Muhammad Faizan, Humair Ahmed, S. M Mohsin Jafri, Syed Asad Ali and Azhar Mahmood
Keywords: Pitting Corrosion, Stainless Steel, Passivation Potential, Pitting Potential, Cyclic Polarization Curve
Abstract:
Pitting corrosion is very menacing and fatal type of localized attack on metallic structures. Customarily pitting corrosion is monitored by pitting potential which could be readily measured by the galvanostatic anodic polarization technique. In current communication, potentiodynamic scan graphs were acquired to find out pitting potential, corrosion potential and passivating potential for stainless steel type 304 sample electrodes while immersed in the 0M, 2M, 3M, 4M and 5M HCl solutions. Comparison of results have concluded that as chloride ion stress increased, it more aggressively attacked on passive layer and induced pitting corrosion at lower threshold potentials thus decreased Pitting potential (Epit) of subject stainless steel type 304 sample electrodes. Moreover pH of electrolyte decreased inside the pit due to hydrolysis of FeCl2 which furnished strong acid (HCl) and weak base (Fe(OH)2) thus further accelerate corrosion process and promote pits penetration. A large ratio between the anode and cathode areas have also favoured propagation of pits depth. In summary, stainless steel type 304 was found vulnerable to pitting corrosion attack under chloride ion stress.
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In-situ Electrochemical Investigations for Monitoring Pitting Corrosion Potential of Passivated Steel under Diverse Cl- Anion Stress and its Micro Structural Evaluation Muhammad Faizan1*, Humair Ahmed1, S. M Mohsin Jafri1, Syed Asad Ali2 and Azhar Mahmood2 1 Department of Materials Engineering, NED University of Engineering & Technology. National University of Sciences and Technology, Islamabad. * Corresponding author e-mail address: mfaizan@neduet.edu.pk 2 Abstract Pitting corrosion is very menacing and fatal type of localized attack on metallic structures. Customarily pitting corrosion is monitored by pitting potential which could be readily measured by the galvanostatic anodic polarization technique. In current communication, potentiodynamic scan graphs were acquired to find out pitting potential, corrosion potential and passivating potential for stainless steel type 304 sample electrodes while immersed in the 0M, 2M, 3M, 4M and 5M HCl solutions. Comparison of results have concluded that as chloride ion stress increased, it more aggressively attacked on passive layer and induced pitting corrosion at lower threshold potentials thus decreased Pitting potential (Epit) of subject stainless steel type 304 sample electrodes. Moreover pH of electrolyte decreased inside the pit due to hydrolysis of FeCl2 which furnished strong acid (HCl) and weak base (Fe(OH)2) thus further accelerate corrosion process and promote pits penetration. A large ratio between the anode and cathode areas have also favoured propagation of pits depth. In summary, stainless steel type 304 is vulnerable to pitting corrosion attack under chloride ion stress. Keywords: Pitting Corrosion, Stainless Steel, Passivation Potential, Pitting Potential, Cyclic Polarization Curve. 1. Introduction Pitting corrosion is very menacing and fatal type of localized attack on metallic constructions. Stainless steels irrespective of their passive ability, are also vulnerable to pitting corrosion in chloride ion solutions [1]. Customarily pitting corrosion is monitored by pitting potential which could be readily measured by the galvanostatic anodic polarization technique [2]. Particularly cyclic slow potentiodynamic anodic polarization method is worth mentioning to apply on Fe, Ni and Co alloys for quality control and material selections as depicted in ASTM G-61 standard [3]. Corrosion is associated with number of structural engineering problems subject to service conditions of fabricating alloy materials. Steel usually undergoes electrochemical corrosion mechanism which induced by various ions in the propinquity of the steel surface. Usually Stainless steels perform well under most of service environment owning to their good corrosion resistance which in turn depends upon constituent alloying elements. However under