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The Formation and Properties of Passive Films on Iron
Abstract
A unified mechanism for the formation of passive films on iron in aqueous solutions is presented. The effects of water, oxygen, and oxidizing and non-oxidizing ions are considered. The γ-Fe2O3 film is formed first by the oxidation of water-formed magnetite and further thickening of the film takes place by the oxidation of diffusing Fe++ ion at the water surface of the oxide film. The main force leading to diffusion is the field set up by the adsorbed negative ion and the positive Fe++ ions at the surface of the metal. Some of the properties of the protective γ-Fe2O3 film and factors leading to its destruction are discussed.
... Special attention has been given to ultrafine particles, due to their connection to adverse health symptoms [55]. More specifically, toner and paper dust from printing devices may disperse in air, generating respirable particles that include ultrafine aerosols [33,48,[56][57][58][59][60]. Particle emissions from different parts of a printer were measured in another study [38] and concluded that ultrafine particles originate from the hightemperature fuser unit but not from the toner. ...
... However, current predictions are not based on a better assumption ( § 4). Calculations in Table 1 assumed that under oxic conditions, iron corrosion is 65 times more rapid than under anoxic conditions [60]. The data in Table 1 show that, assuming uniform corrosion, even under anoxic conditions, a Fe 0 PRB using particle size lesser than 1 mm will not last for 50 years. ...
... The non uniform nature of iron corrosion suggests that any prediction based on reaction stoichiometry is faulty. Moreover, the inaccessibility of the Fe 0 surface and the evidence that the multi-layered oxide scale on Fe 0 surface is insulating (non conductive -despite the inner Fe 3 O 4layer [60]) suggest that secondary (Fe II , H/H 2 ) and tertiary (e.g. Fe 3 O 4 , green rust) reducing agents should be incorporated in models. ...
High air pollution concentrations lead to serious health problems in urbanized and industrialized areas. In Istanbul, Golden Horn is a creek valley that is identified by its
special terrain that makes air pollutants difficult to disperse. The goal of this study is to determine the ozone levels in this region, considering that reducing ozone precursor
emissions accomplishes surface ozone control. Ozone in surface boundary layer is formed by photochemical reactions involving nitrogen oxides and is affected by urbanization, traffic, and industry. In order to investigate the air quality levels in Golden Horn, the surface ozone concentrations and its precursors (NO and NO2) in Alibeykoy and Kagithane regions are temporally analyzed herein. Moreover, the relationship of ozone with precipitation is also determined.
... Accordingly, the case for which reduction products are toxic is still actively discussed [20]. Moreover, the formation of the universal oxide film on The formation of the universal oxide film at the surface of Fe 0 is a characteristic of iron corrosion at pH > 4.5 [24, 25]. The film results from the precipitation of iron (hyrdr)oxides at the surface or in the vicinity of Fe 0 . ...
... In this respect, enhancing the reactivity of Fe 0 by reducing its particle size (μm and nm) for example will not avoid the formation of the universal oxide scale. The oxide scale is necessarily a diffusion barrier for all dissolved species as its shields the Fe 0 surface [24, 25]. Note that the initial oxide scale is porous alloying retarded transport of water and dissolved species including Fe II , H/H 2 and eventually molecular O 2 . ...
... Cohen [25] the film forms 65 times slower under anoxic conditions. Given that adsorbed Fe II and H/H 2 are powerful reducing agents, they will more likely contribute to contaminant reduction (if applicable) than the surface of Fe 0 [30]. ...
A knowledge system (KS) is a knowledge that is unique to a given group of persons. This form of knowledge may have a local or natural origin and is linked to the community that has produced it. On the contrary, the core of mainstream science (MS) is the desire to profoundly understand processes, through sequential studies such as hypothesis formulation, experiment and prediction. Thus, KS is communitarian and MS is universal. KS can be understood and rendered universal through MS. In general, a process discovery (know-how) may be intuitive, accidental, conjectural or inspirational but outcomes should be predictable and repeatable as soon as the know-why is achieved by MS. This paper argues that the technology of using metallic iron for water treatment has all the characteristics of a KS and that promoters of this technology have deliberately rejected scientific arguments leading to the know-why of the fortuitous discovery. Consequently, the technology has developed into an impasse where controversial discoveries are reported on all relevant aspects. It is concluded that the integrity of science in endangered by this communitarian behaviour.
... It 13 can be assumed that the model solutions contained up to 8 mg/L DO. The role of dissolved oxygen 14 in accelerating the kinetics of aqueous iron corrosion is well-documented (e.g. Cohen, 1959; 15 Stratmann and Müller, 1994). Using an oxic solution is a tool to enable the characterization of 16 clogging under relevant conditions at reasonable experimental durations. ...
... 4). The subsequent progressive decrease of the pH value is consistent with slower kinetics of iron 5 corrosion due to the formation of an oxide scale at the Fe 0 surface (Cohen, 1959; Evans, 1969; 6 Aleksanyan et al., 2007; Nesic, 2007). The most important issue fromFig. 2 is that for all Fe 0 - 7 containing systems, the effluent pH value is higher than 5.0. ...
... 6a confirms/shows unambiguously that such a system should contain as 4 less Fe 0 (volumetric proportion) as possible (Caré et al., 2013; Miyajima and Noubactep, 2013). 5 Considering the factor of 65 times to account for the differential kinetics of Fe 0 oxidation under 6 oxic (8 mg/L O 2 ) and anoxic (0 mg/L O 2 ) conditions (Cohen, 1959), it can be argued that the 7 shortest experimental duration (17 days) reported here could corresponds to about 1105 days under 8 anoxic conditions. These are more than 3 years necessary to observe clogging under the 9 experimental conditions of this work after 17 days. ...
Metallic iron (Fe0) is currently used in subsurface and above-ground water filtration systems on a pragmatic basis. Recent theoretical studies have indicated that, to be sustainable, such systems should not contain more than 60 % Fe0 (vol/vol). The prediction was already validated in a Fe0/sand system using methylene blue as an operational tracer. The present work is the first attempt to experimentally verify the new concept using pumice particles. A well-characterized pumice sample is used as operational supporting material and is mixed with 200 g of a granular Fe0, in volumetric proportions, varying from 0 to 100 %. The resulting column systems are characterized (i) by the time dependent evolution of their hydraulic conductivity and (ii) for their efficiency for the removal of CuII, NiII, and ZnII from a three-contaminants-solution (about 0.30 M of each metal). Test results showed a clear sustainability of the long term hydraulic conductivity with decreasing Fe0/pumice ratio. In fact, the pure Fe0 system clogged after 17 days, while the 25 % Fe0 system could operate for 36 days. The experimental data confirmed the view that well-designed Fe0 PRBs may be successful at removing both reducible and irreducible metal species.
