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Assessing the possibility of stress corrosion cracking during acid cleaning
Pergamon Press Ltd. 1980. Printed in Great Britain. Corrosion Science, Vol. 20, pp. 687 to 706
ASSESSING THE POSSIBILITY OF STRESS CORROSION CRACKING DURING ACID CLEANING*t BRYAN POULSON a n d RUSSELL ROBINSON NEI Clarke C h a p m a n Power Engineering Ltd, Gateshead, England Abstract--During chemical cleaning large volumes of inhibited acids are pumped through boilers and there is a possibility of stress corrosion cracking occurring under certain conditions. This paper examines four material/environmental combinations encountered in practice: mild steel in inhibited hydrochloric acid, mild steel, a 970Cr-1 7oMO steel and a sensitized austenitic stainless steel in inhibited citric acid. The electrochemistry of each is examined, various predictability tests performed, and these compared to the results of slow strain-rate stress corrosion tests. No stress corrosion occurred with the material-environmental-stress conditions which should be encountered in practice; however, cracking could be induced to occur by out of specification conditions. The results of the predictability tests did not correlate with the observed cracking susceptibilities. However this lack of agreement could be explained with a detailed knowledge of the electrochemistry involved in each system. INTRODUCTION
A MODERN power station boiler represents a considerable investment. The cost of lost electricity caused by any unplanned shutdown can be very expensive. It is thus very important to ensure that any potentially hazardous corrosion problems are minimized. Thus in recent years there have been large research efforts into both fireside1," and waterside 3'~ corrosion problems. In general this had led to a much better understanding of possible corrosion hazards, and their prevention, during boiler operation. However there is one circumstance when potentially damaging chemicals are deliberately pumped through the boiler. This is during acid cleaning. In general acid cleaning is carried out for one or more of the following reasons: (1) to produce an initially clean surface for subsequent passivation, (2) to remove porous oxides which could cause enhanced corrosion, (3) to remedy hydrodynamic deviations caused by oxide growth or deposition, (4) to reduce activity levels, and (5) to reduce the possibility of tube blockage by exfoliated oxide. Acid cleaning reagents are formulated to minimize metal dissolution and hydrogen adsorption and maximize oxide dissolution. However the use of such potentially aggressive acids is not without its corrosion problems. These have included: pitting after repeated cleaning, 5 pitting caused by copper contamination, 6 attack at crevices, 7 problems with hydrogen damaged steel, ~ attack of stressed regions, 7 preferential corrosion of heat affected zones of welds, '%1° galvanic corrosion at dissimilar metal joints n and stress corrosion cracking (s.c.c.). 11 It is well known that to avoid s.c.c., either during cleaning or in subsequent operation, solutions involving chloride and possibly fluoride should not come into *Manuscript received 1 December 1978. "l-Paper presented at the conference on "Electrochemical Test Methods for Stress Corrosion Cracking", organized by the Working Party on Stress Corrosion Cracking Test Methods of the European Federation of Corrosion and held at Firminy, France, 1%21 September 1978. 687
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BRYANPOULSONand RUSSELLROBINSON
contact with austenitic stainless steels. Similarly, solutions involving hydroxides should be treated with the u t m o s t care to avoid retention in crevices, a n d preferably be avoided. 11 A l t h o u g h it m u s t be emphasized that apart from these situations there have been n o reported cases o f cracking, two areas o f possible concern were identified, which p r o m p t e d this investigation. These were: 1. Solutions where a n o d i c inhibitors are used, since this has been identified as a situation which can cause s.c.c, o f c a r b o n steels. 1~ 2. The presence o f sensitized stainless steels, since this has in n u m e r o u s other instances led to cracking, often involving a p p a r e n t l y i n n o c u o u s e n v i r o n m e n t s ; it has recently been s h o w n la that a 9 % C r - 1 % M o steel c a n be sensitized. Because m o d e r n boilers are usually constructed out o f a n u m b e r of materials, it was felt desirable to examine the possibility o f s.c.c, occurring in the following systems: c a r b o n steel in inhibited hydrochloric acid; c a r b o n steel, a 9 ~ C r - 1 ~ M o steel a n d a sensitized type 316 stainless steel i n inhibited citric acid. EXPERIMENTAL METHOD Steels to be tested were obtained in a number of forms, details and analysis of which are shown in Table 1. Stress corrosion test specimens were of three types: 5 mm wide C-ring specimens from production tubing, stressed to well above yield using a spacer, and 7 in. long tensile specimens with either a 0.5 in. gauge length or a 55° notch in both cases to a reduced cross section of 0.1 in. diameter. Specimens for electrochemical measurements were drilled and tapped to fit a Stern-Makrides compression holder. Mild steel was annealed for 0.5 h at 950°C in an argon atmosphere. 9Cr-I Mo steel was held for 0.5 h at 950°C in an argon atmosphere and then water quenched, followed by tempering in a salt bath for times up to 8 rain or an argon atmosphere for longer times. Type 316 stainless steel was solution annealed at 1050°C for 20 min in an argon atmosphere and then sensitized at 650°C for 16h. TABLE I.
