INTERNATIONAL SYMPOSIUM ON CORROSION FOR SUSTAINABLE DEVELOPMENT

As we all know, atmospheric corrosion is electrochemical corrosion of metal under thin liquid film in wet-dry cycle environment. Because it is difficult to carry out electrochemical measurement under the condition of thin liquid film, monitoring the electrochemical behavior and corrosion rate of atmospheric corrosion has always been a hot spot in the field of corrosion research, and it is a challenging task to carry out in-situ electrochemical monitoring outdoors. In this work, the comb-shaped low carbon steel electrochemical impedance spectroscopy probe was used to monitor the typical atmospheric environment in China for one year. The results show that the real wetting time of metal surface can be calculated by using high-frequency impedance data, while the corrosion rate of metal can be calculated by using low-frequency impedance data, and there is a monotonic linear response relationship between high-frequency impedance and low-frequency polarization impedance. The wetting grade of metal surface reflected by high-frequency solution resistance and the environmental corrosion grade reflected by low-frequency polarization resistance are basically consistent with the classification of corresponding environment made by ISO9223-9226.

Now-a-days, high Si steel with bainitic structure has wide applications in railway and automobile industries due to its properties of high strength, high ductility and high toughness. These properties of the bainitic steels can be varied by the variation of austempering temperature and time [1, 2]. The present work represents the crevice corrosion behavior of a set of newly developed high strength and highly ductile multiphase steels consisting of various fractions of bainite, retained austenite (RA), intercritical ferrite (IF) and pearlite. The steels were made by various combinations of continuous cooling in the ferrite-pearlite zone followed by isothermal heat treatment in the bainite zone. Crevice corrosion tests of the multiphase steel specimens were carried out in 3.5 wt.% NaCl solution for 9 months. Steel rusts after crevice tests were analysed with the help of scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). Ultrasonically cleaned multiphase steel specimens were characterized with the help of SEM and optical profilometry. Multiphase interaction and rust constituents uniquely control the crevice corrosion behaviour of the steels. Considering the effect of various fractions of bainite, RA, IF and pearlite present in the multiphase steels and their tendency to form the galvanic cell corrosion, it has been observed that the multiphase steel isothermally heat treated at 300℃ after 0 s of continuous cooling time from austenitization temperature shows less corrosion. Though, all the multiphase steel specimens have same identical chemical composition, they exhibit different modes and various degrees of corrosion attack due to the difference in the microstructures, morphological distribution and fraction of different constituent phases and their subsequent formation of micro galvanic couples. The combination of high strength and ductility in association with excellent crevice corrosion resistance of the multiphase steels implies its great potential in rail applications.

To meet the sustaining request of enhancing thrust-load ratio, single crystal (SX) superalloy, film cooling and thermal barrier coating system (TBCs) have been perspectively utilized in advanced areo engines. Usually, the coating system consists of an oxidation resistant bondcoat and a heat resistant top coat. During service of SX superalloy components at high temperature, the element interdiffusion between bondcoat and superalloy substrate may instigate serious deterioration of mechanical property (e.g., creep resistance and rupture life) of SX superalloy by forming topologically-close-packed (TCP) phases and second reaction zone (SRZ) [1]. Another impact of such interdiffusion is the loss of beneficial element and undesirable oxidation of refratory elements at surface [2]. Thus, to inhibit or mitigate the interdiffusion is one of the key points to design protective bondcoat for Ni-base SX superalloys [3]. In this study, a Re-base diffusion barrier (DB) has been sucessfully incorporated between the state-of-the-art bondcoat of (Ni,Pt)Al and the Ni-base SX superalloy. In contrast to normal (Ni,Pt)Al coating, the coating with Re-base DB showed better oxidation resistance and less interdiffusion, which further resulted in thinner formation of SRZ and TCP precipitates. Mechanisms responsible for the enhanced performance during the oxidation tests will be intensively discussed.

