Anodic Dissolution of Spheroidal Graphite Cast Iron with Different Pearlite Areas in Sulfuric Acid Solutions

The rate equation of anodic dissolution reaction of spheroidal graphite cast iron in sulfuric acid solutions at 298K has been studied. The cast irons have different areas of pearlite. The anodic Tafel slope of 0.043V decade and the reaction order with respect to the hydroxyl ion activity of 1 are obtained by the linear potential sweep technique. The anodic current density does not depend on the area of pearlite. There is no difference in the anodic dissolution reaction mechanisms between pure iron and spheroidal graphite cast iron. The anodic current density of the cast iron is higher than that of the pure iron.


Introduction
Cast iron is widely used for pipes in neutral environments such as water and soil.A lot of experimental studies of cast iron for practical use were conducted [1][2][3][4][5][6][7][8].A form of corrosion unique to cast irons is a selective leaching attack commonly referred to as graphitic corrosion [9, page 89].A graphite network forms a corrosion-resistant phase called a graphitic layer.Therefore, the corrosion resistance of cast iron is said to be higher than that of steel.That is not necessarily the case.
Laque reported that the corrosion rate of cast irons is lower than that of a steel in the atmosphere above the sea [2].However, Paris and Bruniere [1] and Horikawa et al. [10] found that the corrosion rate of cast irons is nearly equal to that of steel in water and in atmosphere.Furthermore, a book said that graphitic corrosion does not occur in spheroidal graphite cast irons, because the graphite network does not exist [9, page 89], while another author reported that it occurs with both gray and spheroidal graphite cast irons [11].These inconsistent results come from the complexity of graphitic corrosion.To elucidate the mechanisms of corrosion of cast iron, intensive studies are required.In this paper, the electrochemical dissolution of ferrous matrix is discussed.
Cast iron is composed of pearlitic matrix, ferritic matrix, and graphite particles.The area of pearlite influences the physical properties such as tensile strength, elongation, and hardness [12].Pearlitic matrix is a lamellar mixture of ferrite and cementite.Thus, the length of interfaces between ferrite and cementite becomes large with increasing pearlite area.Fontana [9, pages 28-31] and Trethewey and Chamberlain [13] described in their books that grain boundaries are highenergy areas and are more active chemically.The book by Evans said that regions near the grain boundaries may possess electrochemical properties different from those of the grain interiors [14].Therefore, the area of pearlite may influence the mechanism and the rate of anodic dissolution of ferrous matrix.
To the authors' knowledge, there are few studies on the dissolution reaction mechanism of cast iron.The objective of the present investigation is to determine the kinetic parameters and the rate equation of anodic dissolution reaction of spheroidal graphite cast iron having different areas of pearlite in sulfuric acid solutions.The ratios of the pearlite area,   , the graphite particles area,   , and the ferrite area,   , are defined by ( 1), (2), and (3), respectively.

Experimental
2.1.Specimen.The chemical composition of specimens of spheroidal graphite cast iron (hereinafter referred to as cast iron) is given in Table 1.Cast irons which have different areas of pearlite were prepared.A sand mold, given in Figure 1, was used for casting.Specimens were clipped from the plate which was 5 mm thick.The ratio of pearlite area to total surface area,   , was controlled by changing heat treatment process, as shown in Figure 2. The cast irons were composed of pearlitic matrix, ferritic matrix, and graphite particles.  and the ratio of the graphite particles area to total surface area,   , were defined, respectively: = Area of graphite particles Total Surface area .
Pearlitic matrix is a lamellar mixture of ferrite and cementite.
If it is assumed that the area of ferrite in pearlitic matrix is as large as that of cementite, the ratio of the ferrite area to total surface area,   , is expressed by decreases from 0.79 to 0.54 with increasing   from 0 to 0.57, as shown in Table 2, while   does not change.
The average diameter of graphite particles in all specimens was approximately 20 m. Figure 3 shows microscopic photographs of specimens.
The cast iron of 1 cm 2 was grinded with a 1000 grit silicon carbide (SiC) paper, degreased with acetone, etched in 1 M H 2 SO 4 solution for 5 seconds at 323 K, rinsed with distilled water, and dried in air.

