Guide A Candle for a Marine (Always a Marine series Book 18)

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Maghemite is also a defect structure with the Fe:O ratio in the range of 0. XRD presents an important limitation when it comes to differentiating the magnetite phase from the maghemite phase, as both show practically identical diffractograms similar crystalline structures and are very hard to differentiate when mixed with large amounts of other phases lepidocrocite, goethite and akaganeite , as occurs in the corrosion products formed on steel when exposed to marine atmospheres.

Both phases are usually detected in the inner part of the rust adhering to the steel surface, where oxygen depletion can occur [ , ]. It may also be formed by lepidocrocite reduction in the presence of a limited oxygen supply [ ] according to:. With a broader view, Ishikawa et al.

The formation of magnetite can be represented as the following reaction:. Remazeilles and Refait [ ], Nishimura et al. In severe marine atmospheres the spinel phase can be the main rust constituent, as was found by Jeffrey and Melchers [ ] and by Haces et al. There is often uncertainty as to which of the two phases, magnetite or maghemite, is present in AC products, and indeed both species could be present depending on the local formation conditions and the corrosion mechanisms involved in the process.

This lack of definition may also be intimately related with the analytical techniques used for their determination. Many researchers have reported the presence of magnetite in AC products on the basis of XRD data, but much of this data is suspect since the XRD patterns of magnetite and maghemite are very similar. The same happens when the ED method is used [ ]. However, Graham and Cohen [ ] do show convincing evidence on the basis of MS that magnetite is a component of corrosion products on several samples.

However, Leidheiser and Music [ ] and Chico et al. Likewise, Oh et al. In contrast, Nishimura et al. Antony et al. Thus, it seems that the specific nature of each analysis method strongly influences the type of phase identified. The identification of rust amorphous phases as well as the classification of the type of spinel formed magnetite or maghemite are two issues where more research effort is needed.

As Bernal et al. Only some of these transformations were not topotactic and seemed to have dissimilar structures, with renucleation being necessary for the transformation process. According to Matsubara et al. The ideal crystallographic structures of three ferric oxyhydroxides—lepidocrocite, goethite and akaganeite—are described using FeO 6 octahedral units Figure There are also several kinds of GR containing ferric and ferrous ions which have a layered structure as Fe OH 2.

In the structure of GR, the fractions of ferric and ferrous ions in layers of FeO 6 octahedra are variable and different anions and water molecules are intercalated between the layers. Although there are other iron oxide structures including hydroxides, they are fundamentally described in a similar way. The structures are described using FeO 6 octahedral units. Small circles: hydrogen, medium circles: chlorine [ , ]. MS has been used to identify components in corrosion products and to analyse their fine structures.

The results obtained by these methods provide information on the composition, morphology and structure of corrosion products. However, structural information on corrosion products obtained by these methods is limited [ ]. As was seen in Section 3 , in order to characterise corrosion product structures in various scales, Kimura et al. In XAFS, oscillatory modulation near an X-ray absorption edge of a specific element of a specimen provides information in terms of the local structure around an atom Fe, Cl, etc.

However, some reservations should be made regarding information about linkage of the FeO 6 octahedral unit structure, because XAFS data is obtained only from near neighbour atomic arrangements. This strongly suggests the great importance of middle-range ordering MRO for characterising corrosion products. The rust formed on steel when exposed to the atmosphere is usually a complex mixture of several phases. Moreover, each of these phases can take on a wide variety of morphologies depending on their growth conditions.

Thus, the diversity of rust morphologies formed on CS exposed to marine atmospheres is enormous, with a great variety of shapes and sizes of the crystalline aggregates that reflect to a large extent the different growth conditions: chemical characteristics of the aqueous adlayers formed by humidity condensation, rainfall, etc. Table 3 shows the principal habits morphologies of iron oxides according to Cornell and Schwertmann [ 91 ].

Principal habits morphologies of iron oxides according to Cornell and Schwertmann [ 91 ]. As they note, the morphology of AC products is often not describable in terms of typical iron oxide structures but is much more complicated; the component phases in rust formed on steel in outdoor exposure show imperfections in their structures and real component structures appear to diverge from an ideal crystallographic structure of typical iron oxides.

For some time now, articles published on AC studies usually include SEM views of rust formations and in some cases even attribute certain morphologies to specific rust phases without an analytical characterisation. An exception can be seen in the pioneering work of Raman et al. These researchers attempted to indirectly identify the morphologies observed by SEM by comparison with the morphologies of standard rust phases grown in the laboratory and identified by XRD and IRS.

Very recently, the research group of Morcillo et al. Without seeking to be exhaustive, there follows a tentative classification of the different types of morphology observed by the authors in the rust formed on steel exposed to marine atmospheres [ ]:. Tubular: formations in which the crystalline aggregates are constituted by prisms, tubes, or rods, etc. Geode-type: unusual or singular oolitic or globular morphology constituted by fish-egg-like spherical formations. Figure 12 presents typical characteristic morphologies of the four rust phases normally present among the corrosion products formed in marine atmospheres: lepidocrocite, goethite, akaganeite and magnetite.

SEM view of laminar lepidocrocite a ; acicular goethite b ; tubular akaganeite c and toroidal magnetite d formations [ ].

As shown in Figure 13 , from Kimura et al. Nucleation corresponds to the first step of precursor condensation and solid formation. Then the growth process follows, where Fe O,OH 6 octahedra units as cations or smaller sized growing nuclei accumulate to form larger particles. On a colloidal scale, polymerisation of these Fe O,OH 6 octahedra leads to the formation of fine particles of hydroxides, oxyhydroxides or oxides. These particles grow into grains or layers through a long period of ageing processes affected by repeated wet and dry cycles.

Reaction conditions concentration, acidity, temperature, nature of anions, etc. Coagulation and adhesion processes ensue to generate corrosion products, which undergo ageing processes leading the system to stability. During ageing the particles may undergo modifications such as increases in size, changes in crystal type, changes in morphology, etc. Thus, according to Ishikawa et al.

