Effect of surfactants on the coating properties and corrosion behaviour of Ni–P–nano-TiO2 coatings

    Effect of Surfactants on the Coating Properties and Corrosion behaviour of Ni-P-nano-TiO 2 Coatings T.R. Tamilarasan, R. Rajendran, G. Rajagopal, J. Sudagar PII: DOI: Reference:

S0257-8972(15)30120-1 doi: 10.1016/j.surfcoat.2015.07.008 SCT 20381

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

27 June 2014 24 April 2015 6 July 2015

Please cite this article as: T.R. Tamilarasan, R. Rajendran, G. Rajagopal, J. Sudagar, Effect of Surfactants on the Coating Properties and Corrosion behaviour of Ni-P-nanoTiO2 Coatings, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.07.008

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ACCEPTED MANUSCRIPT Effect of Surfactants on the Coating Properties and Corrosion behaviour of Ni-P-nano-TiO2 Coatings

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T.R.Tamilarasana*, R.Rajendrana, G.Rajagopalb, J.Sudagarc a

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Department of Mechanical Engineering, School of Mechanical Sciences, B.S.Abdur Rahman University, Vandalur, Chennai-600048, Tamil Nadu, India. b CSIR – Central Electrochemical Research Institute, Karaikudi-630006, Tamil Nadu, India. c Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai-600036, Tamil Nadu, India.

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*Tel: +91-44-22751347; E-mail: [email protected]

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Abstract

In this paper, the effects of two different types of surfactants on the properties of Ni-P-

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TiO2 coating on low carbon steel substrate were investigated. Sodium dodecyl sulphate (SDS) -

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anionic surfactant and Dodecyl trimethyl ammonium bromide (DTAB) - cationic surfactant were

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used for the deposition. Deposits were characterized by high resolution scanning electron microscope (HR-SEM), energy dispersive X-ray spectrometer (EDS), and X-ray diffraction (XRD) to study the surface morphology, composition and crystal structure of the coatings respectively. In addition, the influence of surfactants on corrosion behaviour of Ni-P-TiO2 coatings was also examined using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt. % sodium chloride solution. The results showed that at an optimum concentration of cationic surfactant DTAB, uniform distribution of TiO2 particles with no defects was observed. The corrosion properties were improved by the incorporation of TiO2 particles in the Ni-P matrix. The increase in corrosion resistance of the Ni-P TiO2 coatings significantly depends on the surfactant and its concentration. Keywords: Electroless deposition; Composite coating; TiO2; Effect of Surfactants; Corrosion resistance. 1

ACCEPTED MANUSCRIPT 1. Introduction The unique combination of high hardness, excellent wear resistance and coating

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uniformity has resulted in widespread application of electroless Ni-P coatings in various

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industrial fields like automotive, oil and petroleum and aerospace industries. Electroless nickel

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plating, also called as autocatalytic reduction process, where coating is produced by the catalytic reduction of nickel ions using sodium hypophosphite as the reducing agent without application

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of electric current. It has proved to be an attractive and alternate method of producing thin and

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uniform deposit on substrate when compared to conventional electroplating [1-3]. Advancement in electroless Ni-P deposition is the co-deposition of various solid particles. Metallic coatings,

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containing the second phase of solid particles can improve the electro chemical and mechanical

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properties of the substrate to a great extent [4]. The properties of these composite coatings depend on various parameters like bath composition, structure of the coatings and the

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characteristics of embedded particles such as type, shape and size [5]. The fact that incorporating nano-sized particles in Ni-P autocatalytic coatings greatly improves their properties and enhances their performance has raised the interest on the metal matrix nano composite coatings [4, 6].

Most recently, the Ni-P composite coatings containing inorganic non-metal nano particles as the reinforcing phase finds wide applications requiring anti-corrosion, anti-wear and anti-friction properties in particular [7]. The combinations that have received considerable attention are electroless nickel in combination with SiC, SiO2, Al2O3, TiO2, CNT, ZrO2 nano particles etc. Among these, introduction of TiO2 particles within the coatings has encouraged a tremendous interest in the research community because of its ever widening applications in the engineering materials discipline. Studies reveal that incorporation of TiO2 particles resulted in enhanced wear 2

ACCEPTED MANUSCRIPT and corrosion resistances and also other properties such as electrocatalysis and photocatalysis [812]. The improved electrocatalytic activity can be used for hydrogen evolution reaction (HER)

