Patent Publication Number: US-2009223829-A1

Title: Micro-Arc Assisted Electroless Plating Methods

Description:
FIELD OF THE INVENTION 
     The invention relates to electroless plating methods which involve a pre-step of plasma electrolytic oxidation. 
     BACKGROUND TO THE INVENTION 
     As the lightest structural metal materials, magnesium (Mg) and Mg alloys are finding increasing application in various industries because of a number of desirable properties. These include high specific strength and stiffness, and excellent castability, machinability and damping properties. The driving force also lies in the greatly improved affordability of commercial Mg alloys. Thus Mg and Mg alloys exhibit great promise. However, the high chemical reactivity of Mg results in poor corrosion resistance. This is one of the main obstacles to the applications of Mg alloys in practical environments. Thus providing a protective surface treatment is an essential part of the manufacturing process for many Mg components. 
     Among various surface treatment techniques, electroless nickel (EN) plating is of particular interest. It has advantages such as uniform deposition, good corrosion and wear resistance, good electrical and thermal conductivity, and good solderability. Electroless nickel (Ni) coatings on steels, Mg alloys, aluminium (Al) and copper (Cu) have been investigated during the past few years. Previous studies of electroless nickel coatings on Mg alloys, such as the Dow method, DeLong et al, described in U.S. Pat. No. 3,152,009 use basic nickel carbonate as the main salt in order to minimize the corrosion tendency of the Mg alloy substrate in the plating bath. This corrosion results in high cost and low efficiency—see Fatigue properties of Keronite coatings on a magnesium alloy (Surface and Coatings Technology, 2004. 182(1): p. 78-84). Gu, C., et al have looked at the use of nickel sulphate solution—see Electroless Ni—P plating on AZ91D magnesium alloy from a sulfate solution (Journal of Alloys and Compounds, 2005. 391(1 2): p. 104-109). However, the prior processing methods generally employ CrO 3 , cyanide and/or hydrofluoric acid during pre-treatment steps for these methods. Such pre-treatment steps are harmful to the operators and unfriendly to the environment. Moreover, galvanic corrosion between the Ni coating and the substrate is always a concern, especially when pores exist in the coating. Therefore, developing low cost and environment-friendly EN plating on metals and alloys such as Mg and Mg alloys with high-performance is an important task. 
     The foregoing discussion has been included here for the purpose of providing a context for the invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the invention as it existed before the priority date. 
     OBJECT OF THE INVENTION 
     It is an object of the present invention to provide an electroless nickel process which overcomes or at least ameliorates some of the abovementioned disadvantages or which at least provides the public with a useful choice. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a method for electroless plating of a substrate comprising the steps of forming a layer of oxide on the substrate by plasma electrolytic oxidation (PEO), and depositing a layer comprising nickel on the substrate by electroless nickel (EN) deposition. 
     Preferably the PEO step of comprises or results in formation of a very thin layer of dense oxide. 
     Preferably the substrate is selected from magnesium, aluminium, titanium, copper and their alloys, and iron alloys. 
     Preferably the PEO step is ceased when the voltage reaches 450V, preferably with current density varying from 0 to 1000 A/m −2 . 
     Preferably or alternatively the PEO step is ceased when a high density of nucleation sites are formed. 
     Preferably between the PEO step and the EN step there is the step of chemical deposition of palladium onto the substrate. 
     Preferably after the step of chemical deposition of palladium, and prior to the EN step there a step of reduction of palladium ions to palladium metal in the vicinity or on the substrate. 
     Preferably prior to the PEO step there is a pre-treatment step of polishing the substrate. 
     Preferably before or after any or all of the steps of polishing, PEO, EN, chemical deposition of palladium and reduction of palladium ions the substrate is washed or cleaned with water. 
     Preferably the EN is carried out by contacting the substrate with a bath containing nickel ions. Preferably the bath includes nickel ions and phosphorous ions. Preferably the pH of the bath is between 6-8 and most preferably the temperature is around 80° C. 
     According to a further aspect of the invention there is provided a method of preparing a plated substrate prepared substantially according to the above method. 
     Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings. 
     As used herein the term “and/or” means “and” or “or”, or both. 
     As used herein “(s)” following a noun means the plural and/or singular forms of the noun. 
     The term “comprising” as used in this specification means “consisting at least in part of”, that is to say when interpreting independent paragraphs including that term, the features prefaced by that term in each paragraph will need to be present but other features can also be present. 
     To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example only and with reference to the drawings in which: 
         FIG. 1 : shows a typical PEO experimental set up 
         FIG. 2 : shows a typical EN experimental set up, 
         FIG. 3 : illustrates potentiodynamic polarization curves for AZ91 alloy, PEO coating and PEO+EN coatings of the invention. 
         FIG. 4 : illustrates potentiodynamic polarization curves for AZ91 alloy, traditional EN and PEO+EN coatings of the invention. 
         FIG. 5 : shows micrographs of surface morphology of (a) AZ91 alloy and (b) PEO coated AZ91 alloy after potentiodynamic polarization tests in 3.5 wt. % NaCl solution. 
         FIG. 6 : shows micrographs of specimens after 168 h of neutral salt spray test (NSST) for (a) AZ91 alloy and PEO coating, (b) traditional EN and PEO+EN coatings, the latter of the invention. 
         FIG. 7 : shows SEM images of sample surface for: (a) Dow method pretreated, (b) PEO pretreated, (c) Dow method EN coating for 4 min, and (d) PEO+EN coating for 4 min, (e) Dow method EN coating for 60 min, (f) PEO+EN for 60 min 
         FIG. 8 : shows the cross-sectional morphology of (a) Dow method EN coating and (b) PEO+EN coating of the invention. 
         FIG. 9 : presents plots of EDS chemical analysis across the interface for: (a) traditional EN coating, and (b) PEO+EN coating of the invention. 
         FIG. 10 : presents a friction plot for the scratch tests of conventional EN on AZ91 alloy substrate. 
         FIG. 11 : shows a micrograph of a scratch track of an adhesion test on a conventional EN coating. 
         FIG. 12 : presents a friction plot for scratch tests of the EN coating prepared by the method of the invention on AZ91 alloy substrate; 
         FIG. 13 : shows a micrograph of a scratch track of an adhesion test on a EN coating of the invention. 
         FIG. 14 : shows micrographs of the ends of scratch tracks on EN coatings: a) conventional EN coating and b) an EN coating of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to providing resistant coatings for metal or metal alloys such as, but not limited to magnesium and magnesium alloys. Coatings may also be formed on titanium, aluminium, copper, and iron and their alloys. 
     Coatings which are successful in protecting a metal from corrosion are ideally uniform, well adhered, pore-free, and have a self-healing ability. 
     In accordance with the invention the invention, prior to carrying out electroless nickel (EN) plating or deposition on a metal or metal alloy substrate there is a pre-step of plasma electrolytic oxidation (PEO). We have found that a plated surface produced according to the method of the invention exhibits superior characteristics and/or properties to those prepared by conventional EN plating methods. In a preferred embodiment of the invention these characteristics and/or properties may include one or more of:
         the film between the nickel coating and the substrate prepared by PEO acting as an effective barrier layer and giving rise to enhanced corrosion resistance of the ultimate coating,   decrease of the corrosion current density. For example, the corrosion current density of the PEO+EN plating on the magnesium alloy AZ91 as indicated by potentiodynamic tests decreased by almost two orders of magnitude compared to the traditional EN coating. Salt fog spray testing further proved this improvement,   the absence of chromium species, cyanide or hydrofluoric acid. The method of the invention does not require the use Cr 6+ , cyanide or HF in its pretreatment, and therefore is a more environmentally friendly process.       

