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The study of electroless Ni–P alloys with different complexing agents on Ck45 steel substrate
⁎Corresponding author. Tel.: +98 9123356295. soheilafaraji@yahoo.com (Soheila Faraji),
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Ck45 Steel was electroless coated with nickel–phosphorus alloy from a bath containing sodium hypophosphite and different complexing agents (sodium citrate, sodium acetate and lactic acid). The effect of different complexing agents on phosphorus content, morphology, structure and hardness of the deposits was studied. The coating compositions deposited were determined by using energy dispersive X-ray spectroscopy (EDX). Scanning electron microscopy (SEM) was used to study the morphology of coatings. The anti-corrosion properties of Ni–P coatings were investigated in 3.5% NaCl solution by the weight loss and potentiodynamic polarization techniques. It has been found that Ni–P coating obtained using sodium citrate complexing agent with the spherical nodular structure and smooth surface showed higher microhardness and anti-corrosion resistance.
Keywords
Electroless Ni (EN)
Complexing agent
Sodium hypophosphite
Corrosion
Hardness
1 Introduction
Deposition of metals without an external current source has become a commonly used process in modern technology. It allows to produce thin layers of pure metals, alloys or composites with uniform thickness and composition on conductive and nonconductive substrates (Faraji et al., 2011a,b,c,2012,2013; Gan et al., 2007; Rudnik and Gorgosz, 2007; Sahoo and Das, 2011). Among a variety of electroless deposited metals, for last 50 years nickel has proved its great significance in many commercial applications. Chemical Ni–P or Ni–B deposits are used as decorative and functional coatings in electronics, machinery, automobile, aerospace, etc. (Mallory and Hadju, 1990; Sahoo and Das, 2011). Four reducing agents have been mainly used for the industrial deposition of autocatalytic composite coatings. These in order of popularity are sodium hypophosphite (NaH2PO2), sodium borohydride (NaBH4), aminoboranes (DMAB) and hydrazine (NH2NH2) (Chen et al., 2003; Contreras et al., 2006; Delaunois et al., 2000). Ninety percentage of the autocatalytic Ni deposition is based on reduction by sodium hypophosphite, because of its good corrosion and wear resistance (Ashassi-Sorkhabi and Rafizadeh, 2004). It is well known that electroless Ni–P coating has a highly plating capability, high bonding strength, excellent weld ability, electrical conductivity, good anti wear and controllable magnetic properties through suitable heat treatment. These properties can be improved further by heat treatment of the coated specimen (Allahkaram et al., 2011; Balaraju et al., 2006a,b; Chen et al., 2010, 2003; Grosjean et al., 2001). The composition of coated alloy strongly influences its properties and is controlled by adjusting the pH, nickel concentration, complexing agent and temperature of the baths. Coating time and heat treatment are important factors that affect the thickness, hardness, structure and morphology of deposit. Ashassi-Sorkhabi and Rafizadeh (2004) studied the effect of coating time and heat treatment on corrosion behavior of electroless Ni–P coated on mild steel specimens in 3.5% NaCl solution. They showed that the corrosion rate of electroless Ni–P coated mild steel specimens decreased with increasing coating time. The electroless Ni–P alloys in various pHs and temperatures were found to have different phosphorus contents. Increasing the pH and the temperature cause a decrease in P content of deposits up to pH 10 (Ashassi-Sorkhabi et al., 2005). Sodium citrate (Na3C6H5O7·2H2O) (Rudnik et al., 2008; Zhao et al., 2004), sodium acetate (CH3COONa) (Zhao et al., 2002) and lactic acid (CH3CHOHCOOH) (Amell et al., 2010; Balaraju et al., 2005; Jiaqiang et al., 2006; Wu et al., 2006a) were added as a complexing agent in bath of electroless Ni coatings. The complex forms of metal increase the metallic ion solubility but decrease the deposit current and avoid hydroxides precipitation due to increase in the stability but the decrease in the deposit current (Ashassi-Sorkhabi et al., 2002; Delaunois et al., 2000). However, the effect of complexing agents on characteristics and corrosion resistances of nickel–phosphorus coating has not been studied widely.
The aim of this work is to investigate the effect of the most appropriate complexing agents such as; sodium citrate, sodium acetate and lactic acid on structure and corrosion behavior of EN plating in 3.5% NaCl by using the weight loss and potentiodynamic polarization techniques.
2 Experimental
2.1 Materials
The Ni–P composite depositions were coated on Ck45 steel substrates (20 mm × 15 mm × 1 mm, having a composition of 0.43% C, 0.35% Mn, 0.15% Si, 0.19% Cr, 0.02% Mo, 0.02% P, 0.02% S and Fe %remaining). The substrate surfaces were metallographically prepared using 400, 600, 800, and 1000-grade SiC papers and then subjected to the following pre-treatment procedure to prepare the substrate for deposition. First, substrates were cleaned with 10% NaOH at 60–80 °C for 10–20 min (to remove grease) and then rinsed with water. They were dipped in 5% HCl solution for 30 s (pickling for the activation of the surface) and then were rinsed with deionised water, before electroless plating (Faraji et al., 2011a,b,c,2012,2013).
