Improved corrosion resistance of permanganate-phosphate conversion coat on steel surface by surfactants

In the present work, we studied the effect of the presence of different concentrations of each of Triton-X-100 and Tween-80 surfactants in the bath of permanganate-phosphate conversion coating (PPC) on the corrosion resistance and the microstructure of the prepared coats. The coats were investigated using a scanning electron microscope (SEM), an energy dispersive X-ray spectrometer (EDX), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization techniques. The SEM results show that, on addition of the surfactants to the PPC bath, the porosity of the coat decreases and the coating layer becomes more compact. EIS results indicated that the presence of 0.01 M Triton-X-100 or 0.01 M Tween-80 in the coating solution caused an increase in the protection efficiency of the coat up to 93.7% and 84.1%, respectively. The potentiodynamic polarization results indicated that the two surfactants mainly act as anodic inhibitors due to the adsorption of their molecules at the anodic sites of the surface of steel and retard its oxidation reaction. The EDX and XPS results confirmed the results of the other techniques. A mechanism for the role of the surfactants in the coating process was proposed using the results of XPS and the other techniques.


Materials and solutions
Mild steel was utilized as a test material.The chemical composition of it was listed in Table 1.
The steel electrode was constructed in a cylindrical shape and covered in epoxy resin such that just one surface with a surface area of 1 cm 2 was exposed, preventing the crevice effect.
Distilled water and chemicals of a high caliber were used to create the stock solutions: 85% H 3 PO 4 , KMnO 4 , NaHPO 4 , and NH 4 NO 3 were acquired from Aldrich chemicals.Tween-80 and Triton-X-100 (Fig. 1) were obtained from Alpha Chemika.Many studies provide valuable information on selection of surfactants in different critical applications.Tween-80 and Triton-X-100 surfactants were found to be safe in low concentrations [39][40][41] .

Application of conversion coat
The conversion coating solution was prepared by mixing 0.75 g of NaHPO 4 , 1 g KMnO 4 and 2 ml H 3 PO 4 in a 100 ml flask with the addition of a certain volume of the Tween-80 or Triton-X-100 and double distilled water to give the desired surfactant concentration.The pH of the coating solution was maintained at value 3. The steel electrode with area 0.2826 cm 2 mild steel was used.The steel electrode was pretreated as follows: hand-polishing using emery paper from 400 up to1000 grit, starting with a coarse grit and working up to a fine one till reaching a mirror finish follows by the pickling process by the identical procedure that was reported in the earlier work 38 .For each experiment, A rotator supported the steel electrode (60 rotates per min.)with an immersion time of 30 min.The coat obtained covered all the surface of steel electrode.Figure 2 provides a detailed description of the treatment procedure including the degreasing process, the phosphoric and soda pickling stage, the conversion treatment, and the final rinse and drying step.

Microstructural characterization
Scanning electron microscopy (SEM) with the JOEL instrument used to examine the surface morphology of the permanganate-phosphate coating without and with Triton x-100 or Tween-80.Coupons with an area of 1 cm 2 steel samples were used in the experiments.The elements of the film that were created on the surface were analyzed by an energy-dispersive X-ray spectrometer (EDS, JEM-2100, Japan).Also, Dry film thickness (PosiTest DFT Ferrous) or coating thickness was measured.

XPS analysis
For X-ray photoelectron spectroscopy (XPS) analysis, 1 cm 2 metallic specimen, after washing with distilled water (conductivity: 6 × 10 −4 Ω −1 m −1 ) and drying, was placed on a specific sample holder then moved to an ultra-high vacuum chamber at 10 −9 torr.The X-ray source (made by VG Microtech, model XR3E2) with Al anode and X-ray energy (1486.6 eV) was used to excite the surface.

