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Application of pure oxygen for the passivation of mild steel in high temperature and pressure Bayer liquor under conditions of disturbed flow
Corrosion Science 50 (2008) 1962–1970
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Application of pure oxygen for the passivation of mild steel in high temperature and pressure Bayer liquor under conditions of disturbed flow Gareth Kear *, Klaus Bremhorst Division of Mechanical Engineering, The University of Queensland, Brisbane, Qld 4072, Australia
a r t i c l e
i n f o
Article history: Received 17 January 2008 Accepted 22 April 2008 Available online 29 April 2008 Keywords: Flow affected corrosion Oxygen Bayer liquor Caustic Disturbed flow
a b s t r a c t The passivation characteristics of sulfuric acid cleaned mild steel in spent Bayer liquor (pH 14.4 and 160 °C) are examined using a high pressure nickel flow loop. An entrained atmosphere of 99.90% v/v oxygen gas is incorporated as an experimental variable along with Reynolds number (141,700 and 50,950) and intensity of fluid flow disturbance. State of passivation has been defined using criteria derived from transient polarisation resistance measurements and large-scale polarisation, linear sweep voltammetry. In the majority of cases, oxygenation introduces instantaneous passivation of the mild steel on contact with the Bayer liquor. In comparison to de-oxygenated and aerated electrolytes, this rapid rate of passivation can lead to up to an order of magnitude reduction in the quantity of charge associated with metal dissolution over 20 h. Although relative rates of corrosion when passive are low and largely independent of the level of flow disturbance, dissolution rates when passive are somewhat larger at the higher Reynolds number. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Under alumina refinery conditions, the corrosion of mild steel in high temperature and pressure 2.75 mol dm3 sodium hydroxide and Bayer liquor electrolytes (pH > 14) has a strong dependency on flow velocity, fluid disturbance and quality of passivation [1– 8]. Acceleration of metallic degradation can occur via either the inhibition or the physical removal/dissolution of the protective ‘scales’ that include iron and/or aluminum based oxides and hydroxides. Moreover, the management of aluminum–siliconbased scale deposits using sulfuric acid-based acid-cleaning also introduces an environment within which iron based films are thermodynamically unstable [9]. As a consequence, the rate of mild steel re-passivation in Bayer liquor has a significant influence over the overall quantity of metal loss experienced during exposure to the caustic cycle. Indeed, the majority of metallic losses occur within the hours immediately following an acid cleaning phase when the mild steel is relatively active. The optimisation of the conditions required for the rapid re-development of passive films on mild steel within high temperature and pressure Bayer liquors post acid-cleaning, therefore, is of primary concern, as passivation characteristics can dictate plant asset management regimes and operating parameters.
* Corresponding author. Present Address: Energy Technology Research Group, University of Southampton, Hampshire SO15 5AE, UK. Tel.: +44 23 80598931; fax: +44 23 80596727. E-mail address: [email protected] (G. Kear). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.04.012
Previous papers have examined the influence of disturbed caustic flow regimes on mild steel passivation using a pressurised nickel flow loop [8–10]. The apparatus simulates disturbed flow with a novel test section which contains a sudden pipe expansion followed by a contraction. Using this equipment the passivation rates of mild steel in both de-oxygenated and aerated electrolytes have been examined using either entrained atmospheres of nitrogen in conjunction with sodium hydroxide [8,9] or nitrogen and air in Bayer liquor [10]. Fe-based films formed over time in strongly alkaline solution at open circuit have been proposed by Rihan and Nešic´ [8], to be composed mainly of magnetite (Fe3O4), the protective nature of which may be dependent to some extent on the mode of formation. For example, the presence of oxygen may lead to the increased stability of ferric species both in the electrolyte and the passive film [8,11]. It has been noted [10] that the presence of oxygen has a potentially positive influence over the quality of the passive film produced on the mild steel in Bayer liquors, although neither the cathodic nor anodic electrochemistry of mild steel was found to be directly controlled by the rate of electrolyte mass transfer. In the current work, a further increase in the level of oxygen is considered for the encouragement of mild steel passivation under flowing conditions with the use of 99.90% v/v oxygen gas. However, there is significant salting out effect in Bayer liquor [12] due to a high dissolved solid content and a value of dissolved oxygen of 0.4 ppm (0.12 104 mol dm3) for Bayer liquor in contact with air (160 °C) [13] can only be multiplied by a factor of approximately five with the use of high purity oxygen. Thus, the concentration of
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dissolved oxygen is limited but the use of an entrained volume of oxygen gas can enable the maintenance of a steady level of dissolved oxygen in the liquor under pressure. Such an application would also be possible within certain process streams within an alumina refinery. As previously in [10], relative rates of corrosion in this work were monitored over time using both polarisation resistance and electrochemical analyses based on linear sweep voltammetry (LSV). Direct comparison is then be made with data produced in de-oxygenated and aerated Bayer liquors. 2. Experimental equipment and procedures 2.1. Materials and apparatus The Bayer liquor contained approximately 2.75 mol dm3 free hydroxide ions (pH > 14) and soluble components of bauxite ore that had completed a plant production cycle. The electrolyte was fully oxygenated prior to use and derived from the same batch used previously [10]. At 160 °C, the associated values of electrolyte density (q), viscosity (l) and kinematic viscosity (m) were 1.231 g cm3, 0.00278 g cm2 s1 and 0.00226 cm2 s1, respectively. A 6% v/v sulfuric acid (H2SO4) electrolyte was produced through dilution of analytical grade concentrated acid of specific gravity 1.84 with distilled water. Commercially supplied amine-based acid mild steel inhibitor (1.5% v/v) was also added. The kinematic viscosity of the combined acid electrolyte at the operating temperature of 60 °C was 0.00554 cm2 s1. All electrolytes were replaced between each individual sequence of measurements. The test section, Fig. 1, has internal diameters of 38.1 mm and 13.7 mm with overall lengths of 500 mm for the large diameter section and 300 mm for the smaller diameter down stream section. Details of the design of the nickel flow loop, pumps, autoclave, electrochemical cell (test section) and monitoring equipment have been described elsewhere [8,10]. Wetted materials included nickel 200 (UNS N02200), hastelloy C (UNS N06022), Monel 400 (UNS N04400), Inconel 601 (UNS N06601) and polytetrafluoroethylene (PTFE). UNS S31600, UNS N04400 and polychloropene tubing and fittings were used throughout the auxiliary systems supporting the loop. A pressure resistant (1.6 MPa at 60 °C) glass encased saturated silver/silver chloride (SSC[sat.]) electrode was used as a reference electrode during the voltammetric measurements and Nickel 200 was used as counter electrode material. The reference electrode port communicated to the centre of the test section via a nickel 200
cooling jacket and AISI 316 (UNS S31600) stainless steel tubing (£ = 6.35 mm). Each reference electrode measurement was corrected for temperature with the application of the Nernst equation [14]. Although a series of 16 patch-electrode positions was contained within the test section (Fig. 1), only electrodes 1 through 8 were incorporated in this current work. The positions of electrodes 1– 8 are given in Table 1, where x is the absolute distance and £ is the internal pipe diameter of the relevant pipe section. The exposed active area of electrodes 1–4 was 35.3 mm2 and the areas of electrodes 5–8 were 36.3 mm2. The working electrodes were manufactured from hot-rolled mild steel round bar conforming to AS/NZS 3679.1-300 [15]. Surface preparation of the electrode active surfaces involved mechanical polishing to 0.3 lm alumina and cleaned with ethanol (C2H5OH). In all cases, the electrodes were removed after each individual measurement and completely re-polished and re-conditioned prior to re-use. Fluid velocities (U) of 0.30 and 2.34 m s1 were applied in the large and small diameter areas of the flow loop test section, respectively. At 160 °C the respective Bayer liquor mean Reynolds numbers (Re = U£/m) were 50,950 and 141,700. The mean test section velocities used during the acid exposure period were 0.20 m s1 (Re = 4990) and 0.56 m s1 (Re = 13,870) at 60 °C. Oxygenation of the electrolytes was achieved through gas diffusion with oxygen (99.90% v/v). The electrolyte was subsequently kept under an atmosphere of oxygen at all times (both within and outside of the loop system). All electrochemical measurements were performed with a Gamry Instruments Inc. PC3 computer controlled potentiostat, ECM8 electrochemical multiplexer and Gamry Instruments Framework Version 3.20 with DC Corrosion Techniques software. Largescale polarisation linear sweep voltammetry measurements were performed at a relatively high potential sweep rate of 5 mV s1 in order to minimize surface disruption and resolve any non-steady state response of the anodic voltammetry. Cathodic and anodic polarisations were performed during separate exposures and were
Table 1 Distances from pipe contraction (electrodes 1–4) and the pipe expansion (electrodes 5–8) Electrode number
1
2
3
4
5
6
7
8
x/£
0.5
1.1
1.6
14.8
2.3
2.5
2.7
7.9
Fig. 1. Sketch of the flow loop test section showing positioning of the eight working electrodes [8]. Fluid transfer is from left to right.
