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Mathematical model for pitting potential of Fe-16% chromium steel
~ )
Corrosion Science, Vol. 36, No. 9, pp. 1569-1574, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 11010-938X/94 $7.00 +11.00
Pergamon
0010-938X(94)00064-6
M A T H E M A T I C A L M O D E L F O R P I T T I N G P O T E N T I A L OF Fe-16% C H R O M I U M S T E E L M. E R G U N
and M. B A L B A S I
Faculty of Engineering and Architecture, Gazi University, Ankara, Turkey Abstract--The effects of chloride concentration, temperature and pH value on the pitting potential, Ep, of Fe-I 6 Cr steel in aqueous solutions were studied by a potentiodynamic method. It was observed that the pitting potential depends linearly on the logarithm of the chloride ion concentration, decreases as the temperature increases and increases linearly with increase in pH. By taking the individual effects of these variables into consideration a mathematical model that correlates the pitting potential with simultaneous variations of CI concentration, temperature and pH was offered. Model parameters were found by the use of experimental data. The resultant expression is: Ep = 1622 - 5.70T- 0.004T- 0.488Tlog [C1 ] where E v is expressed in mV, the temperature T in degrees K and the chloride concentration [C1-] in tool I INTRODUCTION Despite extensive technical and scientific w o r k d o n e up to now, localized corrosion in stainless steels is one of the principal reasons for their failure in industrial plants. M e a s u r e m e n t s of pitting potential provide a rapid m e t h o d for predicting the t e n d e n c y of pitting corrosion in chloride containing e n v i r o n m e n t s and serve as a p a r a m e t e r for pitting t e n d e n c y which m a y be quantitatively used in the d e v e l o p m e n t of pit resistant alloys. A n essential condition for pitting is the presence in the solution of one of the specific anions, particularly C I - , B r - or I - . T h e formation of pits is o b s e r v e d most often in the presence of C I - ions. This is due not only to their greater aggressiveness, but also to their wider distribution in nature, that is, to the greater chance of their being present in aggressive media, including m a n y technically e m p l o y e d solutions. In the studies I-9 dealing with the effect of C I - concentration of pitting potential, it has been f o u n d that the pitting potential is logarithmic function of CI concentration. Increasing C I - concentrations shift this potential to m o r e active values. Studies of the effect of t e m p e r a t u r e on Ep of different stainless steels have been c o n d u c t e d m o r e frequently. 7"s'I°'ll As a rule, Ep decreases with increasing t e m p e r a ture. T h e most p r o n o u n c e d drop in Ep occurs within the range of r o o m t e m p e r a t u r e to - 1 0 0 or 160°C. A further t e m p e r a t u r e increase causes relatively small changes in Ep. Some authors 7'11-14 have studied the effect o f p H on pitting potential. But there is no general a g r e e m e n t concerning the p H effect on pitting potential. As it is seen from previous works, authors have studied the effects of chloride c o n c e n t r a t i o n , t e m p e r a t u r e and p H on pitting potentials independently. But it is i m p o r t a n t to k n o w the value of pitting potentials as a function of t e m p e r a t u r e , p H Manuscript received 27 January 1994. 1569
1570
M. Ergun and M. Balbasi
a n d c o n c e n t r a t i o n of chloride ion for c o n t r o l l i n g pitting corrosion. T h e m a i n objective of the p r e s e n t w o r k is the systematic m e a s u r e m e n t s of the effect of C1c o n c e n t r a t i o n , t e m p e r a t u r e a n d p H of the s o l u t i o n o n p i t t i n g p o t e n t i a l a n d to o b t a i n a m a t h e m a t i c a l m o d e l c o r r e l a t i n g the pitting p o t e n t i a l of a F e - 1 6 Cr stainless steel with these t h r e e process variables.
