Atmospheric Corrosion Mechanism Research Articles

(Invited) Atmospheric Corrosion Studies for Corrosivity Categorization in Alaska

In colder climates, factors like substantial snowfall, frequent freeze-thaw cycles, the use of de-icing salts, and increasing global temperatures add complexities to the established paradigms governing atmospheric corrosion. To elucidate the mechanisms of atmospheric corrosion under colder temperatures, four distinct test sites were established in the state of Alaska, USA. Each site was equipped with a modern, multi-angle exposure test rack coupled with an auxiliary weather station. Within these setups, five commonly used metal alloys were exposed at 0-, 30-, and 45 degrees from the horizontal over 6- and 12-month periods. Weather correlation analysis, mass loss analysis, SEM/EDXA, and optical profilometry studies were conducted following the exposure of the metal specimens to determine corrosion rates (CR) and damage. The measured CR, exposure angle, and accompanying meteorological data—specifically, temperature, relative humidity, time of wetness, precipitation, chlorides, and sulfates—were benchmarked against ISO standards to classify and categorize the corrosivity of the four test sites in Alaska. The PSCA test site (Kodiak, AK) was the most aggressive environment regarding all weather parameters. UAF (Fairbanks, AK) was the least aggressive, and test sites PAA (Port of Anchorage, AK) and UAA (University of Anchorage, AK) showed mild to moderate aggressiveness. Samples exposed at 0° consistently showed higher corrosion rates than those exposed at 30° and 45° across all instances. At the more aggressive PSCA site, a distinct correlation was observed between the exposure angle and CR with samples exposed at 0° showing the highest CR, followed by 30°, then 45°. Optical profilometry studies revealed that the comparison of surface roughness between the exposed samples followed an extremely similar pattern to that of CR. Corrosivity categorization based on CR from experimental mass loss data varied from the estimation of CR through existing dose-response functions and weather parameters. Newer standardized models involving more weather parameters are required to better estimate and predict corrosivity in colder climates. Alaska's growing role as a critical military, cargo, shipping, and refueling hub enhances its importance as an ideal natural laboratory for studying atmospheric corrosion in cold regions, as demonstrated by the developed test sites discussed in this paper.

Outline Design of an Atmospheric Corrosion Data Monitoring System

Atmospheric corrosion monitoring and evaluation are important means for studying the mechanism and behavior of atmospheric corrosion of metals. In view of the long cycle, cumbersome process, and poor data quality of the traditional outdoor hanging method, an innovative atmospheric corrosion data monitoring system has been designed to detect atmospheric environment data and metal corrosion data on site, and display the detection data of the equipment’s service location to users in real-time in the form of a “dynamic corrosion map”. This can provide a differentiated design for the anti-corrosion of important equipment such as power grid transmission and transformation engineering components Provide effective data support for engineering life prediction and operation and maintenance. At present, the system has been piloted and promoted in some network provinces, achieving good social and economic benefits.

A Proposed R-T Model for Atmospheric Corrosion of Metallized Film in AC Capacitors

Atmospheric corrosion is a significant issue affecting the storage quality of metallized film capacitors (MFCs). Oxygen and moisture in the atmosphere permeate among the layers in capacitor, initiating oxidation on the surface of metal layer, which increases the electrode resistance. In this work, a resistance–time model is proposed to study the atmospheric corrosion mechanism of nano-scaled Al–Zn metal layer. The analysis results point out that it is the barrier of metal–oxide interface that affects the anticorrosion ability. Moreover, the metal composition is studied as a factor influencing the anticorrosion ability, revealing that the enhancement gained by adjusting the composition tends to saturate once the Al content exceeds 10 wt.%. Therefore, in terms of MFCs for ac applications, the Al content in electrode should be controlled within 10 wt.% as a compromise between the atmospheric and electrochemical corrosion.

Effects of Actual Marine Atmospheric Pre-Corrosion and Pre-Fatigue on the Fatigue Property of 7085 Aluminum Alloy

Effects of actual marine atmospheric precorrosion and prefatigue on the fatigue property of 7085-T7452 aluminum alloy were investigated by using the methods of marine atmospheric outdoor exposure tests and constant amplitude axial fatigue tests. Marine atmospheric corrosion morphologies, fatigue life, and fatigue fractography were analyzed. After three months of outdoor exposure, both pitting corrosion and intergranular corrosion (IGC) occurred, while the latter was the dominant marine atmospheric corrosion mode. Marine atmospheric precorrosion could result in a dramatical decrease in the fatigue life of the as-received 7085-T7452 aluminum alloy, while selective prefatigue can improve the total fatigue life of the precorroded specimen. The mechanism of the actual marine atmospheric corrosion and its effects on the fatigue life of the 7085-T7452 aluminum alloy were also discussed.

