Effects of phosphorus content on corrosion behavior of trivalent chromium coatings in 3.5 wt.% NaCl solution

Surface & Coatings Technology 244 (2014) 158–165

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Effects of phosphorus content on corrosion behavior of trivalent chromium coatings in 3.5 wt.% NaCl solution H.A. Ramezani-Varzaneh, S.R. Allahkaram ⁎, M. Isakhani-Zakaria School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 29 September 2013 Accepted in revised form 1 February 2014 Available online 9 February 2014 Keywords: Trivalent chromium coating Chromium–phosphorus coating Sodium hypophosphite Corrosion resistance

a b s t r a c t Chromium coatings were electrodeposited from trivalent chromium sulfate based electrolyte, containing sodium hypophosphite as a complexing agent. The effects of this agent on morphology, composition, deposition rate, and corrosion behavior of coatings were investigated. Results indicated that, the addition of (0.0–0.75 M) sodium hypophosphite to the electrolyte: 1) resulted in co-deposition of chromium with (12.0–17.4 wt.%) phosphorus; 2) decreased the deposition rate from 17.0 μm/h for Cr–C to 7.8 μm/h for Cr–17.2 wt.% P coatings; 3) deteriorated the corrosion performance of the coatings due to an increase in micro-crack density and porosity. However, the addition of (0.45 M) sodium hypophosphite to the trivalent chromium chloride based electrolyte, decreased corrosion current density of electrodeposited coatings from 2.8 to 1.4 μA/cm2. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Chromium coatings, electrodeposited from hexavalent chromium (Cr(VI)) electrolytes, are widely used in many industries, due to their excellent corrosion and wear resistance [1–3]. However, these electrolytes exhibit many drawbacks such as intense toxicity of Cr(VI) ions and evolution of toxic fumes during electrodeposition process [4–6]. Thus, numerous attempts have been made to find an appropriate substitute for these coatings [7–9]. A suitable candidate is trivalent chromium (Cr(III)) electroplating process due to less toxicity of its electrolyte. However, this process has several demerits such as low deposition rate [10–12]. In aqueous solutions, Cr(III) ions become hydrated and form rather electrochemically stable [Cr(H2O)6]3+ complex ions with an octahedral structure. Stability of this complex restricts the discharge process of Cr(III) ions, which consequently reduces the deposition rate [13,14]. To resolve this problem, a complexing agent must be added to Cr(III) electrolyte. This agent destructs the octahedral structure of [Cr(H2O)6]3+ complex and forms [Cr(H2O)6 − nLn]3 − n complex ions (L, complexing agent) which, exhibits much higher electrochemical activity [12,13]. However, the addition of this agent causes co-deposition of second phase elements (impurities) such as carbon and oxygen (depending on the nature and concentration of the complexing agent) with chromium which affect electrochemical properties of electrodeposited coatings [2,15,16]. Hence, choosing a suitable complexing agent is very important from viewpoints of both deposition rate and corrosion performance [13,16]. Several authors have studied the influence of complexing

⁎ Corresponding author. Tel./fax: +98 2161114108. E-mail address: [email protected] (S.R. Allahkaram). 0257-8972/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2014.02.002

agent (nature and concentration) on physical properties of Cr(III) coatings [7,17–19]. Li et al. [20] screened several complexing agents for Cr(III) ions in aqueous acidic solution such as Oxalic acid, formic acid, malonic acid and so on (carboxyl acid complexing agents). Authors reported that among the used complexing agents, malonic acid exhibits good complexing ability with Cr(III) ions and gives very good electrodeposition results. Therefore, in this research malonic acid as complexing agent, and the effects of sodium hypophosphite, NaH2PO2, as an additional carbon free ligand on morphology, composition, structure, deposition rate, and corrosion behavior of electrodeposited chromium coatings have been evaluated.