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initiated from Ecorr after complete equilibration in the solution of interest. Polarisation resistance measurements were made at 2-h (7200 s) intervals, at a potential sweep rate of 0.167 mV s1 and a polarisation of Ecorr ± 10 mV (incorporating a negative to positive potential sweep). Analysis of the Rp polarisation response and the derivation of approximated corrosion current density (icorr) values have been described by Rihan and Nešic´ [8] and Kear and Bremhorst [10], respectively.
of physical process variable transients for an acid–Bayer liquor exposure cycle is presented in Fig. 2. Large-scale LSV (cathodic or anodic) was initiated on completion of the polarisation resistance testing. Relatively short-term exposure measurements were also taken during separate experiments, where large-scale polarisation was initiated immediately post attainment of 160 °C and 0.8 ± 0.05 MPa (within a period of approximately 1–2 h from contact of the mild steel with the Bayer liquor).
2.2. Flow loop operation
3. Results and discussion
With the exception of state of oxygenation, each step in the experimental exposure sequences was identical to that published previously for de-oxygenated and aerated Bayer liquor corrosion studies [10]. Initially, the loop and the assembled test section were filled with the acid electrolyte at 25 ± 5 °C and a mean velocity of 0.56 ± 0.02 m s1, with respect to the small diameter portion of the test section, was applied. The temperature of the aqueous acid was ramped over approximately 30 min to 60 ± 2 °C (333.15 ± 2 K). After a further 1 h of circulation, the acid was expulsed from the loop using compressed 99.90% oxygen gas. The loop was flushed with water twice and expulsion was again performed using oxygen. The test section was then isolated from the remainder of the loop in to which the fully oxygenated Bayer liquor was introduced. The temperature of the liquor was ramped to 160 ± 2 °C (433.15 ± 2 K) over approximately 1 h, after which, the test section was opened. A drop in temperature was corrected at a mean applied velocity of 2.36 ± 0.05 m s1 and further pressure was applied with external source of oxygenated Bayer liquor. On attainment of 160 °C and 0.8 ± 0.05 MPa total gauge pressure, a Rp/Ecorr trend measurement program was engaged using the potentiostat and multiplexer system. Data were recorded over a period of 22 ± 2 h. A typical series
Within the test section, the re-circulating flow down stream of the expansion step results in a significant increase in rates of mass transfer [16]; the magnitude of which, will reduce with distance down steam from the re-circulation zone. From Table 1 it may be noted that electrode 8, located at x/£ = 7.9, should experience a flow regime which is much less disturbed than electrodes 5–7. The latter electrodes are closer to a re-circulation zone that is expected to be present immediately down stream of a pipe expansion [17]. The sudden pipe contraction also introduces an increased intensity in flow disturbance within the vicinity of electrodes 1– 3. In addition, the comparatively higher Reynolds number (Re) in this smaller diameter section contributes to a larger overall enhancement in mass transfer. At a location of x/£ = 14.8, electrode 4 experiences flow that is well developed in comparison to electrodes 1–3. Fig. 3 provides an example of a corrosion potential (Ecorr) and an inverse resistance polarisation ðR1 p Þ transient responses measured for electrode 8 in de-oxygenated Bayer liquor [10]. Mild steel passivation is signified by a rapid decrease in the value of R1 p and a corresponding decrease in the negativity of Ecorr. Figs. 4–6 give three examples of Ecorr and R1 trends measured in this work p
Fig. 2. Process variable transients, where P is pressure and T is absolute temperature. The differential pressure was used as an alternative method for the determination of flow rate, the details of which are explained in [8]. The dashed lines indicate the approximate initiation and termination of the period during which polarisation resistance measurements were recorded.
G. Kear, K. Bremhorst / Corrosion Science 50 (2008) 1962–1970
Fig. 3. (a) Corrosion potential and (b) R1 p transient responses measured for electrode 8 during 22 h exposure to de-oxygenated Bayer liquor.
during exposure to the oxygenated spent Bayer liquor at 160 °C. The responses of electrodes 1, 7 and 8, which are at significantly
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different x/£ positions in the flow, are presented for oxygenated Bayer liquor in Figs. 4–6. Passivation of the mild steel appeared to be instantaneous at the temporal resolution of the resistance polarisation measurements, i.e., passivation of the mild steel occurred before polarisation resistance measurements could be initiated (<1 h post exposure to the Bayer liquor). The corrosion potential of the mild steel steadily became more negative with exposure, which was not noted in the de-oxygenated and aerated Bayer liquors [10] and may indicate compositional variation in the passive film with a greater availability of oxygen. The mean times to electrode passivation are compared in Fig. 7 for measurements made in de-oxygenated, aerated and oxygenated Bayer liquors. The responses shown in Fig. 4 are also typical of those measured for electrodes 2–4, which are located in the high velocity compartment post the sudden contraction (Re = 141,700). Those shown in Fig. 5 are typical of the passivation response of electrodes 5 and 6 (Re = 50,950), but not the complete response of electrode 7, which, in one instance, did not immediately passivate. The only significant difference between the two broad types of data (electrodes 1–4 and electrodes 5–8) was the magnitude of the polarisation resistance response when passive. Mean values of the related corrosion rate in terms of corrosion current density (icorr) for all three states of oxygenation are given in Fig. 8. From this figure it can be observed that in the oxygenated Bayer liquor highest values of corrosion were generally observed in the high velocity compartment of the test section. For the oxygenated liquor R1 p values were of the order 0.008 ± 0.002 A V1 cm2 for electrodes 1 through 4 and 0.005 ± 0.002 A V1 cm2 for electrodes 5 through 8. The relative order of these values is very similar to the de-oxygenated Bayer liquor corrosion rates shown in Fig. 8. The high comparative rates of corrosion can be related to a higher rate of
Fig. 4. (a) Corrosion potential and (b) R1 p transients for electrode 1 measured during the 22 h exposure to the oxygenated Bayer liquor. This response is identical to that measured for electrodes 2–4.
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Fig. 5. (a) Corrosion potential and (b) R1 p transient responses measured for electrode 7 during the 22 h exposure to the oxygenated Bayer liquor.
