CC BY
118
Electrochemical investigation on the corrosion of 18%Ni M250 grade maraging steel under welded condition
in sulfuric acid medium
Pradeep Kumar and A. Nityananda Shetty
Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore, Karnataka, India, India PIN — 575025, e-mail: [email protected]
The corrosion behavior of welded maraging steel in sulfuric acid solutions has been studied over a range of acid concentrations and solution temperatures by electrochemical techniques such as Tafel extrapolation and electrochemical impedance spectroscopy. The studies have revealed that the corrosion rate of welded maraging steel increases with the increase in temperature and increase in concentration of sulfuric acid in the medium. The thermodynamic parameters such as activation energy, enthalpy of activation and entropy of activation for the corrosion process are calculated. The results obtained through the two techniques are in good agreement. The surface morphology and surface composition of the corroded samples have been examined by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis, respectively.
Keywords: maraging steel, EIS, polarization, SEM, EDX.
УДК 541.138
INTRODUCTION
Corrosion of metals and alloys is a natural process, but its many and varied consequences are considered to be detrimental for the safe, reliable and efficient operation of equipment and structures. Maraging steel, the most extensively used engineering material, gets corroded under many circumstances, especially in some industrial processes, such as acid cleaning, acid descaling and oil well acidizing. An alloy can be a low-carbon steel that traditionally contains about 18 wt % Ni, substantial amounts of Co and Mo together with small additions of Ti. However, depending on the demands of the market, the composition of the material can be modified [1]. For many of the applications of maraging steels, welding is an important means of fabrication. The unique property of being weldable in the solutionized condition followed by a low temperature (480°C) post-weld maraging treatment makes these steels attractive for fabrication of large structures [2]. According to available literature, atmospheric exposure of 18 Ni maraging steel leads to its corrosion in a uniform manner and makes it completely rust covered [3, 4]. Bellanger et al. [5] have reported that the corrosion behavior of maraging steel at the corrosion potential depends on pH and intermediates remaining on the maraging steel surface in the active region favoring the passivity in radioactive water. Maraging steels were found to be less susceptible to hydrogen embrittlement than common high strength steels due to significantly low diffusion of hydrogen in them [6]. But there seems to be no literature available
that reveals corrosion behavior of 18% Ni M250 grade maraging steel under welded condition in acid medium. So, the corrosion behavior of welded maraging steel in sulphuric acid medium is in the focus of the study in the present paper.
EXPERIMENTAL Material
The experiments were performed with a specimen of the welded maraging steel (18% Ni M250 grade). Percentage composition of 18% Ni M250 grade maraging steel sample is given in Table 1. The maraging steel was welded by GTAW-DCSP, with 5 passes using filler material of composition as given in Table 2. The working electrode was in the form of a rod machined into a cylindrical form embedded in epoxy resin leaving an open surface area of 0.6458 cm2. This coupon was abraded as per standard metallographic practice, belt grinding followed by polishing on emery papers of grade 600, 800, 1000, 1200, 1500, 2000, and finally on polishing wheel using legated alumina abrasive to obtain mirror finish degreased with acetone and dried before immersing in the medium.
Electrolytes
Analytical grade of sulfuric acid and doubly distilled water were used for preparing the test solutions having concentrations 0.1M, 0.5M, 1M, 1.5M and 2M. Analyses were carried out in a calibrated thermostat at temperatures 30°C, 40°C, 50°C and 60°C (±0.5°C).
© Pradeep Kumar, A. Nityananda Shetty, Электронная обработка материалов, 2013, 49(3), 76-83.
Table 1. Composition of the specimen (% by weight)
Element Composition Element Composition
C 0.015% Ti 0.3-0.6%
Ni 17-19% Al 0.005-0.15%
Mo 4.6-5.2% Mn 0.1%
Co 7-8.5% P 0.01%
Si 0.1% S 0.01%
O 30 ppm N 30 ppm
H 2.0 ppm Fe Balance
Table 2. Composition of the filler material used for welding (% by weight)
Element Composition Element Composition
C 0.015% Ti 0.015%
Ni 17% Al 0.4%
Mo 2.55% Mn 0.1%
Co 12% Si 0.1%
Fe Balance
Electrochemical measurements
Electrochemical experiments were performed in a conventional three-electrode system by using an electrochemical work station Gill AC having ACM instrument Version 5 software. The measurements were carried out using a conventional three electrode Pyrex glass cell with platinum foil as the counter electrode and a saturated calomel electrode as the reference electrode. The sample specimen was used as the working electrode.