Electrochemical measurements
Without stirring and with air bubbling, measurements of the electrochemical nature were carried in a 100 ml cell using three electrodes, with the working electrode facing the counter electrode.(a graphite rod) and saturated calomel electrodes (SCE) as the reference electrode.In the text, all potentials are provided in respect to this reference electrode.The electrolyte used was 0.6M NH 4 NO 3 .Electrochemical tests were carried out using a frequency response analyzer (FRA)/potentiostat supplied by Parstat Instrument (PARSTAT 2263.02SN 194).Before performing the electrochemical tests, the working electrode with area 0.2826 cm 2 was put into a 0.6 M NH 4 NO 3 solution and left there for 30 min at room temperature to get the rest potential.
Potentiodynamic polarization measurements were operated with a scan rate of 20 mV/min starting from cathodic potential (E corr − 300 mV) going to anodic direction (E corr + 900 mV).And the measurements of the electrochemical impedance spectrum from 3.2 × 10 4 to 0.1 Hz with 10 mV amplitude around the rest potential.For the consistency of the measurements, each experiment with the same conditions was performed twice and the results had a 2% deviation.surfactant has a slight effect on its concentration on the surface.This behaviour can be discussed on the basis that in presence of low concentrations of the surfactants in the coating solution, the increase of the amount of the surfactant greatly raises the value of the surface coverage; however, in presence of higher concentrations, the steel surface becomes saturated with adsorbed surfactant molecules.Figure 5 shows also that Wt %C on the coat surface in presence of Triton-X-100 surfactant in the coating bath is higher than that in presence of Tween-80 surfactant.This behaviour can be attributed to the presence of a benzene ring in the molecular structure of the Triton-X-100 molecule which leads to the increase of its π-system and its donation of the electrons to metal surface making it strong absorbable 42,43 .

Electrochemical impedance spectroscopy (EIS) results
Nyquist impedance plots of permanganate-phosphate coated steel in 0.6 M NH 4 NO 3 , (the coats contain different concentrations of Triton-X-100 or Tween-80) are depicted in Fig. 6.These plots show three capacitive semicircles; the low-frequency semicircle is associated with charge transfer resistance and double layer capacity, while the high-frequency semicircle is associated with the behaviour of the conversion coat during the corrosion process.Fitting the experimental data to the equivalent circuit model is depicted in Fig. 7.The circuit includes R s which represents the solution resistance; R ct is the charge transfer resistance and CPE is the constant phase element related to the double-layer capacitance.The parallel combination of CPE2 and R2 are related to the capacitance and resistance of the conversion coat.The analysis of impedance spectra was carried out three times to avoid the percentage error of analysis.
The overlaid impedance spectra given in Fig. 6 and Table 2 show that the size of the semicircle and R ct values increase with increasing the concentration of both of the two surfactants in the coating solution which suggests that both surfactants improve the resistances of the permanganate-phosphate coat against corrosion in 0.6 M NH 4 NO 3 solution.
The information in Table 2 demonstrates that when the concentration of the two surfactants increases, the double-layer capacitance decreases (Q dl ) which is attributed to the adsorption of the surfactant molecules on the steel surface.The protection efficiency of the coat (% P) was measured using the equation 42 : where R ct and R ct o are the charge transfer resistances, in the presence and absence of surfactants respectively.The data in the table show that the protection efficiency of the coats increases with raising the concentration of Figure 8 shows the computer fitting of the measured data for Nyquist plots in 0.6 M NH 4 NO 3 solution for permanganate-phosphate coated steel (the coats contain (a): 1 × 10 −5 M Triton -x-100 (b) and 5 × 10 −3 M Tween -80 surfactants).Fitting the EIS data to the equivalent circuit model revealed a good agreement between fitted and measured spectra.The repeatability of all parameters data of measurements are about ± 0.001 to ± 0.18 and the error less than 3%.
Figure 9 shows the variation of the protection efficiency of the coats with concentration of each of surfactants in the coating bath.It is clear that Triton-X-100 has higher protection efficiency than Tween-80 which confirms the EDX results.

Potentiodynamic polarization results
Figure 10a, b shows the potentiodynamic polarization curves of the permanganate-phosphate coated steel (the coats contain different concentrations of surfactants) in 0.6 M NH 4 NO 3 solution.As the figure shows, the anodic portion of the polarization curves is affected rather than the cathodic one when Triton-X-100 or Tween-80 are added to the coating solution indicating that the two surfactants retard the oxidation reaction of the steel at the anodic sites of the surface.Also, it is obvious from the anodic part of the Fig. 10 that the polarization curves show activation behaviour followed by a decrease in the current density obviously at high potentials which means a formation of passive protective film on the steel surface.This passivation is mainly attributed to the formation of the conversion coat.
The electrochemical corrosion parameters including corrosion potential (E corr ), corrosion current density (i corr ), anodic (β a ) and cathodic (β c ) Tafel slopes were determined by extrapolation of the linear Tafel segments of the potentiodynamic polarization curves are presented in Table 3.The protection efficiency (%P) was computed as follows 43 : The tabulated data shows that the i corr values decrease and %P values increase with the increase of the concentrations of both surfactants.Tables 2 and 3 show that there is a good agreement between the values of %P obtained from the impedance and potentiodynamic polarization for the two surfactants.