2. Experimental In this study, Cr(III) coatings were electrodeposited from Cr(III) electrolyte based on sulfate system, containing various concentrations of NaH2PO2 (0.0–0.75 M). The composition of this electrolyte together with the operating conditions is summarized in Table 1. The electrolyte was prepared using AR grade chemicals and double distilled water. After mixing the components, the prepared electrolyte was heated and stirred for 2 h at 70 ± 2 °C. Then, it was cooled to room temperature and stored for 24 h so that formation of Cr(III) complex ions, [Cr(H2O)6 − nLn]3 − n would be completed before the commencement of deposition process. Diluted KOH solution was then used to adjust pH value to 2.6. Copper strips (2 × 2 × 0.2 cm3) were used as substrate (cathode) and Ti/IrO2 electrode was used as anode in order to prevent anodic oxidation of Cr(III) ions. Using, abrasive SiC papers, substrates were mechanically polished down to 1500-grite size. Specimens were then washed with distillated water and acetone, respectively and finally dried in the

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Table 1 Composition of the Cr(III) electrolyte and operating conditions. Electrolyte composition

Function

Concentration (M)

Operating conditions

Cr2(SO4)3·6H2O

Source of Cr

0.15

CH2(COOH)2 NaH2PO2·H2O Na2SO4

Complexing agent Complexing agent Conduction salt

0.45 0.0–0.75 0.56

H3BO3

Buffering agent

0.9

SDS

Wetting agent

0.1 g/L

Anode: Ti/IrO2 Cathode: Copper Temperature: 37 °C pH = 2.6 Current density: 105 mA/cm2 Electrolyte agitation: 400 rpm Electrodeposition time: 1 h

current of hot air. Prior to electrodeposition, specimens were degreased in an alkaline solution (containing NaOH, Na2CO3, and Na3PO4·12H2O) at 65 ± 2 °C for 2–3 min, followed by water rinsing and acid pickling in 5 vol.% H2SO4 solution and finally washed with clean water [21]. To investigate the coating morphologies, composition, deposition rate, and crystal structure, Scanning Electron Microscope (SEM; model CAMSCAN MV2300), equipped with Energy Dispersive Spectrometer (EDS), and X-ray diffractometer (model Philips X Pert Pro), operated at 40 kV and 30 mA with Cu-Kα radiation (λ = 1.541 Å) at a scanning speed of 2.4°/min in the range of 20–100°, were utilized. Prior to EDS analysis, the samples were cleaned in a warm solution of water with soap and then immersed in acetone, while being inside an ultrasonic chamber for 3–5 min so that any carbon-containing impurities on the sample surfaces would be removed. For cross-sectional view, the samples were prepared as follows: (1) cut via wire cut instrument, (2) embedded in epoxy resin, (3) mechanically ground down to 2500 grit SiC paper, and (4) polished with diamond paste. The corrosion resistance of as-deposited coatings was studied using potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) tests. All corrosion tests were carried out in a standard three-electrode cell using platinum electrode and Saturated Calomel Electrode (SCE) as auxiliary and reference electrodes, respectively. Prior to electrochemical tests, the samples were kept in 3.5 wt.% NaCl solution at room temperature and Open Circuit Potential (OCP) was measured and recorded until no further changes were observed (less than 10 mV·h−1). At least two tests were performed for each sample in order to confirm reproducibility of the results. The potentiodynamic polarization tests were carried out, using an EG&G potentiostat/galvanostat (model 273A), at a sweep rate of 1 mV·s−1 with applied potential range from OCP to − 600 and + 100 mV in the cathodic and anodic directions, respectively. The EIS measurements were carried out using a Solartron Model SI 1255 HF Frequency Response Analyzer (FRA) coupled to a Princeton Applied Research (PAR, Model 273A) potentiostat/galvanostat. EIS measurements were obtained at OCP within a frequency range of 0.01 Hz to 10 kHz with an applied AC signal of 5 mV (rms) using the single sine technique. An equivalent circuit simulation program namely “ZView2” was used for EIS data analysis, determination of the equivalent circuit and fitting of the experimental data.

Fig. 1. EDS analysis of electrodeposited coating from Cr(III) electrolyte (a) without and (b) with NaH2PO2 (0.15 M).

Phosphorus is produced from reduction of hypophosphite (H2PO− 2 ) anions at the cathode surface (Eq. (1) [22]) and is co-deposited with chromium. 4H2 PO2

−

þ

−

þ H þ e ¼ 3P þ 3OH þ 2H2 O þ H2 PO3

−

Fig. 2 shows the effect of NaH2PO2 concentration on phosphorus content of the Cr(III) coatings. It is observed that phosphorus content of the coating can be ultimately increased to 17.2 wt.% at maximum concentration of NaH2PO2 (i.e. 0.525 M), beyond which the amount of codeposited phosphorus starts to decrease.