Fig. 6. (a) Corrosion potential and (b) R1 p transients for electrode 8 measured during the 22 h exposure to the oxygenated Bayer liquor. This response is identical to that measured for electrodes 5 and 6.
G. Kear, K. Bremhorst / Corrosion Science 50 (2008) 1962–1970
Fig. 7. Mean electrode passivation times as determined from the inflection of the resistance polarisation transients. (a) de-oxygenated, (b) aerated and (c) oxygenated Bayer liquor. An intrinsic value of ±1 h error is associated with the measurement frequency.
Fig. 8. Resistance polarisation derived, mean corrosion rates for both active and passive behaviour in the de-oxygenated, aerated and oxygenated Bayer liquors for Re = 141,700 (electrodes 1–4) and Re = 50,950 (electrodes 5–8).
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dissolution, or stripping of components of the passive film at higher flow rates [10]. The relative values of corrosion in the oxygenated electrolyte when passive (refer again to Fig. 8) were lower than the rates of corrosion calculated for the mild steel within the aerated electrolyte. Corrosion rates for the non-passivated (active) steel was up to a maximum of 80 times higher than the ‘oxygenated’ passive steel with individual values of 0.1 to 0.5 A V1 cm2. Thus, for most electrodes the rapid passivity induced by oxygenation early in the exposure sequence was able to reduce the relative rate of corrosion by almost 2 orders of magnitude in comparison to non-oxygenated liquors. Due to an apparent instantaneous rate of passivation in all but one instance of exposure to the oxygenated Bayer liquor, the effect of Reynolds number and electrode position on passivation rate could not be directly investigated using the R1 p and Ecorr transient data. This behaviour contrasts with the de-oxygenated and aerated electrolyte passivation rates [10], where immediate passivation of the mild steel was never observed; refer again to Fig. 7 and compare, for example, to the data given for electrode 8 in the de-oxygenated Bayer liquor, Fig. 3. The single transient that displayed active behaviour in the oxygenated Bayer liquor (electrode 7) perhaps provides an indication of the potential instability of the passivation process. The rate of passivation in this case was of a similar order to that measured in the de-oxygenated and aerated electrolytes. Thus, electrode performance may be sensitive to variation in conditions, such as those involving initial surface preparation. The overall R1 p derived, total charge for metal loss over the period of exposure for each working electrode is presented in Fig. 9. These data were produced via integration of the corrosion current density over the total period of Bayer liquor exposure. From the figure, it is clear that the application of high oxygen content atmosphere had a considerable beneficial influence over the quantity of metallic losses experienced by mild steel during exposure to the caustic electrolyte. A difference of up to an order of magnitude in overall materials degradation could be measured in some instances over an approximate 20 h period within the low velocity section of the test section. There was a relatively low value of corrosion resistance measured when passive at higher velocities, but the total quantity of corrosive losses experienced during exposure was still considerably reduced. This was due to the significant
Fig. 9. Total mean charge associated with corrosion of the mild steel in the (a) de-oxygenated, (b) aerated and (c) oxygenated electrolytes over the total period of exposure to the Bayer liquor.
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increase in the rate of passivation in the oxygenated electrolyte, the protective action of which, was at least as effective as that determined for the lower oxygen content liquors. Fig. 10 provides examples of the ‘active’, semi-passive’ and ‘passive’ anodic large-scale polarisation voltammetry responses that were measured for the mild steel in the Bayer liquor. No clear Reynolds number dependence could be observed from an analysis of experimental transpassivity/pitting potentials [18] and electrolyte mass transfer control of the anodic reaction rate under polarisation was not observed in any instance. However, inferences regarding state of passivation could be extracted from the voltammetry. Table 2 provides a summary of the classification of anodic polarisation response for each electrode as a function of state of oxygenation and period of exposure prior to measurement (the characteristics and electrochemistry of each classification of curve have been qualified previously [10]). Only a single active response, as indicated by a high currents, Ecorr < 1.0 V vs. SSC[sat.] and non-steady state peaks I–IV, was measured within the fully oxygenated environment (electrode 5, short-term exposure). Initial corrosion potentials of both the ‘semi-passive’ and ‘passive’ classifications were considerably more positive than 1.0 V vs. SSC[sat.]. Typically, the peaks noted as V, VI and VII (Fig. 10) were observed for the semi-passive state and the dissolution currents densities exhibited more irreversible kinetics compared to the active state. For electrode 7 in the oxygenated Bayer liquor, as shown in Fig. 10, peak VI appeared to include the polarisation of two reactions with similar electrode potentials occurring with a non-steady state dependence. A passive anodic response was assigned to the voltammetry if there was an absence of non-steady state peaks and very low dissolution currents at values
of polarisation within +400 to +500 mV of the initial corrosion potential. Where Ep is the mean empirical value of the peak potential in the active state (given in V vs. SSC[sat.]), and when considering the specific conditions of electrode exposure at pH > 14.4, peaks I through IV have been predicted to involve the anodic polarisation of the following equilibria [10]: EIp ¼ 0:97 0:01 V
Fe þ Al2 O2 4 () FeAl2 O4 þ 2e
1:28 V EIIp
¼ 0:90 0:03 V
ð1Þ
Fe þ 2OH () FeðOHÞ2 þ 2e 0:93 V; or
Table 2 Classification of electrode anodic polarisation response as a function of time of exposure to spent Bayer liquor at 160 °C Condition
Electrode number 1
2
3
4
5
6
7
8
De-oxygenated (short-term) De-oxygenated (long-term)
A SP
A SP
A SP
P SP
A SP
P SP
A SP
P SP
Aerated (short-term) Aerated (long-term)
P SP
P SP
P SP
P SP
A SP
SP SP
SP SP
P SP
Oxygenated (short-term) Oxygenated (long-term)
P P
P P
P P
P P
A SP
P P
SP SP
P SP
A, active; SP, semi-passive; and P, passive.
ð2Þ
Fe þ 4OH () FeðOHÞ 4 þ 3e
0:92 V EIII p
¼ 0:78 0:01 V
ð3Þ
Fe þ 3OH () FeðOHÞ3 þ 3e 0:77 V
EIV p ¼ 0:66 0:03 V
ð4Þ
FeðOHÞ2 þ OH () FeðOHÞ3 þ 1e 0:63 V; or
ð5Þ
3Fe þ 4H2 O () Fe3 O4 þ 8Hþ þ 8e 0:67 V
ð6Þ
The equilibrium potentials at the right of each reaction are shown vs. SSC[sat.] and are corrected from standard conditions [19] to the conditions of exposure experienced during the flow loop Bayer liquor cycle. Peaks V and VI, as measured in the semi-passive state, are likely to involve the same reactions assigned to peaks III and IV, the potentials of which would be shifted to relatively positive values by a significant potential drop due to an increased resistance of the passive film. Finally, EVII p (0.230 ± 0.05 V vs. SSC[sat.]) can be related to: 2Fe þ 3H2 O () Fe2 O3 þ 6Hþ þ 6e
Fig. 10. A typical example of a steel surfaces, measured in the short-term, displaying fully ‘active’ anodic polarisation (de-oxygenated, electrode 7), ‘semi-passive’ (oxygenated, electrode 7) and ‘passive’ behaviour (oxygenated, electrode 8).