Tafel polarization studies
A finely polished welded maraging steel specimen, sealed by epoxy resin with the exposure surface of 0.6458 cm2 (the working electrode) was exposed to the corrosion medium with different concentrations of sulfuric acid (0.1M to 2.0M), at different temperatures (30°C to 60°C), and allowed to establish a steady state open circuit potential. The potentiodynamic current -potential curves were recorded by polarizing the specimen to -250 mV cathodically and +250 mV anodical-ly with respect to the open circuit potential (OCP) at a scan rate of 1 mV s-1.
Electrochemical impedance spectroscopy studies
Electrochemical impedance spectroscopy (EIS), which gives early information about the electrochemical processe at the metal solution interface, was applied in many corrosion studies as reported in [7]. EIS measurements were carried out on the steady open circuit potential (OCP) disturbed with amplitude of 10 mV a.c. sine wave at frequencies from 100 KHz to 10 mHz. Impedance data were analyzed using Nyquist
plots. The charge transfer resistance, Rct was extracted from the diameter of the semicircle in the Nyquist plot.
In all of the above measurements, at least three similar results were considered and their average values were given in [7].
The scanning electron microscope images were recorded to establish the interaction of acid medium with the metal surface using JEOL JSM-6380LA analytical scanning electron microscope.
RESULTS AND DISCUSSION
Tafel polarization measurements
The effect of sulfuric acid concentration and solution temperature on the corrosion rate of welded samples of maraging steel was studied using Tafel polarization technique. The anodic and cathodic current-potential curves are extrapolated up to their intersection at the point where corrosion current density (icorr) and corrosion potential (Ecorr) are obtained [8]. The potentiodynamic polarization curves for the corrosion of welded maraging steel in different concentrations of H2SO4 at 50°C are shown in Fig. 1. As is seen there, the polarization curves are shifted to the high current density region as the concentration of H2SO4 increases, indicating the increase in the corrosion rate with the increase in the concentration of H2SO4. The poten-tiodynamic polarization parameters such as corrosion potential (Ecorr), corrosion current (icorr), anodic and cathodic slopes (ba and bc) and corrosion rate (vcorr) are calculated from the Tafel plots and are listed in Table 3.
Fig. 1. Potentiodynamic polarization curves for the corrosion of welded maraging steel in different concentrations of H2SO4 at 50°C.
The corrosion rate is calculated using Eq. 1 below:
U immy-1) = 3270 X M X (1)
corr ^ y y
p x Z
Table 3. Electrochemical polarization parameters for the corrosion of welded maraging steel in different concentrations of H2SO4 at different temperatures
Molarity of H2SO4 (M) Temperature (K) Corrosion potential (mV vs SCE) Anodic slope (mV dec-1) Cathodic slope (mV dec-1) Corrosion current (mA cm-2) Corrosion rate (mm y-1)
0.1 303 -356 240 -146 1.98 25.52
313 -347 248 -156 2.93 37.76
323 -323 342 -349 6.14 79.14
333 -337 321 -245 9.38 120.91
0.5 303 -322 312 -154 2.47 31.84
313 -318 261 -225 4.71 60.71
323 -312 329 -204 6.94 90.02
333 -308 380 -208 10.45 134.29
1.0 303 -315 233 -84 3.07 39.57
313 -310 314 -186 4.96 63.93
323 -319 369 -241 8.71 112.17
333 -314 327 -232 12.81 164.02
1.5 303 -313 386 -170 3.38 43.43
313 -300 335 -179 5.63 72.57
323 -295 376 -255 9.73 127.56
333 -303 375 -250 15.61 200.34
2.0 303 -293 335 -167 3.72 47.95
313 -290 367 -204 6.38 82.24
323 -283 410 -280 11.27 148.41
333 -276 418 -271 18.02 232.76
where 3270 is the constant that defines the unit of the corrosion rate, icorr is the corrosion current density in A cm-2, p is the density of the corroding material, 8100 kg m-3, M is the atomic mass of the metal, and Z is the number of electrons transferred per metal atom
[9].
