Stress Corrosion Cracking of Carbon Steel in Hot Potassium Carbonate/Bicarbonate

Transcription

1 Technical Paper Boshoku Gijutsu, 36, (1987) UDC : : Stress Corrosion Cracking of Carbon Steel in Hot Potassium Carbonate/Bicarbonate Solutions Z. A. Foroulis* *Exxon Research and Engineering Company This paper summarizes the results of an investigation of the stress corrosion cracking (SCC) behavior of carbon steel in carbonate/bicarbonate solutions encountered in hot carbonate acid gas scrubbing processes for CO2 removal. This work has shown that potassium carbonate solutions which contain dissolved CO2 can cause SCC of carbon steel provided sufficient tensile stresses are present. Carbonate solutions which do not contain dissolved CO2 (HCO3- ions) do not cause stress corrosion cracking. Stress corrosion cracking was found to occur only in the electrochemical potential range of to -0.55V (SCE). Outside this potential region SCC does not occur. Similar behavior was found in carbonate/bicarbonate solutions containing arsenite as activator/inhibitor. In solution containing metavanadate inhibitor at concentrations>0.2 wt NaVO3, SCC does not occur. However, under conditions where passivity breakdown can occur (loss of inhibitor, solution concentration, overheating, etc.) metavanadate containing carbonate/bicarbonate solutions can cause SCC. Keywords: stress corrosion cracking, carbon steel, carbonate solution, metavanadate, inhibitor, electrochemical potential 1. Introduction Stress corrosion cracking (SCC) of carbon steel in hot potassium carbonate/bicarbonate solutions has led to several failures in refinery and ammonia plants which use hot potassium carbonate solutions as a scrubbing medium for removal of CO2 and/or H2S from hydrogen and other petroleum refinery gases1)2). A carbonate/bicarbonate solution generated in-situ from the soil side in regions of disbonded coatings has also been considered the cause of stress corrosion cracking of high pressure gas transmission pipelines3)-5). Previous work reported in the literature6),7) has shown dilute carbonate/ bicarbonate solutions inducing SCC in C-Mn steels. Stress corrosion cracking was reportedly observed in dilute ammonium and sodium carbonate/bicarbonate mixtures, but not in pure dilute carbonate solutions. The present investigation was undertaken to study the stress corrosion cracking behavior of carbon steel in concentrated carbonate/bicarbon- *P. O. Box 101, Florham Park, New Jersey 07932, U.S.A. ate solutions in the presence of CO2 and with the addition of inhibitors/activators such as potassium metavanadate and sodium arsenite which are used in commercial CO2 scrubbing plants. 2. Experimental Approach The experimental technique used was the slow-strain-rate test method in which smooth tensile specimens of carbon steel are strained to failure at a fixed rate in the environment of interest. The test solution was maintained in a cylindrical Teflon cell of one liter capacity which fitted on and around the test rod via a water-tight seal. The cell had a closed Teflon lid equipped with suitable openings for introduction and outflow of purge gas, a platinum counter electrode, a bridge connected to an external saturated calomel reference electrode, a reflux condenser and a thermocouple for temperature control. Heating was done with an external heating mantle. Details of the experimental techniques are given elsewhere8). The test specimens were prepared from cold drawn carbon steel (AISI 1018) rods 0.63