3. Results and discussion 3.1. Compositional analysis EDS analysis (Fig. 1a) shows that electrodeposited coatings from Cr(III) electrolyte without NaH2PO2 (0.0 M), consists of 98.4 wt.% Cr and 1.6 wt.% C (Cr–C coating). However, the addition of NaH2PO2 (0.15 M) to that electrolyte leads to co-deposition of chromium with phosphorus (Fig. 1b), which amount depends on NaH2PO2 concentration.

ð1Þ

Fig. 2. Effect of NaH2PO2 concentration on phosphorus content of Cr(III) coatings.

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An increment in H2PO− 2 anion concentration accelerates the rate of phosphorus formation reaction (Eq. (1)), which increases its amount in the coating. H2PO− 2 anions also participate in another reaction. These anions are oxidized at the anode surface and phosphite (H2PO− 3 ) anions are produced, as shown in Eq. (2) [22]. H2 PO2

−

þ H2 O ¼ H2 PO3

−

þ

þ 2H þ 2e

ð2Þ

Increasing H2PO− 2 anion concentration expedites the rate of oxidation reaction, which leads to an increase in the H2PO− 3 anion concentration. Possibly, when the concentration of H2PO− 3 anion in the electrolyte exceeds a certain value, the rate of phosphorus generation reaction is decelerated, which lowers phosphorus content of the coating, as shown in Fig. 2. 3.2. Morphology and crystal structure 3.2.1. Crystal structure The XRD patterns of the copper substrate and Cr–C coating are shown in Fig. 3a and b, respectively. By comparing Fig. 3a with b and considering the reference pattern of chromium (Fig. 3d, PDF Card No.00-006-0649), it can be deduced that the very broad peak located at around 2θ = 44° corresponds to the Cr(110) plane. Very broad peak is the characteristic of amorphous structures [23] and therefore, Cr–C coating has an amorphous structure. It is believed that the amorphization of this coating is due to the action of carbon alloying element, which incorporates into the chromium crystal lattice and disturbs the setting order of Cr atoms [24]. In the same manner and according to Fig. 3c, it can be said that Cr–P coating containing 12.0 wt.% phosphorus (Cr–12.0 wt.% P) has an amorphous structure, which can be due to the presence of phosphorus in this coating. According to the equilibrium phase diagram of chromiumphosphorus [25], the solid solubility of phosphorus in chromium is negligible at ambient temperature. Therefore, the entrapment of phosphorus into the Cr lattice causes excessive strain. This may result in lattice

disorder, which can cause a change from crystalline to amorphous structure [26]. Similar results were also obtained for other electrodeposited Cr–P coatings, and it is found that the amount of phosphorus does not have a significant effect on crystal structure of Cr–P coatings. A series of sharp peaks are observed in the XRD pattern of Cr–C (Fig. 3b) and Cr–12 wt.% P coatings (Fig. 3c), which correlate to the reflection of the copper substrate for Cu kα (λ = 1.541 Å) radiation. This could be due to the low thickness of the deposit (4.3 μm for Cr–12 wt.% P and 17.0 for Cr–C coatings) with regard to the X-ray penetration depth (≈25 μm [27]).

3.2.2. Morphology Fig. 4 presents the surface morphology of Cr–C (Fig. 4a) and Cr–P coatings with different compositions, viz. Cr–12.0 wt.% P (Fig. 4b), Cr–14.4 wt.% P (Fig. 4c), and Cr–17.2 wt.% P (Fig. 4d). It is observed that Cr–P coatings have higher micro-crack density than that of Cr–C coating. Moreover, among Cr–P coatings, Cr–12.0 wt.% P coating has smaller number of micro-cracks. Higher micro-crack density of Cr–P coatings compared to Cr–C coating can be attributed to the high levels of internal stresses. It has been suggested that high levels of stress in a coating (especially in an amorphous coating) promote cracking and porosity [20]. As mentioned before (see Section 3.2.1), the solid solubility of phosphorus in chromium is low. Therefore, incorporation of phosphorus into the chromium coatings increases its internal stresses, which results in the appearance of micro-cracks. On the other hand, an increase in micro-crack density can be due to the adsorption of chromium hydroxide compounds on the coating surface [20]. The addition of NaH2PO2 to the Cr(III) electrolyte expedites the rate of hydrogen (H+) ion consumption (H+ ions react with H2PO− 2 anions and phosphorus is produced (Eq. (1))) and consequently, the pH around the cathode surface increases faster. High pH conditions (≥4.5 [17]) accelerate olation reaction of Cr(III) complex ions. This leads to the precipitation of chromium hydroxide compounds, which may incorporate into Cr–P coating structure and cause cracking.

