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A statistical evaluation of AISI 316 stainless steel resistance to crevice corrosion in 3.5% NaCl solution and in natural sea water after pre-treatment in HNO3
Corrosion Science, Vol. 27, No. 9, pp. 927-936, 1987 Printed in Great Britain
0010--938X/87 $3.(~)+ 0.00 © 1987 Pergamon Journals Ltd.
A S T A T I S T I C A L E V A L U A T I O N OF AISI 316 S T A I N L E S S STEEL R E S I S T A N C E TO C R E V I C E C O R R O S I O N IN 3.5% NaCI S O L U T I O N A N D IN N A T U R A L S E A W A T E R A F T E R P R E - T R E A T M E N T IN H N O 3 G . SALVAGO
and G. FUMAGALLI
Dipartimento di Chimica Fisica Applicata del Politecnico di Milano, Piazza L, da Vinci 32, 20133 Milano, Italy
and A . MOLLICA and G. VENTURA C . N . R . , Istituto per la Corrosione Marina dei Metalli, Via della Mercanzia 4, 16123 G e n o v a , Italy
Abstract--AISI 316 pre-treatment in nitric acid greatly increases the crevice corrosion induction time both in 3.5% NaCI solution and in natural sea water. Pre-treatment defects do not seem to cause any negative effect on the resistance to crevice corrosion, defects on unshielded surfaces are not influential, while those on shielded surfaces would, at worst, nullify the effects of HNO3 pretreatment. In the conditions adopted for sea water tests, the experimental distribution of corroded samples versus time is described by the following equation
N(t) = NIl(1 - e '~). Pre-treated samples show a m e a n crevice corrosion induction time of 20 days in comparison with r 1.5 day for untreated samples. Stainless steel exposures to sulphides in a concentration that can be found below shielded surfaces exposed to natural sea water reduces, in the long run, the protective effect of the pre-treatment. HNO~ pre-treatment tends to increase the localized attack when crevice corrosion propagates.
INTRODUCTION
A I S I 316 STAINLESS STEEL is one of the materials widely used for marine applications. In such an environment crevice corrosion c o m m o n l y occurs. The pre-treatment of stainless steel in H N O 3 alters the corrosion behaviour, although no full agreement has been reached about its practical usefulness. Generally speaking, m a n y corrosion engineers are sceptical about the effects of stainless steel p r e - t r e a t m e n t in a nitric acid solution. Most accelerated corrosion tests seem to point out that the resistance to localized corrosion is enhanced by pre-treating surfaces in nitric acid;~'2 however, some workers d i s a g r e e ) 4 For stainless steel the pitting potential and corrosion potential in sodium chloride span a rather wide range, that becomes even larger for stainless steel pre-treated in H N O > s'6 The potential is time-dependent even in the absence of localized corroManuscript received 11 February 1986; in a m e n d e d form 10 D e c e m b e r 1986. 927
G. SALVAGOetal.
928
sion. 7'8 Thus, the theoretical comparison between the corrosion behaviour of stainless steel pre-treated or not pre-treated in nitric acid turns out to be quite complicated. In addition, pre-treatment defects or accidental damages may give rise to galvanic coupling between the pre-treated surface and the damaged surface. The results of laboratory tests in NaCI solution and field tests can be different owing to the biological factors which can influence stainless steel corrosion in natural sea water. 9 In order to assess the effect of H N O 3 pre-treatment on the crevice corrosion resistance of stainless steels in sea water, it is advisable to carry out tests in natural sea water and in the presence of pre-treatment defects. With this aim, the following tests were performed: (a) dipping tests in sodium chloride solutions on AISI 316 multicrevice samples with and without HNO3 pre-treatment on shielded and unshielded surfaces; (b) field tests o n shielded samples with and without pre-treatment in HNO3, plunged into anaerobic marine sediments and coupled with unshielded surfaces pre-immersed in natural sea water for 15 days. The procedure followed, during the performance of field tests, attempts to simulate, in a reproducible way, mixed aerobic-anaerobic conditions which can be found on samples after long exposure in sea water. Under operational conditions in sea water the presence of a crevice, either pre-existing or created by deposition of macrofouling, is often associated with the presence of sulphides (Fig. la,b). Sulphides at the same concentration as that found below shielded surfaces are noticed in anaerobic sediments in the presence of Desulphovibrio bacteria (Fig. lc). l° Moreover, the behaviour of unshielded surfaces, that play the role of a cathodic area, depends on the deposition of aerobic bacteria. 8'11This deposition of aerobic bacteria on stainless steel surface immersed in quiet sea water and its effect on the kinetics of oxygen reduction are almost independent of the type of stainless steel. The kinetics of the cathodic process reaches an almost steady value after about two weeks' e x p o s t l r e . 8,11,12
EXPERIMENTAL METHOD The chemical composition of the alloys used is shown in Table 1. For the tests carried out in 3.5% NaC1 solution steel A was used, whereas steel B was utilized for field tests in sea water. All samples were annealed and ground with wet emery paper up to grade 120. The pre-treatment in nitric acid was carried out in 2 M H N O 3 at 85°C for 60 min. Laboratory tests were performed in 3.5% NaCI solution at room temperature. Potentiodynamic tests were carried out at a scanning rate of 1 mV s-t; potentiostatic tests were carried out at a potential of 400 mV(SCE), as this potential is the upper limit of the potential observed for stainless steel in sea water. 8`u Dipping tests in aerated 3.5% NaCI solution for 35 days were performed on 100 x 200 mm steel panels having an Anderson type multicrevice assembly. 16,17The multicrevice assembly was centred at a distance of 50 mm from the lower edge and the samples were partially immersed for 150 ram. The exposed surface/solution volume ratio was 1 dm2/10 litres; losses due to evaporation were compensated by addition of distilled water. No filtering was carried out.
