GI (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scienfiJic.net/AMR.893.397. Corrosion Behavior Of Austenitic Stainless Steel In High Sulphate.
Advanced Materials Research Vol. 893 (2014)pp 397-401 Online available since 2014/Feb/19at ~ 1 ~ ~ ~ ~ . s c i net enf(fic GI (2014)Trans Tech Publications, Switzerland doi:10.4028/www.scienfiJic.net/AMR.893.397
Corrosion Behavior Of Austenitic Stainless Steel In High Sulphate Content
A. lsmail1ra 1
Faculty of Mechanical and Manufacturing Engineering, University Tun Hussein Onn Malaysia, Johor, Malaysia
Keywords: 316L, Sulphate-Chloride ratio, Temperature, Breakdown potential, Passive range, Open Circuit Potential (OCP), Localized corrosion
Abstract. Austenitic stainless steels especially 316L has been used extensively in many sectors including construction, medical and household appliances due to their highly resistance to corrosion attack, reasonable cost and excel in mechanical properties. However, in corrosive media, 316L are susceptible to localised corrosion attack especially in seawater and high temperature. The corrosiveness of media increased as the anions contents increased. This paper presents the corrosion mechanism of 316L exposed to high concentration of sulphate in the salinity of seawater. The solution (media) was prepared according to the same composition as seawater including pH, salinity and dissolved oxygen. The corrosion mechanism were characterized to breakdown potential (Eb) of 316L which are the potential once reaches a sufficiently positive value and also known as pitting potential. This is the most point where localized corrosion susceptibility to evaluate and considered a potential, which could be an appropriate point according to any given combination of rnateriaUambient/testing methods. The Eb value were identified at 4OC, 20°C, 50°C and 80°C and compared with Eb value of 3 16L in seawater. The Eb value of 3 16L in high sulphate are higher compared to seawater in every temperature which elucidate that some anions accelerate corrosion attack whereas some anions such as sulphate behaves as inhibiting effect to 3 16L. Introduction Stainless steels (SS) have been widely used in industrial components for decades especially austenitic stainless steel 316L due to their well-documented resistance to corrosion but yet, stainless steel still suffers for corrosion attack especially in corrosive media such as seawater. Corrosion of SS in seawater is dependent mainly on the salt content (which increase the electrical conductivity) and its oxygen content. A number of variables can influence and complicate the course of corrosion in different ways such as chloride, sulphate and temperature. In the context of corrosion, SS are characterized by their passivity. Under certain conditions, steel is passive, where the corrosion rate for the metal is relatively low. Iron is considered an activepassive metal and, therefore, steel behaves similarly. Passivity can be defined as the loss of chemical reactivity under certain conditions [I]. Steel achieves this by having a passive film form along its surface. The passive film was formed by reaction of chromium with oxygen to produce chromium oxide as a protection from surrounding. Therefore, the characterisation of SS is according to the minimum 10.5% wt% Cr (Chromium) addition to iron [2]. For most SS, the maximum chromium content is about 30% and the minimum iron content is 50%. The stainless characteristic arises from the formation of an invisible and very adherent chromium-rich oxide surface film. This puts the steel in a passive state and when the film is breached, it immediately heals when oxygen is present. It is highly corrosion resistant and shows little or no corrosion if the passive film remains intact.
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As shown in Figure 1, this curve reveals an electrochemical response during anodic polarisation when the potential is scanned in the positive direction fiom E,,, (also known as OCP) and only very small currents (typically 4 0 p4/cm2 ) are recorded until Eb, is reached. At this point the current rises rapidly as a function of potential indicating that corrosion attack has initiated. The current corresponding to the breakdown potential is denoted as breakdown current (current density, ib ( ~lcm~))
The Passive Film. In appropriate conditions, some base metals can develop a surface condition that inhibits interactions with aqueous media. The condition is described as passivity and its development is called passivation. The effect is valuable in conferring corrosion resistance on bare metal surfaces even in aggressive environments. The corrosion resistance of austenitic stainless steels are depends on the formation of natural occurring transparent oxide films. These films may be inpaired by surface contaminations such as organic compounds or metallic or inorganic materials [3]. The formation of these film is by chromium oxide layer and is assumed to be responsible for the effectiveness of the passivation. In mild solutions, the outer layer is reasonably well defined and thick relatively to the dimensions of the whole film. Results and Discussion In anodic polarisation tests, the electrode potential of the material is scanned from the free corrosion potential (Ecorr),also known as the Open Circuit Potential (OCP), in the more positive direction at a fixed rate. When passivity is exhibited, the current initially remains very small. Once the potential reaches the breakdown potential (Eb) (at which the passive film breaks down due to the overpotential driving force), the current increases suddenly. The breakdown potential of the material provides information on the resistance of materials to passivity breakdown [4]. Each anodic ~. polarisation scan was reversed once the current reached a set current (ire,) of 5 0 0 ~ A / c m The degree of the increase in current beyond ire., gives an indication of the propensity for corrosion propagation [5]. An indication