0:27 V:
ð7Þ
The reduction in the number of non-steady state peaks with increasing quality of passivation is the result of increasing irreversibly of reaction rate. When considering the LSV derived data, the oxygenated Bayer liquor produced the greatest number of electrodes exhibiting a fully passive response over short-term exposure periods (Table 2). This was especially true of the high velocity compartment of the test section down stream of the sudden pipe contraction (electrodes 1 through 4). Relative to the aerated liquor, more of the electrodes within the fully oxygenated liquor also tended to maintain or achieve a fully passive voltammetric response after the longer term exposure period. One instance of active corrosion was noted for electrode 5, however, after approximately 1.5 h of exposure to the oxygenated Bayer liquor, which again indicates a finite level of uncertainty associated with the attainment of apparent full- or semi-passivation over short exposure periods. In comparison, and with the exception of electrodes 4, 6 and 8, the short-term character of the mild steel in the de-oxygenated liquor was active corrosion, although long-term exposure led to a global semi-passive character. In an analogous manner to the de-oxygenated and aerated electrolytes [10], cathodic voltammetry in the oxygenated Bayer liquor produced charge transfer controlled kinetics dominated by the reduction of water: 2H2 O þ 2e ! H2 þ 2OH :
ð8Þ
Due to the small differences in absolute value of dissolved oxygen between de-oxygenated and oxygenated states, very little variation in the voltammetry should be expected in terms of the oxygen electrode: O2 þ 2H2 O þ 4e () 4OH :
ð9Þ
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G. Kear, K. Bremhorst / Corrosion Science 50 (2008) 1962–1970 Table 3 Values of cathodic Tafel slope and associated linear correlation coefficient (in parenthesis) for de-oxygenated, aerated and oxygenated Bayer liquor at 160 °C Condition
Tafel slope (mV decade1) (linear correlation coefficient) Electrode number 1
2
3
4
5
6
7
8
De-oxygenated (short-term) De-oxygenated (long-term)
92 (0.999) 167 (0.997)
161 (0.997) 96 (0.997)
170 (0.997) 123 (0.998)
178 (0.998) 115 (0.999)
140 (0.997) 95 (0.999)
149 (0.998) 86 (0.999)
– 86 (0.999)
116 (0.998) 93 (1.000)
Aerated (short-term) Aerated (long-term)
179 (1.000) 108 (1.000)
203 (0.997) 111 (1.000)
197 (0.998) 160 (0.999)
204 (0.999) 168 (0.998)
175 (0.998) 122 (1.000)
175 (0.999) 104 (0.999)
159 (0.999) 125 (0.995)
137 (0.998) 117 (0.999)
Oxygenated (short-term) Oxygenated (long-term)
209 (0.999) 178 (1.000)
194 (0.999) 187 (1.000)
197 (0.998) 165 (0.999)
215 (0.999) 183 (0.999)
– 137 (0.999)
178 (0.999) 161 (1.000)
199 (0.999) 142 (1.000)
189 (0.999) 167 (1.000)
face character could either influence the reaction mechanism of the reduction of water itself, or significantly increase the resistance related to ionic or electronic transfer through surface films [21]. It is also possible that the levels of dissolved oxygen are high enough, in the presence of an atmosphere of entrained oxygen, to make a significant contribution to the overall character of the cathodic response. 4. Conclusions
Fig. 11. Short- and long-term cathodic LSV measured at electrodes 3 and 5 in oxygenated Bayer liquor. Exposure times to the caustic electrolyte are given in parenthesis and Tafel slopes are indicted by the broken lines. These data are typical of those measured at all electrodes.
As the reactions were clearly controlled by charge transfer (Tafel slope [20] responses for each condition are given in Table 3), it can be noted that Reynolds number had no significant influence on the electrochemistry of the cathodic reactions at polarisations 300 to 400 mV relative to Ecorr. A series of examples describing Tafel slope derivation are presented in Fig. 11 for electrodes 3 and 5 in the oxygenated liquor as a function of Bayer liquor exposure period. Considering a simple reduction processes involving a single reaction, at 160 °C, a single exchange of electrons in the rate determining step of the reaction mechanism provides a Tafel slope of approximately 0.172 V decade1. A two-electron exchange will give 0.086 V decade1. The majority of all experimentally determined cathodic Tafel slopes measured were more negative than 0.1 V decade1 (Table 3). For example, the mean value of Tafel slope changed from 0.154 ± 0.021 to 0.100 ± 0.013 V decade1 (de-oxygenated) and 0.178 ± 0.023 to 0.127 ± 0.024 V decade1 (aerated) over approximately 20 h of exposure. However, the most negative values were observed for the fully oxygenated Bayer liquor. In this case, both short- and long-term mean slopes from all eight electrodes were 0.192 ± 0.018 and 0.167 ± 0.018 mV decade1, respectively. All of these values are statistically distinct and, therefore, it is likely that higher levels of oxygen induce a significant modification of the kinetic mechanism of the cathodic half-cell (such a difference is also indicated by the unique character of the oxygenated Bayer liquor corrosion potential transients that were measured during the polarisation resistance testing). The increase in the negative value of Tafel slope in oxygenated Bayer liquor may be due to a variation in surface film composition and/or surface film coverage relative to the aerated and oxygenated cases. This variation in sur-
(1) The contact of atmospheres of oxygen gas with spent Bayer liquor at 160 °C has been shown to facilitate the rapid and effective passivation of acid activated mild steel. In contrast to equivalent results taken in de-oxygenated and aerated electrolytes, passivation was found to occur over liquor exposure times less than 1 h. When passive, somewhat higher dissolution rates were measured using polarisation resistance measurements at greater Reynolds numbers. These latter data emulate the behaviour of de-oxygenated Bayer liquor. No clear distinction, however, could be made between materials performance as a function of disturbed and relatively undisturbed flows in the fully oxygenated liquor. (2) The overall charge associated with the mild steel corrosion process for a period of approximately 20 h exposure could be reduced by up to an order of magnitude relative to the de-oxygenated and aerated cases. The primary factor leading to the increase in corrosion resistance was the apparent instantaneous rate of electrode passivation on contact with the caustic electrolyte. (3) Some limited irreproducibility in the rate of passivation when using oxygenation was observed. This appears to indicate that universal passivation may not be achieved under all conditions experienced in an alumina refinery. The application of high oxygen atmospheres on site may also introduce a considerable hazard to health and safety, the risks associated with which, would require effective management.
Acknowledgements The authors gratefully acknowledge the contribution of materials from The University of Saskatchewan and the initial flow loop development work of S. Nešic´, R.O. Rihan, B. Gammie and S. Coles (The University of Queensland). References [1] U. Meyer, C.C. Brosnan, K. Bremhorst, R. Tomlins, A. Atrens, Wear 176 (1994) 163–171. [2] U. Meyer, A. Atrens, Mater. Perf. 33 (1994) 57–60. [3] U. Meyer, A. Atrens, Wear 189 (1995) 107–116. [4] S. Giddey, C.S. Zheng Ya Lu, D.F.A. Koch, Corrosion of mild steel in turbulent caustic solution, in: Proceedings of 13th International Corrosion Congress, Australasian Corrosion Association, Melbourne, Australia, 1996.
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[5] S. Giddey, B. Cherry, F. Lawson, M. Forsyth, Corros. Sci. 40 (1998) 839–842. [6] S. Giddey, B. Cherry, F. Lawson, M. Forsyth, D.F.A. Koch, High temperature, high pressure equipment for the study of erosion corrosion of metals in corrosive liquors – application to the electrochemical behaviour of AISI 1020 steel in Bayer liquor, in: Proceedings of Corrosion and Prevention’97, Australian Corrosion Association, Melbourne, Australia, 1997. [7] Q. Lu, M.M. Stack, C.R. Wiseman, J. Appl. Electrochem. 31 (2001) 1373–1379. [8] R.O. Rihan, S. Nešic´, Corros. Sci. 48 (2006) 2633–2659. [9] R.O. Rihan, S. Nešic´, Corros. Sci. 48 (2006) 2660–2675. [10] G. Kear, K. Bremhorst, Corrosion 64 (2008) 289–300. [11] G. Rocchini, Corros. Sci. 36 (1994) 2043–2061. [12] R. Battino, R. Rettich, T. Toshihiro, J. Phys. Chem. Ref. Data 12 (1983) 163–178. [13] J.T. Malito, M.L. Roberson, Removal of high molecular weight organic compounds from Bayer process caustic liquor, United States Patent: 4,663,133, 1987. [14] D. Pletcher, A First Course in Electrode Processes, The Electrochemical Consultancy, Romsey, 1991.