All reagents (AR grade) were purchased from Merk (Germany) and used without further purification. Nickel sulfate hexahydrate was used as the source of nickel. Sodium hypophosphite was used as the reducing agent, which also serves as the source of phosphorus in the coating. Thiourea was used as a stabilizer to prevent sodium citrate, sodium acetate and lactic acid to control the rate of release of free metal ions for the reduction reaction. The chemical composition of the plating baths and their operating conditions are given in Table 1. Deposition was done in a 150 mL bath maintained at the temperature 90 ± 1 °C for 80 min. The pH solution was adjusted with NaOH to 5. Magnetic decomposition of the plating bath stirring with rotation rate 300 rpm was used to keep particles from sediment (Jiaqiang et al., 2006; Wu et al., 2006a,b; Zhang et al., 2008a,b).
Bath composition and operating conditions
Bath A
Bath B
Bath C
Nickel sulfate (gL−1)
30
30
30
Sodium hypophosphite (gL−1)
25
25
25
Sodium citrate (gL−1)
20
–
–
Sodium acetate (gL−1)
–
20
–
Lactic acid 88% (mlL−1)
–
–
20
Thiourea (mgL−1)
2
2
2
Temperature (°C)
90 ± 1
90 ± 1
90 ± 1
pH
5
5
5
Nickel (wt.%)
87.80
83.44
80.25
Phosphorus (wt.%)
9.35
10.31
11.45
2.2 Microstructure study of depositions
The surface morphologies of deposited Ni–P composite coatings with different complexing agents of electroless bath (sodium citrate, sodium acetate and lactic acid) were investigated using scanning electron microscope (SERON THECNOLIGY AIS-2100) and also chemical composition of coatings analyzed by wavelength dispersive spectroscopy (WDS).
2.3 Microhardness test
The coating hardness test was carried out using Micro Vickers Hardness Tester, Model Strvers Durmin (Denmark), by subjecting the samples to a load of 50 g for 10 s. The hardness test results are based on an average of five indentations.
2.4 Corrosion rate measurements
2.4.1 Weight loss technique
The steel samples were polished with 400, 600, 800, 1000 and 1200 grades of abrasive papers, washed thoroughly with distilled water, degreased with acetone and dried at room temperature (30 °C). The polished and pre-weighed Ni–P coating samples were immersed in 3.5% NaCl solution for 480 h (20 days) (Faraji et al., 2012, 2013) then the specimens were taken out, washed, dried and weighed. The procedure was repeated with Cu–P coatings and carbon steel substrates. The weight loss of Ni–P and carbon steel with immersion time t (hours) were expressed as mg cm2 h.
2.4.2 Potentiodynamic polarization studies
The corrosion parameters of various Ni–P composite coatings were studied using potentiodynamic polarization in 3.5% sodium chloride solution. The potentiodynamic polarization experiments were conducted by sweeping the potential at a scan rate of 1 mVs−1 in the range of ±600 versus open circuit potential (OCP) using an EG&G Potentiostat/Galvanostat Model 273A and the results were analyzed via Tafel extrapolation theory. Tafel plots were obtained from the data and the corrosion current density (Icorr) was determined by extrapolating the straight-line section of the anodic and cathodic Tafel lines. The Icorr values were used to calculate the corrosion inhibition efficiency (IE%) of the coatings according to Eq. (1) (Faraji et al., 2011b,2013):
3 Results and discussion
3.1 Surface analysis
The element compositions of electroless Ni–P with different complexing agents of electroless bath (sodium citrate, sodium acetate and lactic acid) were measured using EDX. The Ni and P contents on the composite coatings are tabulated in Table 1. Table 1 shows the effects of complexing agents on phosphorus content determined by EDX. Fig. 1 shows the microstructure of the deposits and steel surface morphology observed by SEM. The deposits show different morphologies when deposited in different complexing agents. In these figures a definite dependence of surface morphology on P content can be observed. Table 1 and Fig. 1 show the effects of complexing agents on phosphorus content determined by EDX. The surface morphology of Ni–P deposit obtained using bath A showed a spherical nodular structure as shown in Fig. 1a. This observation is similar to previous work (Balaraju and Rajam, 2005). The Ni–P deposits obtained using baths B and C exhibited a coarse nodular structure as shown in Fig. 2b and c. Fig. 1a–c shows that the grain size and nodule structure of nickel–phosphorus deposits decreased with increasing phosphorus content, which is in good agreement with the report of Lu and Zangari (2002).
