Composition Distribution and Electrochemical Behavior of an Ni 2 Al 3 Coating on Q 235 Steel

An Ni2Al3 coating was prepared via the electrodeposition of nickel followed by pack aluminization. Polarization curve and electrochemical impedance spectroscopy were performed to study the room temperature corrosion behavior of the coating in a 3.5 wt. % NaCl solution. The long-term impedance of the coating was also investigated after immersion for 30 days. Results show that the Ni2Al3 coating possessed lower corrosion current density (Icorr) and higher polarization resistance (Rp) than the substrate in the solution. The Bode plots of the coating showed two time constants after the long-term immersion. Pitting corrosion was found on the coating surface.


Introduction
Nickel-aluminide type coatings, such as Ni 2 Al 3 , NiAl, and Ni 3 Al, are widely used for protecting the nickel-base superalloy against high-temperature oxidation [1][2][3] or carburization [4].Further, the nickel-aluminide coatings can also protect other metal substrates with different preparation methods.For example, the SHS (self-propagating high-temperature synthesis) casting route can prepare a Ni 3 Al coating to protect carbon steel [5], a plasma-sprayed Ni 3 Al coating can protect boiler tube steels [6,7], a mechanical alloying to format nanostructure NiAl can defend carbon steel [8], and a two-step process to make nickel aluminide/nickel hybrid coatings can shield alloy steel [9] or TiAl alloy [10].Compared to the other methods mentioned above, the two-step process can aluminize at low temperature by pack aluminization of preliminarily nickel-coated samples [9].Due to the simple composition and single structure of the nickel layer, the Ni/Al diffusion coefficient is higher than that of a superalloy [11].
The Ni 2 Al 3 coating has been previously prepared by the two-step method mentioned above [9,12].The main concern, therefore, is with high temperature oxidation property rather than the room corrosion [9].Studying the corrosion behavior of the coating in Cl ´-solution can explore its potential application in the seawater.Compared with the results of ocean exposure experiment, the NaCl (3.5 wt.%) solution is widely used to simulate the ocean environment [13][14][15][16].An electrochemical technique is commonly adopted to characterize the corrosion behavior of material [13,[17][18][19][20][21][22][23].
The aim of the present work is to investigate the room temperature corrosion behavior of the Ni 2 Al 3 coating in a Cl ´solution (3.5 wt.% NaCl).Polarization curve and electrochemical impedance spectroscopy (EIS) were used to clarify the electrochemical behavior of the Ni 2 Al 3 coating.

Sample Preparation
The Q235 steel serving as the substrate was cut to a size of 10 mm ˆ10 mm ˆ2 mm (L ˆW ˆH).All of the specimens were ground with emery paper from 120 to 800 grits, then polished with alumina suspension, cleaned with acetone and distilled water, and finally immediately dried.The substrate was first plated with nickel using direct current electrodeposition in a Watts bath.After plating, the specimens were aluminized using pack cementation at 650 ˝C for 20 h.The pack powder mixture was composed of Al, AlCl 3 (anhydrous), and Al 2 O 3 powder.

Electrochemical Tests
The electrochemical tests were conducted with the electrochemical working station (CHI604D, Chenhua, Shanghai, China).All measurements were tested in a Cl ´solution, namely, a 3.5 wt.% NaCl solution at room temperature.A platinum electrode and a saturated calomel electrode (SCE) were used as the counter and the reference electrode, respectively.The specimens were embedded in epoxy resin with an exposed working area of 1 cm 2 (L ˆW) as the working electrodes.Before electrochemical tests, the coated samples were soaked for 30 min in the electrolyte solution.A polarization curve measurement was performed at potentials from ´500 to +500 mV versus open circuit potential.In order to study the coating failure mechanism, it was immersed for 30 days to test the impedance spectrum.

Coating Characterization
Figure 1 shows the XRD pattern of the surface coating on the substrate.It indicates that the surface coating mainly consisted of the Ni 2 Al 3 phase.Figure 2 presents the morphology and composition of the Ni 2 Al 3 coating surface.As shown in Figure 2a, the coating surface is smooth.The atomic percentage of Ni is 41.17 at.%, while the content of Al is 58.83 at.% (Figure 2b).The atomic ratio of Ni to Al is close to 2/3, which is consistent with the result of XRD.The coating cross-section morphology is smooth with no apparent pore defects, as can be seen in Figure 3a.The composition of Ni and Al are stable from the surface to about 30 µm into the internal, then the Ni decreases and the Al increases in the diffusion zones, where Fe is present, as shown in Figure 3b.The content of Ni and Al are gradually reduced to zero deep into the substrate (about 55 µm).The rest is Fe.The results show that the Ni 2 Al 3 coating was successfully prepared by using the two-step process; the thickness of the coating is about 30 µm, while the diffusion zone is 20 µm thick.

