"S.C. Ikpeseni1 and B.O Onyekpe2"
Keywords: Stainless steel, pitting, corrosion, chloride environment.
Abstract:
The susceptibility of austenitic (AISI 301) stainless steel to pitting corrosion was evaluated in sodium chloride (NaCl) solutions - 0.1M, 0.2M, 0.3M, 0.5M and 0.7M and 1.0M. Tensile tests and microscopic examinations were performed on samples prepared from the steel after exposure in the various environments. It was revealed that AISI 301 steel suffers from pitting corrosion in all the investigated solutions. This ultimately led to reduction in tensile properties with increased concentration as a function of time.
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ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 Pitting Corrosion Susceptibility of AISI 301 Stainless Steel in Chloride Environments IKPESENI, S.C.1 AND ONYEKPE, B.O.2 1 Department of Mechanical Engineering, Delta State University, P.M.B. 1, Abraka, Delta State, Nigeria. E-Mail: Sunnychukwuyem@yahoo.com 2 Department of Mechanical Engineering, University of Benin, Benin- City, Edo state, Nigeria. E-Mail: bemonk99@yahoo.co.uk Abstract The susceptibility of austenitic (AISI 301) stainless steel to pitting corrosion was evaluated in sodium chloride (NaCl) solutions - 0.1M, 0.2M, 0.3M, 0.5M and 0.7M and 1.0M. Tensile tests and microscopic examinations were performed on samples prepared from the steel after exposure in the various environments. It was revealed that AISI 301 steel suffers from pitting corrosion in all the investigated solutions. This ultimately led to reduction in tensile properties with increased concentration as a function of time. Keywords: Stainless steel, pitting, corrosion, chloride environment. Introduction Pitting corrosion is a localized form of attack which produces holes in the metal. It is one of the most destructive and insidious form of corrosion and causes premature failure of equipment, with only a small percentage of weight loss [1, 2]. Among the halides, the most aggressive and thus, the most frequently investigated is the chloride ions, particularly its effect on pit formation in 18/8 stainless steel [1 – 3]. Chloride ions also aggravate stress corrosion cracking (SCC) failure of most martensitic stainless steels and high strength low alloy (HSLA) steels [4, 5, 6,]. Equipment / facility failure as a result of pitting corrosion is catastrophic especially those used to handle (process, transport or store) flammable fluids and aggressive chemicals. Pitting corrosion has been described as: 1 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 “under certain conditions, particularly involving high concentrations of chlorides (such as sodium chloride in sea water), and moderately high temperatures and exacerbated by low pH (i.e. acidic conditions), very localized corrosion can occur leading to perforation of pipes and fittings etc. This is not related to published corrosion data as it is extremely localized and severe corrosion which can penetrate right through the cross section of the component” [7]. Due to its better corrosion resistance than the straight chromium steels, austenitic steels are widely specified for the more severe corrosive conditions such as those encountered in the processing industries. They are rustresistant in the atmosphere and find use for architectural purposes in the kitchen, food manufacturing and dispensing and for application where contamination (rust) is undesirable. Though it has high resistance to general corrosion it may suffer from localized corrosion. Hence, in this work, the pitting corrosion susceptibility of AISI 301 stainless steel in various NaCl environments over time is investigated. Experimental Procedure The AISI 301 stainless steel used for this investigation as obtained from Petroleum Training Institute (P.T.I.), Effurun, Delta State, Nigeria. The Chemical composition of the steel is presented in Table 1.0. Reagent grade NaCl (99.9% NaCl after ignition) was used in preparation of the environments. The as-received stainless steel was machined into standard tensile test specimens and microscopic examination samples. Six different molar concentrations of the NaCl (0.1M, 0.2M, 0.3M, 0.5M, 0.7M and 1.0M were prepared from the regent grade NaCl and deionized water in equilibrium with the atmosphere. The specimens were exposed to these solutions by complete immersion. The specimens were prepared using well established procedures contained elsewhere [8, 9]. 