of the extent of propagation is therefore obtained by consideration of ,,,i which represents the maximum current attained should the current not begin to fall immediately after scan reversal. This experiment was conducted to quantify the effect of sulphate to corrosion attack on 3 16L. For passive alloy including austenitic stainless steels (3 16L), corrosion resistance is provided by a very thin surface film, known as passive fihn that is an invisible film of oxide, formed by the metal reacting with the ambient environment. Normally these films are fi-ee of pores, but their stability may be weakened locally in the environment contained aggressive anions. Figure 1 shows 3 16L in a solution that has higher sulphate content compared to seawater but same salinity at 4"C, 20°C, 50°C and 80°C. The cyclic polarisation performs in positive hysteresis at all temperatures indicates that after passive film destroyed, the material does not repair itself. Therefore, the corrosion attack propagate and localised corrosion will start. The breakdown potential at every temperature are higher compared to 3 16L in seawater. This indicates that sulphate has an inhibiting effect on pitting corrosion [6,7]. In addition, the presence of sulphate causes the distribution of available pit sites to be shifted to a higher potential, which causes pit propagation in both metastable and stable states
PI. Figure 2 present the cyclic polarisation of 3 16L in seawater. This is to compare the performance of 3 16L between seawater and seawater with high sulphate content. Similar to 3 16L in seawater of high sulphate content, the polarisation presents as positive hystheresis in 4"C, 20°C, 50°C and 80°C. There is a significant reduction in Eb as the temperature increases which explain that reduction in corrosion resistance of material to corrosion attack. In addition, the positive hysteresis in all solutions proved that sulphate ion inhibite 316L from further localised attack. Apart of that, the positive hystheresis could indicate that the passive film damage is not repaired and allowed pits to initiate [9]. However, the Eb value presents lower than the Eb value in high suphate content which reveals that 3 16L are more susceptible to localised attack in seawater compared to high sulphate
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containing chloride solution. The drastic change in the nature and the properties of oxide films on passive alloys i n water in increasing temperature is attributed to a breakdown of passivity [10,11].
Figure 1: Cyclic polarisation of 316L in high sulphate content
In both solutions, the Open Circuit Potential (OCP) values of 3 16L shifted towards inore active values as the temperature increased, while the breakdown corrosion potential presented the opposite tendency. Figure 3 presents potential as a hnction of current for 3 16L in high sulphate content and in seawater. The co~nparisonas shown in Figure 4 clearly shows that 3 16L has higher Eb value in high sulphate content compared to seawater. This explained that 316L has better corrosion resistance in high sulphate content compared to seawater. The severity of localised corrosion attack is shown in Figure 5 by optical microscope observation.
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Figure 2: Cyclic polarisation of 316L in seawater
Figure 3: Cyclic polarisation of Potential versus Current for 316L in (left) high sulpahe content and (right) seawater
+Seawater
Figure 4: Comparison of breakdown potential of 316L in high sulphate content and seawater
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Figure 5: Optical microscope image of corrosion attack on 316L in high sulphate content
Summary It is generally accepted that the risk for pitting decreases with increasing sulfate ( ~ 0 4 content ~3 in seawater. There are also indications that higher sulfate/chloride ionic ratio is needed to inhibit the pitting corrosion of stainless steels at higher chloride concentrations [12].Stainless steel 316L are prone to localised corrosion attack or pitting corrosion in seawater compared to high suphate content of seawater. Non halide anions such as sulphate ( ~ 0 4 can ~ 3reduce risk to pitting in chloride containing solutions. Their inhibiting effect depends on their concentration and chloride content of the solution. However, some anions may behave as inibition due to competitive adsorption with chloride ions on metal surface. Due to these competition, some anions behaves as protection to metal whereas some anions may behaves contradictly. References
[ l ] M.G. Fontana,Corrosion Engineering, McGraw-Hill, 1987, pp. 3-13. [2] Smith W.F., Principles of materials science and engineeringl996: McGraw Hill. [3] Roberg P.R., Corrosion engineering: Principles and practice: McGraw-Hill, 2008, pp 5, 169177. [4] Thompson N. G. and Payer J.H., DC electrochemical test methods. Vol. 6. 1998. 98. [5] Sedriks A.J., Corrosion stainless stee11979: John Wiley & Sons, New York. 282 [6] Dexter S. C. and Gao G.Y., Corrosion, 1988.44: p. 7 17. [7] Laitinen T., Localized corrosion of stainless steel in chloride, sulfate and thiosulfate containing environments. Corrosion Science, 2000. 42(3): p. 421-441. [8] Pistorius P.C. and Burstein G.T., Growth of corrosion pits on stainless steel in chloride solution containing dilute sulphate. Corrosion science, 1992. 33(12): p. 1885-1897. [9] Tait W.S., An introduction to electrochemistry corrosion testing for practicing engineers and scientist, 1994, PairODocs Publications. p. 119. [lo] Stellwag B., The mechanism of oxide film formation on austenitic stainless steels in high temperature water. Corrosion Science, 1998. 40(2/3): p. 337-370. [ I 11 Garfias-Mesias L. F., Sykes J.M.., Metastable pitting in 25 Cr duplex stainless steel. Corrosion Science, 1999.41(5): p. 959-987. [12] Kirkov P., The influence of halide ion concentration on the effects of surface active inhibitors in strong electrolytes. Corrosion Science, 1973, 13(10): p. 697-706.
