Impact of some inorganic anions on the corrosion of nickel in a solution containing Na₂SO₄ and NaClO₄

Abstract

Potentiodynamic study was carried out on nickel in Na₂SO₄ solution in the presence of ClO₄^–, WO₄^2–, MoO₄^2–, NO₂^– and NO₃^– ions. The anodic excursion spans of the metal nickel in a solution of Na₂SO₄ are marked by the appearance of clearly defined anodic peak, passive region, and transpassive shoulder. According to the data, the anodic peak current density (I_PAI) rise from 1.82 to 8.12 mA cm^–2 as the concentration of the Na₂SO₄ solution rises from 0.2 to 1.0 M. It is clear that as scan rate increases, the I_PAI rises reaching to 11.8 mA cm^–2. The apparent activation energy of nickel corrosion in Na₂SO₄ is 33.25 kJ mol^–1. ClO₄^– anion addition speeds up nickel’s active dissolution, as well tends to break down the passive layer, and causes pitting penetration. It was found that, the pitting potential (E_pit) of nickel in solutions containing the two anions ClO₄^– and SO₄^2– shifts to the positive direction by addition of WO₄^2–, MoO₄^2–, NO₂^– anions and shifts to the negative direction by addition NO₃^- anion. E_pit increased by 0.67, 0.37 and 0.15 V in the presence of WO₄^2–, MoO₄^2– and NO₂^–, respectively. WO₄^2– > MoO₄^2– > NO₂^– was the order in which the inhibitors were most effective.

Introduction

Nickel has attracted many investigations of its electrochemical and other properties in different electrolytes^1,2,3,4, due to its wide speared industrial applications. Nickel is a metal with strong resistance to corrosion according to many circumstances; however, it may still corrode under some conditions. Nickel corrosion behavior is affected by environmental variables, including the existence of corrosive chemicals, and the specific type of nickel (for instance, native nickel, nickel alloys).

Nickel may corrode in corrosive conditions, especially those including halides e.g. chlorides. The incorporation of halides can hasten the corrosion process by breakdown the passivity of the nickel surface^5,6,7. The proposed mechanisms in which chlorides ions are adsorbed on the nickel surface in competition with water molecules as the following:

$${\text{Ni }} + {\text{ H}}_{{2}} {\text{O }} \to {\text{ Ni}}\left( {{\text{H}}_{{2}} {\text{O}}} \right)_{{{\text{ads}}}} ,$$

(1)

$${\text{Ni }} + {\text{ Cl}}^{ - } \to {\text{ Ni}}\left( {{\text{Cl}}^{ - } } \right)_{{{\text{ads}}}} ,$$

(2)

$${\text{Ni}}\left( {{\text{Cl}}^{ - } } \right)_{{{\text{ads}}}} \to {\text{ NiCl}} + {\text{ e}}^{ - } ,$$

(3)

$${\text{NiCl }} + {\text{ Cl}}^{ - } \to {\text{ NiCl}}_{{{2}({\text{aq}}.)}} + {\text{ e}}^{ - } .$$

(4)

In the presence of other different species, including perchlorate, sulphate, and nitrate ions, localized corrosion of nickel can also happen⁸. Several corrosion preventative methods can be used to reduce nickel corrosion that includes:

Coatings: Using protection coatings like nickel plating, electroless metal coatings, as well as organic coatings might give an extra shield towards corrosion⁹.

Alloying: To improve corrosion resistance in certain settings, the alloy of nickel with additional elements can be used. Because of their superior resistance to different corrosive chemicals, nickel-based alloys such as Inconel or Hastelloy alloy are frequently used in corrosive settings¹⁰.

Cathodic protection: Using cathodic protection methods including sacrificial electrodes system or impressed current structures, nickel may be protected from corrosion¹¹.

Nickel corrosion resistance may be significantly improved by chromate ions^12,13. Chromate is first adsorbed on the cathodic sites of the metal surface, followed by electrochemical cathodic reduction to give some kinds of Cr oxides. This oxide layer acts as a barrier from further corrosion by blocking corrosive substances such as oxygen and moisture from accessing the underlying metal. By encouraging the creation of a stable and protective chromium oxide layer, the chromate ion passivation technique can improve the corrosion resistance of nickel. In alkaline or somewhat acidic situations, chromate passivation is especially effective. But as a result to their toxicity, hexavalent chromium material, such as chromate ions have recently caused environmental issues. As a consequence, studies regarding new, environmentally conscious corrosion inhibitors and passivation approaches have changed.