The human activity that we know as “science” is based upon two broad philosophies; empiricism and determinism. Empiricism is the philosophy that everything that we can ever know we must have experienced, whereas determinism posits that the future can be predicted form the past upon the basis of the natural laws, which are condensations of all previous scientific experience. Viewed in this light, “science” is clearly the process of conversion from empiricism to determinism. Thus, observations (experiments) are made empirically and the results eventually lead to the formulation of new “natural” laws that then are used to constrain deterministic prediction to what is “physically real”. The impediment to this process is “complexity”, which is measured by the number of degrees of freedom in a system. Complexity can be likened to a fog that limits the field of view and obscures physico-chemical detail. The advancement of science occurs via the lifting of that fog. Thus, complexity is overcome by using more discerning tools and sensors. Perhaps the greatest tool in lifting the shroud of complexity has been the development of the high-speed, digital computers that are now capable of performing billions of individual calculations per second. Computers have been responsible for the creation of more scientific knowledge over the past several decades than had been created in all preceding human history. Consider the problem of describing the behavior of a cluster of atoms, which in physics is commonly referred to as a “many bodied problem”. The Hamiltonian, which describes the motion of each atom in the system, for a system of 100 atoms was an insurmountable challenge just a few decades ago; now, using supercomputers, clusters of tens of thousands of atoms can be accurately described. Another useful concept in combatting the debilitating effect of complexity is the average property approximation. Consider the problem of describing the propagation of a crack in a piece of iron. For convenience, let us assume that the mass of the metal is 55.5 g. This piece of metal contains 6.023x1023 atoms and formulation of the Hamiltonian to describe the motion of all atoms in the system, including those atoms at the tip of the crack that are responsible for crack advance, is clearly an impossible task, even with today’s most powerful super computers. However, crack advance is due only to the motion of a relatively few atoms in the vicinity of the crack tip. The remainder of the piece of iron, whose atoms are not involved in the crack advance process, may be assigned “average” properties, thereby greatly decreasing the complexity of the system. It is this approximation that enables deterministic description of physico-chemical phenomena in practical systems (e.g., crack propagation in nuclear power plant coolant piping). An example of the deterministic prediction of damage due to stress corrosion cracking (SCC) in reactor piping is shown in Figure 1, in which the depth of a crack in Type 304 stainless steel in the core shroud of a Boiling Water (Nuclear) Reactor is displayed as a function of various operating protocols as might be chosen by the reactor operator. Thus, “normal water chemistry” is the standard operating protocol for a BWR in which no attempt is made to modify the redox properties of the coolant (water at 288 oC) so as to reduce the driving force of the crack, which is the corrosion potential of the steel. Under this protocol the crack grows by about 2.2 cm over the operating period of ten years. The addition of hydrogen to the coolant water in the Hydrogen Water Chemistry (HWC) operating protocol that results in a reduction in the driving force for crack propagation and hence in the crack propagation rate, was originally developed in Sweden as a means of combatting SCC in the coolant circuits of BWRs. As seen, if HWC is implemented at the start of operation, the increase in crack length is reduced to 0.6 cm, which is substantial with significant financial implications for the operator and consumer alike. On the other hand, if HWC is implemented after five years, the crack is predicted to grow by 1.9 cm, which is only moderately better than if HWC had never been implemented at all. The reader will note that this “law of decreasing returns” situation is due to the shape of the crack length vs time correlation, corresponding to a decrease in the crack growth rate as the crack grows. This important feature was predicted by deterministic modeling and has since been verified experimentally. The reader will also note that the crack length vs time curves are not smooth. The discontinuities are not due to calculational error but reflect various outages of the reactor, including refueling outages. From the above, the reactor operator might conclude that, if HWC is to be implemented, then it should be put into effect as soon as possible, if the maximum benefit is to be realized. This example illustrates the benefits of deterministically modeling corrosion phenomena in “real world” scenarios.