[15] AS/NZS 3679.1, Structural steel – hot-rolled bars and sections, Standards Australia/Standards New Zealand, Homebush, Australia/Wellington, New Zealand, 1996. [16] G. Kear, K. Bremhorst, S. Coles, S.-H. Huáng, Corros. Sci., in press, doi: 10.1016/ j.corsci.2008.04.001. [17] S. Nesic, J. Bienkowski, K. Bremhorst, K.-S. Yang, Corrosion 56 (2000) 1005– 1014. [18] ASTM G61, Standard test method for conducting cyclic potentiodynamic polarization measurements for localized corrosion susceptibility of iron-, nickel-, or cobalt-based alloys, ASTM International, West Conshohocken, USA, 2003. [19] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. [20] J. Tafel, K. Schmitz, K. Naremann, B.B. Emmert, Z. Phys. Chem. 50 (1905) 641– 712. [21] B.D. Craig, Fundamental Aspects of Corrosion Films in Corrosion Science, Plenum Press, London, 1991.
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Application of pure oxygen for the passivation of mild steel in high temperature and pressure Bayer liquor under conditions of disturbed flow Gareth Kear *, Klaus Bremhorst Division of Mechanical Engineering, The University of Queensland, Brisbane, Qld 4072, Australia
a r t i c l e
i n f o
Article history: Received 17 January 2008 Accepted 22 April 2008 Available online 29 April 2008 Keywords: Flow affected corrosion Oxygen Bayer liquor Caustic Disturbed flow
a b s t r a c t The passivation characteristics of sulfuric acid cleaned mild steel in spent Bayer liquor (pH 14.4 and 160 °C) are examined using a high pressure nickel flow loop. An entrained atmosphere of 99.90% v/v oxygen gas is incorporated as an experimental variable along with Reynolds number (141,700 and 50,950) and intensity of fluid flow disturbance. State of passivation has been defined using criteria derived from transient polarisation resistance measurements and large-scale polarisation, linear sweep voltammetry. In the majority of cases, oxygenation introduces instantaneous passivation of the mild steel on contact with the Bayer liquor. In comparison to de-oxygenated and aerated electrolytes, this rapid rate of passivation can lead to up to an order of magnitude reduction in the quantity of charge associated with metal dissolution over 20 h. Although relative rates of corrosion when passive are low and largely independent of the level of flow disturbance, dissolution rates when passive are somewhat larger at the higher Reynolds number. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Under alumina refinery conditions, the corrosion of mild steel in high temperature and pressure 2.75 mol dm3 sodium hydroxide and Bayer liquor electrolytes (pH > 14) has a strong dependency on flow velocity, fluid disturbance and quality of passivation [1– 8]. Acceleration of metallic degradation can occur via either the inhibition or the physical removal/dissolution of the protective ‘scales’ that include iron and/or aluminum based oxides and hydroxides. Moreover, the management of aluminum–siliconbased scale deposits using sulfuric acid-based acid-cleaning also introduces an environment within which iron based films are thermodynamically unstable [9]. As a consequence, the rate of mild steel re-passivation in Bayer liquor has a significant influence over the overall quantity of metal loss experienced during exposure to the caustic cycle. Indeed, the majority of metallic losses occur within the hours immediately following an acid cleaning phase when the mild steel is relatively active. The optimisation of the conditions required for the rapid re-development of passive films on mild steel within high temperature and pressure Bayer liquors post acid-cleaning, therefore, is of primary concern, as passivation characteristics can dictate plant asset management regimes and operating parameters.
* Corresponding author. Present Address: Energy Technology Research Group, University of Southampton, Hampshire SO15 5AE, UK. Tel.: +44 23 80598931; fax: +44 23 80596727. E-mail address: [email protected] (G. Kear). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.04.012
Previous papers have examined the influence of disturbed caustic flow regimes on mild steel passivation using a pressurised nickel flow loop [8–10]. The apparatus simulates disturbed flow with a novel test section which contains a sudden pipe expansion followed by a contraction. Using this equipment the passivation rates of mild steel in both de-oxygenated and aerated electrolytes have been examined using either entrained atmospheres of nitrogen in conjunction with sodium hydroxide [8,9] or nitrogen and air in Bayer liquor [10]. Fe-based films formed over time in strongly alkaline solution at open circuit have been proposed by Rihan and Nešic´ [8], to be composed mainly of magnetite (Fe3O4), the protective nature of which may be dependent to some extent on the mode of formation. For example, the presence of oxygen may lead to the increased stability of ferric species both in the electrolyte and the passive film [8,11]. It has been noted [10] that the presence of oxygen has a potentially positive influence over the quality of the passive film produced on the mild steel in Bayer liquors, although neither the cathodic nor anodic electrochemistry of mild steel was found to be directly controlled by the rate of electrolyte mass transfer. In the current work, a further increase in the level of oxygen is considered for the encouragement of mild steel passivation under flowing conditions with the use of 99.90% v/v oxygen gas. However, there is significant salting out effect in Bayer liquor [12] due to a high dissolved solid content and a value of dissolved oxygen of 0.4 ppm (0.12 104 mol dm3) for Bayer liquor in contact with air (160 °C) [13] can only be multiplied by a factor of approximately five with the use of high purity oxygen. Thus, the concentration of
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dissolved oxygen is limited but the use of an entrained volume of oxygen gas can enable the maintenance of a steady level of dissolved oxygen in the liquor under pressure. Such an application would also be possible within certain process streams within an alumina refinery. As previously in [10], relative rates of corrosion in this work were monitored over time using both polarisation resistance and electrochemical analyses based on linear sweep voltammetry (LSV). Direct comparison is then be made with data produced in de-oxygenated and aerated Bayer liquors. 2. Experimental equipment and procedures 2.1. Materials and apparatus The Bayer liquor contained approximately 2.75 mol dm3 free hydroxide ions (pH > 14) and soluble components of bauxite ore that had completed a plant production cycle. The electrolyte was fully oxygenated prior to use and derived from the same batch used previously [10]. At 160 °C, the associated values of electrolyte density (q), viscosity (l) and kinematic viscosity (m) were 1.231 g cm3, 0.00278 g cm2 s1 and 0.00226 cm2 s1, respectively. A 6% v/v sulfuric acid (H2SO4) electrolyte was produced through dilution of analytical grade concentrated acid of specific gravity 1.84 with distilled water. Commercially supplied amine-based acid mild steel inhibitor (1.5% v/v) was also added. The kinematic viscosity of the combined acid electrolyte at the operating temperature of 60 °C was 0.00554 cm2 s1. All electrolytes were replaced between each individual sequence of measurements. The test section, Fig. 1, has internal diameters of 38.1 mm and 13.7 mm with overall lengths of 500 mm for the large diameter section and 300 mm for the smaller diameter down stream section. Details of the design of the nickel flow loop, pumps, autoclave, electrochemical cell (test section) and monitoring equipment have been described elsewhere [8,10]. Wetted materials included nickel 200 (UNS N02200), hastelloy C (UNS N06022), Monel 400 (UNS N04400), Inconel 601 (UNS N06601) and polytetrafluoroethylene (PTFE). UNS S31600, UNS N04400 and polychloropene tubing and fittings were used throughout the auxiliary systems supporting the loop. A pressure resistant (1.6 MPa at 60 °C) glass encased saturated silver/silver chloride (SSC[sat.]) electrode was used as a reference electrode during the voltammetric measurements and Nickel 200 was used as counter electrode material. The reference electrode port communicated to the centre of the test section via a nickel 200