Fig. 3. XRD pattern of (a) copper substrate, (b) Cr–C coating, (c) Cr–P coating containing 12.0 wt.% phosphorus (Cr–12.0 wt.% P) and (d) reference XRD pattern of chromium (PDF Card No.00-006-0649).

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Fig. 4. SEM micrographs of (a) Cr–C, (b) Cr–12.0 wt.% P, (c) Cr–14.4 wt.% P and (d) Cr–17.2 wt.% P coatings.

3.3. Deposition rate Fig. 5 shows the correlation between the deposition rate and NaH2PO2 concentration. Initially, a small addition of (0.15 M) NaH2PO2 decreases the deposition rate from 17.0 to 4.3 μm/h. This phenomenon can be attributed to the reduction of H2PO− 2 anions on the

Fig. 5. Correlation between the deposition rate and NaH2PO2 concentration.

cathode surface (Eq. (1)). A portion of current is consumed by this reaction, which diminishes the available electron for discharge of Cr(III) ions and consequently the deposition rate is decreased. On the other hand, chromium hydroxide compounds are formed by the addition of NaH2PO2 to the electrolyte (see Section 3.2.2), which causes depletion of active chromium ions, [Cr(H2O)6 − nLn]3 − n, in the vicinity of the cathode surface and thus, the deposition rate is decreased. Besides, adsorption of these compounds on the coating surface hinders further discharge of Cr(III) ions to Cr(s) [18] which, leads to lower deposition rate. However, deposition rate of the coating slightly rises from 4.3 to 7.8 μm/h (Fig. 5), with increasing NaH2PO2 concentration up to 0.525 M, which is due to high amounts of electrochemically active chromium complex ions. In the preparation step of Cr(III) electrolyte, which contains malonic acid (CH2(COO)2H2) and NaH2PO2, [Cr(H2O)6]3 + complex ions are gradually converted to [Cr(H2O)4(CH2(COO)2)]+ and 2+ [Cr(H2O)5(H2PO− complex ions (Eqs. (3) and (4)). Therefore, by in2 )] creasing H2PO− concentration, the amount of electrochemically active 2 2+ [Cr(H2O)5(H2PO− complex ions in the electrolyte (especially 2 )] around the cathode surface) is increased, which results in higher deposition rate.  3þ  2− 
þ CrðH2 OÞ6 þ CH2 ðCOOÞ2 ¼ Cr H2 ðCOOÞ2 þ 2H2 O;

ð3Þ

  3þ − − 2þ þ H2 PO2 ¼ CrðH2 OÞ5 ðH2 PO2 Þ þ H2 O: CrðH2 OÞ6

ð4Þ

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On the other hand, H2PO− 2 anions have the ability to reduce Cr(III) ions [28]. Thus, increasing the concentration of H2PO− 2 anion enhances the chromium reduction process (Eq. (5)) and consequently the deposition rate is increased. 3þ

2Cr

þ 3H2 PO2

−

þ 3H2 O ¼ 2CrðsÞ þ 3H2 PO3

−

þ

þ 6H

ð5Þ

A decline in deposition rate is observed (Fig. 5) as the NaH2PO− 2 concentration is reached beyond 0.525 M, which could be due to the precipitation of chromium phosphite (Cr(H2PO3)2) around the cathode [20]. − H2PO− 3 anion is generated by oxidation of H2PO2 anion during electrodeposition process (see Section 3.1, Eq. (2)). However, the solubility of this anion is decreased by increasing its concentration [22], which results in the precipitation of Cr(H2PO3)2. This phenomenon is augmented in the vicinity of the cathode, due to the high levels of pH value. Therefore, the amount of electrochemically active chromium ions and consequently the deposition rate is decreased. 3.4. Corrosion behavior of electrodeposited coatings 3.4.1. Electrochemical Impedance Spectroscopy (EIS) studies The impedance spectra obtained for the electrodeposited coatings in 3.5 wt.% NaCl solution at their respective OCP, are shown as Nyquist diagrams in Fig. 6a. It is seen that, all plots have an arc like structure, which resembles depressed semicircles and in turn suggests the effect of overlapping time constants or frequency dispersion (constant phase element, CPE, behavior) [27]. In order to distinguish between these two behaviors, the Bode and Bode-phase diagrams were plotted, as shown in Fig. 6. The presence of a single step in Bode diagrams (Fig. 6b) confirms that there is a single time constant in electrochemical process of corrosion. Moreover, according to the phase angle values in Bode-phase diagrams (Fig. 6c) that approaches but does not reach −90° (with variation in frequency), it can be concluded that, CPE behavior exist at the coating/electrolyte interface which, could have occurred due to the inhomogeneity of the coating surface, viz. variation in surface composition, micro-cracks, porosity, and roughness [29,30]. The CPE impedance is given by Eq. (6): s