certain harsh conditions exposure stainless steel undergo corrosion attack because of passive film cracking. These defective small area points activate associated metal surface while large metal area remains passivated. This difference in relative areas promotes the corrosion, causing the pits to penetrate deeper. Aggressiveness of this phenomenon is determined by pH, nature of electrolyte and metal composition [4]. Several researchers have worked on stainless steel to find the extent of pitting corrosion in various electrolytes [5, 6, 7]. In current communication, potentiodynamic polarization attributes of stainless steel was studied in various chloride ions concentration and were reported in terms of passivation potential, pitting potential and corrosion potential of stainless steel. 2. Experimental Five austenitic stainless steel type 304 sample coupons in rectangular shape of size 1×1 cm and of composition C 0.08%, S 0.03%, Mn 1.33%, Ni 9.88%, Mo 2.44% and Cr 16.72% were employed for subject study. Electrodes preparation were involved epoxy mounting of sample metals and their subsequent drying for 24 Hrs. Afterward electrodes were grinded on MOPAO 260 E Grinder Machine by employing emery papers of range 180, 220, 800 and 1000 mesh size. In order to remove all kinds of grinding marks, Benetec Polishing Machine were used. Afterward samples were washed with de-ionized water, rinsed with acetone before drying and stored in desiccators. Copper wire was soldered onto the sample surface and the soldered sample then cold mounted in epoxy (Figure 1). Figure 1: Mounted soldered Sample for Potentiostat experiment After mounting m th he sample e, thermal conductivitty of samp ple was me easured to o check the continuity of electrical connectio on with the e sample through t diigital multi meter. After the t workin ng electrod des prepa aration, sample area a was me easured by y using stereo microscop pe. The pre epared sam mples were e then ana alysed for ccyclic pola arization in the Computeri C zed Poten ntiostat G750-350 series. Corro osion cell w was comprised of three electrodes e s (i) working electro ode i.e. Stainless S S Steel sample electro ode (ii) counte er electrode (Graphitte) and (iiii) the refe erence ele ectrode (S Saturated calomel c electro ode). Poten ntiodynamic scans we ere run witth these th hree electro odes imme ersed in the 0M M, 2M, 3M, 4M and 5M 5 HCl so olution. Mic crograph analysis a wa as carried out via Olympu us GX51 for f the dettermination n of pit forrmation on n these samples in chloride c ion con ntaining ele ectrolyte. 3. Results R and d Discuss sion Potentiodynamic scan gra aphs were acquired for stainle ess steel ttype 304 sample electro odes while immersed d in the 0M M, 2M, 3M, 4M and 5M 5 HCl so olutions an nd were depicte ed in figure es. Figure 2 showed the variatiion of pote ential (E) w with Curren nt (I) for stainlesss steel 30 04 in dilute hydrochloric acid. The line AB A represe ents the cathodic c behaviour of the sample while w line BC B located in active zone z i.e. a anodic diss solution reactio on. At pote entials morre positive than B, corrosion c ra ate increasses, and reaches r maximum at the e point C, which is passivation n potential (Eps). Th he transitio on from active dissolution n occurs in n the regio on C to D. A protecttive film be egins to fo orm and breakdown n of the causess a sudden drop in corrosion current density. At point D b protecttive film be egins which h correspo ond to pittin ng potentia al (Epit) and d promotes s pitting conseq quently spe ecimen faillure. Figure e 2: Cyclic Polarizatio on curve (E E vs I) of sttainless ste eel in dilute e HCl soluttion Figuress 3 to 7 had represe ented pola arization cu urve relatio onships be etween currrent (I) and Po otential (E)) for stainle ess steel type t 304 sample s electrodes in the 0M, 2M, 2 3M, 4M an nd 5M HC Cl solution ns respecttively. In figure f 3 (0M HCl/ deionized water) corrosion potentiial (Ecorr) was w -217.5 5 mV, pass sivation po otential (Ep) was 856 6 mV & the pittting potential (Epitt) was found to o be 7.141 mV. Figure 4 (2M H HCl) had ex xhibited the corrrosion pottential (Ecoorr) at -205..6 mV, pas ssivation potential (E Ep) at 808 mV m and