Voids of different sizes form between the fine particles in the rust layer. Scheme of iron corrosion in the atmosphere according Kimura et al. Triangle pairs represent Fe O,OH 6 octahedra. Resulting from these complicated processes, corrosion products are generally classified as coarse or fine grains, both of which are composed of crystallites and inter-crystallites. The morphology takes the form of grains or particles, agglomerates of grains, flakes, and even exfoliations layers or laminates [ ] Figure The texture of rust is seen to vary according to the atmospheric aggressivity Figure A more heterogeneous surface appearance and coarser granulometry is found in more aggressive atmospheres industrial and marine [ ].

In marine atmospheres, the granulometries are coarser and become more accentuated with airborne salinity and exposure time Figure These results confirm the observations of Ishikawa et al. Type of rust morphologies formed on carbon steel exposed to marine atmospheres [ ]. Granulometries of outermost rusts formed on skyward- facing side of carbon steel exposed for 5 years at different type of atmospheres.

Granulometries of outermost rusts formed on skyward- facing side of carbon steel exposed for 6 and 12 months at marine atmospheres of different aggressivity [ 64 ]. The compactness of the rust layers depends on the morphology of the rust particles; smaller particles form more compact and less permeable layers. However, as Ishikawa et al. Ishikawa et al. It was revealed that the specific surface area SSA obtained by N 2 adsorption decreased with increasing airborne salinity Figure In contrast, in a low salinity environment fine rust particles assemble to form densely packed rust layers with high corrosion resistance.

This finding suggests that NaCl promotes rust particle growth, resulting in the formation of larger pores as voids between larger particles in the rust layer and facilitating further corrosion. When the thin layer of corrosion products has grown to cover the whole surface, further growth requires reactive species from the aqueous adlayer to be transported inwards through the rust layer while metal ions are transported outwards. In addition to this, electrons must be transported from anodic to cathodic sites on the surface, so that those produced in the anodic reaction can be consumed in the cathodic reaction.

As long as the metal substrate is covered only by a thin oxide film, electron transportation through the film is generally not a rate-limiting step. However, when the corrosion products grow in thickness, electron transportation may become rate-limiting [ 8 ]. This section considers the different physical and chemical properties of corrosion product layers. It starts by addressing the organoleptical properties of rust layers, such as their colour and texture, before going on to consider other properties more related with their protective capacity: stratification, stabilisation, adhesion, thickness, and porosity and their evaluation using different indices.

CS exposed to the atmosphere develops ochre-coloured rust which becomes dull brown as the exposure time increases. Lighter rust colours are seen in atmospheres with greater salinity more corrosive and darker rusts in less aggressive atmospheres [ 64 ]. In marine atmospheres, the colour of rust varies not only with the salinity of the atmosphere, but also according to the steel type, exposure time, etc. Rust color rating: 1 lightest —5 darkest. In Section 5. In part, this property is closely related with the texture of the outer surface of the rust layer.

Sense of touch is used to determine aspects of texture such as smoothness, unevenness and roughness. Doctoral Thesis of I. Cano [ ] reported one to three-year exposure of a variety of CS in different types of atmospheres, where differences in texture were observed in the rust layers formed. There is controversy about the stratification of the rust layer in different sublayers on unalloyed CS [ 97 ].

In their investigations these authors have found the presence of two sublayers in all rust films: an uncoloured dark grey inner layer and an orangey-brown-coloured outer layer Figure Thus, the dual nature of the rust layer is not an exclusive characteristic of WS since plain CS with less AC resistance also generates a stratified rust. Optical micrograph obtained by polarized light. The outer orange-coloured layer is mainly lepidocrocite while the inner greyish layer is mainly goethite and magnetite [ ].

According to Suzuki [ ], rust layers usually present considerable porosity, spallation, and cracking. Cracked and non-protective oxide layers allow corrosive species easy access to the metallic substrate, and is the typical situation in atmospheres of high aggressivity. However, compact oxide layers formed in atmospheres of low aggressivity favour the protection of the metallic substrate.

As time elapses, the number and size of defects may decrease due to compaction, agglomeration, etc. Bibliographic information on this aspect is highly erratic and variable. The gradual development of a corrosion layer takes several years before steady-state conditions are obtained, though the exact time taken to reach a steady state of AC will obviously depend on the environmental conditions of the atmosphere where the steel is exposed.

Previously it was confirmed that the corrosion rate y plotted against the exposure time x fitted an exponential decrease equation:. The rust layer stabilisation time decreases as the corrosivity category ISO [ 30 ] of the atmosphere rises. However, a shorter stabilisation time does not imply a greater protective capability of the rust. In this respect, stabilisation of the rust layer occurs faster in marine atmospheres, due to their greater corrosivity, but the protective value of this rust is lower than that of rusts formed in less aggressive atmospheres rural, urban, etc.

The steady-state corrosion rate increases in line with the corrosivity of the atmosphere in both rural, urban, industrial, and marine atmospheres Figure 20 [ 87 ]. Relationship between steady-state corrosion rate of carbon steel and atmospheric corrosivity category according ISO for different type of atmospheres [ 87 ]. According to Honzak [ ] it is possible to differentiate three layers in rust: surface rust that is easily removable e.

Not all steel corrosion products become incorporated in the rust layer. Some examples [ ] include:. Leaching of soluble components of the rust layer iron chlorides in marine atmosphere by rainwater, and. It has long been known that not all the corroded metal becomes part of the measurable rust product [ ], but there have been very few attempts to quantify this part. According to these authors, the protective properties of rust on CS in a given corrosive environment depends on the characteristics of the adherent rust, i. The thickness of the rust layer increases with time of exposure and the aggressivity of the atmosphere.

A direct linear relationship is found between the rust layer thickness and the substrate corrosion rate [ ] Figure Corrosion rate versus rust thickness for low carbon steel exposed during two years in atmospheres of different aggressivity [ ]. The thickness of the rust layer formed in marine atmospheres is not usually uniform, being thicker in some areas than in others, and the attack profile of the underlying steel generally shows the abundant formation of pits of variable depths [ 64 ].