In the current work, alternative inorganic materials, WO₄^2–, MoO₄^2–, NO₂^–, NO₃^– ions are being explored for their anti-corrosion properties on nickel in Na₂SO₄ solution in the presence of perchlorate ions. These ions are considered as a less toxic alternative to hexavalent chromium compounds. Ongoing research aims to optimize the use of these inorganic materials and develop effective passivation materials for nickel and its alloys.

Experimental

Polarization measurements were performed on cylindrical electrodes machined nickel rod (99.98%, surface area 0.463 cm^–2). Every time, the nickel surface was cleaned up by using emery paper for polishing from no. 400 to no. 2500. Finally, the working electrode was washed with double distilled water and degreased with acetone. Studies on potentiodynamic polarization were conducted using a computer and a potentiostat instrument (Gamry-3000). The tests have been carried out in a standard three-compartment cell employing a saturated calomel electrode to be the reference electrode, a platinum foil to be the auxiliary electrode, and nickel to be the working electrode. The polarization curves were swept from − 2.0 to 2.0 V. Potential scan rate range from 20 to 100 mV s^–1.

Chemicals of analytical grade and three-time distilled pure water were used to create solutions. For all studies, various concentrations of Na₂SO₄ solutions were prepared using laboratory-grade Na₂SO₄ (Merck) and triple distilled water. Sigma-Aldrich Company supplied the inorganic compounds (NaClO₄, Na₂WO₄, NaNO₂, NaNO₃), which were utilized without any extra purification. By submerging the cell in a water thermostat, the temperature of the experiment’s solution was regulated with a temperature range of 30–70 °C. All experiments were conducted in aerated solutions.

The surface morphology of some samples after immersion for 120 h at 30 °C was performed using Scanning Electron Microscope (SEM) ZEISS Gemini-SEM Microscopy.

Results and discussion

Electrochemical polarization of nickel in Na₂SO₄ solution

Typical polarization curves of nickel in various concentrations of Na₂SO₄ solution (0.2 to 1.0 M) are given in Fig. 1. The curves were swept from -2.0 V to 2.0 V at scan rate υ = 20 mVs^–1 and at 30 °C. On positive going scan, the cathodic current density decreases continually reaching corrosion potential (E_corr) at zero current density. Prior to the oxygen evolution reaction, each anodic curve shows one dissolution peak (I), a definitive passive state, and a transpassive state (II). The dissolution peak (I) of nickel is related to electroformation of hydrous Ni(OH)₂^14,15,16 layer, which is transformed chemically and electrochemically to produce the passive NiO layer in the end^17,18,19. When the concentration of nickel oxide exceeds the solubility product of NiO, precipitation of a porous oxide film occurs on the electrode surface. The current decreases to an extremely low value, signalling the beginning of primary passivation, when the surface is completely covered with a nonporous passive film. The passivation current start to increase markedly, forming transpassive state prior to oxygen evaluation. Sato and Kamoto^20,21,22,23 attributed this behaviour to the dissolution of Nickel yielding Ni²⁺ ions, through active spots in the passive film.

Figure 1

Polarization curve for nickel electrode in Na₂SO₄ solution at 30 °C with scan rate of 20 mVs^–1.

The anodic peak current density (I_PA1) rise from 1.82 to 8.12 mA cm^–2 as the concentration of the Na₂SO₄ solution rises from 0.2 to 1.0 M, as shown in Fig. 1. Furthermore, as the concentration of the Na₂SO₄ solution increases, the anodic peak potential relatively shifts in a less positive direction.

Figure 2 depicts the findings of this study on the effect of potential scan rate (υ) on the potentiodynamic polarization of a nickel electrode in 0.4 M Na₂SO₄ solution at 30 °C. It is clear that as υ increases, the peak current density (I_PA1) rises reaching to 11.8 mA cm^–2 and its peak potential (E_PA1) moves towards more negative values. Good linearity of the (I_PA1) verses υ^1/2 plot was observed (Fig. 3), but do not pass through the origin. It illustrates that a combined operation including direct film formation and diffusion-controlled dissolve may be used to explain the oxide layer formation process in the potential area of peak A1.