... The role of dissolved oxygen in accelerating the kinetics of aqueous iron corrosion is well-documented (e.g. Cohen, 1959;Stratmann and Müller, 1994). Using an oxic solution is a tool to enable the characterization of clogging under relevant conditions at reasonable experimental durations. ...
... The subsequent progressive decrease of the pH value is consistent with slower kinetics of iron corrosion due to the formation of an oxide scale at the Fe 0 surface (Cohen, 1959;Evans, 1969;Aleksanyan et al., 2007;Nesic, 2007). The most important issue from Fig. 2 is that for all Fe 0 -containing systems, the effluent pH value is higher than 5.0. ...
... Considering the factor of 65 times to account for the differential kinetics of Fe 0 oxidation under oxic (8 mg/L O 2 ) and anoxic (0 mg/L O 2 ) conditions (Cohen, 1959), it can be argued that the shortest experimental duration (17 days) reported here could corresponds to about 1105 days under anoxic conditions. These are more than 3 years necessary to observe clogging under the experimental conditions of this work after 17 days. ...
Metallic iron (Fe(0)) is currently used in subsurface and above-ground water filtration systems on a pragmatic basis. Recent theoretical studies have indicated that, to be sustainable, such systems should not contain more than 60% Fe(0) (vol/vol). The prediction was already validated in a Fe(0)/sand system using methylene blue as an operational tracer. The present work is the first attempt to experimentally verify the new concept using pumice particles. A well-characterized pumice sample is used as operational supporting material and is mixed with 200 g of a granular Fe(0), in volumetric proportions, varying from 0 to 100%. The resulting column systems are characterized (i) by the time dependent evolution of their hydraulic conductivity and (ii) for their efficiency for the removal of Cu(II), Ni(II), and Zn(II) from a three-contaminants-solution (about 0.3 mM of each metal). Test results showed a clear sustainability of the long term hydraulic conductivity with decreasing Fe(0)/pumice ratio. In fact, the pure Fe(0) system clogged after 17 days, while the 25% Fe(0) system could operate for 36 days. The experimental data confirmed the view that well-designed Fe(0) PRBs may be successful at removing both reducible and non-reducible metal species.
... It has been demonstrated/recalled that under environmental conditions, the Fe 0 /H2O interface does not exist [101][102][103][104][105][106]. Rather, there is a minimum of two interfaces: Fe 0 /Fe-oxides and Fe-(hydr)oxide/H2O, with the material comprising a "core-shell" structure [80,81,[107][108][109][110][111][112]. Moreover, the (hydr)oxide layer comprises the location for H/H2 and Fe 2+ formation which is driven by Fe 0 corrosion. ...
... This approach would have been beneficial for the procurement of new systems as well as the modification and operation of existing systems (e.g., three-pitcher household filter). It is of vital importance that these systems are designed according to the intrinsic properties of Fe 0 [101,104,108,109], and known principles for designing conventional granular filters [146][147][148]. Moreover, the intrinsic properties of contaminants should be considered because classifications like "organic contaminants (e.g., dyes, pesticides, and pharmaceuticals/drugs)" or "industrial organic wastes (e.g., phenols and aromatic amines)" say nothing about the chemical reactivity or the affinity of the species of concern for the Fe 0 /H2O system. ...
There are many factors to consider for the design of appropriate water treatment systems including: cost, the concentration and type of biological and/or chemical contamination, concentration limits at which contaminant(s) are required to be removed, required flow rate, level of local expertise for on-going maintenance, and social acceptance. An ideal technology should be effective at producing clean, potable water; however it must also be low-cost, low-energy (ideally energy-free) and require low-maintenance. The use of packed beds containing metallic iron (Fe0 filters) has the potential to become a cheap widespread technology for both safe drinking water provision and wastewater treatment. Fe0 filters have been intensively investigated over the past two decades, however, sound design criteria are still lacking. This article presents an overview of the design of Fe0 filters for decentralized water treatment particularly in the developing world. A design for safe drinking water to a community of 100 people is also discussed as starting module. It is suggested that Fe0 filters have the potential for significant worldwide applicability, but particularly in the developing world. The appropriate design of Fe0 filters, however, is site-specific and dependent upon the availability of local expertise/materials.
... Reactions after Eq. 2 and Eq. 3 could be catalysed by the Fe 0 surface but more likely by the surface of in-situ generated iron corrosion products (including nascent oxides and oxyhydroxides) building the oxide scale. The oxide scale is built by a myriad of precipitation, crystallisation processes that will not be addressed herein [22,24,25,44,45]. Amonette et al. [46] has demonstrated quantitative dechlorination of CCl 4 by Fe II associated with goethite. ...
... However, the formation of Fe II -EDTA avoids/delays the formation of an oxide scale on Fe 0 such that direct reduction is likely to occur [50]. However, in real world situations an oxide scale will constantly shield the Fe 0 surface [21][22][23][24][25]. ...
The research community on using metallic iron (Fe0) for environmental remediation is virtually divided in two schools, characterized each by a different research tradition. The two are arbitrarily termed the ‘conventional’ and ‘critical’ schools. The conventional school has discovered Fe0 for environmental remediation and the critical school has conciliated the discovery with the mainstream corrosion science. It is very difficult to understand how both schools are suffering from a ‘dialogue of the deaf’. This communication clarifies the view of the critical school and demonstrates that there is no need for a third approach to conciliate both schools. All is needed is an holistic approach of the Fe0/H2O system, obeying to the laws of chemical thermodynamics.
Cite as:
Ebelle T.C., Makota S., Tepong-Tsindé R., Nassi A., Noubactep C. (2019): Metallic iron and the dialogue of the deaf. Fresenius Environmental Bulletin 28, 8331–8340.
... When a reactive Fe 0 particle is introduced in water, Fe 0 undergoes oxidative dissolution and [15,16]. On the other hand, the primary iron corrosion products (Fe II and H 2 /H) are reducing agents which reductive capacity is increased by the catalytic effects of the surface of oxides (secondary corrosion products) [17]. ...
... Fe 0 research was erroneously introduced as a new field of research. However, important work on aqueous iron corrosion under environmental conditions had been going on for decades [15,16,[41][42][43][44][45][46][47]. Moreover, fundamental knowledge on aqueous iron corrosion is available since the 1930s [23]. ...
The premise of this research note is that current research on metallic iron (Fe0) for environmental remediation and water treatment has started on a biased basis. Before expecting experienced researchers to correct flawed approaches compromising the future of the technology, the attention of new researchers should be drawn on the prevailing flawed conceptual models. There are guides on how to select good research topics, to perform good literature review, to select good mentors, and to write good scientific papers. But critically reviewing the published material is part of the competence of any new researcher in a given field of research. This research note summarizes the most critical issues of research on Fe0 for water treatment as asks some key questions which would help research beginners to find their way.
