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Evaluation of electrochemical properties for CrMoV steel in acidic solutions to determine embrittlement—I. Open circuit potential measurements in acidic aqueous solutions of various organic compounds
EVALUATION OF ELECTROCHEMICAL PROPERTIES FOR CrMoV STEEL IN ACIDIC SOLUTIONS TO DETERMINE EMBRITTLEMENT-I. OPEN CIRCUIT POTENTIAL MEASUREMENTS IN ACIDIC AQUEOUS SOLUTIONS OF VARIOUS ORGANIC COMPOUNDS NORIKO SATO and MASAYUKI SUZUKI Toshiba Research and Development center, Toshiba Corporation, 1. Komukai Toshiba
Saiwai-ku,
15 December 1987; in revised form 3 March 1988)
Abstract-The embrittlement of CrMoV steel used for a steam turbine rotor, which had been in service over 139,ooO h, has been investigated by open circuit potential measurements in acidic aqueous solutions of various organic compounds. The open circuit potential shifted positively in nitrophenol and nitrobenaoic acid solutions as the embrittlement level of the CrMoV steel increased, while it shifted negatively in aliphatic and aromatic acids without the nitro group. The possibility for embrittlement evaluation was also demonstrated by obtaining the rate of change in open circuit potential when solution stirring was stopped. The mechanism for the open circuit potential shift is discussed based on polarization measurements.
INTRODUCTION Low alloy steels have been used extensively in power plants. However, long time use of these alloys in the range of 400-500°C leads to degradation of their mechanical properties. One typical example of such material degradation is the temper embrittlement of CrMoV steels due to high temperature operation. This phenomenon has been thought to be caused by segregation of impurities such as phosphorus to the grain boundaries[ l-31. There is considerable current interest in being able to detect embrittlement in these steels and the present paper discusses the work we have undertaken with a view to developing a practical methdd for testing the degree of embrittlement of rotor steels in service. Surface analytical methods have previously been applied to obtain the correlation between impurity segregation at the grain boundaries and the embrittlement of the material[2-51. Electrochemical measurements have also been adopted on account of their high sensitivity in detecting selective dissolution of grain boundaries in suitable etching solutions. Kiippcr et a/.[63 measured polarization curves for FeP alloys in a calcium nitrate solution and found a correlation between thecorrosion current density and phosphorus content at the grain boundaries. Bruemmer[7] measured polarization curves for NiCrMoV steels of different phosphorus levels in saturated picric acid solution. He found the change in peak active current density due to the difference in phosphorus level. Shoji and Takahashi[S] found a correlation between passive current density and embrittlement level of CrMoV steel by polarization measurements in a special picric acid solution. In general, however, polarization measurements encounter difficulties such as poor reproducibility and may give rise to changes in surface condition, especially EA
33:10-I
when the specimen is polarized to the passive region. Selective etching is another method that has often been used to detect temper embrittlement by means of developing grain boundaries[P-13-J. The technique is based on the selective dissolution phenomenon in the low overvoltage area on the surface at the open circuit potential and seems to have good reproducibility. The difficulty with this method lies in quantitative evaluation of the etching involved. In the present study, in order to investigate the effect of temper embrittlement for CrMoV steels on their electrochemical behaviour, the open circuit potential was measured in aqueous solutions containing various kinds of organic compounds. It was found that good sensitivity and reproducibility are obtainable in a correlation between the open circuit potential and embrittlement level for a CrMoV steel when appropriate solutions are used.
EXPERIMENTAL Specimens
and chemicals
The CrMoV steel specimens of various embrittlement levels were obtained from several parts of a steam turbine rotor which was operated for over 139,000 h. The bulk composition of this material is listed in Table 1. Charpy impact tests were carried out on these specimens to determine the fracture appearance transition temperature (FATT) values[ 141. The difference in the FATT values between embrittled and control specimens, AFATT, was used as the measure for the embrittlement level. The AFATT value for the most embrittled sample was 98°C in this study. The control specimen, whose AFATT value is regarded to be 0°C. was sampled from the coupling of the rotor, as usual [S], which was located outside of the turbine casing
1383
N. SATO and M. SUZUKI
1384
Table 1. CrMoV steel chemical composition (%(w/w)) C
Si
Mn
P
S
Ni
Cr
MO
V
Cu
Sn
As
Sb
0.38
0.35
0.80
0.023
0.043
0.45
1.01
1.02
0.23
0.14
< 50 pptn
< 1OOppm
< 20 ppm
and was free from embrittlement. Steam temperatures during operation and AFATT values for the specimens are listed in Table 2. Specimens for the electrochemical measurements were cut from the Charpy impact specimens, degreased with acetone and mechanically polished with 600 mesh Sic grit paper. Then, the specimens were wrapped in parallin film except for the testing area of 8 mm in diameter. Commercially available analytical grade chemicals were used without further purification. Testing solutions were prepared using deionized distilled water. Solution compositions are listed in Table 3. The organic compounds were chosen for their potential ability to etch low alloy steels moderately.
Electrochemical
measurements
The open circuit potential measurements and potentiodynamic measurements were performed in a 200 ml single compartment three electrode glass cell, a saturated calomel electrode (see) and a platinum plate electrode being used as the reference and auxiliary electrodes, respectively. A probe-type cell was also used to examine the solution agitation effect on the open circuit potential. The cell body contains 40 ml of testing solution, which iscontacted with the specimen through the window of 8 mm in diameter. The specimen temperature was kept constant by flowing water fed from a water bath through the specimen holder. Solution agitation was performed with an stirring blade driven by a motor. All measurements were conducted without removing dissolved oxygen from the solution. Temperature was kept constant at 30 f 0.5”C, unless otherwise specified. Potentiodynamic measurements were carried out with a potentiostat (Hokuto Denko Ltd. HA-104) and a potential sweeper (Hokuto Denko Ltd. HB-104). Potential and current data from the potentiostat were converted to digital data by digital multimeters and processed by a personal microcomputer (Hewlett Packard Series 9000).
