| For decades, NiWP alloys had been studied widely for their excellent properties, such as appearances, hardness, wear resistance, corrosion resistance and et al.The NiWP alloy can be obtained by means of various methods, such as electroless plating,electrodeposition(direct current and pulse plating) and sputtering technique. The NiWP alloys possess various properties for their different structures. The NiWP alloy with tungsten content >20 wt.% is rather hard to be obtained using deposition methods and the reports about the alloy can be hardly found.Fortunately,a NiWP alloy with tungsten content being about 30 wt.% was successfully electrodeposited in an ammoniacal citrate bath using proper plating parameters.The detailed research work was listed as follows:1. Electrodeposition of Ni-30W-2P(wt.%) alloy from a citrate bath In this dissertation, the effects of operating parameters on NiWP alloy deposition in a citrate bath are studied in details.The results show that tungsten content in the alloy and the current efficiency increase with pH. When the ph is 7.0, the tungsten content can reach 30.28 wt.%.The pH should be controlled in the range of 6.5~8.0.The W content is 30.75wt.% in the deposit when the temperature of the solution is 70℃。Considering the higher temperature can bring on the decrease of current density, the temperature should be controlled in the range of 6.5~8.0. The W content in the alloy increases and then decreases with the increasing of current density. The bright and smooth Ni-30W-2P(wt.%) alloy can be electrodeposited in the citrate bath using best operating parameters.The alloy coating is unknit and uniform. The cross-sectional morphology of the alloy indicates the deposit is compact and combined with the substrate well.2. The study of the structure of the Ni-30W-2P(wt.%) alloy Firstly ternary Ni-30W-2P (wt.%) alloy coatings were electrodeposited on E235 steel in an ammoniacal citrate bath. Then their microstructure, crystal structure, crystallization process and the relation between the structure and properties were studied in detail, respectively. The corrosive morphology indicates that the crystal shape in the alloy is mainly hexagons and pentagons. The distribution of tungsten element on the deposit surface is symmetrical. The experimental results show that when heat treatment temperature is below 500℃, Ni-30W-2P(wt.%) alloy deposits present a Ni(111) characteristic peak at about 2θ44o,and it shows thermal stability in some extent. Annealed at 500℃for 180 min, the amounts of metastable phases in the deposit increase and achieve transformation to form stable phases finally, meanwhile the deposits possess the highest microhardness. Potentiodynamic polarization experiments conducted in a 3.5wt.% NaCl solution indicates the icorr of Ni-30W-2P (wt.%) alloy annealed at 300℃for 1 h is closed to Cr coating and exhibits excellent corrosion resistance. The HCl solution (VHCl:VH2O=1:1) immersion tests indicates that the Ni-30W-2P (wt.%) alloy possesses outstanding corrosion acid -resistant.3. Study on galvanic corrosion resistance of the NiWP alloy The galvanic corrosion behaviors between the deposits(NiWP,FeNiW,NiP and Cr) and substrate(Q235A steel) are studied in 3.5 wt.% NaCl solution by means of weight loss, photography of corrosive morphologies and electrochemical polarization test. The results show that in these four kinds of couples Q235A steel acts as the anode, whose galvanic corrosion behavior is mainly controlled by the cathodic polarization and all the deposits act as the cathode, respectively. The corrosion rate of all the couples has the same trend with the corrosive time. The galvanic effect of these couples has such a relation as:γNiP >γNiWP(140℃) >γFeNiW(600℃) >γNiWP(320℃) >γNiWP(500℃) >γCr.The NiWP deposit annealed at 500℃possesses excellent galvanic corrosion resistance which is close to that of Cr one. The NiP alloy has the poorest galvanic corrosion resistance and the FeNiW alloy exhibits slight weaker property than NiWP one for its structure with some microcrack.The galvanic corrosion behavior of NiWP alloy coating coupled with 30CrMo, 35CrMo, 40Cr and 45# steel in 3.5 wt.% NaCl solution was studied by means of weight loss and photography. The results show that NiWP was protected for acting as cathode and the steels as anode were corroded severely. The galvanic effect of these couples has such a relation as:γ30CrMo >γ40Cr >γ35CrMo >γ45# steel. The NiWP alloy coupled with 30CrMo is highly susceptible to galvanic corrosion and the one coupled with 45# steel is least susceptible to galvanic corrosion in corrosive environment. |
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