EXPERIMENTAL
METHOD
Working electrodes were prepared from a stainless steel bar of composition: C, 0.75; Mo, 1.44; Cr, 15.96; V, 0.14; Si, 2.64; Mn, 0.63; W, 0.97; S, 0.002; Fe, balance. The rods were cut into thin sections, 20 mm in.length. Copper wires were threaded through the back of the specimens after which the wire was enclosed in a glass sleeve and the specimen was completely embedded in an epoxy resin cured at 70°C. Boundaries between metal surface and epoxy resin were carefully shielded with a resin paint. The exposed surface area was a circle of about 1 cm2. Prior to each experiment specimens were ground with 3/0 emery paper and then polished with alumina. They were degreased with acetone and washed in water. A conventional three electrode system using platinum counter electrode and saturated calomel reference electrode (SCE) were employed. All potentials are referred to the SCE. The oxygen content of the solutions was limited to that from natural aeration. A heating magnetic stirrer was used for using agitation and controlling the temperature with a thermistor probe in conjunction with it. NaOH was used in adjusting the pH of the solutions prepared with distilled water and reagent grade chemicals. Each experiment was done in duplicate or triplicate if necessary. Potentiodynamic polarization tests were conducted using a scan rate of 1 mV s- ~commencing at the free corrosion potential after the samples had been immersed for 30 min in the test solutions. The pitting potentials were determined by measuring anodic polarization curves at this fixed scan rate and noting the potential at which the current exhibits a certain increase. Anodic potentiostatic polarization measurements were obtained using an EG&G Model 362 Scanning Potentiostat. A Servagor 120 X-Y recorder was used for the simultaneous measurement of potential and current.
EXPERIMENTAL RESULTS AND DISCUSSION
The effect of chloride ion concentration on pitting potential T h e effect of the chloride ion c o n c e n t r a t i o n o n the pitting p o t e n t i a l was s t u d i e d in p H 9 NaC1 solutions. T h e m e a s u r e d values are r e p o r t e d in T a b l e 1. It was g e n e r a l l y o b s e r v e d that pitting p o t e n t i a l of a m e t a l follows r e l a t i o n of the following type: 1-9 Ep = a + b log [ C I - ] , w h e r e a a n d b are c o n s t a n t s a n d [C1-] is the chloride ion c o n c e n t r a t i o n . T h e results of T a b l e 1 also exhibit the similar d e p e n d e n c y with a b coefficient of - 1 2 2 . 5 m V : Ep = - 6 9 . 7 - 122.5 log [ C I - ] , w h e r e Ep is expressed in m V , [C1-] is in mol 1-1. I n a p r e v i o u s p u b l i c a t i o n , A I S I 316L 1 y i e l d e d a b coefficient of - 0 . 0 4 4 V (at Table 1. Pitting potentials in solutions at pH 9.0 and 25°C Test No.
[CI ] (mol 1-1)
Ep (mV)
1 2 3 4
0.00513 0.0513 0.2565 0.513
+185 +120 +5 -60
Mathematical model for pitting potential of Fe-16Cr steel
1571
90°C), and a coefficient of - 0 . 1 8 3 V for a 12% chromium stainless steel 2. The value of the b depends on many factors affecting the same metal. For iron, b was found to be - 0 . 1 3 V in borate buffer ( p H = 8), 3 - 0 . 0 8 V in phthalate buffer ( p H = 5) and - 0 . 3 1 9 in 0.002 N C r O 2- containing NaCI solutions. 4 In our previous work 5 with carbon steel, the slope changed between - 0 . 1 2 and - 0 . 1 9 V depending upon the inhibitor type in the NaCI solutions. For 304 SS, Ep is shifted in the noble direction by 70 m V for each tenfold decrease in chloride ion concentration. 6 Leckie and Uhlig 7 reported a shift of 90 m V for the same change in chloride ion concentration. O t h e r reported work 8 with 304 SS found that both a and b are t e m p e r a t u r e dependent. In the 20-80°C range, b was - 0 . 1 5 V while at 150 and 200°C, it was - 0 . 1 0 V. A much steeper slope was obtained for Mo containing steel type 304L 9 at high t e m p e r a t u r e (70°C) than the slope summarized above for iron and stainless steels.