ANALYSTS OF STEELS USED
Composition Designation
C
Mild steel rod 0.1 Mild steel tube 0.08 5Cr-lMo 0.1 9Cr-I Mo tube 0.1 9Cr-l Mo rod 0.1 Type 316 stainless 0.05 Inconel 600 0.04
Mn
Si
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0.02 0.03 0.004 0.017 0.008 0.012 0.008
0.03 0.04 0.013 0.017 0.014 0.025 0.01
Ni
12.04 7.4
Cr
Mo
4.97 9.07 8.55 17.63 16.1
0.56 0.1 0.99 2.53
Others
9.4Fe
The inhibited citric acid (3 or 3.5 wt. ~o citric acid + 0.05 wt. ~o Starmine LTP and ammonia to pH 3.5 to 4 at 80 or 90°C) and the inhibited hydrochloric acid (5 wt. %HCI + 0.25 wt. ~o Armohib 28 at 75°C) were both de-oxygenated using white spot nitrogen. Tensile slow strain-rate s.c.c, tests were performed in a PTFE cell in a motorized hard beam tensometer with load recording facilities. All other tests were performed in a multi-necked 1 1. flask. Potentials were measured with an external saturated calomel reference electrode (SCE) at room temperature, and are reported on that scale. Potential-time curves were recorded on a chart recorder using a potentiostat as a high impedance voltmeter. Fast and slow anodic polarization curves were obtained using a linear sweep generator to drive the potentiostat, the current being recorded on the chart recorder by manual switching between ranges. Fast straining electrode tests were performed in the PTFE cell mounted in a motorized Hounsfield tensometer. Specimens for constant potential etching were metallographically polished and inserted into the cell at the required potential. All specimens for examination in the scanning electron microscope (SEM) were ultrasonically cleaned in methanol immediately the test finished and stored until required.
Assessing the possibility of stress corrosion cracking during acid cleaning
689
EXPERIMENTAL RESULTS Stress corrosion tests (a) M i l d steel in inhibited hydrochloric acid. Typical stress-time curves obtained
during slow strain-rate (1.2 × 10-5 s -1) stress corrosion tests on smooth specimens in inhibited hydrochloric acid are shown in Fig. 1. It might be thought that cracking had occurred at -- 400 mV (SCE). Metallographic examination indicated otherwise; in particular failure was due to pitting (Fig. 2). At more negative potentials some end grain attack occurred after failure and prior to specimen removal, but no cracking. (b) M i l d steel in inhibited citric acid. Annealed mild steel notched samples were tested at a slow strain rate (crosshead velocity of 1.1 / 10-6 in. s -1) over a range of potentials in inhibited citric acid, the results being shown in Fig. 3. It can be seen that in this most severe test cracking occurred at potentials below ca. - - 400 mV (SCE). Such failures had fractographic featm es (Fig. 4) which were typical of hydrogen embrittlement in low strength carbon steels32 Tests on highly stressed C-rings failed to reveal any cracking susceptibility, even at potentials down to -- 800 mV (SCE). (c) 9Cr-I Mo in inhibited citric acid. A number of preliminary tests had shown that tests under freely corroding conditions were not very reproducible. Therefore the effect of potential on the s.c.c, of sensitized (950°C WQ + 1 min at 750°C) C-rings was examined. The results are shown in Fig. 5 and it can be seen that stress corrosion occurred over the potential range -- 550 to -- 200 mV (SCE). Outside this range neither pitting nor general corrosion was apparent. C-ring specimens which had been quenched and then tempered for 30 min at 750°C showed no indication of cracking at any potential. Quenched C-ring samples failed due to hydrogen embrittlement below -- 850 mV (SCE): in the more severe slow strain-rate test quenched specimens failed by hydrogen embrittlement under freely corroding conditions. A more detailed examination of the effects of heat treatment was performed by exposing C-ring specimens to inhibited citric acid at the potential of maximum susceptibility to s.c.c. -- 300 mV (SCE). The effects of varying the tempering temperature for a 30 min tempering time is shown in Fig. 6 which also includes the results of some hydrogen embrittlement studies. The results of all such heat-treatments can be presented in the form of a parametric plot as shown in Fig. 7. In an attempt to examine the possible occurrence of a static threshold stress both constant displacement and slow strain-rate tests were performed on 9Cr-1Mo tensile specimens (950°C WQ, tempered 30 rain at 470°C) in inhibited citric acid at -- 300 mV (SCE). The results of these tests are presented in Fig. 8. While there are a limited number of tests it is clear that cracking occurs at very low stresses. This is reflected in the time-to-fail ratio (time to fail in citric acid/time to fail in air) of 0.4 obtained in slow strain-rate tests. C-ring specimens which had failed due to s.c.c, typically contained a number of intergranular cracks (Fig. 9a) with some secondary cracking along martensite lath boundaries (Fig. 9b). The amount of this secondary cracking varied as is shown in Fig. 9c, d and appeared to increase with decreasing hardness. C-ring specimens which had failed by hydrogen embrittlement contained fewer cracks, usually one or two (Fig. 10a). These tended to be transgranular (Fig. 10b) and the fracture surface was a mixture of quasi-cleavage and pseudo-dimples (Fig. 10c)
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FIG. 11. Potential time curves and potential ranges promoting SCC of sensitized 97ooCr-1~Mo steel in inhibited citric acid. although some intergranular cracking was usually present, particularly in quenched specimens (Fig. 10d). (d) Sensitized type 316 stainless steel in inhibited citric acid. Sensitized type 316 stainless steel notched samples were tested at a slow strain rate (cross head velocity of 1.1 x 10 -6 in. s -1) over a range of potentials in inhibited citric acid. Apart from one specimen which showed mild susceptibility when tested at -- 100 mV no cracking was observed; attempts to reproduce this susceptibility were unsuccessful. There was reason to believe that the inhibited citric acid had been overheated, possibly partially destroying the inhibitor, and tests in boiling inhibited citric acid at -- 100 mV SCE reproduced the observed intergranular cracking. Electrochemical measurements Potential-time curves. Potential-time curves for polished, and oxidized (950°C for