     Important steps of the method of the invention comprise: plasma electrolytic oxidation, and electroless nickel plating. 
     Plasma Electrolytic Oxidation 
     Plasma electrolytic oxidation (PEO) or deposition has been used in the prior art to prepare coatings on Fe, Al, Ti and Mg metal and/or alloys. PEO is a spark-anodizing oxidation method. It involves the modification of a conventionally anodically grown film by the application of an electric field greater than the dielectric breakdown field for the oxide. Discharges occur and the resulting plasma-chemical reactions contribute to the growth of the coating. The technique is friendly to the environment because no chromate solution is needed. 
     Conventional PEO involves the formation of a dense and thick ceramic coating. This ceramic coating is directly used to provide protection to the substrate alloys. Therefore, porosity in the coatings will greatly harm the protective ability of the coatings. 
     In the method of the invention PEO is carried out to produce a very thin layer of preferably dense and preferably continuous oxide. Without being bound by any particular theory we believe the oxide of the PEO step provides high density of nucleation sites for the next stage electroless coating. 
     A typical PEO set up is illustrated in  FIG. 1  and comprises power supply  1 , working electrodes  2  and electrolyte  3 . This is in a bipolar micro-arc oxidation treatment mode where both electrodes are the substrates to be treated. No counter electrode is requited. 
     The degree of PEO sufficient to be effective will depend upon the substrate and the conditions however, for Mg and Mg alloys the voltage usually progresses from low to high, 0 to 450 V; current density from 0 to 1000 A/m 2  over 2 to 10 minutes. This will also apply for many types of electrolyte which can produce a dense, continuous protective layer on substrates such as aluminium, titanium and steels. 
     We use the applied voltage to decide when to terminate the PEO process. In a preferred embodiment of the invention on Mg or Mg alloys, using the electrolyte as listed in Table 2 we believe when the voltage reaches ˜450 V, there will be enough nucleation sites for the subsequent EN treatment. 
     In general the preferred conditions for the PEO process include one or more of the following:
         the electrolyte is an alkaline-based solution,   the concentration for each chemical is no more than 5 g per litre (very dilute), and   the current density should be kept in the region up to 1000 A/m 2 , and ideally not exceeding 1000 A/m 2 , for at least 2 minutes.   The electrolyte must be kept under 45° C.,       