The effect of temperature on pitting potential In determining the dependence of pitting potential on temperature for fixed values of NaCI concentration and p H , temperature was changed between 25 and 65°C. From the data given in Table 2, the following equations were obtained by linear regression with regression constants of 0.985, 0.978 and 0.975, respectively: Ep = 63.4 - 4.91T (K), (mV) (0.513 M NaC1, p H = 7.0) Ep = 49.6 - 4 . 1 9 T ( K ) , (mV) (0.513 M NaC1, p H = 8.0) Ep = 54.3 - 5.14T (K), (mV) (0.513 M NaC1, p H = 9.0). In general, Ep decreases with increase steels, although the particular dependence steel. According to the data for 18Cr-8Ni t e m p e r a t u r e influence on pitting potential
in t e m p e r a t u r e for all types of stainless on temperature depends on the grade of steel 7 in 0.1 N NaCI, there is a greater below 25 and above 70°C. In the range
Table 2. Ep values measured at various pH in 0.513 M NaCI solution as a function of temperature Test No.
pH
T (°C)
Ep (mY)
5 6 7 8
7 7 7 7
25 45 55 65
-50 -180 -200 -250
9 10 11 12
8 8 8 8
25 45 55 65
-85 -145 -220 -280
13 14 15 16
9 9 9 9
25 45 55 65
-60 -205 -230 -245
1572
M. Ergun and M. Balbasi
of 30-70°C a linear relation was observed between temperature and Ep. In other work 5'1° with AIS1304 stainless steel, a shift of Ep in the negative direction occurs as temperature increases from 20 to 80°C. At higher temperatures, relatively smaller shifts of Ep in the negative direction were observed. For type 430 stainless steel 11 in neutral 3% NaC1, it was found that the increase of temperature by 10°C shifts the Ep in the active direction by about 30 mV. In this work this value changes between 30 and 50 mV. This difference must result from the presence of molybdenum. In the literature, 9A2 it has been reported that protective efficiency of molybdenum alloyed in steels decreases with an increase in temperature. Consequently steels containing high molybdenum content are more likely to experience pitting attack when the service temperature is high. The decrease in pitting potential as a result of increase in temperature indicates that the resistance to pitting corrosion of these alloys decreases with increasing temperature. Two possibilities may be taken into consideration. First, the porosity of the film increases with temperature are often assumed. Second, the passive film undergoes an intrinsic modification of its chemical composition and/or physical structure.
The effect of pH on pitting potential Table 3 shows the pH effect on Epin 0.513 M NaC1 solution at 25°C in the pH range of 7-10. By linear regression the following relation was found between Ep and pH: Ep = 101 + 4pH (mV). There has been only a small number of studies devoted to the effect of pH on pitting potential and also there is no general agreement concerning the pH effect on pitting potential. Leckie and Uhlig 7 stated that for 18 Cr-8 Ni stainless steel Ep is not affected appreciably in the acid 0.1 N NaCI at 25°C for pH values ranging from i to 7. In alkaline solutions of pH 7-10, however, Ep becomes much more noble corresponding to increased resistance to pitting. It was reported that Ep for type 430 and 304 stainless steels 11 in the range of 1.6-12.7 changes less than 10 mV per unit of pH that is in agreement with the present work in which change in Ep per unit of pH was found to be 4 mV. For 18Cr-8Ni-Ti, 13 Cr and 17 Cr stainless steels in 3% NaC1 solution, Ep was constant at pH from 4 to 9.11 In another study, 1° AIS1304 SS in 3% NaC1 containing chromate ion resulted in a change of 50 mV/pH. A recent study on AISI 316 SS 13using a spent bleach solution in which pH was changed from 2.3 to 6.5, pitting potential was found to change by an amount equal to 26 mV/pH. Alvarez and Galvele 14worked with pure iron in the 1.0 M NaC1 solutions with pH values ranging from 7.0 to 12.0. The pitting
Table 3. Pittingpotentialsin 0.0513 M NaCI solutions at 25°C Test No.
pH
Ep (mV)
17 18 19 20
7 8 9 10
+110 +115 + 120 + 140
Mathematical model for pitting potential of Fe-16Cr steel
1573
potential was found to be constant and independent of p H values between 7 and 10. Between p H 10 and 12, on the other hand, the pitting potential was a function of pH and the higher the pH, the higher the pitting potential measured. Model for pitting potential By taking individual effects of variables solution pH, solution temperature and chloride ion concentration, into consideration four different correlation models which are thought to represent the effects of variables together were offered. (Model 1)
Ep = a + b T + c T p H + d T l o g [C1-]
(Model 2)
Ep=a+bT+cTpH+dpHlog[Cl-]
(Model 3)
Ep = a + b T + c T log[Cl-] + d pH log [C1-]
(Model 4)
Ep=a+bpH+cTpH+dpHlog[Cl-]
Model parameters a, b, c and d values, were determined by the use of the experimental data given in Tables 1, 2 and 3 and with a computational program. The values of these constants are given in Table 4. Table 4. Parameters for different pitting potential models Model No.