Data Mining to Atmospheric Corrosion Process Based on Evidence Fusion

An electrical resistance sensor-based atmospheric corrosion monitor was employed to study the carbon steel corrosion in outdoor atmospheric environments by recording dynamic corrosion data in real-time. Data mining of collected data contributes to uncovering the underlying mechanism of atmospheric corrosion. In this study, it was found that most statistical correlation coefficients do not adapt to outdoor coupled corrosion data. In order to deal with online coupled data, a new machine learning model is proposed from the viewpoint of information fusion. It aims to quantify the contribution of different environmental factors to atmospheric corrosion in different exposure periods. Compared to the commonly used machine learning models of artificial neural networks and support vector machines in the corrosion research field, the experimental results demonstrated the efficiency and superiority of the proposed model on online corrosion data in terms of measuring the importance of atmospheric factors and corrosion prediction accuracy.

Measuring Aerosol Chlorides for Atmospheric Corrosion Studies in Cold Alaskan Climate

Atmospheric corrosion is a complex process, which involves chemical, electrochemical, and physical changes to the metal exposed. Atmospheric corrosion occurs when a metal surface is under a thin layer of moisture, but not completely immersed, and the metal surface corrodes while exposed to environmental factors. Atmospheric corrosion of metal alloys in cold environments is assumed to be negligible and limited corrosion data are available in cold climates. However, studies in the Arctic and Antarctic regions have shown significant corrosion damages when exposed to cold conditions. The rate of atmospheric corrosion on various alloys is affected by environmental parameters that include temperature, rainfall, humidity, chloride-ion deposition rate, and time of wetness (TOW). Two important factors that affect atmospheric corrosion rates are aerosol chlorides and TOW. The main goal of this research is to monitor and measure the aerosols chlorides in the atmosphere, measure TOW and correlate the degradation of carbon steel alloys that are widely used in land, sea, and aerospace transportation, the oil and gas, fisheries, and mining applications. The main objectives achieved are design of Combined Chloride and Corrosion Portable Racks (C3PR) and deployment of C3PR racks in four different Alaskan environments. Chloride deposition rate measured using ASTM standards for three different exposure periods along with TOW, ambient temperature and RH data are correlated to the atmospheric corrosion rate of carbon steel to understand the underlying atmospheric corrosion mechanisms. The results from this research work will give a better understanding on the atmospheric corrosion studies in cold arctic/sub-arctic environment and lead to more externally funded projects.

Atmospheric Corrosion Protection Performance and Mechanism of Superhydrophobic Surface Based on Coalescence-Induced Droplet Self-Jumping Behavior.

The coalescence-induced droplet self-jumping behavior on the superhydrophobic surface (SHS) provides a new way to achieve atmospheric corrosion protection. This work controls the droplet self-jumping behavior by regulating the SHS's surface energy and analyzes the relevant mechanism from the energy perspective, revealing the key pathway by which the surface energy impacts the droplet self-jumping behavior. On this basis, the electrochemical impedance spectroscopy testing technique is used to evaluate the effect of the droplet self-jumping behavior on the SHS corrosion protection performance, and the SHS atmospheric corrosion protection mechanism based on the coalescence-induced droplet self-jumping behavior is revealed. This study provides theoretical guidance for the development of SHS-based anticorrosion protection.