1 h) 9 C r - l M o samples as well as 9Cr-lMo/316, 9Cr-lMo/Inconel 600, 9 C r - l M o / 5 C r - l M o and 9Cr-1Mo/mild steel couples in inhibited citric acid are shown in Fig. 11, together with the potential range promoting s.c.c, of sensitized 9Cr-lMo. Heat treatment of 9 C r - l M o steel had only a slight effect on this behaviour; increasing tempering caused the free corrosion potential to shift to more negative values. It is clear from Fig. 11 that the free corrosion potential can lie in the range causing cracking especially if the sample is oxidized, coupled to type 316 stainless or [nconel 600, or if small amounts of oxygen are present. It is also clear that the effect of oxygen on the free corrosion potential is large and not immediately reversible. Anodic polarization curves. Potentiodynamic anodic polarization curves for mild steel in both inhibited and uninhibited hydrochloric acid are shown in Fig. 12. It appears that Armohib 28 acts as an anodic inhibitor. There is considerable hysteresis between positive and negative sweeping slow polarization curves, suggesting that pitting is occurring. This results in higher currents being measured at slow sweep rates than fast rates, above about -- 390 mV (SCE). Stepped polarization curves (20 mV every 10 rain) were carried out. The results are shown in Fig. 13. It can be seen that at potentials of -- 460 mV (SCE) and below the current decreases with time: -- 420 mV (SCE) and above it increases with time, again indicative of pitting. It would appear that under these conditions there is no induction time for pitting. Fast (1000 mV/min) and slow (10 mV/min) anodic polarization curves for mild steel, 5Cr, 316, [nconel 600, and 9 C r - l M o after different heat treatments were
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Assessing the possibility of stress corrosion cracking during acid cleaning
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obtained in inhibited citric acid. I m p o r t a n t points observed were: (a) All materials except mild steel exhibit art active-passive transition with increasing potential (Fig. 14) or are passive under freely corroding conditions. With 5Cr steel the active to passive potential is outside the range of potentials of interest. (b) There are only small effects of heat treatment on the behaviour of 9 C r - l M o (Fig. 15). (c) O f the materials studied the largest differences between the peak currents of slow and fast polarization curves were for 5Cr and 9 C r - l M o steels (Table 2). (d) The passive film formed on 9 C r - l M o material at anodic potentials appeared quite stable during negative going potential sweeps from the passive range and no "reactivation" occurred. TABLE 2.
ELECTROCHEMICAL
P A R A M E T E R S FROM
ANODIC
POLARIZATION
Fast
Slow
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Alloy Mild steel 5Cr-lMo 9Cr-1Mo quenched 9Cr-I Mo sensitized 9Cr-I Mo correctly heat treated 316
29.88 0.085 0.075 0.146 0.017
CURVES
Emax [mV(SCE)]
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Emax [mV(SCE)]
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Potentiostatic etching (a) Mild steel in inhibited hydrochloric acid. The rate of dissolution increased with increasing potential (Fig. 12) and above a certain potential pitting occurred. In all tests sulphide inclusions dissolved rapidly with the carbides remaining unattacked. When pitting occurred it invariably initiated from a sulphide and then proceeded to form a cavity with a covering veil of intact metal. Representative surface morphologies are shown in Fig. 16.
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(b) Mild steel in inhibited citric acid. Although very small pits occurred at the free corrosion potential (Fig. 17a) more positive potentials promoted general dissolution rather than the deep localized attack observed in inhibited hydrochloric acid. Again sulphide inclusions dissolved preferentially and crystallographic etch pits formed during the initial stage of dissolution at high potentials (Fig. 17b). (c) 9Cr-lMo in inhibited citric acid. On metallographically polished samples exposed to inhibited citric acid at potentials promoting cracking (of sensitized samples), interference thickness oxide films formed and specimens were lightly etched. Intergranular corrosion did not occur on sensitized specimens but some cracking did, due to residual quenching stresses--again indicating the low threshold stress for cracking. (d) Sensitized type 316 stainless steel in inhibited citric acid. Specimens dissolved at a very low rate at all potentials with no noticeable etching. Straining electrode experiments Tensile 9Cr-lMo s.c.c, specimens were held unstressed at -- 300 mV (SCE) in inhibited citiic acid for 1 h during which time the current decayed to nearly steady state conditions. The specimen was then rapidly strained to fracture at 200 %/min. and the current recorded. A typical result is shown in Fig. 18. If it is assumed that the metal is ductile and the protective film brittle then the per cent bare metal exposed at any strain can be calculated from ref 14. L,y, % bare metal : 100 1 -- L~e] L ~ = initial specimen length; L f : final specimen length or for small strains % bare metal approx = ½% strain.
Assessing the possibility of stress corrosion cracking during acid cleaning
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Thus from the peak current obtained in the yielding electrode experiment the current density on the newly created bare metal cart be calculated. The results of such calculations have been tabulated in Table 3. It is apparent that these dissolution rates are more than adequate in explaining the observed cracking rates. However, the differences between specimens having different heat treatment are relatively small and do not reflect the cracking tendency of the sensitized material.
TABLE3. RESULTSOFYIELDINGELECTRODEEXPERIMENTSON9Cr-1Mo Calculated bare metal dissolution rate at --300 mV (SCE)
Heat treatment Quenched Sensitized (950°C WQ + 30 min at 475°C) Tempered (950°C WQ + 30 min at 750°C
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DISCUSSION
Practical aspects Although cracking could be induced in mild steel, 9Cr-1Mo and type 316 stainless steel in inhibited citric acid, this is most unlikely to occur in practice, but for different reasons in each system.
