New Insight into Adherence of Ni-P Electroless Deposited Coatings on AA6061 Alloy through Al₂O₃ Ceramic

Abstract

The adhesion quality of Ni-P coatings on aluminum is important for mechanical and anticorrosion properties. In this study, the adhesion of Ni-P coatings on nanoporous Al₂O₃ ceramic (NAC) was evaluated by impact testing. NAC was fabricated on AA6061 alloy by anodizing in sulfuric acid. The deposition of Ni-P coating was carried out on NAC with and without zincate pretreatment. It was found that zincate activation of Al₂O₃ accelerates the formation of Ni-P coating. A cross-sectional analysis using energy-dispersive X-ray spectroscopy showed that the mechanical properties and impact resistance of the Ni-P coating are strongly related to the chemical composition in the vicinity of its interface with Al₂O₃. The course of the formation process of Ni-P coating and its mechanisms are also very important. Although the formation of Ni-P coating was slower without zincate treatment, its stronger adhesion to NAC led to superior impact resistance compared to zincate-treated Al₂O₃. Improved durability of items with Ni-P coatings can benefit many applications.

1. Introduction

The automotive industry needs aluminum (Al) components with good mechanical properties. Ni-P coatings produced by electroless deposition provide good protection against corrosion, friction resistance, paint compatibility and often increase mechanical strength [1,2]. Impact resistance is especially important in car bodies and other automotive applications. Their durability is mostly dictated by the adhesion between a Ni-P coating and the substrate, which frequently involves pretreated Al, e.g., anodic Al₂O₃. Researchers studied the effects of double zincate pretreatment and the adhesion of electroless Ni-P coating (9% P) onto 1000, 2000 and 5000 series Al alloys without anodization [3]. They found that without zincate treatment Ni-P coatings could not form on 1000 series alloys, while adhesion on 2000 and 5000 series alloys was poor—0.07 and 0.04 kN∙m⁻¹, respectively, obtained by a peeling test. In all cases, Ni-P adhesion increased to 0.52, 0.13 and 0.06 kN∙m⁻¹, respectively, after the 1st zincate treatment. The 2nd zincate treatment was only beneficial for 2000 and 5000 series alloys.

Other scientists investigated surface preparation for electroless Ni-P deposition on anodized Al5052 alloy [4]. They showed that sensitization and activation using SnCl₂ and PdCl₂, respectively, before Ni-P deposition increased surface hardness and scratch resistance by more than two times. The influence of surface pretreatment on the dry sliding wear behavior of electroless Ni-P coatings deposited on a 7075-T6 Al alloy was also studied [5]. They found that the wear performance and coefficient of friction (COF) of heat-treated Ni-P coatings depends on pretreatment conditions, and friction recedes according to this sequence: Ni, zincate and hypophosphite. The deposition of Ni–P–fly ash/zincate composite on the 5083 Al substrate resulted in a significant decrease in the COF and an increase in the surface hardness and roughness, with superior adhesion strength [6].

Studies of electroless Ni-P coatings on the anodized Al alloy AA1050 revealed that nickel fluoride (NiF₂) is not only used as a sealing agent to close the nanopores of anodic Al₂O₃ but also could be used as an activator [7]. Treatment with NiF₂ increases the adhesion between the anodic layer and the Ni-P coating by about 1.4 times. Unfortunately, AA1050 is a commercially pure alloy, which is rarely used in practice.

The double zincate treatment is the standard metal finishing technique of Al alloys before nickel plating in which a thin layer of zinc with a thickness from 10 to 40 nm is deposited [8]. This produces uniform Ni-P coatings with high deposition rates and improved adhesive strength [3,7]. Relatively smooth surfaces and higher adhesion are beneficial for decorative properties in industrial applications. Nevertheless, the high adhesion strength of Ni-P coatings could be achieved without zincate pretreatment. Yazdi et al. [9] investigated the adhesion of Ni-P coating with double zincate pretreatment on Al 3004 that was anodized in sulfuric acid solution by the bend test. The authors demonstrated that direct adhesion of Ni-P on the anodic layer is higher when compared to the non-anodized surfaces with or without double zincate pretreatment. This effect could be explained by higher surface roughness and increased mechanical interlocking. On the other hand, zincate treatment produces relatively smooth surfaces [9]. These findings are well correlated with another study of Ni-P adhesion on 3000 series Al alloy tested by a scratch method [10]. Anodic Al₂O₃ with higher surface roughness showed better adhesion of Ni-P when compared to the one deposited directly onto the alloy. Besides, anodic surfaces promote Ni–P particles embedding in the pores, resulting in a more compact coating with a smaller grain size and distribution. The ability to deposit Ni-P directly on anodic Al₂O₃ has several advantages. First of all, this allows for increased productivity and reduces the number of technological operations, including HNO₃-pickling and multiple washing procedures. Second, optimized technological processes reduce water contamination, which is relevant due to ecological and economic reasons. Ni-P coatings are beneficial for many applications where high mechanical resistance is required, including the automotive, electronics and aerospace industries. In case of better properties, thinner Ni-P coatings might be sufficient to achieve the required performance, leading to a lower consumption of Ni. Because of its excellent mechanical properties and good corrosion resistance [1,4], Ni-P coatings have been frequently applied to engines, valves, frames, cases and other equipment under different operational conditions.

The purpose of this work was to investigate the influence of zincate double pretreatment on Ni-P adhesion, focusing on AA6061 alloy, which is widely used in the automotive industry. We showed the difference between zincated and plain Ni-P coatings with respect to their formation on anodic Al₂O₃, surface morphology and adhesion properties. The importance of the chemical composition in the vicinity of the interface between Al₂O₃ and Ni-P for impact resistance was also highlighted. A standardized procedure, ASTM D2794, was selected for impact tests.

2. Materials and Methods

2.1. Anodization and Electroless Ni-P Deposition

AA6061 alloy specimens (plates of 100 mm × 50 mm × 2 mm) were cleaned in UniClean 151 (Atotech) alkaline solution for 7 min at 55 °C, rinsed in DI water, then cleaned in Alklean AC-2 (Atotech) acid solution for 30 s at 30 °C and rinsed again. Anodization was performed in 195 g/L H₂SO₄ with 9 g/L Al³⁺ by applying 15 V DC at 19 °C to obtain an NAC coating of ~20 µm thickness for the subsequent Ni-P deposition, containing mostly nanoporous Al₂O₃. Ni-P coating was deposited with or without zincate treatment, resulting in ZNiP (“Zincated Ni-P”) or pNiP (“plain Ni-P”) specimens, respectively. pNiP specimens were produced by cleaning and drying NAC plates and immersing them into Marquee BMP (Columbia chemical, Brunswick, OH, USA) of pH 4.7 for 40 min at 88–92 °C. ZNiP specimens were obtained by pretreating NAC plates in solution Alumseal NCY X2 for 20 s at 20 °C, 40 wt.% HNO₃ pickling for 30 s at 20 °C, rinsing and repeating the same procedure once more. After drying, Ni-P coating was deposited in the same manner as for pNiP specimens. Afterwards, specimens were rinsed, dried in air and stored at room temperature until testing.

2.2. Ni-P Adhesion

Qualitative evaluation of Ni-P adhesion on the Al₂O₃ layer was carried out by a standard impact test (ASTM D 2794). An 8 mm diameter spherical ball was used for evaluating the resistance of Ni-P coating to the deformation caused by a falling weight. The opposite side of the impact site of Ni-P coating was analyzed by an optical microscope.

2.3. Microscopy and Elemental Analysis

A dual-beam Helios Nanolab 650 Scanning Electron Microscope (SEM, FEI, Eindhoven, The Netherlands) system equipped with an energy-dispersive X-ray spectrometer (EDS) (Oxford Instruments, Xmax 20 mm² detector, INCA 4.15 software, Abingdon, UK) was used for sample surface and cross-section investigation. Cross-sections of the samples were prepared by a standard mounting, grinding and polishing procedure using Secotom-15 and Tegramin-25 equipment (Struers). The samples were vacuum chromium-coated and examined in the secondary electron imaging (SEI) mode at 5 kV acceleration voltage and 0.1/0.8 nA beam current. The elemental analysis and mapping were performed using 20 kV acceleration voltage and 1.6 nA beam current.

3. Results and Discussion

The plates of AA6061 alloy were anodized to form an Al₂O₃ coating of 20 ± 1.6 µm thickness with the nanopores size of 20 nm diameter on their surface (Figure 1a). Then, a Ni-P coating of 12 ± 3.0 µm thickness was deposited with or without double zincate pretreatment. The grain sizes on pNiP (without zincate) were relatively large (Figure 1b), while those on ZNiP were more than two times smaller (Figure 1c).

The grains of Ni-P with a diameter of ~20 µm tend to form individually on the pNiP surface (Figure 1b). This difference can be explained by the fact that zincate pretreatment of NAC creates many nucleation points, resulting in a much smoother formation of Ni-P coating when compared to the Ni-P deposited directly on the NAC. These results agree well with those of similar studies investigating the effect of zincate pretreatment additives on the structural properties of electroless Ni-P coating on AA6061 [11].