From the data summarized in Table 3 it is clear that the corrosion rate of the welded maraging steel specimen increases with an increase in the strength of sul-furic acid in the solution. It is also deduced from the results that the corrosion potential is shifted towards less negative values as the concentration of sulfuric acid increases.
The corrosion of steel normally proceeds via two partial reactions in acid solutions. The partial anodic reaction involves the oxidation of metal and formation of soluble Fe2+ ions, while the partial cathodic reaction involves the evolution of hydrogen gas [10]:
Fe ^Fe2+ + 2e- (Oxidation) 2H+ + 2e~ ^ H21 (Reduction)
(2) (3)
The overall chemical reaction can be expressed as follows [11]:
Fe + H2SO4 ^ FeSO4 + H21 (4)
Although the overall corrosion rate of the welded maraging steel in concentrated sulfuric acid is low, still some corrosion procedes. The reaction products of
the corrosion process are iron sulfate and hydrogen gas.
Electrochemical impedance spectroscopy
The corrosion behavior of the welded maraging steel was also investigated by EIS with different concentrations of sulfuric acid at different temperatures. The Nyquists plots for the corrosion of the welded maraging steel in sulfuric acid solutions of different concentrations at 50°C are shown in Fig. 2. The point where the semicircle of the Nyquist plot intersects the real axis at high frequency (close to the origin) yields the solution resistance (Rs). The intercept on the real axis at the other end of the semicircle (low frequency) gives the sum of the solution resistance and the charge transfer resistance (Rct). Hence the charge transfer resistance value is simply the diameter of the semicircle [12]. The diagonal region in between the high frequency and low frequency regions has a negative slope due to the capacitive behavior of the electrochemical double layer.
The corrosion current density is calculated using the Stern Geary Equation 5 as in [13]:
bb
2.303(b + b)R
^ a c ' ct
(5)
Figure 2 demonstrates that the diameter of the semicircle decreases with the increase in the concentration of H2SO4, thus indicating the decrease in Rct value
and the increase in the corrosion rate. The fact that impedance diagrams have semicircular appearance indicates that the corrosion of the welded maraging steel is controlled by a charge transfer process and the mechanism of dissolution of metal in H2SO4 is not altered with the change in the H2SO4 concentration [14]. Also Fig. 2 shows the Nyquist plots as not perfect semicircles. The deviation has been attributed to frequency dispersion. The "depressed" semicircles have a center under the real axis and can be seen as depressed capacitive loops. Such phenomena often correspond to surface heterogeneity, which may be the result of surface roughness, dislocations, distribution of the active sites or adsorption of molecules [15].
6 5
£ u
a
o
r
N
0-1
• 0,! M M 1.0 M — 1.5 M 2.0 M
/tX 1\ X * i i
6
ohm cm2
10
12
Fig. 2. Nyquist plots for the corrosion of welded maraging steel in different concentrations of H2SO4 at 50°C.
The results obtained can be interpreted in terms of the equivalent circuit of the electrical double layer as shown in Fig. 3. The circuit fitment was done by ZSimpWin software of version 3.21. The equivalent circuit consists of solution resistance Rs, the charge transfer resistance Rct and a constant phase element (CPE)/ parallel to Rct. The constant phase element (Qd/) is substituted for the capacitive element to give a more accurate fit, as most capacitive loops are depressed semicircles rather than regular semicircles [16].
The impedance of the constant phase is given by the expression:
Zq = Vorn (6)
where Y0 is the proportional factor, n has the meaning of a phase shift, and j = (-1)12. The value of the double layer capacitance (Cd/) can be obtained from the following equation:
Cdl = Yo(<)^ (7)
where ®nm is the frequency at which the imaginary
part of the impedance has maximum [16].
The results of EIS measurement are summarized in Table 4. They show that the charge transfer resistance
(Rct) value decreases and the double layer capacitance (Cd/) increases with the increase in the concentration of sulfuric acid. The Nyquist plots obtained in the real system represent general behavior where the double layer on the interface of metal/solution does not behave as a real capacitor. On the metal side, electrons control the charge distribution whereas on the solution side it is controlled by ions. As ions are much larger than electrons, the equivalent ions to the charge on the metal will occupy quite a large volume on the solution side of the double layer. Increase in the capacitance, which can result from an increase in the local dielectric constant and/or a decrease in the thickness of the electrical double layer, suggests that the sulfate ions act by adsorption at the metal/solution interface [17].