TABLE 1.
CHEMICAL COMPOSITION OF STAINLESS STEELS INVESTIGATED
AISI316
C
Cr
Ni
Mo
Si
Mn
S
19
N
A B
0.040 0.044
16.7 17.4
10.6 11.2
2.1 2.2
0.43 0.35
1.5 1.6
0.008 0.023
0.027 0.025
0.029 --
(b) FIG. 1. Local presence of sulphides revealed by black spots (PbS) on lead containing white PVC exposed in sea water. (a) Sulphides under macrofouling (barnacles); (b) sulphides on shielded surface. (Continued) 929
FIG. 1 continued.
Sulphides on strips partially dipped into anaerobic sediments.
930
AISI 316 stainless steel resistance to crevice corrosion
931
To assess the effect of HNO 3 pre-treatment with different defects and damage, dipping tests were performed according to the following schedule: Dipping tests in 3.5% NaCI solution Tests
Shielded surface
Unshielded surface
A B C D
not pre-treated not pre-treated pre-treated pre-treated
not pre-treated pre-treated not pre-treated pre-treated
B: sample obtained by HNO 3 pre-treatment after installation of Anderson type multicrevice assembly. C: sample obtained by repeating manual grinding with wet emery paper after installation of Anderson type multicrevice assembly. Field tests in marine environment were performed on a sample assembled as shown in Fig. 2. AISI 316 discs (¢p 25) assembled on to an India rubber latex gasket (q 12) were dipped in anaerobic sediments drawn from Genoa port through 1 mm mesh screen. The immersion depth into the anaerobic sediment vessel was 111 cm because, according to Ref. 10, this depth ensures the maximum sulphide ccncentration (some hundred ppm). The natural sea water over anaerobic sediments (about 100 1) was quiet and completely rechanged by continuous flow every 6 h. The temperature was set at 30°C. Each AISI 316 sample was coupled, by means of the resistor R = 100 ,Q, with a 100 cm 2 254 SMO unsbielded surface which was previously immersed in sea water for 15 days. The super-austenitic 254 SMO stainless steel (20Cr-18Ni~Mo) was chosen to minimize the chances for localized corrosion occurring on this surface, taking into account that the cathodic behaviour in sea water is almost independent of the kind of stainless steel used. 12The potential difference measured at the erLds of the resistor R was used to calculate galvanic currents. The initiation of crevice corrosion was cc nsidered to be at that time when galvanic current exceeded 10 p,A (potential difference at the ends of R, lmV). In order to evaluate the effects of HNO3 pre-treatment and the effects of the time of exposure to sulpbides on the resistance to crevice corrosion, four tests were performed according to the following schedule: Field tests Tests
Nsamples
Ii
2(I
F G
2(I 10
tI
10
AIS1316 shielded surface pre-treated in HNO 3prc-immersed in anaerobic sediments for 15 days pre-treatedin HNO~,not pre-immerscd not pre-treated, pre-immersed in anaerobic sediments for 15 days not pre-trcated, not pre-immersed
EXPERIMENTAL
RESULTS AND
254 SMO unshieldcd surfacc pre-immersed in natural sea water for 15 days ibid. ibM.
ibM.
DISCUSSION
L a b o r a t o r y tests F i g u r e 3 i l l u s t r a t e s t h e p o t e n t i a l b e h a v i o u r o f A I S I 316 s a m p l e s d u r i n g p r o t r e a t m e n t in H N O 3 . T h r e e d i f f e r e n t s t a g e s c a n b e i d e n t i f i e d : (a) d i s s o l u t i o n p h e n o m e n a c a n b e c l e a r l y n o t i c e d ; t h e p o t e n t i a l b e h a v i o u r is s e v e r e l y a f f e c t e d b y t h e p r e s e n c e o f i r o n i o n s in n i t r i c a c i d ; ( b ) a g r a d u a l p o t e n t i a l i n c r e a s e o c c u r s - - t h e d u r a t i o n o f t h i s s t a g e d e p e n d s o n t h e s t e e l s u r f a c e t y p e o f f i n i s h i n g as well as o n t h e
932
G. SALVAGOetal.
254 SMO
sea v,~ter
anaerobic
sediment FIG. 2.
specimen
holder
Scheme of e q u i p m e n t for field test.