704
BRYAN POULSON and RUSSELL ROBINSON
Notched mild steel specimens failed in slow strain-rate tests---due to hydrogen embrittlement--in inhibited citric under freely corroding conditions. However no failures occurred when carbon steel tubing was highly stressed over a range of potentials down to -- 800 mV (SCE). Similarly, sensitized 9Cr-lMo steel failed at very low stresses and at potentials which could occur in practice. This is not worrying if 9Cr-lMo is used in the correctly heat-treated condition. Thus both tempering and post-weld heat treatments are specified to lie in the safe region of Fig. 19, with a maximum hardness criteria of about 260 Hv. This is obviously compositional dependent and is being examined for the higher Si compositions currently favoured for their increased resistance to breakaway oxidation in CO2 based coolants. The occurrence of some cracking in highly sensitized type 316 stainless steel in inhibited citric acid is not fully understood. It does appear to require above normal temperatures which conceivably could promote inhibitor breakdown. Some support for this could be inferred from the observation that when cracking did occur it was associated with above normal corrosion rates. This cracking appears to require high stresses, a sensitized structure and some degradation of the inhibited citric; it is thus unlikely to be of concern. However it does emphasize that sensitized stainless steels are prone to intergranular s.c.c, and any proposed cleaning solution should be assessed for its aggressivity if sensitized material is present. NO ATTACK 750
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Assessing the possibility of stress corrosion cracking during acid cleaning
705
THEORETICAL ASPECTS From a predictive viewpoint the electrochemical measurements suggested: 1. Mild steel should suffer s.c.c, in inhibited hydrochloric acid. 2. Mild steel should not crack in inhibited citric acid. 3. 9 C r - l M o steel should not crack in inhibited citric acid. 4. Sensitized type 316 should not crack in inhibited citric acid. However, as Parkins 16 has repeatedly emphasized there are also structural factors which must be taken into account in predicting the possibility of cracking. Thus the mild steel does not crack in inhibited hydrochloric acid because there is neither a pre-existing or strain induced active path which can suffer localized restricted dissolution in inhibited hydrochloric acid; a similar situation has previously been reported for mild steel in concentrated sulphuric acid. 17 Furthermore in inhibited hydrochloric acid, the solution which forms in pits or crevices is more aggressive than the bulk solution, possibly due to inhibitor poisoning, and it is unlikely that restricted localized dissolution could occur. The occurrence of cracking in sensitized 9 C r - l M o in inhibited citric acid--when none was predicted--is due to a region depleted in chromium influencing the localized dissolution/repassivation behaviour but not the bulk electrochemical properties. Earlier it was suggested 1~ that electrochemical measurements, and in particular weight loss measurements after potentiostatic etching at + 100 mV (SCE) or reactivation potentials, obtained from polarization curves, would indicate sensitized material. In Fig. 20 crack velocities in citric acid are compared with various measures of sensitization. It can be seen that susceptibility to s.c.c, is displaced in relation to tbe weight loss curve. In particular, specimens quenched and then tempered at low tepperatures (e.g. down to 300°C) are susceptible to s.c.c, but this is not expected from their weight loss. This apparent anomaly is thought to be due to appreciable carbide precipitation being required to caused increased weight loss, while intergranular s.c.c, can occur before this stage is reached. This is still under investigation.
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706
BRYANPOULSONand RUSSELLROBINSON CONCLUSIONS
1. The possibility o f s.c.c, occurring during acid cleaning has been assessed in the following systems: mild steel in inhibited hydrochloric acid, mild steel, a 9 C r 1Mo steel and a sensitized stainless steel in inhibited citric acid. N o cracking occurred with the material/environmental/stress conditions which should be encountered in practice. 2. However, cracking could be induced to occur in inhibited citric acid by: (a) Mild steel: testing under unrealistically severe mechanical conditions, i.e. a slow strain-rate test using a notched sample. (b) 9 C r - I M o : if the material is hard or sensitized. (c) Sensitized type 316 stainless steel: if the environment was degraded by boiling. 3. Electrochemical s.c.c, predictability tests were unsuccessful in that they predicted cracking o f mild steel in inhibited hydrochloric acid and did not indicate the possibility o f sensitized 9 C r - l M o failing in inhibited citric acid. 4. It is recommended that although electrochemical measurements can give an indication if possible reactions, slow strain rate s.c.c, tests should always be carried out over a range o f potentials, and possibly including out o f specification heattreatment and environmental conditions. Tests under realistic conditions can then be used to decide if cracking will occur. Acknowledgements--The authors thank the management of NEI Clarke Chapman Power Engineering
Ltd and the CEGB who have been associated with this work, for permission to publish the outcome of the developments and the results. Thanks also must go to the staff of the SEM in the Department of Chemistry, University of Neweastle-upon-Tyne for their help. REFERENCES 1. Proceedings of Conference on Corrosion of Steels in CO~, BNES, London (1974). 2. C. S. TEDMAN(Ed.), Corrosion Problems in Energy Conversion and Generation, Electrochemical
Soc., Princeton (1974). 3. Proceedings of Conference on Effects of Environment on Materials Properties in Nuclear Systems,
Inst. of Civil Engineers, London (1971). 4. Proceedings of Conference on Ferritic Steels for Fast Reactor, BNES, London (1978). 5. J. K. RICE, Combustion 33, 45 (1961). 6. A. S. PEARCEand R. N. PARKINS,Br. Corros. J. 3, 70 (1968). 7. J. K. RICE and C. M. LOUCKS,Materials, Prot. 4, 14 (1965). 8. C. SETTLE,P. J. MITCHELand D. FISHER, Corros. Sci. 6, 33 1966). 9. L. T. OVERSTREET,Materials Prot. 2, 48 (1963).
10. B. POULSON,in preparation. 11. J. A. AYRES,Decontamination of Nuclear Reactors and Equipment (Edited by J. A. AYRES).Ronald, New York (1970). 12. B. POULSON,Corros. Sci. 15, 469 (1975). 13. B. POULSON,Corros. Sci. 18, 371 (1978). 14. S. F. BUBORand D. A. VERMILYEA,.jr electrochem. Soc. 113, 892 (1966). 15. R. L. COWAN on C. S. TEDMAN, Intergranular Corrosion of Iron-Nickel-Chromium Alloys. Advances in Corrosion Science, Vol. 3. Plenum, New York (1973). 16. R. N. PARK1NS,The Theory of Stress Corrosion Cracking in Alloys (Edited by J. C. SCULLY),p. 167. NATO, Brussels (1971). 17. B. POULSON,in Proceedings of Conference Testing and Monitoring. Inst. of Corrosion Sci. and Technology, London (1978).