Effect of temperature
The effect of temperature on the corrosion rate of the welded maraging steel was studied by measuring the corrosion rate at the temperature range of 30°C-60°C. Figs. 4 and 5 represent the potentiody-namic polarization curves and Nyquist plots, respectively, at different temperatures for the corrosion of welded maraging steel sample in 1M H2SO4 solution. Similar plots were obtained in other concentrations of solutions also. The Tafel polarization results and EIS results at different temperatures are listed in Tables 3 and 4, respectively. From Figs 4 and 5 and from the results in Tables 3 and 4 it is evident that the corrosion rate increases with the increase in temperature. This may be attributed to the fact that the hydrogen evolution overpotential decreases with the increase in temperature that leads to an increase in cathodic reaction rate [18]. The values of bc and ba change with the increase in acid concentration and also with the increase in temperature, which indicates the influence of acid concentration and temperature on the kinetics of both hydrogen evolution and metal dissolution.
Activation energy (Ea) for the corrosion process of the welded maraging steel in sulfuric acid was calculated from the Arrhenius equation (Eq. 8) as in [3, 4]:
E
lnu = B--a
corr RT
(8)
where B is a constant that depends on the metal type, and R is the universal gas constant. The plot of ln (ucorr) versus reciprocal of absolute temperature (1/T) gives a straight line with the slope = -Ea/R, from which the activation energy values for the corrosion process were calculated. The Arrhenius plots for the welded specimen are shown in Fig. 6.
The enthalpy and entropy of activation (AH* & AS*) were calculated from the transition state theory (see Eq. 9 as in [19]):
Fig. 3. Equivalent circuit model used to fit the experimental data for the corrosion of Welded maraging steel in 0.1M H2SO4 solution at 50°C.
Table 4. Impedance parameters for the corrosion of welded maraging steel in different concentrations of H2SO4 at different temperatures
Molarity of H2SO4 (M) Temperature (K) Charge transfer resistance (Q cm2) Double layer capacitance (^F cm-2) Corrosion rate (mm y-1)
0.1 303 20.12 1327.0 25.26
313 14.11 1341.0 38.03
323 10.24 1513.0 78.11
333 6.53 1833.0 119.10
0.5 303 18.73 1320.0 30.81
313 11.41 1358.0 59.29
323 7.90 1396.0 89.09
333 5.39 1635.0 131.41
1.0 303 16.01 679.9 38.02
313 10.34 677.7 62.77
323 7.28 571.0 108.17
333 4.34 1162.0 158.32
1.5 303 15.93 962.2 41.37
313 9.37 902.2 69.61
323 6.94 788.8 124.38
333 4.16 764.1 193.25
2.0 303 13.87 562.0 44.89
313 9.17 482.0 80.05
323 6.48 622.5 143.16
333 4.03 781.6 226.32
Q-
-100-
pj -200 -
3
B)
> -300 -E
BÎ-400 --500-600 -
1 I M......1 I I IIIIII-1 I I NIM!-1 I I HUM-1 .....Ill-
¡E-3 0.01 0.! I 10 100
iT m A cm-'
10 ¡2 14 16
Z', ohtn cm2
Fig. 4. Potentiodynamic polarization curves for the corrosion of Fig. 5. Nyquist plots for the corrosion of welded maraging steel welded maraging steel in 1M H2SO4 at different temperatures. in 1M H2SO4 at different temperatures.
0.00300 ' 0.00310 ' 0.00320 0.00330
r-1
1/T, K-
-0,2
^ -0.8;
E
£
^ -1.4 H
S
p.
-2.0-
-2.6
0.00300 ' 0.00310 ' 0.00320 ' 0.00330 1/T, K-1
Fig. 6. Arrhenius plots for the corrosion of welded maraging steel Fig. 7. Plots of ln(uco,./T)vs1/T for the corrosion of welded ma-
in H2SO4
raging steel in H2SO4
¡000 900 »00 TOO 600 51)0
o
■loo 300 21)0 100 0
0(11
0.00
xoo
Fig. 8. SEM images of (a) freshly polished surface, (b) corroded surface.
001
z
■3 i3
'J1* . ,-J o y ; ^ t X
! S U £ ! XU
I Li. 1 1
i 1 1 1 ■ 1 1
4 .on
6.00
ti.00
ksV
Fig. 9a. EDX spectra of freshly polished surface.