They saw the aforementioned thick and thin areas within the rust layers and found that akaganeite was preferentially located in the thick areas and was scarce in the thin areas of the rust layers. The rust present inside the pits formed on carbon steel substrates when exposed to severe marine atmospheres is almost entirely composed of akaganeite: a cross section affecting a deep pit; b Raman spectrum of rust inside the pit. In not highly aggressive marine atmospheres, consistent consolidated , adherent, and continuous rust layers present a two-sublayer organisation, as has been seen above Figure However, the exposure of CS in very aggressive marine atmospheres can in certain circumstances lead to the formation of heterogeneous and anomalous thick rust layers.

These thick rust layers tend to become detached from the steel substrate exfoliated , leaving it uncovered and without protection and thus accelerating the metallic corrosion process [ ]. The rust exfoliation phenomenon can only take place if such anomalous thick rust layers are formed, as has also been observed in studies carried out by other researchers [ 42 , ].

The exfoliated rust layers are composed of multiple rust strata, this can clearly be seen in the cross section of Figure 23 and Figure Observation by optical microscopy shows that in general the thick rust layer contains one or more strata of compact rust, exhibiting a greyish colouring and a metallic shine, whose number varied according to the area of the rust layer observed. With regard to the rust exfoliation mechanism, it is recommended to consult recent publications by the authors [ , , ]. The characteristic of the different rust sublayer within the rust multilayer are described in Figure 24 [ ].

Schematic illustration of different rust sublayers in exfoliated rust Figure The denomination and characteristic of each rust layer is given [ ]. Voids of different sizes are formed among the rust particles in the corrosion layer, whose compactness depends on the rust particle size. Important parameters for the protective ability of rust layers include their thickness, porosity and Specific Surface Area SSA. Thus, these characteristics of the rust layer directly influence the AC mechanisms.

Despite this fact, few studies of rust pore structures have been reported. The pore size of atmospheric rust is in the range of up to 15 nm, and the highest peak always appears below 5 nm [ ]. However, as the authors note, these methods do not provide quantitative data on the three-dimensional distribution of pores in the rust layer, their tortuosity or their connectivity; three other parameters about which information is desirable for AC modelling [ 41 ].

The SSA of rust layers is evaluated by fitting the BET equation [ ] to the adsorption isotherms of nitrogen S N and water molecules S W and using the cross-sectional area of nitrogen and water molecules 0. Among the results obtained using this technique, attention is drawn to the following which are considered particularly relevant:.

Compact rust layers with a high S N or a small particle size exhibit high corrosion resistance Figure The corrosion resistance of rust layers is a result of pore filling by the adsorption and capillary condensation of water [ ], according to the proposed scheme shown in Figure Trend plot of specific surface area SSA against corrosion rate for the rusts formed by exposing carbon steels at different bridges in Japan for 17 years [ ].

Schematic representation of pore filling by adsorption and capillary condensation of water, according to Ishikawa et al. Figure 27 depicts the pore size distribution trend curve of rusts formed on steels, as calculated by the Dollimore-Heal D-H method [ ] from N 2 adsorption isotherms. Trend plot of pore size distributions of the rust formed on steels exposed to coastal conditions [ 48 ].

V: adsorbed amount of N 2 ; D: pore diameter. Rusts formed in saline environments, such as marine or coastal regions or districts where deicing salts are used, show larger particle sizes than rusts formed in rural and urban areas, resulting in the formation of larger pores which act as voids between larger particles in the rust layer and facilitate further corrosion.

Thus, the formation and growth of rust particles is influenced by both parameters. Adsorption isotherms of nitrogen a and water b on the rusts generated by exposing a carbon steel for 3 months at two atmospheres with different NaCl deposition rates [ ]. The SSA determined by N 2 adsorption Figure 17 decreases as the NaCl content of the atmosphere increases the micropores volume MPV shows a similar tendency , indicating that the rusts formed at coastal sites are agglomerates of large particles which give rise to large pores, consistent with the fast corrosion rate at the coast.

This finding again suggests that NaCl promotes rust particle growth. According to Yamashita and Misawa [ ]. Kamimura et al. Despite the fact that magnetite is a conducting phase, Dillmann et al. As has been noted in the introduction to this review, it is rather surprising that despite the great practical importance of this issue it has only recently that it has started to attract the interest of corrosion scientists.

It is well known that the presence of atmospheric pollutants natural or anthropogenic notably accelerates the AC process of CS. The two most common pollutants, which have drawn the majority of research efforts, are SO 2 and marine chlorides. In principle most of the attention has been focused on SO 2 , and considerable progress has been made in this respect [ 4 ].

However, as Nishimura et al. Until then very little research was undertaken in this field, the most notable being the work of Keller [ ], Feitknecht [ 24 ], Henriksen [ ] and Misawa [ , , ]. Keller in [ ] reported three basic chlorides obtained by partial precipitation from FeCl 2 solution in various concentrations, noting that these three basic chlorides were presumably the precursors of GR1.

Feitknecht [ 24 ] reported the existence of chloride accumulations nests containing FeCl 2 in the rust layers formed on steel exposed in coastal areas and their role in stimulating AC. Henriksen [ ], using autoradiography, noted that the AC of CS in marine atmospheres starts at weak spots in the oxide film. Misawa et al. As noted in [ 23 ], this is a feedback mechanism, sometimes referred to as autocatalytic. According to Misawa et al. Fe II hydroxo-complexes thus formed are oxidised by dissolved oxygen, resulting in lepidocrocite through an intermediate GR.

GR1 is converted to black magnetite by slow oxidation in solution and this reaction is considered to correspond to the formation of magnetite in the underlying rust layer where the oxygen supply is limited. Worch et al. Releasing HCl. It should be noted that this represents a reaction cycle in which rereleased HCl will react with iron to form fresh FeCl 2. Once started, therefore, the cycle will be independent of incoming HCl. A fundamental advance in relation with the role played by akaganeite in the AC process of steel in marine atmospheres was made by Nishimura et al.

Akaganeite was reduced to an amorphous intermediate oxide during the wet stage of the cycle and reproduced in the dry stage, giving rise to the proposal of the following rusting model of iron in wet and dry corrosion in the presence of NaCl Figure Rusting model of iron in wet and dry corrosion condition containing NaCl [ 39 ].