Research on the electrical contact degradation model of plug-in connectors under corrosion and wear

Failures due to corrosion are common for connectors operating under atmospheric environment. Results of previous studies lacked universal applicability and neglected the degradation process of contact resistance. Also, wear is rarely considered in studies on corrosion degradation, which is an inevitable mechanical process for plug connectors. Considering these problems, the atmospheric corrosion process and copper dynamics were analyzed. The consistency of the atmospheric corrosion mechanism was used to study the local corrosion degradation law and its influencing factors. The wear mechanism on corrosion degradation was determined through the analysis of the influencing factors. The corrosion model of the gold-plated parts under atmospheric wear was established. To study the degradation process of electrical contacts, a degradation model of contact resistance based on the multi-spot contact mechanism was established combined with the previous corrosion degradation model. Experimentally, the corrosion spot density increases as a function of time and varies with plated thickness, whereas the corrosion spot size distribution is still relatively independent of time. The skew phenomenon appears in the cumulative distribution probability of contact resistance as exposure time increases. Whereas the degradation of electrical contact resistance increases as a function of time, the median remains relatively unchanged. A brief analysis of the contact reliability under wear and corrosive environments was also carried out.

Long-term corrosion behavior of the 7A85 aluminum alloy in an industrial-marine atmospheric environment

The atmospheric corrosion behavior and mechanism of the 7A85 aluminum alloy exposed to the Qingdao industrial-marine atmospheric environment for five years was investigated using the weight-loss method, mechanical property analysis, morphology observations, scanning Kelvin probe force microscopy, and corrosion product analysis. The results showed that the mechanical properties of the exposed 7A85 Al alloy deteriorated considerably, which was mainly induced by pitting corrosion and intergranular corrosion. Al2CuMg and Al7Cu2Fe intermetallic particles were the main precipitates in the 7A85 Al alloy, and the particles not only induced a defective film but also acted as cathode phases in the precipitate–matrix galvanic corrosion couple, which led to pitting corrosion initiation during atmospheric corrosion in Qingdao. Moreover, intergranular corrosion initiated and then propagated owing to micro-galvanic corrosion along the grain boundaries.

(Invited) Atmospheric Corrosion Monitoring in Cold Arctic Climate Using Modular Corrosion Test Racks

Atmospheric corrosion is a complex process, which involves chemical, electrochemical, and physical changes to the metal exposed. Atmospheric corrosion occurs when a metal surface is under a thin layer of moisture, but not completely immersed, and the metal surface corrodes while exposed to environmental factors. The arctic and sub-arctic region identified by the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) has an average temperature of -18oC or less during winter. It is commonly assumed that there is very little to no corrosion in cold environments. However, previous studies in the Antarctic and Arctic regions have shown significant corrosion damage when exposed to cold conditions. Studies in the sub-arctic region of Canada, Norway, and Russia show extensive atmospheric corrosion rates (when compared to Antarctica) due to human developments and the resulting increase in mining and metallurgical industries. Experimental and theoretical work has shown that the electrochemical process proceeds at temperatures as low as -25oC to -20oC . Moreover, very little corrosion data are available for metal alloys exposed to cold arctic and sub-arctic conditions. The two important factors that affect atmospheric corrosion rates are aerosol chlorides (or salt-laden snow from the marine environment or deicing salts on roads) and time of wetness (TOW) along with other climatic parameters such as rainfall, temperature, humidity. Factors that drive the atmospheric corrosion in cold climates are winds that can bring in salt-laden snow from the marine environment and the use of de-icing salts can also contribute to high levels of chlorides. The eutectic point or the freezing point of de-icing salts can be lowered to -50oC, melting the ice/snow layer on top of metal samples. This phenomenon keeps metal samples moist for much longer periods, thus increasing the TOW. In the presence of chlorides and moisture, extensive atmospheric corrosion damages can be observed on metal samples. Another contributing factor to high corrosion rates is low rainfall, which in turn cannot periodically wash off the deposited chlorides and SO2 on top of the samples. In addition, ever-increasing ambient temperatures due to climate change in recent years affect the snow presence on top of the metal samples. The temperature of the samples is not too high to evaporate the snow/ice deposited, but high enough to cause melt and sustain moisture for longer periods of time. This leads to the formation of varying thickness of wet ice/snow layers on the metal surface. Long hours of sunlight in the summer also increase the surface temperature of metal samples beyond the ambient temperatures, causing dew formation and condensation, which in turn results in higher TOW. The atmospheric corrosion damage in cold environments is close to the main human activity, which is concentrated near the coastal areas. The substantial human growth and climate change in the arctic and sub-arctic region pushes for a renewed better understanding of the atmospheric corrosion mechanisms that can lead to good choice of materials selection and better design practices for infrastructure and other applications. The combination of urbanization and proximity to marine environments make arctic and sub-arctic regions in North America, particularly Alaska, an important natural laboratory to study atmospheric corrosion in cold regions. Design and establishment of modular corrosion test racks equipped with weather stations will be discussed. The angle of exposure can be changed to 0, 30 and 45-degrees to the horizontal (see the attached figure). This will allow us to test the metals alloys exposed to different angles for the same exposure period and study the effect of snow/ice retention on top of the metal surface. Figure 1