nitric acid concentration and temperature; and (c) some instability of the potential may appear. The above-described behaviour has been observed for other stainless steels too, and, in correspondence with the third stage, localized corrosion could be observed. J.2 Limiting HNO3 treatment to the second stage, electrochemical tests carried out in aerated 3.5% NaCI solution show a general increase in the free corrosion potential and in the pitting potential as well as a reduction of passivity currents. If the treatment is performed at 85°C with 2 M nitric acid, the second stage lasts 78 h and pre-treated AISI 316 samples do not show pitting corrosion during potentiodynamic tests. Moreover, the potentiostatic test carried out at +400 mV(SCE) proved that this localized corrosion induction time was not lower than 100 h for the same samples. The results of potentiodynamic and potentiostatic tests, after controlled pretreatment of AISI 316 in H N O 3, a r e similar to those normally obtained on materials having higher chromium and molybdenum content and greater resistance to localized corrosion. From the dipping test in aerated 3.5% NaCI solution carried out on multicrevice samples, the following number of etching sites was detected Iv and
OS.
CO
w 0
I
n tog t
FIG. 3.
Schematic time behaviour of the AISI 316 corrosion potential in H N O 3 solution.
AISI 316 stainless steel resistance to crevice corrosion
051 E vs SCE(V)
~°,~t,r
0
10 "~,,.~ _
_
{
_
....
I
1,0 k ..,,...~ [ (days)
0.5
start of crevice corrosion
t coupting
500 II' ('uA) 0" . . . . 0
Fie,. 4.
933
_ j :
10
t (days)
Trend of corrosion potential and galwmic current before and after coupling between shielded (AISI 316) surface and unshielded (254 SMO) surface.
microscopically confirmed ~8(over a total of 64 crevice positions for each sample): test A, untreated samples, etching sites = 4 test B, only unshielded surface pre-treated in HNO3, etching sites = 3 test C, only shielded surface pre-treated in HNO3, etching sites = 0 test D, samples completely pre-treated in HNO3, etching sites = 0. No pitting corrosion was observed. Although dipping tests are not very significant due to the low number of etching sites observed, they seem to indicate: (a) a clear beneficial effect of the pre-treatment in nitric acid when the surface is not damaged or when only the unshielded surface is damaged, and (b) the absence of negative effects when also shielded surfaces are damaged. Field tests
A typical trend of the current and potential for an AISI 316-254 SMO couple, chosen from test E, is shown in Fig. 4. Before coupling, as for 254 SMO unshielded surfaces immersed in renewed natural sea water, a free corrosion potential increase was observed from its initial value up to over 300 mV(SCE), thus confirming the presence of aerobic microbial activity on them, according to Refs 8, 11, 13; as for AISI 316 shielded surfaces pre-immersed in anaerobic sediments (tests E and G), a corrosion potential drop could be noted from its initial value down to -450 to - 4 8 0 mV(SCE) regardless of pre-treatment in HNO3, thus confirming the reducing conditions present in the sediments. After coupling, mixed potentials and galvanic currents were periodically checked. The initiation of corrosion was considered to be the time when galvanic current exceeded the value of 10/~A, along with a simultaneous potential drop. The type of corrosion observed was crevice corrosion. Figure 5 shows the percentage of corroded samples versus time. To interpolate test results, reference was made to Weibull statistics. The reliability function R(t) = e x p ( ( t - ~ z y ) / ~ ) where cz, fl and V are scale, shape and position parameters, was optimizcd to
G. SALVAGOetal.
934 100
o ~.~-~
i"
,~...'-";""- " ::o
~50
¢ #t •
#
/
~
'A #
"¶/
oo .
.
"r= 1.5
.
;
1/
.l
~J •
ss A •
,
.
.
.
.
.
F16. 5. Corroded samples percentages vs time after coupling between shielded and unshielded surface: [] shielded surface untreated and pre-immersed into anaerobic sedim e n t s for 15 days; z5 shielded surface untreated and not pre-immersed; • shielded surface treated in HNO~ and pre-immersed; • shielded surface treated in H N O 3 and not preimmersed.
experimental data, assuming as time-origin the moment corresponding to anode cathode coupling. The parameters ), and fl, that gave the best fit to the data, correspond to ), = 0, fl = 1. Under these conditions, assuming a = ~, the general distribution is reduced to an exponential distribution.
N(t)
= N0(1 - e -'/~)
where N(t) means the number of corroded samples in a given (t) time. The curves shown in Fig. 5 were calculated by optimizing r value in the different cases. The experimental data fit the exponential distribution, indicating that the crevice corrosion mean induction time is not dependent on time. This observation is at least formally similar to that already suggested in the case of pitting corrosion.a4 From these experimental results, it was possible to conclude that: (a) The mean induction time for crevice corrosion on untreated samples, pre-immersed or not in sulphides (tests G and H) is very short (1.5 day). (b) HNO3 pre-treatment, in agreement with, is lengthens the time necessary for localized corrosion to start. For AISI 316 samples pre-treated and not pre-immersed in anaerobic sediments before coupling (test F), the mean induction time is about one order of magnitude (20 days) longer than untreated AISI 316 samples. (c) The pre-immersion of treated AISI 316 samples in mud having reducing conditions deriving from the presence of sulphides at a concentration that can be found in natural environments reduces the crevice corrosion induction time. Under test conditions, pre-immersion into anaerobic sediments for 15 days (test E) reduces to a half the mean induction time (from 20 days to 9 days). The maximum depth of attack observed on each single sample 18 at the end of the test versus coulombs is shown in Fig. 6. The results indicate that in the case where attack developed, corresponding to the passage of 100 C charge, the depth of attack tends to vary, assuming the lower values for untreated samples. This can be attributed to the possibly greater resistance to the superficial extension of crevice corrosion, deriving from the greater resistance to depassivation of pre-treated surfaces.