Figure 2

Polarization curve for nickel electrode in 0.4 M of Na₂SO₄ solution at 30 °C at different scan rate.

Figure 3

Relation between (I_PAI) vs. the square root of scan rate for nickel electrode in 0.4 M of Na₂SO₄ solution at 30 °C.

Figure 4 shows the effect of solution temperature on the potentiodynamic polarization response of nickel in 0.4 M Na₂SO₄ solution at scan rate of 20 mVs^–1. The data clearly show that, the rise of temperature increases the value of I_PA1 and slightly shifts its peak potential towords more negative values. This is results can be interpreted on the bases of the fact that, at high temperature, porous oxide film is then, soft, and non-protective. This is because, the solubility of the oxide film by the electrolyte increase with increasing temperature, thus enhancing the anodic dissolution processes²⁴. Rise in temperature in temperature also accelerates the transport of reagent and reaction products to and from electrode surface²⁵.

Figure 4

Polarization curve for nickel electrode in 0.4 M of Na₂SO₄ solution with scan rate of 20 mVs^–1 at different temperatures.

In Fig. 5, the values of log I_PA1 are displayed as variables of (1/T)(K) (Arrhenius graph) for several temperatures. Based on the slope of that Arrhenius plotting, the apparent activation energy E_a for anodic reaction was determined. The data provide a value for the apparent activation energy, E_a = 33.25 kJ mol^–1.

Figure 5

Relation between log (I_PAI) vs. (1/T) for nickel electrode in 0.4 M of Na₂SO₄ solution with scan rate of 20 mVs^–1.

Effect ClO₄ ^– ions on the electrochmeical behavior of nickel

It was investigated how different NaClO₄ addition concentrations (0.1–0.5 M) affected the potentiodynamic, E/I, polarization of nickel in 0.4 M Na₂SO₄ at 30 °C and 20 mVs^–1 scan rate (see Fig. 6). The data shows that, the progressive additions of NaClO₄ cause an increase in the peak current density (I_PA) of the anodic peaks and shifts its potential to more negative. These findings imply that perchlorate ions disrupt the oxidation operations of nickel electrodes. Competing with SO₄^2– ions for adsorption on the bare metal surface, ClO₄^– ions can take part in electrode dissolution processes directly²⁶. However, the ClO₄^- in the sulphate liquid increases I_pass and has a tendency to dissolve the passive layer. The passive current goes up abruptly and strongly at a specific critical potential (E_pit), signifying the breakdown of the passive barrier and the beginning and progression of the pitting. As the amount of ClO₄^- gets higher, the pitting potential (E_pit) moves to an increasingly negative (active) location. Figure 7 data demonstrates that, in accordance with Eq. (5), E_pit is in a linear manner linked to log [ClO₄^–].

$${\text{E}}_{{{\text{pit}}}} = {\text{ x}} - {\text{y log}}\left( {{\text{ClO}}_{{4}}^{ - } } \right),$$

(5)

where x and y are constants and depends on the condations and nature of the electrode.

Figure 6

Effect of addition ClO₄^– ion on the potentiodynamic polarization curve for nickel electrode in 0.4 M of Na₂SO₄ solution with scan rate of 20 mVs^–1 at 30 °C.

Figure 7

Relation between (E_pit) vs. log [ClO₄^–] for nickel electrode in 0.4 M of Na₂SO₄ solution with scan rate of 20 mVs^–1 at 30 °C.

The adsorption of ClO₄^– on the oxide wrapped layer, specifically at its weakness points and flaws, can be attributed to the breakdown of the definitive passive barrier and the beginning of the pitting corrosion. Pit growth begins earlier when ClO₄^- concentration increases due to its mobility and acidity increases as a result of metal cation hydrolysis within pits²⁷.

In Fig. 8, the significance of the scan rate (υ) regarding the E/I relation of a nickel electrode in a 0.4 M solution of Na₂SO₄ containing 0.3 M NaClO₄ are shown. The anodic peak current is enhanced and has a more negative peak potential as υ increases. Furthermore, the increase in υ shifts E_pit towards a more positive value. Incubation time is used to clarify this attitudes²⁸. Once the scan has become high, pitting beginning takes place at higher positive potentials, which relate to a short enough incubation period—that is, the period of time required to penetrate the passive layering system.