2 690 Boshoku Gijutsu Table 1 Slow strain-rate tests (at 1x106 sec-1) of carbon steel (AISI 1018) in 25% K2C03/ C02-sat'd at 90C (194F). NC=No Cracking T=Transgranular I=Intergranular Cracking Cracking cm diameter. The rods were cut into 33cm long pieces and machined into tensile specimens having a gage diameter of 0.25cm and a gage length of 1.27cm. After machining the specimens were used without any further heat treatment. Before the tests, the specimens were surface polished with 600 grit SiC paper, rinsed with water and finally degreased with acetone. Slow-strain-rate tests were carried out in potassium carbonate and potassium carbonate/bicarbonate solutions which were produced by CO2 absorption in carbonate solutions, in the temperature range of 90 to 120C. Several of the solutions tested were saturated with CO2 and also contained inhibitors/activators which are found in commercial acid gas scrubbing processes. Most of the tests were conducted at a strain rate of 1x10-6sec-1. A few tests were also carried out at strain rates of 2.5x10-6, 4x 10-6, and 4x10.7sec-1. These rates were selected because experience in this and other laboratories indicated that cracking of carbon steel generally occurs at strain rates between about 10.5 and 10.7sec-1. The fracture and side surfaces of all specimens tested were examined with a scanning electron microscope (SEM) following catholic cleaning of the specimens using a proprietary alkaline solution. In addition, metallographic examination of longitudinal sections through the fractured test pieces was also carried out for evidence of side cracking. The assessement of the results was carried out in terms of the nature of fracture (micro-void coalescence vs. multiple cracking with intergranular or transgranular facets), morphology of side cracks, percent reduction in area, time to failure and maximum nominal stress calculated on the basis of the original cross section area. Average crack propagation rates were determined from the deepest crack measured on the final fracture surface and dividing this by the total test time. This assumes that cracking occurs continuously from the initiation of tests, whereas cracking probably does not initiate until the yield stress is approached. The error introduced by this assumption is likely to be small, less than ten percent. 3. Results Several experiments were carried out using 25% K2CO3/N2 sat'd and 25% K2CO3/CO2 sat'd at 90C (194F). In addition, several experiments were carried out with CO2 sat'd carbonate solution which also contained different concentrations of sodium metavanadate and sodium arsenite. These conditions were selected because they represent typical conditions which are likely to occur in the absorber and regenerator environments encountered in acid gas scrubbing plants using potassium carbonate and activated potassium carbonate processes. 4. Carbonate/Bicarbonate Mixtures Have a Tendency to Cause Stress Corrosion Cracking Tests were carried out at the free corrosion potential of carbon steel and at several controlled potentials within the range of electrochemical potentials where the carbon steel surface is reactive as far as corrosion3). This potential range is mainly between to -0.55V (SCE). Data of slow-strain-rate tests carried out in 25% K2CO3/CO2 and N2 sat'd at

3 Vol. 36, No Fig. 1 Cross section microphotograph showing the transgranular nature of secondary cracks initiated in the neckdown area in specimen (Test 1, Table 1) tested in 25% K2CO3/CO2 sat'd at 90C at -0.8 V(SCE). 2% Nital, 320X Fig. 2 Cross section microphotograph showing secondary cracks initiated in the neckdown area of specimen (Test 4, Table 1) tested in 25%, K2CO3/CO2 sat'd at 90C at -0.65V(SCE). 2% Nital, 400x 90C are summarized in Table 1. The data shows that in CO, saturated solution, SCC was observed in the potential range -0.8 to -0.6 V (SCE) as well as at the free corrosion potential. At potentials higher than as indicated with tests at -0.4V (SCE), SCC was not observed. Typical appearance of the cracking observed in 25% KZCO3 solution sat'd with CO, is shown in Figures 1 and 2. the cracking is predominantly transgranular at lower potentials and tends to be intergranular at higher potentials. The average crack growth rate observed in the CO2 sat'd solutions appeared to vary with the electrochemical potential achieving a maximum rate of about 1.4x10-2mm/hr in the poten- Fig. 3 The effect of potential on average crack growth rate of carbon steel (AISI 1018) in 25% K2CO3/CO2 sat'd at 90C. tial of about -0.75V (SCE) as shown in Figure 3. In the absence of dissolved CO2, potassium carbonate solutions were not aggressive toward carbon steel as shown with the Test No. 8, Table 1. The lack of SCC in pure carbonate solutions without CO2 is attributed to the high ph of these solutions (-12) which tends to promote passivation and to lower cathodic reaction rates8). 5. Metavanadate Inhibitor Can Prevent Stress Corrosion Cracking Potassium metavanadate has been used as a corrosion inhibitor in several activated potassium carbonate CO, scrubbing processes such as Catacarb and Benfield processes. Several tests were therefore carried out to establish the effect of metavanadate ions on the tendency of carbonate/bicarbonate solutions to promote SCC of carbon steel under the influence of tensile stresses. Several tests using the slowstrain-rate method were carried out using 25% K2CO3/CO2 sat'd at C in which different concentrations of potassium metavanadate inhibitor were added. A summary of these tests No. 1 to 8 which were carried out at the free corrosion potential is given in Table 2. In all tests except test No. 4 the test specimens were prepassivated in the test solution prior to running the slow-strain-rate test. In test No. 4 the specimen was cathodically activated at -1.5V (SCE) prior to testing. The results summarized in Table 2 indicate that in the presence of sufficient concentration