AISI 316 stainless steel resistance to crevice corrosion
935
t5.
:'j° g 1.0. E
D []
•
A~D
:¢
o
1:313
D
~= 0.5.
I o
o15
i
f5
O(kC) FIG. 6.
Maximum depth of corrosive attack on AISI 316 vs coulombs passed at the end of the test: • • samples pretreated in HNO3; [] A untreated samples.
CONCLUSIONS F r o m the results of dipping tests in 3.5% NaCi solution and sea water p e r f o r m e d on A I S I 316 stainless steel samples pre-treated or not pre-treated in 2 M H N O ~ , the following conclusions can be drawn. (a) In the conditions used for l a b o r a t o r y dipping tests: (i) p r e - t r e a t m e n t in H N O 3 can p r e v e n t crevice corrosion in 3.5% NaC1 solution; (ii) defects on pre-treated surfaces do not seem to cause any negative effect on the resistance to crevice corrosion; defects on unshielded surfaces are not influential, while those on shielded surfaces would, at worst, nullify the effects of H N O 3 p r e - t r e a t m e n t . (b) In the conditions used for sea water tests: (i) the experimental distribution of c o r r o d e d samples versus time is described by the following e q u a t i o n N(t)
= N()(I - e -'/~)
thus pointing out that the crevice corrosion m e a n induction time is not timed e p e n d e n t ; (ii) pre-treated samples show a crevice corrosion m e a n induction time of 20 days in c o m p a r i s o n with 1.5 day for u n t r e a t e d samples; (iii) the c o n t i n u o u s e x p o s u r e of pre-treated surfaces to a sulphide c o n c e n t r a t i o n that can be f o u n d below shielded surfaces i m m e r s e d in natural sea water, reduces, in the long run, the protective effect of the p r e - t r e a t m e n t ; after the p r e - i m m e r s i o n in anaerobic sediments for 15 days of treated samples, the mean induction time decreases to 9 days; (iv) once the crevice corrosion has started, H N O 3 pre-treated surfaces show a greater localization of corrosive attack than untreated surfaces.
REFERENCES
1. 2. 3. 4. 5. 6.
G. SALVAGOand G. FUMAGALLI,Proc. Interfinish, Vol. 84, p. 441. Jerusalem, Israel (1984). G. SALVAGOand G. FUMAGALLI,Proc. Electrochim. 85, 6b-22. Firenze, Italy (1985). J. M. DEFRANOUX,Corrosion Treatments 19,404 (1971). M. A. BARBOSA,Corros. Sci. 23, 1293 (1983). T. SHIBATAand T. TAKEYAMA,Corrosion 33,243 (1977). R. FRATES1,Corrosion 41,114 (1985).
936
G. SALVAGOet al.
7. G. SALVAGO,Proc. Convegno Conclusivo Progetto Finalizzato Metallurgia C.N.R., 142, Terni, Italy (1985). 8. A. MOLLICA,A. TREVIS, E. TRAVERSO,G. VENTURA,V. SCOTTO,G. ALABISO,G. MARCENARO,U. MONTINI, G. DE CAROLISand R. DELLEPIANE,6th Int. Cong. Marine Corrosion and Fouling, p. 269. Athens (1984). 9. F. L. LAQUE,Mat. Perf. 21, 13 (1982). 10. E. MOR, A. MOLLICA,V. SCOTrO and F. FERRARONE,Proc. IV Int. Cong. Marine Corrosion and Fouling, p. 379. Juan-Les-Pins (1976). 11. V. SCOTTO,R. DI CINTIOand G. MARCENARO,Corros. Sci. 25,185 (1985). 12. R. JOHNSENand E. BARDAL,Corrosion 41,296 (1985). 13. A. MOLLICAand A. TREVIS,Proc. IVInt. Cong. Marine Corrosion and Fouling, p. 351, Juan-Les-Pins (1976). 14. D. E. WILLIAM,C. WESTCOTFand M. FLEISHMANN,Proc. 1XInt. Cong. Metallic Corrosion, Vol. 2, p. 173. Toronto (1984). 15. D. SINIGAGLIA,G. SALVAGO,G. FUMAGALLI,G. TACCANI,G. RONDELLIand B. VICENTINI,Proc. I X Int. Cong. Metallic Corrosion, Vol. 1, p. 245. Toronto (1984). 16. D. ANDERSON,Galvanic and Pitting Corrosion-Field and Laboratory Studies, STP 576, American Society for Testing and Materials, Philadelphia, p. 231 (1976). 17. ASTM G78-83, Crevice Corrosion Testing of Iron-base and Nickel-base Stainless Alloys in Seawater and Other Chloride Containing Aqueous Environments, American Society for Testing and Materials, Philadelphia (1983). 18. ASTM G46-76, Examination and Evaluation of Pitting Corrosion, American Society for Testing and Materials, Philadelphia (1976).