2 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 Tensile strength of the specimens in the various concentrations mentioned above was determined at intervals of 240 hours (10 days) for a period of 1200 hours (50days). The specimens were tested to fracture at room temperature using universal tensile testing machine. Microscopic examination was carried out with an Olympus optical metallurgical microscope fitted with photographic devices. Number of pits on each view was noted and recorded. Table 1.0: Chemical Composition of the investigated AISI 301 steel Element C Si Mn P S Cr Mo Content (Wt %) 0.047 0.288 1.28 0.0056 0.0084 18.59 1.27 Element Ni Al Co Cu Nb Ti V Content (Wt %) 6.56 <0.0010 0.246 0.204 0.042 0.016 0.106 Element W Pb Sn Fe Content (Wt %) 0.058 <0.0050 0.034 71.25 Results and Discussion Table 2.0 shows the tensile properties of the investigated material in air (X o) and in the chloride environments (A10 – F10 to A50 – F50). As can be observed, all the properties 3 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 Table 2.0: Tensile Test Result TIME SAMPLE CONC YS UTS FS % EL % RA eenv /eair (Mol/dm3) (N/mm2) (N/mm2) (N/mm2) (Days) X0 780 978 890 47.1 60.1 1.000 10 A10 0.1 780 978 890 47.0 60.0 0.998 B10 0.2 780 977 889 47.1 60.0 1.000 C10 0.3 779 976 888 46.8 59.2 0.994 D10 0.5 779 976 887 46.5 58.0 0.987 E10 0.7 779 976 886 46.7 58.1 0.991 F10 1.0 778 975 885 46.3 58.0 0.983 20 A20 0.1 779 977 888 46.5 58.3 0.987 B20 0.2 779 975 886 46.5 57.0 0.987 C20 0.3 778 975 885 46.0 56.2 0.977 D20 0.5 778 974 885 45.8 55.6 0.972 E20 0.7 778 973 884 45.5 55.4 0.966 F20 1.0 778 971 881 44.0 55.0 0.934 30 A30 0.1 778 976 886 46.2 56.7 0.981 4 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 B30 0.2 778 974 883 45.2 55.8 0.960 C30 0.3 777 974 882 45.0 55.0 0.955 D30 0.5 777 973 883 44.6 54.6 0.947 E30 0.7 777 972 882 44.2 54.3 0.938 F30 1.0 776 970 880 44.0 54.0 0.934 40 A40 0.1 777 975 884 45.4 55.4 0.964 B40 0.2 776 973 881 44.0 55.0 0.934 C40 0.3 776 973 880 44.0 54.8 0.934 D40 0.5 776 972 878 43.5 52.8 0.923 E40 0.7 776 971 876 43.1 52.6 0.915 F40 1.0 775 968 873 43.0 52.0 0.913 50 A50 0.1 777 975 884 45.3 55.3 0.962 B50 0.2 776 973 879 43.7 53.2 0.928 C50 0.3 776 972 878 43.5 53.0 0.923 D50 0.5 776 972 874 43.0 52.4 0.913 E50 0.7 775 970 873 42.8 52.1 0.909 F50 1.0 775 968 871 42.6 52.0 0.904 5 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 measured decreased with increased time and concentration. Figures 1 – 2 present the ductility parameter [i.e. elongation parameter (eenv /eair)] versus concentration and time respectively. Again the parameter decreased e env /e air with increased concentration and time. 1.020 1.000 0.980 0.960 0.940 0.920 0.900 0.880 10days 20days 30days 40days 50days 0.1M 0.2M 0.3M 0.5M 0.7M 1.0M NaCl Concentration(mol/dm 3) Fig. 1: Elongation Parameter Vs Concentration 1.020 1.000 /e 0.940 e 0.960 env air 0.980 0.920 0.1M 0.2M 0.900 0.3M 0.880 0.5M 10 20 30 40 50 Time (Days) 0.7M 1.0M Fig. 2: Elongation Parameter Vs Time 6 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 Figures 3 and 4 showed that the number of pits on the specimen increased with increased concentration and time. Plates 1 – 6 revealed the pits covered by the corrosion products (dark spots). 7 6 No. OF PITS 5 10 Days 20 Days 30 Days 4 3 40 Days 50 days 2 1 0 0.1 0.2 0.3 0.5 0.7 1 NaCl CONCENTRATION (Mol/dm3) Fig. 3: No. of Pits Vs Concentration. 7 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 7 6 5 0.1M 0.2M No. OF PITS 4 0.3M 0.5M 3 0.7M 1.0M 2 1 0 10 20 30 40 50 TIME (DAYS) Fig. 4: No. of Pits Vs Time. MICROPHOTOGRAPH OF SOME SAMPLES x400 8 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 Plate 1: Optical Micrograph of A20 (i.e the Sample in 0.1M NaCl solution at the 20 th day) x400 Plate 2: Optical Micrograph of B20 (i.e. the sample in 0.2M NaCl solution at the 20th day) x400 Plate 3: Optical Micrograph of C40 (i.e. the sample in 0.3M NaCl solution at the 40th day) 9 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 x400 Plate 4 Optical Micrograph of D30 (i.e. the sample in 0.5M NaCl solution at the 30th day) x400 Plate 5 Optical Micrograph of E50 (i.e. the sample in 0.7M NaCl solution at the 50th day) x400 10 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 Plate 6 Optical Micrograph of F50 (i.e. the sample in 1.0M NaCl solution at the 50th day) All the results showed that increasing chloride concentration has adverse effect on the