Effect of Process Time in the Deposition and Corrosion Resistance of Ni-Zn-P Coating Formed on Mild Steel Surface

In this investigation, uniform and full coverage of Ni-Zn-P coatings were electroless deposited successfully on mild steel substrates. The coatings were electroless formed at 85ºC for various deposition time (10, 30, 60 and 90 min). The coating morphology were observed via optical microscope and the corrosion resistance of obtained coatings were evaluated by weight loss test in a 3.5% NaCl solution. The results revealed that the coating deposition increased as the process time increased. The optimal uniform coating deposition was formed at deposition time of 60 & 90 min. Accordingly, the anti-corrosion performance was improved and the best corrosion resistance was obtained at process time of 90 min. The weight loss was 0.55 g/m 2 after 24 h immersion time for the coating formed with process time of 90 min.

Compared to electrodeposition coatings, electroless deposition technique produce coatings with uniform and full coverage regardless of the sample shape.

Journal of Alasmarya University: Basic and Applied Sciences
The electroless coatings performance depend on many factors including process parameters (deposition temperature, pH of the solution and process time) and its microstructure [9,10].The inclusion of zinc on the chemical composition of electroless Ni-Zn-P coating was reported by Abdel Hamid et al. [11].The codeposition of Zn with Ni-P resulted in transformation of the coating structure from amorphous to crystalline.It is also noted that the corrosion resistance was significantly improved as the percentage of the zinc reached to 7.4% in the coating composition.Isaac Fayomi et al. [12] have investigated the effect of deposition time on the anticorrosion and microstructural properties of Zn-Ni-P coating formed by electrodeposition technique on mild steel surface.They found that, higher corrosion performance was provided as the time of deposition increased.Remarkably, a predominant refine crystal orientation on the morphology was noticed, which produced coatings with free of stresses, porosity, and cracks at the optimum time.Therefore, optimization the electroless deposition parameters such as time and temperature is very important to obtain coating with uniform deposition, full coverage and good corrosion performance.In this study, the Ni-Zn-P coatings were formed on mild steel samples by using electroless deposition method with alkaline bath.The effect of deposition time in the coating formation and corrosion resistance were evaluated and addressed.

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MATERIALS AND METHODS Mild steel samples with dimension of 10×15×1.5 mm and chemical composition as listed in Table 1 were used in this study to produce Ni-Zn-P coatings by electroless plating from alkaline solutions.The chemical composition and operating conditions for the alkaline bath used in this study are shown in Table 2.The pH of the coating solution was hosen (9.5) based on previous literatures which recommended the range of pH should be between 8 and 11 [5,11].The values of pH higher than 11 and lower than 8 provide coatings with less deposition and coverage.The pH in this study was adjusted by adding a 50 % NaOH solution.
The temperature also has a strong effect on the coating deposition, and based on some preliminary experiments and results (not shown here) the temperature fixed at 85ºC.This was in agreement with reported study by Abdel Hamid, et al. [11], where the best deposition temperature for Ni-Zn-P coating was in the range of 75 to 90ºC.
The mild steel samples were firstly abraded with emery SiC papers till 420 grits, and then degreased in acetone and cleaned thoroughly in deionized water respectively.Thereafter, the substrates immersed in the alkaline coating bath for different processing time.Eventually, the coated samples were cleaned thoroughly by deionized water and dried by compressed air.The experimental set up of coating deposition is presented in Fig. 1.The surface morphology of the obtained coatings was photographed and observed using optical microscope.The corrosion performance of the obtained coatings was evaluated by total immersion test in an aerated 3.5% sodium chloride solution at room temperature.The calculations were taken by measuring the substrate weight before immersion in the corrosive solution (Wʹ).After completing the test period, the samples were taken out, rinsed with deionized water, dried with compressed air and then weighed again (Wʺ).The weight loss in the sample weight after 24 and 168 hours of immersion, which related to the corrosion performance, was determined according to the following equation [13,14]: where, W loss is the weight loss in the coated sample (g/cm 2 ), and A is the surface area of the substrate (cm 2 ).All the measurements were performed by analytical balance with an accuracy of ± 0.0001g, and the results were the average of three times.the surface of coated substrates demonstrated a very fine coating with a uniform and full coverage.The formation of electroless Ni-Zn-P coating on mild steel surface occurred though some cathodic reactions which start at metal-bath interface.These reactions resulted in reduction of Zn, Ni, and P on the metal surface as follows [11]:

Corrosion resistance of Ni-Zn-P coatings evaluated by weight loss test
The corrosion performance, evaluated by weight loss test, of mild steel sample and Ni-Zn-P coatings obtained at various deposition time are shown in Table 3.It is observed that the deposited coatings provided a barrier protection to the mild steel substrates from the corrosive media of sodium chloride solution.The weight loss of bare steel sample after 24 and 168 h of immersion time was 3.25 and 16.93 g/m2 respectively, whilst it was 0.55 and 5.39 g/m2 for the coating deposited with 90 min of process time.It was also noticed that, as the process time increased the corrosion performance of coated substrates was improved indicating a remarkable effect of process time in the coating deposition and corrosion resistance.These results clearly revealed that the Ni-Zn-P coating provided an excellent corrosion protection to the mild steel surface and the best corrosion resistance obtained with deposition time of 90 min.

CONCLUSIONS
Ni-Zn-P coatings formed on mild steel surface by electroless method at various deposition time were addressed in this study.The optical microscope micrographs showed the fine and dense coatings were formed at process time of 60 and 90 min.The weight loss test demonstrated that the corrosion resistance of steel substrate was significantly improved after coating formation.The deposition time has a strong effect on the coating formation and its anticorrosion performance.The best coating coverage and corrosion resistance was obtained for Ni-Zn-P coating deposited for 90min.

Figure 1 . 1 Volume ( 6 )
Figure 1.experimental set up for Ni-Zn-P coating deposited on mild steel surface.

Figure 2 .
Figure 2. Photos of (a) Ni-Zn-P coating on mild steel surface performed at 85ºC for 90 min, and (b) bare mild steel surface.

Table 1 .
Chemical composition of the mild steel substrate

Table 3 .
Corrosion resistance of Ni-Zn-P coatings evaluated by immersion in 3.5% NaCl solution