Table
temperatures 2. Steam Ak-ATT values
steam Temperature (“C) 50-80 (Coupling) 350 420 490 510
and
AFATT 0 50 72 85 98
(“C)
RESULTS
Measurements
on open circuit potential,
E,
Open circuit, potentials were measured for several specimens with different AFATT values in the testing solutions listed in Table 3. In most cases, the open circuit pot’ential reached the steady state value, E,, about 10-20 min after immersion. In some cases, such as when using picric acid solutions, the open circuit potential moved gradually showing a broad peak or a plateau several minutes after immersion. In such cases, the peak or the plateau potential was defined as the open circuit potential, E,. Figure 1 shows the relations between E, and AFATT for the CrMoV steel in picric acid solutions with threedifferent concentrations. Linear correlations were found between both values; ie, E, shifted in the positive direction as the embrittlement level increased for every picric acid concentration. During the measurement, the test piece surface was gradually covered with black and homogeneous corrosion products. It is noticeable that no hydrogen evolution was observed at all in spite of acidic solutions
Table 3. Compositions
Compound Nitrophenols 2,4,6_Trinitrophenol (Pi&c acid) CNitrophenol 2,CDinitrophcnol 2,4,6-Trinitroresorcinol Nitrobenzoic acids 2+Dinitrobenzoic acid (2,4-DNBA)
2,6-DNBA 3,5-DNBA 2-Nitrobenzoic acid Other aromatic acids Salicylic acid 2,6-Dihydroxybenzoic Phthalic acid Aliphatic acids Acetic acid Oxalic acid Citric acid Inorganic acid Perchloric acid Saturated. +200/!MeOH
l
solution.
acid
of various solutions Concentration (mM)
PH
4.36 6.55 13.1 50.0 If 6.50
2.5 2.2 2.0 4.3 3.3 2.2
4.71 6.50 7.07 9.43 16.5 7.07 7.07+ 12.0
2.5 2.3 2.2 2.2 2.0 2.2 2.7 2.2
7.24 10.9 6.49 13.0 6.02
2.7 2.5 2.3 2.0 2.7
6.58 6.5 1 6.55
3.5 2.3 2.7
6.58
2.2
Electrochemical properties of CrMoV steel-I
1385
/
-150 *18
/I-.’ ;,oo~A?
2
,.H
w
0
50
f3' -2oow vi > $
:::LL-
100
and AFATT in picric acid 6.55 mM;-A-4.36 mM.
(pH < 3), where hydrogen evolution is usually seen in iron and steel corrosion. In order to investigate whether or not the above results are characteristic of only picric acid, other nitrophenols were examined. Figure 2 shows the results with 4-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitroresorcinof solutions. Though E, values in these solutions shifted to positive potential with increasing embrittlement level (ie, larger AFATT value), the slopes were far less steep than those for picric acid solutions. Particularly, E, was almost independent oft he embrittlement level in Cnitrophenol, in which the test piece surface was covered with reddish brown corrosion product due to high pH value of the solution (pH 4.3). No signs of hydrogen evolution, ie, bubble formation on the test piece surface, were found in these solutions as in picric acid solutions. Nitrobenzoic acids were also examined in the same manner (Fig. 3). A marked correlation was found in the 7.07 mM 2,4_dinitrobenzoic acid (2,4-DNBA) solution, the slope of E, USAFATT being comparable to that in 6.55 mM picrlc acid solution. The dependence of the
w
-200
I-=-
/_ -250-/B _.e*-
8 w
AFATT. "C
Fig. 1. Relation between E, solutions: -O13.1 mM;-o-
/
50 AFATT,
0
IOC
-2
Fig. 3. Relation between E, and AFATT in several nitroben7.07 mM ZPDNBA; -I7.07 zoic acid solutions: + mM 2,6-DNBA;[email protected] mM 3.SDNBA;-A12.0 mM 2-nitrobenzoic acid. slope on the 2,4-DNBA concentration is shown in Fig. 4. In this figure, the longitudinal axis, AE, means the values between the specimen of difference in E, AFATT = 98°C and that of AFATT = 0°C. As seen from the figure, the maximum AE, value was found for around 7mM of 2,4-DNBA (ca 95 mV). This value was the largest found throughout the present study. In all nitrobenzoic acid solutions examined, no indications of hydrogen evolution were observed. and the sample surface after measurements was covered with grey corrosion product. From the above results, it is interesting to note that the number and the position of nitro group have a and significant effect on the relation between E, AFATT for the CrMoV steel. Further, it is likely that hydroxyl or carboxyl group is not essential to obtain relation, but the the large slope of Em us AFATT coexisting nitro group is. In order to confirm this suggestion, acidic solutions of aromatic compounds having no nitro group were examined in the same way. Figure 5 shows the results with phthalic acid, salicylic acid and 2,6-dihydroxybenzoic acid solutions. In these solutions, open circuit potentials for the CrMoV steel
M r 2 x w
-250 -350
-400
2
60 ii w a 40
o-o-
-0
I 0
50 AFATT.
100
i 20
“C
Fig. 2. Relation between E, and AFATT in several ‘nitrop henol solutions: -Clsaturated 2,Qdinitrophenol; -A50.0 mM 46.50 mM 2,4,6_trinitroresorcinol; -onitrophenol.
0 t 2.4-DNBA.
Fig. 4. AE, dependence
mM
on 2+DNBA
concentration.
1386
N.
SAT0
and M. %JZUKl
s’ AFATT. OC Fig. 5. Relation between E, and AFATT in several aromatic acid solutions: -A - 10.9 mM salicylicacid;-A-7.24 mM salicylic acid; - q - 6.49 mM 2,6-dihydroxybenzoic acid; I13.0 mM 2,6-dihydroxybenzoic acid; + 6.02 mM
ts TIME Fig. 7. Changes in E, with lapsed time before and after solution stirring was stopped in 7.07 mM 2,4-DNBA solution: (A) embrittled CrMoV steel (AFATT = 98°C); (B) unembrittled CrMoV steel (AFATT= 0°C). Stirring was stopper at 2,.
phthalic acid.