... It has been demonstrated/recalled that under environmental conditions, the Fe 0 /H 2 O interface does not exist [101][102][103][104][105][106]. Rather, there a minimum of two interfaces: Fe 0 /Fe-oxides and Fe-(hydr)oxide/H 2 O, with the material comprising a "core-shell" structure [80,81,[107][108][109][110][111][112]. Moreover, the (hydr)oxide layer comprises the location for H/H 2 and Fe 2+ formation which is driven by Fe 0 corrosion. ...
... three-pitcher household filter). It is of vital importance that these systems are designed according to the intrinsic properties of Fe 0 [101,104,108,109], and known principles for designing conventional granular filters [146][147][148]. Moreover, the intrinsic properties of contaminants should be considered because classifications like "organic contaminants (e.g., dyes, pesticides, and pharmaceuticals/drugs)" or "industrial organic wastes (e.g., phenols and aromatic amines)" say nothing about the chemical reactivity or the affinity of the species of concern for the Fe 0 /H 2 O system. ...
There are many factors to consider for the design of appropriate water treatment systems including: cost, the concentration and type of biological and/or chemical contamination, concentration limits at which contaminant(s) are required to be removed, required flow rate, level of local expertise for on-going maintenance, and social acceptance. An ideal technology should be effective at producing clean, potable water; however it must also be low-cost, low-energy (ideally energy-free) and require low-maintenance. The use of packed beds containing metallic iron (Fe0 filters) has the potential to become a cheap widespread technology for both safe drinking water provision and wastewater treatment. Fe0 filters have been intensively investigated over the past two decades, however, sound design criteria are still lacking. This article presents an overview of the design of Fe0 filters for decentralized water treatment particularly in the developing world. A design for safe drinking water to a 100 people community is also discussed as starting module. It is suggested that Fe0 filters have the potential for significant worldwide applicability, but particularly in the developing world. The appropriate design of Fe0 filters, however, is site-specific and dependent upon the availability of local expertise/materials.
... In fact, the model of oxide scale generation and evolution sustaining the 'reductive transformation' concept [9] was proven inconsistent by Odziemkowski and colleagues [10][11][12]. Moreover, although magnetite films (Fe 3 O 4 ) are electronic conductive in nature, the whole oxide scale on Fe 0 is always made of several layers including layers of Fe III -species [14][15][16]. This evidence makes quantitative electron transfer from the metal body to species adsorbed at the outer surface of the oxide scale unlikely. ...
... The critical issue of this discussion is the mechanism of contaminant removal. This issue is elegantly solved by simply properly considering the iron corrosion literature [14][15][16]. ...
Running title: Interdisciplinary approach gone astray? Save the peer-review system.
... Accordingly the film must be electronic conductive. However, no such conductive film is expected in nature [6,15,16]. Moreover, the concept regarding oxide-scale as curse is built on the premise that Fe 0 is a strong reducing agent. ...
... For the sake of clarity the diffusion barrier in the Fe 0 remediation will first be presented. surface is necessarily the limiting step for the corrosion process which is said to be "diffusion controlled" [7,16]. If, the rate of iron corrosion were limited by the adsorption or electron transfer steps, the reaction would be said to be "chemical controlled", "surface controlled", or "reaction controlled" (reaction-limited). ...
The further development of Fe0-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe0/H2O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both process is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe0 as remediation agent compared to thermodynamically more favourable Al0 and Zn0. The validity of the adsorption/co-precipitation concept is corroborated.
... Accordingly the film must be electronic conductive. However, no such conductive film is expected in nature [6,15,16]. Moreover, the concept regarding oxide scale as curse is built on the premise that Fe 0 is a strong reducing agent. ...
... (i) diffusion of the oxidizing agent (H + , O 2 , contaminant) to the Fe 0 surface, (ii) adsorption of the oxidizing agent onto the iron surface, (iii) the reduction of the oxidizing agent, and (iv) diffusion of reaction products (including Fe II species) away from the reactive site on Fe 0 . Because aqueous iron corrosion (at pH > 4.5) is always coupled to the formation of an oxide scale on the Fe 0 surface, the rate of the oxidizing agent diffusion to the iron surface is necessarily the limiting step for the corrosion process which is said to be " diffusion controlled " [7,16]. If, the rate of iron corrosion were limited by the adsorption or electron transfer steps, the reaction would be said to be " chemical controlled " , " surface controlled " , or " reaction controlled " (reaction-limited). ...
The further development of Fe(0)-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe(0)/H(2)O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both processes is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe(0) as remediation agent compared to thermodynamically more favourable Al(0) and Zn(0). The validity of the adsorption/co-precipitation concept is corroborated.
... Therefore, the metal surface is covered with hydroxide or oxide in aqueous solutions [14][15][16]. Depending on the type of metallic material, oxide films are formed on the metal in the atmosphere or hydroxides are formed in the solution may become a highly protective film. This is generally called oxide films (passive films), and the reason why aluminum, titanium, stainless steel has high corrosion resistance is attributable to these oxide films. ...
... Fe 3 O 4 and Fe 2 O 3 are formed on the steel surface at the high alkalinity of the pore solution, as shown in Fig. 2. These passive films on the steel surface vary in thickness (from 1 to 10 nm) and composition, depending on the availability of oxygen, water, alkalinity and type of ions in the pore solution [32,33]. Passive films are 'self-healing' in nature but the presence of anions such as chlorides can locally destroy the films and initiate corrosion. ...
Alkali activated materials (AAMs) have been recognised as potential alternatives to Portland cement concretes in specific applications in the construction industry due to their environmental benefits such as substantially reduced CO2 emissions and utilisation of industrial wastes. While many studies reported the superior performance of AAM concretes over Portland cement concretes in protecting steel from corrosion, some other studies indicated an opposite view. Hence, there is a need for further research on the long-term corrosion studies of AAM concretes in the laboratory as well as in the field. Among many important areas of investigation is the resistance of AAMs to chloride induced corrosion of steel reinforcement which is not well understood. In this paper, the above aspect is reviewed including chloride ingress, chloride binding and chloride induced corrosion rate of steel reinforcement in AAMs. Chloride ingress in AAMs involves both open and closed pore systems. Chloride binding in AAMs is predominantly physical and not chemical. The chloride threshold levels initiating steel corrosion in AAMs are significantly different in comparison to Portland cement concretes.
... The oxygen content in the initial blended powder is already quite high as compared to industrial alloys. Here, the oxygen comes from thin passivated layers in the case of Al and Cr [35,36] and from less stable layers in the case of Co, Ni and Fe [37,38]. The standard deviation indicates that there may remain some regions richer in certain elements and particularly O-rich, despite the fact that the powders were blended for 2h. ...