ZCPE ¼ 1=Y0 ðjωÞ ;

ð6Þ

where ω is the angular frequency (rad/s), n is the power number defined as n = α(π / 2), α is the constant phase angle of CPE (rad), and Y0 is CPE constant (sn·Ω−1·cm−2), which is directly proportional to the active surface area exposed to the electrolyte. The term “n” shows how far the interface is from the ideal capacitor and can vary between −1 and 1 [31,32]. Interpretation of impedance spectrums (data) requires the use of an appropriate electrical circuit (including resistors, capacitors, constant phase element, etc.) which, produce similar spectrums (i.e., good fitness between the calculated and experimental impedances. Various Equivalent Electrical Circuits, EEC, (such as series R–C circuit, parallel R–C circuit, etc.), were proposed to fit the impedance data. Among proposed EECs, the most probable EEC which is matched closely to the impedance data (obtained for Cr–C and Cr–P coatings) is shown in Fig. 6d. In this electrical circuit, Rs is the uncompensated solution resistance, CPE is the constant phase element that indicates the non-ideal frequency response, and Rct is the charge transfer resistance (the diameter of Nyquist curve), which is equal to the polarization resistance (Rp) in the absence of inductive behavior. The Rs value depends on the distance between working and reference electrode in the electrolyte and was kept constant in all the experiments. The Rp and CPE parameter values estimated from the impedance spectra fitting analysis are summarized in Table 2. It is seen that all the parameters follow a specific trend with an increase in the amount of phosphorus content. Rp is decreased from 9552 to 2579 Ω·cm2 and Y0 is increased from 7.297E− 05 to 13.432E − 05 Ω−1·cm− 2·sn. An increase in Y0 indicates that, more surface

Fig. 6. (a) Nyquist, (b) Bode and (c) Bode-phase diagrams of electrodeposited coatings in 3.5 wt.% NaCl solution. (d) Equivalent electrical circuit model.

area of the coating is exposed to corrosion, which can be due to an increase in micro-crack density (Fig. 4) or porosity of the coatings (see Section 3.4.2). On the other hand, n and maximum phase angle value (Fig. 6c) decrease gradually with increasing phosphorus content of the coatings.

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Table 2 Electrochemical parameters extracted from EIS data of electrodeposited coatings in 3.5 wt.% NaCl solution. Coating/parameter

Rp (Ω·cm2)

Y0 (Ω−1·cm−2·sn)

n

Cr–C Cr–12.0 wt.% P Cr–14.4 wt.% P Cr–17.2 wt.% P

9552 8434 3846 2579

7.297E−5 9.014E−5 9.373E−5 13.43E−5

0.833 0.857 0.845 0.822

The decrease in these parameters demonstrates an increase in the inhomogeneity of the coating surface, which could be due to the variation in phosphorus content and/or an increase in the micro-crack density of the coatings. Therefore, according to the EIS results, it can be concluded that the corrosion resistance of Cr–P coatings are lower than that of Cr–C coatings which, is characterized by lower Rp and higher Y0.