Figure 8

Potentiodynamic polarization curve for nickel electrode in (0.4 M Na₂SO₄ + 0.3 M ClO₄^–) solution at 30 °C at different scan rate.

Plotting I_PA vs. υ^1/2 yielded a linear trend depicted in Fig. 9.The resulting straightaway fails to pass through the origin. In theory, the plotting I_PA vs. υ^1/2 shows that diffusion is partially controlling the anodic dissolution operations on nickel in the existence of ClO₄^– anion²⁹.

Figure 9

Relation between (I_PAI) vs. the square root of scan rate for nickel electrode in (0.4 M Na₂SO₄ + 0.3 M ClO₄^–) solution at 30 °C.

Effect of added inorganic anions on pitting corrosion

The impact of WO₄^2–, MoO₄^2–, NO₂^– and NO₃^– ions as corrosion-inhibiting agents has been investigated with the goal to learn more concerning the particular function that certain inorganic inhibitors serve on the preventing corrosion mechanisms taking place between the interface between the pure nickel electrode and the electrolyte. E/I curves for nickel in a solution of (0.4 M NaSO₄ + 0.3 M NaClO₄) at 30 °C are shown in Figs. 10 and 11, which show the effects when adding different amounts of WO₄^2– and NO₃^– anions in turn. Figure 12 depicts the dependent relationship of E_pit on logarithmic inorganic salts concentration.

Figure 10

Effect of addition WO₄^2– ions on the potentiodynamic polarization curve for nickel electrode in (0.4 M Na₂SO₄ + 0.3 M ClO₄^–) solution with scan rate of 20 mVs^–1 at 30 °C.

Figure 11

Effect of addition NO₃^– ions on the potentiodynamic polarization curve for nickel electrode in (0.4 M Na₂SO₄ + 0.3 M ClO₄^–) solution with scan rate of 20 mVs^–1 at 30 °C.

Figure 12

Relationship between (E_pit) and inorganic inhibitors concentrations for nickel in a solution containing 0.4 M Na₂SO₄ and 0.3 M ClO₄^–.

Analyses of data in Figs. 10, 11, 12 show that, based on the kind and amount of the inhibitor, the existence of all of these anions (except NO₃^-), hinders the overall corrosion of nickel to a particular extent. The decline in I_PA and the positive departure of the E_pit demonstrate this clearly (see Tables 1 and 2). It is evident that these agents have a decreasing effect on nickel’s active dissolution in the following order: WO₄^2– > MoO₄^2– > NO₂^–. The adsorption competition of the inhibitor SO₄^2– and ClO₄^– ions on the the outermost layer of the electrode is thought to hinder corrosion. Adsorption and founding of SO₄^2– and ClO₄^– are hindered by the adsorption inhibitor ions. As a result, the SO₄^2– and ClO₄^– anions’surface coverage diminished by the adsorbed inhibitor ions, and the total number of areas of defect at the film’s surface that the anions could enter was also decreased. The difference of the molar polarisation of these ions, which is in the order WO₄^2– > MoO₄^2– > NO₂^–, may be responsible for the different anions' inhibitory properties³⁰. Because anion molar polarisation is believed to be entirely correlated to ions adsorption³¹, the inhibitory impact should improve with enhancing ions adsorption over the electrode. Surface active ions with higher defarmability are likely to adsorb onto the nickel surface, reducing the actual area that can be used by redox ions for reaction. Figure 11 data show that the NO₃^– anion promotes the I_PAI and moves the E_pit. in a negative way. These impacts may be clarified by the NO₃^– anion’s lower polarizability value with respect to the severe SO₄^2– and ClO₄^– ions³².

Table 1 The pitting potential (E_pit) for nickel electrode in (0.4 M Na₂SO₄ + 0.3 M ClO₄^–) solution in the absence and presence of amounts of WO₄^2–, MoO₄^2–, NO₂^– and NO₃^– anions.

Table 2 The anodic peak current density (I_PA1) for nickel electrode in (0.4 M Na₂SO₄ + 0.3 M ClO₄^–) solution in the absence and presence of amounts of WO₄^2–, MoO₄^2–, NO₂^– and NO₃^– anions.