Electrochemical Behavior
The polarization curves of different material in the 3.5 wt.% NaCl solution are shown in Figure 4.The anodic slope β a and cathodic slope β c can be obtained from the Tafel region of the polarization curves.The corrosion current density I corr and polarization resistance R p can also be directly obtained (Table 1).The Stern-Geary constant B [24] is calculated by the equation: The relation between I corr and R p is given [24]:  The B values can be calculated by Equation (1).From the calculated data, it can be seen that the relationship between I corr , R p and B is roughly in accordance with Equation ( 2).The coating corrosion current density (I corr ) is about 1/28 of that of the substrate (see Table 1).At the same potential, the I corr of the coating is much lower than that of the substrate.Meanwhile, the inflection point (point P) of the polarization curves in Figure 4 should be noticed, since the current density suddenly increases after the point P. In other words, the passive state (before point P) is converted into the active state (after point P) according to the change of the current density.
The corrosion resistance can also be evaluated by means of the electrochemical impedance spectroscopy (EIS) measurement.Figure 5 shows the equivalent circuit models in order to analyze the EIS results.Each parameter of the models is represented as following: R s is the solution resistance, C dl is the double layer capacitance, R ct is the charge transfer resistance at the surface/electrolyte interface, C c is the coating capacity, and R c is the coating resistance.Y 0 (ohm¨cm ´2¨s n ) and n (0 ď n ď 1) are the two parameters that characterize both C dl and C c .Y 0 represents the base admittance of the constant phase element (CPE), where n is the exponent, which can be used as a gauge of surface heterogeneity [25].Figures 6 and 7 show the Nyquist and Bode plots of the experimental and fitted EIS curves of the coating at different times.The extracted impedance parameters analyzed by the EIS analyzer software from the plots are summarized in Table 2.According to the EIS results, it can be understood that at the first 2 h of immersion, the Nyquist and Bode plots of the coating show only one time constant that is typically an indication of uniform corrosion occurrence on the coating surface (Figures 6 and 7b).The results indicate that there is only one reaction interface between the coating and solution.As time passes by, there are two time constants emerged in Figure 7b.These two time constants represent two interfaces: one is the oxide film and coating surface, and the other is the oxide film and electrolyte solution.These results are consistent with the above-mentioned equivalent circuit models (Figure 5).

Time
There are two interfaces indicating two interface reactions.One is the formation of oxide film, and the other is the destruction of the oxide film.At the beginning of the corrosion process, the formation of oxide film is faster than that of the destruction.Then, the destruction of oxide film is accelerated due to the constant infiltration of the corrosion solution.At this period, the value of R ct decreases (see Table 2).Finally, the destruction of oxide film is still faster than that of formation.Therefore, the coating corrosion resistance increases, which is caused by the accumulation of the corrosion products; corrosion products inhibit the solution ions (Cl ´), penetrating the coating to corrode the substrate, which leads to an increase in the value of R ct .
The variation tendency of R ct verified our analysis of the coating corrosion process very well (Table 2).The larger the R ct , the better the corrosion resistance.
After 30 d of immersion, the coating surface became gray and dark.The XRD pattern of the corrosion products on the coating surface is shown in Figure 8  The morphology of the corrosion products surface is shown in Figure 9a.They randomly distributed in granular form.When the corrosion products are removed, it is found that pitting occurs (Figure 9b).

Conclusions
The Ni 2 Al 3 coating was successfully prepared on Q235 steel, and its room temperature corrosion behavior in a 3.5 wt.% NaCl solution was studied.Polarization curve and electrochemical impedance spectroscopy measurements were analyzed.It was shown that the Ni 2 Al 3 coating has lower corrosion current density (I corr ) and higher polarization resistance (R p ) compared to the substrate steel.Pitting corrosion was found in the coating surface.

Figure 1 .
Figure 1.The XRD pattern of the Ni 2 Al 3 coating.

Figure 2 .
Figure 2. The morphology (a) and composition (b) of the Ni 2 Al 3 coating surface.

Figure 3 .
Figure 3. Cross section (a) and composition (b) of a line scan of the Ni 2 Al 3 coating.

Figure 4 .
Figure 4.The polarization curves of the Ni 2 Al 3 coating and Q235 steel in the 3.5 wt.% NaCl solution.

Figure 5 .
Figure 5.The equivalent circuits to fit long-term EIS results for: (a) early stage; (b) middle-late stage.

Figure 6 .
Figure 6.The Nyquist of the Ni 2 Al 3 coating in different soaking times.The symbols delegate the experimental results, and the lines delegate the model results.

Figure 7 .
Figure 7.The Bode of the Ni 2 Al 3 coating in different soaking times.The symbols delegate the experimental results, and the lines delegate the model results.(a) Impedance diagram; (b) phase diagram.
. The main corrosion products are made up of Ni 2 Al 3 , Al 2 O 3 , and AlCl 3 O 12 .The Al 2 O 3 oxide film is formed on the surface in contact with the aqueous solution.The existence of AlCl 3 O 12 shows that the corrosion is mainly caused by the Cl ´ion.

Figure 8 .
Figure 8. XRD pattern of the corrosion products on the coating surface after 30 d immersion.

Figure 9 .
Figure 9.The morphology of corrosion products on the coating surface.(a) Low image; (b) high image.

Table 1 .
Electrochemical corrosion parameter values for the Ni 2 Al 3 coating and Q235 steel in the 3.5 wt.% NaCl solution by polarization curve.

Table 2 .
EIS parameters for the Ni 2 Al 3 coating at different immersion times.