3.4.2. Potentiodynamic polarization studies EIS results showed that phosphorus had deleterious effects on corrosion performance of Cr(III) coatings. For further examination, potentiodynamic polarization tests were performed in 3.5 wt.% NaCl solution. SEM micrograph of the coating surface after anodic polarization is shown in Fig. 7. It can be seen that Cr–P coatings (Fig. 7a) are corroded more severely in comparison with Cr–C coatings (Fig. 7b) and the substrate dissolution is also initiated as well which can be attributed to the presence of microcracks and pores on coating surface. These defects may also act as favorable paths for chloride (Cl) ions to penetrate through the coating and cause corrosion in the substrate. The polarization curves of Cr–C and Cr–P coatings are shown in Fig. 8. Using Tafel extrapolation method, the corresponding electrochemical parameters were extracted from polarization curves which are listed in Table 3. To determine the corrosion rate from polarization measurement, Tafel regions (linear regions on the slope of E vs. log i curve) are extrapolated to the corrosion potential, Ecorr, (the potential that voltmeter indicates prior to the application of current). This point corresponds to the corrosion rate in terms of current density [33]. As observed in Table 3, the Ecorr for all the electrodeposited coatings is approximately equal (Ecorr ≈ −292 mV). Moreover, Cr–P coatings (in contrast to Cr–C coatings) have higher icorr and lower anodic Tafel slope (ba). Lower ba of Cr–P coatings indicates that the electrochemical dissolution of chromium coatings is more favored in the presence of phosphorous.

Fig. 8. Polarization plots of Cr–C and Cr–P coatings in 3.5 wt.% NaCl solution.

The difference in icorr of electrodeposited coatings could be partially ascribed to differences in their morphology. Hence, accelerated corrosion of the cracked Cr–P coatings would be expected due to the penetration of Cl ions through the cracks, which become preferential sites for corrosion initiation. On the other hand it could be due to the presence of numerous pores on the coating surface. Pores are the unique path for the electrolytic current to flow more easily [34]. Saravanan and Mohan [12] used an electrochemical technique to determine the porosity of the chromium coatings, via the following equation:   −ð½ ΔEcorr=baÞ PorosityðPÞ% ¼ Rp;s =Rp;c  10 ;

ð7Þ

where Rp,s is the substrate polarization resistance (Ω·cm2), Rp,c is the coating polarization resistance (Ω·cm2), ΔEcorr is the corrosion potential difference between the coating and the substrate (mV), and ba is the anodic Tafel slope for the substrate (mV/dec). Using Eq. (7), and considering the values of Rp,s = 322 Ω·cm2 and ba = 80 mV/dec, the porosity percentage of the coatings was calculated and summarized in Table 3. It is observed that the porosity percentage is increased from 0.9 to 4.8%, which could be due to the reaction of H2PO− 2 anion with water

Fig. 7. SEM micrograph of corrosion surface of (a) Cr–C and (b) Cr–P coatings.

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Table 3 Corrosion parameters extracted from polarization plots (Fig. 8). Coating/parameter

Ecorr (mV, SCE)

icorr (μA/cm2)

ba (mV/dec)

Porosity (%)

Cr–C Cr–12.0 wt.% P Cr–14.4 wt.% P Cr–17.2 wt.% P

−283 −308 −296 −283

0.64 1.4 2.3 4.1

67 45 40 43

0.9 2.0 3.0 4.8

± ± ± ±

9 16 4 13

± ± ± ±

0.07 0.2 0.1 0.3

molecules on the cathode surface and formation of hydrogen atoms (Eq. (8) [22]). These combine to form hydrogen molecules (Eq. (9)), which appear on the coating surface in the form of bubbles of hydrogen gas. This phenomenon augments the porosity of coating. High porosity exposes more area of the coating surface to the corrosive media that results in a higher icorr. H2 PO2

−

þ H2 O ¼ H2 PO3

−

þ 2Had ;

ð8Þ ð9Þ

Fig. 10. Polarization curve of Cr–C (Cr–C–Cl) and Cr–P (Cr–P–Cl) coatings, which electrodeposited from chloride base electrolyte.

Anodic polarization curve reveals the coating dissolution process. As it can be seen in Fig. 8, a decrease in the chromium dissolution current for Cr–P coatings at E ≈ −120 mV corresponds to the onset of passive film formation on the coating. But, as potential is shifted toward positive direction, the current begins to rise, which indicates the dissolution of the passive film. This phenomenon does not occur in Cr–C coating. Therefore, it can be inferred that phosphorus stimulates the passive film formation on Cr–P coatings. However, this film is not stable, which could be due to the presence of micro-cracks on the coating surface. Micro-cracks act as channels for chlorine ions to penetrate, which erode the chromium bulk. Electrochemical corrosion tests show that Cr–C coatings exhibit better corrosion resistance in 3.5 wt.% NaCl solution than Cr–P coatings, which is characterized by lower icorr and ba. Therefore, it can be inferred that NaH2PO2 is not a suitable complexing agent for Cr(III) electrolyte based on sulfate system and deteriorates anti-corrosion performance of the electrodeposited coatings in 3.5 wt.% NaCl solution. Zeng et al. [35] reported that the corrosion performance of chromium coatings in Cl ion containing corrosion media (i.e. 10% HCl solution) could be improved by the addition of (0.25 M) NaH2PO2 to the electroplating bath. This discrepancy in obtained results could be due to the difference in Cr–P coating composition (i.e. lower phosphorus content, 9.8 wt.%, in Zeng et al. research). According to Section 3.2.2, in Cr–P coatings; the lower the phosphorus content, the lower the