ASSESSING THE POSSIBILITY OF STRESS CORROSION CRACKING DURING ACID CLEANING*t BRYAN POULSON a n d RUSSELL ROBINSON NEI Clarke C h a p m a n Power Engineering Ltd, Gateshead, England Abstract--During chemical cleaning large volumes of inhibited acids are pumped through boilers and there is a possibility of stress corrosion cracking occurring under certain conditions. This paper examines four material/environmental combinations encountered in practice: mild steel in inhibited hydrochloric acid, mild steel, a 970Cr-1 7oMO steel and a sensitized austenitic stainless steel in inhibited citric acid. The electrochemistry of each is examined, various predictability tests performed, and these compared to the results of slow strain-rate stress corrosion tests. No stress corrosion occurred with the material-environmental-stress conditions which should be encountered in practice; however, cracking could be induced to occur by out of specification conditions. The results of the predictability tests did not correlate with the observed cracking susceptibilities. However this lack of agreement could be explained with a detailed knowledge of the electrochemistry involved in each system. INTRODUCTION
A MODERN power station boiler represents a considerable investment. The cost of lost electricity caused by any unplanned shutdown can be very expensive. It is thus very important to ensure that any potentially hazardous corrosion problems are minimized. Thus in recent years there have been large research efforts into both fireside1," and waterside 3'~ corrosion problems. In general this had led to a much better understanding of possible corrosion hazards, and their prevention, during boiler operation. However there is one circumstance when potentially damaging chemicals are deliberately pumped through the boiler. This is during acid cleaning. In general acid cleaning is carried out for one or more of the following reasons: (1) to produce an initially clean surface for subsequent passivation, (2) to remove porous oxides which could cause enhanced corrosion, (3) to remedy hydrodynamic deviations caused by oxide growth or deposition, (4) to reduce activity levels, and (5) to reduce the possibility of tube blockage by exfoliated oxide. Acid cleaning reagents are formulated to minimize metal dissolution and hydrogen adsorption and maximize oxide dissolution. However the use of such potentially aggressive acids is not without its corrosion problems. These have included: pitting after repeated cleaning, 5 pitting caused by copper contamination, 6 attack at crevices, 7 problems with hydrogen damaged steel, ~ attack of stressed regions, 7 preferential corrosion of heat affected zones of welds, '%1° galvanic corrosion at dissimilar metal joints n and stress corrosion cracking (s.c.c.). 11 It is well known that to avoid s.c.c., either during cleaning or in subsequent operation, solutions involving chloride and possibly fluoride should not come into *Manuscript received 1 December 1978. "l-Paper presented at the conference on "Electrochemical Test Methods for Stress Corrosion Cracking", organized by the Working Party on Stress Corrosion Cracking Test Methods of the European Federation of Corrosion and held at Firminy, France, 1%21 September 1978. 687
688
BRYANPOULSONand RUSSELLROBINSON
contact with austenitic stainless steels. Similarly, solutions involving hydroxides should be treated with the u t m o s t care to avoid retention in crevices, a n d preferably be avoided. 11 A l t h o u g h it m u s t be emphasized that apart from these situations there have been n o reported cases o f cracking, two areas o f possible concern were identified, which p r o m p t e d this investigation. These were: 1. Solutions where a n o d i c inhibitors are used, since this has been identified as a situation which can cause s.c.c, o f c a r b o n steels. 1~ 2. The presence o f sensitized stainless steels, since this has in n u m e r o u s other instances led to cracking, often involving a p p a r e n t l y i n n o c u o u s e n v i r o n m e n t s ; it has recently been s h o w n la that a 9 % C r - 1 % M o steel c a n be sensitized. Because m o d e r n boilers are usually constructed out o f a n u m b e r of materials, it was felt desirable to examine the possibility o f s.c.c, occurring in the following systems: c a r b o n steel in inhibited hydrochloric acid; c a r b o n steel, a 9 ~ C r - 1 ~ M o steel a n d a sensitized type 316 stainless steel i n inhibited citric acid. EXPERIMENTAL METHOD Steels to be tested were obtained in a number of forms, details and analysis of which are shown in Table 1. Stress corrosion test specimens were of three types: 5 mm wide C-ring specimens from production tubing, stressed to well above yield using a spacer, and 7 in. long tensile specimens with either a 0.5 in. gauge length or a 55° notch in both cases to a reduced cross section of 0.1 in. diameter. Specimens for electrochemical measurements were drilled and tapped to fit a Stern-Makrides compression holder. Mild steel was annealed for 0.5 h at 950°C in an argon atmosphere. 9Cr-I Mo steel was held for 0.5 h at 950°C in an argon atmosphere and then water quenched, followed by tempering in a salt bath for times up to 8 rain or an argon atmosphere for longer times. Type 316 stainless steel was solution annealed at 1050°C for 20 min in an argon atmosphere and then sensitized at 650°C for 16h. TABLE I.
ANALYSTS OF STEELS USED
Composition Designation
C
Mild steel rod 0.1 Mild steel tube 0.08 5Cr-lMo 0.1 9Cr-I Mo tube 0.1 9Cr-l Mo rod 0.1 Type 316 stainless 0.05 Inconel 600 0.04
Mn
Si
S
P
0.43 0.58 0.46 0.47 0.43 1.58
0.21 0.15 0.27 0.44 0.59 0.5 0.4
0.02 0.03 0.004 0.017 0.008 0.012 0.008
0.03 0.04 0.013 0.017 0.014 0.025 0.01
Ni
12.04 7.4
Cr
Mo
4.97 9.07 8.55 17.63 16.1
0.56 0.1 0.99 2.53
Others
9.4Fe
The inhibited citric acid (3 or 3.5 wt. ~o citric acid + 0.05 wt. ~o Starmine LTP and ammonia to pH 3.5 to 4 at 80 or 90°C) and the inhibited hydrochloric acid (5 wt. %HCI + 0.25 wt. ~o Armohib 28 at 75°C) were both de-oxygenated using white spot nitrogen. Tensile slow strain-rate s.c.c, tests were performed in a PTFE cell in a motorized hard beam tensometer with load recording facilities. All other tests were performed in a multi-necked 1 1. flask. Potentials were measured with an external saturated calomel reference electrode (SCE) at room temperature, and are reported on that scale. Potential-time curves were recorded on a chart recorder using a potentiostat as a high impedance voltmeter. Fast and slow anodic polarization curves were obtained using a linear sweep generator to drive the potentiostat, the current being recorded on the chart recorder by manual switching between ranges. Fast straining electrode tests were performed in the PTFE cell mounted in a motorized Hounsfield tensometer. Specimens for constant potential etching were metallographically polished and inserted into the cell at the required potential. All specimens for examination in the scanning electron microscope (SEM) were ultrasonically cleaned in methanol immediately the test finished and stored until required.