Neutral salt spray test and polarization test were used to compare the corrosion resistance of Co-containing maraging stainless steels with that of other three commercial stainless steels. The test results showed that the maraging stainless steel with high content of Co showed poor corrosion behavior. Observation on microstructure of the maraging stainless steels proved that the segregation of Cr in the matrix deteriorated its corrosion resistance. The surface morphology of the aged maraging stainless steel with high content of Co indicated that during passive process the new formed passive film with sinusoidal distribution could be easily broken down by the invade medium and finally caused poor corrosion resistance. Meantime, it was proved by first principle calculations that Co increased Fe-Fe ferromagnetic interaction which increased the clustering tendency of Cr atoms and facilitated the formation Cr-rich clusters.

One of the major problems faced by the biomedical industry is the lack of long-term corrosion resistant materials compatible to the human body environment. Such incompatibility causes several problems such as implant loosening, osteolysis, joint pain and Alzheimer. Surface modifications of the implant materials has shown to help resolve these issues to a greater extent. In this regard, several coating techniques have been used by the researchers, among which thermal spray (TS) is one of the most widely accepted coating techniques to mitigate the above-mentioned problems. However, high processing temperature in thermal spray is a point of concern. To reduce high-temperature effects, we developed coatings by cold spray (CS) technique. Cold spray is a solid-state coating technique utilizing a processing temperature that is far below the melting point of the coating materials. Hence, temperature sensitive materials like titanium and hydroxyapatite can be sprayed without any phase change. Recent studies show that hydroxyapatite can be replaced by a calcium silicate compound (baghdadite –〖Ca〗_3 Zr〖Si〗_2 O_9). Pham et al. [1] showed baghdadite provides superior mechanical and biological properties compared to hydroxyapatite. We developed a novel coating to enhance the corrosion resistance of the implant materials. Titanium/Baghdadite composite coatings are fabricated using high pressure cold gas spray system. Initial characterizations are performed on Scanning electron microscopy (SEM), X-Ray diffraction (XRD), and Energy dispersive spectroscopy (EDS). Furthermore, electrochemical corrosion behavior of the coatings is also performed to evaluate the corrosion behaviour of the developed coatings.

Mg-Li based alloys are the lightest alloys and hence, are attractive for aerospace applications [1]. We have optimized Li content as 8 wt% and Zn content as 2 wt% in Mg to obtain tensile properties better than the existing commercial and experimental wrought Mg-Li based alloys. The optimization of alloy composition, processing parameters and tensile properties have already been reported elsewhere [2]. However, the corrosion resistance of Mg-Li alloys is poorer than the other Mg-alloys, which restricts its applications [3]. The 2 wt% Zn addition not only improved the tensile properties of Mg-8Li (LZ80) alloy but also improved its corrosion resistance. Here, we present the electrochemical corrosion behavior of the two cold-rolled alloys, i.e., Mg-8Li and Mg-8Li-2Zn (LZ82). The microstructure of the alloys was examined by scanning electron microscope before and after corrosion tests in order to explain the observed corrosion behaviour. Both the alloys exhibited localized pitting corrosion, but it was less severe in the case of LZ82 alloy. The pitting corrosion initiated at the interface of α-hcp and β-bcc phases due to micro-galvanic coupling between the two phases, and Zn addition lowered the potential difference between these phases, as revealed by scanning Kelvin probe force microscopy.