AlCoCrFeNi High Entropy Alloys were here synthesized by the combination of Planetary Ball Milling and Spark Plasma Sintering at 1100 °C. The relatively low rotating speed led to a peculiar agglomerate state referred to as “Mechanical Activated”. The reactive sintering of activated agglomerates leads to a unique dualphase microstructure: the sintered sample exhibited a distinctive nanostructured lamellar microstructure consisting of two main phases (FCC and BCC). Atom Probe Tomography (APT) was used to ensure that the sintered sample was chemically homogeneous at the nanoscale in each phase. APT also revealed the presence of a Cr-rich sigma phase and oxide nanoprecipitates. X-ray Photoelectron Spectrometry (XPS) results demonstrated that most of the oxygen originated from the commercial powders. Calphad calculations revealed that the presence of oxides could alter the microstructure by modifying the global chemical composition.
... Historically ample literature has been produced on the oxidation mechanism of iron and phase transformations of iron oxides from bulk materials to thin films down to nanoparticles [16,17,[26][27][28][29][30][31]. Phase diagrams for the Fe-O system in bulk materials are well known [32], but they are not directly applicable to nanoscale objects. ...
In this work, we present an in situ transmission electron microscopy (TEM) study of Fe thin films to Fe nanoparticle formation and their oxidation to single-crystal magnetite nanoparticles. Amorphous Fe thin films were prepared by sputtering on TEM carbon grids. The thin Fe films were continuously heated in situ from room temperature to 700 °C under vacuum (4 × 10–4 Pa). With the increase in temperature, the continuity of the thin film starts breaking, and Fe nanoparticle nucleation centers are formed. At 600 °C, the thin film transforms into metallic Fe nanoparticles (NPs) with a small presence of different Fe oxide NPs. Further increase in the temperature to 700 °C resulted in the full oxidation of the NPs (i.e., no core–shell were found). Zero-loss energy filtered diffraction and HRTEM analysis of the lattice spacing reveals that all NPs have fully transformed into single-phase magnetite NPs. The structural study of the magnetite NPs shows that magnetite NPs are free of antiphase domain boundary defects. This work demonstrates that under low partial pressure of oxygen at elevated temperatures a complete oxidation of Fe NPs into magnetite single-crystal nanoparticles can be achieved.
... Therefore, in H 2 SO 4 solution, the active ingredients of Eu leaf extract inhibit the dissolution of mild steel by direct adsorption over the mild steel surface. However, in phosphoric acid the mechanism of dissolution is totally different, there is a formation of a stable black iron phosphate film [50,51]. Hence, the active ingredients are not adsorbed directly to the mild steel surface, but adsorbed over an iron phosphate film, resulting in a slightly lower inhibition efficiency in phosphoric acid solutions, ...
The adsorption mechanism and inhibitive action of the Eucalyptus plant leaf extract (Eu) on the corrosion of mild steel in 0.5 M H2SO4 and 0.5 M H3PO4 solutions were investigated by potentiodynamic polarization curves measurements and electrochemical impedance spectroscopy technique. Potentiodynamic polarization curves revealed that the Eucalyptus leaf extract acts as a mixed type inhibitor in both acidic solutions. The impedance responses indicated that the corrosion process occurs under activation control. Fourier transform infrared spectroscopy has been used to predict the possible major chemical constituent of the leaf extract. Four adsorption isotherms including Langmuir, kinetic–thermodynamic, Flory–Huggins and Temkin model were used to investigate the mode of inhibition of Eucalyptus leaf extract. The free energy of adsorption showed that the corrosion inhibition takes place by spontaneous physical adsorption of Eucalyptus leaf extract molecules on the mild steel surface. The obtained data indicated that Eucalyptus leaf extract is a more efficient inhibitor of mild steel corrosion in 0.5 M H2SO4 than in 0.5 M H3PO4 solutions. Thermodynamics activation parameters were also calculated and discussed.
... MEA at 100:1 additions by volume in the test solution (Fig .6c) [18,19]. This could be enough O2. ...
... 13) (Bilardi 2013, Domga et al. 2015. For instance, it was reported that, in oxygenated waters, Fe 0 can be corroded up to 65 times faster compared to oxygen-free waters (Cohen 1959). Similarly, the degree of Fe 0 corrosion within the PRB clearly decreased in groundwater with lower oxygen concentrations (3.5-5 mg/L), located at greater depths, compared to groundwater with higher oxygen concentrations (5-6 mg/L), situated alt lower depths (Flury et al 2009). ...
Chromium (Cr) is an important metal used in a variety of industrial applications, which significantly contribute to pollution of air, soil, and waters. In natural environments, chromium can exist mainly in two of its most stable oxidation states, (+III) and (+VI). Among them, Cr(VI) is the most hazardous due to its high mobility in the environment and severe harmful effects exerted on all living matters. Therefore, it should be removed from all contaminated waters. During the last 25 years, there has been great interest in using metallic iron (Fe0) for the abatement of Cr(VI) pollution. The first mechanism, known as the reductive-precipitation mechanism, was proposed at the beginning of the nineties, and attributed the efficiency of Fe0 in removing Cr(VI) mainly to the direct electron transfer from the Fe0 surface to Cr(VI), followed by precipitation of the resulted cations as simple hydroxides and/or mixed Fe(III)-Cr(III) (oxi)hydroxides. Recently, new perspectives were added to this early mechanism. A new concept, known as the adsorption-coprecipitation mechanism, suggests that direct reduction with Fe0, if applicable, is less important than had previously been assumed by the reductive-precipitation mechanism; accordingly, contaminants are quantitatively removed in Fe0/H2O systems principally by adsorption, coprecipitation, and size exclusion, while reduction, when possible, is mainly the result of indirect reducing agents
produced by Fe0 corrosion. In spite of the substantial research work that has proven the capability of metallic iron as a reactive material to remove Cr(VI), there is no consensus at this time in what regards the mechanism of this process. Therefore, after providing an overview of chromium occurrence, chemistry, and toxicity, this work will critically review the existing knowledge on this subject, clearly demonstrating that mechanism of Cr(VI) removal with Fe0 is more complex than the simple reductive precipitation.
... There are conflicting views on which particular type of oxide confers passivity to a metal. Cohen (1959) Leach, 1975), on the other hand, argued that only one or two monolayers are required. ...
... Permeability loss is further accelerated by the presence of dissolved oxygen (O 2 ). According to Cohen [67] Fe 0 may corrode up to 65 times faster under oxic conditions compared to anoxic conditions. Working with MB and under oxic conditions is regarded as a powerful tool to 'naturally' shorten the experimental time while still obtaining ...
