Corrosion Science Research Articles

Microstructure and Electrochemical Characterizations of Mn-Ce Conversion Coating on LZ91 Magnesium Alloy

In the past few years, the energy crisis and environmental issues have been emphasized. In order to reduce the greenhouse effect and enhance the efficiency of energy consumption, each country has enacted relative laws to decline the emission of carbon dioxide year by year. Magnesium alloys, the lightest structural metal, have found their potential use in the aerospace, automobile, tool, and electronic industries due to their attractive damping capacity, high specific strength and specific stiffness, as well as good electromagnetic shielding characteristics. In comparison to the AZ (with aluminum and zinc as the major alloy elements) series magnesium alloys, the LZ (with lithium and zinc as the major alloy elements) series magnesium alloys have been developed by substituting aluminum with lithium. However, because of the difference in the standard reduction potential between the α phase and the β phase and the presence of active lithium, Mg-Li alloys display poor corrosion resistance. Surface modification is thus essential for Mg-Li alloys. In this study, the main theme is aimed at the improvement of phosphate/permanganate conversion coating. With substituting phosphate for cerium (III), magnesium dissolved more magnesium ions and released more electrons to enhance the oxidation-reduction reaction. The conversion coating showed better adhesion and less severe crack by virtue of increasing the chemical reactivity and lowering the immersion time, expecting to replace hexavalent chromate conversion coating and reduce the cost. The LZ91 magnesium alloys were mechanically grinded up to 1200 grade silica emery paper, rinsed with deionized water, and dried with an air stream. The conversion coating treatment was conducted in the solution composed potassium permanganate (KMnO4) and cerium nitrate (Ce(NO3)3). The pH of the solution was about 1.5-4.5. After conversion coating treatment, the LZ91 plate was rinsed in deionized water and left drying overnight at room temperature. Based on the microstructural, and XPS, the formation and deposition mechanism of the Mn-Ce conversion coating on LZ91 is discussed in detail. The corrosion resistance of the coating was characterized by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and salt spray tests (SST). The results of the electrochemical measurement and SST demonstrated that the corrosion resistance of LZ91 alloy was markedly improved by the Mn-Ce conversion treatment. Figure 1. shows the potentiodynamic polarization curves of bare LZ91 and the various Mn-Ce coated LZ91 plates (for pH=1.5) in 0.05 M NaCl and 0.1 M Na2SO4 solution. In general, the cathodic reaction in the polarization curve is related to the evolution of hydrogen and the anodic polarization curve is the characteristics corresponding to the corrosion resistance of the coating. Both the cathodic and anodic reactions were inhibited in the presence of the conversion coating, indicating the coating effectively enhances the corrosion resistance of the LZ91. And, the values of electrochemical corrosion parameters were shown in Table 1. In contrast with the un-coated LZ91 alloy, the corrosion current density Icorr of the Mn-Ce coated LZ91 plates was decreased by one orders of magnitude (with the value from 46.9 µA/cm2 to 1.78 µA/cm2), which functioned as a barrier. References S. Y. Jian, Y. R. Chu, C. S. Lin, Corrosion Science 93(2015) 301-309 C. Wang, F. Jiang, F. Wang, Corrosion Science 46 (2004) 75–89G. Bikulčius, A. Ručinskienė, A. Sudavičius, V. Burokas, A. Grigucevičienė, Surface & Coatings Technology 203(2008) 115–120 H. Zhang, G. Yao, S. Wang, Y. Liu, H. Luo, Surface & Coatings Technology 202 (2008) 1825–1830 L. Yang, J. Li, X. Yu, M. Zhang, X. Huang, Applied Surface Science 255 (2008) 2338–2341 C. S. Lin, S. K. Fang, Journal of The Electrochemical Society, 152 (2) B54-B59 (2005) Figure 1

Oxide Scale Analysis of the Carbon Steel Exposed to Bentonite after Electrochemical Test