Test Solutions.
Sulfuric acid solutions at pH = 1.0, 2.0, and 3.0 were prepared.Those test solutions contained Na 2 SO 4 as a supporting electrolyte so as to keep ionic strength at 0.5 mol dm −3 .The test solutions were deaerated with pure  nitrogen and agitated with a stirring bar.The rotating speed of the stirring bar did not affect the anodic current densities.

Measurements.
The specimen was set in an empty cell.Pure nitrogen was passed through the cell for 3600 seconds.The specimen was not exposed to the air after it was set in the cell.The test solution deaerated with pure nitrogen in a reservoir was introduced to the cell.The corrosion potential was measured for 600 seconds after the specimen was exposed to the test solution.Subsequently, the linear potential sweep from the corrosion potential started.The sweep rate was ranged from 0.005 to 0.1 V s −1 .A new test piece was used for each measurement.The counter electrode was platinum.A KCl saturated Ag-AgCl reference electrode was used.The solution resistance between the specimen and the reference electrode was measured by ac impedance method, with a frequency of 2 kHz and an amplitude of 0.010 V.The electrode potential was expressed on the SHE scale after the correction of the ohmic potential drop.All electrochemical measurements were performed at 298 K. Areas of cementite and graphite particles are assumed not to take part in the anodic dissolution.The anodic current density was calculated from where   is the anodic current density,   is the observed anodic current, and  is the total surface area.

Anodic Current Density due to Dissolution of Cast Iron.
The steady-state anodic current can be measured when the sweep rate is lower than 0.1 V s −1 .Figure 4 shows the anodic polarization curves of the specimen no. 3 in a sulfuric acid solution at pH = 2.0.The anodic current densities do not depend on the sweep rates in the range of 0.005 to 0.1 V s −1 .The sweep rates in this range do not affect the polarization curves at other pHs.  Figure 5 illustrates the steady-state anodic polarization curves of the specimen no. 3 at pHs 1.0 to 3.0, with a potential sweep rate of 0.01 V s −1 .Each curve has a Tafel region covering more than one decade.The anodic Tafel slopes are 0.043 V decade −1 .
It is clear that the anodic current density due to the dissolution of the cast iron at a constant electrode potential depends on pH, as shown in Figure 5.The anodic current density is expressed by (5) in which the reaction order, , will be discussed in Section 3.3.Consider where   is the rate constant of the anodic reaction,   OH − is the hydroxyl ion activity at the electrode surface, and  is the electrode potential.
The anodic dissolution reaction is controlled by surface reaction process, because convective conditions do not affect polarization curves.Therefore, the hydroxyl ion activity at the electrode surface is the same as that in the bulk solution.The Tafel slope of 0.043 V decade −1 in ( 5) is very close to (2/3) ⋅ (2.3/).The anodic current density,   , of the cast iron can be expressed by where  * OH − is the hydroxyl ion activity in the bulk solution,  is the Faraday constant,  is the gas constant, and  is the absolute temperature.

Effect of the Ratio of the Pearlite Area on the Anodic Current Density.
A relation of the ratio of the pearlite area,   , to the anodic current density,   , is discussed.Figure 6 shows the plots of log   against   at a few constant electrode potentials and pHs.The effect of   on the anodic current density is not observed.This means that the acceleration of anodic dissolution at interfaces between ferrite and cementite can be negligible.

Determination of the Reaction Order with respect to the
Hydroxyl Ion Activity.Three approaches for determining the reaction order with respect to the hydroxyl ion activity are examined.

First Approach.
The reaction order, , is given by partially differentiating log   with respect to pH in (6).Consider Figure 7 shows the plots of log  against pH for any specimens nos.from 1 to 4 at  = −0.30V.As the figure shows, a straight line with a slope of 1 is obtained.