High entropy alloys are one of the advanced materials with five or more elements forming a single phase. Earlier studies have shown that these alloys get their advanced properties due to four core effects: (i) high entropy effect, (ii) severe lattice distortion effect, (iii) sluggish diffusion effect, and (iv) cocktail effect. The available HEAs mainly contain d-block elements, such as Al, Cr, Mn, Fe, Co, Ni, Ti, Mo, etc.
Microstructures, morphology, and distribution of constituent phases, like bi-modal microstructure, can improve the properties of materials to a great extent. In the last few years, the group of K. Ameyama of Ritsumeikan University, Japan, has come up with a new type of bi-modal microstructure, known as harmonic microstructure, where very fine and coarse grains distribute in a regular and periodic fashion [1]. It has been found that harmonic structured 316L stainless steel and 304L stainless steel show better corrosion resistance and wear resistance than their non-harmonic counterparts [2-5].
Our study deals with Cantor alloy (FeCrMnNiCo), whose grain size has been modified to harmonic microstructure. Scanning electron micrographs show the distinguished regions containing clusters of larger grains known as core and the network of fine grains known as shell. It has been confirmed by X-ray diffraction that a single FCC phase is present in the core and the shell regions. Corrosion properties of the Cantor alloy in simulated body fluid solutions are yet to be explored in literature. Hence, corrosion behavior has been carried out on the harmonic structured Cantor alloy and compared with its non-harmonic counterpart and harmonic structured 316L stainless steel, which is well known for its biological applications and harmonic structured 316L shows similar corrosion behavior but better wear resistance than the conventional 316L stainless steel. It has been found out in our study that the corrosion resistance of the harmonic structured Cantor alloy is greater than the non-harmonic one and comparable to that of the harmonic 316L stainless steel. Enrichment of Cr in the fine-grained shell region of harmonic structured Cantor alloy is attributed to the greater corrosion resistance of the alloy.

Thin-film corrosion is a severe issue in almost every sector. Thus, corrosion simulation under thin electrolyte films has always been of high interest as experimental studies are often challenging. For microbial corrosion, the existence of a biofilm on the metal surface impacts corrosive species' production and transportion, which is yet understood poorly. Therefore, almost all the present corrosion models are risk-based and meant for ranking potential threats to the industrial assets, rather than trying to quantify those threats. Thus far, especially for atmospheric corrosion, progress has been made to model the effect of several essential factors on thin-film corrosion rates. Some of these parameters are electrolyte thickness, electrolyte composition, chemical reactions in the electrolyte, electrode size and change in electrode size, environmental parameters, and corrosion products deposition. However, these parameters are mainly drawn from different studies and have not been modeled concurrently in a single simulation study, making the thin film corrosion model far from being complete yet. Our research aims to resolve the problems mentioned above by employing finite element analysis using the corrosion module of COMSOL Multiphysics software. We have developed a multi-species multi-reaction moving boundary (MSMRMB) model, which not only provides flexibility for modeling altering corrosive environments but can quantify the corrosion rate. Quantification of corrosion rate will enable the industries to apply fitness for service (FFS) to the corroded pressure vessels based on the maximum pit size, operating pressure, lifetime, etc. We are aware that there is a long way to achieve a complete model, but our developed MSMRMB model is promising for this journey.

In this study[1], we investigated the galvanic corrosion performance of an Aluminum–Boron Nitride (Al–BN) abradable seal coating system (with a Ni5Al bond layer and a 0Cr17Ni4Cu4Nb substrate) in chloride solution by electrochemical methods. Galvanic interaction of the coating system during corrosion has been confirmed, while the Al–BN layer assumes anodic character, while the bond NiAl and substrate act as cathodes. Negative difference effect (NDE)[2] was tested during the anodic dissolution of Al-BN top layer, which indicated that about 13% of the anodic current of the Al–BN layer was compensated by hydrogen evolution by NDE. In addition, a three-stage process occurred during the anodic dissolution of the coupled coating system, consisting of a spontaneous pitting stage I under charge transfer control with a decreasing rate, a corrosion developing stage ІІ under mass transfer control with an increasing rate, and a final steady stage III. Precipitation of Al(OH)3 restricts[3] the oxygen transport process to the cathode and induces localized acidification of the occluded pores of the Al–BN layer, which was the mechanism that could explain the changes of corrosion performance during the three immersion stages of Al–BN coating system. The study suggests that galvanic corrosion of the porous multi-layer Al–BN abradable coating system is mostly influenced by its corrosion product deposition.