Later, in the year , continuing with the in-situ XRD technique but here in combination with alternating current impedance, Nishimura et al. After 60 min in the dry process of the cycle the presence of GR1 was detected. GR1 could still be detected after min Figure 32 , but disappeared after 12 h of testing, indicating that the transformation to akaganeite was complete. Quantitative analysis of the identified phases was carried out using an XRD standard method. The other large phase was goethite. In contrast, the amounts of lepidocrocite and magnetite were low.

This spinel oxide may have been formed by reduction of akaganeite during the wet process of the cycle. In marine atmospheres, corrosion is generally driven by the deposition of hygroscopic sea salt aerosols that absorb moisture from the environment and form salt droplets. These aerosols range from a few angstroms to several hundred microns in diameter. Lan et al. Li and Hihara [ 55 ] studied salt particle deposition and the initial stage of MAC at severe marine test sites.

The corrosion products were identified as lepidocrocite. These same researchers [ , ], in a laboratory study in which they manually deposited NaCl droplets of different diameters on CS steel, used RS to analyse the very initial stage of NaCl particle-induced corrosion. At larger NaCl droplet diameters, corrosion initiated quickly under the droplets in the form of pitting.

In-situ and ex-situ Raman spectra show the formation of GR in regions close to the anodic sites and the precipitation of lepidocrocite clusters over cathodic sites surrounding the GR region. Magnetite was detected mostly in the rust clusters formed in the transitional region from GR to lepidocrocite. Upon exposure to ambient air, GR transformed to the more stable lepidocrocite due to oxidation. Li and Hihara underline the need for more research effort in droplet electrochemistry [ ].

Risteen et al. They also observed the dependence of the occurrence of corrosion on drop size, noting that this behaviour appears to be strongly dependent on the microstructure and surface finish: corrosion initiation on steel was dominated by manganese sulfide inclusions when a mirror surface finish was maintained. In contrast, for high purity iron, initiation was dominated by surface roughness. The NaCl solution was first dripped and immediately dried in a vacuum desiccator. A Raman spectrum was recorded every 15 min.

The Raman spectra of the rust surface in the presence of 0. The Raman spectra further changed after 30 h of exposure, when lepidocrocite again emerged. When the amount of NaCl deposit is decreased to 0. Once the corrosion process has started on the steel surface it will be necessary to consider the possible mechanisms that take place in the formation and growth of the rust layer, where, as is known, the corrosion products transform from one compound to a more stable form and may involve any of a number of processes including hydrolysis, nucleation, crystallisation, precipitation, dehydration, thermal transformation, dehydroxylation, etc.

Temperature, time and pH are the main factors governing such transformations [ 97 ]. In , Evans [ ] formulated the first electrochemical method for atmospheric rusting, in which the oxidation of iron wet periods. Later, after partial drying of the pore structure of the rust dry period , magnetite is reoxidised by oxygen that now has free access through the pores due to gas diffusion.

The autocatalytic cycle responsible for the fact that rust promotes further rusting involves alternate reduction and reoxidation of the preexisting rust. Subsequently, Stratmann et al. Stratmann et al. Thus, Stratmann [ 38 ] proposed dividing the AC mechanism of pure iron into the following three stages: wetting of the dry surface, wet surface, and drying-out of the surface see Figure 2. Misawa [ ] notes the following mechanism for the rusting process Figure 34 :. In the first stage of rusting the aerial oxidation of ferrous ions, dissolved from the steel into a slightly acidic thin water layer formed by rain on the steel surface, leads to the precipitation of lepidocrocite.

Fine weather accelerates the precipitation and crystallisation of lepidocrocite by drying. The lepidocrocite is formed on the steel surface and transformed to amorphous ferric oxyhydroxide and goethite during the atmospheric rusting process. The amorphous ferric oxyhydroxide transforms to goethite by deprotonation using hydroxyl ions provided by the rainwater. Mechanism for the rusting process according to Misawa [ ]. Another important aspect to consider is how the rust layer grows. Horton [ ] in observed that rust layers grow by several mechanisms: i by iron ions diffusing outward through the rust to form fresh rust at the air-rust interface; ii at the steel-rust surface; and iii within the rust layer to fill pores and cracks.

It was the first time that this observation was reported in scientific literature. Years later, Burger et al. With regard to a , they observed a significant contribution of inward diffusion of oxidant through the corrosion product layer. As these cyclic electrochemical reactions require electrical contact between the reactive phase and the metallic substrate, and given the complex morphology of the corrosion patterns, an important outlook is to take into account the connectivity and conductivity of the different phases constituting the corrosion product layer and their influence on its structural evolution.

In the last decade great advances have been made in the understanding of AC mechanisms. As has been mentioned above, many of these advances have been due to the French research groups of Professors Legrand and Dillmann [ 40 , 41 , 44 , 47 , 93 , , , ]. Both groups have made important advances in: a the electrochemical reactivity of the ferric phases that constitute rust; b the localisation of oxygen reduction sites; c the decoupling of anodic and cathodic reactions; d in-situ characterisation of reduction and reoxidation processes, etc. With a view to the development of a model of the AC process that can predict the long term AC behaviour of iron, they note the need to consider several important parameters in order to describe rust layer: average lepidocrocite fraction, thickness, average porosity, tortuosity and specific area, connectivity of the phases inside the rust layer, etc.

As these researchers note, there is still a long way to go before long-term AC mechanisms are fully clarified. As has been mentioned several times in this paper, considerable advances have been made in the knowledge of AC mechanisms in atmospheres polluted with SO 2 e. In addition to the proposals of Nishimura et al. In Nomura et al. Poorly crystalline FeOOH is considered to deposit on the iron surface by the initial corrosion reaction listed above. Before the start of the polymerisation process to form rust according to the following equation:.

It is relevant to note at this point the important laboratory studies carried out by Refait and Genin [ , ] and subsequently by Remazeilles and Refait [ , ] on the formation conditions of Fe II hydroxychlorides, GR1 and akaganeite previously mentioned in Section 5. More recently, Ma et al. After six months of exposure the akaganeite content and the corrosion rate decrease and akaganeite is gradually transformed into maghemite until it completely disappears.