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and it is applied as a catalytic support, reinforcement and inert filler [13]. The electrocatalytic

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oxidation of small organic molecules can be used for fuel cell applications [14]. The electroless Ni-P TiO2 coatings reduced the adhesion of bacterial strains for about 75%, as compared with

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Stainless Steel and Ni-P coatings [15]. The essential outcomes of such particle reinforcement can

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only be achieved if the solid phase is well dispersed in the metal coating [16]. Undoubtedly, the properties of Ni-P composite coatings is highly dependent on the stable dispersion of the nano

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particles in the plating bath, otherwise uniform distribution of particles will get affected, owing to segregation and agglomeration of nano particles with high surface energy and activity in the

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plating bath [17]. In order to maintain nano powder suspension in the solution, ultrasonication and magnetic stirring are widely used. Surfactants are often used to help particle separation in the

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plating baths [18, 19]. Surfactants are responsible for enhancing the stability and uniformity of a suspension by increasing the wettability and their ability to modify the surface charge. Also, they improve the adsorption of suspended particles on the cathode by increasing their net positive charge [20].

Many attempts have been made to deposit TiO2 within the Ni-P matrix by Electroless deposition method. However, there is lack of information in literature on the effect of surfactant types and concentration of surfactants on Ni-P-TiO2 composite coatings. The aim of this investigation was to explore the effects of two different types of surfactants namely sodium dodecyl sulphate (SDS) as anionic and Dodecyl trimethyl ammonium bromide (DTAB) as cationic surfactant on Ni-P-TiO2 coatings. The study focuses mainly on the effects on surface morphology,

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ACCEPTED MANUSCRIPT composition and deposition rate of Ni-P-TiO2. In addition, the influence of surfactants on corrosion behaviour of Ni-P-TiO2 coatings was also investigated.

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2. Experimental procedure

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2.1. Substrate processing

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In this work, low carbon steel specimens with dimensions of 20 mm X 20 mm X 3 mm

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were used as substrate materials for the deposition of coatings. All the samples were finished by grinding followed by disc polishing. The surface finish values were measured using a stylus

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instrument and the average roughness value of the finished samples was 0.56µm. Then, the sample were subjected to ultrasonic cleaning in acetone for 15 min, rinsed with distilled water,

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degreased by ethanol for 10 min and activated using 50 vol% HCl solution for 30 sec. Finally,

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2.2 Plating methods

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the specimens were rinsed with deionized water and immediately transferred to the plating bath.

In the plating bath, Nickel sulphate was used as the source of nickel, sodium hypophosphite was used as the reducing agent, lactic and propionic acids were used as the complexing agents. Thio urea was used as the stabilizer to prevent the decomposition of the plating bath. The bath compositions and operating conditions are given in Table 1. All chemicals used were of analytical grade. The pH value of the bath was measured by Eco Testr pH meter and was maintained at 6 during the plating process by using ammonia solution as the pH regulator. The stirring rate and the temperature of the plating bath were maintained at 400 rpm and 88 ± 2 °C respectively. The TiO2 nano particles (anatase) were purchased from NANOSHEL Co Ltd with an average particle size of 25nm. The TiO2 concentration in the plating bath was kept constant at 2 g/l. The amount of dispersed TiO2 particles in the electroless nickel bath 4

ACCEPTED MANUSCRIPT greatly influences the incorporation level in the deposit. It was previously reported, at a concentration level above 2 g/l, there seems to be saturation though the concentration is varied

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up to a level of 10 g/l [21].

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Various concentrations of sodium dodecyl sulphate (SDS) and dodecyl trimethyl ammonium

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bromide (DTAB) were used as the surfactants. The different surfactant concentrations were arrived based on critical micelle concentration (CMC) values of SDS and DTAB as shown in

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Table 2. Before composite plating, the TiO2 nano particles were dispersed by ultrasonic agitation

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for 30 min just before being added to the plating bath. Initially, Ni-P coating was deposited for 15 min and then TiO2 was added to the plating bath and deposition continued for 45 min. The

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total plating time was 60 min. For the purpose of comparison, plain Ni-P coating and Ni-P-TiO2

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composite coating without surfactants were deposited using the above procedure. After plating,

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the deposits were cleaned with distilled water and air dried at room temperature. 2.3 Coating characterizations