The amount of inorganic salts needed to produce a significant positive shift in E_pit (as indicated inhibition) improves in the following sequence: NO₂^– > MoO₄^2– > WO₄^2– (see Fig. 12). Because of their particular adsorbility, these inhibitors are able to remove ClO₄^– ion from the locations where it more readily enters the passive coating, which improves pitting resistance against corrosion^33,34.

Furthermore, the reduced modes of these inhibitors (WO₂ and MoO₂), for example, transform into component of the passivating oxide and have a tendency to block its pores and weaknesses getting more effectively shielding capabilities. on the additional one, nitrite ions had been found to be reduced to ammonia in which oxygen that remained on the surface triggered the oxide reaction³⁵. Figure 12 data demonstrate that the incorporation of NO₃^- ions moves E_pit to a more negative direction, demonstrating that NO₃^- ions encourage pitting corrosion. The finding could be explained by the relatively low polarizability of NO₃^– ions³⁶ in comparison to other ions.

The inhibition corrosion efficiency (IE%) of various inorganic ions was estimated as follows³⁷:

$${\text{IE}}\,\% = \frac{{{\text{I}}_{{{\text{PA}}(0)}} - {\text{I}}_{{{\text{PA}}}} }}{{{\text{I}}_{{{\text{PA}}(0)}} }} \times 100,$$

(6)

where I_PA(0) is the anodic current density in the absence inorganic ions. The inhibitory efficiency improves as the concentration of inorganic ions increases (see Table 3). The sequence of increase in the inhibitors' inhibitory effectiveness is WO₄^2– > MoO₄^2– > NO₂^–.

Table 3 The corrosion inhibition efficiency of inorganic inhibitors for nickel in (0.4 M Na₂SO₄ + 0.3 M ClO₄^–) solution.

Surface studies

The surface morphology caused by immersion in the corrosive solution (0.4 M Na₂SO₄ + 0.3 M NaClO₄) in the presence and absence of the compounds under consideration (ClO₄^–, WO₄^2–, MoO₄^2–, NO₂^– and NO₃^– ions, 0.03 M) were compared using the SEM methodology on nickel specimens.

The nickel specimens were subjected to (0.4 M Na₂SO₄ + 0.3 M NaClO₄) for 120 h around 30 degrees Celsius, and then they were taken from the solution and allowed to sit for an additional hour to dry before examination. According to Fig. 13a depicts the image of the untreated sample in the corrosive solution (0.4 M Na₂SO₄ + 0.3 M NaClO₄). In the absence of inhibitors, the large damaged regions, evident deteriorations, and pitting holes are visible.

Figure 13

SEM images of Nickel samples in corrosive medium (0.4 M NaSO₄ + 0.3 M NaClO₄) (image (a)) and in the presence of the inhibitors, WO₄^2– (image (b)), MoO₄^2– (image (c)), NO₂^– (image (d)) and NO₃^– (image (e)). Immersion time: 120 h. Temperature: 30 °C.

In contrast, the microscope images of the tested nickel samples in the presence of the inhibitors, WO₄^2–, MoO₄^2–, NO₂^– and NO₃^–, in Fig. 13b–d, and e, respectively, showed highly effective corrosion inhibition efficiency as evidenced by the disappearance of deterioration areas, the disappearance of localized corrosion areas, and the fit (except NO₃^–).

The nickel surface was found to be very corrosive in the presence of NO₃^-ions (Fig. 13e). The present SEM results and the prior electrochemical technique results both show that the examined inhibitors (with the exception of NO₃^–) have effective corrosion control.

Conclusion

The current study investigates the anti-corrosion capabilities of various inorganic materials, such as WO₄^2–, MoO₄^2–, NO₂^– and NO₃^–, on nickel in Na₂SO₄ solution in the presence of perchlorate ions. Potentiodynamic curves show one dissolution peak (I), definitive passive, and transpassive state (II) regions. The data reveal that the anodic current density increase with increase in Na₂SO₄ solution concentration, potential scan rate (υ) and solution temperature. The progressive additions of NaClO₄ cause an increase in corrosion of nickel electrode and tend to breakdown the passive layer. The presence of the inorganic anions studied (Except NO₃^–) inhibits both anodic dissolution and pitting corrosion of nickel. The sequence of increase in the inhibitors' inhibitory effectiveness is WO₄^2– > MoO₄^2– > NO₂^–.