cooling jacket and AISI 316 (UNS S31600) stainless steel tubing (£ = 6.35 mm). Each reference electrode measurement was corrected for temperature with the application of the Nernst equation [14]. Although a series of 16 patch-electrode positions was contained within the test section (Fig. 1), only electrodes 1 through 8 were incorporated in this current work. The positions of electrodes 1– 8 are given in Table 1, where x is the absolute distance and £ is the internal pipe diameter of the relevant pipe section. The exposed active area of electrodes 1–4 was 35.3 mm2 and the areas of electrodes 5–8 were 36.3 mm2. The working electrodes were manufactured from hot-rolled mild steel round bar conforming to AS/NZS 3679.1-300 [15]. Surface preparation of the electrode active surfaces involved mechanical polishing to 0.3 lm alumina and cleaned with ethanol (C2H5OH). In all cases, the electrodes were removed after each individual measurement and completely re-polished and re-conditioned prior to re-use. Fluid velocities (U) of 0.30 and 2.34 m s1 were applied in the large and small diameter areas of the flow loop test section, respectively. At 160 °C the respective Bayer liquor mean Reynolds numbers (Re = U£/m) were 50,950 and 141,700. The mean test section velocities used during the acid exposure period were 0.20 m s1 (Re = 4990) and 0.56 m s1 (Re = 13,870) at 60 °C. Oxygenation of the electrolytes was achieved through gas diffusion with oxygen (99.90% v/v). The electrolyte was subsequently kept under an atmosphere of oxygen at all times (both within and outside of the loop system). All electrochemical measurements were performed with a Gamry Instruments Inc. PC3 computer controlled potentiostat, ECM8 electrochemical multiplexer and Gamry Instruments Framework Version 3.20 with DC Corrosion Techniques software. Largescale polarisation linear sweep voltammetry measurements were performed at a relatively high potential sweep rate of 5 mV s1 in order to minimize surface disruption and resolve any non-steady state response of the anodic voltammetry. Cathodic and anodic polarisations were performed during separate exposures and were
Table 1 Distances from pipe contraction (electrodes 1–4) and the pipe expansion (electrodes 5–8) Electrode number
1
2
3
4
5
6
7
8
x/£
0.5
1.1
1.6
14.8
2.3
2.5
2.7
7.9
Fig. 1. Sketch of the flow loop test section showing positioning of the eight working electrodes [8]. Fluid transfer is from left to right.
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initiated from Ecorr after complete equilibration in the solution of interest. Polarisation resistance measurements were made at 2-h (7200 s) intervals, at a potential sweep rate of 0.167 mV s1 and a polarisation of Ecorr ± 10 mV (incorporating a negative to positive potential sweep). Analysis of the Rp polarisation response and the derivation of approximated corrosion current density (icorr) values have been described by Rihan and Nešic´ [8] and Kear and Bremhorst [10], respectively.
of physical process variable transients for an acid–Bayer liquor exposure cycle is presented in Fig. 2. Large-scale LSV (cathodic or anodic) was initiated on completion of the polarisation resistance testing. Relatively short-term exposure measurements were also taken during separate experiments, where large-scale polarisation was initiated immediately post attainment of 160 °C and 0.8 ± 0.05 MPa (within a period of approximately 1–2 h from contact of the mild steel with the Bayer liquor).
2.2. Flow loop operation
3. Results and discussion
With the exception of state of oxygenation, each step in the experimental exposure sequences was identical to that published previously for de-oxygenated and aerated Bayer liquor corrosion studies [10]. Initially, the loop and the assembled test section were filled with the acid electrolyte at 25 ± 5 °C and a mean velocity of 0.56 ± 0.02 m s1, with respect to the small diameter portion of the test section, was applied. The temperature of the aqueous acid was ramped over approximately 30 min to 60 ± 2 °C (333.15 ± 2 K). After a further 1 h of circulation, the acid was expulsed from the loop using compressed 99.90% oxygen gas. The loop was flushed with water twice and expulsion was again performed using oxygen. The test section was then isolated from the remainder of the loop in to which the fully oxygenated Bayer liquor was introduced. The temperature of the liquor was ramped to 160 ± 2 °C (433.15 ± 2 K) over approximately 1 h, after which, the test section was opened. A drop in temperature was corrected at a mean applied velocity of 2.36 ± 0.05 m s1 and further pressure was applied with external source of oxygenated Bayer liquor. On attainment of 160 °C and 0.8 ± 0.05 MPa total gauge pressure, a Rp/Ecorr trend measurement program was engaged using the potentiostat and multiplexer system. Data were recorded over a period of 22 ± 2 h. A typical series
Within the test section, the re-circulating flow down stream of the expansion step results in a significant increase in rates of mass transfer [16]; the magnitude of which, will reduce with distance down steam from the re-circulation zone. From Table 1 it may be noted that electrode 8, located at x/£ = 7.9, should experience a flow regime which is much less disturbed than electrodes 5–7. The latter electrodes are closer to a re-circulation zone that is expected to be present immediately down stream of a pipe expansion [17]. The sudden pipe contraction also introduces an increased intensity in flow disturbance within the vicinity of electrodes 1– 3. In addition, the comparatively higher Reynolds number (Re) in this smaller diameter section contributes to a larger overall enhancement in mass transfer. At a location of x/£ = 14.8, electrode 4 experiences flow that is well developed in comparison to electrodes 1–3. Fig. 3 provides an example of a corrosion potential (Ecorr) and an inverse resistance polarisation ðR1 p Þ transient responses measured for electrode 8 in de-oxygenated Bayer liquor [10]. Mild steel passivation is signified by a rapid decrease in the value of R1 p and a corresponding decrease in the negativity of Ecorr. Figs. 4–6 give three examples of Ecorr and R1 trends measured in this work p
Fig. 2. Process variable transients, where P is pressure and T is absolute temperature. The differential pressure was used as an alternative method for the determination of flow rate, the details of which are explained in [8]. The dashed lines indicate the approximate initiation and termination of the period during which polarisation resistance measurements were recorded.