The surface morphology of the electroless Ni-P-TiO2 coatings using different surfactant concentrations was examined by means of high resolution scanning electron microscope (HRSEM) equipped with energy dispersive X-ray spectrometer (EDS), which was used for the determination of chemical composition of the coatings. The crystal structure of the coatings was determined using X-ray diffraction method (XRD, Shimadzu, X’Pert) by scanning in the range of 2θ = 10 to 90°. Surface roughness of the coating was measured using a stylus instrument and averaged for five trials per sample. The deposition rate of the electroless Ni-P-TiO2 is expressed in terms of the weight gain during the deposition process. The sample as-deposited was dried and

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ACCEPTED MANUSCRIPT weighed for repeatability so as to eliminate the effect of moisture in the gradual drying of the Ni-

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P-TiO2 deposit. The deposition rate can be expressed as the following equation:

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(Deposition Rate)

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Where R is the deposition rate (µm/h), w is the weight gain (g), d is the deposit (g/cm3), A is the area of the deposit, and t is the deposition time in hours. The experimental outcomes mentioned

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in this paper are the average values of a number of runs, which are reproducible within ± 15 µg. Sudagar et al. [22] employed the same to find the deposition rate of electroless coatings.

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Corrosion resistance of the deposits in 3.5 wt. % NaCl solution at ambient temperature was conducted by potentiodynamic polarization and electrochemical impedance spectroscopy

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(EIS). The corrosion tests were conducted using PARSTAT potentiostat/galvanostat/FRA model 2273. Three electrode configuration was employed consisting of the sample as the working

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electrode, saturated calomel electrode (SCE) as reference electrode and a platinum as counter electrode. Potentiodynamic polarization studies were carried out at a scan rate of 0.5 mV/s in the range of -0.2 to +0.2 V versus open circuit potential (OCP). Tafel plot was transformed from the recorded data and the corrosion current density (Icorr ) and corrosion potential (Ecorr) were determined directly from these log (i)-E curves. Thus corrosion rate in mills (0.001 in.) per year (mpy) can be determined by using the following equation: Corrosion rate (mpy) = Icorr K (1/ρ) Ɛ Where K is a constant, unit-less and equal to 0.12866, ρ is the density of corroding metal in units of g cm-3, Icorr is given in units of µA/cm2, and Ɛ is the gram equivalent of corroding metal. Thus corrosion current density (Icorr) can serve as a measure of the corrosion rate for general cases.

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ACCEPTED MANUSCRIPT The EIS studies were conducted using a Frequency response analyser (FRA) coupled to a Princeton applied research (PARSTAT potentiostat/galvanostat/FRA Model 2273). The EIS

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measurements were carried out in a frequency range of 100 KHz to 10 MHz with an applied AC

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signal of 10 mV. Powersuite software was used for analysing the acquired EIS data.

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3. Results and Discussion

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3.1 Characteristics of the deposits

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Fig. 1-2 shows the surface morphology of the electroless Ni-P-TiO2 composite coating with different amounts of cationic and anionic surfactants. The given microstructures permit us

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to evaluate the relationship between a Ni-P coating (Fig. 1a), Ni-P-TiO2 coating (Fig. 1b) and Ni-

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P-TiO2 coatings at various concentrations of the surfactants (Figs. 1c-1f, and 2). These microstructures reveal that the morphology of the composite coating is considerably affected

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after the addition of surfactants. In the case of Ni-P-TiO2 coating without surfactant (Fig. 1b), a nodular structure was observed with the presence of microvoids. The agglomerated TiO2 particles were also observe at some areas. The agglomeration was analogous to the inferences reported by Esmaeel Nad et al. [23]. The embedded TiO2 nanoparticles in Ni-P layer have an effect on the heterogeneity of the surface and distinctly increase the boundaries between Ni and other particles in the matrix. In the presence of anionic surfactant SDS, the embedded TiO2 grains into the matrix significantly enhancing the surface expansion of the composite Ni-P-TiO2 layer compared to that of coating without surfactant. When the surfactant concentration is equal to or above CMC, smooth surface is formed and microvoids are reduced (Figs. 1d,1e), which is also confirmed by the roughness

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ACCEPTED MANUSCRIPT measurements shown in Table 3. This is in complete agreement with the investigations of Alsari et al. and Elansezhian et al. [24, 25].