Second Approach.
The partial differentiation of  with respect to pH is expressed by Equation ( 8) presents that the electrode potential at a constant anodic current density is a linear function of pH.If  = 1 in ( 8), the value of the right side is equal to −0.040 V pH −1 at 298 K.The relationships between the electrode potential at   Kelly have revealed that the cathodic current density of pure iron is as follows [15,16]: where   is the cathodic current density and   is the rate constant of the cathodic reaction.The present authors have confirmed that the cathodic current density of spheroidal graphite cast iron is also given by (9).The cathodic current is the product of   and total surface area.The anodic current is calculated by ( 4) and (6).The corrosion potential is represented in (10), by combining (4), (6), and (9).Consider where  corr is the corrosion potential and   is ionization constant of water.Therefore, the effect of pH on the corrosion potential is given by By assuming that  = 1, the value of the right side in ( 11) is equal to −0.060 V pH −1 at 298 K. Figure 9 shows the plots of  corr versus pH for any specimens of nos. 1 to 4. The linear relation between  corr and pH is confirmed, as shown in the figure.The observed slope is precisely −0.060 V pH −1 .It is proved that the three approaches reach the same result of  = 1.

Mechanism of Anodic Dissolution Reaction of Cast Iron.
The mechanisms of iron dissolution reaction in acid solutions have been proposed by many researchers .The kinetic parameters of typical mechanisms are summarized in Table 3.
In the mechanism by Bonhoeffer and Heusler, the anodic Tafel slope is 0.030 V decade −1 at 298 K, and the reaction order with respect to the hydroxyl ion activity is 2 [18][19][20].
According to Bockris et al., the anodic dissolution reaction is characterized by the anodic Tafel slope of 0.040 V decade −1 at 298 K and the first order dependency on the hydroxyl ion activity [15].Abdul Azim and Sanad reported that the anodic Tafel slope was 0.060 V decade −1 at 298 K, and the reaction order with respect to the hydroxyl ion activity was 1 [37].These results refer to different experimental conditions.Lorenz et al. studied the effect of heat treatment of iron on the anodic dissolution rate [21,38].They found that the anodic Tafel slope was 0.030 V decade −1 when the specimen was annealed at a temperature lower than 673 K.The anodic Tafel slope was 0.040 V decade −1 for the specimen annealed at a temperature higher than 1023 K.
In the present study, the specimens experienced heat treatment at 993 K and 1173 K to change the pearlite area.The iron wire electrodes used in the Bockris study were heated in a furnace for 15 minutes at temperatures between 873 and 1173 K before electrochemical measurements.The Tafel slope of 0.040 V decade −1 obtained by the present authors' and Bockris' experiments substantiates the findings by Lorenz et al.The authors' results support the following mechanism of the anodic dissolution of iron proposed by Bockris et al. [15].Consider Fe + OH − = (FeOH) ads + e (12) The anodic current density is expressed from the plots in Figures 5 to 8 For pure iron, Kelly has revealed that the rate equation of the anodic dissolution of iron is as follows [16]: The anodic current density of the cast iron is higher than that of the pure iron.Alloyed sulfur and phosphorus tend to promote the anodic reaction, even if the content of these elements is lower than 0.02% [11,39].The cast iron in the present study contains 0.01% sulfur and 0.013% phosphorus.The anodic current density of the cast iron is considered to be stimulated by these elements.

Conclusions
The anodic dissolution of spheroidal graphite cast iron with different pearlite areas in sulfuric acid solutions is studied.The main results are as follows.
(1) The anodic current density of cast iron is expressed by which is consistent with that of pure iron proposed by Bockris et al.
(2) The anodic current density does not depend on the area of pearlite.The acceleration of anodic dissolution at interfaces between ferrite and cementite can be negligible.
(3) No difference in anodic dissolution reaction mechanisms between pure iron and spheroidal graphite cast iron is found.

Figure 1 :
Figure 1: Sand mold and clipping position of specimens.

Figure 2 :
Figure 2: Heat treatment processes of specimens.

Figure 5 :
Figure 5: Anodic polarization curves of the specimen no. 3 in sulfuric acid solutions at pH = 1.0, 2.0, and 3.0, with a potential sweep rate of 0.01 V s −1 .

Figure 6 :
Figure 6: Anodic current densities as a function of the ratio of the pearlite area,   , at a few constant electrode potentials and pHs.

Table 1 :
Chemical composition of the spheroidal graphite cast iron (mass%).

Table 2 :
Ratios of the pearlite area, the graphite particles area, and the ferrite area.

Table 3 :
Kinetic parameters of typical mechanisms for iron dissolution reaction.  is the anodic Tafel slope, and  is the reaction order with respect to the hydroxyl ion activity. Note: