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Electrochemical and corrosion behaviour of laser modified iron surfaces
Materials Chemistry 5 ( 1 9 8 0 ) 73 - 78 © CENFOR S.R.L. - Printed in Italy
PRELIMINARY NOTE
ELECTROCHEMICAL AND CORROSION BEHAVIOUR OF LASER MODIFIED IRON SURFACES It is well known that high power laser irradiation o f materials causes structural changes in the heat affected regions 1 . A recent work 2 showed the possibility of obtaining a non-crystalline phase at the surface o f polierystalline A1 samples by irradiating them with very short (15-ns time duration) pulses from a ruby laser (~, = 0.694/am). The electrochemical and corrosion behaviour of the laser irradiated A1 samples was studied as a function of the energy density o f the pulses in the range between 1 and 5 J/cm 2 and it resulted noticeably improved with respect to unirradiated specimens 3 . The best results were obtained for an energy density of about 3 J/cm 2 . In this paper some preliminary results are presented concerning pure iron (Johnson-Matthey, 99,99*/,) disks, 0.5 cm in diameter. Their surfaces were submit-
Fig. 1 - Iron surface, polished to a mirror finish (a}, then laser irradiated at 2 d/cm 2 (b} "and 3 d/cm 2 (c). Magnification: 500 X.
74 ted to a mirror finishing and then irradiated in air at room temperature with a Qswitched 15ns pulsed ruby laser. The energy densities of the pulses were 2 and 3 J/cm 2 . Figure 1 shows the SEM micrographs of the untreated (a) and irradiated at 2 J/cm 2 (b) and at 3 J/cm 2 (c) iron surfaces. There is evidence of some finishing defects which, the higher the energy densities are, cause larger and more numerous bowls to appear. The electrochemical and corrosion behaviour of irradiated specimens was investigated following different techniques:
1)
Potentiodynamic current/voltage curves were carried out in a passivating solution, namely 0.15 N H3BO3 + 0.15 N Na~B407 • 10H20 stagnant deaerated solution, still specimen at the temperature of 25"C. The specimen was kept at - 1200 mV vs. the saturated sulphate electrode (SSE) for 10 minutes, in order to reduce all the surface oxyde layers, a voltage scan was then carried ,~
.
3oo
-I A,
lo
.~ -
/4Acre
1
5.1
~,
CUS'RENT DENSITY cathodic amodi¢
1
lo
p,A cm-2
Fig. 2 - Potentiodynamic (720 m V/h) current~voltage plots in a deaerated borate buffer (pH = 8.41} solution for untreated [0), laser irradiated at 2 J/cm 2 ( i ) , and 3 J/cm 2 (&) pure iron specimens.
7~ out at the rate of 720 mV/h up to the incipient oxygen evolution. Plots are shown in Fig. 2. Polarization resistance measurements were performed in water, in equilibrium with atmospheric oxygen, imposing galvanostaticaUy current triangle waveforms at the sweeping frequency of 0.8 c/min, the current maxima being tentatively chosen in order to get potential displacements not greater than +- 10 mV for a time duration of 92 hours, on rotating disks (1500 r.p.m.). The conductivity of the water was slightly increased by the addition of 0.1% of
2)
Na2 SO4. Table 1 collects the values of Rp, the calculated Icorr (by means Table 1 - Summary of electrochemical and corrosion data of iron in water. Ecorr. = = open circuit stationary corrosion potential; Rp = polarisation resistance AEfiSI: Icorr ' = calculated corrosion current4 ; Rc = calculated corrosion rate, mm/year s, (a) untreated, (b) laser irradiated at 2 J/cm 2 and (c) at 3 J/cm 2 pure iron specimens. TIME (h)
Corrosion Treatment 19
Ecorr.
- 970
- 1036
-
1040
mV vs. S.S.E.
~'~ . c m 2
1040
-
- 1040 1012
-
438
-
825
-
835
-
870
-
-
660
-
966
-
950
-
970
-
304
Rp
92
28
24
2.30
Data
54.103
280 8.64.103
2.16.10 a
280
272
264
7.36, 10 a 6.72" 103
996
288 3.2" 10 a
272
264
232
0.0735
0.0757
0.0694
0.0657
0.0714
0 . 3 7 . 1 0 -3
2 . 3 1 . 1 0 -3
mA" cm "2
0.0092
0.0714
0.0735
0.0757
0.0862
Rc
0.762
0.827
0.850
0.880
0.802
0.00429
0.0268
0.0315
0.0344
0.0724
0.107
0.827
0.850
0.880
0.990
lcorr.
m m
•
y - i
2.71 • 10-3 2.97" 10"a 6.25 • 10"a
76 o f the Heitz and Schwenk approximate equation) 4 and the calculated corrosion rate expressed as mm/year obtained from the formula: d/t = 3.2706 M • i/z • p = 11.5905 i ~. Fig. 3 reports the trends o f calculated weight losses N _ ~§10 ]
o
,--, 10
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
TIME
__L
90
HOURS
Fig. 3 - Plots o f calculated weight losses in water,vs, ttme. Untreated iron (0), laser irradiated at 2 J/cm 2 (m).
vs. time. They were obtained by means o f an approximate integration o f the progressive corrosion rate values. SEM micrographs (Fig. 4) show the attack morphology due both to elec.rochemical anodic polarization and to fluid conditions.
77
iiii:i! i!
Fig. 4 - Corrosion morphology in water o f laser irradiated at 2 J/cm 2 (a) and untreated (b) pure iron. Magnification: 50 X.
The above described experimental results are undoubtedly preliminary but not exhaustive and more research work is in progress in our laboratory. Infact, we can notice that the initial mechanical surface finishing is inadequate to the heat treatment given by the laser irradiation, the remaining scratches and defects being sources of local fusion, which cause an impervious final surface to settle, especially at the highest laser light energy densities. It follows that the beneficial effect of the rapid cooling on the structure is partly hindered by the presence of local active sites, as it is clearly shown in Fig. 2 where the current/voltage curves present increasing difficulty in reaching the passive state, from the untreated iron to the two irradiated specimens. Nevertheless the current in the passive state is considerably lower on irradiated than on untreated specimens, the lowest values being associated to samples irradiated at the highest energy density, which also show the largest passivation current peak. The presence of bowls and other surface defects hinders the study of pitting corrosion in the passive state. This matter will be the subject of future work which will be performed on electrolitically polished iron surfaces. The calculations of the corrosion rate and the analysis for attack morphologies have underlined.some interesting results which support the hypothesis of amorphous surface layers, which are more corrosion resistant. Once more, it is noteworthy that the effeots of the surface finishing are critical, in fact the highest irradiation energy is almost ineffective in lowering the corrosion rate, because of the high number of active sites on the surface. It is however noteworthy that the corrosion potential in water, which has the value of
78 - 9 7 0 mV (SSE) for unirradiated iron and gets the much nobler value o f - 4 3 8 mV (SSE) for a sample "irradiated at 2 J/cm 2 , is still - 6 6 0 mV (SSE) at 3 J/cm 2, and the initial Rp value of 304~ cm 2 on unirradiated iron reaches 54 K~2 cm 2 on the 2 J/cm 2 irradiated specimens and is still 2 Kfl cm 2 at the higher energy irradiated samples. After a few hours, however, the 3 J/cm 2 irradiated specimens show the behaviour of the untreated iron, while at 2 J/cm 2 the effect of the heat treatment lasts for 92 hours and more.
P.L. Bonora, M. Bassoli, P.L. De Anna Centro Studi C N R di Chimica Applicata - Fiera del Mare, Pad. D - (I) - 16129 G E N O VA - l t a l y .
G. Battaglin, G. Della Mea, P. Mazzoldi Unitd G N S M - C N R , lstituto di Fisica dell'Universitd - Via Marzolo, 8 - 3 5 1 0 0 P~4D O VA -ltaly.
E. Jannitti Centro Gas Ionizzati, C N R , Universitd di Padova - Via Gradengo, 6a - 3 5 1 0 0 PAD O VA - Italy.
REFERENCES 1. 2. 3. 4. 5.
E.M. BREINAN, B.H. KEAR, C. BANAS - Physics Today, Nov. 1976, p. 44. P. MAZZOLDI, G. DELLA MEA, G. BATTAGLIN, A. MIOTELLO, M. SERVIDORI, D. BACCI, E. JANNITTI - Phys. Rev. Lett., 44, 88, 1980. P.L. BONORA, M. BASSOLI, P.L. DE ANNA, G. BATTAGLIN, G. DELLA MEA, P. MAZZOLDI, A. MIOTELLO - Electrochimica A c t a , in press. E. HEITZ, W. SCHWENK - Brit. Corr. J., 11, 74, 1976. L.L. SHREIR - Corrosion, Newnes-Butterworths, London, 1976, p. 21-64.
PRELIMINARY NOTE
ELECTROCHEMICAL AND CORROSION BEHAVIOUR OF LASER MODIFIED IRON SURFACES It is well known that high power laser irradiation o f materials causes structural changes in the heat affected regions 1 . A recent work 2 showed the possibility of obtaining a non-crystalline phase at the surface o f polierystalline A1 samples by irradiating them with very short (15-ns time duration) pulses from a ruby laser (~, = 0.694/am). The electrochemical and corrosion behaviour of the laser irradiated A1 samples was studied as a function of the energy density o f the pulses in the range between 1 and 5 J/cm 2 and it resulted noticeably improved with respect to unirradiated specimens 3 . The best results were obtained for an energy density of about 3 J/cm 2 . In this paper some preliminary results are presented concerning pure iron (Johnson-Matthey, 99,99*/,) disks, 0.5 cm in diameter. Their surfaces were submit-
Fig. 1 - Iron surface, polished to a mirror finish (a}, then laser irradiated at 2 d/cm 2 (b} "and 3 d/cm 2 (c). Magnification: 500 X.
74 ted to a mirror finishing and then irradiated in air at room temperature with a Qswitched 15ns pulsed ruby laser. The energy densities of the pulses were 2 and 3 J/cm 2 . Figure 1 shows the SEM micrographs of the untreated (a) and irradiated at 2 J/cm 2 (b) and at 3 J/cm 2 (c) iron surfaces. There is evidence of some finishing defects which, the higher the energy densities are, cause larger and more numerous bowls to appear. The electrochemical and corrosion behaviour of irradiated specimens was investigated following different techniques:
1)
Potentiodynamic current/voltage curves were carried out in a passivating solution, namely 0.15 N H3BO3 + 0.15 N Na~B407 • 10H20 stagnant deaerated solution, still specimen at the temperature of 25"C. The specimen was kept at - 1200 mV vs. the saturated sulphate electrode (SSE) for 10 minutes, in order to reduce all the surface oxyde layers, a voltage scan was then carried ,~
.
3oo
-I A,
lo
.~ -
/4Acre
1
5.1
~,
CUS'RENT DENSITY cathodic amodi¢
1
lo
p,A cm-2
Fig. 2 - Potentiodynamic (720 m V/h) current~voltage plots in a deaerated borate buffer (pH = 8.41} solution for untreated [0), laser irradiated at 2 J/cm 2 ( i ) , and 3 J/cm 2 (&) pure iron specimens.
7~ out at the rate of 720 mV/h up to the incipient oxygen evolution. Plots are shown in Fig. 2. Polarization resistance measurements were performed in water, in equilibrium with atmospheric oxygen, imposing galvanostaticaUy current triangle waveforms at the sweeping frequency of 0.8 c/min, the current maxima being tentatively chosen in order to get potential displacements not greater than +- 10 mV for a time duration of 92 hours, on rotating disks (1500 r.p.m.). The conductivity of the water was slightly increased by the addition of 0.1% of
2)
Na2 SO4. Table 1 collects the values of Rp, the calculated Icorr (by means Table 1 - Summary of electrochemical and corrosion data of iron in water. Ecorr. = = open circuit stationary corrosion potential; Rp = polarisation resistance AEfiSI: Icorr ' = calculated corrosion current4 ; Rc = calculated corrosion rate, mm/year s, (a) untreated, (b) laser irradiated at 2 J/cm 2 and (c) at 3 J/cm 2 pure iron specimens. TIME (h)
Corrosion Treatment 19
Ecorr.
- 970
- 1036
-
1040
mV vs. S.S.E.
~'~ . c m 2
1040
-
- 1040 1012
-
438
-
825
-
835
-
870
-
-
660
-
966
-
950
-
970
-
304
Rp
92
28
24
2.30
Data
54.103
280 8.64.103
2.16.10 a
280
272
264
7.36, 10 a 6.72" 103
996
288 3.2" 10 a
272
264
232
0.0735
0.0757
0.0694
0.0657
0.0714
0 . 3 7 . 1 0 -3
2 . 3 1 . 1 0 -3
mA" cm "2
0.0092
0.0714
0.0735
0.0757
0.0862
Rc
0.762
0.827
0.850
0.880
0.802
0.00429
0.0268
0.0315
0.0344
0.0724
0.107
0.827
0.850
0.880
0.990
lcorr.
m m
•
y - i
2.71 • 10-3 2.97" 10"a 6.25 • 10"a
76 o f the Heitz and Schwenk approximate equation) 4 and the calculated corrosion rate expressed as mm/year obtained from the formula: d/t = 3.2706 M • i/z • p = 11.5905 i ~. Fig. 3 reports the trends o f calculated weight losses N _ ~§10 ]
o
,--, 10
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
TIME
__L
90
HOURS
Fig. 3 - Plots o f calculated weight losses in water,vs, ttme. Untreated iron (0), laser irradiated at 2 J/cm 2 (m).
vs. time. They were obtained by means o f an approximate integration o f the progressive corrosion rate values. SEM micrographs (Fig. 4) show the attack morphology due both to elec.rochemical anodic polarization and to fluid conditions.
77
iiii:i! i!
Fig. 4 - Corrosion morphology in water o f laser irradiated at 2 J/cm 2 (a) and untreated (b) pure iron. Magnification: 50 X.
The above described experimental results are undoubtedly preliminary but not exhaustive and more research work is in progress in our laboratory. Infact, we can notice that the initial mechanical surface finishing is inadequate to the heat treatment given by the laser irradiation, the remaining scratches and defects being sources of local fusion, which cause an impervious final surface to settle, especially at the highest laser light energy densities. It follows that the beneficial effect of the rapid cooling on the structure is partly hindered by the presence of local active sites, as it is clearly shown in Fig. 2 where the current/voltage curves present increasing difficulty in reaching the passive state, from the untreated iron to the two irradiated specimens. Nevertheless the current in the passive state is considerably lower on irradiated than on untreated specimens, the lowest values being associated to samples irradiated at the highest energy density, which also show the largest passivation current peak. The presence of bowls and other surface defects hinders the study of pitting corrosion in the passive state. This matter will be the subject of future work which will be performed on electrolitically polished iron surfaces. The calculations of the corrosion rate and the analysis for attack morphologies have underlined.some interesting results which support the hypothesis of amorphous surface layers, which are more corrosion resistant. Once more, it is noteworthy that the effeots of the surface finishing are critical, in fact the highest irradiation energy is almost ineffective in lowering the corrosion rate, because of the high number of active sites on the surface. It is however noteworthy that the corrosion potential in water, which has the value of
78 - 9 7 0 mV (SSE) for unirradiated iron and gets the much nobler value o f - 4 3 8 mV (SSE) for a sample "irradiated at 2 J/cm 2 , is still - 6 6 0 mV (SSE) at 3 J/cm 2, and the initial Rp value of 304~ cm 2 on unirradiated iron reaches 54 K~2 cm 2 on the 2 J/cm 2 irradiated specimens and is still 2 Kfl cm 2 at the higher energy irradiated samples. After a few hours, however, the 3 J/cm 2 irradiated specimens show the behaviour of the untreated iron, while at 2 J/cm 2 the effect of the heat treatment lasts for 92 hours and more.
P.L. Bonora, M. Bassoli, P.L. De Anna Centro Studi C N R di Chimica Applicata - Fiera del Mare, Pad. D - (I) - 16129 G E N O VA - l t a l y .
G. Battaglin, G. Della Mea, P. Mazzoldi Unitd G N S M - C N R , lstituto di Fisica dell'Universitd - Via Marzolo, 8 - 3 5 1 0 0 P~4D O VA -ltaly.
E. Jannitti Centro Gas Ionizzati, C N R , Universitd di Padova - Via Gradengo, 6a - 3 5 1 0 0 PAD O VA - Italy.
REFERENCES 1. 2. 3. 4. 5.
E.M. BREINAN, B.H. KEAR, C. BANAS - Physics Today, Nov. 1976, p. 44. P. MAZZOLDI, G. DELLA MEA, G. BATTAGLIN, A. MIOTELLO, M. SERVIDORI, D. BACCI, E. JANNITTI - Phys. Rev. Lett., 44, 88, 1980. P.L. BONORA, M. BASSOLI, P.L. DE ANNA, G. BATTAGLIN, G. DELLA MEA, P. MAZZOLDI, A. MIOTELLO - Electrochimica A c t a , in press. E. HEITZ, W. SCHWENK - Brit. Corr. J., 11, 74, 1976. L.L. SHREIR - Corrosion, Newnes-Butterworths, London, 1976, p. 21-64.