(Invited) Atmospheric Corrosion of Magnesium Alloys: Influence of Aluminium Content

Magnesium has gained considerable interest as a structural material for automotive and aerospace applications due to its low density and high specific strength. The use of magnesium alloys in engineering applications is, however, mainly limited by their unsatisfactory surface properties and their poor corrosion resistance. In recent years, significant improvements have been made in achieving a better corrosion resistance particularly by reducing the contents of impurities such as Fe, Cu, and Ni. Intermetallic phases, a result of the casting process, play an important role in the corrosion process. The role of these intermetallic phases has been addressed in a number of papers [1–3]. The role of aluminum in the atmospheric corrosion of magnesium alloys has less addressed in the literature [4]. Esmaily et al have shown recently that the rate of corrosion of magnesium alloys decreased when increasing the Al content. However these experiments were performed on commercial magnesium alloys produced high pressure die casting under constant RH exposure at 90% RH.In our work the corrosion behavior of binary Mg-Al alloys as well as that of commercial magnesium alloys with different aluminum contents have studied both under cyclic corrosion exposure in the laboratory and in field exposures in France and in Vietnam. Analyses of corrosion products have also been conducted. Additional in-situ measurements have been performed in order to better understand the atmospheric corrosion mechanisms of Mg alloys.An important decrease in the maximum pit depth can be observed after different laboratory exposures when increasing the aluminum content in the material. Similar results have been obtained after 9 months exposure in Field. Hence, from these results it is obvious that an increase of the aluminum concentration in Mg-Al alloys decreased the corrosion rate of the alloy. This lead in lower mass loss and less deep corrosion attacks in the magnesium grains. The analyses of corrosion products formed after the cyclic corrosion tests and field exposures revealed the presence of Magnesium carbonate and MgCO3(OH)4,4H20, respectively. In both cases magnesium hydroxide (Mg(OH)2 was also found in the corrosion products. The results will be discussed in relation with the microstructure of the cast materials. O. Lunder, J.E. Lein, S.M. Hesjevik, T.K. Aune, K. Nisancioglu, Werkstoffe und Korrosion 45 (6) (1994) 331–340. O. Lunder, J.H. Nordlien, K. Nisangliou, Corrosion Reviews 15 (3–4) (1997) 439–469.G. Song, A. Atrens, M. Dargusch, Corrosion Science 41 (1999) 249–273.M. Esmailly, D.B. Blucher, J.E. Svensson, M. Halvarson and LG Johansson, Scripta Materiala, 115 (2016), 91-96

Analysis of Environmental Factors Affecting the Atmospheric Corrosion Rate of Low-Alloy Steel Using Random Forest-Based Models.

As one of the factors (e.g., material properties, surface quality, etc.) influencing the corrosion processes, researchers have always been exploring the role of environmental factors to understand the mechanism of atmospheric corrosion. This study proposes a random forest algorithm-based modeling method that successfully maps both the steel’s chemical composition and environmental factors to the corrosion rate of low-alloy steel under the corresponding environmental conditions. Using the random forest models based on the corrosion data of three different atmospheric environments, the environmental factors were proved to have different importance sequence in determining the environmental corrosivity of open and sheltered exposure test conditions. For each exposure test site, the importance of environmental features to the corrosion rate is also ranked and analyzed. Additionally, the feasibility of the random forest model to predict the corrosion rate of steel samples in the new environment is also demonstrated. The volume and representativeness of the corrosion data in the training data are considered to be the critical factors in determining its prediction performance. The above results prove that machine learning provides a useful tool for the analysis of atmospheric corrosion mechanisms and the evaluation of corrosion resistance.

