Microstructure and corrosion behavior of electroless Ni--P coatings on 6061 aluminum alloys.
Abstract The microstructure and corrosion behavior of electroless Ni--P alloy plating on 6061 aluminum alloys substrate in an alkaline plating bath with sodium hypophosphite as reducing agent were investigated. The effects of bath temperature on the plating rate, compositions, and microstructure of the electroless Ni--P deposits were studied. The results showed that the deposition rate and the P content of the electroless Ni--P deposits increased with the rise of the bath temperature. Scanning electron microscopy (SEM) of the deposits showed nodular structure for binary deposits. X-ray diffraction patterns of all the deposits revealed a single and broad peak which indicated the amorphous structure of the deposits. Corrosion resistance of the Ni--P coatings was evaluated by potentio-dynamic polarization. The results indicated that electroless Ni--P plating could obviously improve the corrosion resistance of 6061 aluminum alloy.
Keywords Electroless nickel plating, 6061 aluminum alloys, Microstructure, Corrosion
The 6061 aluminum alloy contains magnesium and silicon as its major alloying elements and is available in several different grades. This alloy constitutes a very important engineering material widely employed in the aircraft and aerospace industry for the manufacturing of different parts and components due to its excellent mechanical strength, low specific weight, good form-ability, and relatively low cost. (1) Unfortunately, the widespread use of these materials has been limited by their poor corrosion properties in some environments. (2), (3) Therefore, various anticorrosive strategies should be developed to improve the practical usage of such aluminum alloys. In recent years, surface treatment technology has attracted a great deal of attention to improve mechanical and electrochemical properties of aluminum alloys such as wear resistance, friction coefficient, strength, and ductility as well as corrosion resistance. (4-6) Many elements have been used as protective coatings of aluminum alloy substrates, such as chromium, (7), (8) titanium, (9) and nickel. (10) Among them, nickel is an interesting transition element applied to improve the surface properties of aluminum alloy because of its excellent properties including good hardness, ductility, and corrosion-resistance.
To obtain desirable surface characteristics of aluminum alloys, a variety of coating techniques has been developed including electroless, anode oxidation, painting, and electroplating. (11-14) Electroless Ni--P coatings are often considered engineering coatings owing to their exceptionally high hardness, remarkable wear, and corrosion resistance properties. (15-17) Although electroless Ni--P plating technology has been used on ordinary aluminum alloys, (3) the study of electroless Ni--P plating on 6061 aluminum alloy and resultant microstructure and corrosion resistance has not been revealed.
The aim of the present study is to develop an effective coating and increase the corrosion resistance of 6061 aluminum alloy in harsh condition. The direct electroless Ni--P plating on 6061 aluminum alloy was carried out and the effects of bath temperature on deposition rate, surface morphology, crystal structure, chemical composition, and corrosion resistance of Ni--P plated 6061 aluminum alloy were investigated.
In the present investigation, Ni-P alloy was deposited on 6061 aluminum alloy at different bath temperatures by an electroless process. The detail procedures are described below.
The specimens of 6061 aluminum alloy were pretreated in three steps prior to the electroless deposition process. Step one: the specimens were precleaned with a 5% non-ionic detergent at pH 7 and 40[degrees]C for 20 min. Step two: the specimens were immersed in a NaOH base solution (pH 10) for 5 min, and cleaned with deionized water. Step three: a nitric acid solution (20% [HNO.sub.3]) was used to pickle the surface of the specimens for 5 min, and the specimens were cleaned with deionized water again. After the above pretreatment procedure, the electroless Ni-P plating process was carried out immediately to produce the deposits on the substrates. The plating bath composition and operating condition of electroless Ni-P deposition are listed in Table 1. The pH of the plating bath was adjusted by using the sodium hydroxide aqueous solution.
Table 1: Compositions and operating conditions of electroless Ni-P plating Composition of bath and plating parameters Specifications [NiSO.sub.4] * [6H.sub.2]O (g/L) 15 [Na.sub.3][C.sub.6][H.sub.5][O.sub.7] * [2H.sub.2]O 20 (g/L) [H.sub.3][BO.sub.3] (g/L) 30 [NaH.sub.2][PO.sub.2] * [H.sub.2]O (g/L) 30 pH 8.5 Temperature ([degrees]C) 60,70,80 Time (min) 20
In the post-treatment stage, all the Ni-P plated Al alloy samples were rinsed with deionized water at 40[degrees]C for 20 min immediately after the metallizing reaction of electroless nickel plating and then dried in an oven at 60[degrees]C.
Electroless Ni-P deposition rate was calculated from the weight gain of the specimen before and after the electroless Ni-P plating procedure and plating time. The units are expressed as mg/[cm.sub.2] h. The crystal structure of the Ni-P deposits was investigated by using X-ray diffraction (XRD, Cu Ka radiation and graphite filter at 40 kV and 40 mA). A field emission scanning electron microscope (SEM, JSM-6335F at 3.0 kV) was used to characterize the surface morphology of the deposits. The chemical composition of the Ni-P deposits was determined by using an energy dispersive X-ray (EDX) analyzer that was attached to the SEM.
A common three-electrode cell was used to perform polarization curves in 3.5% NaCl solutions at room temperature. A platinum electrode and saturated calomel electrode (SCE) were used as counter and reference electrode, respectively.