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In the presence of DTAB, the surfactant tends to form globular aggregates at the surface of the

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specimen due to electrostatic attraction between the substrate’s negative charge and the cationic

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head groups of the surfactant. At lower concentration of DTAB surfactant, nodular structures are formed and as the concentration increases (above 0.5 X CMC), the surface morphology slowly

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changes from non smooth nodular structure to a smooth surface with uniform distribution of

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TiO2 particles and no voids appear in the surface of the coating (Figs. 2b,2c) which is aided by the surface roughness measurements shown in Table 3. These results display a similar trend as

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stated in the investigation of Medina-Valtierra et al. [26] who studied the effect of CTAB on the

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roughness of TiO2 films formed by sol-gel process.

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The results of elemental composition obtained by EDX analysis are given in Table 4 confirming the presence of Ni, P and TiO2 elements in the coatings. It is clearly evident that the Ni content has decreased and P content has been fluctuating with increasing TiO2 content. The XRD patterns of Ni-P-TiO2 composite coatings with and without surfactants are shown in Fig. 3. A broad peak at 2θ = 45° can be seen in all the coatings which indicates the amorphous structure of Ni-P deposits. The small TiO2 peak at around 2θ = 25° in XRD patterns also confirms the presence of crystalline TiO2 particles embedded in the Ni-P matrix in the presence of SDS and DTAB surfactants. 3.2 Effect of surfactants on the deposition rate Fig.4 represents the impact of varying the concentration of cationic and anionic surfactants on the deposition rate of Ni-P-TiO2 composite coatings. It is interpreted from the 8

ACCEPTED MANUSCRIPT graph that concentration of the surfactants (anionic and cationic) affects the deposition rate to a great extent. The deposition rate of the specimen without the use of surfactant was measured to

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be 34 µm/hr. But the rate seemed to decrease with increasing concentrations of the anionic and

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cationic surfactants in the presence of 2 g/l TiO2 in the plating bath. This could be due to the addition of surfactants to the solution, as a result of which the surfactant particles are more prone

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to enveloping the cathode surface, thereby slowing down the diffusion of Ni 2+ ions over the

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surface, subsequently affecting the rate of co-deposition [27]. For a given surfactant concentration, the anionic surfactant SDS shows lower rates of deposition compared to that of

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the cationic surfactant DTAB. The difference in behavior and nature of the surfactant over the deposition rate can be owing to the electrostatic charge borne on the cationic and anionic

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surfactants which slow down the reduction of Ni2+ ions, further reducing the Ni content, which

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acts as the catalyst for the deposition.

3.3. Effect of surfactants on the TiO2 content Table 4 shows the variation of weight percentage of the codeposited TiO2 particles with respect to different levels of surfactant concentration. It is noted that the weight percentage of TiO2 increases with increasing concentration of surfactant. A maximum weight percent of 6.95 wt. % of TiO2 was attained for a DTAB concentration of level (1 X CMC). Similarly, for a SDS concentration of about (1.5 X CMC) 4.88 wt. % of TiO2 concentration was found to be codeposited. Any further addition of the surfactants above these corresponded levels of concentration will tend to decrease the amount of TiO2 content because of the excess surfactant molecules which engulf the TiO2 particles. In both the cases of surfactants, there is an optimum level of concentration at which the weight of the embedded TiO2 is maximum. At this particular concentration level of surfactants, namely (1 X CMC) of DTAB and (1.5 X CMC) for SDS, there 9

ACCEPTED MANUSCRIPT is a decrease in the contact angle leading to better wettability of the particles. Also, the plating bath becomes saturated with excess surfactant which means that all the TiO2 particles would be

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completely surrounded by surfactant molecules. The excessive surfactant will be absorbed by the

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cathode and it will invariably affect the deposition rate of both Ni2+ and TiO2 particles. By reason of slower deposition at this concentration, only a smaller amount of TiO2 particles will be

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embedded in the Ni-P Matrix at levels higher than the optimum surfactant concentration.