Porosity of coat
Permanganate-phosphate conversion coating is mainly composed of insulating hopeites; the pores in the coat are generally regarded as the exposed area of the substrate, which can be measured from the results-obtained from the potentiodynamic polarization and electrochemical impedance spectrophotometry techniques 44,45 .Using the following equation 46 : where R ps and R p are the polarization resistances of the bare and coated steel samples, respectively.ΔE corr is the difference between the corrosion potentials of the bare and coated steel samples.β a is referred to slope of the anodic Tafel line derived from the polarization curves.The porosity values of the permanganate-phosphate coat   on the steel surface in absence and presence of different concentrations of Triton-X-100 or Tween-80 surfactants were calculated using the electrochemical data presented in Tables 2 and 3 and given in Table 4.
Variation of the porosity of the coat with the concentration of each of Triton-X-100 and Tween-80 are presented in Fig. 11.It is clear that, in presence of low concentrations (< 0.001 M) of the surfactants, in the coating bath porosity of the coat sharply decrease.However, in presence of higher concentrations of the surfactant in the solution, it has a slight effect on the porosity of the coat.this behaviour confirms the EDX results.

Thickness of the coat
Steel coupons with area 2 cm 2 rectangular mild steel and with the same chemical composition of steel samples used in the electrochemical measurements were used in the experiment.The thickness of the permanganatephosphate-coats for varied concentration of the two surfactants are shown in Table 5.It is clear that the thickness of the coat drops as the concentration of surfactant raised, which indicates that the presence of the surfactant molecules in the coating bath enhancing the coating process making the coat less porous.The results show also that in presence of Triton-X-100 the thicknesses of the coats are less than those in presence of Tween-80 which confirm the results of SEM, EDX and porosity.

Coat stability
Figure 12 shows the Nyquist diagrams of the permanganate-phosphate coated steel (the coats contain 1 × 10 -2 M Triton -x-100 or 1 × 10 -2 M Tween -80) in 0.6 M NH 4 NO 3 at different immersion times.As can be observed, the capacitive semicircle's size gradually shrinks as immersion duration increases.The previously utilized equivalent circuit (Fig. 7) was used to evaluate these graphs.The electrochemical parameters values obtained through EIS for Permanganate -phosphate coated steel at different immersion times are given in Table 6.
Figure 13 shows the dependence of the protection efficiency of the coat (% P) on the immersion time of per-manganate_phosphate coated steel contains 1 × 10 -2 M of surfactants in 0.6 M NH 4 NO 3 solution.It is clear that increasing immersion time up to 120 h, leads to a minor decrease in the protection efficiency, demonstrating the remarkable stability of the Permanganate_phosphate coated steel.

XPS results
Figure 14 shows the full XPS spectra of (A) coated steel free from surfactants, (B) coated steel free from surfactants after immersion in 0.6 M NH 4 NO 3 for 12 h, (C) coated steel its coating bath contains 1 × 10 -2 M Triton-X-100 after immersion in 0.6 M NH 4 NO 3 for 12 h, (D) coated steel its coating bath contains 1 × 10 -2 M Tween-80 after immersion in 0.6 M NH 4 NO 3 for 12 h.
Table 7 summarizes the results in this Fig. 14 and represents the intensities of the peak of the different elements in the four samples.
The XPS results of the coated steel free from surfactants (Fig. A) show six peaks at 156.2, 179.6, 488.7, 561.3, 766.8 and 998.6 eV.These peaks correspond to the elements and their respective chemical states present on the surface of the coated steel.
After immersion in NH 4 NO 3 solution for 12 h (Fig. B) a new peak appeared at 408.7 eV which corresponds to N1s state of nitrate.This indicates that the corrosive anions adsorbed and reacted with the coating forming a layer of the corrosion products on the surface.When Triton-X-100 is added to the coating bath (Fig. C) two new peaks appeared at 206.7 and 300 eV correspond to C1s and C12p of the surfactant.This indicates that the surfactant is incorporated into the coating layer.Additionally, the peak intensities of the P, K and Mn increased indicating that the presence of Triton-X-100 in the coat increased its protection efficiency, The peak of N is disappeared which indicates that the presence of the surfactant in the coating bath prevent the adsorption of the (NO 3 − ) ion at the steel surface due to its high absorbability.However, when Tween-80 is added (Fig. D) to the coating bath, the two carbon peaks appeared with low intensities indicating that this surfactant is less efficient

Mechanism of action of the surfactants in the conversion coating process
A complex series of chemical and electrochemical processes occur at the steel/solution interface in the coating bath.The formation of the coat mainly takes place through the following steps which represented in Fig. 15 [47][48][49] .