Corrosion and its mitigation costs dearly (any developed economy loses 3-4% of GDP due to corrosion, which translates to ~$250b to annual loss USA). In spite of traditional approaches of corrosion mitigation (e.g., use of corrosion resistance alloys such as stainless steels and coatings), loss of infrastructure due to corrosion continues to be a vexing problem. So it is technologically as well as commercially attractive to explore disruptive approaches for durable corrosion resistance.

Graphene has triggered unprecedented research excitement for its exceptional characteristics. The most relevant properties of graphene as corrosion resistance barrier are its remarkable chemical inertness and impermeability and toughness, i.e., the requirements of an ideal surface barrier coating for corrosion resistance. However, the extent of corrosion resistance has been found to vary considerably in different studies. The author’s group has demonstrated an ultra-thin graphene coating to improve corrosion resistance of copper by two orders of magnitude in an aggressive chloride solution (similar to seawater). In contrast, other reports suggest the graphene coating to actually enhance corrosion rate of copper, particularly during extended exposures. Authors group has investigated the reasons for such contrast in corrosion resistance due to graphene coating as reported by different researchers. On the basis of the findings, author’s group has succeeded in demonstration of durable corrosion resistance as result of development of suitable graphene coating. The presentation will also assess the challenges in developing corrosion resistant graphene coating on most common engineering alloys, such as mild steel, and presents results demonstrating circumvention of these challenges.

Strong interdiffusion between protective coatings and superalloy substrates at high temperatures leads to formation of secondary reaction zone (SRZ), which may seriously weaken the creep and fatigue properties of the coated components[1-3]. In this talk, a serial of nanocrystalline coatings will be introduced to show their high substrate-coating compatibility in addition to excellent oxidation resistance at 1100 ºC. The coatings were prepared on Ni-base single crystal superalloy Rene N5 using magnetron sputtering, and their chemical composition were derived from Rene N5. The thermally grown oxide (TGO) grew on the coating with chemical composition same as Rene N5 is alumina with dispersive TaOx, which is harmful to the oxidation resistance. Increasing the Al concentration to decrease the Ta content in the γ' phase of the coating can reduce the amount of the TaOx in TGO while either making a single γ' phase coating or decreasing the Ta in the coating can prevent the formation of TaOx in TGO. The β phase in the coatings should be prohibited, since it is the driving force to form SRZ.

Mild steels or their derivative steels in the carbon range of 0.1-0.25%C are used in almost all the common engineering applications, such as construction, bridges, household, agriculture, etc. On the other hand, constructions in off-shore or river always experience hostile environment leading to severe corrosion of the steel structures as well as reinforced bars embedded in concrete structures [1]. Therefore, quick assessment of corrosion in laboratory scale is extremely important in order to facilitate the selection of suitable materials for varied corrosion environments in off-shore area [2, 3].
Hence, the present work concentrates on the quick evaluation of corrosion of a normalized mild steel under the influence of water line change due to wave as well as natural evaporation. A novel simulated marine environment device has been developed to evaluate the corrosion behavior in various marine zones (atmosphere zone, splash zone, and immersion zone). The Raman spectroscopic analysis of the corrosion products, formed at various zones, has revealed the formation of α-FeOOH, γ-FeOOH, β-FeOOH and Fe2O3 in a unique way. The morphologies of the corrosion product formed in various zones, as characterized by scanning electron microscope (SEM), show that the corroded region of the splash zone contains cracks and porous rust layer in comparison to that of the other regions, indicating instability of the corrosion products. Moreover, the thickness of the corrosion product layer is more in the splash zone in comparison to that of the other zones due to cyclic wetting and drying. In addition, open circuit potential (OCP) and linear polarization (LP) measurements, conducted on the corroded surface of various zones without the removal of the rust layer, suggest strong relationship of rust layer composition and phase fraction with OCP and LP.