G. Kear, K. Bremhorst / Corrosion Science 50 (2008) 1962–1970
Fig. 3. (a) Corrosion potential and (b) R1 p transient responses measured for electrode 8 during 22 h exposure to de-oxygenated Bayer liquor.
during exposure to the oxygenated spent Bayer liquor at 160 °C. The responses of electrodes 1, 7 and 8, which are at significantly
1965
different x/£ positions in the flow, are presented for oxygenated Bayer liquor in Figs. 4–6. Passivation of the mild steel appeared to be instantaneous at the temporal resolution of the resistance polarisation measurements, i.e., passivation of the mild steel occurred before polarisation resistance measurements could be initiated (<1 h post exposure to the Bayer liquor). The corrosion potential of the mild steel steadily became more negative with exposure, which was not noted in the de-oxygenated and aerated Bayer liquors [10] and may indicate compositional variation in the passive film with a greater availability of oxygen. The mean times to electrode passivation are compared in Fig. 7 for measurements made in de-oxygenated, aerated and oxygenated Bayer liquors. The responses shown in Fig. 4 are also typical of those measured for electrodes 2–4, which are located in the high velocity compartment post the sudden contraction (Re = 141,700). Those shown in Fig. 5 are typical of the passivation response of electrodes 5 and 6 (Re = 50,950), but not the complete response of electrode 7, which, in one instance, did not immediately passivate. The only significant difference between the two broad types of data (electrodes 1–4 and electrodes 5–8) was the magnitude of the polarisation resistance response when passive. Mean values of the related corrosion rate in terms of corrosion current density (icorr) for all three states of oxygenation are given in Fig. 8. From this figure it can be observed that in the oxygenated Bayer liquor highest values of corrosion were generally observed in the high velocity compartment of the test section. For the oxygenated liquor R1 p values were of the order 0.008 ± 0.002 A V1 cm2 for electrodes 1 through 4 and 0.005 ± 0.002 A V1 cm2 for electrodes 5 through 8. The relative order of these values is very similar to the de-oxygenated Bayer liquor corrosion rates shown in Fig. 8. The high comparative rates of corrosion can be related to a higher rate of
Fig. 4. (a) Corrosion potential and (b) R1 p transients for electrode 1 measured during the 22 h exposure to the oxygenated Bayer liquor. This response is identical to that measured for electrodes 2–4.
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Fig. 5. (a) Corrosion potential and (b) R1 p transient responses measured for electrode 7 during the 22 h exposure to the oxygenated Bayer liquor.
Fig. 6. (a) Corrosion potential and (b) R1 p transients for electrode 8 measured during the 22 h exposure to the oxygenated Bayer liquor. This response is identical to that measured for electrodes 5 and 6.
G. Kear, K. Bremhorst / Corrosion Science 50 (2008) 1962–1970
Fig. 7. Mean electrode passivation times as determined from the inflection of the resistance polarisation transients. (a) de-oxygenated, (b) aerated and (c) oxygenated Bayer liquor. An intrinsic value of ±1 h error is associated with the measurement frequency.
Fig. 8. Resistance polarisation derived, mean corrosion rates for both active and passive behaviour in the de-oxygenated, aerated and oxygenated Bayer liquors for Re = 141,700 (electrodes 1–4) and Re = 50,950 (electrodes 5–8).
1967
dissolution, or stripping of components of the passive film at higher flow rates [10]. The relative values of corrosion in the oxygenated electrolyte when passive (refer again to Fig. 8) were lower than the rates of corrosion calculated for the mild steel within the aerated electrolyte. Corrosion rates for the non-passivated (active) steel was up to a maximum of 80 times higher than the ‘oxygenated’ passive steel with individual values of 0.1 to 0.5 A V1 cm2. Thus, for most electrodes the rapid passivity induced by oxygenation early in the exposure sequence was able to reduce the relative rate of corrosion by almost 2 orders of magnitude in comparison to non-oxygenated liquors. Due to an apparent instantaneous rate of passivation in all but one instance of exposure to the oxygenated Bayer liquor, the effect of Reynolds number and electrode position on passivation rate could not be directly investigated using the R1 p and Ecorr transient data. This behaviour contrasts with the de-oxygenated and aerated electrolyte passivation rates [10], where immediate passivation of the mild steel was never observed; refer again to Fig. 7 and compare, for example, to the data given for electrode 8 in the de-oxygenated Bayer liquor, Fig. 3. The single transient that displayed active behaviour in the oxygenated Bayer liquor (electrode 7) perhaps provides an indication of the potential instability of the passivation process. The rate of passivation in this case was of a similar order to that measured in the de-oxygenated and aerated electrolytes. Thus, electrode performance may be sensitive to variation in conditions, such as those involving initial surface preparation. The overall R1 p derived, total charge for metal loss over the period of exposure for each working electrode is presented in Fig. 9. These data were produced via integration of the corrosion current density over the total period of Bayer liquor exposure. From the figure, it is clear that the application of high oxygen content atmosphere had a considerable beneficial influence over the quantity of metallic losses experienced by mild steel during exposure to the caustic electrolyte. A difference of up to an order of magnitude in overall materials degradation could be measured in some instances over an approximate 20 h period within the low velocity section of the test section. There was a relatively low value of corrosion resistance measured when passive at higher velocities, but the total quantity of corrosive losses experienced during exposure was still considerably reduced. This was due to the significant
Fig. 9. Total mean charge associated with corrosion of the mild steel in the (a) de-oxygenated, (b) aerated and (c) oxygenated electrolytes over the total period of exposure to the Bayer liquor.
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increase in the rate of passivation in the oxygenated electrolyte, the protective action of which, was at least as effective as that determined for the lower oxygen content liquors. Fig. 10 provides examples of the ‘active’, semi-passive’ and ‘passive’ anodic large-scale polarisation voltammetry responses that were measured for the mild steel in the Bayer liquor. No clear Reynolds number dependence could be observed from an analysis of experimental transpassivity/pitting potentials [18] and electrolyte mass transfer control of the anodic reaction rate under polarisation was not observed in any instance. However, inferences regarding state of passivation could be extracted from the voltammetry. Table 2 provides a summary of the classification of anodic polarisation response for each electrode as a function of state of oxygenation and period of exposure prior to measurement (the characteristics and electrochemistry of each classification of curve have been qualified previously [10]). Only a single active response, as indicated by a high currents, Ecorr < 1.0 V vs. SSC[sat.] and non-steady state peaks I–IV, was measured within the fully oxygenated environment (electrode 5, short-term exposure). Initial corrosion potentials of both the ‘semi-passive’ and ‘passive’ classifications were considerably more positive than 1.0 V vs. SSC[sat.]. Typically, the peaks noted as V, VI and VII (Fig. 10) were observed for the semi-passive state and the dissolution currents densities exhibited more irreversible kinetics compared to the active state. For electrode 7 in the oxygenated Bayer liquor, as shown in Fig. 10, peak VI appeared to include the polarisation of two reactions with similar electrode potentials occurring with a non-steady state dependence. A passive anodic response was assigned to the voltammetry if there was an absence of non-steady state peaks and very low dissolution currents at values
of polarisation within +400 to +500 mV of the initial corrosion potential. Where Ep is the mean empirical value of the peak potential in the active state (given in V vs. SSC[sat.]), and when considering the specific conditions of electrode exposure at pH > 14.4, peaks I through IV have been predicted to involve the anodic polarisation of the following equilibria [10]: EIp ¼ 0:97 0:01 V
Fe þ Al2 O2 4 () FeAl2 O4 þ 2e
1:28 V EIIp
¼ 0:90 0:03 V
ð1Þ
Fe þ 2OH () FeðOHÞ2 þ 2e 0:93 V; or
Table 2 Classification of electrode anodic polarisation response as a function of time of exposure to spent Bayer liquor at 160 °C Condition
Electrode number 1
2
3
4
5
6
7
8
De-oxygenated (short-term) De-oxygenated (long-term)
A SP
A SP
A SP
P SP
A SP
P SP
A SP
P SP
Aerated (short-term) Aerated (long-term)
P SP
P SP
P SP
P SP
A SP
SP SP
SP SP
P SP
Oxygenated (short-term) Oxygenated (long-term)
P P
P P
P P
P P
A SP
P P
SP SP
P SP
A, active; SP, semi-passive; and P, passive.