Packed beds of metallic iron (Fe0) and sand are tested for their efficiency at discolouring an aqueous methylene blue (MB) solution (2.0 mg L-1) in gravity driven systems for up to 95 days. The aim was to determine the optimal Fe0/sand ratio for sustainable filters. Six different Fe0/sand volumetric ratios were explored: 0/100, 20/80, 30/70, 40/60, 60/40 and 100/0. The columns were characterized by (i) the time-dependant extent of MB discoloration and (ii) the evolution of the hydraulic conductivity (permeability). Results clearly showed increased permeability loss with increasing Fe0 ratio. The Fe0/sand ratio dependent extent of MB discoloration was not monotone. These observations corroborated the working hypothesis that properly designing a Fe0/sand filter is finding a compromise between: (i) increased sustainability by lowering Fe0 ratios and (ii) decreased efficiency by lowering Fe0 ratios. This work provided the first experimental evidence for an optimal Fe0/sand volumetric ratio of 25/75. This result will accelerate efforts for non site-specific system design.
... From the huge available literature on iron corrosion it is known that under environmental pH conditions, the Fe 0 surface is always covered by an oxide-film (Cohen 1959, Schmuki 2002, Stratmann and Müller 1994, Wilson 1923. Thus, investigations regarding the processes of contaminant removal in Fe 0 -H 2 O systems should be performed under conditions favouring the formation of oxide-films on Fe 0 materials. ...
... However, from open literature on corrosion it is known that under natural conditions (nearneutral pH, slowly flowing groundwater) such an interface does not exist due to the ubiquitous presence of iron oxide that coats the metal surface [24][25][26][27] suitable to characterize Fe 0 reactivity and the effects of operational conditions in systems exempt from in situ generated oxide-films [31]. As a strong iron complexing agent without redox properties EDTA has been used successfully for this purpose [32,33]. ...
In an attempt to characterize material intrinsic reactivity, iron dissolution from elemental iron materials (Fe0) was investigated under various experimental conditions in batch tests. Dissolution experiments were performed in a dilute solution of ethylenediaminetetraacetate (Na2-EDTA - 2 mM). The dissolution kinetics of eighteen Fe0 materials were investigated. The effects of individual operational parameters were assessed using selected materials. The effects of available reactive sites [Fe0 particle size (≤2.0 mm) and metal loading (2-64 g L–1)], mixing type (air bubbling, shaking), shaking intensity (0-250 min–1), and Fe0 pre-treatment (ascorbate, HCl and EDTA washing) were investigated. The data were analysed using the initial dissolution rate (kEDTA). The results show increased iron dissolution with increasing reactive sites (decreasing particle size or increasing metal loading), and increasing mixing speed. Air bubbling and material pre-treatment also lead to increased iron dissolution. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A unified experimental procedure for the investigation of processes in Fe0/H2O systems is suitable. Alternatively, a parameter (τEDTA) is introduced which could routinely used to characterize Fe0 reactivity under given experimental conditions.
... It is interesting to notice that the observed effect of shaking speed on the Fe 0 reactivity is qualitatively the same as the often-enunciated effect of mixing intensity on reaction rate constant to demonstrate the possibility of mass transfer limitations for reactions with elemental metals in batch systems [39]. Thereafter, the overall rate of contaminant reduction [20,44,[63][64][65][66][67], the results of the present study suggest that the rate of contaminant reduction by Fe 0 materials is always mass-transfer limited. Moreover, the reported reaction mechanism difference at slow and high mixing speeds is likely to be the result of the interference of iron precipitation on the removal process. ...
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe0/H2O systems) are not really understood. One of the main reasons for this is the lack of a unified procedure for conducting batch removal experiments. This study gives a qualitative and semi-quantitative characterization of the effect of the mixing intensity on the oxidative dissolution of iron from two Fe0-materials (material A and B) in a diluted aqueous ethylenediaminetetraacetic solution (2 mM EDTA). Material A (fillings) was a scrap iron and material B (spherical) a commercial material. The Fe0/H2O/EDTA systems were shaken on a rotational shaker at shaking intensities between 0 and 250 min-1 and the time dependence evolution of the iron concentration was recorded. The systems were characterized by the initial iron dissolution rate (kEDTA). The results showed an increased rate of iron dissolution with increasing shaking intensity for both materials. The increased corrosion through shaking was also evidenced through the characterization of the effects of pre-shaking time on kEDTA from material A. Altogether, the results disprove the popular assumption that mixing batch experiments is a tool to limit or eliminate diffusion as dominant transport process of contaminant to the Fe0 surface.
... Fe 0 oxidation releases dissolved iron species (Fe II , Fe III ) which hydrolyse with increasing pH and precipitate primarily as hydrous oxides (oxide-film) or corrosion products (CP). Oxide-films (CP) of varied composition and thickness develop at all aqueous Fe 0 /H 2 O interfaces [13,14]. ...
Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe0/H2O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe0 and Fe0 corrosion products (CP), (ii) co-precipitation with in-situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO2) is not expected. The kinetics of MB discoloration by Fe0, Fe2O3, Fe3O4, MnO2, and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe0 source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in-situ generated iron CP as main discoloration mechanism. Under high shaking intensities (> 150 min-1), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in-situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe0/H2O systems is reiterated.
... The kinetics of Fe 0 oxidation by O 2 is more rapid. According to Cohen [52] the reaction is 65 times more rapid than under anoxic conditions. However, oxidation with O 2 is coupled with the formation of non conductive oxides (e.g. ...
The use of metallic iron as environmental remediation medium was based on an incorrect interpretation of experimental observations. Since then, faced with seemingly contradictory data, researchers have substantially revised their models but controversial reports are still current, suggesting that a substantial revision is unavoidable. This communication analyses redox processes in Fe0/H2O systems and demonstrates that the current paradigm even contradicts textbook knowledge on aqueous iron corrosion that was available before the advent of the Fe0 technology. Accordingly, the use of metallic iron for environmental remediation should be regarded as a classical case where scientists are entrenched in a false paradigm. An immediate correction is recommended before a questionable ‘novelty’ is transferred into standard textbooks.
... The low affinity of MB for iron corrosion products suggests that the retention time of MB in a Fe 0 -based system will be minimal (rapid breakthrough). The breakthrough time is further lowered by working under atmospheric conditions (oxic conditions) where voluminous corrosion products are generated [30] [38]. Despite low affinity to iron oxides, MB is removed in Fe 0 based filters mainly by adsorptive size-exclusion. ...