0010--938X/87 $3.(~)+ 0.00 © 1987 Pergamon Journals Ltd.
A S T A T I S T I C A L E V A L U A T I O N OF AISI 316 S T A I N L E S S STEEL R E S I S T A N C E TO C R E V I C E C O R R O S I O N IN 3.5% NaCI S O L U T I O N A N D IN N A T U R A L S E A W A T E R A F T E R P R E - T R E A T M E N T IN H N O 3 G . SALVAGO
and G. FUMAGALLI
Dipartimento di Chimica Fisica Applicata del Politecnico di Milano, Piazza L, da Vinci 32, 20133 Milano, Italy
and A . MOLLICA and G. VENTURA C . N . R . , Istituto per la Corrosione Marina dei Metalli, Via della Mercanzia 4, 16123 G e n o v a , Italy
Abstract--AISI 316 pre-treatment in nitric acid greatly increases the crevice corrosion induction time both in 3.5% NaCI solution and in natural sea water. Pre-treatment defects do not seem to cause any negative effect on the resistance to crevice corrosion, defects on unshielded surfaces are not influential, while those on shielded surfaces would, at worst, nullify the effects of HNO3 pretreatment. In the conditions adopted for sea water tests, the experimental distribution of corroded samples versus time is described by the following equation
N(t) = NIl(1 - e '~). Pre-treated samples show a m e a n crevice corrosion induction time of 20 days in comparison with r 1.5 day for untreated samples. Stainless steel exposures to sulphides in a concentration that can be found below shielded surfaces exposed to natural sea water reduces, in the long run, the protective effect of the pre-treatment. HNO~ pre-treatment tends to increase the localized attack when crevice corrosion propagates.
INTRODUCTION
A I S I 316 STAINLESS STEEL is one of the materials widely used for marine applications. In such an environment crevice corrosion c o m m o n l y occurs. The pre-treatment of stainless steel in H N O 3 alters the corrosion behaviour, although no full agreement has been reached about its practical usefulness. Generally speaking, m a n y corrosion engineers are sceptical about the effects of stainless steel p r e - t r e a t m e n t in a nitric acid solution. Most accelerated corrosion tests seem to point out that the resistance to localized corrosion is enhanced by pre-treating surfaces in nitric acid;~'2 however, some workers d i s a g r e e ) 4 For stainless steel the pitting potential and corrosion potential in sodium chloride span a rather wide range, that becomes even larger for stainless steel pre-treated in H N O > s'6 The potential is time-dependent even in the absence of localized corroManuscript received 11 February 1986; in a m e n d e d form 10 D e c e m b e r 1986. 927
G. SALVAGOetal.
928
sion. 7'8 Thus, the theoretical comparison between the corrosion behaviour of stainless steel pre-treated or not pre-treated in nitric acid turns out to be quite complicated. In addition, pre-treatment defects or accidental damages may give rise to galvanic coupling between the pre-treated surface and the damaged surface. The results of laboratory tests in NaCI solution and field tests can be different owing to the biological factors which can influence stainless steel corrosion in natural sea water. 9 In order to assess the effect of H N O 3 pre-treatment on the crevice corrosion resistance of stainless steels in sea water, it is advisable to carry out tests in natural sea water and in the presence of pre-treatment defects. With this aim, the following tests were performed: (a) dipping tests in sodium chloride solutions on AISI 316 multicrevice samples with and without HNO3 pre-treatment on shielded and unshielded surfaces; (b) field tests o n shielded samples with and without pre-treatment in HNO3, plunged into anaerobic marine sediments and coupled with unshielded surfaces pre-immersed in natural sea water for 15 days. The procedure followed, during the performance of field tests, attempts to simulate, in a reproducible way, mixed aerobic-anaerobic conditions which can be found on samples after long exposure in sea water. Under operational conditions in sea water the presence of a crevice, either pre-existing or created by deposition of macrofouling, is often associated with the presence of sulphides (Fig. la,b). Sulphides at the same concentration as that found below shielded surfaces are noticed in anaerobic sediments in the presence of Desulphovibrio bacteria (Fig. lc). l° Moreover, the behaviour of unshielded surfaces, that play the role of a cathodic area, depends on the deposition of aerobic bacteria. 8'11This deposition of aerobic bacteria on stainless steel surface immersed in quiet sea water and its effect on the kinetics of oxygen reduction are almost independent of the type of stainless steel. The kinetics of the cathodic process reaches an almost steady value after about two weeks' e x p o s t l r e . 8,11,12
EXPERIMENTAL METHOD The chemical composition of the alloys used is shown in Table 1. For the tests carried out in 3.5% NaC1 solution steel A was used, whereas steel B was utilized for field tests in sea water. All samples were annealed and ground with wet emery paper up to grade 120. The pre-treatment in nitric acid was carried out in 2 M H N O 3 at 85°C for 60 min. Laboratory tests were performed in 3.5% NaCI solution at room temperature. Potentiodynamic tests were carried out at a scanning rate of 1 mV s-t; potentiostatic tests were carried out at a potential of 400 mV(SCE), as this potential is the upper limit of the potential observed for stainless steel in sea water. 8`u Dipping tests in aerated 3.5% NaCI solution for 35 days were performed on 100 x 200 mm steel panels having an Anderson type multicrevice assembly. 16,17The multicrevice assembly was centred at a distance of 50 mm from the lower edge and the samples were partially immersed for 150 ram. The exposed surface/solution volume ratio was 1 dm2/10 litres; losses due to evaporation were compensated by addition of distilled water. No filtering was carried out.
