Structure and Corrosion Behavior of Nano-Crystalline Ni-P Alloy Containing Tungsten

The whole world is interested in the metal industry and its permanent development. One of these metals is carbon steel. Therefore, scientists tend to improve the properties of this metal, in this research we have improved the properties of carbon steel through electroless plating process of Ni-P and Ni-W-P alloys. In different industries, electroless nickel-phosphorus Ni-P and nickel-tungsten-phosphorus Ni-W-P deposits have been commonly used as engineering safety coatings. In our research, Ni-P and Ni-W-P were deposited on low carbon steel by using acid bath. To study the improvement of the properties of the coats, microstructure analysis investigated by thin film (XRD), coat’s morphology by electron microscope scan (SEM), analyzing the coat by X-ray dispersive energy (EDX) and protection of corrosion of the coats were determined by potentiodynamic polarization measurements in artificial sea water (3.5% NaCl solution). The results indicated that the phases formed from the electroless coating give excellent corrosion resistance of low carbon steel and also indicated that the alloy formed in the presence of tungsten through the electroless bath give higher corrosion protection than that formed without it. As the concentration of tungstate increase in the bath, coat has higher corrosion protection i.e. Ni-W-P III>Ni-W-P II >Ni-W-P I>Ni-P.


Introduction
Electroless process is an autocatalytic technique that required action of the deposited metal and reduced by the electron obtained [1]. Also have advantages over electrolytic process such as uniform coating and non-conductive materials can be coated [2]. The resistance of corrosion and character of the crystalline deposit Ni-P could affected by the composition of the alloy and heat treatment [3]. Electroless plated Ni-P coatings are used in a wide variety of sectors, these have several attributes as nice wear, protection of corrosion and a strong degree of hardness [4][5][6][7][8]. Insertion of hard nanoparticles (e.g., C, Ti-Si-C, W, Ti-N, etc.) improve the properties of the alloy [9][10][11][12][13][14][15][16][17][18][19]. As the tungsten content increase the life time of the alloy increase, since Ni-W-P barrier have longer life time than Ni-P barrier [20]. Ni based coatings temperature resistance improved by adding tungsten that has high melting point. As tungsten increase the phosphorus decrease and make enhancement changes in nanocrystalline phase composition [21]. By heating the deposits of the binary and ternary alloys, the hardness increase [22]. At 400°C show crystallinity of Ni [23].

Experimental Procedure
The substrate material was low carbon steel for its low cost. The sample was (2.5x2x0.1cm 3 ) with chemical composition as shown in Table 1. The samples have been mechanically polished, cleaned with sodium carbonate, degreased in a degreasing alkaline solution, chemically etched in dilute 10% HCl and washed down in running water and deionized water, dried and then hanged in the electroless bath.

Bath Composition
The electroless bath was included in 100 ml glass container which was stored in a water bath at a steady temperature at 85°C. All the deposits were plated for one hour at pH= 4. After deposition, the samples were again washed down in running water and deionized water, dried and kept for characterization. The coatings were deposited from the most stable bath containing, the source of Ni+2ions (nickel sulphate), reducing agent(sodium hypophosphite), complexing agent (Sodium acetate), lactic acid, propionic acid and the surfactant (sodium lauryl sulphate) then adding different concentration of sodium tungstate 20, 40 and 60 g/l.

Thin Film (XRD), SEM and (EDX) Measurements
A thin film (XRD) measurement was made in as deposited conditions and with heat treatment using an x-ray diffractometer (Panalytical coX'pert PRO, Holland). Scanning electron microscope (SEM) (Quanta 250 FEG, Taiwan). And with x-ray fluorescence (XRF) energy dispersive analysis of model ARL 9400 (EDX).

Electrochemical Study
To find out the electrochemical polarization behavior of binary and ternary alloy coatings, potentiodynamic polarization studies were tested in 3.5% NaCl solution at 30°C with scan rate of 0.01 V/sec within a potential range of 0 to 250 mV. The polarization resistance (RP) was evaluated using the polarization resistance technique in the region of ± 50 mv with respect to Ecorr.