The influence of metallic iron (Fe0) amendment on the efficiency of sand to discolor a 2.0 mg L-1 methylene blue (MB) solution was investigated in column studies. MB was used as an indicator to identify the optimum Fe0/sand ratio for efficient filtration systems. Columns contained 0, 100 or 200 g of a Fe0 material. The volumetric proportion of Fe0 in the reactive layer of the columns with 100 g of material varied from 10 to 100 %. Results showed that, Fe0 amendment significantly impaired MB discoloration by sand for experiments lasting for up to 132 days. Early MB breakthrough in Fe0/sand columns delineated the paramount importance of particle cementation, which has caused preferential flow with a negative impact on discoloration efficiency. The most efficient Fe0/sand mixtures were the ones with 30 to 50 % Fe0 (v/v). These volumetric ratios correspond 33 to 41 % weight ratios showing that the commonly used 1:1 weight ratio (50 %) may not be optimal. Further research with compounds exhibiting different affinities to both Fe0 and sand is needed before this observation can be generalized.
... [75,76]. In particular, O 2 adsorption (pH > 4.0) is accompanied by metal oxyhydroxide precipitation [74,77,78]. The pH range of natural waters (4 ≤ pH ≤ 10) is exactly the area of remediation with M 0 /H 2 O systems and corresponds to slow dissolution 4 kinetics of Al 0 , Fe 0 and Zn 0 . ...
Metallic iron (Fe0) is often reported as a reducing agent for environmental remediation. There is still controversy as to whether Fe0 plays any significant direct role in the process of contaminant reductive transformation. The view that Fe0 is mostly a generator of reducing agents (e.g. H, H2 and FeII) and Fe oxyhydroxides has been either severely refuted or just tolerated. The tolerance is based on the simplification that, without Fe0, no secondary reducing agents could be available. Accordingly, Fe0 serves as the original source of electron donors (including H, H2 and FeII). The objective of this communication is to refute the named simplification and establish that quantitative reduction results from secondary reducing agents. For this purpose, reports on aqueous contaminant removal by Al0, Fe0 and Zn0 are comparatively discussed. Results indicated that reduction may be quantitative in aqueous systems containing Fe0 and Zn0 while no significant reduction is observed in Al0/H2O systems. Given that Al0 is a stronger reducing agent than Fe0 and Zn0, it is concluded that contaminant reduction in Fe0/H2O systems results from synergic interactions between H/H2 and FeII within porous Fe oxyhydroxides. This conclusion corroborates the operating mode of Fe0 bimetallics as H/H2 producing systems for indirect contaminant reduction.
... The low affinity of MB for iron corrosion products suggests that the retention time of MB in a Fe 0-based system will be minimal (rapid breakthrough). The breakthrough time is further lowered by working under atmospheric conditions (oxic conditions) where voluminous corrosion products are generated [30,38]. Despite low affinity to iron oxides, MB is removed in Fe 0-based filters mainly by adsorptive size-exclusion. ...
The influence of metallic iron (Fe0) amendment on the efficiency of sand to discolor a 2.0 mg L�1 methylene
blue (MB) solution was investigated in column studies. MB was used as an indicator to identify the
optimum Fe0/sand ratio for efficient filtration systems. Columns contained 0, 100 or 200 g of a Fe0 material.
The volumetric proportion of Fe0 in the reactive layer of the columns with 100 g of material varied
from 10% to 100%. Results showed that, Fe0 amendment significantly impaired MB discoloration by sand
for experiments lasting for up to 132 days. Early MB breakthrough in Fe0/sand columns delineated the
paramount importance of particle cementation, which has caused preferential flow with a negative
impact on discoloration efficiency. The most efficient Fe0/sand mixtures were the ones with 30–50%
Fe0 (v/v). These volumetric ratios correspond 33–41% weight ratios showing that the commonly used
1:1 weight ratio (50%) may not be optimal. Further research with compounds exhibiting different affinities
to both Fe0 and sand is needed before this observation can be generalized.
In this work, the black rice husk ash (BRHA), a pozzolanic material, was used as a partial replacement in concrete with the weight percentages of 0%, 10%, 20%, 30%, 40% and 50% for enhancing the corrosion resistivity in the marine environment. The compressive strength, corrosion by accelerated corrosion test by impressed voltage (ACTIV), and chloride (Cl ⁻ ) penetration of concrete specimens were investigated after 28 days of curing. For corrosion and chloride penetration analyses, the 20% of BRHA replacement specimen was the most effective concrete specimen because the deformation was not observed within 19 days of the test. The cement specimens with lower BRHA percentages were cracked due to the development of stress by the rust formation. For higher BRHA percentages, the protective Fe 2 O 3 was dissolved due to the acidic environment caused by higher chloride accumulation in the cement specimens. The steel rebar was then aggressively attacked by the chloride and it was finally broken. Therefore, the optimization of the BRHA percentage is needed to minimize corrosion. However, the longer curing time of 20% BRHA replacement specimen is required for increasing the compressive strength because its compressive strength is slightly lower than the standard.
The passivation phenomenon and its explanation, the passive films and their structures of pure iron are briefly reviewed. The nano-thick passive film of Fe is usually composed of an inner layer of Fe2+ compound and an outer layer of Fe3+ compound, such as a Fe3O4/γ-Fe2O3 or α-Fe2O3 bilayer.
This study involves development of a low-cost cathode catalyst suitable for algae-based microbial fuel cell (MFC) application. The electrodes comprise of nanoparticles (NPs) of CuO/MnO2/Fe3O4 mixed with activated graphite. The study compares the performance of composite electrodes with Pt coated C-cloth. The graphite/CuO supported highest power & current density, algal growth, and electrocatalytic activity while Pt/C electrode failed in all aspects as algae ceased to grow in the system. The graphite/CuO cathode led to 6 W/m³ of power density, 25 A/m³ of current density, and 0.256 d⁻¹ as a specific algal growth rate. The composite graphite/CuO electrode showed better catalytic activity towards oxygen reduction reaction (ORR) in the cathode. Furthermore, the Pt was found out to be toxic for the algal growth. Hence, the Pt-MFC exhibited low power output due to failure of cathodic algal growth. In addition to this, the GC-MS analysis of FAME (Obtained from cathodic algae) revealed the optimum presence of both saturated and unsaturated fatty acids, indicating its suitability for biodiesel application.
Introduction. Corrosion of reinforcement in concrete structures is rather expensive for the economy of any country. Current statutory documents offer two main directions — primary and secondary protection. One of the promising areas of primary protection methods is the use of complex anti-corrosion additives in concrete due to its technological simplicity and economic efficiency. Passivators and surfactants (SAS), as components of such additives, are of particular interest. Sodium nitrite is offered as a passivator, and polycarboxylate molecules (PCE) — as a SAS. The additive of sodium nitrite together with PCE has a visual synergistic effect.
Materials and methods. Identification of structural characteristics of the molecules of the obtained PCE polymers was determined by methods of exclusion gel-permeation chromatography and 13C NMR spectroscopy. To study the synergetic effect of NaNO2 + PCE additive, the samples were kept in the models of pore liquid, after which the morphology of their surface was studied by methods of scanning electron microscopy and electron probe microanalysis.