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In addition to this, the major reason which attributes to the higher amount of codeposited TiO2

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content using DTAB systems than to that of anionic surfactant SDS is that the cationic surfactant varies the zeta potential of the TiO2 particles to a more positive level. There is an improvement

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in the probability of electrostatic absorption of the suspended particles on the matrix of cathode

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[28]. The addition of anionic surfactant SDS weakens the rate of incorporation of TiO2 in the coating by altering the zeta potential towards a more negative value. This negative charge borne

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on SDS draws towards its molecules the Ni2+ ions hindering it to actively take part in forming in the Ni-P matrix

3.4 Corrosion behaviour of the Ni-P-nano-TiO2 coatings Fig. 5 shows the polarization curves for the substrate, Electroless Ni-P coated specimen, composite coated specimen (Ni-P-TiO2) with and without the use of surfactants DTAB and SDS. The values of corrosion potential and current density were arrived using Tafel Plot after experimenting the samples in 3.5 wt. % NaCl solution. The corrosion potential and current density for electroless Ni-P nano TiO2 composite coating was measured as -362 mV and 8.52 µA/cm2.Though, tests with varying concentrations of the surfactants were performed to evaluate the corrosion parameters, among those, the level of surfactant concentration attaining enhanced

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ACCEPTED MANUSCRIPT corrosion behaviour were at (1 X CMC) level for DTAB and (1.5 X CMC) level for SDS. With the use of cationic surfactant DTAB (1 X CMC) concentration the corrosion potential and the

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current density were measured as -318 mV and 5.38 µA/cm2 whereas for coatings using anionic

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surfactant SDS (1.5 X CMC) concentration, the values were -337 mV and 6.99 µA/cm2 correspondingly. It is very clear from the above values, that the addition of surfactant in plating

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bath significantly affects the corrosion behavior. Basically, Electroless Ni-P coatings have an

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improved corrosion resistance than its plain substrate; however the contribution of the coatings towards less corrodible behavior is dependent on factors such as phosphorous content and the

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nature of the corroding solution [7]. Mukherjee et al. [29] has reported that the high phosphorous amorphous Ni-P coatings show good corrosion resistance. In this study, a similar behavior is

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noticed as all the coatings have an amorphous structure internally (Fig. 3). Fig. 5 clearly indicated that the coatings with the higher concentration of TiO2 particles exhibited

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better corrosion properties in the presence of both SDS and DTAB systems. The reason behind the increase in amount of TiO2 embedded in the Ni-P matrix under the influence of the surfactant (concentration in terms of CMC) is due to the decrease in surface tension between the hydrogen produced at the surface during the plating process. The Fig. 5 shows the measure of the corrosion parameters for the coated specimens with the optimum concentration levels of surfactants. In the absence of surfactant, electroless Ni-P coating and plain substrate, the difference in the corrosion current and potential between these specimens seem to be significant. However, the cationic surfactant DTAB arrived at a lower value of current density than the base specimen, Ni-P, Ni-PTiO2 without surfactant and the same with the anionic surfactant SDS. Hence, from the Icorr values, the corrosion rate has been calculated and is shown in Table 5. The addition of DTAB surfactant at this optimal concentration level (1 X CMC) improves the corrosion resistance with 11

ACCEPTED MANUSCRIPT a belief that the molecules of the surfactant prevent agglomeration of TiO2 particles which leads to an uniform distribution of particles in the coating matrix of Ni-P. Thus it leads to a more

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uniform surface with less defects making it an effective barrier against the diffusion of the

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corrosive ions through the metal surface.

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The results obtained from the polarization curves of the specimen are confirmed by Nyquist plots (Fig. 6). All the plots are simple semi-circles which indicate a single time constant for each

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coating. The curves appear to have a similarity in their shape, but differ extensively in their size.

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This may account for the similar process occurring on all the coatings, but over a different effective area in each case. An equivalent circuit model (Fig. 6) consisting of solution resistance

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(Rs), Double layer capacitance (Cdl) and charge transfer resistance (Rct) was utilized to determine

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the corrosion kinetic parameters for the coatings. A similar circuit model was used by

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Afroukhteh et al. [18] to study the corrosion behaviour of electroless Ni-P coated steel in 3.5 wt. % NaCl solution.

The Rct and Cdl values are tabulated in Table 5. The Cdl values are associated with the porosity or presence of micro voids in the coatings. Ni-P-TiO2 coating shows higher Cdl values than Ni-P coating which indicates the presence of micro voids in Ni-P-TiO2 coatings. This coincides with the previous findings of Novakovic [9]. After addition of SDS and DTAB surfactants to the NiP-TiO2 coatings, the Cdl values have reduced considerably. It means that the coating deposited in presence of surfactant is more stable and less porous. Presence of surfactant increases the Rct value of the coatings compared to that of the coatings without using surfactant. Cationic surfactant DTAB exhibits highest TiO2 incorporation and high Rct value compared to SDS surfactant in the Ni-P-TiO2 system.

