Corrosion Resistance and Hardness Properties of Electroless NiP/NiB Duplex Coating

1. Introduction Electroless nickel coating is a technique in which nickel atoms are obtained from an aqueous solution containing a reducing agent through autocatalytic chemical reduction and applied to the material surface [1]. Electroless nickel coatings find wide application in the aerospace, automotive, chemical and electronics industries due to their solderability, high hardness, wear and corrosion resistance [2,3]. Through the electroless coating method, in addition to pure metal coatings, coatings with binary and multiple alloy structures can be obtained. In line with the desired properties of the coating, coatings with the same or different alloy structures can be applied as single-layer, two-layer or multi-layer. The electroless coating method can be applied not only to metal surfaces but also to materials of different shapes and sizes such as glass, ceramics and plastics [4]. Due to their superior properties, electroless NiP and NiB coatings are particularly widely applied. Electroless NiP alloy coatings are applied in many fields due to their excellent properties such as high corrosion and wear resistance, good lubricity and high hardness [5]. NiB alloy coatings are preferred due to their high hardness and superior wear resistance [6,7]. However, electroless coatings are sensitive to thermal effects, and heat treatments have a strong effect on the structural properties, hardness and corrosion resistance of the coatings. Studies show that heat treatment applied especially at 400°C causes notable changes in the structure of electroless coatings and therefore in their properties.

2. Experimental

In the experimental study, iron-based powder metal (PM) parts were used as substrate material. PM parts were coated in electroless NiP and NiB coating baths with composition and working parameters given in Table 1 and Table 2. Given the service conditions of iron-based powder metal compacts, hardness and wear resistance become important. For this reason, the coating structure was designed with NiB coating film, which stands out with hardness and wear resistance, as a thick top layer, and NiP coating film, which stands out with corrosion resistance, as a thin intermediate layer. Table 1. Ni-P coating bath and working parameters Table 2. Ni-B coating bath and working parameters The structure, morphology and chemical composition of the coatings were analyzed by scanning electron microscopy (SEM/EDX). The phase structures of the coatings were determined by X-ray diffraction analysis (XRD). Hardness values were determined by microhardness tests applied to the coating cross-section. Corrosion resistance was determined using a Parstat 4000 potentiostat/galvanostat system in a 3.5% NaCl solution.

3. Experimental Results and Discussion

3.1. Coating Morphology and Structure

The cross-sectional views of the electroless NiP/NiB duplex coating applied to PM parts are shown in Figure 1. The thin NiP film (<500 nm) formed a compatible interface between the PM compact and NiB. The NiB coating, showing columnar growth, was found to have a coating thickness of 20 μm. Figure 2 shows SEM images of the cross-section and surface morphology of the NiP/NiB coating film. Electroless NiB coating has a well-known cauliflower-like surface morphology. Under friction conditions, the cauliflower-like surface structure provides a natural lubricity property that reduces the contact area of the coating, thereby increasing wear resistance. Chemical composition analyses revealed that the Ni-P coating contained 4.8% P and 95.2% Ni by weight, and the Ni-B coating contained 5.7% B and 94.3% Ni by weight.

Figure 1. Cross-sectional morphology of NiP/NiB coating.

Figure 2. Surface and cross-sectional morphology of NiP/NiB coating film.

Figure 3 shows the XRD diffraction patterns of samples in the as-coated state and those subjected to heat treatment at 400°C. The XRD diffraction pattern of the as-coated coating yielded a broad peak characteristic of an amorphous phase. After heat treatment at 400°C for 1 hour, the NiB coating crystallized and Ni, Ni2B and Ni3B phases formed in the NiB coating (Figure 3). The Ni3B and Ni2B phases precipitated in the nickel matrix caused a notable increase in the coating hardness. No phases related to the NiP sublayer were detected in the XRD analyses.

3.2. Coating Hardness

The hardness value of 668 HV100 in the as-coated state reached 1143 HV100 as a result of heat treatment through precipitation hardening caused by the formation of Ni3B and Ni2B intermetallic phases (Figure 4). This increase in surface hardness of the PM compacts will also positively affect their wear resistance.

Figure 4. Microhardness of PM compacts and coatings.

3.3. Corrosion resistance of NiP/NiB coating

Polarization curves of uncoated PM compact and electroless NiP/NiB coated and heat-treated compacts are given in Figure 5. As can be seen from the curves, the Tafel line of the NiP/NiB coating in the as-coated state is obtained at lower polarization current density values compared to the PM compact and coated and heat-treated sample. This result shows that the corrosion resistance of the NiP/NiB coating is higher than that of the PM compact and NiP/NiB coated and heat-treated sample. As can be seen from the XRD results, the crystallization of the amorphous structure as a result of heat treatment caused a decrease in the corrosion resistance of the coating.

Conclusions

In this study, electroless NiP/NiB duplex coatings were examined using microstructure analysis, microhardness, SEM, XRD and corrosion testing techniques. NiP/NiB duplex coating was successfully applied using the electroless coating method. Heat treatment causes a notable increase in the microhardness of the coatings due to precipitation hardening caused by the formation of Ni3B and Ni2B phases in the nickel matrix. Structural changes caused by heat treatment reduce the corrosion resistance of the coating.   Dr. Associate Professor Ulaş Matik Karabük University Department of Mechanical and Metal Technologies Metallurgical Program      

References [1] Kılıçarslan A, Toptan F and Kerti I 2010 Electroless nickel coating method and its application to ceramic particles 33–7. [2] Matik U 2016 Structural and wear properties of heat-treated electroless Ni-P alloy and Ni-P-Si3N4 composite coatings on iron based PM compacts Surf. Coatings Technol. 302 528–34. [3] Abdel-Gawad S A, Sadik M A and Shoeib M A 2019 Preparation and properties of a novel nano Ni-B-Sn by electroless deposition on 7075-T6 aluminum alloy for aerospace application J. Alloys Compd. 785 1284–92. [4] Delaunois F, Petitjean J P, Lienard P and Jacob-Duliere M 2000 Autocatalytic electroless nickel-boron plating on light alloys Surf. Coatings Technol. 124 201–9. [5] Hsu C I, Hou K H, Ger M Der and Wang G L 2015 The effect of incorporated self-lubricated BN(h) particles on the tribological properties of Ni-P/BN(h) composite coatings Appl. Surf. Sci. 357 1727–35. [6] Bekish Y N, Poznyak S K, Tsybulskaya L S and Gaevskaya T V. 2010 Electrodeposited Ni-B alloy coatings: Structure, corrosion resistance and mechanical properties Electrochim. Acta 55 2223–31. [7] Matik U 2018 Improving surface properties of iron-based powder metal compacts with electroless Ni-B coating Gazi University Faculty of Engineering and Architecture Journal 2018 1603–10.