Electrochemical dissolution of steel
Steel instantly forms a microgalvanic couple when submerged in the acidic conversion coating bath.The dissolution of iron is done through the following reaction at the microanodes.
The dissolved Fe 2+ is speedily oxidized to Fe 3+ by the potassium permanganate in the conversion coating bath The hydrogen evolution occurs simultaneously at the microcathodes was done via the following reaction:

Deposition of insoluble salts at steel surface
An amorphous film of both oxides and phosphates of iron is deposited Also, it is reported that The Mn species were mostly existing as Mn 4+ in the coating 28 .

Growth of the deposited coat
The deposition of both FePO 4 and Fe 2 O 3 on the steel surface is done via two steps: nucleation and crystal growth.The deposited coat behaviour is varied according to whether the coat is formed in the presence or the absence of surfactants.The surfactants play a weighty influence in the microstructure of the deposited coat.Adsorption of surfactants on the steel surface retarding the dissolution of metal, the low concentration of Fe 2+ in the conversion coating bath allows higher rate of nucleation process over the crystal growth and so a uniform coat is formed.This mechanism is confirmed by measuring the coat thickness and porosity of the coats in the absence and presence of the surfactants and by the XPS results.

Conclusion
1. Electrochemical, SEM, EDX and XPS results indicated that the presence of each of Triton-X-100 or Tween-80 surfactants in the coating bath of the permanganate-phosphate coat effectively improved its corrosion resistance.2. EIS results demonstrated that the presence of 0.01 M of each of Triton-X-100 or Tween-80 in the coating solution increased the protection efficiency of the coat up to 93.0% and up to 84.1%, respectively.The greater of that in presence of Triton-X-100 is believed to the presence of a benzene ring in its molecular structure.3. A mechanism for the action of the surfactants in the coating process is proposed using the findings of the different techniques.

Figure 3 Figure 2 .
Figure 3 shows the micrographs of SEM graphs of (A): Bare steel, (B): coated steel, (C): coated steel in presence of 5 × 10 −5 M Tween-80, (D): coated steel in presence of 5 × 10 −3 M Tween-80, (E): coated steel in presence of 5 × 10 −5 M Triton-X-100 and (F) coated steel in presence of 5 × 10 −3 M Triton-X-100.It is clear that both the surfaces of the bare steel and the coat B have significant porosity in the absence of surfactants.Upon progressively adding more of the surfactants during the coating process (C, D, E, F), the coating layer becomes more dense and the porosity of the coat diminishes.The SEM images demonstrate that the presence of surfactants reduces the number of surface holes, and that the shape of the surface becomes uniform.The EDX results are represented in Fig. 4. Variations of the weight percent of carbon (Wt % C) on the coated surface with the concentration of the surfactants in the coating solution are shown in Fig. 5.It is clear that in presence of low concentrations (≤ 0.0001 M) of the surfactants in the solution, Wt % C on coat surface sharply increases.However, in the presence of higher concentrations in the solution, change of the concentration of the

Figure 5 .
Figure 5. Variations of the weight percent of carbon (Wt % C) on the coated surface with the concentration of each of Triton-X-100 or Tween-80 present in the coating solution.

Figure 6 .
Figure 6.Nyquist plots of permanganate-phosphate coated steel in 0.6 M NH 4 NO 3 solution (the coats contain different concentrations of each of Triton -x-100 (a) and Tween -80 (b) surfactants).

Figure 7 .
Figure 7. Schematic for the equivalent circuit of coated steel.

Figure 9 .
Figure 9.Effect of the concentration of Triton-X-100 and Tween-80 surfactants on the protection efficiency of the permanganate_phosphate coats in 0.6 M NH 4 NO 3 solution.

Figure 11 .Table 5 .Figure 12 .
Figure 11.Relation between the porosity of the coat and concentration of each of Triton-X-100 and Tween-80 present in the coating bath.

Table 2 .
Electrochemical impedance parameters of Permanganate -phosphate coated steel in 0.6 M NH 4 NO 3 solution in the absence and presence of different concentrations of surfactants in the coating solution at 30 °C.

Table 3 .
The electrochemical parameters for the coated steel that was performed using a bath containing different concentrations of surfactants in the coating solution at 30 °C.

Table 4 .
The electrochemical parameters used in the determination of the porosity of permanganatephosphate coat on steel in the absence and presence of different concentrations of each of Triton-X-100 or Tween-80 in the coating bath.

Table 7 .
Intensity of the peaks of the elements in the coats and coat after immersion in NH 4 NO 3 solution for 12 h.