properties of the material. The loss of these properties could be attributed to the initiation and growth of pits as could be verified from figures 3 and 4. This can better be explained by Uhlig’s [10] Oxygen adsorption theory of passivity; chloride ion adsorbed on the metal surface is in competition with dissolved oxygen. This adsorption increases the exchange current and critical current density for anodic dissolution, leading to breakdown of passivity. The dissolution of the passive film is controlled by increasing hydrogen ion (H+) concentration and the presence of chloride ion (Cl-) has stronger capability to cause breakdown of the film [11,12]. Chloride ion causes breakdown of passive state leading to the formation of pits. Figures 1 – 2 showed decline in ductility parameters. The most useful parameter to evaluate SCC susceptibility is the elongation parameter, which compares a test performed under corrosive condition with a test performed under inert conditions (air or oil). A ration (eenv/einert) is evaluated and if the ratio is near 1.0, there is no susceptibility to SCC [6, 13]. Though, figure 1 – 2 portray deviation from mechanical failure, the deterioration of the ductility parameter cannot be attributed to SCC as the elongation parameter only decreased from 1.0 to 0.904 within the investigated concentration and time. But this could rather be attributed mainly due to the nucleation and growth of pits as evident in figures 3 and 4 and in plates 1 – 6. It takes longer time for pit to initiate in lower concentrations as can be seen in figures 3 – 4 having no pit for sample in 0.1M within the first 240 hours (10 days). Growth of the pits after initiation cause greater harm to the investigated material. Inclusions act as stress raisers for pit initiation. This could be verified from plates 5 – 6, which revealed that some of the pits were formed around inclusions as shown by the white ring – like structures enclosed in the pits. 11 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 Conclusion From the foregoing, it is therefore concluded that the investigated material (AISI 301) is susceptible to pitting corrosion in chloride environments. The susceptibility increases with increased chloride concentration as the time of exposure increases. It was equally observed that the initiation and growth of pits resulted to increased loss of tensile properties of the steel within the investigated time and concentrations. 12 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 References [1] “Elements of Corrosion and Protection Theory”, Ijeomah, M.N.C., Auto-Century Publishing Co. Ltd, Enugu, Nigeria, (1991). [2] “Corrosion Engineering”, Fontana, M.G., 3rd Ed., Tata McGraw – Hill Publishing Co. Ltd., New Delhi, India, (2005). [3] Foyie, R.T. Corrosion Science,. 26, pp18, (1970). [4] “Corrosion Doctors: Stress Corrosion Cracking”, [Home page on the internet]. Available form: http://www.corrosion-doctors.org./forms-scc/sec.htm. [5] “Stress Corrosion Cracking of Martensitic Stainless steel for Transmutation Application”, Roy, A.K.; Hosaain M.K.; O’Toole B.J., The 10th Int’l High-level Radioactive Waste Management Conference, paper no. 69425; March 30th – April 1st. Las Vegas, Nevada, Available form: http:www.aaa.nevadaedu/pdffiles/Gudipati:pdf, 2003. [6] “Stress Corrosion Cracking of Dual-Phase steel in carbonate / Bicarbonate solutions”, John, S.; Bradford, S.A., NACE Journal of Corrosion Science and Engineering, 41, 8, 1985. [7] “Stainless Steel Trap Corrosion”, Memarzadeh, F, Division of Technical Resources, Office of research facilities, Uk, 2005. [8] “ The Effect of Chloride Concentration on Tensile Fracture of AISI 301 Stainless Steel”, Ikpeseni, S.C., M.Eng. Thesis Submitted to P.G. School, University of Benin, Benin City, Nigeria, 2008. 13 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 6 submitted 8 January 2012 [9] “The structure, Properties and Heat Treatment of Metals”, Davis, D.J.; Oelmann, L.A., Pitman Books Ltd., London, 1983. [10] Corrosion Handbook, Uhlig, H.H, John Wiley and Sons Inc. New York, 1948. [11] Shreir, L.L.: Corrosion Metal / Environment Research,1, Butterworths & Co. Pub. Ltd., London, 1979. [12] Ijeomah, N.N.C., Journal of Electrochemical society, 134, pp2960, 1987. [13] “ Stress Corrosion Cracking: The Slow Strain Rate Technique”, Payer, J.H.; Berry, W.E.; Boyd, W.K., ASTM, Philadelphia, Pennsylvania, 1979. 14 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.