     There are no special requirements concerning the nature of the electrodes in the PEO process except the electrodes need to be conductive and the surface should be kept clean. Example electrodes include Al, Ti and Mg and Fe. It is their ability to form oxide coatings which makes them suitable. 
     In one form of the invention single oxides (eg Al 2 O 3 , TiO 2  or MgO) will be formed, but it is possible that mixed oxides will be produced on the substrate surface. For example the use of a set of phosphate based alkaline electrolytes plus additives can produce mixed oxide coatings. For example sodium hydroxide 2-5 g, sodium phosphate 2-5 g, and sodium silicate 0.5-1 g, produce magnesium oxide content and a small amount of hydroxides and silicates. Some hydroxide and oxide may benefit the subsequent nickel nucleation. 
     Our preferred conditions are listed in Stage 1 of Table 2. 
     Electroless Nickel Plating 
     Electroless nickel (EN) plating is a chemical reduction process which depends upon the catalytic reduction of Ni ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of Ni—P alloys or Ni—B alloys without the use of electrical energy. 
     Ni—P alloy coatings are more common in the art. The deposits typically contain P in the range 3-13% by weight. 
     Ni—B alloy coatings are more common in industrial wear applications for their “as-plated” hardness, which is higher than Ni—P. 
     “Poly” alloys are a combination of Ni, B or P and other metals such as Co, Fe, W, Re or Mo. 
       FIG. 2  illustrates a typical EN set up comprising a heating element  4 , the substrate to be coated  5 , electrolyte  6 , and an agitator or stirrer  7 . 
     In the method of the invention suitable electrolytes may include all kinds of traditional alkaline and acidic based electroless nickel plating electrolytes as is well known in the art. Nickel sulphate or carbonate are preferred as in nickel salts. A preferred electrolyte is as documented in Stage 4 of Table 1. This will result in a phosphorous concentration of the final Ni—P coating is 6-10 wt %. However other phosphorous concentrations are possible, within the scope of the invention. 
     General EN process conditions include control of temperature of the electrolyte, pH of the electrolyte, and chemical concentrations. 
     A preferred embodiment of the invention has an electrolyte temperature of around 80° C., and a pH between 6-8. The process is preferably carried out under ambient atmospheric conditions with some gentle stirring. 
                     TABLE 1                  Preferred Plating Process Stages                         Stage               No.   Constituent or condition   Value or range                                     1   PEO   NaOH   2-5 g/L               NaH 2 PO 4 •H 2 O   3-5 g/L               Na 2 SiO 3     0.5-1 g/L               Time   2-10 min               Current density   500-1000 A/m 2         2   Activation   PdCl 2     0.2-0.5 g/L               HCl   3-5 ml/L               Temperature   Ambient (298 K)               Time   1-2 min       3   Reduction   NaH 2 PO 2 •H 2 O   30 g/L               Temperature   Ambient (298 K)               Time   0.5-1 min       4   Electroless nickel   Nickel sulphate   15 g/L           plating &amp; operating   Citric acid   5 g/L           conditions   Ammonium bifluoride   10 g/L               Sodium hypophosphite   20 g/L               Ammonium   30 ml/L               hydroxide 25%               pH (colorimetric)   4.5-6.8               Temperature   349-353 K               Agitation required   Mild mechanical                    
Preferably there is a water rinse step after each step in this table.
 