The kinetics of redox reactions, such s the hydrogen electrode reaction (HER) and the Oxygen Electrode Reaction (OER) on the noble metals, such as Pt, are commonly treated as though they occur on a mare metal substrate with the only involvement of the metal being a source of electrons vis orbital overlap. This case has been treated to great extent by Marcus, who received a Nobel Prize for his work. However, noble metal surfaces are not “bare, depending upon the potential. Thus, experiment show that Pt is covered with an oxide “passive” layer at potentials down to the hydrogen evolution region so that, in the case of the OER, at least, the reaction occurs on a PtO surface. The involvement of the oxide is seldom taken into account even though charge carriers (electrons and holes) must transfer through the oxide from the metal to the redox center on the solution side of the oxide/solution interface. By combining the Generalized Butler-Volmer equation with quantum-mechanical tunneling theory and the Point Defect Model for the growth of passive films, a method has been devised to access the kinetic parameters for the redox reaction occurring on a passive metal surface. This allows interpretation of the kinetic data free from the complication of the presence of the oxide film and hence renders the system closer to the charge transfer theory developed by Marcus and other. The possible involvement in surface oxygen vacancies in the OER on platinum is discussed.

Our civilization is based upon the reactive metals, such as aluminum, iron, nickel, chromium, titanium, and so forth. All these metals and their alloys react with oxygen and water with considerable negative changes in the Gibbs energy, indicating that the reactions are thermodynamically spontaneous and many of the reactions occur at considerable rate; some violently so (e.g., the burning of Al or Mg in air). The resulting destruction of the alloy, known as corrosion, exacts an enormous cost on society that has been estimated at about 3.5 % of the GDP for industrialized countries like the US. Given that the GDP of the US is about $21 trillion ($21x1012), the annual cost of corrosion is approximately $735 billion. On a worldwide basis, the cost is estimated to be $2.2 trillion. Corrosion is an electrochemical process comprising at least two partial reactions, one of which is the electrodissolution (destruction) of the metal or alloy substrate to produces electrons that are quantitatively consumed by a cathodic partial reaction, such as the reduction of oxygen or the evolution of hydrogen via the reduction of water. The rate of the electrodissolution reaction in the active state increases exponentially with the electrochemical potential, so that even modest changes in the potential can result in massive changes in the rate. Fortunately, once the potential exceeds a critical value, known as the Flade or passivation potential, the rate drops precipitously to values that are sufficiently low (< 1 µm/a) that the metals and their alloys may be used to fabricate machines that retain their precise dimensions over useful service lifetimes (40 – 100 a). This if known as the passive state in which the thermodynamically highly reactive metals attain kinetic stability because of the formation of a nanometer thick oxide film on the surface that separates the reactive substrate from the corrosive environment. In this presentation, I will review the scientific baes for the phenomenon of passivity within the framework of the Point Defect Model (PDM) and define precisely the condition that must be achieved for passivity to occur. Indeed, the occurrence of our metals-based civilization can be expressed as a simple inequality that has profound implications for life as we know it. I will also discuss how the PDM predicts the breakdown of passivity that is responsible for the $2.2 trillion annual cost of corrosion. These predictions will be illustrated with practical examples, such as the corrosion and failure of airframes, the failure of oil/gas pipelines, and the failure of nuclear reactor coolant piping.

