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Our study was concerned with the electroless nickel bath containing sodium hypophosphite as reducing agent and citric acid as complexing agent, it can be possible to deposit Ni-P, Ni-W-P, Ni-Sn-P, Ni-W-Sn-P coatings at bath temperature (90°C) and pH = 8.5. We study the effect of blackening of Ni-P, Ni-W-P, Ni-Sn-P and Ni-W-Sn-P on mechanical and electrochemical properties of the surface and solar properties of the blackening surface.
In the first part, electroless Ni-P and Ni-W-P coatings were prepared using alkaline bath and concentrations of sodium tungstate is 2g/l . The plating temperature (90̊C) is carried out for 60 minutes. Also we study the effect of blackening process of Ni-P, Ni-W-P.
- We study the kinetics of electroless nickel bath. The deposition rate was increased as citric acid concentration increased up to 10g/l, but it was slightly the same at higher concentration. Also, the deposition rate increased with increasing sodium hypophosphite concentration up to 10 g/l. This because hypophosphite is capable of reducing nickel ions over a wide pH rang (pH˃3).The deposition is increased up to 60 minute, by increase the time the deposition rate decrease and the coating is not homogenous and grossy this because of exhausting the complexing agent and basic nickel salts precipitated. The deposition rate of the bath temperature increased with increasing temperature up to 90°C. Above this temperature the deposition rate decreases. The deposition rate increase till pH= 8.5.At pH >8.5, the deposition rate decrease and the operating solution turns turbid as result of precipitation of basic nickel salt.
- X-ray diffraction pattern indicates that the structures of the coatings Ni-P was amorphous. The reflection corresponds to III plane of face-centered cubic (fcc) phase of nickel. The grain size of the as deposited Ni-P is 5.5 nm. Heating to 400°C for one hour, the peaks are sharper due to formation of fine nickel and tetragonal nickel phosphide Ni3P metastable phases where the grain size is 31.8 nm. XRD for the ternary Ni-W-P as deposited and after heat treatment. It was observed that three broad peaks corresponding to Ni (III) which are sharper than that of the as deposited binary Ni-P alloy. The grain size of the ternary alloy Ni-W-P is 21.4 nm. Hence the XRD patterns of ternary coating revealed only a prominent Ni III peak. After heat treatment, sharp peaks were induced as aresult of the formation of Ni(III) plane face-centered cubic (fcc) phase of nickel ,tetragonal nickel phosphide Ni3P and monoclinic nickel tungsten oxide NiWO4 ,were the grain size is 46.8 nm. XRD for black Ni-P layer showed five sharp peaks corresponding to cubic nickel. Black Ni-W-P coat showed five peaks , all related to cubic nickel.
- The surface morphology of the as deposited Ni-P and Ni-W-P showed a dense coating with spherical nodular structure with very smooth and high coalescence. The surface morphology of black coat (Ni-P and Ni-W-P) showed more dense coating with spherical nodular structure. The particles are in close contact with each other which improve corrosion resistance.
- The cross section of Ni-P and black Ni-P showed the thickness of the layers. The cross section of Ni-Sn-P 5.845µm which become after blackening equal to 2.545 µm and black layer is equal to 3.45 µm. The cross section of Ni-W-P is equal to 4.34µm which become after blackening equal to 3.15 and black layer is equal to 5.71 nm.
- The optical properties were measured via solar absorbance and ultra violet emittance of the black coating was detected.for black binary coat Ni-P, the absorption percentage was 99.45%. nickel tungsten phosphorous the absorption percentage of the black Ni-W-P was 99.51%. The high absorption indicates that the black film can be used as an absorbing material. the presence of tungsten in the ternary coat improves the absorption properties.XPS show that the composition of black film should be Ni-O , Ni(H2PO4)2, Ni2O3 and WO3, metallic Ni and W/nickel content in binary black film. It can be concluded that dissolution of Ni and formation of metallic oxides are main processes during blackening.