Introduction Geological disposal is the most realistic method in high-level radioactive waste (Hereafter HLW) in Japan. To consider long-term stability for geological disposal, several types of material Engineer barrier with host rock is studied. In such studies, the combination of carbon steel and bentonite is proposed one of idea for use. Carbon steels show uniform corrosion in many cases1. Magnetite scale is formed on carbon steel when it is exposed to low-oxygen partial pressure. And the Magnetite scale may serve as corrosion resistant film. Corrosion under film, however, is known even if the film is a paint film2. Therefore we have to investigate the long-term corrosion tendency of carbon steel. For this purpose, the electrochemical test for accelerated corrosion. Experimental A sample composed of SS400 class carbon steel (ISO 630 equivalent) and bentonites (Kinigel V1: 21% pts. Mass% water and 1.6g/cm3 dry density) is prepared. Two carbon steel plates having t=2mm thickness and 60mm diameter are emplacement between both ends of bentonite column. One is used for a working electrode and the other is used for a counter. The sample is surrounded by an acrylic container to limit the oxygen diffusion. This is a model of HLW disposal container into the underground. Regarding oxide scale of the surface carbon steel, it is remained considering to in its natural state for Overpack made of carbon steel before the disposal. For measuring the potential of sample, we used the silver-silver chloride electrode. At first, a polarization test is carried out. Scanned overvoltage is -250mV to 250mV. The scan rate was 1mV/s3. After that, we carried out +100mV electrolysis of the same sample for 13 days. After electrolysis3, we analyzed the steel/ bentonite interface with SEM, EDX and Raman Spectroscopy. Results and Discussion The corrosion potential measured by polarization test was -0.6VvsSSE3. This is the mixed potential of steel and scale. Hara and co-researchers report that the corrosion potential of iron in supernatant of bentonite water solution is -0.4VvsSSE and that of magnetite, main composition of the oxide scale, is -0.8VvsSSE4. Therefore the potential we obtained is the mixed potential of iron and magnetite. After electrolysis test, we obtained that the steel corroded under the oxide scale confirmed3. The interface of dummy canister shows same as the electrolysis test This corrosion phenomenon was also found at Engineer barrier experiment test in Switzerland for study HLW disposal5. Those results indicate that at least a part of the inner steel metal is connected to outside bentonite and corroded at mixed potential of magnetite and steel. This means that the steel metal may be a sacrifice anode. The scale is confirmed to be FeOOH by micro Raman spectroscopy. The scale is trivalent iron. In the other case of a lab experiment, Hara et al. 4, carbon steel (SM400B) with Fe3O4 film into bentonite (Kunigel V1) showed that the corrosion product was a mixture of FeCO3 and Fe2(OH)2CO3. At first stage, Fe3O4 film-coated carbon steel in compressed bentonite is reacting as the self-reduction reaction. Next step, this reaction is a trigger to destroy Fe3O4. As a result, the internal iron is corroded and bivalent iron is created. The difference of corrosion product, trivalent or bivalent iron, is determined by the presence of the internal state oxygen during the test. As the next step, the authors are planning to make a lab test under the conditions of oxygen and total current density controlled. Reference E. McCafferty, Introduction to Corrosion Science, Chapter1,Springer (2010)Y. Akage, H.Saito, ECS transactions, 50(50),pp.7-11(2013)H.Saito, M. Uyama,et.al., 67th ISE Proseeding (now printing)T.Tsushima, N.Hara, K.Sugimoto,Zairyo-to-Kankyo,52,pp545-553(2003) in JapaneseNagra Grimsel Test Site HP (FEBEX-DP), http://www.grimsel.com/gts-phase-vi/febex-dp/febex-dp-introduction

Corrosion Protection of Galvanized Steel Using Smart-Release Hydrotalcite Pigments Containing Benzotriazolate Derivatives