TABLE 1.
CHEMICAL COMPOSITION OF STAINLESS STEELS INVESTIGATED
AISI316
C
Cr
Ni
Mo
Si
Mn
S
19
N
A B
0.040 0.044
16.7 17.4
10.6 11.2
2.1 2.2
0.43 0.35
1.5 1.6
0.008 0.023
0.027 0.025
0.029 --
(b) FIG. 1. Local presence of sulphides revealed by black spots (PbS) on lead containing white PVC exposed in sea water. (a) Sulphides under macrofouling (barnacles); (b) sulphides on shielded surface. (Continued) 929
FIG. 1 continued.
Sulphides on strips partially dipped into anaerobic sediments.
930
AISI 316 stainless steel resistance to crevice corrosion
931
To assess the effect of HNO 3 pre-treatment with different defects and damage, dipping tests were performed according to the following schedule: Dipping tests in 3.5% NaCI solution Tests
Shielded surface
Unshielded surface
A B C D
not pre-treated not pre-treated pre-treated pre-treated
not pre-treated pre-treated not pre-treated pre-treated
B: sample obtained by HNO 3 pre-treatment after installation of Anderson type multicrevice assembly. C: sample obtained by repeating manual grinding with wet emery paper after installation of Anderson type multicrevice assembly. Field tests in marine environment were performed on a sample assembled as shown in Fig. 2. AISI 316 discs (¢p 25) assembled on to an India rubber latex gasket (q 12) were dipped in anaerobic sediments drawn from Genoa port through 1 mm mesh screen. The immersion depth into the anaerobic sediment vessel was 111 cm because, according to Ref. 10, this depth ensures the maximum sulphide ccncentration (some hundred ppm). The natural sea water over anaerobic sediments (about 100 1) was quiet and completely rechanged by continuous flow every 6 h. The temperature was set at 30°C. Each AISI 316 sample was coupled, by means of the resistor R = 100 ,Q, with a 100 cm 2 254 SMO unsbielded surface which was previously immersed in sea water for 15 days. The super-austenitic 254 SMO stainless steel (20Cr-18Ni~Mo) was chosen to minimize the chances for localized corrosion occurring on this surface, taking into account that the cathodic behaviour in sea water is almost independent of the kind of stainless steel used. 12The potential difference measured at the erLds of the resistor R was used to calculate galvanic currents. The initiation of crevice corrosion was cc nsidered to be at that time when galvanic current exceeded 10 p,A (potential difference at the ends of R, lmV). In order to evaluate the effects of HNO3 pre-treatment and the effects of the time of exposure to sulpbides on the resistance to crevice corrosion, four tests were performed according to the following schedule: Field tests Tests
Nsamples
Ii
2(I
F G
2(I 10
tI
10
AIS1316 shielded surface pre-treated in HNO 3prc-immersed in anaerobic sediments for 15 days pre-treatedin HNO~,not pre-immerscd not pre-treated, pre-immersed in anaerobic sediments for 15 days not pre-trcated, not pre-immersed
EXPERIMENTAL
RESULTS AND
254 SMO unshieldcd surfacc pre-immersed in natural sea water for 15 days ibid. ibM.
ibM.
DISCUSSION
L a b o r a t o r y tests F i g u r e 3 i l l u s t r a t e s t h e p o t e n t i a l b e h a v i o u r o f A I S I 316 s a m p l e s d u r i n g p r o t r e a t m e n t in H N O 3 . T h r e e d i f f e r e n t s t a g e s c a n b e i d e n t i f i e d : (a) d i s s o l u t i o n p h e n o m e n a c a n b e c l e a r l y n o t i c e d ; t h e p o t e n t i a l b e h a v i o u r is s e v e r e l y a f f e c t e d b y t h e p r e s e n c e o f i r o n i o n s in n i t r i c a c i d ; ( b ) a g r a d u a l p o t e n t i a l i n c r e a s e o c c u r s - - t h e d u r a t i o n o f t h i s s t a g e d e p e n d s o n t h e s t e e l s u r f a c e t y p e o f f i n i s h i n g as well as o n t h e
932
G. SALVAGOetal.
254 SMO
sea v,~ter
anaerobic
sediment FIG. 2.
specimen
holder
Scheme of e q u i p m e n t for field test.
nitric acid concentration and temperature; and (c) some instability of the potential may appear. The above-described behaviour has been observed for other stainless steels too, and, in correspondence with the third stage, localized corrosion could be observed. J.2 Limiting HNO3 treatment to the second stage, electrochemical tests carried out in aerated 3.5% NaCI solution show a general increase in the free corrosion potential and in the pitting potential as well as a reduction of passivity currents. If the treatment is performed at 85°C with 2 M nitric acid, the second stage lasts 78 h and pre-treated AISI 316 samples do not show pitting corrosion during potentiodynamic tests. Moreover, the potentiostatic test carried out at +400 mV(SCE) proved that this localized corrosion induction time was not lower than 100 h for the same samples. The results of potentiodynamic and potentiostatic tests, after controlled pretreatment of AISI 316 in H N O 3, a r e similar to those normally obtained on materials having higher chromium and molybdenum content and greater resistance to localized corrosion. From the dipping test in aerated 3.5% NaCI solution carried out on multicrevice samples, the following number of etching sites was detected Iv and
OS.