     Other Steps 
     As is illustrated in Table 1 and as described in the description of experimental below, other steps may be included in the preferred process of the invention. Whilst not mandatory, these steps are useful for product quality and economy reactions (for example steps a) and b) prolong the life of the electrolyte). 
     a) Pre-Polishing, Prior to PEO 
     
         
         
           
             This is useful for improving the adhesion strength of the oxide film and the electroless nickel coatings. It also increases the life of the electrolyte. 
           
         
       
    
     b) Cleaning/Washing Steps 
     
         
         
           
             These can be prior to the method and in between each step of the method if desired. They are again important for improving die adhesion strength of the oxide film and the electroless nickel coatings. They can also prolong the life-time of the electrolyte. 
           
         
       
    
     c) Activation and Reduction Steps—Between the PEO and EN Steps
         The activation step again assists in preparation of dense and effective catalytic nucleation sites for the subsequent electroless nickel plating. This step involves the chemical deposition of Pd metal onto the substrate surface. Well-distributed fine Pd particles are formed on the surface. To activate the surface other materials such as nickel may be alternatively used.       

     Our preferred process may include the use of PdCl 2  to provide the source of Pd adatoms for the subsequent catalytic sites. 
     The reduction step is mainly to control the amount of Pd deposition. It can reduce the effect of possible over-doping of Pd ions in the electroless nickel plating bath or nickel atoms on the substrate (such as Mg) surface, which will again prolong the lifetime of the bath and improve the quality of nickel plating. In the preferred process it involves the use of sodium hypophosphite to reduce the Pd ions into Pd atoms on the surface. 
     Experimental 
     Rectangular specimens of 15×10×2 mm were cut from sand-cast magnesium AZ91 alloy. The nominal chemical composition of the alloy is given in Table 2. A 2 mm diameter hole was drilled in the middle of one edge of the specimens for hanging in the solution during treatments. The surfaces of the specimens were mechanically polished by SiC sand papers up to 1200 grit to ensure the same surface toughness, followed by washing with ethanol in an ultrasonic bath. The operation conditions of the PEO treatment and EN plating are shown in Table 1. The initial voltage for PEO treatment was 80 V DC, after which pulsed DC voltage was gradually increased with time to a maximum of 430 V at 500 Hz of frequency and 50% duty ratio. For comparison purposes, conventional Dow method EN plating also was applied to AZ91 alloy. 
     The following description of experimental work further illustrates the invention. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Chemical composition of AZ91 Mg alloy (wt. %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Al 
                 Zn 
                 Mn 
                 Ni 
                 Cu 
                 Fe 
                 Si 
                 Ca 
                 K 
                 Mg 
               
               
                   
               
               
                 8.9 
                 0.74 
                 0.17 
                 0.001 
                 0.001 
                 &lt;0.001 
                 &lt;0.01 
                 &lt;0.01 
                 &lt;0.01 
                 Bal 
               
               
                   
               
            
           
         
       
     