Ferritic stainless steels have been widely employed as solid oxide fuel cell (SOFC) interconnects owing to their low cost, coefficient of thermal expansion match with other SOFC components, and good oxidation resistance and acceptable electrical conductivity of Cr2O3. However, they are confronted with several problems during operation in SOFC cathode working condition such as the evaporation of Cr2O3 which results in cathode Cr-poisoning and subsequent degradation of cell performance [1-5]. Therefore, it is necessary to develop electrically conductive coating on them in order to block Cr2O3 evaporation. So far, CuFe2O4 spinel coating is a promising coating to improve the electrical conductivity of the surface oxide scale thermally formed on the steel. In present paper, CuFe alloy layer has been deposited on ferritic stainless steel (SUS 430) by magnetron sputtering method. The coated steels were evaluated in air at 800C corresponding to SOFC cathode environment. It was found that the coated steel initially experienced a large mass gain, followed by slight increase with time. The CuFe alloy layer was mainly converted into CuFe2O4 spinel layer beneath which a Cr-rich layer was grown from the steel substrate upon thermal exposure. The Cr-free outer layer not only suppressed Cr migration outward but also reduced the surface oxide scale area specific resistance (ASR) of the coated steel.
Keywords: (CuFe)3O4 coating, Solid oxide fuel cell interconnect, Oxidation, Electrical property

Corrosion properties of Ag and Au nanoparticles in water-based environment were compared using electrochemical techniques hyphenated with quartz-crystal microbalance. In addition, quantum chemical calculations of Ag nanoparticles and Ag nanoparticles/ascorbic acid hybrid system were performed to find out how theoretical approach can contribute as support for laboratory experiments. An assessment of dissolution rate from measured corrosion parameters for both bare and inhibited nanoparticles was determined in Hank solution as simulated human body fluid. The dramatic change of redox behaviour for Ag nanoparticles was observed in the presence of antioxidant molecules of ascorbic acid (AsA). Inhibition effect in dissolution of Ag NPs initiated by AsA was theoretically proofed by performing quantum chemical calculations of redox properties of studied hybrid system. The quantum-chemical calculation has showed that the electrons are mainly transferred from AsA molecule to nanoparticle and the free radicals are suppressed. Combination of these techniques help to study unknown thermodynamics, corrosion of nanoobjects and finally assessment of the life-time of NPs applied in environment of interest.
What will audience learn from your presentation?
• The aim is to highlight problem related with opening “Pandora´s box” regarding reactivity of NPs and their interaction with environment due to the changes of redox properties by surface modification.
• Prediction of life-time of metallic nanobjects in application environment by monitoring of corrosion phenomena.

Corrosion is an issue that has led to widespread cost and even danger. Thus, it is of importance to investigate the nature of corrosion initiation and/or propagation mechanisms; one such way is via analysing the nobility and electrochemical changes of the metal microstructure through their Volta potential (Ψ), which is the potential difference between a point just outside the surface of the metal and a point infinitely far away from the surface [1]. The Ψ gives information about the surface electrons, and hence can be used as a gauge of a material’s tendency towards physicochemical reactions such as corrosion [2].
Using a recently developed Scanning Kelvin Probe Force Microscopy (SKPFM) setup that incorporates an ability to vary the relative humidity [3,4], the earliest stages of localised corrosion on aluminium alloys, such as AA5083, under chloride-contaminated thin-film electrolytes were investigated at ambient temperature over the humidity range 20%-85% RH. The in-situ time-lapse SKPFM investigation elucidated magnesium silicide particle regions in AA5083, which initially showed positive Volta potential (vs matrix), suffering from severe, sharp nobility loss due to the ongoing dissolution of magnesium and the formation of rod-like corrosion products [5]. The corrosion product growth, most likely a form of nesquehonite, was highly favoured when exposed to humidities >80% RH. Transient events that occurred during the corrosion were also captured by the in-situ time-lapse SKPFM method, further demonstrating that it is necessary to measure the Volta potential during corrosion to reflect the true relationship between the Volta potential and corrosion potential or breakdown potential of a material.
Furthermore, the phenomenon of nobility adoption is discussed, detailing instances when local sites adopted the nobility of their neighbouring region. Nobility adoption seemingly occurs at elevated humidity (>80% RH) and within close proximity of a particle with large Volta potential difference relative to the matrix. Different behaviours of nobility adoption were also seen in the analysis, when the aluminium alloys were exposed to thin-film electrolyte over extended period, suggesting that the localised corrosion mechanism evolves over time.

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