were considerably negative, compared with those in aromatic nitroeompound solutions, and shifted in the
negative direction as the steel became more brittle. No clear correlation was found between E, and AFATT. Aliphatic acids and perchloric acid solutions were also examined (Fig. 6). These solutions gave results similar to those for solutions of aromatic compounds having no nitro group. It should be noted that hydrogen evolution was seen in the solutions of these aliphatic and aromatic compounds without nitro group as well as in the perchloric acid solution. Agitation
effect
The above mentioned results suggest that the open circuit potential measurement can be used for a non-
destructive evaluation of embrittlement. However, in all solutions examined, open circuit potential, E,, was sensitive to solution movement. So, to clarify whether or not reproducible E, values are obtainable, solution stirring effect on E, values was investigated using the probe-type cell in the 7.07 mM 2,4-DNBA solution. Figure 7 shows typical changes in E, with time in 7.07 mM 2,4-DNBA solution for both embrittled (line A) and unembrittled (line B) specimens, before and after
solution stirring was stopped. Though E, values for both specimens were kept almost constant shifting to considerably positive potentials during solution stirring, the difference in E, values between the two specimens became so small that sensitive detection of material degradation was thought to be dillicult (Fig. 8). On the other hand, when the solution stirring was stopped, the open circuit potential returned rapidly to the negative potential. It can be seen from the two curves in Fig. 7 that the rate of change in E, immediately after stopping agitation is greatly different for the two samples. So, the rate of change in E,, defined as R,, was evaluated. The result is shown in Fig. 9, where R, was determined by fitting a straight line to the initial decay, as shown by broken lines in Fig. 7. A satisfactory correlation between R, and AFATT was obtained (correlation coefficient = - 0.990). Furthermore, the temperature dependence of R, was examined for four specimens with different embrittlement levels in the same solution (7.07 mM 2,4DNBA). Their AFATT values were 0, SO,72 and 98”C, respectively. Figure 10 illustrates semilogarithmic plots of R, us 1/T in the temperature range of 2040°C. An exceedingly linear relationship was found for each specimen. This implies that an accurate temperature
-500r----w
=: r
2
-u----+-_Q._ -550
--*\a_a_*
a_
-o%o -“_
;: w -600
I
-O--o--o_ 0
I
0
o
50
--a_ o_ 100
I
AFATT, “C Fig. 6. Relation between E, and AFATT in aliphatic acid and perchloric acid solutions: -A6.55 mM citric acid; ~ 3 ~ 6.58 mM acetic acid; -iI6.51 mM oxalic acid; -c-6.58 mM perchloric acid.
AFATT. “C during solution Fig. 8. Relation between E, and AFATT agitation by stirring in 7.07 mM 2,4-DNBA solution: stirring blade; -Aagitation by air bubbling; - 0 agitation by N, bubbling.
Electrochemical
AFATT. YI Fig. 9. Relation between Rr a,nd AFATT in 7.07 mM 2,4DNBA solution: agrtatron by stirring blade; -Aagitation by air bubbling; -Oagitation by N, bubbling.
D-
o-
3-
I
I
/
33
3.2
$xlOs.
1387
properties of CrMoV steel-1
3.4 K-’
Fig. 10. Semilogarithmic plots of R, vs l/T in 7.07 mM 2,4DNBA solution: AFATT: --o0°C: -W50°C; -A72°C: -.98°C.
compensation is possible to determine the embrittlement level from the R, measurement. The effect of agitation caused by bubbling air or N, into the solution wascompared with that caused by the, stirring blade. E, and R, values measured for air or N, bubbling are plotted in Figs 8 and 9, respectively, and are seen to lie on the same curve as for stirring blade. That is, these parameters were not susceptible to the agitation means or the kind of bubbling gas used. It is obvious that oxygen has no effect on the electrochemical reaction for the CrMoV steel in 2,4-DNBA.
Figure 11 shows polarixation curves for specimens with AFATT = 0°C (solid line) and 98°C (broken line). It is noticeable that an increase in the overvoltage can be observed in the anodic branch with the embrittlement progress whereas little difference is seen in the cathodic region (see below - 400 mV). Consequently, it can be concluded that the difference in E, between embrittled and unembrittled materials should be attributed to the change in the anodic dissolution rate of iron. This suggests that the microstructure changes which brought about the steel embrittlement did not affect the cathodic reaction but suppressed the anodic dissolution rate of the steel. As mentioned above, in this solution no hydrogen evolution was observed at all, so, some reactant must be reduced on the CrMoV steel surface, whose reaction is thought to have a limiting current. Aliphatic and aromatic acids having no nitro group gave the opposite relation between open circuit potential and the steel embrittlement level us aromatic nitrocompounds. Besides, hydrogen evolution was observed in the solutions of such compounds. Then, for comparison, polarization curves were measured in 6.55 mM citric acid solution (Fig. 12). In this case, no clear differences were observed between two specimens for both anodic and cathodic branches. Further, these curves were dissimilar to the curves obtained in the 2,4DNBA solution. The solution stirring effect on the polarization behavior for CrMoV steel in 2,4-DNBA solution was also examined. Figure 13 shows the result. In the cathodic branch, a large current enhancement for both embrittled and unembrittled specimens arose from solution stirring and the plateau in the -600 to -950 mV range for the stagnant solution disappeared, which reveals that the cathodic reaction was so fast that it was susceptible to mass transfer. Also, it is noticeable that the anodic active dissolution was essentially independent of solution stirring. This indicates that the anodic steel in this solution is not dissolution of the CrMoV diffusion controlled, but instead, it is dependent on the anodic dissolution overvoltage, which becomes larger with embrittlement progress.
2-
Polarization measurements In order to clarify what caused the changes in E, or R, with the difference in embrittlement level of the CrMoV steel, polarization measurements were performed for two specimens with different embrittlement levels (AFATT = 0 and 98’C) in 7.07 mM 2,4-DNBA solution. The glass cell was used and solution stirring was carried out by a magnetic stirrer.
Potential,
mV
vs.
SCE
Fig. 11. Current us potential curves for embrittled and unembrittled CrMoV steels in 7.07 mM 2,4-DNBA solution: unembrittled steel (AFATT = 0°C); - ~ ~ embrittled steel (AFATT = 98°C).
1388
N.
SATO and
E
-ait
-700
-308
-SEW
Potential,
mV
vs.
SCE
Fig. 12. Current us potential curves for embrittled and unembrittled CrMoV steels in6.55 mMcitricacid solution:embrittled unembrittled steel (AFATT = OOC); steel (AFATT = 98°C).
-IEBB
-EBB
-688
Potent]
-488
-288
al,
mV
E vs.
200
d
SCE
Fig. 13. Current us potential curves for embrittled and unembrittled CrMoV steels in stirred 7.07 mM 2,4-DNBA
solution: -
unembrittled steel (AFATT = 0°C);. embrittled steel (AFATT = 98°C).
..
.
M. SUZUKI
group 1 compound solutions can be used to detect the CrMoV steel embrittlement nondestructively. In particular, the 7.07 mM 2,4-DNBA solution seems to be the most suitable because of steepness, linearity and good reproducibility in the E, vs AFATT relation. On the basis of the polarization behaviors in 2,4DNBA solution, an interpretation ofthe changes in E, and R, with CrMoV steel embrittlement level was considered. Figure 14 presents a kinetic diagram schematically to explain the effects of embrittlement and solution stirring. Lines A and B denote anodic dissolution reactions for embrittled and unembrittled steels, respectively. These lines mean that the overvoltage for the anodic reaction increases with embrittlement. The solution stirring effect on anodic reaction was neglected based on the experimental results. Lines C and D stand for cathodic reactions in the absence and presence of solution stirring, respectively. Also, based on the results in Figs 11 and 13, cathodic reactions are considered to be identical for both embrittied and unembrittled steels regardless of whether or not there is solution stirring. The four open circuit potentials can be indicated at the crossing point of each line in Fig. 14. The change in E, with embrittlement is explained by the mixed potentials E,, and E,,. That is, the mixed potential shifts to a positive potential as the embrittlement level increases. When the solution is stirred, mixed potentials for both steels shift to positive and the difference in potential becomes small (E,, and E,,), corresponding to the result shown in Fig. 8. This diagram explains the results shown in Fig. 7 as follows. When the solution is initially stirred and then made stagnant, the mixed potential for the embrittled steel moves from E,, to more negative E,,. However, the potential shift rate is expected to be slower than that for the unembrittled steel, ie, the change rate from E,, to E,,, because the current density for embrittled steel is always lower than that for unembrittled steel. This in Fig. 9. with the results shown coincides to reflect the Consequently, R, can be considered apparent rate for the diffusion layer formation at the steel surface. Details of each reaction will be presented in a succeeding paper [15].
DISCUSSION It is obvious that there are correlations betwen E,, and AFATT in various organic acid solutions. Based on the E, us AFATT relation, thecompounds tested in the present study can be classified into two groups: group I ~-nitrophenols and nitrobenzoic acids; group II-aromatic acids having no nitro group, aliphatic acids and inorganic acid (perchloric acid). In the solutions of group I, E, shifted in the positive direction with increasing AFATT. No hydrogen evolution was observed and the steel surface was covered with grey to black corrosion products. On thecontrary, E, shifted in the negative in group II solutions, direction with increasing AFATT. In general, Ew values for group II solutions were in the more negative region than those for group I solutions. The most characteristic behavior for group II solutions was that gas evolution due to hydronium ion reduction occurred. These results suggest that the E, measurement in
+
w
_
Jog
Ii I
Fig. 14. Kinetic diagram to explain the correlation between open circuit potential and steel embrittlement: (A) anodic polarization curve for embrittled steel: (B) anodic polarization curve for unembrittled steel; (C)cathodicpolarizationcurve in stagnant solution; (D) cathodic polarization curve in stirred solution. E,,, E,,, E,,, Eac_denote the mixed potential, respectively.
Electrochemical
pr0pe.11 :iesof CrMoV steel-1
CONCLUSIONS Relations between the embrittlement level and open circuit potential for a CrMoV rotor steel were investigated in acidic solutions of various organic compounds. From the measured correlation between the open circuit potential and the steel embrittlement level, the compounds were classified into two groups. The first is nitrophenols and nitrobenzoic acids, in which the open circuit potential of the steel shifted to a positive potential with an increase in theembrittlement level, and no hydrogen evolution was observed. Other examined compounds, ie, aliphatic acids, aromatic acids having no nitro group and perchloric acid, belong to the second group. In these compounds, the direction of potential shift was opposite to that for the first group solutions and hydrogen evolution was observed in all experiments. A steep slope for the open circuit potential us the CrMoV steel embrittlement level relation was found in 2+dinitrobenzoic acid and picric acid solutions, particularly for 7 mM 2,4_dinitrobenzoic acid solution. Furthermore, it was found, from an observation on pdtential shift due to solution stirring using 2,4dinitrobenzoic acid solution, that the rate of change in open circuit potential when solution stirring was stopped became small as the embrittlement level increased. This result can be used in non-destructive evaluation to determine the embri!tlement 1eveL The presence of a peculiarity in the solufion composition used was confirmed by polarization measure-
1389
ments, where clear differences were recognized between 2+dinitrobenzoic acid and citric acid for both anodic and cathodic reactions.
REFERENCES 1. M. Guttmann, Sur$ Sci. 53, 213 (1975). 2. 2. Ou and K. H. Kuo. Metal. Trans. 12A. 1333(1981). 3. Q. zhe, S. C. Fu and C.j. McMahon Jr, EPki Rep&t Ck 2242,February(1982). 4. A. H. Ucisik. C. J. McMahon Jr. and H. C. Feng, Metal. Trans. 9A, 321 (1978). 5. T. Ogura, C. J. McMahon, Jr., H. C. Feng and V. Vitek.
Acta Metal. 26, 1317 (1978). 6. J. Kiipper, H. Erhart and H. J. Grabke, Cow. Sci. 21,227 (1981). 7. S. M. Bruemmer, Corrosion 42, 180 (1986). 8. T. Shoji and H. Takahashi, ‘Non-destructive evaluation of materials degradation during service operation by means of electro-chemical method’, EPRI Conference on life extension and assessment of fossil plants, June 2-4 (1986). 9. R. S. Archer, Trans. Am. Inst. Min. Engrs 62, 754 (1920). 10. J. B. Cohen, A. Hurlich and M. Jacobson, Trans. Am. Sot. Metals. 39. 109 (1947). 11. G. A. Dreykr, D. h. Au&in. and W. D. Smith, Metal Prog. 86. 116 (1974L 12. ?‘:bgu& A. ftlakino and T. Masumoto, Scripto Metal. 14, 887 (1986). 13. T. Ogura, A. Makino and T. Masumoto, Metal. Trans. 15A, 1513 (1984). 14. Annual Book of ASTM Standards, E23-82. 15. N. Satoand M. Suzuki, Electrochim. Acta 33,1391(1988).
Saiwai-ku,
15 December 1987; in revised form 3 March 1988)
Abstract-The embrittlement of CrMoV steel used for a steam turbine rotor, which had been in service over 139,ooO h, has been investigated by open circuit potential measurements in acidic aqueous solutions of various organic compounds. The open circuit potential shifted positively in nitrophenol and nitrobenaoic acid solutions as the embrittlement level of the CrMoV steel increased, while it shifted negatively in aliphatic and aromatic acids without the nitro group. The possibility for embrittlement evaluation was also demonstrated by obtaining the rate of change in open circuit potential when solution stirring was stopped. The mechanism for the open circuit potential shift is discussed based on polarization measurements.
INTRODUCTION Low alloy steels have been used extensively in power plants. However, long time use of these alloys in the range of 400-500°C leads to degradation of their mechanical properties. One typical example of such material degradation is the temper embrittlement of CrMoV steels due to high temperature operation. This phenomenon has been thought to be caused by segregation of impurities such as phosphorus to the grain boundaries[ l-31. There is considerable current interest in being able to detect embrittlement in these steels and the present paper discusses the work we have undertaken with a view to developing a practical methdd for testing the degree of embrittlement of rotor steels in service. Surface analytical methods have previously been applied to obtain the correlation between impurity segregation at the grain boundaries and the embrittlement of the material[2-51. Electrochemical measurements have also been adopted on account of their high sensitivity in detecting selective dissolution of grain boundaries in suitable etching solutions. Kiippcr et a/.[63 measured polarization curves for FeP alloys in a calcium nitrate solution and found a correlation between thecorrosion current density and phosphorus content at the grain boundaries. Bruemmer[7] measured polarization curves for NiCrMoV steels of different phosphorus levels in saturated picric acid solution. He found the change in peak active current density due to the difference in phosphorus level. Shoji and Takahashi[S] found a correlation between passive current density and embrittlement level of CrMoV steel by polarization measurements in a special picric acid solution. In general, however, polarization measurements encounter difficulties such as poor reproducibility and may give rise to changes in surface condition, especially EA
33:10-I
when the specimen is polarized to the passive region. Selective etching is another method that has often been used to detect temper embrittlement by means of developing grain boundaries[P-13-J. The technique is based on the selective dissolution phenomenon in the low overvoltage area on the surface at the open circuit potential and seems to have good reproducibility. The difficulty with this method lies in quantitative evaluation of the etching involved. In the present study, in order to investigate the effect of temper embrittlement for CrMoV steels on their electrochemical behaviour, the open circuit potential was measured in aqueous solutions containing various kinds of organic compounds. It was found that good sensitivity and reproducibility are obtainable in a correlation between the open circuit potential and embrittlement level for a CrMoV steel when appropriate solutions are used.
EXPERIMENTAL Specimens
and chemicals
The CrMoV steel specimens of various embrittlement levels were obtained from several parts of a steam turbine rotor which was operated for over 139,000 h. The bulk composition of this material is listed in Table 1. Charpy impact tests were carried out on these specimens to determine the fracture appearance transition temperature (FATT) values[ 141. The difference in the FATT values between embrittled and control specimens, AFATT, was used as the measure for the embrittlement level. The AFATT value for the most embrittled sample was 98°C in this study. The control specimen, whose AFATT value is regarded to be 0°C. was sampled from the coupling of the rotor, as usual [S], which was located outside of the turbine casing
1383
N. SATO and M. SUZUKI
1384
Table 1. CrMoV steel chemical composition (%(w/w)) C
Si
Mn
P
S
Ni
Cr
MO
V
Cu
Sn
As
Sb
0.38
0.35
0.80
0.023
0.043
0.45
1.01
1.02
0.23
0.14
< 50 pptn
< 1OOppm
< 20 ppm
and was free from embrittlement. Steam temperatures during operation and AFATT values for the specimens are listed in Table 2. Specimens for the electrochemical measurements were cut from the Charpy impact specimens, degreased with acetone and mechanically polished with 600 mesh Sic grit paper. Then, the specimens were wrapped in parallin film except for the testing area of 8 mm in diameter. Commercially available analytical grade chemicals were used without further purification. Testing solutions were prepared using deionized distilled water. Solution compositions are listed in Table 3. The organic compounds were chosen for their potential ability to etch low alloy steels moderately.
Electrochemical
measurements
The open circuit potential measurements and potentiodynamic measurements were performed in a 200 ml single compartment three electrode glass cell, a saturated calomel electrode (see) and a platinum plate electrode being used as the reference and auxiliary electrodes, respectively. A probe-type cell was also used to examine the solution agitation effect on the open circuit potential. The cell body contains 40 ml of testing solution, which iscontacted with the specimen through the window of 8 mm in diameter. The specimen temperature was kept constant by flowing water fed from a water bath through the specimen holder. Solution agitation was performed with an stirring blade driven by a motor. All measurements were conducted without removing dissolved oxygen from the solution. Temperature was kept constant at 30 f 0.5”C, unless otherwise specified. Potentiodynamic measurements were carried out with a potentiostat (Hokuto Denko Ltd. HA-104) and a potential sweeper (Hokuto Denko Ltd. HB-104). Potential and current data from the potentiostat were converted to digital data by digital multimeters and processed by a personal microcomputer (Hewlett Packard Series 9000).
Table
temperatures 2. Steam Ak-ATT values
steam Temperature (“C) 50-80 (Coupling) 350 420 490 510
and
AFATT 0 50 72 85 98
(“C)
RESULTS
Measurements
on open circuit potential,
E,
Open circuit, potentials were measured for several specimens with different AFATT values in the testing solutions listed in Table 3. In most cases, the open circuit pot’ential reached the steady state value, E,, about 10-20 min after immersion. In some cases, such as when using picric acid solutions, the open circuit potential moved gradually showing a broad peak or a plateau several minutes after immersion. In such cases, the peak or the plateau potential was defined as the open circuit potential, E,. Figure 1 shows the relations between E, and AFATT for the CrMoV steel in picric acid solutions with threedifferent concentrations. Linear correlations were found between both values; ie, E, shifted in the positive direction as the embrittlement level increased for every picric acid concentration. During the measurement, the test piece surface was gradually covered with black and homogeneous corrosion products. It is noticeable that no hydrogen evolution was observed at all in spite of acidic solutions
Table 3. Compositions
Compound Nitrophenols 2,4,6_Trinitrophenol (Pi&c acid) CNitrophenol 2,CDinitrophcnol 2,4,6-Trinitroresorcinol Nitrobenzoic acids 2+Dinitrobenzoic acid (2,4-DNBA)
2,6-DNBA 3,5-DNBA 2-Nitrobenzoic acid Other aromatic acids Salicylic acid 2,6-Dihydroxybenzoic Phthalic acid Aliphatic acids Acetic acid Oxalic acid Citric acid Inorganic acid Perchloric acid Saturated. +200/!MeOH
l
solution.
acid
of various solutions Concentration (mM)
PH
4.36 6.55 13.1 50.0 If 6.50
2.5 2.2 2.0 4.3 3.3 2.2
4.71 6.50 7.07 9.43 16.5 7.07 7.07+ 12.0
2.5 2.3 2.2 2.2 2.0 2.2 2.7 2.2
7.24 10.9 6.49 13.0 6.02
2.7 2.5 2.3 2.0 2.7
6.58 6.5 1 6.55
3.5 2.3 2.7
6.58
2.2
Electrochemical properties of CrMoV steel-I
1385
/
-150 *18
/I-.’ ;,oo~A?
2
,.H
w
0
50
f3' -2oow vi > $
:::LL-
100
and AFATT in picric acid 6.55 mM;-A-4.36 mM.
(pH < 3), where hydrogen evolution is usually seen in iron and steel corrosion. In order to investigate whether or not the above results are characteristic of only picric acid, other nitrophenols were examined. Figure 2 shows the results with 4-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitroresorcinof solutions. Though E, values in these solutions shifted to positive potential with increasing embrittlement level (ie, larger AFATT value), the slopes were far less steep than those for picric acid solutions. Particularly, E, was almost independent oft he embrittlement level in Cnitrophenol, in which the test piece surface was covered with reddish brown corrosion product due to high pH value of the solution (pH 4.3). No signs of hydrogen evolution, ie, bubble formation on the test piece surface, were found in these solutions as in picric acid solutions. Nitrobenzoic acids were also examined in the same manner (Fig. 3). A marked correlation was found in the 7.07 mM 2,4_dinitrobenzoic acid (2,4-DNBA) solution, the slope of E, USAFATT being comparable to that in 6.55 mM picrlc acid solution. The dependence of the
w
-200
I-=-
/_ -250-/B _.e*-
8 w
AFATT. "C
Fig. 1. Relation between E, solutions: -O13.1 mM;-o-
/
50 AFATT,
0
IOC
-2
Fig. 3. Relation between E, and AFATT in several nitroben7.07 mM ZPDNBA; -I7.07 zoic acid solutions: + mM 2,6-DNBA;[email protected] mM 3.SDNBA;-A12.0 mM 2-nitrobenzoic acid. slope on the 2,4-DNBA concentration is shown in Fig. 4. In this figure, the longitudinal axis, AE, means the values between the specimen of difference in E, AFATT = 98°C and that of AFATT = 0°C. As seen from the figure, the maximum AE, value was found for around 7mM of 2,4-DNBA (ca 95 mV). This value was the largest found throughout the present study. In all nitrobenzoic acid solutions examined, no indications of hydrogen evolution were observed. and the sample surface after measurements was covered with grey corrosion product. From the above results, it is interesting to note that the number and the position of nitro group have a and significant effect on the relation between E, AFATT for the CrMoV steel. Further, it is likely that hydroxyl or carboxyl group is not essential to obtain relation, but the the large slope of Em us AFATT coexisting nitro group is. In order to confirm this suggestion, acidic solutions of aromatic compounds having no nitro group were examined in the same way. Figure 5 shows the results with phthalic acid, salicylic acid and 2,6-dihydroxybenzoic acid solutions. In these solutions, open circuit potentials for the CrMoV steel
M r 2 x w
-250 -350
-400
2
60 ii w a 40
o-o-
-0
I 0
50 AFATT.
100
i 20
“C
Fig. 2. Relation between E, and AFATT in several ‘nitrop henol solutions: -Clsaturated 2,Qdinitrophenol; -A50.0 mM 46.50 mM 2,4,6_trinitroresorcinol; -onitrophenol.
0 t 2.4-DNBA.
Fig. 4. AE, dependence
mM
on 2+DNBA
concentration.
1386
N.
SAT0
and M. %JZUKl
s’ AFATT. OC Fig. 5. Relation between E, and AFATT in several aromatic acid solutions: -A - 10.9 mM salicylicacid;-A-7.24 mM salicylic acid; - q - 6.49 mM 2,6-dihydroxybenzoic acid; I13.0 mM 2,6-dihydroxybenzoic acid; + 6.02 mM
ts TIME Fig. 7. Changes in E, with lapsed time before and after solution stirring was stopped in 7.07 mM 2,4-DNBA solution: (A) embrittled CrMoV steel (AFATT = 98°C); (B) unembrittled CrMoV steel (AFATT= 0°C). Stirring was stopper at 2,.
phthalic acid.
were considerably negative, compared with those in aromatic nitroeompound solutions, and shifted in the
negative direction as the steel became more brittle. No clear correlation was found between E, and AFATT. Aliphatic acids and perchloric acid solutions were also examined (Fig. 6). These solutions gave results similar to those for solutions of aromatic compounds having no nitro group. It should be noted that hydrogen evolution was seen in the solutions of these aliphatic and aromatic compounds without nitro group as well as in the perchloric acid solution. Agitation
effect
The above mentioned results suggest that the open circuit potential measurement can be used for a non-
destructive evaluation of embrittlement. However, in all solutions examined, open circuit potential, E,, was sensitive to solution movement. So, to clarify whether or not reproducible E, values are obtainable, solution stirring effect on E, values was investigated using the probe-type cell in the 7.07 mM 2,4-DNBA solution. Figure 7 shows typical changes in E, with time in 7.07 mM 2,4-DNBA solution for both embrittled (line A) and unembrittled (line B) specimens, before and after
solution stirring was stopped. Though E, values for both specimens were kept almost constant shifting to considerably positive potentials during solution stirring, the difference in E, values between the two specimens became so small that sensitive detection of material degradation was thought to be dillicult (Fig. 8). On the other hand, when the solution stirring was stopped, the open circuit potential returned rapidly to the negative potential. It can be seen from the two curves in Fig. 7 that the rate of change in E, immediately after stopping agitation is greatly different for the two samples. So, the rate of change in E,, defined as R,, was evaluated. The result is shown in Fig. 9, where R, was determined by fitting a straight line to the initial decay, as shown by broken lines in Fig. 7. A satisfactory correlation between R, and AFATT was obtained (correlation coefficient = - 0.990). Furthermore, the temperature dependence of R, was examined for four specimens with different embrittlement levels in the same solution (7.07 mM 2,4DNBA). Their AFATT values were 0, SO,72 and 98”C, respectively. Figure 10 illustrates semilogarithmic plots of R, us 1/T in the temperature range of 2040°C. An exceedingly linear relationship was found for each specimen. This implies that an accurate temperature
-500r----w
=: r
2
-u----+-_Q._ -550
--*\a_a_*
a_
-o%o -“_
;: w -600
I
-O--o--o_ 0
I
0
o
50
--a_ o_ 100
I
AFATT, “C Fig. 6. Relation between E, and AFATT in aliphatic acid and perchloric acid solutions: -A6.55 mM citric acid; ~ 3 ~ 6.58 mM acetic acid; -iI6.51 mM oxalic acid; -c-6.58 mM perchloric acid.
AFATT. “C during solution Fig. 8. Relation between E, and AFATT agitation by stirring in 7.07 mM 2,4-DNBA solution: stirring blade; -Aagitation by air bubbling; - 0 agitation by N, bubbling.
Electrochemical
AFATT. YI Fig. 9. Relation between Rr a,nd AFATT in 7.07 mM 2,4DNBA solution: agrtatron by stirring blade; -Aagitation by air bubbling; -Oagitation by N, bubbling.
D-
o-
3-
I
I
/
33
3.2
$xlOs.
1387
properties of CrMoV steel-1
3.4 K-’
Fig. 10. Semilogarithmic plots of R, vs l/T in 7.07 mM 2,4DNBA solution: AFATT: --o0°C: -W50°C; -A72°C: -.98°C.
compensation is possible to determine the embrittlement level from the R, measurement. The effect of agitation caused by bubbling air or N, into the solution wascompared with that caused by the, stirring blade. E, and R, values measured for air or N, bubbling are plotted in Figs 8 and 9, respectively, and are seen to lie on the same curve as for stirring blade. That is, these parameters were not susceptible to the agitation means or the kind of bubbling gas used. It is obvious that oxygen has no effect on the electrochemical reaction for the CrMoV steel in 2,4-DNBA.
Figure 11 shows polarixation curves for specimens with AFATT = 0°C (solid line) and 98°C (broken line). It is noticeable that an increase in the overvoltage can be observed in the anodic branch with the embrittlement progress whereas little difference is seen in the cathodic region (see below - 400 mV). Consequently, it can be concluded that the difference in E, between embrittled and unembrittled materials should be attributed to the change in the anodic dissolution rate of iron. This suggests that the microstructure changes which brought about the steel embrittlement did not affect the cathodic reaction but suppressed the anodic dissolution rate of the steel. As mentioned above, in this solution no hydrogen evolution was observed at all, so, some reactant must be reduced on the CrMoV steel surface, whose reaction is thought to have a limiting current. Aliphatic and aromatic acids having no nitro group gave the opposite relation between open circuit potential and the steel embrittlement level us aromatic nitrocompounds. Besides, hydrogen evolution was observed in the solutions of such compounds. Then, for comparison, polarization curves were measured in 6.55 mM citric acid solution (Fig. 12). In this case, no clear differences were observed between two specimens for both anodic and cathodic branches. Further, these curves were dissimilar to the curves obtained in the 2,4DNBA solution. The solution stirring effect on the polarization behavior for CrMoV steel in 2,4-DNBA solution was also examined. Figure 13 shows the result. In the cathodic branch, a large current enhancement for both embrittled and unembrittled specimens arose from solution stirring and the plateau in the -600 to -950 mV range for the stagnant solution disappeared, which reveals that the cathodic reaction was so fast that it was susceptible to mass transfer. Also, it is noticeable that the anodic active dissolution was essentially independent of solution stirring. This indicates that the anodic steel in this solution is not dissolution of the CrMoV diffusion controlled, but instead, it is dependent on the anodic dissolution overvoltage, which becomes larger with embrittlement progress.
2-
Polarization measurements In order to clarify what caused the changes in E, or R, with the difference in embrittlement level of the CrMoV steel, polarization measurements were performed for two specimens with different embrittlement levels (AFATT = 0 and 98’C) in 7.07 mM 2,4-DNBA solution. The glass cell was used and solution stirring was carried out by a magnetic stirrer.
Potential,
mV
vs.
SCE
Fig. 11. Current us potential curves for embrittled and unembrittled CrMoV steels in 7.07 mM 2,4-DNBA solution: unembrittled steel (AFATT = 0°C); - ~ ~ embrittled steel (AFATT = 98°C).
1388
N.
SATO and
E
-ait
-700
-308
-SEW
Potential,
mV
vs.
SCE
Fig. 12. Current us potential curves for embrittled and unembrittled CrMoV steels in6.55 mMcitricacid solution:embrittled unembrittled steel (AFATT = OOC); steel (AFATT = 98°C).
-IEBB
-EBB
-688
Potent]
-488
-288
al,
mV
E vs.
200
d
SCE
Fig. 13. Current us potential curves for embrittled and unembrittled CrMoV steels in stirred 7.07 mM 2,4-DNBA
solution: -
unembrittled steel (AFATT = 0°C);. embrittled steel (AFATT = 98°C).