4 692 Boshoku Gijutsu Table 2 Slow-strain-rate tests (1x10-8 sec-1) of carbon steel (AISI 1018) in 25% K2CO3 sat'd With CO2 Containing Different Concentrations of NaVO3. X=Temperature overrun to solution evaporation. =Specimen was activated at -1.5V (SCE) prior to testing. Fig. 4 Side view h otomicrograph of carbon steel specimen after test in 25% K2CO3/ CO2 sat'd containing 0.05% NaVO3, at 90C, at the free corrosion potential (Test 12, Table 2). 400X of sodium metavanadate (0.2 to 0.45% NaVO3) carbon steel will not SCC at the free corrosion Fig. 5 Cross section microphotograph showing intergranular cracking and pitting of carbon steel in the neckdown area in specimen Test 12, Table 2. 2% Nital, 400x potential. the beneficial effect of metabanadate inhibitor is attributed to shifting of the free corrosion potential to potentials within the passive potential region [E>-0.55V(SCE)] where a protective iron oxide film can be maintained. This film suppresses the rate of metal dissolu-

5 Vol. 36, No Table 3 Slow-strain-rate tests 1x10-6 sec-1 of carbon steel (AISI 1018) in 25% K2C03/CO2 sat'd and containing different concentrations of sodium arsenite at 90C. Based on the results presented in Table 2 and Figures 4 and 5 it is concluded that carbonate/ bicarbonate solutions containing metavanadate inhibitor in concentrations greater than 0.2% NaVO3 suppress the tendency to SCC of carbon steel subjected to tensile stresses. However, the data also indicate that under conditions leading to passivity breakdown such as when the inhibitor concentration is lower than 0.2% NaVO3, SCC can occur. A more detailed discussion of the effect of metavanadate oxyanions on SCC of carbon steel in carbonate/bicarbonate solutions is given elsewhere9). Fig. 6 Free corrosion potential of carbon steel in 25% K2C03/C02 sat'd at 90C and different concentrations of sodium metavanadate inhibitor. tion and prevents SCC. At much lower inhibitor concentrations and particularly when the steel surface is activated prior to testing or when a temperature overrun occurs leading to passivity breakdown, stress corrosion cracking may occur as shown in Tests No. 4 to 6 and 8. Figures 4 and 5 are a side view photomicrophotograph and cross section microphotograph respectively of specimen Test 12 (Table 2) showing intergranular cracks/pits. Figure 6 shows the influence of metavanadate inhibitor concentration on the free corrosion potential of carbon steel in CO2 sat's 25% K2CO3. The data shows that very low inhibitor concentration under condition which can cause loss of passivity (e. g., overheating) may shift the corrosion potential within the cracking zone. Scuh condition can produce SCC. 6. Carbonate/Bicarbonate Solutions Containing Arsenite Have a Tendency To SCC Another commercial hot potassium carbonate process for CO2 scrubbing contains arsenite (which is added as As2O3) as a process activator and corrosion inhibitor. This process is known as Giammarco-Vetrocoke process and is used extensively outside the USA. Practical experience with several plants using this process indicates that welded carbon steel equipment which have not been post weld heat treated can be susceptible to SCC1),2). Several tests were therefore carried out to gain an understanding of the effect of As2O3 on the tendency for SCC in carbonate/bicarbonate solutions. A number of tests were carried out using 25% K2CO3/CO2 sat'd with the addition of various concentration of NaAsO2. These tests were carried out at 90C at the free corrosion potential using slow-strain-rates of 1x10-6 sec-1. A summary of these test results is presented in Table 3. The data indicate that CO2 sat'd 25% K2CO3