micro-crack density. In this case, the continuity of the coating surface is sufficient to form a stable passive film and therefore, the corrosion performance of the coatings is improved. On the other hand, the electrolyte nature (i.e. chloride or sulfate base) could have an influence on corrosion performance of deposited Cr–P coatings. For more investigation, Cr–C and Cr–P coatings were electrodeposited from chloride base electrolyte hereinafter referred to as Cr–C–Cl and Cr–P–Cl coatings. The bath composition and operating parameters are the same as summarized in Table 1. But, instead of Cr2(SO4)·6H2O, CrCl3·6H2O (in same molar concentration) was used as a source of Cr(III) ions. Fig. 9 shows the surface morphology of Cr–C–Cl and Cr–P–Cl coatings. As it can be seen, the surface of Cr–C–Cl coating is more porous (Fig. 9a) and has some blisters around the edge. But, these flaws are disappeared by the addition of NaH2PO2 (0.45 M) to the electrolyte (Fig. 9b), which results in co-deposition of chromium with (8.9 wt.%) phosphorous. Moreover, the surface of Cr–P–Cl is almost crack free, which could be due to the lower phosphorus content. Therefore, it can be anticipated that corrosion performance of Cr–P–Cl is better than those of Cr–C–Cl coatings. Polarization curves of Cr–C–Cl and Cr–P–Cl coatings are shown in Fig. 10. The polarization curve of Cr–C coatings electrodeposited from sulfate base electrolyte is also inserted for comparison. Using Tafel

Hads þ Hads ¼ H2 :

Fig. 9. Surface morphology of (a) Cr–C [Cr–C–Cl], (b) Cr–P [Cr–P–Cl], which electrodeposited from chloride base electrolyte.

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Acknowledgements

Table 4 Corrosion parameters extracted from polarization plots (Fig. 10). Coating/parameter

Ecorr (mV, SCE)

icorr (μA/cm2)

ba (mV/dec)

Cr–C Cr–C–Cl⁎

−283 ± 9 −234 ± 15 −310 ± 10

0.64 ± 0.07 2.8 ± 0.4 1.4 ± 0.2

67 60 60

Cr–P–Cl (8.9 wt.% P)

165

Cl: electrodeposited from chloride base electrolyte.

extrapolation method, the corresponding electrochemical parameters were extracted from polarization curves and are summarized in Table 4. It is seen that, Cr–C–Cl coatings in comparison with Cr–C coatings have higher icorr. So, it can be inferred that electrolyte nature has a significant influence on corrosion performance of chromium coatings. However, the addition of 0.45 M NaH2PO2 to the electrolyte improves corrosion resistance of chromium coatings, electrodeposited from chloride base electrolyte. icorr is decreased from 2.8 to 1.4 μA/cm2 due to the lower microcrack density and porosity of Cr–P–Cl coatings. In summary, it can be deduced that NaH2PO2 is not a suitable complexing agent for Cr(III) electrolyte based on sulfate system and deteriorates the corrosion performance of electrodeposited coatings. However, this agent may act contrariwise and improves the corrosion resistance of chromium coatings electrodeposited from chloride base electrolyte.

4. Conclusion Chromium coatings were electrodeposited from trivalent chromium sulfate base electrolyte containing various concentrations of sodium hypophosphite. The results showed that: 1. The addition of sodium hypophosphite to the sulfate base electrolyte increases micro-crack density and porosity of the coatings. 2. Sodium hypophosphite has detrimental effects on electrochemical behavior of chromium coatings, which is characterized by a higher icorr and lower ba). 3. The presence of phosphorus in chromium coatings stimulates the passive film formation. But, it is not stable due to the discontinuity of coating surface. 4. The addition of 0.45 M sodium hypophosphite to the chloride base electrolyte improves the corrosion resistance of electrodeposited coatings.

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