The authors speculate on the need to exceed a critical chloride threshold in order for akaganeite to form. It is unanimously accepted that lepidocrocite is the primary crystalline corrosion product formed in the atmosphere. As the exposure time increases and the rust layer becomes thicker, the active lepidocrocite is partially transformed into goethite and magnetite. In mildly acidic solutions lepidocrocite is transformed into goethite. Schwertmann and Taylor established that the transformation occurs in solution through different steps: dissolution of lepidocrocite, formation of goethite nuclei, and nuclei growth [ ].

Magnetite may be formed by oxidation of Fe OH 2 or intermediate ferrous-ferric species such as GR [ ], but also by lepidocrocite reduction in the presence of a limited oxygen supply [ , ]:. Thus it is not surprising that magnetite is usually detected in the inner part of rust adhering to the steel surface, where oxygen depletion may occur. The formation of magnetite rust can be represented by the following cathodic reaction:. In marine atmospheres, where the surface electrolyte contains chlorides, akaganeite is also formed.

How does akaganeite form? The oxidation process of ferrous hydroxychloride which leads to akaganeite formation passes through different steps via the formation of GR1 intermediate compounds. The whole oxidation process can be summarised as follows [ 99 , , , , ]:. Thus requiring a relatively long time. There has been much speculation about the need to exceed a critical atmospheric salinity threshold for akaganeite formation to take place.

Corrosion rate of mild steel during the first year of atmospheric exposure as a function of the annual average chloride deposition rate at the exposure site. The points of the graph, represented by circles, include an indication of the annual average RH at exposure site. Blue circle represent test site where akaganeite had been identified, and white circle represent test sites where it had not been possible to identify it by XRD [ ]. This fact is well seen in Figure 37 , which shows rusts formed after 3 months exposure of CS in marine atmospheres with different salinities [ 95 ].

Variation of rust phases content in rusts formed on mild steel exposed during 3 months in marine atmospheres with different levels of salinity [ 95 ]. Variation of rust phases content on mild steel exposed during one year in test sites with different chloride deposition rate [ 64 , , ]. Akaganeite could be reduced electrochemically in the corrosion process, being consumed in the wetting of the metallic surface [ 10 ].

Lair et al. Rust layers present considerable porosity and cracking. NaCl promotes rust particle growth, resulting in the formation of larger pores and voids between larger particles in the rust layer and facilitating further corrosion [ , ]. The SSA of rusts decreases as salinity increases, enlarging the diameter of the pores and forming less and less compact rust layers with low protective properties. Thus the compactness of the corrosion product layers formed is dependent on the salinity of the atmosphere at the exposure site.

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In Figure 38 it is possible to see the variation in the structure of the rust layer as the atmospheric salinity rises. Carbon steel specimens were exposed for three months in different marine atmospheres. The circles indicate the content of different phases in the rust, information obtained by XRD RIR of powdered rust [ 64 ].

L: lepidocrocite; G: goethite; A: akaganeite; M: magnetite and Mh: maghemite. Figure 38 also indicates how the content of the different phases in the rust layers varies as the atmospheric salinity rises. The lepidocrocite phase decreases while the goethite and akaganeite phase contents increase [ 64 ]. The base steel shows the formation of pits when exposed to marine atmospheres. As the atmospheric salinity rises, pitting becomes more significant and the Cl signal obtained by EDS inside the pits also rises Figure 39 [ 64 ].

As has been seen in Section 6 Figure 22 , there is a strong presence of akaganeite in the interior of the pits formed on the base steel in severe marine environments. Formation of pits in steel substrate exposed to two atmospheres of different salinities. The EDS signal for Cl is more intense in the atmosphere of higher salinity [ 64 ]. The steel corrosion process consists of the following reactions:.

Under this basic mechanism, the steel corrosion rate will be highly influenced by the concentration of ionisable substances in the aqueous adlayer, as in the case of chlorides present in marine atmospheres. In contrast, the exposure of CS to severe marine atmospheres can lead in certain circumstances to the formation of thick rust layers.

These thick rust layers tend to become detached from the steel substrate, leaving it uncovered and without protection and thus accelerating the metallic corrosion process. The formation of anomalous thick rust layers and the accompanying exfoliation phenomenon has also been observed in studies carried out by the authors and other researchers [ 42 , , , ]. In studies by Chico et al. Exfoliated rust layers are composed of multiple rust strata, as can clearly be seen in the cross-section of Figure The characteristics of the different rust sublayers within the rust multilayer are described in Figure 24 : the outermost rust layer OR rich in lepidocrocite and goethite , and a succession of alternating strata of fragile compact rust CR and loose interlayer rust LIR layers [ 12 , , , ].

The CRs present high goethite and maghemite contents, low lepidocrocite contents and the practical absence of akaganeite. The mechanism that is proposed for the formation of CRs consists of two stages: i the formation of magnetite by electrochemical reduction of lepidocrocite and akaganeite phases wet stage ; and ii the solid-state transformation of magnetite into maghemite dry stage.

The LIR presents high goethite and akaganeite contents along with low lepidocrocite and spinel contents. It is proposed that the akaganeite and lepidocrocite phases will be electrochemically reduced to magnetite maghemite at a later stage and the formation of the CR layer takes place by consumption of the akaganeite and lepidocrocite phases leading to the complete disappearance of the interlayer rust stratum. An extremely dry period may cause the corrosion process to end without fully exhausting the interlayer rust stratum.

Subsequently, once the extremely dry period has come to an end and a new wet period starts, the formation of a second CR layer would begin, and so on, giving rise to the formation of a sandwich-type structure constituted by alternate CR and LIR layers. A scheme of a feasible multilayered rust formation and rust exfoliation mechanism for CS exposed to severe marine atmospheres is shown in Figure 40 [ , ].

Scheme of a feasible multilayered rust formation mechanism of carbon steel exposed to severe marine atmospheres [ , ]. The detachment exfoliation of multilayered rust from the steel substrate takes place after complete drying of the whole rust layer, creating expansion stresses that exceed the adhesion forces which keep the multilayered rust joined to the steel substrate.

