Electroless nickel-plating on AZ91D magnesium alloy: effect of substrate microstructure and plating parameters

doi:10.1016/S0257-8972(03)00866-1

Surface and Coatings Technology

Volume 179, Issues 2–3, 23 February 2004, Pages 124-134

https://doi.org/10.1016/S0257-8972(03)00866-1 Get rights and content

Abstract

Electroless nickel-plating on AZ91D magnesium alloy has been investigated to understand the effect of substrate microstructure and plating parameters. The initial stage of the deposition was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis on substrates plated for a very short interval of time. The early stage of growth was strongly influenced by the substrate microstructure. Plating was initiated on β-phase grains probably due to the galvanic coupling of β and eutectic α-phase. Once the β-phase was covered with the coating, it then spread onto eutectic α and primary α-phase. The coating produced with the optimised bath showed 7 wt.% phosphorus with a hardness of approximately 600–700 VHN. The optimum ligand to metal ion ratio was found to be 1:1.5, while the safe domain for thiourea (TU) was in the range of 0.5–1 mg/l. Fluoride was found to be an essential component of the bath to plate AZ91D alloy with an optimum value of 7.5 g/l. The presence of 0.25–0.5 mg/l mercapto-benzo-thiosole (MBT) found to accelerate the plating process.

Introduction

Magnesium and its alloys, with one quarter of the density of steel and only two-thirds that of aluminium and a strength to weight ratio that far exceeds either of these, fulfill the role admirably as an ‘ultra light weight’ alloy. Hence, these alloys have obviously become the choice for weight reduction in portable microelectronics, telecommunications, aerospace and automobile applications etc.

The magnesium–aluminium system has been the basis of the most widely used magnesium alloys since these materials were introduced in Germany during the First World War. Most of these alloys contain 8–9% aluminium with small amounts of zinc [1], [2].

A serious limitation for the potential use of several magnesium alloys and AZ91 in particular, is their susceptibility to corrosion. Magnesium alloys, especially those with high purity, have good resistance to atmospheric corrosion [3]. However, the addition of alloying elements modifies the corrosion behaviour in such a way that it can be beneficial or deleterious. The standard electrochemical potential of magnesium is −2.4 V vs. NHE, even though in aqueous solutions magnesium shows a potential of −1.5 V due to the formation of Mg(OH)₂ film [4]. Consequently, magnesium dissolves rapidly in aqueous solutions by evolving hydrogen below pH 11.0, the equilibrium pH value for Mg(OH)₂ [4].

Although the addition of several alloying elements such as aluminium, zinc and rare earths have been reported [5], [6], [7], [8], [9], [10], [11], [12] to improve the corrosion resistance, technologically that does not satisfy the requirement for several applications. Hence, the application of a surface engineering technique is the most appropriate method to further enhance the corrosion resistance. Among the various surface engineering techniques that are available for this purpose, coating by electroless nickel is of special interest especially in the electronic industry, due to its conductivity and several other engineering properties. Electroless nickel is well known for its corrosion resistance and hardness [13], [14], [15], [16], [17], [18]. However, the nickel/Mg system is a classical example of cathodic coating on an anodic substrate. Hence, the porosity in the coating might influence the corrosion behaviour and service lifetime of the electroless nickel-plated magnesium. The protective ability of electroless nickel on many engineering materials is limited by the porosity in the coating [19], [20], [21], [22], [23], [24], [25], [26].

Being a highly active metal, electroless plating of magnesium alloy needs special bath formulations and pre-cleaning treatments. Hence, the direct plating of magnesium is still a challenge for the researchers. The process becomes more complicated on AZ91 alloy due to the microstructural heterogeneity owing to the unequal distribution of aluminium within the three constituent phases namely primary α, eutectic α and β phases [26]. Therefore, the substrate material is electrochemically heterogeneous and each constituent behaves differently to the plating bath. The available information on electroless nickel-plating of magnesium alloys is very limited. This paper reports the work carried out on electroless nickel-coating of AZ91D magnesium alloy, more specifically, the effect of substrate microstructure on coating nucleation and the effect of various bath parameters.

Section snippets

Experimental

The substrate material used for the present investigation was AZ91D ingot-cast alloy. The chemical composition of the alloy is given in Table 1. Rectangular coupons of size 20×40×4 mm were used for the investigation. The surface of the substrate material was wet-ground (using water) on 1000 grade SiC paper and polished on a diamond wheel using 6-μm diamond paste. The polished specimens were thoroughly washed with water before passing through the pre-cleaning schedule as shown in Table 2. The

Microstructure of substrate material

Fig. 1a shows microstructure of the substrate material AZ91D ingot. The microstructure consisted of primary α, eutectic α and β-phases (marked in the figure). β-phase is an intermetallic with the stoichiometric composition of Mg₁₇Al₁₂. Coring during solidification resulted in considerable variation in the distribution of aluminium and zinc in the microstructure of AZ91D alloy. Previously, we have reported the variation of aluminium and zinc concentration adjacent to β-phase in AZ91 ingot-cast

Early stage growth and coating morphology

Direct plating of magnesium alloys with electroless nickel is still a challenge. Only a limited amount of literature is available [28], [29] on the electroless nickel-plating of magnesium alloys and applications. The process is more complicated when the substrate contains second phase particles as for AZ91, which makes the alloy electrochemically heterogeneous. The three microstructural constituents in AZ91 alloy (Fig. 1) namely β, eutectic α and primary α-phase have different electrochemical

Conclusions

1
The electroless nickel-coating deposited on AZ91D alloy in optimised bath showed amorphous structure with 7 wt.% P and a hardness value of 600–700 VHN. The microstructure of the coating in the transverse direction showed lamellar structure with a phosphorus content varying in a sinusoidal manner.
2
A strong influence of substrate microstructure was found. Initially, the coating was nucleated preferentially on β-phase. The coating spread over to primary α-phase, once the β-phase and eutectic

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Electroless nickel-plating on AZ91D magnesium alloy: effect of substrate microstructure and plating parameters

Abstract

Introduction

Section snippets

Experimental

Microstructure of substrate material

Early stage growth and coating morphology

Conclusions

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Mater. Chem. Phys.

Surf. Coat. Technol.

Corros. Sci.

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Corrosion

Met. Powder rep.

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Corros. Rev.