     In the PEO step the NaOH provides the pH value required and produces magnesium oxides NaH 2 PO 4  helps to produce magnesium phosphate on the surface. Sodium silicate acts as a corrosion resistant agent for the magnesium alloy. 
     In the EN step the Nickel sulphate provides the source of nickel ions for the nickel plating. A very small amount of HF helps to dissolve the nickel sulphate and acts as corrosion resistant buffer. This small amount is not detrimental. In other forms of the invention nickel carbonate is used. The citric acid acts as a complexant (to reduce the plating speed and keep the bath stable), ammonium bifluoride acts as buffer, sodium hypophosphite is the main reductant, and ammonium hydroxide is used to adjust the pH value of the electroless nickel bath. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 The other EN plating electrolyte 
               
            
           
           
               
               
               
               
            
               
                 Constituent and Conditions 
                 1. Low P 
                 2. Medium P 
                 3. High P 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Basic nickel sulfate 
                 10 
                 g/L 
                 10 
                 g/L 
                 10 
                 g/L 
               
               
                 Citric acid 
                 5 
                 g/L 
                 5 
                 g/L 
                 20 
                 g/L 
               
               
                 Ammonium bifluoride 
                 10 
                 g/L 
                 10 
                 g/L 
                 10 
                 g/L 
               
               
                 Sodium hypophosphite 
                 10 
                 g/L 
                 20 
                 g/L 
                 40 
                 g/L 
               
               
                 Ammonium hydroxide 25% 
                 40 
                 ml/L 
                 30 
                 ml/L 
                 50 
                 ml/L 
               
               
                 Thiourea 
                 1 
                 mg/L 
                 1 
                 mg/L 
                 1 
                 mg/L 
               
            
           
           
               
               
               
               
            
               
                 pH (colorimetric) 
                 6.5-7.5 
                 4.5-6.8 
                 4.5-6.0 
               
               
                 Temperature (K) 
                 349-353 
                 349-353 
                 349-353 
               
               
                 Agitation required 
                 Mild 
                 Mild 
                 Mild 
               
               
                   
               
            
           
         
       
     