CO
w 0
I
n tog t
FIG. 3.
Schematic time behaviour of the AISI 316 corrosion potential in H N O 3 solution.
AISI 316 stainless steel resistance to crevice corrosion
051 E vs SCE(V)
~°,~t,r
0
10 "~,,.~ _
_
{
_
....
I
1,0 k ..,,...~ [ (days)
0.5
start of crevice corrosion
t coupting
500 II' ('uA) 0" . . . . 0
Fie,. 4.
933
_ j :
10
t (days)
Trend of corrosion potential and galwmic current before and after coupling between shielded (AISI 316) surface and unshielded (254 SMO) surface.
microscopically confirmed ~8(over a total of 64 crevice positions for each sample): test A, untreated samples, etching sites = 4 test B, only unshielded surface pre-treated in HNO3, etching sites = 3 test C, only shielded surface pre-treated in HNO3, etching sites = 0 test D, samples completely pre-treated in HNO3, etching sites = 0. No pitting corrosion was observed. Although dipping tests are not very significant due to the low number of etching sites observed, they seem to indicate: (a) a clear beneficial effect of the pre-treatment in nitric acid when the surface is not damaged or when only the unshielded surface is damaged, and (b) the absence of negative effects when also shielded surfaces are damaged. Field tests
A typical trend of the current and potential for an AISI 316-254 SMO couple, chosen from test E, is shown in Fig. 4. Before coupling, as for 254 SMO unshielded surfaces immersed in renewed natural sea water, a free corrosion potential increase was observed from its initial value up to over 300 mV(SCE), thus confirming the presence of aerobic microbial activity on them, according to Refs 8, 11, 13; as for AISI 316 shielded surfaces pre-immersed in anaerobic sediments (tests E and G), a corrosion potential drop could be noted from its initial value down to -450 to - 4 8 0 mV(SCE) regardless of pre-treatment in HNO3, thus confirming the reducing conditions present in the sediments. After coupling, mixed potentials and galvanic currents were periodically checked. The initiation of corrosion was considered to be the time when galvanic current exceeded the value of 10/~A, along with a simultaneous potential drop. The type of corrosion observed was crevice corrosion. Figure 5 shows the percentage of corroded samples versus time. To interpolate test results, reference was made to Weibull statistics. The reliability function R(t) = e x p ( ( t - ~ z y ) / ~ ) where cz, fl and V are scale, shape and position parameters, was optimizcd to
G. SALVAGOetal.
934 100
o ~.~-~
i"
,~...'-";""- " ::o
~50
¢ #t •
#
/
~
'A #
"¶/
oo .
.
"r= 1.5
.
;
1/
.l
~J •
ss A •
,
.
.
.
.
.
F16. 5. Corroded samples percentages vs time after coupling between shielded and unshielded surface: [] shielded surface untreated and pre-immersed into anaerobic sedim e n t s for 15 days; z5 shielded surface untreated and not pre-immersed; • shielded surface treated in HNO~ and pre-immersed; • shielded surface treated in H N O 3 and not preimmersed.
experimental data, assuming as time-origin the moment corresponding to anode cathode coupling. The parameters ), and fl, that gave the best fit to the data, correspond to ), = 0, fl = 1. Under these conditions, assuming a = ~, the general distribution is reduced to an exponential distribution.
N(t)
= N0(1 - e -'/~)
where N(t) means the number of corroded samples in a given (t) time. The curves shown in Fig. 5 were calculated by optimizing r value in the different cases. The experimental data fit the exponential distribution, indicating that the crevice corrosion mean induction time is not dependent on time. This observation is at least formally similar to that already suggested in the case of pitting corrosion.a4 From these experimental results, it was possible to conclude that: (a) The mean induction time for crevice corrosion on untreated samples, pre-immersed or not in sulphides (tests G and H) is very short (1.5 day). (b) HNO3 pre-treatment, in agreement with, is lengthens the time necessary for localized corrosion to start. For AISI 316 samples pre-treated and not pre-immersed in anaerobic sediments before coupling (test F), the mean induction time is about one order of magnitude (20 days) longer than untreated AISI 316 samples. (c) The pre-immersion of treated AISI 316 samples in mud having reducing conditions deriving from the presence of sulphides at a concentration that can be found in natural environments reduces the crevice corrosion induction time. Under test conditions, pre-immersion into anaerobic sediments for 15 days (test E) reduces to a half the mean induction time (from 20 days to 9 days). The maximum depth of attack observed on each single sample 18 at the end of the test versus coulombs is shown in Fig. 6. The results indicate that in the case where attack developed, corresponding to the passage of 100 C charge, the depth of attack tends to vary, assuming the lower values for untreated samples. This can be attributed to the possibly greater resistance to the superficial extension of crevice corrosion, deriving from the greater resistance to depassivation of pre-treated surfaces.