ð2Þ
Fe þ 4OH () FeðOHÞ 4 þ 3e
0:92 V EIII p
¼ 0:78 0:01 V
ð3Þ
Fe þ 3OH () FeðOHÞ3 þ 3e 0:77 V
EIV p ¼ 0:66 0:03 V
ð4Þ
FeðOHÞ2 þ OH () FeðOHÞ3 þ 1e 0:63 V; or
ð5Þ
3Fe þ 4H2 O () Fe3 O4 þ 8Hþ þ 8e 0:67 V
ð6Þ
The equilibrium potentials at the right of each reaction are shown vs. SSC[sat.] and are corrected from standard conditions [19] to the conditions of exposure experienced during the flow loop Bayer liquor cycle. Peaks V and VI, as measured in the semi-passive state, are likely to involve the same reactions assigned to peaks III and IV, the potentials of which would be shifted to relatively positive values by a significant potential drop due to an increased resistance of the passive film. Finally, EVII p (0.230 ± 0.05 V vs. SSC[sat.]) can be related to: 2Fe þ 3H2 O () Fe2 O3 þ 6Hþ þ 6e
Fig. 10. A typical example of a steel surfaces, measured in the short-term, displaying fully ‘active’ anodic polarisation (de-oxygenated, electrode 7), ‘semi-passive’ (oxygenated, electrode 7) and ‘passive’ behaviour (oxygenated, electrode 8).
0:27 V:
ð7Þ
The reduction in the number of non-steady state peaks with increasing quality of passivation is the result of increasing irreversibly of reaction rate. When considering the LSV derived data, the oxygenated Bayer liquor produced the greatest number of electrodes exhibiting a fully passive response over short-term exposure periods (Table 2). This was especially true of the high velocity compartment of the test section down stream of the sudden pipe contraction (electrodes 1 through 4). Relative to the aerated liquor, more of the electrodes within the fully oxygenated liquor also tended to maintain or achieve a fully passive voltammetric response after the longer term exposure period. One instance of active corrosion was noted for electrode 5, however, after approximately 1.5 h of exposure to the oxygenated Bayer liquor, which again indicates a finite level of uncertainty associated with the attainment of apparent full- or semi-passivation over short exposure periods. In comparison, and with the exception of electrodes 4, 6 and 8, the short-term character of the mild steel in the de-oxygenated liquor was active corrosion, although long-term exposure led to a global semi-passive character. In an analogous manner to the de-oxygenated and aerated electrolytes [10], cathodic voltammetry in the oxygenated Bayer liquor produced charge transfer controlled kinetics dominated by the reduction of water: 2H2 O þ 2e ! H2 þ 2OH :
ð8Þ
Due to the small differences in absolute value of dissolved oxygen between de-oxygenated and oxygenated states, very little variation in the voltammetry should be expected in terms of the oxygen electrode: O2 þ 2H2 O þ 4e () 4OH :
ð9Þ
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G. Kear, K. Bremhorst / Corrosion Science 50 (2008) 1962–1970 Table 3 Values of cathodic Tafel slope and associated linear correlation coefficient (in parenthesis) for de-oxygenated, aerated and oxygenated Bayer liquor at 160 °C Condition
Tafel slope (mV decade1) (linear correlation coefficient) Electrode number 1
2
3
4
5
6
7
8
De-oxygenated (short-term) De-oxygenated (long-term)
92 (0.999) 167 (0.997)
161 (0.997) 96 (0.997)
170 (0.997) 123 (0.998)
178 (0.998) 115 (0.999)
140 (0.997) 95 (0.999)
149 (0.998) 86 (0.999)
– 86 (0.999)
116 (0.998) 93 (1.000)
Aerated (short-term) Aerated (long-term)
179 (1.000) 108 (1.000)
203 (0.997) 111 (1.000)
197 (0.998) 160 (0.999)
204 (0.999) 168 (0.998)
175 (0.998) 122 (1.000)
175 (0.999) 104 (0.999)
159 (0.999) 125 (0.995)
137 (0.998) 117 (0.999)
Oxygenated (short-term) Oxygenated (long-term)
209 (0.999) 178 (1.000)
194 (0.999) 187 (1.000)
197 (0.998) 165 (0.999)
215 (0.999) 183 (0.999)
– 137 (0.999)
178 (0.999) 161 (1.000)
199 (0.999) 142 (1.000)
189 (0.999) 167 (1.000)
face character could either influence the reaction mechanism of the reduction of water itself, or significantly increase the resistance related to ionic or electronic transfer through surface films [21]. It is also possible that the levels of dissolved oxygen are high enough, in the presence of an atmosphere of entrained oxygen, to make a significant contribution to the overall character of the cathodic response. 4. Conclusions
Fig. 11. Short- and long-term cathodic LSV measured at electrodes 3 and 5 in oxygenated Bayer liquor. Exposure times to the caustic electrolyte are given in parenthesis and Tafel slopes are indicted by the broken lines. These data are typical of those measured at all electrodes.
As the reactions were clearly controlled by charge transfer (Tafel slope [20] responses for each condition are given in Table 3), it can be noted that Reynolds number had no significant influence on the electrochemistry of the cathodic reactions at polarisations 300 to 400 mV relative to Ecorr. A series of examples describing Tafel slope derivation are presented in Fig. 11 for electrodes 3 and 5 in the oxygenated liquor as a function of Bayer liquor exposure period. Considering a simple reduction processes involving a single reaction, at 160 °C, a single exchange of electrons in the rate determining step of the reaction mechanism provides a Tafel slope of approximately 0.172 V decade1. A two-electron exchange will give 0.086 V decade1. The majority of all experimentally determined cathodic Tafel slopes measured were more negative than 0.1 V decade1 (Table 3). For example, the mean value of Tafel slope changed from 0.154 ± 0.021 to 0.100 ± 0.013 V decade1 (de-oxygenated) and 0.178 ± 0.023 to 0.127 ± 0.024 V decade1 (aerated) over approximately 20 h of exposure. However, the most negative values were observed for the fully oxygenated Bayer liquor. In this case, both short- and long-term mean slopes from all eight electrodes were 0.192 ± 0.018 and 0.167 ± 0.018 mV decade1, respectively. All of these values are statistically distinct and, therefore, it is likely that higher levels of oxygen induce a significant modification of the kinetic mechanism of the cathodic half-cell (such a difference is also indicated by the unique character of the oxygenated Bayer liquor corrosion potential transients that were measured during the polarisation resistance testing). The increase in the negative value of Tafel slope in oxygenated Bayer liquor may be due to a variation in surface film composition and/or surface film coverage relative to the aerated and oxygenated cases. This variation in sur-
(1) The contact of atmospheres of oxygen gas with spent Bayer liquor at 160 °C has been shown to facilitate the rapid and effective passivation of acid activated mild steel. In contrast to equivalent results taken in de-oxygenated and aerated electrolytes, passivation was found to occur over liquor exposure times less than 1 h. When passive, somewhat higher dissolution rates were measured using polarisation resistance measurements at greater Reynolds numbers. These latter data emulate the behaviour of de-oxygenated Bayer liquor. No clear distinction, however, could be made between materials performance as a function of disturbed and relatively undisturbed flows in the fully oxygenated liquor. (2) The overall charge associated with the mild steel corrosion process for a period of approximately 20 h exposure could be reduced by up to an order of magnitude relative to the de-oxygenated and aerated cases. The primary factor leading to the increase in corrosion resistance was the apparent instantaneous rate of electrode passivation on contact with the caustic electrolyte. (3) Some limited irreproducibility in the rate of passivation when using oxygenation was observed. This appears to indicate that universal passivation may not be achieved under all conditions experienced in an alumina refinery. The application of high oxygen atmospheres on site may also introduce a considerable hazard to health and safety, the risks associated with which, would require effective management.
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