     It should be noted that the EN processes are complex, and especially EN coating on Mg alloys are not fully understood. Therefore our discussion of the roles of the constituents above are to the best of our knowledge and we do not wish to be bound by this. 
     To evaluate the effect of the PEO coating, the potentiodynamic polarization measurement in 3.5 wt. % NaCl solutions was carried out to compare the corrosion behaviour of the uncoated Mg alloy, PEO coated and PEO+EN coated samples. The polarization range was from −2.0 to +0.2 V versus saturated calomel electrode. The polarization scanning rate was 40 mV/min. The neutral salt spray test (NSST) was also carried out for 168 hours according to ASTM B117-97 standard. Specimens were inspected daily. 
     The morphology and microstructure of the deposits were examined by means of optical microscopy and scanning electron microscopy (SEM, Philips XL30S) with a field emission gun. Energy-dispersive spectroscopy (EDS) and XPS were used to analyse the chemical composition and states. The structure of the deposits was also determined by using a D8 advanced X-ray diffractometer. 
     Results and Discussion 
     Al and Zn are the main alloying elements in AZ91 alloy (Table 2). The alloy consists of two phases as shown in  FIG. 6   a . The α-Mg matrix is a Mg—Al—Zn solid solution with the same crystal structure as pure Mg, and the β precipitates are intermetallic phase Mg 17 A 112 . This intermetallic compound, segregated at grain boundaries, has a free corrosion potential of −1.0 V, while the α-phase has a free corrosion potential of −1.73 V. Therefore, the intermetallic compound would provide some advantages for the nucleation during the PEO treatment. The PEO coating clearly shows a few peaks of MgO by means of XRD pattern result. 
     Potentiodynamic polarization testing:  FIG. 3  shows potentiodynamic polarization curves for the uncoated Mg alloy and the specimens with PEO coating and PEO+EN coating in 3.5 wt. % NaCl solution at room temperature. For the Mg alloy and the alloy treated by PEO, an activated-controlled cathodic process occurred in the cathodic branch, and the main reaction was hydrogen evolution. The surface of the specimen changed little in the cathodic process. When the applied potential increased into the anodic branch, an activation-controlled anodic process was observed. The polarization current increased with increasing applied anodic potential, and no passivation occurred. Although the intersection point of the anodic and cathodic curve (E corr ) showed a little shift to the positive direction, the polarization current density decreased by two orders of magnitudes under the same potential for the specimen treated by PEO. This can be explained as the PEO coating acted as an insulating barrier which can reduce the current density significantly. 
     For the alloy with the new PEO+EN coating, the cathodic reaction was still hydrogen evolution, but an obvious passivation occurred in the anodic branch. The surface morphology of the two types of specimens after the potentiodynamic polarization is shown in  FIG. 5 . For the uncoated Mg alloy, the morphology of attack changed from pitting corrosion to overall corrosion with increasing potential ( FIG. 5   a ). For the alloy with PEO coating, the attack was pitting corrosion, which gradually increased with the increase of applied potential ( FIG. 5   b ). But the degree of attack was much lower than that on the bare Mg alloy. Chlorine content was high around the corroded area in  FIG. 5  by EDS analysis, indicating that Cl− is corrosive and associated with MgO and Mg(OH) 2 . However, for the alloy with PEO+EN coating, the corrosion resistance was greatly improved, and passivation occurred during anodic polarization. There is no obvious pitting corrosion when the applied potential reaches 200 mV. 
     The electrochemical behaviour of the new coating was also compared with a traditional EN coating on AZ91 ( FIG. 4 ). It can be clearly seen that the E corr  is almost same, probably due to the two EN coatings having the same chemical composition. However, the corrosion current density (i corr ) decreased by more than two orders of magnitudes under the same potential for the specimen with PEO+EN coating, indicating that the new EN coating has less porosity and better corrosion resistance. It can be seen from the PEO pretreated surface in  FIG. 8   b  that the average pore size is no more than 5 μm, and that there is a more uniform pore distribution than that of the traditional process in  FIG. 8   a . Moreover, overall the PEO coating in this work is a denser and thinner oxidized layer as compared to the results reported by Yerokhin, A. L. 
     Salt spray testing:  FIG. 6  shows photos of the typical morphologies of specimens after 168 h neutral salt spray testing. There is no noticeable galvanic corrosion pits on the surface of the PEO and PEO+EN coatings, demonstrating that the PEO coating and the new PEO+EN coating have better corrosion resistance than that of Mg alloy and the conventional EN coating. As discussed above, the two phases, α-Mg matrix and the β-Mg 17 A 112 , have very different corrosion behaviour. Therefore galvanic corrosion took place around the phase boundaries as shown in  FIG. 6   a.    
     For the PEO coating, an effective barrier layer was formed, which prevent the penetrating of the salt solution. From the cross section SEM, we can see that the PEO layer is only 5 μm thick. This indicates that the PEO layer must be very compact. 