Assessing the possibility of stress corrosion cracking during acid cleaning
689
EXPERIMENTAL RESULTS Stress corrosion tests (a) M i l d steel in inhibited hydrochloric acid. Typical stress-time curves obtained
during slow strain-rate (1.2 × 10-5 s -1) stress corrosion tests on smooth specimens in inhibited hydrochloric acid are shown in Fig. 1. It might be thought that cracking had occurred at -- 400 mV (SCE). Metallographic examination indicated otherwise; in particular failure was due to pitting (Fig. 2). At more negative potentials some end grain attack occurred after failure and prior to specimen removal, but no cracking. (b) M i l d steel in inhibited citric acid. Annealed mild steel notched samples were tested at a slow strain rate (crosshead velocity of 1.1 / 10-6 in. s -1) over a range of potentials in inhibited citric acid, the results being shown in Fig. 3. It can be seen that in this most severe test cracking occurred at potentials below ca. - - 400 mV (SCE). Such failures had fractographic featm es (Fig. 4) which were typical of hydrogen embrittlement in low strength carbon steels32 Tests on highly stressed C-rings failed to reveal any cracking susceptibility, even at potentials down to -- 800 mV (SCE). (c) 9Cr-I Mo in inhibited citric acid. A number of preliminary tests had shown that tests under freely corroding conditions were not very reproducible. Therefore the effect of potential on the s.c.c, of sensitized (950°C WQ + 1 min at 750°C) C-rings was examined. The results are shown in Fig. 5 and it can be seen that stress corrosion occurred over the potential range -- 550 to -- 200 mV (SCE). Outside this range neither pitting nor general corrosion was apparent. C-ring specimens which had been quenched and then tempered for 30 min at 750°C showed no indication of cracking at any potential. Quenched C-ring samples failed due to hydrogen embrittlement below -- 850 mV (SCE): in the more severe slow strain-rate test quenched specimens failed by hydrogen embrittlement under freely corroding conditions. A more detailed examination of the effects of heat treatment was performed by exposing C-ring specimens to inhibited citric acid at the potential of maximum susceptibility to s.c.c. -- 300 mV (SCE). The effects of varying the tempering temperature for a 30 min tempering time is shown in Fig. 6 which also includes the results of some hydrogen embrittlement studies. The results of all such heat-treatments can be presented in the form of a parametric plot as shown in Fig. 7. In an attempt to examine the possible occurrence of a static threshold stress both constant displacement and slow strain-rate tests were performed on 9Cr-1Mo tensile specimens (950°C WQ, tempered 30 rain at 470°C) in inhibited citric acid at -- 300 mV (SCE). The results of these tests are presented in Fig. 8. While there are a limited number of tests it is clear that cracking occurs at very low stresses. This is reflected in the time-to-fail ratio (time to fail in citric acid/time to fail in air) of 0.4 obtained in slow strain-rate tests. C-ring specimens which had failed due to s.c.c, typically contained a number of intergranular cracks (Fig. 9a) with some secondary cracking along martensite lath boundaries (Fig. 9b). The amount of this secondary cracking varied as is shown in Fig. 9c, d and appeared to increase with decreasing hardness. C-ring specimens which had failed by hydrogen embrittlement contained fewer cracks, usually one or two (Fig. 10a). These tended to be transgranular (Fig. 10b) and the fracture surface was a mixture of quasi-cleavage and pseudo-dimples (Fig. 10c)
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FIG. 11. Potential time curves and potential ranges promoting SCC of sensitized 97ooCr-1~Mo steel in inhibited citric acid. although some intergranular cracking was usually present, particularly in quenched specimens (Fig. 10d). (d) Sensitized type 316 stainless steel in inhibited citric acid. Sensitized type 316 stainless steel notched samples were tested at a slow strain rate (cross head velocity of 1.1 x 10 -6 in. s -1) over a range of potentials in inhibited citric acid. Apart from one specimen which showed mild susceptibility when tested at -- 100 mV no cracking was observed; attempts to reproduce this susceptibility were unsuccessful. There was reason to believe that the inhibited citric acid had been overheated, possibly partially destroying the inhibitor, and tests in boiling inhibited citric acid at -- 100 mV SCE reproduced the observed intergranular cracking. Electrochemical measurements Potential-time curves. Potential-time curves for polished, and oxidized (950°C for
1 h) 9 C r - l M o samples as well as 9Cr-lMo/316, 9Cr-lMo/Inconel 600, 9 C r - l M o / 5 C r - l M o and 9Cr-1Mo/mild steel couples in inhibited citric acid are shown in Fig. 11, together with the potential range promoting s.c.c, of sensitized 9Cr-lMo. Heat treatment of 9 C r - l M o steel had only a slight effect on this behaviour; increasing tempering caused the free corrosion potential to shift to more negative values. It is clear from Fig. 11 that the free corrosion potential can lie in the range causing cracking especially if the sample is oxidized, coupled to type 316 stainless or [nconel 600, or if small amounts of oxygen are present. It is also clear that the effect of oxygen on the free corrosion potential is large and not immediately reversible. Anodic polarization curves. Potentiodynamic anodic polarization curves for mild steel in both inhibited and uninhibited hydrochloric acid are shown in Fig. 12. It appears that Armohib 28 acts as an anodic inhibitor. There is considerable hysteresis between positive and negative sweeping slow polarization curves, suggesting that pitting is occurring. This results in higher currents being measured at slow sweep rates than fast rates, above about -- 390 mV (SCE). Stepped polarization curves (20 mV every 10 rain) were carried out. The results are shown in Fig. 13. It can be seen that at potentials of -- 460 mV (SCE) and below the current decreases with time: -- 420 mV (SCE) and above it increases with time, again indicative of pitting. It would appear that under these conditions there is no induction time for pitting. Fast (1000 mV/min) and slow (10 mV/min) anodic polarization curves for mild steel, 5Cr, 316, [nconel 600, and 9 C r - l M o after different heat treatments were
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FIG. 9.