The development of credible, environmentally-friendly replacements for toxic, inhibitive pigments based on sparingly soluble Cr(vi) containing salts remains one of the major challenges in the field of corrosion science. One of the most promising alternative technologies presently under investigation is the use of ion-exchange materials as smart-release pigments incorporated within an organic polymer binder. Anion-exchange layered double hydroxides, such as hydrotalcite (HT, Mg6Al2[OH]16CO3.4H2O) have received significant recent attention and have been previously shown to act as highly effective inhibitors of chloride-induced filiform corrosion on organic coated AA2024 alloy surfaces 1,2. The efficiency is principally attributed to their ability to sequester aggressive chloride ions and moderate underfilm pH, although specific effects due to the nature of the stored inhibitor anion type are also observed. Of the numerous potential inhibitors evaluated within HT pigments, benzotriazolate (BTA) anions were previously shown to be among the most effective in forestalling FFC 3. In this work, BTA-containing HT pigments are investigated as inhibitors of corrosion-driven organic coating delamination from a hot-dip galvanized steel (HDG) surface. A series of model coatings, prepared by dispersing various volume fractions of the BTA-HT pigments in a polyvinyl butyral (PVB) binder, were applied to HDG coupons and an in-situ scanning Kelvin probe technique was employed to evaluate their efficiency in inhibiting corrosion driven coating failure via cathodic disbondment. A marked and progressive decrease in delamination rate with increasing pigment loading, as shown in Figure 1, was obtained. In addition the plots of delamination distance (xdel) with time in this figure also demonstrate that increasing pigment volume fraction also produce a progressively longer time delay to the initiation of underfilm corrosion. Figure 1 also demonstrates a transition from parabolic to linear kinetics, suggesting that the inhibition mechanism may in part be due to an underfilm interaction of BTA- with the underlying zinc, leading to a significant blocking of cathodic oxygen reduction. Identical experiments were carried out using stored BTA derivatives, including 5-methyl benzotriazole and benzotriazole-5-carboxylic acid to assess whether further improvements in performance could be obtained. Pigments based on 5-methyl benzotriazole produced a marginally more profound inhibition of cathodic disbondment when compared with BTA-HT, while in-coating 5-carboxy-benzotriazolate inhibitor acted far less effectively. It will also be demonstrated that BTA acts as a powerful inhibitor when released from the coating into a corrosive electrolyte contacting the bare zinc surface at a penetrative coating defect. A combination of in-situ-scanning vibrating electrode technique (SVET) and potentiodynamic studies, along with mass loss experiments were carried out on galvanized specimens immersed in chloride solutions. A comparison of results obtained in corrosive electrolyte containing various concentration of dissolved BTA confirmed highly efficient inhibition of the exposed substrate at concentrations of greater than 0.01 mol dm-3.

Cover Picture: Materials and Corrosion. 8/2016

Cover:EUROCORR, the annual event of the European Federation of Corrosion, is the premier Corrosion Congress in Europe. EUROCORR2016, organized by CEFRACOR in cooperation with Chimie ParisTech, will take place in Montpellier, France, from 11 to 15 September 2016. It will cover all aspects of corrosion science, technology, and engineering, with emphasis on a main theme: Advances in linking science to engineering.This theme is illustrated on the cover by a new multi‐technique platform designed to study in situ the evolution of material surfaces during their interaction with gaseous or liquid environments. It combines, into a unique machine, in situ electron spectroscopy for chemical analysis, in situ scanning tunneling and atomic force microscopies, electron diffraction, electrochemistry, and surface preparation facilities. The background is a STM image with atomic resolution of an austenitic stainless steel surface.

Aleksandr Georgievich Tyurin (1953–2015) and his research in corrosion science

This paper is a tribute to one of the leading Russian scientists in the field of corrosion science and my supervisor, Aleksandr Georgievich Tyurin, who suddenly and unexpectedly passed away in 2015. A short biographical note is given together with an outline of his contributions to corrosion and electrochemical science.