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ACCEPTED MANUSCRIPT The high values of the charge transfer resistance (Rct) obtained for the coatings in the present study correspond to the better corrosion protective ability of electroless Ni-P-TiO2 in the

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presence of the anionic and cationic surfactants [30]. From Fig.6 it is seen that the charge

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transfer resistance of the Ni-P coatings increased to ~ 2600 Ω cm2 as TiO2 particles were introduced in the system. In the presence of SDS surfactants, Ni-P-TiO2 coatings exhibited the

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charge transfer resistance of ~ 4100 Ω cm2. In the presence of DTAB surfactant, Ni-P-TiO2

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coatings exhibited the charge transfer resistance of ~ 5400 Ω cm2. At an optimum concentration of DTAB, the charge transfer resistance of Ni-P-TiO2 coatings increased four times to that of Ni-

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P deposits which implies the subsequent improvement in corrosion protective ability of the

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coatings using DTAB.

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4. Conclusion

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The effects of cationic and anionic surfactants on composite coatings were examined, which reveals that the surfactant type and its level of concentration have a major impact on the process. In this paper, Ni-P and Ni-P-TiO2 coatings were prepared by electroless deposition method and the effects of SDS and DTAB surfactants on the properties of the deposits were studied. The concentration of surfactants was optimised for enhanced anti-corrosive property. The addition of DTAB to the plating bath indicates a higher rate of deposition when compared to that of SDS. Also the amount of TiO2 content in the coating is high when DTAB is used as surfactant. The studies on the surface morphology of Ni-P-TiO2 coatings reveal that uniform distribution TiO2 was observed at an optimum concentration of DTAB where as nodular structures of TiO2 was observed for the other surfactant. The corrosion protective ability of Ni-PTiO2 coating is considerably increased as a result of increase in the charge transfer resistance (Rct) four times higher to that of Ni-P coatings. 13

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I. R. Mafi, C. Dehghanian, Studying the effects of the addition of TiN nanoparticles to Ni–P electroless coatings, Appl. Surf. Sci. 258 (2011) 1876-1880.

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M-D Ger, B.J. Hwang, Effect of surfactants on codeposition of PTFE particles with electroless Ni-P coating, Mater. Chem. Phys. 76 (2002) 38-45.

29.

D. Mukherjee, D.Rajagopal, Electrodeposition of amorphous nickel alloys, Met. Finish. 90 (1992) 15.

30.

J.N. Balaraju, T.S.N. Sankara Narayanan, S.K. Seshadri, Evaluation of the corrosion resistance of the electroless Ni-P and Ni-P composite coatings by electrochemical impedance spectroscopy, J Sold State Electrochem. 5(2001) 334-338.

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ACCEPTED MANUSCRIPT Tables: Table 1: Bath compositions and operating conditions of the electroless Ni-P coating and Ni-P-

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nano-TiO2 composite coating.

Quantity

Nickel Sulphate

30 (g/l)

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Constituent

40 (g/l)

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Sodium Hypophosphite Lactic Acid

36 (ml/l)

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Propionic Acid Stabilizer

1 ppm 0 and 2 (g/l)

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Nano TiO2

2.4 (ml/l)

(0.25 - 2) x CMC

pH

6

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SDS or DTAB Surfactant

Temperature

88 °C (± 2 ° C)

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ACCEPTED MANUSCRIPT Table 2: Different concentrations of two types of surfactants used. CMC (g/l)

Concentration (g/l)

2.393

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Anionic / SDS

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Type of the bath /surfactant

1.0 x CMC 1.5 x CMC 2.0 x CMC 0.5 x CMC 1.0 x CMC

4.347 1.5 x CMC 2.0 x CMC

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Cationic / DTAB

0.5 x CMC

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ACCEPTED MANUSCRIPT Table 3: Roughness measurements of the deposits as a function of surfactant type and

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concentration in the plating bath.

