affecting the microhardness value. On the other hand, we also found that particle size varied with process parameters. Generally, the particle becomes smaller when higher frequency, larger current density and smaller t on /t off is applied. Above results suggest that pulse parameters have great effects on chemical composition, thickness and microstructure of the Zn Ni coatings. Since the anticorrosion properties of a Zn Ni coating depend mainly on its chemical composition, thickness and microstructure, above results are of significant importance for selection of process parameters Study of corrosion variation with time At the beginning, corrosion initiated from some protruded particles, as labeled A in Fig. 4a. As corrosion is processed, some particles were totally surrounded by corroded gaps, as labeled B in Fig. 4b. At last, some isolated particles presented on the upper layer when surrounding materials were totally corroded, as labeled A in Fig. 4c. These isolated particles appear as white rust under SEM. The area fraction occupied by white rust increases along with time, as shown in Fig. 5. As illustrated by the curve, the corrosion rate was slow at first. It became faster as corrosion proceeded with time. This can be attributed to that corrosion occurred from more sites as time proceeded. X-ray analysis carried out after exposure in the salt spray chamber, and also showed that main corrosion products for all coatings are ZnCl 2 4Zn(OH) 2 [9,10]. This process is confirmed by EDX for chemical analysis to one of particle across center from one brim to another as labeled A in Fig. 6a. The Ni content of particle in different spots varies from brim to center. In addition, the Ni content of particle first increases from one brim, up to peak in center, then decreases until to another brim as shown in Fig. 6b. The change of Ni content for Zn Ni coatings leads to the formation of potential differential [7]. This may give rise to micro-cells reaction due to the presence of micro-potential differential. Therefore, corrosion does not penetrate inwards along the grain boundaries, because the grainboundary phases are invariably cathodic to the grain interior. Corrosion tends to be concentrated in the area adjoining the grain boundary until eventually the grain may be undercut and fall out. This agrees to previous results obtained in the SEM images. Fig. 7 shows the anodic and cathodic polarization curves obtained in aerated 3.5% NaCl solution and indicates how the corrosion potential values (E corrosion ) and the intensity of the corrosion current (I corrosion ) are determined. The cathodic polarization curves confirm that the Zn Ni coatings obtained in alkaline bath have a noble free corrosion potential and a polarization resistance Bonding strength In order to examine the bonding strength between Zn Ni coatings and substrate of magnesium alloy, two methods are selected to apply. One is thermoshock method, another is tensile method. Fig. 6. The variation of Ni contents for a particle in sample No. 4 in Table 1: (a) site of particle; (b) dependence of Ni content on appointed site.

6 398 Y.F. Jiang et al. / Surface & Coatings Technology 191 (2005) machine for the measurement of the tensile bonding strength. The result shows that the bonding strength is above 14.8 MPa, which is obviously higher than that of electroless nickel on magnesium alloy [11]. 4. Conclusions Fig. 7. The polarization curve obtained in 3.5% NaCl solution for sample No. 1 in Table 1. The plated sample is firstly heated up to 200 jc, and then constant temperature preservations for 30 min, finally quenching in water at ambient temperature. If this process is cycled, the test can be performed 10 times, and it is observed that there is no degradation of any kind on the surface of test specimens. It could attain applied standard. As to adhesive tensile method, the sample and two assistant pieces is machining according to size in Fig. 8. Then each plated test specimen and two loading fixtures are attached to the face of the plated coatings using special adhesive glue with an adhesive strength of about 60 MPa in term of Fig. 8 and the assembly is loaded in tensile test 1. In the pulse-electrodeposition process, the density and frequency of electric current, processing time and t on /t off have effects on chemical composition, thickness, microhardness, sizes and arrangement of particles of Zn Ni coatings plated on AZ91 magnesium alloy. 2. The pulse-electrodeposition process can produce a Zn Ni coating with compact structure. This compact structure is another reason for the good corrosion resistance of Zn Ni coating besides the intrinsic properties of Zn Ni alloy. 3. Corrosion occurs preferentially from Zn Ni particles interface. The particles are strong to alkaline corrosion. 4. Thermoshock test can be carried above 10 times, and adhesive tensile test give result of above 14.8 MPa for bonding strength. Acknowledgements Financial support for this work was provided by the Key Project of National 863 High Technology Programs (Grant No. 2001AA331030) from the Ministry of Science and Technology of the P.R. China. Fig. 8. Schematic illustration of adhesive tensile test for bonding strength.

7 Y.F. Jiang et al. / Surface & Coatings Technology 191 (2005) References [1] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [2] M.M. Avedesian, H. Baker, Magnesium and Magnesium alloys, ASM International, USA, [3] N. Zaki, Met. Finish. 87 (6) (1989) 57. [4] R.R. Sizelove, Plating Surf. Finish. 78 (3) (1991) 26. [5] M. Pushpavanam, S.R. Natarajan, K. Balakrishan, L.R. Sharma, J. Appl. Electrochem. 21 (1991) 642. [6] S.O. Pagotto Jr., C.M. de Alvarenga Freire, M. Ballester, Surf. Coat. Technol. 122 (1999) 10. [7] C. Muller, M. Sarret, M. Benballa, Electrochim. Acta 46 (2001) [8] D.E. Hall, Plating Surf. Finish. 70 (11) (1983) 59. [9] G. Kong, J.T. Lu, J.H. Chen, Q.Y. Xu, L.X. Liu, Chin. J. Nonferr. Met. 73 (8) (Chinese). [10] A.Z. Xu, W.B. Hu, B. Shen, W.J. Ding, Electroplat. Poll. Cont. 20 (3) (2000) 1 (Chinese). [11] A.K. Sharma, M.R. Suresh, H. Bhojraj, H. Narayanamurthy, R.P. Sahu, Met. Fin. 96 (1998) 10.