PQ completes merger with Eco Services

Corrosion of petrochemical equipment is one of the reasons that affect the process safety and sustainability of the petrochemical industry. To understand the research status of corrosion in petrochemical equipment and explore the knowledge base and hot trends, 1887 studies from 2000 to 2020 on the WOS retrieval platforms SCIE, SSCI, A & HCI and CPCI-S were used as data samples. Co-occurrence analysis, keyword mutation analysis, and co-citation analysis were used to analyze the current status of petrochemical equipment corrosion research using the method of informetrics. The number of papers published in China ranks first among countries/regions, and the Universidade de Lisboa ranks first among relevant institutions. Corrosion resistance structure analysis, corrosion loss model analysis and steel structure corrosion analysis are the basis of knowledge in the field of petrochemical equipment corrosion research, and scholars mostly take oil tankers as the object of analysis. Engineering Failure Analysis, Marine Structure, Ocean Engineering, Corrosion Science and Corrosion are the core journals in this field. At present, the basic theory and research systems in the field of petrochemical equipment corrosion have been largely completed, and many research directions and broad frontier branches are currently emerging. “Research on corrosion mechanism and anti-corrosion mechanism”, “microbial corrosion research” and “corrosion detection and monitoring research” are the three core development paths in this field. In the process of evolution, the research path is gradually becoming refined, and research directions are gradually changing to nano anti-corrosion materials. Simulation, biofuels and composites are the new research frontiers of research on corrosion of petrochemical equipment.

Biosourced polyolefins: Sabic opens research and development centre in the Netherlands

Corrosion of petrochemical equipment is one of the reasons that affect the process safety and sustainability of the petrochemical industry. To understand the research status of corrosion in petrochemical equipment and explore the knowledge base and hot trends, 1887 studies from 2000 to 2020 on the WOS retrieval platforms SCIE, SSCI, A & HCI and CPCI-S were used as data samples. Co-occurrence analysis, keyword mutation analysis, and co-citation analysis were used to analyze the current status of petrochemical equipment corrosion research using the method of informetrics. The number of papers published in China ranks first among countries/regions, and the Universidade de Lisboa ranks first among relevant institutions. Corrosion resistance structure analysis, corrosion loss model analysis and steel structure corrosion analysis are the basis of knowledge in the field of petrochemical equipment corrosion research, and scholars mostly take oil tankers as the object of analysis. Engineering Failure Analysis, Marine Structure, Ocean Engineering, Corrosion Science and Corrosion are the core journals in this field. At present, the basic theory and research systems in the field of petrochemical equipment corrosion have been largely completed, and many research directions and broad frontier branches are currently emerging. “Research on corrosion mechanism and anti-corrosion mechanism”, “microbial corrosion research” and “corrosion detection and monitoring research” are the three core development paths in this field. In the process of evolution, the research path is gradually becoming refined, and research directions are gradually changing to nano anti-corrosion materials. Simulation, biofuels and composites are the new research frontiers of research on corrosion of petrochemical equipment.

Cover Picture: Materials and Corrosion. 7/2016

Cover:EUROCORR, the annual event of the European Federation of Corrosion, is the premier Corrosion Congress in Europe. EUROCORR2016, organized by CEFRACOR in cooperation with Chimie ParisTech, will take place in Montpellier, France, from 11 to 15 September 2016. It will cover all aspects of corrosion science, technology, and engineering, with emphasis on a main theme: Advances in linking science to engineering.This theme is illustrated on the cover by a new multi‐technique platform designed to study in situ the evolution of material surfaces during their interaction with gaseous or liquid environments. It combines, into a unique machine, in situ electron spectroscopy for chemical analysis, in situ scanning tunneling and atomic force microscopies, electron diffraction, electrochemistry, and surface preparation facilities. The background is a STM image with atomic resolution of an austenitic stainless steel surface.