RT AR# -u =-e R e
corr Nh
AH ' RT
¡0.00
(9)
loon wo koo 700 M>0 500 400 .100 2ÛD 100 0
2.00
_J
o
0 -J
i—i o
es
Qj u_ *>* H' IZ a
:2 in 1 J. a lO Ja Ii
fë t £ ü. - U z
Jill! A1 j T 1 * 1 Ii 1
4.00
(1.01.1
IteV
Fig. 9b. EDX spectra of corroded surface.
10.00
Table 5. Activation parameters for the corrosion of welded maraging steel in sulfuric acid
where h is Planck's constant, N is Avogadro's number and R is the ideal gas constant. A plot of ln(ucorr/T) versus 1/T gives a straight line with the slope = -AH#/R and intercept = ln(R/Nh) + AS*/R. The plots of ln(vcorr/T) versus 1/T for the corrosion of the welded maraging steel in different concentrations of sulfuric acid are depicted in Fig. 7.
Molarity of H2SO4 (M) Energy of activation (kJ mol-1) Enthalpy of activation (kJ mol-1) Entropy of activation (J K-1 mol-1)
0.1 43 46 -21
0.5 37 40 -37
1.0 38 39 -34
1.5 41 43 -24
2.0 42 45 -18
The activation parameters calculated are listed in Table 5. The activation energy values indicate that the corrosion of the alloy is controlled by the surface reaction, since the values of the activation energy for the corrosion process exceed 20 kJ mol-1 [20]. The entropy of activation is negative. This implies that the activated complex in the rate-determining step represents association rather than dissociation, indicating that a decrease in randomness takes place when going from reactants to the activated complex [17].
SEM and EDX examinations of the e/ectrode surface
The SEM images were recorded to establish the interaction of the acid solution with the metal surface. The SEM image of a freshly polished surface of the welded maraging steel sample (Fig. 8a) shows the un-corroded surface with few scratches due to polishing. Fig. 8b shows the SEM image of the welded maraging steel surface after being immersed for 3 h in 2.0 M H2SO4. The SEM images demonstrate that the specimen without being immersed in the acid solutions is in a better condition having a smooth surface, while the metal surface immersed in 2.0 M H2SO4 is deteriorated due to the acid action. The corroded surface shows detachment of particles from the surface.
EDX survey spectra were used to determine the surface composition of the specimen before and after the exposure to the acid solution. Fig. 9a reveals the fresh surface of the specimen with an intense peak of Fe. The spectrum in Fig. 9b shows that the Fe peaks are considerably suppressed relative to the fresh specimen; and new peaks have appeared for oxygen and sulfur. The suppression of the Fe lines indicates that the specimen has undergone corrosion in the presence of sulfuric acid. The presence of sulfur and oxygen peaks indicate the presence of sulfate ion on the surface the corrosive medium and this could be due to the formation of FeSO4 during corrosion.
CONCLUSIONS
From the above results and discussion, the main conclusions can be summarized in the following points:
1. The corrosion rate of the welded maraging steel specimen in sulfuric acid medium is quite high.
2. The corrosion rate is influenced by temperature and concentration of sulfuric acid medium. The corrosion rate of the specimen under investigation increases with the increase in solution temperature and in the concentration of sulfuric acid.
3. The corrosion kinetics follows the Arrhenius law.
4. The results obtained from the Tafel polarization
curves and electrochemical impedance spectroscopies
are in good agreement.
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Received 28.05.12 Accepted 27.06.12
Реферат
Коррозийное поведение сварной мартенситностаре-ющей стали в растворах серной кислоты было изучено при различных концентрациях кислоты и температурах раствора с помощью таких электрохимических методов, как экстраполяция Тафеля и электрохимический импеданс спектроскопии. Исследование показало, что скорость коррозии сварной мартенситностареющей стали возрастает с увеличением температуры и концентрации серной кислоты в среде. Были рассчитаны такие термодинамические параметры, как активирующая энергия, энтальпия активации и энтропия активации для коррозионных процессов. Результаты, полученные при применении обоих методов, находятся в хорошем согласии. Морфология и состав поверхности корродированных образцов были изучены с помощью сканирующей электронной микроскопии (SEM) и энергодисперсионным рентгеновским анализом (EDX).
Ключевые слова: мартенситностареющая сталь, EIS, поляризация, SEM, EDX.