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Dissolution morphologies of carbon sleel tested in inhibited hydrochloric acid at constant potential.
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Assessing the possibility of stress corrosion cracking during acid cleaning
701
obtained in inhibited citric acid. I m p o r t a n t points observed were: (a) All materials except mild steel exhibit art active-passive transition with increasing potential (Fig. 14) or are passive under freely corroding conditions. With 5Cr steel the active to passive potential is outside the range of potentials of interest. (b) There are only small effects of heat treatment on the behaviour of 9 C r - l M o (Fig. 15). (c) O f the materials studied the largest differences between the peak currents of slow and fast polarization curves were for 5Cr and 9 C r - l M o steels (Table 2). (d) The passive film formed on 9 C r - l M o material at anodic potentials appeared quite stable during negative going potential sweeps from the passive range and no "reactivation" occurred. TABLE 2.
ELECTROCHEMICAL
P A R A M E T E R S FROM
ANODIC
POLARIZATION
Fast
Slow
/max (mAitre ~)
Alloy Mild steel 5Cr-lMo 9Cr-1Mo quenched 9Cr-I Mo sensitized 9Cr-I Mo correctly heat treated 316
29.88 0.085 0.075 0.146 0.017
CURVES
Emax [mV(SCE)]
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Emax [mV(SCE)]
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Potentiostatic etching (a) Mild steel in inhibited hydrochloric acid. The rate of dissolution increased with increasing potential (Fig. 12) and above a certain potential pitting occurred. In all tests sulphide inclusions dissolved rapidly with the carbides remaining unattacked. When pitting occurred it invariably initiated from a sulphide and then proceeded to form a cavity with a covering veil of intact metal. Representative surface morphologies are shown in Fig. 16.
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(b) Mild steel in inhibited citric acid. Although very small pits occurred at the free corrosion potential (Fig. 17a) more positive potentials promoted general dissolution rather than the deep localized attack observed in inhibited hydrochloric acid. Again sulphide inclusions dissolved preferentially and crystallographic etch pits formed during the initial stage of dissolution at high potentials (Fig. 17b). (c) 9Cr-lMo in inhibited citric acid. On metallographically polished samples exposed to inhibited citric acid at potentials promoting cracking (of sensitized samples), interference thickness oxide films formed and specimens were lightly etched. Intergranular corrosion did not occur on sensitized specimens but some cracking did, due to residual quenching stresses--again indicating the low threshold stress for cracking. (d) Sensitized type 316 stainless steel in inhibited citric acid. Specimens dissolved at a very low rate at all potentials with no noticeable etching. Straining electrode experiments Tensile 9Cr-lMo s.c.c, specimens were held unstressed at -- 300 mV (SCE) in inhibited citiic acid for 1 h during which time the current decayed to nearly steady state conditions. The specimen was then rapidly strained to fracture at 200 %/min. and the current recorded. A typical result is shown in Fig. 18. If it is assumed that the metal is ductile and the protective film brittle then the per cent bare metal exposed at any strain can be calculated from ref 14. L,y, % bare metal : 100 1 -- L~e] L ~ = initial specimen length; L f : final specimen length or for small strains % bare metal approx = ½% strain.
Assessing the possibility of stress corrosion cracking during acid cleaning
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Thus from the peak current obtained in the yielding electrode experiment the current density on the newly created bare metal cart be calculated. The results of such calculations have been tabulated in Table 3. It is apparent that these dissolution rates are more than adequate in explaining the observed cracking rates. However, the differences between specimens having different heat treatment are relatively small and do not reflect the cracking tendency of the sensitized material.
TABLE3. RESULTSOFYIELDINGELECTRODEEXPERIMENTSON9Cr-1Mo Calculated bare metal dissolution rate at --300 mV (SCE)
Heat treatment Quenched Sensitized (950°C WQ + 30 min at 475°C) Tempered (950°C WQ + 30 min at 750°C
59 mA/cm2 63 mA/cm2 30 mA/cm~
DISCUSSION
Practical aspects Although cracking could be induced in mild steel, 9Cr-1Mo and type 316 stainless steel in inhibited citric acid, this is most unlikely to occur in practice, but for different reasons in each system.