AISI 316 stainless steel resistance to crevice corrosion
935
t5.
:'j° g 1.0. E
D []
•
A~D
:¢
o
1:313
D
~= 0.5.
I o
o15
i
f5
O(kC) FIG. 6.
Maximum depth of corrosive attack on AISI 316 vs coulombs passed at the end of the test: • • samples pretreated in HNO3; [] A untreated samples.
CONCLUSIONS F r o m the results of dipping tests in 3.5% NaCi solution and sea water p e r f o r m e d on A I S I 316 stainless steel samples pre-treated or not pre-treated in 2 M H N O ~ , the following conclusions can be drawn. (a) In the conditions used for l a b o r a t o r y dipping tests: (i) p r e - t r e a t m e n t in H N O 3 can p r e v e n t crevice corrosion in 3.5% NaC1 solution; (ii) defects on pre-treated surfaces do not seem to cause any negative effect on the resistance to crevice corrosion; defects on unshielded surfaces are not influential, while those on shielded surfaces would, at worst, nullify the effects of H N O 3 p r e - t r e a t m e n t . (b) In the conditions used for sea water tests: (i) the experimental distribution of c o r r o d e d samples versus time is described by the following e q u a t i o n N(t)
= N()(I - e -'/~)
thus pointing out that the crevice corrosion m e a n induction time is not timed e p e n d e n t ; (ii) pre-treated samples show a crevice corrosion m e a n induction time of 20 days in c o m p a r i s o n with 1.5 day for u n t r e a t e d samples; (iii) the c o n t i n u o u s e x p o s u r e of pre-treated surfaces to a sulphide c o n c e n t r a t i o n that can be f o u n d below shielded surfaces i m m e r s e d in natural sea water, reduces, in the long run, the protective effect of the p r e - t r e a t m e n t ; after the p r e - i m m e r s i o n in anaerobic sediments for 15 days of treated samples, the mean induction time decreases to 9 days; (iv) once the crevice corrosion has started, H N O 3 pre-treated surfaces show a greater localization of corrosive attack than untreated surfaces.
REFERENCES
1. 2. 3. 4. 5. 6.
G. SALVAGOand G. FUMAGALLI,Proc. Interfinish, Vol. 84, p. 441. Jerusalem, Israel (1984). G. SALVAGOand G. FUMAGALLI,Proc. Electrochim. 85, 6b-22. Firenze, Italy (1985). J. M. DEFRANOUX,Corrosion Treatments 19,404 (1971). M. A. BARBOSA,Corros. Sci. 23, 1293 (1983). T. SHIBATAand T. TAKEYAMA,Corrosion 33,243 (1977). R. FRATES1,Corrosion 41,114 (1985).
936
G. SALVAGOet al.
7. G. SALVAGO,Proc. Convegno Conclusivo Progetto Finalizzato Metallurgia C.N.R., 142, Terni, Italy (1985). 8. A. MOLLICA,A. TREVIS, E. TRAVERSO,G. VENTURA,V. SCOTTO,G. ALABISO,G. MARCENARO,U. MONTINI, G. DE CAROLISand R. DELLEPIANE,6th Int. Cong. Marine Corrosion and Fouling, p. 269. Athens (1984). 9. F. L. LAQUE,Mat. Perf. 21, 13 (1982). 10. E. MOR, A. MOLLICA,V. SCOTrO and F. FERRARONE,Proc. IV Int. Cong. Marine Corrosion and Fouling, p. 379. Juan-Les-Pins (1976). 11. V. SCOTTO,R. DI CINTIOand G. MARCENARO,Corros. Sci. 25,185 (1985). 12. R. JOHNSENand E. BARDAL,Corrosion 41,296 (1985). 13. A. MOLLICAand A. TREVIS,Proc. IVInt. Cong. Marine Corrosion and Fouling, p. 351, Juan-Les-Pins (1976). 14. D. E. WILLIAM,C. WESTCOTFand M. FLEISHMANN,Proc. 1XInt. Cong. Metallic Corrosion, Vol. 2, p. 173. Toronto (1984). 15. D. SINIGAGLIA,G. SALVAGO,G. FUMAGALLI,G. TACCANI,G. RONDELLIand B. VICENTINI,Proc. I X Int. Cong. Metallic Corrosion, Vol. 1, p. 245. Toronto (1984). 16. D. ANDERSON,Galvanic and Pitting Corrosion-Field and Laboratory Studies, STP 576, American Society for Testing and Materials, Philadelphia, p. 231 (1976). 17. ASTM G78-83, Crevice Corrosion Testing of Iron-base and Nickel-base Stainless Alloys in Seawater and Other Chloride Containing Aqueous Environments, American Society for Testing and Materials, Philadelphia (1983). 18. ASTM G46-76, Examination and Evaluation of Pitting Corrosion, American Society for Testing and Materials, Philadelphia (1976).