a
b
c
1 2 3 4
1622 1640 272 -111
5.70 -5529 -0.174 145
-4.43 10 3 -2.34 10 2 1.721 -0.646
d 0.488 _5.4610-2
80.8 -0.161
The calculated Ep values from these models were compared with the experimental values. It was observed that Model 1 satisfies these data best for all four relations. So the resultant expression is given as follows: Ep = 1622 - 5 . 7 0 T - 4.43 × 1 0 - 3 T p H - 4.88 x 1 0 - 1 Tlog [CI-]. Table 5 represents the pitting potential values calculated from the above equation and the experimental Ep values. This equation is valid for temperatures between 25 and 65°C, for alkaline pH values ranging from 7 to 10, and the chloride ion concentrations range considered in this study. CONCLUSION By the use of the model developed, the pitting potential values and also pitting potential variations depending upon the changes in system variables, pH, temperature and C I - ion concentration, can be easily calculated for 16% chromium stainless steel. Knowledge of the pitting potential value makes it possible to control material damage by pitting.
1574
M. Ergun and M. Balbasi
Table 5.
Comparison of experimental and calculated Ep values
Test No.
Ep (Experimental)
E o (Calculated)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2O
+185 +120 +5 -60 -50 -180 -200 -250 -85 -145 -220 -280 -60 -205 -230 -265 +110 +115 +120 +140
+243 +98 -2 -48 -45 -157 -213 -269 -46 -158 -214 -270 -48 -160 -216 -272 +101 +108 +140 +130
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
T. Suzuki and Y. Kimatura, Corrosion 28, 1 (1972). A. Atrens, Corrosion 39,483 (1983). H . H . Strehlow and B. Titze, Corros. Sci. 17,461 (1977). E. McCafferty, J. electrochern. Soc. 137, 3791 (1990). M. Ergun and A. Y. Turan, Corros. Sci. 32, 1137 (1991). H. P. Leckie, J. electrochem. Soc. 117, 1152 (1970). H. P. Leckie and H. H. Uhlig, J. electrochem. Soc. 113, 1262 (1966). J. H. Wang, C. C. Su and Z. Szklarska-Smialowska, Corrosion 44,733 (1978). G. Ruijini, S. C. Srivastava and M. B. Ives, Corrosion 45,874 (1988). M. Ergun and M. Balbasi, Korozyon 3, 101 (1991). Z. Szklarska-Smialowska, Corrosion 27,223 (1971). J. A. M. Kolotyrkin, corrosion 19, 261t (1963). G. Matamalar, Corrosion 43, 97 (1987). M. G. Alvarez and J. R. Galvele, Corros. Sci. 24, 27 (1984).
Corrosion Science, Vol. 36, No. 9, pp. 1569-1574, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 11010-938X/94 $7.00 +11.00
Pergamon
0010-938X(94)00064-6
M A T H E M A T I C A L M O D E L F O R P I T T I N G P O T E N T I A L OF Fe-16% C H R O M I U M S T E E L M. E R G U N
and M. B A L B A S I
Faculty of Engineering and Architecture, Gazi University, Ankara, Turkey Abstract--The effects of chloride concentration, temperature and pH value on the pitting potential, Ep, of Fe-I 6 Cr steel in aqueous solutions were studied by a potentiodynamic method. It was observed that the pitting potential depends linearly on the logarithm of the chloride ion concentration, decreases as the temperature increases and increases linearly with increase in pH. By taking the individual effects of these variables into consideration a mathematical model that correlates the pitting potential with simultaneous variations of CI concentration, temperature and pH was offered. Model parameters were found by the use of experimental data. The resultant expression is: Ep = 1622 - 5.70T- 0.004T- 0.488Tlog [C1 ] where E v is expressed in mV, the temperature T in degrees K and the chloride concentration [C1-] in tool I INTRODUCTION Despite extensive technical and scientific w o r k d o n e up to now, localized corrosion in stainless steels is one of the principal reasons for their failure in industrial plants. M e a s u r e m e n t s of pitting potential provide a rapid m e t h o d for predicting the t e n d e n c y of pitting corrosion in chloride containing e n v i r o n m e n t s and serve as a p a r a m e t e r for pitting t e n d e n c y which m a y be quantitatively used in the d e v e l o p m e n t of pit resistant alloys. A n essential