Nuclear Corrosion: Achievements and Challenges

Corrosion science faces new challenges in various nuclear environments. Three main areas may be identified where increases of knowledge and understanding have been done and are still needed to face the technical needs: (i) the extension of the service time of nuclear power plants from 40 years, as initially planned, to 60 years and probably more as expected now, (ii) the prediction of long term behaviour of metallic materials in nuclear waste disposal where the corrosion processes have to be predicted over large periods of time, some thousands years and more, (iii) the choice of materials for use at very high temperatures as expected in Generation IV power plants in environments like gas (helium), supercritical water, liquid metals or salts. Service time extension, deep geological waste repositories and high temperature reactors sustain researches and developments to model corrosion phenomena at various scales, from atoms to components.

Electrochemical Study of Corrosion in High Temperature Molten Salts

Concentrating solar power systems are projected to need to produce energy with a cost of $0.06/kWh when implemented on a very large scale. It is generally believed that temperatures of the heat transfer fluids will need to be higher than 800oC to drive an advanced Brayton cycle or Rankine cycle with superheated steam to reach this target. However, current state-of knowledge of corrosion science indicates a lack of published data on the impact of molten salts at temperatures above 800°C on corrosion of molten salt contacting materials such as steels or high temperature alloys and ceramics. This work investigates the corrosion mechanism of selected alloys and ceramics in chloride and fluoride molten salts used for high operating temperature heat transfer fluids using a combination of electrochemical techniques, theoretical calculations, and thermodynamic predictions. The electrochemical characterizations included linear polarization resistance scans, electrochemical impedance spectroscopy, and cyclic potentiodynamic polarization. Results show that the Fe-Ni-Cr alloys tested have high general corrosion and require corrosion inhibition. Advanced materials, such as SiC-SiC and TZM, have been shown to have much lower corrosion and good passivation behavior. Experimental tests using coupon immersion in the molten salts support the mechanism determined from electrochemical testing that the main element being oxidized is Cr. Analysis of samples also indicates selective Cr oxidation concentrates along grain boundaries and creates porous networks in the sample. The electrochemical impedance tests suggest that the corrosion is diffusion controlled and this assumption correlates well with corrosion modeling studies. The electrochemical testing data combined with thermodynamic modeling led to the development of a reaction mechanism for high temperature corrosion in molten salts.

Detection of corrosion degradation using electrochemical noise (EN): review of signal processing methods for identifying corrosion forms

Electrochemical noise (EN), as one of the most promising in situ electrochemical methods in corrosion and electrochemical science, has been developing rapidly in recent years with the advancements in instrumentation and signal processing methods. One advantage of EN is its application in long-term or early stage corrosion process monitoring because it instantly detects corrosion rate and corrosion forms. Investigators have applied various mathematical methods to extract characteristic parameters from EN. In this paper, identifying corrosion forms using parameters obtained from time domain, frequency domain and time–frequency domain is reviewed, and the correlation between parameters and corrosion forms is discussed. Finally, other in situ techniques are recommended to be employed synchronously with EN measurement in order to obtain reliable analyses.

A review on conducting polymer coatings for corrosion protection

Corrosion of metals is one of the most important problems in the manufacturing industries. Many corrosion control methods use coatings of conducting polymers and conversion layers that contain toxic and environmentally hazardous materials, especially chromium compounds. These objectives have led to the development of new protective coating strategies that employs nanocomposites and carbon-based materials. In recent years, conducting polymers have attracted much attention because of their wide range of industrial applications and economic viability. Polymers possess long-chain carbon linkages and therefore, upon adsorption are able to block large areas of the corroding metal surfaces. The thin films adsorbed on the metal substrate provide a barrier effect between the metal and its environment. This review article summarizes the different techniques used in corrosion protection of metals, conducting polymers and nanomaterials, nanocomposites, and carbon-based materials in corrosion science.