The difference in density between the sublayers involved CR and LIR suggests that compactness combined with mechanical properties may play an important role in the triggering of rust exfoliation. A closer look at the molar volume of the rust phases, i. Molar volume of different rust phases. Taking this difference in molar volume into consideration, it is possible to anticipate a great volume contraction and consequent void formation when the least compact phase akaganeite is structurally transformed into the much more compact spinel phase during primarily wet periods.

Similarly, great volume expansion and stress introduction is induced when lepidocrocite is transformed into spinel. Hence, considering the molar volume data, it is not surprising that the compact rust sublayer contains the rust phases with the lowest molar volume goethite and spinel while the loose rust interlayer is dominated by akaganeite with a higher molar volume than the main phases in the solid rust sublayer [ ]. Thus it is suggested that rust exfoliation is the result of frequent phase transformations, together with great variations in compactness between the rust phases involved.

At some critical point the changes in compactness, compressive stresses and void formation become too large and the whole rust sublayer collapses mechanically and results in a fracture along the innermost rust layer [ ]. The atmosphere of many coastal cities in developing countries is polluted with SO 2 due to the growth of industry, and in many cases formerly pure marine atmospheres can now be categorised as marine-industrial. The small amount of research that has been performed on this matter has been carried out in field tests by Corvo [ 80 ], Allam [ ], Almeida et al.

Corvo [ 80 ], after 6 months of atmospheric exposure of steel at marine testing stations in Cuba, found the following damage function:. SO 2 also influences weight loss with a lower coefficient, but in a quadratic form. Allam [ ], on the basis of results obtained in a study carried out at the shoreline on the western coast of the Arabian Gulf, with the presence of SO 2 10 ppb and H 2 S 70 ppb in the atmosphere, formulated the following mechanism for tests from 10 h to 12 months in duration, in which advanced surface analysis was used to characterise the corrosion products.

During the initial stage, the formation of iron sulfate FeSO 4 takes place concurrently with the formation of iron chlorides. This competitive effect has been reported in different papers [ 70 , , ]. As blisters grow to form a thick continuous corrosion product layer, the formation of iron chlorides will eventually decrease compared to that seen during the initial stages. Almeida et al.

1. Introduction

After passing a certain threshold in the concentration of both pollutants, the attack of the steel seems to decrease, but this observation will need to be confirmed in a greater number of atmospheres with very high concentrations of both pollutants. Liang et al. Wang et al. Some laboratory studies have also been carried out on the joint effect of both pollutants acting in combination. Which acquires an acid pH, and the ion capturing capacity of the corrosion products that are formed. They find an additivity of effects when both pollutants act together. Effect of chlorides, SO 2 and the combination of both pollutants on iron corrosion kinetics.

Finally, Chen et al. From the kinetic point of view this finding is not in accordance with the commonly held idea [ 30 ] that a higher SO 2 content in the atmosphere should lead to a higher steel corrosion rate. Accordingly, greater research efforts are needed on both aspects. For socio-economically advanced societies with heavy infrastructure investments in coastal regions, steel corrosion may be a considerable problem. Thus it is fundamental for engineers and political policy-makers to be able to predict AC well into the future 25, 50, years.

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It must be considered that in some highly developed countries efforts are now being made to design civil structures such as bridges and other load-bearing structures for 50— years of service without any maintenance. Data mining and modelling tools can help to improve AC forecasts and anti-corrosive designs, but despite great progress in the development of damage functions dose-response in wide-scale international cooperative research programmes there is still a way to go for such long-term modelling of AC processes.

The nature of the rust constituents is barely affected by the exposure time; in fact, the same species are usually detected at a given site however long the exposure.


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The time factor only alters the proportions of the constituents, or at most determines the appearance or disappearance of intermediate or minor compounds [ 97 ]. Akaganeite is a typical component of rust developed in marine atmospheres. On contacting the steel surface, akaganeite is gradually transformed into magnetite [ , ], in such a way that in severe marine atmospheres this substance can become the main component of the corrosion layer [ ].

There are numerous published damage functions on CS corrosion and environmental parameters, both meteorological air temperature, RH, rainfall, TOW, etc. In Section 4 it was seen that the annual corrosion of steel accelerates as the saline content in the atmosphere rises Figure 7 ; the magnitude of the attack in marine atmospheres normally exceeds that found in other types of atmospheres. For long-term AC, most of the experimental data has been found to adhere to the following kinetic relationship:.

Thus, corrosion penetration data is usually fitted to a power function involving logarithmic transformation of the exposure time and corrosion penetration:. This power function also called the bilograrithmic law is widely used to predict the AC behaviour of metallic materials even after long exposure times, and its accuracy and reliability have been demonstrated by a great number of authors.

Figure 43 , obtained from CS corrosion data after different exposure times in marine atmospheres at different test sites [ 87 ], confirms the verification of the power function Equation Data obtained from the reference [ 87 ]. According to Benarie and Lipfert [ ], Equation 41 is a mass-balance equation, showing that the diffusion process is rate-determining, and this rate depends on the diffusive properties of the layer separating the reactants. The exponential law with n close to 0. This situation seems to occur in slightly polluted inland atmospheres.

On the other hand, n values of more than 0. Conversely, n values of less than 0. However, many of these may be real values and not be due to error in mass loss determinations. The reason for this behaviour lies in the fact that in highly severe marine atmospheres, Equation 41 , based on diffusion mechanisms, can sometimes not be applicable.

The acceleration of the attack as exposure time advances is evident [ 12 ]. Values of exponent n and correlation coefficient R have been obtained from log C vs. According to Ishikawa et al. Data on the corrosion resistance of metals over long periods of time is important for determining the service life of metal structures and for developing the methods and means for their protection and preservation. Reliable estimates of corrosion resistance can be provided by corrosion tests under natural conditions. Such tests are time-consuming and expensive.

In view of this, researchers pay great attention to the development of models that allow long-term forecasts without requiring testing under natural conditions. It has been seen that the power function Equation 41 is widely used in long-term forecasts of the AC of metals. Table 6 sets out average values of exponent n for plain CS in different types of atmospheres, and Figure 46 shows the corresponding box-whisker plots of n values.