condition for pitting is the presence in the solution of one of the specific anions, particularly C I - , B r - or I - . T h e formation of pits is o b s e r v e d most often in the presence of C I - ions. This is due not only to their greater aggressiveness, but also to their wider distribution in nature, that is, to the greater chance of their being present in aggressive media, including m a n y technically e m p l o y e d solutions. In the studies I-9 dealing with the effect of C I - concentration of pitting potential, it has been f o u n d that the pitting potential is logarithmic function of CI concentration. Increasing C I - concentrations shift this potential to m o r e active values. Studies of the effect of t e m p e r a t u r e on Ep of different stainless steels have been c o n d u c t e d m o r e frequently. 7"s'I°'ll As a rule, Ep decreases with increasing t e m p e r a ture. T h e most p r o n o u n c e d drop in Ep occurs within the range of r o o m t e m p e r a t u r e to - 1 0 0 or 160°C. A further t e m p e r a t u r e increase causes relatively small changes in Ep. Some authors 7'11-14 have studied the effect o f p H on pitting potential. But there is no general a g r e e m e n t concerning the p H effect on pitting potential. As it is seen from previous works, authors have studied the effects of chloride c o n c e n t r a t i o n , t e m p e r a t u r e and p H on pitting potentials independently. But it is i m p o r t a n t to k n o w the value of pitting potentials as a function of t e m p e r a t u r e , p H Manuscript received 27 January 1994. 1569
1570
M. Ergun and M. Balbasi
a n d c o n c e n t r a t i o n of chloride ion for c o n t r o l l i n g pitting corrosion. T h e m a i n objective of the p r e s e n t w o r k is the systematic m e a s u r e m e n t s of the effect of C1c o n c e n t r a t i o n , t e m p e r a t u r e a n d p H of the s o l u t i o n o n p i t t i n g p o t e n t i a l a n d to o b t a i n a m a t h e m a t i c a l m o d e l c o r r e l a t i n g the pitting p o t e n t i a l of a F e - 1 6 Cr stainless steel with these t h r e e process variables.
EXPERIMENTAL
METHOD
Working electrodes were prepared from a stainless steel bar of composition: C, 0.75; Mo, 1.44; Cr, 15.96; V, 0.14; Si, 2.64; Mn, 0.63; W, 0.97; S, 0.002; Fe, balance. The rods were cut into thin sections, 20 mm in.length. Copper wires were threaded through the back of the specimens after which the wire was enclosed in a glass sleeve and the specimen was completely embedded in an epoxy resin cured at 70°C. Boundaries between metal surface and epoxy resin were carefully shielded with a resin paint. The exposed surface area was a circle of about 1 cm2. Prior to each experiment specimens were ground with 3/0 emery paper and then polished with alumina. They were degreased with acetone and washed in water. A conventional three electrode system using platinum counter electrode and saturated calomel reference electrode (SCE) were employed. All potentials are referred to the SCE. The oxygen content of the solutions was limited to that from natural aeration. A heating magnetic stirrer was used for using agitation and controlling the temperature with a thermistor probe in conjunction with it. NaOH was used in adjusting the pH of the solutions prepared with distilled water and reagent grade chemicals. Each experiment was done in duplicate or triplicate if necessary. Potentiodynamic polarization tests were conducted using a scan rate of 1 mV s- ~commencing at the free corrosion potential after the samples had been immersed for 30 min in the test solutions. The pitting potentials were determined by measuring anodic polarization curves at this fixed scan rate and noting the potential at which the current exhibits a certain increase. Anodic potentiostatic polarization measurements were obtained using an EG&G Model 362 Scanning Potentiostat. A Servagor 120 X-Y recorder was used for the simultaneous measurement of potential and current.