Results. The texture and surface color of the presented micrographs indicate surface formations on the samples stored in the models of pore liquid with different anti-corrosion additives. Electron probe microanalysis showed increased concentrations of carbon, oxygen and sodium on the surface of samples stored in pore concrete liquid models with the addition of a complex anti-corrosion additive of sodium nitrite and PCE, which indicates increased concentrations of PCE and possible increased pH on the steel surface. Based on the data acquired, the mechanism of formation of a protective film layer using complex anti-corrosion additive NaNO2 + PCE is offered.
Conclusions. Justification of synergetic effect of complex polycarboxylate anti-corrosion additive in the model of concrete pore liquid opens up prospects for research of such additives to the concrete model.
Elemental iron (Fe0) has been widely used in groundwater/soil remediation, safe drinking water provision and wastewater treatment. It is still mostly reported that a surface-mediated reductive transformation (direct reduction) is a relevant decontamination mechanism. Thus, the expressions "contaminant removal" and "contaminant reduction" are interchangeably used in the literature for reducible species (contaminants). This contribution reviews the scientific literature leading to the advent of the Fe0 technology and shows clearly that reductive transformations in Fe0/H2O systems are mostly driven by secondary (FeII, H/H2) and tertiary/quaternary (e.g. Fe3O4, green rust) reducing agents. The incidence of this original mistake on the Fe0 technology and some consequences for its further development are discussed. It is shown in particular that characterizing the intrinsic reactivity of Fe0 materials should be the main focus of future research.
Polypyrrole (PPy) coating was electrochemically synthesized on carbon steel using sulfonic acids as dopants: p-toluene sulfonic acid (p-TSA), sulfuric acid (SA), (±) camphor sulfonic acid (CSA), sodium dodecyl sulfate (SDS), and sodium dodecylbenzene sulfonate (SDBS). The effect of acidic dopants (p-TSA,
SA, CSA) on passivation of carbon steel was investigated by linear potentiodynamic and compared with morphology and corrosion protection performance of the coating produced. The types of the dopants used were significantly affecting the protection efficiency of the coating against chloride ion attack on the metal surface. The corrosion performance depends on size and alignment of dopant in the polymer backbone. Both p-TSA and SDBS have extra benzene ring that stack together to form a lamellar sheet like barrier to chloride ions thus making them appropriate dopants for PPy coating in suppressing the corro- sion at significant level. Further, adhesion performance was enhanced by adding long chain carboxylic acid (decanoic acid) directly in the monomer solution. In addition, PPy coating doped with SDBS displayed excellent biocidal abilities against Staphylococcus aureus. The polypyrrole coatings on carbon steels with dual function of anti-corrosion and excellent biocidal properties shows great potential application in the industry for anti-corrosion/antimicrobial purposes.
This article critically evaluates recent review articles on using metallic iron (Fe(0)) for environmental remediation in order to provide insight for more efficient Fe(0)-based systems. The presentation is limited to peer-reviewed articles published during 2014 and 2015, excluding own contributions, dealing mostly with granular Fe(0). A literature search was conducted up to June 15th 2015 using Science Direct, SCOPUS, Springer and Web of Science databases. The search yielded eight articles that met the final inclusion criteria. The evaluation clearly shows that seven articles provide a narrative description of processes occurring in the Fe(0)/H20 system according to the concept that Fe(0) is a reducing agent. Only one article clearly follows a different path, presenting Fe(0) as a generator of adsorbing (hydroxides, oxides) and reducing (Fe(II), H/H2) agents. The apparent discrepancies between the two schools are identified and extensively discussed based on the chemistry of the Fe(0)/H20 system. The results of this evaluation indicate clearly that research on 'Fe(0) for environmental remediation' is in its infancy. Despite the current paucity of reliable data for the design of efficient Fe(0)-based systems, this review demonstrates that sensible progress could be achieved within a short period of time, specific recommendations to help guide future research are suggested.
Copyright © 2015 Elsevier Ltd. All rights reserved.
The past two decades have witnessed a boom of sci-entific articles on relevant processes governing aqueous contaminant removal in the presence of metallic iron (Fe0). Nonetheless, transforming accumulated data into useful knowledge is difficult. The major limitation is that a the-ory of the system is yet to be established or accepted. Generally, the theory of the system is established during the period between a discovery and its market introduction (‘valley of death’). This communication argues that the too short ‘valley of death’ has harmed progresses in Fe0 technology. The introduction of this technology was cou-pled with the consensus that Fe0 is a reducing agent. However, the question as to whether the reductive trans-formation theory was worthy of pursuit remained inade-quately addressed. The aim of this paper is to offer an answer to this question. A critical evaluation of the reduc-tive transformation theory is presented. It is established that pursuing the reductive transformation theory was irrational. It is shown that it is worthy to base future work on the concept that contaminants are removed during the dynamic process of Fe0 oxidative dissolution and subse-quent hydroxide/oxide precipitation.
Despite two decades of intensive laboratory investigations, the removal mechanism of several contaminants from aqueous solutions by elemental iron (e.g. in Fe0/H2O systems) are not really elucidated. Two of the major reasons for this are: (i) the failure to consider Fe0/H2O systems as consisted of the elemental iron material (Fe0) covered by a layer of corrosion products (oxide-film), and (ii) the failure to treat properly the combined problem of mass transport and chemical reaction in these complex systems. Well-mixed batch experiments that have been undertaken in order to circumvent the mass-transport problem associated with bulk solutions have not always adequately addressed these key issues. Mixing intensity may not only affect the hydrodynamic but also the chemical dynamics, in particular the formation of the oxide-film. The present work presents a critical review on the process of oxide-film formation and its impact on the process of mass-transport to the Fe0 surface. It is shown that well-mixed batch systems are not necessarily an effective tool for investigating the mechanism of contaminant removal by Fe0 since mixing may increase corrosion rate, avoid/delay the formation of oxide-films and/or provoke their abrasion. This discussion suggests that quantitative abiotic contaminant reduction in Fe0/H2O systems may mostly occur within the oxide-film as result of: (i) electron transfer from Fe0 surface, (ii) catalytic activity of secondary reductants (FeII, H2/H). Non-shaken batch experiments are proposed as a simple tool to investigate mass-transport limitation through oxide-films at laboratory scale. Working with stationary Fe0 samples and controlled stirring speeds may allow the investigation of oxide-film effect under more realistic conditions.