..
.
M. SUZUKI
group 1 compound solutions can be used to detect the CrMoV steel embrittlement nondestructively. In particular, the 7.07 mM 2,4-DNBA solution seems to be the most suitable because of steepness, linearity and good reproducibility in the E, vs AFATT relation. On the basis of the polarization behaviors in 2,4DNBA solution, an interpretation ofthe changes in E, and R, with CrMoV steel embrittlement level was considered. Figure 14 presents a kinetic diagram schematically to explain the effects of embrittlement and solution stirring. Lines A and B denote anodic dissolution reactions for embrittled and unembrittled steels, respectively. These lines mean that the overvoltage for the anodic reaction increases with embrittlement. The solution stirring effect on anodic reaction was neglected based on the experimental results. Lines C and D stand for cathodic reactions in the absence and presence of solution stirring, respectively. Also, based on the results in Figs 11 and 13, cathodic reactions are considered to be identical for both embrittied and unembrittled steels regardless of whether or not there is solution stirring. The four open circuit potentials can be indicated at the crossing point of each line in Fig. 14. The change in E, with embrittlement is explained by the mixed potentials E,, and E,,. That is, the mixed potential shifts to a positive potential as the embrittlement level increases. When the solution is stirred, mixed potentials for both steels shift to positive and the difference in potential becomes small (E,, and E,,), corresponding to the result shown in Fig. 8. This diagram explains the results shown in Fig. 7 as follows. When the solution is initially stirred and then made stagnant, the mixed potential for the embrittled steel moves from E,, to more negative E,,. However, the potential shift rate is expected to be slower than that for the unembrittled steel, ie, the change rate from E,, to E,,, because the current density for embrittled steel is always lower than that for unembrittled steel. This in Fig. 9. with the results shown coincides to reflect the Consequently, R, can be considered apparent rate for the diffusion layer formation at the steel surface. Details of each reaction will be presented in a succeeding paper [15].
DISCUSSION It is obvious that there are correlations betwen E,, and AFATT in various organic acid solutions. Based on the E, us AFATT relation, thecompounds tested in the present study can be classified into two groups: group I ~-nitrophenols and nitrobenzoic acids; group II-aromatic acids having no nitro group, aliphatic acids and inorganic acid (perchloric acid). In the solutions of group I, E, shifted in the positive direction with increasing AFATT. No hydrogen evolution was observed and the steel surface was covered with grey to black corrosion products. On thecontrary, E, shifted in the negative in group II solutions, direction with increasing AFATT. In general, Ew values for group II solutions were in the more negative region than those for group I solutions. The most characteristic behavior for group II solutions was that gas evolution due to hydronium ion reduction occurred. These results suggest that the E, measurement in
+
w
_
Jog
Ii I
Fig. 14. Kinetic diagram to explain the correlation between open circuit potential and steel embrittlement: (A) anodic polarization curve for embrittled steel: (B) anodic polarization curve for unembrittled steel; (C)cathodicpolarizationcurve in stagnant solution; (D) cathodic polarization curve in stirred solution. E,,, E,,, E,,, Eac_denote the mixed potential, respectively.
Electrochemical
pr0pe.11 :iesof CrMoV steel-1
CONCLUSIONS Relations between the embrittlement level and open circuit potential for a CrMoV rotor steel were investigated in acidic solutions of various organic compounds. From the measured correlation between the open circuit potential and the steel embrittlement level, the compounds were classified into two groups. The first is nitrophenols and nitrobenzoic acids, in which the open circuit potential of the steel shifted to a positive potential with an increase in theembrittlement level, and no hydrogen evolution was observed. Other examined compounds, ie, aliphatic acids, aromatic acids having no nitro group and perchloric acid, belong to the second group. In these compounds, the direction of potential shift was opposite to that for the first group solutions and hydrogen evolution was observed in all experiments. A steep slope for the open circuit potential us the CrMoV steel embrittlement level relation was found in 2+dinitrobenzoic acid and picric acid solutions, particularly for 7 mM 2,4_dinitrobenzoic acid solution. Furthermore, it was found, from an observation on pdtential shift due to solution stirring using 2,4dinitrobenzoic acid solution, that the rate of change in open circuit potential when solution stirring was stopped became small as the embrittlement level increased. This result can be used in non-destructive evaluation to determine the embri!tlement 1eveL The presence of a peculiarity in the solufion composition used was confirmed by polarization measure-
1389
ments, where clear differences were recognized between 2+dinitrobenzoic acid and citric acid for both anodic and cathodic reactions.
REFERENCES 1. M. Guttmann, Sur$ Sci. 53, 213 (1975). 2. 2. Ou and K. H. Kuo. Metal. Trans. 12A. 1333(1981). 3. Q. zhe, S. C. Fu and C.j. McMahon Jr, EPki Rep&t Ck 2242,February(1982). 4. A. H. Ucisik. C. J. McMahon Jr. and H. C. Feng, Metal. Trans. 9A, 321 (1978). 5. T. Ogura, C. J. McMahon, Jr., H. C. Feng and V. Vitek.
Acta Metal. 26, 1317 (1978). 6. J. Kiipper, H. Erhart and H. J. Grabke, Cow. Sci. 21,227 (1981). 7. S. M. Bruemmer, Corrosion 42, 180 (1986). 8. T. Shoji and H. Takahashi, ‘Non-destructive evaluation of materials degradation during service operation by means of electro-chemical method’, EPRI Conference on life extension and assessment of fossil plants, June 2-4 (1986). 9. R. S. Archer, Trans. Am. Inst. Min. Engrs 62, 754 (1920). 10. J. B. Cohen, A. Hurlich and M. Jacobson, Trans. Am. Sot. Metals. 39. 109 (1947). 11. G. A. Dreykr, D. h. Au&in. and W. D. Smith, Metal Prog. 86. 116 (1974L 12. ?‘:bgu& A. ftlakino and T. Masumoto, Scripto Metal. 14, 887 (1986). 13. T. Ogura, A. Makino and T. Masumoto, Metal. Trans. 15A, 1513 (1984). 14. Annual Book of ASTM Standards, E23-82. 15. N. Satoand M. Suzuki, Electrochim. Acta 33,1391(1988).