EXPERIMENTAL RESULTS AND DISCUSSION
The effect of chloride ion concentration on pitting potential T h e effect of the chloride ion c o n c e n t r a t i o n o n the pitting p o t e n t i a l was s t u d i e d in p H 9 NaC1 solutions. T h e m e a s u r e d values are r e p o r t e d in T a b l e 1. It was g e n e r a l l y o b s e r v e d that pitting p o t e n t i a l of a m e t a l follows r e l a t i o n of the following type: 1-9 Ep = a + b log [ C I - ] , w h e r e a a n d b are c o n s t a n t s a n d [C1-] is the chloride ion c o n c e n t r a t i o n . T h e results of T a b l e 1 also exhibit the similar d e p e n d e n c y with a b coefficient of - 1 2 2 . 5 m V : Ep = - 6 9 . 7 - 122.5 log [ C I - ] , w h e r e Ep is expressed in m V , [C1-] is in mol 1-1. I n a p r e v i o u s p u b l i c a t i o n , A I S I 316L 1 y i e l d e d a b coefficient of - 0 . 0 4 4 V (at Table 1. Pitting potentials in solutions at pH 9.0 and 25°C Test No.
[CI ] (mol 1-1)
Ep (mV)
1 2 3 4
0.00513 0.0513 0.2565 0.513
+185 +120 +5 -60
Mathematical model for pitting potential of Fe-16Cr steel
1571
90°C), and a coefficient of - 0 . 1 8 3 V for a 12% chromium stainless steel 2. The value of the b depends on many factors affecting the same metal. For iron, b was found to be - 0 . 1 3 V in borate buffer ( p H = 8), 3 - 0 . 0 8 V in phthalate buffer ( p H = 5) and - 0 . 3 1 9 in 0.002 N C r O 2- containing NaCI solutions. 4 In our previous work 5 with carbon steel, the slope changed between - 0 . 1 2 and - 0 . 1 9 V depending upon the inhibitor type in the NaCI solutions. For 304 SS, Ep is shifted in the noble direction by 70 m V for each tenfold decrease in chloride ion concentration. 6 Leckie and Uhlig 7 reported a shift of 90 m V for the same change in chloride ion concentration. O t h e r reported work 8 with 304 SS found that both a and b are t e m p e r a t u r e dependent. In the 20-80°C range, b was - 0 . 1 5 V while at 150 and 200°C, it was - 0 . 1 0 V. A much steeper slope was obtained for Mo containing steel type 304L 9 at high t e m p e r a t u r e (70°C) than the slope summarized above for iron and stainless steels.
The effect of temperature on pitting potential In determining the dependence of pitting potential on temperature for fixed values of NaCI concentration and p H , temperature was changed between 25 and 65°C. From the data given in Table 2, the following equations were obtained by linear regression with regression constants of 0.985, 0.978 and 0.975, respectively: Ep = 63.4 - 4.91T (K), (mV) (0.513 M NaC1, p H = 7.0) Ep = 49.6 - 4 . 1 9 T ( K ) , (mV) (0.513 M NaC1, p H = 8.0) Ep = 54.3 - 5.14T (K), (mV) (0.513 M NaC1, p H = 9.0). In general, Ep decreases with increase steels, although the particular dependence steel. According to the data for 18Cr-8Ni t e m p e r a t u r e influence on pitting potential
in t e m p e r a t u r e for all types of stainless on temperature depends on the grade of steel 7 in 0.1 N NaCI, there is a greater below 25 and above 70°C. In the range
Table 2. Ep values measured at various pH in 0.513 M NaCI solution as a function of temperature Test No.
pH
T (°C)
Ep (mY)
5 6 7 8
7 7 7 7
25 45 55 65
-50 -180 -200 -250
9 10 11 12
8 8 8 8
25 45 55 65
-85 -145 -220 -280
13 14 15 16
9 9 9 9
25 45 55 65
-60 -205 -230 -245
1572
M. Ergun and M. Balbasi
of 30-70°C a linear relation was observed between temperature and Ep. In other work 5'1° with AIS1304 stainless steel, a shift of Ep in the negative direction occurs as temperature increases from 20 to 80°C. At higher temperatures, relatively smaller shifts of Ep in the negative direction were observed. For type 430 stainless steel 11 in neutral 3% NaC1, it was found that the increase of temperature by 10°C shifts the Ep in the active direction by about 30 mV. In this work this value changes between 30 and 50 mV. This difference must result from the presence of molybdenum. In the literature, 9A2 it has been reported that protective efficiency of molybdenum alloyed in steels decreases with an increase in temperature. Consequently steels containing high molybdenum content are more likely to experience pitting attack when the service temperature is high. The decrease in pitting potential as a result of increase in temperature indicates that the resistance to pitting corrosion of these alloys decreases with increasing temperature. Two possibilities may be taken into consideration. First, the porosity of the film increases with temperature are often assumed. Second, the passive film undergoes an intrinsic modification of its chemical composition and/or physical structure.