The precessional magnetization dynamics of high quality epitaxial magnetite (Fe3O4) thin films growth on MgO are investigated by inductive magnetization dynamic measurements in time and frequency domain. An upper bound for the intrinsic Gilbert damping parameter of α0 = 0.037±0.001 is derived, which is significantly lower than previously reported for epitaxial Fe3O4 on GaAs. With increasing film thickness from 5 up to 100 nm a strong increase in the effective damping up to 0.2 is observed which cannot be explained by simple nonuniform spin wave excitations. Possible origins of this effect are discussed.
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe(0)/H2O systems) are not really understood. One of the main reasons for this is the lack of a unified procedure for conducting batch removal experiments. This study gives a qualitative and semi-quantitative characterization of the effect of the mixing intensity on the oxidative dissolution of iron from two Fe(0)-materials (materials A and B) in a diluted aqueous ethylenediaminetetraacetic solution (2 mM EDTA). Material A (fillings) was a scrap iron and material B (spherical) a commercial material. The Fe(0)/H2O/EDTA systems were shaken on a rotational shaker at shaking intensities between 0 and 250 min(-1) and the time dependence evolution of the iron concentration was recorded. The systems were characterized by the initial iron dissolution rate (k(EDTA)). The results showed an increased rate of iron dissolution with increasing shaking intensity for both materials. The increased corrosion through shaking was also evidenced through the characterization of the effects of pre-shaking time on k(EDTA) from material A. Altogether, the results disprove the popular assumption that mixing batch experiments is a tool to limit or eliminate diffusion as dominant transport process of contaminant to the Fe(0) surface.
Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe(0)/H(2)O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe(0) and Fe(0) corrosion products (CP), (ii) co-precipitation with in situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO(2)) is not expected. The kinetics of MB discoloration by Fe(0), Fe(2)O(3), Fe(3)O(4), MnO(2), and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe(0) source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in situ generated iron CP as main discoloration mechanism. Under high shaking intensities (>150 min(-1)), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe(0)/H(2)O systems is reiterated.
A combination of two techniques was employed to investigate the surface condition of passive iron in acidic and alkaline 0.1 M Na2SO 4 solutions at 5°C. Theoretical interpretation of oxygen overpotential curves suggested the nature of the adsorbed intermediate species in the oxygen evolution reaction. Forced coulometric decay traces from oxygen evolution to hydrogen evolution showed that the extent of any passive film was less than one-fifth of a monolayer for any reasonable chemical species. This low coverage precludes species other than those involved in oxygen evolution, i.e., hydroxyl radicals in acidic solution and hydroxide ions in alkaline solution.
The effect of chloride ion on the potential of iron in sodium nitrite solutions has been measured. This has been related to the surface composition and topography by the techniques of electron diffraction and electron microscopy. The effect of initial surface preparation has also been investigated.
It is shown that in the absence of chloride ion the potential is noble and steady and that the surface remains practically unchanged. A film of is shown to be present by electron diffraction. When chloride ion is added to the system the potential becomes unsteady and less noble. Inclusions of a second phase grow in the oxide layer. This is shown to be or lepidocro‐cite. The lower and unsteady potential is probably due to the decrease in resistance of the anodic areas, which correspond to the inclusions in the oxide layer.
The reaction between pure iron and oxygen‐free water has been investigated at 25°, 60°, and 300°C. At room temperature, the primary product of reaction appears to be ; no evidence was found to indicate the presence of . Formation of , however, occurs readily in the iron‐water system at both 60° and 300deg;C. The possibility that this is produced via formation and subsequent decomposition of is discussed; the conclusion is reached that the mechanism of formation of from the reaction of iron and water cannot be decided definitely at this time.
Apparatus and technique for cathodic reduction of α-Fe2O3 films is described. Measurements of the reduction efficiency in buffered and unbuffered electrolytes are given. Effects of pH of the electrolyte, dissolved oxygen current density, and film thickness are noted and discussed. Electrolytes containing “ferrous-complexing” ions have also been investigated. Methods of determining quantities of a-Fe2O3 in thin films are suggested and their limits of accuracy considered.
The weights of ferric phosphate present in films formed by the passivation of iron in 0.1M solutions of disodium and trisodium phosphate were determined using a radioactive tracer, and the thickness of oxide was estimated by an electrochemical method. The weights of phosphate were found to decrease with a rise in pH value of the solution. Surface pretreatments, involving the removal of the original air-formed oxide film by dipping in acid, resulted in higher phosphate contents than surface pretreatments involving abrasion, either with or without subsequent prolonged exposure to dry air.
Inhibitors having relatively strong oxidizing anions, such as sodium Chromate and nitrite, passivate iron both in the presence of air and in deaerated solutions. Much weaker oxidizing agents such as sodium tungstate and molybdate behave similarly to Chromate and nitrite in the presence of air but do not prevent corrosion in deaerated solutions despite the fact that potential/time and polarization curves indicate that slow film formation is occurring. Tungstate ions, however, are effective oxidizing agents toward iron when discharged anodically at high current density. Solutions of sodium acetate, benzoate, carbonate, hydroxide, orthophosphate, and silicate, which do not contain oxidizing anions, passivate iron only when they contain dissolved air. When these solutions are deaerated they attack iron very slowly, potential/time and polarization curves indicating that this attack is mainly under cathodic control. It is postulated that oxygen dissolved in solution is mainly responsible for passivity by virtue of its heterogeneous reaction with surface iron atoms to form a thin film of γ-Fe2O3, approximately 200 A2 thick, in a manner similar to that by which oxide films are formed in air. This film, if kept in constant repair, prevents iron ions from the metal passing into solution. It is considered that, in inhibitors containing oxidizing anions, the passivity film is formed mainly by dissolved oxygen, whereas in other inhibitors, which do not contain oxidizing anions, the oxide is formed entirely by dissolved oxygen.
Many alloying elements that confer resistance to high‐temperature oxidation also prevent low‐temperature aqueous corrosion;
they produce oxide films that only slowly transmit cations. Conversely sulfur, which introduces lattice defects into the films
and thus favors the transmission, accelerates high‐temperature oxidation and favors the onset of aqueous corrosion. It is
therefore suggested that the onset of aqueous corrosion is controlled by the relative ease with which cations can pass through
the original air‐formed oxide film on the metal. A rapid supply of dissolved oxygen to all parts of the surface may arrest
cations as they emerge, thus thickening the film and preventing corrosion; a far more rapid supply of oxygen is needed to
prevent corrosion on iron or zinc than on stainless steel, but the supply needed is diminished if an inhibitor is present.
The weakest places in the film are those where cations can most readily pass through; they become the initial corrosion anodes.
The further development of the anodic zones depends on the production of anodic acidity.
The passive film formed by sodium nitrite is γ-Fe 2O 3 with a small amount of γ-Fe 2O 3·H 2O. The oxide film is formed by a reaction between the nitrite (and oxygen) and the metal at the liquid-metal interface, with adsorption of the inhibitor as a probable intermediate step.
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