Impact testing is a standardized procedure to measure coating adhesion and resistance to rapid deformation, which is beneficial for rough and porous materials, e.g., anodic Al₂O₃. In this investigation, hardness, elasticity or other dynamic-mechanical parameters of the Ni-P coatings were not measured since their deposition methods were the same, except for the zincate pretreatment. Therefore, it was assumed that the adhesion between the Ni-P coating and Al₂O₃ is the major factor dictating the impact test results. Nevertheless, the brittleness of the Ni-P coating should not be disregarded when assessing the overall resistance to impact stress. The morphology of the Ni-P after the impact tests is shown in Figure 2. The ZNiP coating (deposited on NAC with zincate treatment) cracks or peels off completely at a hammer lift height of 10 cm (Figure 2a). Meanwhile, the pNiP coating (deposited on NAC without zincate treatment) does not change significantly (Figure 2d).

The impact damage on the latter becomes apparent only when the hammer lift height is increased to 25 cm (Figure 2e). These data are in agreement with the results obtained by other researchers [9]. They tested the adhesion of Ni-P to AA3004 alloy and found that Ni-P coatings showed lower adhesion on AA3004 with or without a zincate interlayer when compared to Ni-P coatings on anodized AA3004. The authors explain this effect by a mechanical interlocking mechanism between Ni-P and Al₂O₃. They also noted that the Al₂O₃ substrate performs a stress reduction function in the interface layer. The adhesion strength of the Ni-P layer to Al₂O₃ is an important parameter because a weakly adhered layer can easily delaminate and produce crack propagation under applied stress. The formation of this type of crack is quite evident on the ZNiP coatings.

In the case of zincate pretreatment, the adhesion of the Ni-P coating to the anodized surface of AA6061 can take place via the formation of multiple active spots on Al₂O₃, which accelerates the formation of Ni-P coating (Figure 3a,c,e, see ZNiP) compared to the formation of Ni-P coating on Al₂O₃ without zincate treatment (Figure 3b,d,f, see pNiP).

In the case of Ni-P coating without zincate treatment, a weaker mechanical adhesion to Al₂O₃ might appear likely. This might be implied by the interface data (Figure 4). In the first case, the width of the interface layer is about 8.0 µm (Figure 4a), and in the second the width is about 4.0 µm (Figure 4b).

However, the tests show better impact resistance of pNiP, which suggests that several factors might be responsible for the improved adhesion to Al₂O₃. Chemical composition in the vicinity of the NiP-Al₂O₃ interface is different, based on the EDS analysis. The penetration of the deposited layer into the NAC nanopores might significantly improve mechanical and tribological resistance [12,13]. In addition, Krishnan et al. reported that the tensile strength and other mechanical properties strongly depend on P contents in the coating [14]. A cross-sectional analysis of ZNiP showed a quite gradual recession of P content. For pNiP coatings, a slightly higher concentration of P was recorded in the interface layer, although it might not be very significant if Al and O contents are excluded. Nevertheless, this difference might still affect the adhesion properties.

Internal stresses in the interface vicinity might be another very important factor. Most likely, they are quite different when comparing pNiP and ZNiP. In the case of zincate pretreatment, the formation of Ni-P is faster, which might produce higher internal stresses on the NiP-Al₂O₃ interface. This might result in rapid crack propagation upon impact and reduced mechanical resistance. The complexity of chemical composition and internal stresses of the interface layer indicates that many aspects still need further studies, and their progress might lead to significant advancement of Ni-P deposition techniques on anodic coatings.

4. Conclusions

In this study, Ni-P coatings were deposited by the electroless method on anodic Al₂O₃ produced on an AA6061 alloy. The Ni-P coatings were deposited with or without zincate pretreatment. Their surface structure, morphology, adhesion and impact resistance were evaluated. The results showed that zincate-treated surfaces accelerate the formation of Ni-P coating by creating multiple active spots on Al₂O₃ and result in smooth surfaces. The thickness of the obtained interface layer reaches 8.0 µm. On the other hand, plain Ni-P coatings demonstrate the formation of rough surfaces with individual grains of ~20 µm diameter and an interface layer of around 4.0 µm. Higher surface roughness and increased mechanical interlocking appear beneficial for adhesion properties, as evaluated by impact testing according to the ASTM D2794 procedure. As the results show, the impact resistance of the electroless Ni-P coatings on anodic surfaces can be increased by double or more by omitting the zincate pretreatment procedure. According to cross-sectional and EDS analyses, the chemical composition of the interface between Ni-P and anodic Al₂O₃ is very important, in particular the P content. The course of the formation of Ni-P coating and its mechanisms are also significant factors that influence mechanical properties and impact resistance. The improved mechanical resistance of Ni-P coatings suggests promising opportunities in industrial applications involving both technological and ecological aspects, with the ability to reduce the number of technological operations and simplify washing procedures.