704
BRYAN POULSON and RUSSELL ROBINSON
Notched mild steel specimens failed in slow strain-rate tests---due to hydrogen embrittlement--in inhibited citric under freely corroding conditions. However no failures occurred when carbon steel tubing was highly stressed over a range of potentials down to -- 800 mV (SCE). Similarly, sensitized 9Cr-lMo steel failed at very low stresses and at potentials which could occur in practice. This is not worrying if 9Cr-lMo is used in the correctly heat-treated condition. Thus both tempering and post-weld heat treatments are specified to lie in the safe region of Fig. 19, with a maximum hardness criteria of about 260 Hv. This is obviously compositional dependent and is being examined for the higher Si compositions currently favoured for their increased resistance to breakaway oxidation in CO2 based coolants. The occurrence of some cracking in highly sensitized type 316 stainless steel in inhibited citric acid is not fully understood. It does appear to require above normal temperatures which conceivably could promote inhibitor breakdown. Some support for this could be inferred from the observation that when cracking did occur it was associated with above normal corrosion rates. This cracking appears to require high stresses, a sensitized structure and some degradation of the inhibited citric; it is thus unlikely to be of concern. However it does emphasize that sensitized stainless steels are prone to intergranular s.c.c, and any proposed cleaning solution should be assessed for its aggressivity if sensitized material is present. NO ATTACK 750
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Assessing the possibility of stress corrosion cracking during acid cleaning
705
THEORETICAL ASPECTS From a predictive viewpoint the electrochemical measurements suggested: 1. Mild steel should suffer s.c.c, in inhibited hydrochloric acid. 2. Mild steel should not crack in inhibited citric acid. 3. 9 C r - l M o steel should not crack in inhibited citric acid. 4. Sensitized type 316 should not crack in inhibited citric acid. However, as Parkins 16 has repeatedly emphasized there are also structural factors which must be taken into account in predicting the possibility of cracking. Thus the mild steel does not crack in inhibited hydrochloric acid because there is neither a pre-existing or strain induced active path which can suffer localized restricted dissolution in inhibited hydrochloric acid; a similar situation has previously been reported for mild steel in concentrated sulphuric acid. 17 Furthermore in inhibited hydrochloric acid, the solution which forms in pits or crevices is more aggressive than the bulk solution, possibly due to inhibitor poisoning, and it is unlikely that restricted localized dissolution could occur. The occurrence of cracking in sensitized 9 C r - l M o in inhibited citric acid--when none was predicted--is due to a region depleted in chromium influencing the localized dissolution/repassivation behaviour but not the bulk electrochemical properties. Earlier it was suggested 1~ that electrochemical measurements, and in particular weight loss measurements after potentiostatic etching at + 100 mV (SCE) or reactivation potentials, obtained from polarization curves, would indicate sensitized material. In Fig. 20 crack velocities in citric acid are compared with various measures of sensitization. It can be seen that susceptibility to s.c.c, is displaced in relation to tbe weight loss curve. In particular, specimens quenched and then tempered at low tepperatures (e.g. down to 300°C) are susceptible to s.c.c, but this is not expected from their weight loss. This apparent anomaly is thought to be due to appreciable carbide precipitation being required to caused increased weight loss, while intergranular s.c.c, can occur before this stage is reached. This is still under investigation.
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706
BRYANPOULSONand RUSSELLROBINSON CONCLUSIONS
1. The possibility o f s.c.c, occurring during acid cleaning has been assessed in the following systems: mild steel in inhibited hydrochloric acid, mild steel, a 9 C r 1Mo steel and a sensitized stainless steel in inhibited citric acid. N o cracking occurred with the material/environmental/stress conditions which should be encountered in practice. 2. However, cracking could be induced to occur in inhibited citric acid by: (a) Mild steel: testing under unrealistically severe mechanical conditions, i.e. a slow strain-rate test using a notched sample. (b) 9 C r - I M o : if the material is hard or sensitized. (c) Sensitized type 316 stainless steel: if the environment was degraded by boiling. 3. Electrochemical s.c.c, predictability tests were unsuccessful in that they predicted cracking o f mild steel in inhibited hydrochloric acid and did not indicate the possibility o f sensitized 9 C r - l M o failing in inhibited citric acid. 4. It is recommended that although electrochemical measurements can give an indication if possible reactions, slow strain rate s.c.c, tests should always be carried out over a range o f potentials, and possibly including out o f specification heattreatment and environmental conditions. Tests under realistic conditions can then be used to decide if cracking will occur. Acknowledgements--The authors thank the management of NEI Clarke Chapman Power Engineering
Ltd and the CEGB who have been associated with this work, for permission to publish the outcome of the developments and the results. Thanks also must go to the staff of the SEM in the Department of Chemistry, University of Neweastle-upon-Tyne for their help. REFERENCES 1. Proceedings of Conference on Corrosion of Steels in CO~, BNES, London (1974). 2. C. S. TEDMAN(Ed.), Corrosion Problems in Energy Conversion and Generation, Electrochemical
Soc., Princeton (1974). 3. Proceedings of Conference on Effects of Environment on Materials Properties in Nuclear Systems,
Inst. of Civil Engineers, London (1971). 4. Proceedings of Conference on Ferritic Steels for Fast Reactor, BNES, London (1978). 5. J. K. RICE, Combustion 33, 45 (1961). 6. A. S. PEARCEand R. N. PARKINS,Br. Corros. J. 3, 70 (1968). 7. J. K. RICE and C. M. LOUCKS,Materials, Prot. 4, 14 (1965). 8. C. SETTLE,P. J. MITCHELand D. FISHER, Corros. Sci. 6, 33 1966). 9. L. T. OVERSTREET,Materials Prot. 2, 48 (1963).
10. B. POULSON,in preparation. 11. J. A. AYRES,Decontamination of Nuclear Reactors and Equipment (Edited by J. A. AYRES).Ronald, New York (1970). 12. B. POULSON,Corros. Sci. 15, 469 (1975). 13. B. POULSON,Corros. Sci. 18, 371 (1978). 14. S. F. BUBORand D. A. VERMILYEA,.jr electrochem. Soc. 113, 892 (1966). 15. R. L. COWAN on C. S. TEDMAN, Intergranular Corrosion of Iron-Nickel-Chromium Alloys. Advances in Corrosion Science, Vol. 3. Plenum, New York (1973). 16. R. N. PARK1NS,The Theory of Stress Corrosion Cracking in Alloys (Edited by J. C. SCULLY),p. 167. NATO, Brussels (1971). 17. B. POULSON,in Proceedings of Conference Testing and Monitoring. Inst. of Corrosion Sci. and Technology, London (1978).