It is possible to see a clear tendency towards higher n values in marine atmospheres. Panchenko et al. They find a stochastic relationship between exponent n of the power function Equation 41 and corrosion losses over the first year, and make a forecast of corrosion losses based on a power function using the n values calculated from the identified stochastic relationships.

McCuen and Albrecht [ ] proposed improving the power model by replacing it with two different approaches: numerical model and power-linear model, the latter consisting of a power function at the initial stage Equation 41 and a linear function:. As to whether this law provides a better prediction of the AC of WS for exposure times of at least 20 years, McCuen et al. Albrecht and Hall [ ], by refinement of the power-linear model, have proposed a new bi-linear model based on ISO [ ], called modified ISO , as well as an adjustment of this new bi-linear model that accounts for a modified corrosion rate during the first year of exposure and a steady state in subsequent years.

Finally, Melchers [ , ] suggests a bi-modal model for long-term forecasts of the corrosion loss of WS and grey cast iron in marine atmospheres. The model consists of a number of sequential corrosion phases, each representing the corrosion process that is dominant at that time and which controls the instantaneous corrosion rate. The phases are summarised in Figure The important difference from conventional models is that the bi-modal model has longer-term corrosion governed by microbial activity. Melchers has successfully applied this model to different sets of data points for long-term exposures at different test sites.

Bi-modal model for corrosion loss showing the changing behavior of corrosion process, according to Melchers [ , ]. Nowadays most countries have ample meteorological databases covering their entire territories which would allow estimations of TOW. Furthermore, information on atmospheric SO 2 concentrations is increasingly available.

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However, there tends to be very little information available on atmospheric salinity in different coastal areas, and reliable databases on this subject do not generally exist. It would be desirable to include this data in the numerous published damage functions between steel corrosion and environmental factors. This would make it possible to estimate AC simply from environmental data, without having to carry out natural corrosion tests at a specific site, which involve long waiting times and considerable expense. TOW can generally be defined as the amount of time a metal surface remains wet during atmospheric exposure.

A number of limitations of the ISO definition have been pointed out in the literature and even in the ISO standard itself: a One major issue is that by definition precipitation and dewing events are excluded in ISO A study by Cole et al. Wetting phenomena associated with AC along with TOW definitions and determination methods were overviewed by Schindelholz and Kelly [ 14 , 15 , 16 ].

According to Cole et al. They note the need for a more flexible method for predicting not only TOW but also the cycles of moisture accumulation and depletion. Given the established role of hygroscopic salts in promoting wetting in particulates, whether airborne or on surfaces, a model of surface wetting should address the role of deposited salts. The basic principle of the model is that the TOW of metal surfaces fully exposed to the environment can be approximated by the time of condensation TCD plus rain periods, i.

Thus, these researchers propose a method for estimating the wetting of a surface based on a comparison of surface RH and the deliquescence of salts that may pollute the surface, deriving relatively simple rules for wetting based on the DRH model and ISO classifications. These rules predict the total TOW to a high degree of accuracy. Studies have shown that the two main sources of salt aerosol carried by the wind are ocean waves and breaking surf [ 60 , 66 , , , ]. To mention just a few see Section 4 : wave height at high sea and at the coast; presence or absence of a surf zone on the coast; direction, speed and persistence of marine winds; height above sea level; distance from the shore; topographical effects; etc.

Thus it has been seen in Section 4 that for instance wave height values of 1. There is abundant literature on this topic. Perhaps a less studied aspect has been the degree of shielding or sheltering on wind speed.

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In relation with this matter, Nakajima [ ] has developed a mapping method based on grids with topographic factors to assess the influence of several factors on average wind speed at locations near a sea coast. The concept integrates the geographical texture of the environment in all directions with its effect on airflow. The most significant factor affecting sea wind is the degree of shielding. Similarly, Klassen and Roberge [ ] have measured and modelled by CLIMAT units [ ] the influence of wind effects on local atmospheric corrosivity considering various degrees of wind sheltering.

They found a fold difference between the average mass loss of the most wind-protected and the least wind-protected points. As noted by Roberge et al. In this respect, Cole et al. Salt aerosol sinks, such as gravity, rain and trees obstacles , were accounted for. The study also noted that a high surface RH and a lower cloud height led to decreased penetration inland, and further reductions occurred in areas of high rainfall. This work illustrated that complex variables such as airspeed, ground roughness vegetation , surface air RH, cloud height and rainfall could be incorporated into a model.

A good correlation was reported to exist between the model and empirical results from a limited data set. Despite the high number of factors involved, it is hoped that the growing flow of knowledge on this subject will soon lead to the desirable goal of being able to make a rough estimate of atmospheric salinity in a given geographic area without the need for measurements involving marine aerosol capturing techniques. The prediction of atmospheric corrosivity at a given site is even more complex, as it is necessary to take into account an even greater number of variables.

Thus, Roberge et al. AC has been extensively researched over the last one hundred years, and as a result the effects of meteorological and pollution variables on AC are now well known. Even so, our knowledge on this issue still holds many gaps, such as how to accurately estimate the total TOW of metallic structures, and the effects of climate change and acid rain, etc. Thus it is surprising that marine atmospheric corrosion MAC has until recently received relatively little attention by corrosion scientists.

This is therefore a relatively young scientific field, where there continue to be great gaps in knowledge. In this review, we have noted a number of aspects in relation with which greater research efforts would seem to be necessary. To mention just a few:. It would also be necessary to undertake more research in the case of marine-industrial atmospheres, where there are great discrepancies among researchers. This is a particularly important issue in developing countries where factories are often located in coastal regions.

One currently unresolved question is concerned with the presence of the amorphous phase in the rust layer and the evolution of its amount with exposure time. With regard to the role played in corrosion mechanisms by the less crystalline phases of rust ferrihydrite, feroxyhyte, etc. Another matter that generates a great deal of uncertainty is the differentiation of magnetite and maghemite phases, both of which are very similar in many of their characteristics, but which can play a different role in the MAC process.