The effect of pH on pitting potential Table 3 shows the pH effect on Epin 0.513 M NaC1 solution at 25°C in the pH range of 7-10. By linear regression the following relation was found between Ep and pH: Ep = 101 + 4pH (mV). There has been only a small number of studies devoted to the effect of pH on pitting potential and also there is no general agreement concerning the pH effect on pitting potential. Leckie and Uhlig 7 stated that for 18 Cr-8 Ni stainless steel Ep is not affected appreciably in the acid 0.1 N NaCI at 25°C for pH values ranging from i to 7. In alkaline solutions of pH 7-10, however, Ep becomes much more noble corresponding to increased resistance to pitting. It was reported that Ep for type 430 and 304 stainless steels 11 in the range of 1.6-12.7 changes less than 10 mV per unit of pH that is in agreement with the present work in which change in Ep per unit of pH was found to be 4 mV. For 18Cr-8Ni-Ti, 13 Cr and 17 Cr stainless steels in 3% NaC1 solution, Ep was constant at pH from 4 to 9.11 In another study, 1° AIS1304 SS in 3% NaC1 containing chromate ion resulted in a change of 50 mV/pH. A recent study on AISI 316 SS 13using a spent bleach solution in which pH was changed from 2.3 to 6.5, pitting potential was found to change by an amount equal to 26 mV/pH. Alvarez and Galvele 14worked with pure iron in the 1.0 M NaC1 solutions with pH values ranging from 7.0 to 12.0. The pitting
Table 3. Pittingpotentialsin 0.0513 M NaCI solutions at 25°C Test No.
pH
Ep (mV)
17 18 19 20
7 8 9 10
+110 +115 + 120 + 140
Mathematical model for pitting potential of Fe-16Cr steel
1573
potential was found to be constant and independent of p H values between 7 and 10. Between p H 10 and 12, on the other hand, the pitting potential was a function of pH and the higher the pH, the higher the pitting potential measured. Model for pitting potential By taking individual effects of variables solution pH, solution temperature and chloride ion concentration, into consideration four different correlation models which are thought to represent the effects of variables together were offered. (Model 1)
Ep = a + b T + c T p H + d T l o g [C1-]
(Model 2)
Ep=a+bT+cTpH+dpHlog[Cl-]
(Model 3)
Ep = a + b T + c T log[Cl-] + d pH log [C1-]
(Model 4)
Ep=a+bpH+cTpH+dpHlog[Cl-]
Model parameters a, b, c and d values, were determined by the use of the experimental data given in Tables 1, 2 and 3 and with a computational program. The values of these constants are given in Table 4. Table 4. Parameters for different pitting potential models Model No.
a
b
c
1 2 3 4
1622 1640 272 -111
5.70 -5529 -0.174 145
-4.43 10 3 -2.34 10 2 1.721 -0.646
d 0.488 _5.4610-2
80.8 -0.161
The calculated Ep values from these models were compared with the experimental values. It was observed that Model 1 satisfies these data best for all four relations. So the resultant expression is given as follows: Ep = 1622 - 5 . 7 0 T - 4.43 × 1 0 - 3 T p H - 4.88 x 1 0 - 1 Tlog [CI-]. Table 5 represents the pitting potential values calculated from the above equation and the experimental Ep values. This equation is valid for temperatures between 25 and 65°C, for alkaline pH values ranging from 7 to 10, and the chloride ion concentrations range considered in this study. CONCLUSION By the use of the model developed, the pitting potential values and also pitting potential variations depending upon the changes in system variables, pH, temperature and C I - ion concentration, can be easily calculated for 16% chromium stainless steel. Knowledge of the pitting potential value makes it possible to control material damage by pitting.
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M. Ergun and M. Balbasi
Table 5.
Comparison of experimental and calculated Ep values
Test No.
Ep (Experimental)
E o (Calculated)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2O
+185 +120 +5 -60 -50 -180 -200 -250 -85 -145 -220 -280 -60 -205 -230 -265 +110 +115 +120 +140
+243 +98 -2 -48 -45 -157 -213 -269 -46 -158 -214 -270 -48 -160 -216 -272 +101 +108 +140 +130
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