Results and discussion
Temperature is one of the important factors for electroless Ni-P plating, and the Ni-P deposition rate at different temperatures is shown in Fig. 1. The results showed that the bath temperature had a significant effect on the Ni-P deposition rate. For a given bath concentration in the solution and pH value, a tendency shown was that coating growth rate became faster as temperature became higher. This was in accordance with another finding. (18)
[FIGURE 1 OMITTED]
Effects of temperature on nickel and phosphorus are shown in equations (1) and (2), respectively. (19)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
where E, [E.sub.0], F, R, T, and a are electromotive force, standard electromotive force, Faraday constant (96 485.3399 C [mol.sup.-1]), gas constant (8.314 J/K mol), temperature, and activity, respectively. Therefore, higher temperature can increase electromotive force (E) which has an effect on the reaction rate. A higher electromotive force can promote Ni-P plating.
Surface morphology and composition of electroless Ni-P plating
The surface micrographs of Ni-P coated 6061 aluminum alloy at temperatures of 60, 70, and 80[degrees]C, respectively, are presented in Fig. 2. The Ni-P plating layers exhibited a nodular feature with a typical cauliflower-like structure. The Ni-P deposits had clearly different surface morphology and their nodule sizes increased with increasing temperature. In addition, the surface morphology of as-deposited deposit was not compact under the temperature of 60[degrees]C. Rather, the deposits became more compact, which indicated better properties under the temperature of 70[degrees]C and 80[degrees]C.
[FIGURE 2 OMITTED]
According to polymorphous transformation kinetics, (20)
D = exp(-Q/kT), (3)
where D, Q, k, and T are particle size, activation energy, the Boltzmann constant, and temperature, respectively. The equation indicates that particle size is a function of temperature; that is, when the temperature is lowered at constant activation energy, the particle size is also lowered.
The effect of temperature on the composition of Ni-P alloy deposits is shown in Fig. 3. The results showed that the percent of phosphorus in the deposit increased slightly with increasing temperature of reaction. However, all the Ni-P coating was found to contain more than 10.0% P. With an increase of temperature of reaction, deposition rate of phosphorus increased more' rapidly than that of nickel. Consequently, the proportion of phosphorus in the deposit increased. (19)
[FIGURE 3 OMITTED]
Figure 4 shows the XRD patterns for the as-plated samples, which exhibited only a single broad peak at around 2[theta] of 45[degrees], indicating the amorphous nature of the coating. According to previous papers, co-deposited phosphorus content plays an important role in determining the structure of the Ni-P deposit. (21) XRD studies have shown that deposits with more than 7 wt% are amorphous. (22), (23)
[FIGURE 4 OMITTED]
The mechanism of the formation of Ni-P coatings suggests that during Ni-P deposition, random capturing of P on Ni atoms occurs and that the variation in the rate of segregation of Ni and P atoms determines the crystal-structure of the resultant coating. Between these two atoms, the diffusion rate of P is relatively small compared to that of Ni. (24) Hence, a Ni-P coating with a higher P content involves the movement of more P atoms from a given area per unit time during deposition to achieve segregation of Ni and p. (25), (26) As mentioned before, all the Ni-P deposits contain more than 7.0% phosphorus. Since the required P segregation is very large, it prevents the nucleation of the face-centered-cubic (FCC) Ni phase which results in an amorphous structure in the as-deposited condition.
Corrosion resistance of electroless Ni--P plating
The even and compact Ni--P coatings protect the Al alloy against attack by the corrosion medium. Polentiodynamic polarization measurements were employed to evaluate the effect of the coatings on the corrosion resistance characteristics of aluminum alloys. The corrosion potential [E.sub.corr] and corrosion current density [i.sub.corr] were calculated from the intersection of the cathodic and anodic Tafel curves extrapolated cathodic and anodic polarization curves. For both of the polarization curves, active reaction dominates the anodic side and oxygen reduction dominates the cathodic side.
Figure 5 shows the polarization curves of the bare Al alloy and the Ni-P coating plated at 70[degrees]C in 3.5 wt% NaCl solution at room temperature. The corrosion potentials ([E.sub.corr]) of the bare aluminum alloy and the Ni-P coated alloy were -0.69 and -0.40 V, respectively. The corrosion potential was increased by 0.29 V in the noble direction after electroless Ni-P plating. The corrosion current density of the coated alloy also was lower than that of the bare alloy. For the two curves, once the scanned potential exceeded [E.sub.corr], the corrosion current density of the samples continually increased. However, the increase of the corrosion current density of the coated alloy was less than that of the bare alloy. This implied that the anodic dissolution reaction of the Ni-P coating was restrained, which effectively decreased the corrosion sensibility of the coated sample in NaCl solution.
[FIGURE 5 OMITTED]
The effect of bath temperature of electroless plating on the formation and characteristics of Ni-P deposits obtained from an alkaline hypophosphite reduced electroless nickel bath was investigated in this article. As bath temperature rose, the deposition rate and P content of the Ni-P layer increased. In addition, the nodule size of the Ni-P layer increased with increasing temperature. XRD analyses revealed that the as-plated Ni-P deposits were in an amorphous phase. Furthermore, the results of potentiodynamic polarization curves demonstrated that the coating improved the corrosion resistance of 6061 aluminum alloy.
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W. Qin ([??])
Aviation Engineering Institute, Civil Aviation Flight University of China, Guanghan 618307, China
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