Thin Film-(XRD)
Figure 1 a-d shows X-ray diffraction pattern of the as deposited Ni-P and Ni-W-P deposits. The pattern of the diffraction of the as deposited coatings has only a single wide peak for all the deposits. The peak broading can be due to the deposit's amorphous existence. The reflections corresponding to Nickel phosphide tetragonal Ni3P phase and nickel phosphide Rhombohedral Ni8P3 phase for Ni-P alloy. Nickel phosphide tetragonal Ni3P, three face centered cubic plane nickel (fcc), nickel tungstate phosphide NiW2P3 monoclinic and nickel phosphide rhombohedral Ni8P3 for Ni-W-P coatings.   There was a small rise in grain size for ternary alloys from Table 3. Ternary alloy coatings grain size increased attributed to the co-deposition of Win the nickel-tungsten-phosphorus deposits. The rise in grain size may be due to the presence of larger atoms of tungsten (r=2.02 Ao) in face centered cubic nickel matrix.

Morphology-SEM
Figure 3 a-d shows the surface morphology of binary and ternary alloys (Ni-W-P I, Ni-W-P II and Ni-W-PIII). All these surface morphology of Ni-P and Ni-W-P deposits has a similar spherical nodular structure and are big nodule including many fine nodules. It also can be seen that the slight difference of Ni-W-P coatings that the boundary between fine nodule in one big nodule is disappeared gradually with increasing the amount of sodium tungstate in plating bath.

Figure 3. Surface morphology of as coated (a) Ni-P, (b) Ni-W-P (I), (c) Ni-W-P (II) and (d) Ni-W-P (III).
Nodular deposition in a coating depends on nucleation rate and the growth of the deposit. These SEM observations indicated obviously that the amount of sodium tungstate in plating bath will affect the nucleation rate and growth of deposit. Also we can notice that the addition of sodium tungstate make surface morphology fine, more compact grain and highly coalescence smooth deposit surface than that of nickel phosphorous deposit. Annealing of binary and ternary alloys to 400°C for one hour, where the plating layer is finer and more coalescence due to the changes of the amorphous into crystalline alloy. The surface morphology of the heat treated coating binary and ternary alloys are shown in Figure  4 a-d respectively. The analysis of the content of the as deposited electroless nickel coatings (EDX) are shown in table 4.

Dispersive Energy study of X-ray Fluorescence (EDX)
The decrease of phosphorus content as tungsten is added to electroless baths is due to produce of Ni-W-P films as shown from thin film XRD.
Competitive reaction between tungsten and phosphorus compounds can occur or the codeposition of phosphorus during deposition could be prevented by the formation of complex compounds. For all the plating baths, the nickel content was near 88 percent.

Potentiodynamic Polarization Studies
To understand the corrosion behavior of these coatings detail, potentiodynamic polarization studies were made in 3.5% NaCl solution at 30°C. Figure 5 a-d shows the polarization curves for binary and ternary coatings in 3.5% sodium chloride solution and are compared with low carbon steel substrate (straight line). The parameters of electrochemical corrosion obtained from Tafel curves are tabulated in table 5. It's clearly seen from the table that the corrosion current density value for all the coatings in 3.5% NaCl solution is from 15.84 x 103 to 10.939 x 103nA/cm2. For mild steel substrate (15.84 x 103nA/cm2) a higher corrosion current density value is obtained. The corrosion rate (MPY) calculated for these coatings also show similar trend as shown in the table 5.  The preferential dissolution of nickel leading to the enrichment of phosphorous on the surface layer is evident from literature on Ni-P coatings. This enriched phosphorous interacts with water to form a coating of adsorbed hypophosphite anions (H 2 PO 2 -), which in turn blocks water supply to the electrode surface preventing nickel hydration which is known to be the first step in the formation of either Ni +2 species or a passive nickel film. So, the more resistant to corrosion obtained for electroless Ni-P and poly alloy coating because of the enrichment of phosphorous on the electrode surface. The protection of corrosion follow the sequence Ni-W-P III<Ni-W-P II<Ni-W-P I<Ni-P.

Conclusion
Electroless binary and ternary Ni-P, Ni-W-P I, Ni-W-P II and Ni-W-P III films were prepared using acidic bath. Inclusion of an alloying element influenced the composition of the deposit, XRD results revealed that all the as plated deposits had a single wide peak of Ni III. The grain size of the as deposited Ni-P, Ni-W-P I, Ni-W-P II and Ni-W-P III were 3, 3.6, 6.1, and 14 respectively. As plated deposits exhibited nodular appearance. Presence of tungsten improved the corrosion resistance in 3.5% NaCl solution. Annealing binary and ternary alloys show nodular structure also and sharp peaks of X-Ray diffraction.