0.5 x CMC

1 x CMC

Ra (µm)

0.478

0.435

0.289

0.264

0.279

Rt (µm)

4.453

5.798

6.308

5.928

5.798

Rz (µm)

3.036

3.400

4.115

2.544

3.400

Measurements

1.5 x CMC

2 x CMC

DTAB Concentration

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Measurements

SDS Concentration

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Roughness

0.50 x CMC

1 x CMC

1.5 x CMC

2 x CMC

Ra (µm)

0.478

0.446

0.295

0.339

0.341

Rt (µm)

4.453

6.503

5.547

5.721

5.239

Rz (µm)

3.036

4.388

3.881

3.036

2.874

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ACCEPTED MANUSCRIPT Table 4: Deposition composition (determined by EDS analysis) as a function of surfactant type and concentration in the plating bath. SDS Concentration

Composition

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Deposit 0.5 x CMC

1 x CMC

Ni (wt.%)

93.82 ± 1.5

90.99 ± 1.2

90.38 ± 0.8

P (wt.%)

4.98 ± 0.4

6.67 ± 0.3

TiO2 (wt.%)

1.2 ± 0.2

2.34 ± 0.2

2 x CMC

93.62 ± 0.7

6.08 ± 0.4

6.58 ± 0.2

5.36 ± 0.5

3.54 ± 0.2

4.88 ± 0.4

1.02 ± 0.2

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88.54 ± 0.6

DTAB Concentration 0.5 x CMC

1 x CMC

1.5 x CMC

2 x CMC

Ni (wt.%)

93.82 ± 1.5

91.09 ± 1.1

86.13 ± 0.7

88.73 ± 0.8

90.94 ± 1.0

P (wt.%)

4.98 ± 0.4

6.38 ± 0.2

6.92 ± 0.5

7.03 ± 0.5

7.03 ± 0.3

TiO2 (wt.%)

1.2 ± 0.2

2.53 ± 0.3

6.95 ± 0.2

4.24 ± 0.2

2.03 ± 0.3

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ACCEPTED MANUSCRIPT Table 5: Corrosion characteristics of Ni-P and Ni-P-nano-TiO2 composite coatings

Rct (Ω)

Cdl (µF)

NiP

1251

43.7

4.31

Ni-P-TiO2

2527

51.2

3.64

30.7

3.66

29.8

3.18

28.6

2.99

29.9

3.34

4602

27.1

3.49

5381

25.2

2.3

1.0 x CMC

3849

1.5 x CMC

4076

2.0 x CMC

3718

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0.5 x CMC

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Ni-P-nano-TiO2 +(SDS)

0.5 x CMC

Corrosion rate (mpy)

RI

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Sample

1.5 x CMC

4713

26.5

2.69

2.0 x CMC

4821

26.9

3.06

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ACCEPTED MANUSCRIPT List of figures: Figure 1. Surface morphology of composite coatings: Without surfactant [(a) Ni-P, (b) Ni-P-

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nano-TiO2]; With SDS surfactant [(c) 0.5 x CMC, (d) 1 x CMC, (e) 1.5 x CMC, (f) 2 x CMC].

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Figure 2. Surface morphology of composite coatings: With DTAB surfactant [(a) 0.5 x CMC,

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(b) 1 x CMC, (c) 1.5 x CMC, (d) 2 x CMC].

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Figure 3. XRD Pattern of electroless coatings: (a) Ni-P, (b) Ni-P-nano-TiO2, (c) Ni-P-nano-TiO2

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+ (1.5 x CMC of SDS), (d) Ni-P-nano-TiO2 + (1 x CMC of DTAB). Figure 4. Effect of different surfactant and its concentration on Ni-P-nano-TiO2 deposition rate.

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Figure 5. Polarization curves of different specimens like substrate, Ni-P, Ni-P-nano-TiO2, Ni-P-

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nano-TiO2 + (1.5 x CMC of SDS), Ni-P-nano-TiO2 + (1 x CMC of DTAB).

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Figure 6. Nyquist plots of the specimens in 3.5 wt. % NaCl solution with schematic equivalent circuit diagram.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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ACCEPTED MANUSCRIPT Highlights Ni-P-nano-TiO2 composite were deposited on mild steel by electroless method.



Effects of anionic and cationic surfactants in the deposits were analyzed.



In DTAB presence, uniform distribution of TiO2 particles was obtained.



At CMC value, DTAB had higher deposition rate and TiO2 content than SDS.



Corrosion protective ability of Ni-P-nano-TiO2 reached almost four times than Ni-P

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deposit.

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