Galvanic corrosion of the anodized 7050 aluminum alloy coupled with the low hydrogen embrittlement Cd[sbnd]Ti plated 300M steel in an industrial-marine atmospheric environment

The industrial-marine atmospheric corrosion behavior and mechanism of anodized 7050 Al alloy coupled with low hydrogen embrittlement CdTi (LHE CdTi) plated 300M steel were investigated by means of morphology observations, corrosion product analysis and mechanical property testing. The results showed that the CdTi coating reduced the potential difference between the cathode and the anode, and greatly weakened the galvanic corrosion. Therefore, the self-corrosion of anodized 7050 Al and LHE CdTi plated 300M steel dominated the corrosion process. The pitting and intergranular corrosion (IGC) of the anodized Al were severe, and significantly reduced the strength and elongation. The corrosion products composed of AlOOH, Al(OH)3 and Al2(SO4)3 distributed non-uniformly on the surface. The high concentration of Cl− in the industrial-marine atmosphere destructed the anodized films and induced the pitting corrosion of the Al alloy. Due to the pitting and the micro-galvanic corrosion at the grain boundary, the IGC initiated and propagated. In addition, the sulfide dissolved into the thin electrolyte film and aggravated the pitting and IGC. For the LHE CdTi plated steel, the sacrificial CdTi coating was preferentially corroded, and the further corrosion was inhibited due to the corroded surface composed of cadmium hydroxides, chlorides and sulfates.

New Insights into Synthetic Copper Greens: The Search for Specific Signatures by Raman and Infrared Spectroscopy for Their Characterization in Medieval Artworks

A systematic investigation of medieval copper green pigments was carried out based on written sources: 21 manuscripts, dating from 50–70 to 1755 AD, were sourced and 77 recipes were selected, translating into 44 experiments. Reconstructions from medieval recipes were prepared and characterized through a multianalytical approach to disclose the original pigment formulation that is often described as verdigris. Based on the results obtained, we propose three main groups of copper green pigments, group 1, in which only Cu(CH3COO)2·H2O is formed; group 2, where this acetate is found together with copper oxalates; group 3, in which atacamite is present as the major green component or as a signature compound. The products formed are in perfect agreement with that predicted by the state-of-the-art research on the mechanisms of atmospheric corrosion of copper. This knowledge, together with our experience on craft recipes to prepare medieval paint materials, allowed us to recover a lost medieval recipe to produce a copper green pigment based mainly on atacamite, a basic copper chloride, which has been recently detected, by Raman and infrared spectroscopy, in artworks ranging from Catalonia and the Crown of Aragon panel painting to Islamic manuscripts.

Physicochemical approaches to gold and silver work, an overview: Searching for technologies, tracing routes, attempting to preserve

AbstractGold alloys and silver alloys have always been widely employed in the production of significant objects. With high reflectivity, precious metals are perceived as both materials and colours, and can be skilfully combined to produce metallic polychrome effects. Because their structure and composition contain information on their manufacture, use, disclaim and degradation, items in gold and in silver enclose major information on the technologies employed by past societies and on exchange networks. This information can be acquired using appropriate analytical protocols, established according to the nature of the query and the characteristics of the objects.By using physicochemical techniques, it is possible to identify the technologies, materials and tools used by the artisan and, in particular cases, to situate the sources of raw materials and the workshops producing the objects, as well as to follow the trade routes. The aim of this work is to outline major achievements in the study of goldwork and silverwork based on the different physicochemical methods that are available, and to refer the analytical difficulties that have to be faced when studying objects made from precious metals. Based on several examples, three topics are addressed. The first concerns the major role of the techniques of exam when describing shaping, decorating, assembling and finishing; the second considers the search for metallic polychrome effects in some cultural areas; and the third discusses the challenging question of fingerprinting. A fourth section is dedicated to a short reflection on the difficulties related to the identification of the atmospheric corrosion mechanisms of precious metals.