the pittting poten ntial (Epit) was at -10.87 mV. In figure 5 (3M HC Cl) the co orrosion potential (Ecorr) was w found to be -194 4 mV, passivation po otential (Ep) was 760 0 mV & the pittting potential (Epitt) was w -84 mV. m Where eas figure 6 (4M HC Cl) had dis splayed corrosion potentiial (Ecorr) at a -165 mV V, passiva ation poten ntial (Ep) a at 689 mV V & the pitting potential (E ( pitt) at-14 45 mV. In figure 7 (5 5M HCl) th he corrosio on potentia al (Ecorr) ound to be e -151.8 mV, m passivvation pote ential (Ep) was 600 mV & the e pitting was fo potential (Epitt) wa as -161.8 mV. m Figure e 3. Cyclic Polarizatio on Curve of Stainless s Steel Sam mple 1 in d deionized water w (0 M HCl)) Figure e 4: Cyclic Polarizatio on Curve of o Stainless s Steel Sam mple 2 in 2 2M HCl Solution Figure e 5: Cyclic Polarizatio on Curve of o Stainless s Steel Sam mple 3 in 3 3M HCl Solution. Figure e 6: Cyclic Polarizatio on Curve of o Stainless s Steel Sam mple 4 in 4 4M HCl Solution. Figure e 7: Cyclic Polarizatio on Curve of o Stainless s Steel Sam mple 5 in 5 5M HCl Solution. Figure 8 depicte ed variatio on trend of o Passiva ation Potentials of sstainless steel s in differen nt chlorine e concentra ations and d data valu ues were also a tabula ated in tab ble 1. It showed d that as the molarr concentrration of th he chloride e electrolyyte increas sed the passiva ation pote ential decrreased i.e.. potentiall required to passivvate the material m decreased. Figure 9 displayed relationships established between measured pitting potential (mV) of stainless steel sample electrode and HCl solution concentration while data values were also tabulated in table 2. It has showed that as the molar strength of chloride ion solution increased the pitting potential of stainless steel sample electrode decreased that is sample steel electrodes were more prone to pitting corrosion under high chloride ion stress. 900 Passivation Potential (mV) 850 800 750 700 650 600 550 0 2 3 4 5 Molar Concentration (M) Figure 8: Variation of Passivation Potentials with Chloride Concentrations 20 0 Pitting Potential (mV) ‐20 0 2 3 4 ‐40 ‐60 ‐80 ‐100 ‐120 ‐140 ‐160 ‐180 Molar Comcnetration (M) Figure 9: Variation of Pitting Potentials with Chloride Concentration 5 Figure 10 has illustrated the micrographs (at 500× & 1000× magnification) of stainless steel sample electrode after acquiring anticipated pitting potential state during potentiodynamic scan with immersion in 5 M HCl solution. By analysis of micrograph it was revealed that significant pitting attack occurred on the surface of Stainless Steel in chloride ion solution which confirmed that pits were forming at computed pitting potentials. Pits were formed at defective points of passive layer by oxidation of metal which accumulated positive charges in the form of Fe2+. It would then attract negatively charged chloride ions. The resulting ferrous chloride has hydrolysed to produce an insoluble ferrous hydroxide plus excess hydrogen and chloride ions. These ions have further promote the corrosion to propagate depth of pit as described by Loto [2]. Figure 10: Micrograph of a Pitted Sample in Chloride Ion Solution. 4. Conclusion Potential state at which pitting of metal initiated is called Pitting potential (Epit). It was recognized by sudden sharp rise of anodic current in polarization curve of figures 4-8 under various chloride ion stresses. Comparison of graphs concluded that as chloride ion stress increased, it more aggressively attacked on passive layer and induced pitting corrosion at lower threshold potentials thus decreased Pitting potential (Epit) of subject stainless steel type 304 sample electrodes. Moreover pH of electrolyte decreased inside the pit due to hydrolysis of FeCl2 which furnished strong acid (HCl) and weak base (Fe(OH)2) thus further accelerate corrosion process and promote pits penetration. A large ratio between the anode and cathode areas also favoured propagation of pits depth. Acknowledgment The authors wish to acknowledge the support provided for this research by Prof. Dr. M.Tufail Dean CPE, M. Abdul Ghani Chishty for Cyclic Polarization analysis and M.Mohsin Jafri for Micrograph analysis. 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