No Scientific Debate in the Zero‐Valent Iron Literature

Since the 1930s, the modern science of aqueous iron corrosion has been established by scientists like J. O'M. Bockris, M. Cohen, U. R. Evans, Y. M. Kolotrykin, G. Okamoto, M. Pourbaix, H. H. Uhlig, or K. J. Vetter. Yet, the electrochemical nature of aqueous iron corrosion is seriously questioned by flawed science. Especially, metallic iron (Fe0) was mistakenly introduced in the 1990s as an environmental reducing agent. Fe0 was then successfully used in subsurface permeable reactive barriers to treat contaminated groundwater. This commentary recalls that observed contaminant degradation/reduction is not the cathodic reaction simultaneous to iron oxidative dissolution. It is concluded that the Fe0 remediation community is working against creativity and innovation.

ECS Classics: Making the Phone System More Reliable: Battery Research at Bell Labs

Underlying the evolution of the telecommunications network and the communications system of AT&T, research played a key role by bringing together the evolving technical needs and opportunities with experimental science and applications. Research in electrochemistry is an example of the work done at Bell Laboratories that was informed by real work problems while contributing to our basic knowledge of materials and processing. As Albert M. Clogston, Executive Director-Research, Physics, and Academic Affairs Division, Bell Laboratories, November, 1981 wrote in the volume of A History of Engineering and Science in the Bell System, “the mission of Bell Labs [was] to supply the technology the Bell System need[ed] to do its job in the long term as well as in the short term.”1 Some context is needed as the telecommunications landscape has changed so dramatically from the system that developed following the invention of the telephone. Soon after the invention of a working telephone by Alexander Graham Bell, a large number of companies formed to offer phone services across the U.S. The Census of 1880 showed 148 telephone companies. Alexander Graham Bell and two investors, Gardiner Hubbard and Thomas Sanders, formed the Bell Telephone Company, which in few years became the American Bell Telephone Company, which also acquired interest in a manufacturing unit, the Western Electric Company. The enterprise became collectively known as the Bell System. The American Telephone and Telegraph Company (AT&T) was incorporated on March 3, 1885 as a wholly owned subsidiary of American Bell. AT&T acquired a controlling interest in Western Electric, a key supplier of telephone equipment, in 1882. Eventually, many of the companies providing telephone service in the U.S. were acquired into the AT&T network, which came to be operated as a regulated monopoly. As AT&T built its telecommunications network, essentially a large analog telephony system in the first decades, brand new technologies were invented that we take for granted and are widely commercially available today. As large segments of communications network were deployed, attention turned increasingly more to exploration of promising fundamental areas of science. In 1925 4,000 scientists and engineers were dedicated fully to research in the newly created Bell Telephone Laboratories. Among the technologies that came out of Bell Labs research (and, of course, research at other industrial, governmental, and academic research organizations) were electroplating and related coating technologies, thin-film material, fundamental corrosion science, basic battery research, and an array of related fundamental research. Electrochemical processes were widely used in the manufacture of much of the equipment used in the Bell System. Indeed, much of the technology being utilized in the Bell System was based on the physics and chemistry of materials. Additionally, electrochemical processes, especially corrosion and tarnishing, were important degradation mechanisms for the equipment making up the system. Further, backup energy systems based on battery technology provided the reliability that the Bell System advertised to its customers and its regulators. Research on Batteries That Provided Reliability to the Bell System

長期予測の視点から今後の腐食防食技術への期待

長期予測の視点から今後の腐食防食技術への期待

Long-Term Corrosion Processes of Iron and Steel Shipwrecks in the Marine Environment: A Review of Current Knowledge

Methodologies for examining the corrosion behavior of iron and steel shipwrecks have steadily progressed since the 1970s, but the analytical techniques utilized since then are comparatively site-specific, and the overall quantity of data available for independent review is seemingly limited. Laudable advancements in the fields of maritime archaeology, oceanography, and corrosion science support the determination that microbiologically-influenced corrosion primarily controls the degradation rates of iron and steel shipwrecks over archaeological timescales. Future in situ analyses performed on these shipwreck sites need to consider the overreaching impacts that microbiological metabolism have on long-term corrosion rates. The corrosion behavior of an iron or steel archaeological shipwreck site should also not be readily applied to similar sites or to other wrecked vessels that are in close proximity.