In the rust layers formed, aspects such as decoupling of the anodic and cathodic corrosion reactions, localisation, connectivity and reactivity of the different rust phases inside the corrosion layers, as well as characteristics such as porosity, tortuosity, etc. Although great advances have recently been made in this field, there are still a number of basic aspects that remain to be clarified in order for a complete comprehension of rust exfoliation phenomena.

Finally, a matter of enormous technical importance for engineers and political policy-makers is to be able to predict steel corrosion rate well into the future 20, 50, years. Data mining and modelling tools can help to improve forecasts and anti-corrosive designs, but despite great progress in the development of damage functions dose-response in wide-scale international cooperative research programmes, there is still a long way to go for such long-term modelling of atmospheric corrosion processes.

In this sense, better scientific knowledge is needed towards the desirable goal of being able to estimate atmospheric salinity in a given geographic area without the need for measurements involving marine aerosol capturing techniques. National Center for Biotechnology Information , U.

Journal List Materials Basel v. Materials Basel. Published online Apr Yong-Cheng Lin, Academic Editor. Author information Article notes Copyright and License information Disclaimer. Received Mar 9; Accepted Apr 7. This article has been cited by other articles in PMC. Abstract The atmospheric corrosion of carbon steel is an extensive topic that has been studied over the years by many researchers.

Keywords: atmospheric corrosion, marine environment, carbon steel. Introduction Steel is the most commonly employed metallic material in open-air structures, being used to make a wide range of equipment and metallic structures due to its low cost and good mechanical strength. Basic Concepts The AC of metals is an electrochemical process which is the sum of individual processes that take place when an aqueous adlayer forms on the metal. Open in a separate window. Figure 1. Saltwater Aerosols The deposition of salt particles on a metallic surface accelerates its corrosion, especially, as in the case of chlorides, if they can give rise to soluble corrosion products rather than the only slightly soluble products formed in pure water.

Hydrogen Chloride Vapours Askey et al. Figure 2. The Marine Atmosphere From the point of view of MAC, the marine atmosphere is characterised by the presence of marine aerosol. Atmospheric Salinity Atmospheric salinity is a parameter related with the amount of marine aerosol present in the atmosphere at a certain geographic point. Production of Marine Aerosol Cole et al. Figure 3. Figure 4. Entrainment of Marine Aerosol Inland Aerosol particles can be entrained inland by marine winds winds proceeding from the sea , settling after a certain time and after travelling a certain distance.

Figure 5. Figure 6. Effect of Salinity on Steel Corrosion For salt to accelerate corrosion the metallic surface must be wet. Steel Corrosion versus Salinity In studies of MAC a direct relationship is generally established between corrosion and the saline content of the atmosphere. Figure 7. Figure 8. Steel Corrosion versus Distance from the Shore The influence of the distance from the sea is one of the most important aspects of MAC in coastal areas.

Figure 9.

Measurement of Atmospheric Salinity Airborne salinity is the amount of marine aerosol present in a given marine atmosphere, and a value that is commonly measured in corrosion studies. Figure Salt Lake Atmospheres Very little research work has focused on steel corrosion in salt lake environments, though several papers have recently been published in relation with Qinghai salt lake in north-west China [ 85 , 86 ].

Deicing Salts Although typically associated with marine environments, NaCl is actually more prevalent in the environment from the use of road deicing salt. Atmospheric Corrosion Products Atmospheric corrosion products of iron, referred to as rust, comprise various types of oxides, hydroxides, oxyhydroxides and miscellaneous crystalline and amorphous substances chlorides, sulfates, nitrates, carbonates, etc.

Table 1 Iron corrosion species according Cornell and Schwertmann [ 91 ]. Table 2 Iron corrosion species containing chloride [ 92 ]. Morphology The rust formed on steel when exposed to the atmosphere is usually a complex mixture of several phases. Table 3 Principal habits morphologies of iron oxides according to Cornell and Schwertmann [ 91 ]. The Rust Layer When the thin layer of corrosion products has grown to cover the whole surface, further growth requires reactive species from the aqueous adlayer to be transported inwards through the rust layer while metal ions are transported outwards.

Organoleptical Properties 6. Colour CS exposed to the atmosphere develops ochre-coloured rust which becomes dull brown as the exposure time increases. Texture In Section 5. Stratification of Rust Layers There is controversy about the stratification of the rust layer in different sublayers on unalloyed CS [ 97 ].

Adhesion According to Honzak [ ] it is possible to differentiate three layers in rust: surface rust that is easily removable e. Some examples [ ] include: a. Thickness and Internal Structure The thickness of the rust layer increases with time of exposure and the aggressivity of the atmosphere. Porosity Voids of different sizes are formed among the rust particles in the corrosion layer, whose compactness depends on the rust particle size.

Mechanisms of Steel in MAC 7. First Researches As has been noted in the introduction to this review, it is rather surprising that despite the great practical importance of this issue it has only recently that it has started to attract the interest of corrosion scientists. The Fundamental Role of Akaganeite in the Atmospheric Corrosion Process of Steel in Marine Atmospheres A fundamental advance in relation with the role played by akaganeite in the AC process of steel in marine atmospheres was made by Nishimura et al. Initial Stages of MAC In marine atmospheres, corrosion is generally driven by the deposition of hygroscopic sea salt aerosols that absorb moisture from the environment and form salt droplets.

Formation and Growth of the Corrosion Layer Once the corrosion process has started on the steel surface it will be necessary to consider the possible mechanisms that take place in the formation and growth of the rust layer, where, as is known, the corrosion products transform from one compound to a more stable form and may involve any of a number of processes including hydrolysis, nucleation, crystallisation, precipitation, dehydration, thermal transformation, dehydroxylation, etc.

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OH reduced lepidocrocite ,. Proposal of an Overall Mechanism for the MAC Process of Steel The composition of the rust layer depends on the conditions in the aqueous adlayer and thus varies according to the type of atmosphere. Table 4 Variation of rust phases content on mild steel exposed during one year in test sites with different chloride deposition rate [ 64 , , ].

Table 5 Molar volume of different rust phases. Quiz-summary 0 of 18 questions completed Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 To start just click the button below. Good luck! You have already completed the quiz before. Hence you can not start it again. Quiz is loading You must sign in or sign up to start the quiz. Answered Review.