6 694 Boshoku Gijutsu detailed discussion of the effect of arsenite on SCC behavior of carbon steel in carbonate/bicarbonate solutions is given elsewhere10). Fig. 7 Effect of arsenite inhibitor concentration on free corrosion potential of carbon steel in 25% K2C03/CO2 sat'd at 90C. Fig. 8 Cross section microphotograph showing transgranular nature of secondary crack initiated in the neckdown area in specimen Test 1, Table 4, Tested in 25%, K2CO3/CO2 sat'd with 0.5% NaAsO2 at Ecror, 90C. 2% Nital, 400x solutions containing sodium arsenite in the concentration 0.5 to 5% cause SCC of carbon steel. In this concentration range the arsenite additions maintain the corrosion potential of steel within the cracking potential zone as shown in Figure 7 and thus promotes SCC. The mode of cracking appears to be transgranular (Figure 8). At these concentrations, arsenite tends to maintain the corrosion potential at potentials where hydrogen enhanced cracking can become significant as indicated by the somewhat reduced ductility of carbon steel (Table 3). A more 7. Conclusion Based on the results discussed in this report the following conclusions can be drawn: (1) Aqueous solutions of potassium carbonate (which do not contain CO2) with and without inhibitors do not cause SCC of carbon steel. These solutions have a ph of about 12 which promote passivity and does not support cracking. (2) Mixtures of potassium carbonate/bicarbonate solutions which are produced by CO2 absorption in carbonate solutions have a strong tendency to cause SCC of carbon steel under the influence of tensile stresses. The maximum average crack growth rate is of the order of 1.4x10-2mm/hr. The SCC mode is transgranular at lower potentials and intergranular in the higher potential range. Cracking was found to occur in the potential range to -0.55V (SCE) but not outside this potential zone. (3) Potassium metavanadate inhibitor in the concentration range greater than 0.5 wt% as NaVO3 can prevent SCC of carbon steel at the free corrosion potential. The effect of the vanadate inhibitor is to shift the free corrosion potential in the passive potential zone E>-0.55V (SCE) where a protective iron oxide film can be maintained. However, under conditions where passivity breakdown does occur (loss of inhibitor, overheating, etc.), metavanadate containing carbonate/bicarbonate solutions at active potentials [-0.85 to -0.55V (SCE)] can cause SCC of carbon steel. (4) Carbonate/bicarbonate solutions containing arsenite inhibitor/activator have strong tendency to cause SCC of carbon steel. SCC was found to occur in the potential range to -0.55V (SCE) and at the free corrosion potential which lies within the cracking potential zone. The cracking mode is primarily transgranular. (5) Experiences with these systems has shown that adequate post weld heat treatment (PHWT) which eliminates the welding residual stresses can prevent stress corrosion cracking11)

7 Vol. 36, No Acknowledgement The author would like to acknowledge the contributions of D. Rewick and D. Lattig who performed the experimental part of this work. (Received May 6, 1987) References 1) K. T. G. Atkins, D. Fyfe & J. D. Rankin: AIChE Symposium on Saefety in Ammonia Plants and Related Facilities, CEP Technical manual, 16, 39 (1974). 2) K. Naito, T. Hashimoto & T. Kihara: Japan Petr. Inst., 14, 672 (1971). 3) R. L. Wenk: 5th Symposium on Line Pipe Research, American Gas Association, Catalogue No. L30174 (1974). 4) R. R. Fessler: Proceedings Interpipe 76, Houston, TX, January (1976). 5) R. N. Parkins & R. R. Fessler: Materials in Engineering Applications, 1, 80 (1978). 6) J. M. Sutcliffe, R. R. Fessler, W. K. Boyd & R. N. Parkins: Corrosion, 28, 313 (1972). 7) J. H. Payer, W. E. Berry & R. N. Parkins: in "Stress Corrosion Cracking -The Slow Strain- Rate Technique", ASTM STP 665, American Society for Testing and Materials 1979, p ) Z. A. Foroulis: Corrosion, submitted. 9) Z. A. Foroulis: Metallurgical Transactions, submitted. 10) Z. A. Foroulis: Corrosion Science, Submitted. 11) I. Ishimaru & T. Takegawa: Ammonia Plant Safety, AIChE, 22, 170 (1980).