Predicting Geologic Corrosion with Electrodes

Ever since humans discovered how to separate metal from its ore mineral, preserving its metallic luster has been a driving force in the advancement of materials science. In modern times, developing materials that will contain and isolate nuclear waste has pushed corrosion science to new limits. We must now predict corrosion rates over geologic time scales, upwards of a million years. This article reviews the electrochemical basics that underpin metal and mineral corrosion and uses that to understand the case study of copper corrosion in nuclear-waste containers. Electrochemistry can also explain electron-transfer processes on mineral surfaces and so offer insight into weathering and environmentally relevant natural redox processes, such as those forming supergene metal deposits.

Oxidation kinetics of oxide film on bubble surface of aluminum foams produced by gas injection foaming process

In the range of 620–710 °C, air was blown into A356 aluminum alloy melt to produce aluminum foams. In order to study the influence of temperature on the thickness of oxide film on bubble surface, Auger electron spectroscopy (AES) was used. Based on the knowledge of corrosion science and hydrodynamics, two oxidation kinetics models of oxide film on bubble surface were established. The thicknesses of oxide films produced at different temperatures were predicted through those two models. Furthermore, the theoretical values were compared with the experimental values. The results indicate that in the range of 620–710 °C, the theoretical values of the thickness of oxide film predicted by the model including the rising process are higher than the experimental values. While, the theoretical values predicted by the model without the rising process are in good agreement with the experimental values, which shows this model objectively describes the oxidation process of oxide film on bubble surface. This work suggests that the oxidation kinetics of oxide film on bubble surface of aluminum foams produced by gas injection foaming process follows the Arrhenius equation.

Corrosion of Galvanically Coupled Magnesium

New environmental regulations, good mechanical properties, abundant availability and reduced cost have generated renewed interest in the wider use of magnesium (Mg), especially in the automobile and aerospace industries [1]. The main obstacle to the use of Mg is its susceptibility to corrosion. Fortunately, its corrosion rate is limited to some degree by the relatively slow rate of hydrogen evolution on the surface of magnesium. However, the rate of corrosion increases significantly when Mg is galvanically coupled to another metal, such as steel, which coupling leads to increased rates of hydrogen evolution [2]. Although galvanic coupling of Mg results in increased corrosion, self-corrosion does not stop entirely and varies throughout the corrosion process. In this study, we examine the relative roles of self-corrosion and galvanic corrosion for magnesium that is galvanically coupled to carbon steel. As described previously [3] samples tested consisted of the exposed end of an Mg rod (99.95%, GalliumSource), 3mm in diameter, that was insulated and surrounded by a mild steel electrode. The steel electrode was a cylinder with an inside diameter just large enough to accommodate the insulated Mg rod and an outer diameter of either 8, 12 or 16mm. This structure (Mg surrounded by steel) was cast into epoxy (EpoThinTm2 Epoxy System, Buehler) for testing. After casting, samples were cross-sectioned with a diamond saw, and each surface was polished prior to submersion in the 5 wt% NaCl electrolyte. The concentric Mg and steel electrodes were galvanically coupled by electrically connecting them external to the solution through a zero resistance ammeter, which also permitted direct measurement of the galvanic current. Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) was used to directly measure the amount of Mg dissolved during the experiment. The difference between the total corrosion, as measured by magnesium dissolution, and the total amount of galvanic corrosion (from the integral of the galvanic current) provided the amount of self-corrosion. We have already seen that the fraction of self-corrosion at the end of 10 minutes was roughly one-third of total corrosion [3-4]. However, fraction of self-corrosion does not remain constant over the entire experiment. As seen in Fig. 1, the fraction of self-corrosion is initially low for galvanically coupled Mg, but subsequently increases to a peak value, after which the rate decreases once again. The galvanic portion of the corrosion steadily decreases with time, possibly due to increased solution pH [3-4]; however, the self-corrosion behaves quite differently. This is also reflected in the surface morphology where galvanically coupled samples experience disc-shaped corrosion while self-corrosion of uncoupled samples is filiform in nature (see Fig. 2). Previous attempts to model Mg corrosion have noted a discrepancy between the measured and predicted corrosion rates [5-6]. One possible reason for this might be the influence of the changing rate of self-corrosion, which represents a significant fraction of the corrosion rate for galvanically coupled systems. In this study, we combine experiments and numerical simulations to investigate the roles of self-corrosion and galvanic corrosion for Mg in NaCl solutions. In particular, the influence of the active surface area and changing hydrogen reaction rates are explored. Acknowledgments The authors wish to thank CD-adapco for their generous funding of this work. References Kulekci, Mustafa. The Int. J. of Advanced Manufacturing Tech. 2008: 851-65.Williams, G., N. Birbilis, and HN McMurray. Electrochemistry Communications 2013: 1-5.Banjade D., Harb J. Electrochemical Society Spring Meeting 2015-. Banjade D., Harb J. “The role of self-corrosion during the dissolution of galvanically coupled magnesium-To be submitted manuscript”Jia, J. Materials and corrosion 2005: 56-7 2005. Deshpande, K. Corrosion Science 2010: 3514-22. Figure 1