Surface morphology of the electroless Ni–P coatings obtained using baths A–C (a) bath A (sodium citrate), (b) bath B (sodium acetate) and (c) bath C (lactic acid).
Comparison of the corrosion rates of Ni–P composite coatings with different complexing agents and substrate in 3.5% NaCl solutions.
3.2 Effect of different complexing agents on the coating microhardness
Table 2 shows the effect of complexing agent on the microhardness of Ni–P deposits after heat treatment at 400 °C. It is evident that the micro hardness value of the coatings significantly increased using sodium citrate as complexing agent which is attributed to the decrease of phosphorous content. This trend is similar to that reported by Ashassi-Sorkhabi and Rafizadeh (2004).
3.3 Corrosion rate measurements
3.3.1 Weight loss technique
The anti-corrosion performance of the Ni–P with different complexing agents in 3.5% NaCl solution was compared with substrates via the weight loss method (Fig. 2). Fig. 2 shows the comparison of the corrosion rates of carbon steel substrates, Ni–P composite coatings in 3.5% NaCl solution during the 480 h (20 days) corrosion tests. For the substrate an almost linear increase of weight loss with respect to time was observed. On the other hand, a minimum increase in weight loss for the Ni–P coatings was observed only after 200 h of immersion. These curves also show that after 480 h immersion in 3.5% NaCl solution, steel has the highest weight loss of 4.90 mg cm−2 while Ni–P obtained using bath A showed the lowest weight loss of 1.45 mg cm−2 in 3.5% NaCl solution.
3.3.2 Potentiodynamic polarization studies
Fig. 3 shows the electrochemical results obtained from polarization studies for electroless Ni–P coatings with different complexing agents in 3.5% sodium chloride solutions. The electrochemical corrosion parameters obtained from the Tafel polarization curves are tabulated in Table 3. The amounts of phosphorus present in Ni–P deposits from the baths A, B and C are 9.35, 10.31 and 11.45 wt.%, respectively. The decrease in phosphorus content in the deposits is mainly due to the presence of nickel. However, all coatings have showed relatively good resistance to corrosion in sodium chloride solution. This is because of the absence of the defects in crystalline alloys such as grain boundaries, dislocations, stacking faults and segregation. In general, the corrosion resistance of any alloy depends on the speed of formation of a surface protective film. Phosphorus can make the corrosion potential increase and the corrosion current decrease, and it promotes the anodic and cathodic reactions during the corrosion process, thereby increasing the anodic dissolution of nickel. Accelerated corrosion of nickel provides prerequisites for concentrating P and thereby for the formation of Ni3P and NixPy stable intermediate compounds.
Potentiodynamic polarization curves of electroless Ni–P composite coating in 3.5% NaCl solution obtained using (a) bath A, (b) bath B, (c) bath C and (d) substrate.
Specimens
HV50
Ni–P (used bath A)
970
Ni–P (used bath B)
856
Ni–P (used bath C)
788
Substrate
165
Specimens
Ecorr
(mV vs SCE)
Rp (KΩ cm2)
Icorr
(μA cm−2)Corrosion rate (mm year−1)
IE%
Ni–P
(used bath A)−353.2
3.4
0.47
31
82
Ni–P
(used bath B)−388.6
3.2
0.58
37
78
Ni–P
(used bath C)−416.3
3.1
0.65
40
75
Substrate
−614.1
1.6
2.65
125
0.0
It can be seen that Icorr and annual corrosion rate of the coatings decreased more using bath A while Rp increased and the corrosion inhibition efficiency (IE%) of these coatings reached 82 [Table 3].
Electroless Ni–P coating obtained by bath A resulted in a homogenous surface profile and less nodule boundaries, dislocations, kink sites and other surface defects (Fig. 1a). These characteristics can also account for the high corrosion resistance of the electroless Ni–P coating deposited under this bath condition. The other alloys resulted by baths B and C have the low corrosion resistance, because they have surface inhomogeneities (grain boundaries), which are active sites for corrosion attack (Ashassi-Sorkhabi and Rafizadeh, 2004 and Lu and Zangari 2002).
The potentiodynamic polarization results also confirm that the Ni–P composite coating (used bath A) elevates the anti-corrosion performance of specimens remarkably consistent with the results of the weight loss test.
4 Conclusion
The complex sodium citrate in the bath decreased the contents of P atoms in the depositions, and increased the deposit processes of Ni atoms.
The Ni–P composite coating obtained using sodium citrate complexing agent with the homogenous surface and less nodular structure showed higher anti-corrosion resistance.
The microhardness of coatings increased with decreasing P content.
Acknowledgments
The authors acknowledge the Amir Kabir University of Technology for the use of their instruments facilities.
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