- The corrosion protection of all coats were tested in 3.5% NaCl solution. The corrosion potential E corr of the black Ni-W-P coatings was more positive than that of Ni-W-P coating and substrate. The results indicated that the corrosion resistance follows the sequence black Ni-W-P > black Ni-P > Ni-W-P > Ni-P> substrate.
- In the second part , electroless Ni-Sn-P and Ni-W-Sn-P coatings were prepared using alkaline bath and concentrations of sodium tungstate is (2g/l)and with SnCl2 (0.01-0.5g/l) in Ni-Sn-P and 0.5g/l in Ni-W-Sn-P . The plating temperature (90̊C) is carried out for 60 minutes. We study the effect of blackening process of Ni-Sn-P and Ni-W-Sn-P.
- X-ray diffraction patterns of the as deposited Ni-Sn-P and black Ni-Sn-P, showed that the structure of this type of coatings are amorphous. The diffraction patterns of the as deposited Ni-Sn-P showed broad peak which is attributed to amorphous nature of the as deposited Ni-Sn-P. The reflection corresponds to III plane of face centered cubic (fcc) nickel. The grain size of the as deposited Ni-Sn-P is 5.3nm. Phase identification of the blackened Ni-Sn-P samples, four peaks all corresponding to cubic nickel and the grian size is 5.2 nm.
- X-ray diffraction patterns of the as deposited Ni-W-Sn-P and black Ni-W-Sn-P coatings were discussed.
- The surface morphology of Ni-Sn-P and black Ni-Sn-P showed the similar spherical nodule structure and one big nodule including many fine nodule. The diameter of nodule of of Ni-Sn-P and black Ni-Sn-P composition are 0.909nm and 5.123µm respectively.
- The surface morphology of Ni-W-Sn-P and black Ni-W-Sn-P had a spherical nodular structure. The diameter of nodules were 0.5357 µm and 5.6 µm respectively.
- XPS spectra of the black Ni-Sn-P with 6.95%Sn. It can be observed that Ni, Sn, P, O and C elements were contained in the black film. According to atomic ratio and valence of elements, The compositions of the black film should be NiO, Ni(H2PO4)2, Ni2O3, SnO, metallic Ni and Sn.
- XPS spectra of the black Ni-W-Sn-P deposit, with 3.32 % W and 3.02 % Sn .In case of black Ni-W-Sn-P, it was observed Ni, P, O and carbon elements were contained in the black film. According to atomic ratio and valence of element, the composition of black film should be Ni-O, Ni(H2PO4)2 , Ni2O3, metallic Ni, W and Sn. It can be concluded that dissolution of Ni and formation of metallic oxides are main during blackening.
- The cross section of Ni-Sn-P and black Ni-Sn-P showed the thickness of the layers. the cross section of Ni-Sn-P is equal to5.09 nm which become after blackening equal to 3.2 nm and black layer is equal to 1.63 nm.
- The thickness of Ni-W-Sn-P and black Ni-W-Sn-P can be measured by cross section. The cross section of Ni-W-Sn-P is equal to 5.8 µm which become after blackening equal to 4.1 µm and black layer is equal to 0.9982 µm.
- The rate of the reflectance of the black coating of Ni-Sn-P was 0.7 in the range of the wave length 250-550 nm. The absorption percentage range of the black Ni-Sn-P range was 99.3%. The high absorption indicates that the black film can be used as an absorbing material. The black colour of Ni-Sn-P is due to the formation of the oxidized alloy at the surface of the Ni-Sn-P black layer, nickel oxides (NiO, Ni2O3) and nickel phosphorus Ni-P (confirmed by composition detection of black Ni-Sn-P.
- The corrosion protection of all the coating were discussed.the corrosion resistance follows the sequence black Ni-W-Sn-P > black Ni-Sn-P > Ni-W-Sn-P > Ni-Sn-P > substrate.