Switching from Homogeneous to Localized Corrosion in Stainless-Type Amorphous and Nano-Crystalline Steels

Amorphous stainless-type steels with high Cr content provide next to some applications as corrosion protective coatings an interesting playground for fundamental studies of electrochemical corrosion behavior (1-4). Starting from an ultimately homogeneous glassy version, partially and fully nanocrystalline states with large elemental heterogeneity depending on the crystallization can be derived by simple heat treatments (1,2). The amorphous materials Fe65-xCrxMo14C15B6show relative low passive currents which significantly increase after partial crystallization owing in part to a related nano-scaled roughness of the passive films (Fig.1). For low-Cr contents we have as well observed a clear switch from the typical homogeneous corrosion of these alloys to a clearly localized attack. We will discuss this phenomenon in the light of atom probe tomography (Fig.2), microscopy, and electrochemical measurements. 1. M. J. Duarte, A. Kostka, J. A. Jimenez, P. Choi, J. Klemm, D. Crespo, D. Raabe, F. U. Renner, Crystallization, phase evolution, and corrosion of Fe-based metallic glasses: an atomic-scale structural and chemical characterization study, 2014, Acta Materialia 71 (2014), 20-30. 2. M.J. Duarte, J. Klemm, S. O. Klemm, S. Borodin, K. Mayrhofer, M. Stratmann, A.H. Romero, D. Crespo, J. Serrano, P. Choi, D. Raabe, F. U. Renner, Element-Resolved Corrosion Analysis of Stainless-Type Glass-Forming Steels, Science 341 (2013), 372-376. 3. M. Madinehei, P. Bruna, M.J. Duarte, S. J. A. Figueroa, E. J. Lede, J. Lengsfeld, F. U. Renner, E.Pineda, Glass-formation and corrosion properties of Fe-Cr-Mo-C-B glassy ribbons with low Cr content, J. Alloy. Comp. 615 (2014), 128 - 131. 4. J. Klemm, M.J. Duarte, S. O. Klemm, P.Choi, K.J.J. Mayrhofer, F.U. Renner, Multi-element-resolved electrochemical corrosion analysis. I. Dissolution behavior and passivity of amorphous Fe50Cr15Mo14C15B6, Corrosion Science 89 (2014), 59-68. Figure 1