     SEM, EDS, XPS and XRD were used to characterise the coatings.  FIG. 7  shows SEM micrographs of the samples at different treatment stages.  FIGS. 7   a  and  7   b  show a AZ91 sample surface pretreated by Dow pre-treatments ( 7   a ) and after PEO pretreatment ( 7   b ). It can be clearly seen from  FIGS. 7   a  and  7   b  that the traditional treatment does not produce a uniform and dense protective film before EN plating. More importantly, the PEO processing only requires one-step pre-treatment rather than three-step operations which are typical of the prior art, and eliminates the use of hazardous chemicals such as chromium acid and hydrofluoric acid. Furthermore, we found that the nucleation mechanism is also different.  FIGS. 7   c  and  7   d  show the surface after conventional EN coating (4 minutes) ( 7   c ) and after PEO, +EN coating for 4 minutes ( 7   d ). The prior art EN coatings have been discussed as being preferentially nucleated on the β-phase or in the vicinity, resulting in non-uniform distribution. On the contrary, as is shown in  7   d , the nucleation of the new EN processing is quite uniform on the PEO pre-treated surface, and the nucleation density is higher than that of the traditional process. This indicates that the PEO pre-treatment eliminates the effect of the electrochemically heterogeneous AZ91 substrate surface. Consequently, the final surface of the new EN coating ( FIG. 7  is smoother and more uniform than that of the Dow EN coating as shown in  FIG. 7   e.    
     Moreover, it can be found that the nodular size in  FIG. 7   f  is about 10 μm or less whereas the nodular size of the traditional EN surface is often bigger than 50 μm, as shown in  FIG. 7   e . The coarse nodular structure in  FIG. 7   e  probably contains more pores around the nodular and substrate grain boundaries. Therefore, it can be concluded that the new PEO+EN coating provides a less porous duplex coating than the traditional one, and hence reduces the possible galvanic corrosion between the Ni coating and Mg substrate significantly. 
       FIG. 8  shows SEM cross-sectional morphology of two types of EN coatings on AZ91 alloy. It can be seen that the traditional EN process produces a tough and heterogeneous interface ( 8   a ) between the EN and the substrate due to the strong etching effect of chromium acid (CrO 3 ). However the interface between the new EN and the substrate ( 8   b ) is relatively smooth, and the coating has a more uniform thickness and smoother surface, as shown in  FIGS. 6   b  and  6   f.    
       FIGS. 9   a  and  9   b  show the EDS chemical analysis along the white lines in  FIGS. 8   a  and  8   b . It can be seen that oxygen concentration is lower than fluorine around the interface region in  FIG. 9   a . It can be seen from  FIG. 8   b  that oxygen content is higher than fluorine and the range of oxygen is much wider (5 μm) than that of the traditional EN (˜2 μm, indicating that PEO pre-treatment in the new EN process produces an oxide film of ˜5 μm thick. Moreover, it can also be found that the oxygen concentration gradually increased to ˜25 wt. % around the interface region in  FIG. 9   b , indicating that the PEO technique can produce gradient coatings. 
     Scratch adhesion strength: These results are discussed with reference to  FIGS. 10 to 14 .  FIGS. 10 and 12  are plots of the measured friction force (Fx) vs. loading (Fz) for two coatings.  FIG. 9  is the plot for the scratch tests of a conventional EN on AZ91 substrate, whilst  FIG. 12  is for an EN coating prepared on AZ91 substrate in accordance with the invention. 
     An increasing load was applied via a diamond tip which was moving on the top surface of the coatings. The measured Fx shows the increasing wear force with increasing load. The transition point (critical load, Lc) indicates the penetrating of the coating, which can be considered as reflecting the adhesion strength of the coating. 
     The conventional electroless nickel (EN) on AZ91 Mg alloy shows not only a tough surface, but also lower adhesion strength (Critical Load, Lc=10.8 N) than that of the novel EN coating (14.6 N) on AZ91. 
       FIG. 11  shows the scratch track of the adhesion test on the conventional EN coating whilst  FIG. 13  shows the track for the coating of the invention. 
     In general these results illustrate that the coating of the invention shows a higher critical load, and a smoother friction force than the conventional EN coating. 
     Furthermore, it can be seen from  FIG. 14 , which illustrates the ends of scratch tracks on EN coatings (a) conventional EN, (b) the invention EN (the arrows point in the scratching direction), that the failure behaviour is different for the two coatings, probably due to the different interface structure for the two coatings (a: MgF 2 , b: MgO). The scratch track on the EN coating produced by the method of the invention is narrower than that on the conventional EN. This may be attributed to the different hardness of the interlayer. The PEO film as the interlayer has a higher hardness than the conventional one. 
     Experimental Conclusions 
     Salt fog spray and potentiodynamic polarization testing demonstrate that the PEO treatment produces a dense, well adhered oxide coating on the AZ91 Mg alloy. The presence of the PEO film between the nickel plating and the substrate acted as an effective barrier layer, and also provided high density nucleation sites after activation treatment for the subsequent EN coating, which can significantly reduce the porosity of the nickel coating. Therefore, the coatings produced via PEO+EN process possess superior corrosion resistance to salt spray testing as compared to the traditional EN coatings. Potentiodynamic polarization tests also indicated that the corrosion current density of the new coating on AZ91 decreased by at least two orders of magnitudes. This new coating process does not need Cr 6+  and HF, and is therefore more environmentally friendly. 
     Although the invention has been described by way of example and with reference to particular embodiments, it is to be understood that modifications and/or improvements may be made without departing from the scope or spirit of the invention as described in the accompanying claims.