Corrosion Driven Drying Phenomena Occurring during the Atmospheric Corrosion of Magnesium Containing Galvanized Steel

The corrosion behaviour of various Zn-Mg-Al hot-dip galvanised coatings on steel substrates were studied using a combination of time-lapse photography (TLP), the scanning Kelvin probe (SKP) and non-contact height profiling. In a standard experiment 1uL electrolytic droplets comprising varying concentrations of aqueous NaCl were deposited onto the surface of the Zn-Mg-Al alloy coating held in a humid air atmosphere maintained in isopiestic equilibrium (equilibrium with respect to the vapour pressure of water) with a large remote reservoir of aqueous electrolyte which had the same composition as the original droplet. The progress of atmospheric corrosion was then followed using TLP with photographs taken at 2-minute intervals for a period of up to 60 hours. When required, in-situ SKP was used to produce area maps of electrochemical potential if in the vicinity of the electrolytic droplets over a similar period of time. The SKP probe could also be used as a non-contact height profilometer which allowed the topography of the electrolytic droplet and the surrounding Zn-Mg-Al alloy surface to be mapped. This made it possible to study time-dependent changes in droplet volume and shape as atmospheric corrosion proceeds. In most cases the phenomenon known as cathodic secondary spreading was observed at the droplet periphery, where a thin film (microns) of electrolyte develops and advances away from the droplet as time proceeds. The presence of this thin electrolyte film changes the reflectivity of the Zn-Mg-Al alloy surface with respect to visible light and also produces a marked reduction (drop) in local electrochemical potential. The kinetics of secondary spreading could therefore be followed using a combination of TLP and in-situ SKP. Results are presented which show the dependence of spreading rate on: time, original NaCl concentration and Zn-Mg-Al alloy composition. The role of aluminium and magnesium in modifying (reducing) rates of secondary spreading is discussed in relation to their relative electrocatalytic activities for cathodic oxygen reduction and their distribution within the microstructure of the various alloy coatings. It is shown, using in-situ profilometry, that at lower initial concentrations of NaCl (and consequently higher relative humidity under isopiestic conditions) the original electrolyte droplets dry out within approximately 2-3 hours of being deposited on the alloy surface. The mechanism of this drying phenomenon is discussed in relation to the relevant mechanisms of atmospheric corrosion. It is proposed that as corrosion proceeds all the Na+ cations originally present in the droplet migrate out into the region of secondary cathodic spreading. Conversely, Cl- anions migrating towards the anodic sites at the droplet centre become sequestered as insoluble (or poorly soluble) zinc corrosion products such as Simonkolleite, Zn(OH)Cl. (The presence of Simonkollite is confirmed using grazing angle Z-ray diffraction.) As these corrosion products precipitate the activity of water in the droplet (and its associated vapour pressure) increase and water evaporates from the droplet as it tries to maintain isopiestic equilibrium with the experimental atmosphere. Corrosion-driven drying was also seen for higher initial concentrations of NaCl but at considerably slower rates. The drying effect, caused by the secondary spreading phenomena, may be an important self-limiting mechanism of atmospheric corrosion affecting Zn-Mg-Al alloy coatings.

Atmospheric corrosion comparison of antirust aluminum exposed to industrial and coastal atmospheres

The atmospheric corrosion behavior of antirust aluminum alloys 3A21 and 5A05 in industrial and coastal atmospheres was investigated and compared. Weight loss results, together with corrosion morphology characterization, suggest that the coastal atmosphere is much more corrosive to antirust aluminum than the industrial atmosphere. 5A05 alloy has better localized corrosion resistance than 3A21 alloy, and the corrosion rate shows a decreasing tendency with exposure time. After 3 years of exposure, the antirust aluminum samples only suffer pitting corrosion attack and retain their mechanical properties well. In the two atmospheres, the corrosion products are mainly oxides, aluminum hydroxychloride, aluminum sulfate hydrate, and basic aluminum sulfate. Besides, the atmospheric corrosion mechanism and the corrosion comparison between the two atmospheres were also discussed.

Atmospheric Corrosion Behavior and Mechanism of a Ni-Advanced Weathering Steel in Simulated Tropical Marine Environment

Corrosion behavior of Ni-advanced weathering steel, as well as carbon steel and conventional weathering steel, in a simulated tropical marine atmosphere was studied by field exposure and indoor simulation tests. Meanwhile, morphology and composition of corrosion products formed on the exposed steels were surveyed through scanning electron microscopy, energy-dispersive x-ray spectroscopy and x-ray diffraction. Results indicated that the additive Ni in weathering steel played an important role during the corrosion process, which took part in the formation of corrosion products, enriched in the inner rust layer and promoted the transformation from loose γ-FeOOH to dense α-FeOOH. As a result, the main aggressive ion, i.e., Cl−, was effectively separated in the outer rust layer which leads to the lowest corrosion rate among these tested steels. Thus, the resistance of Ni-advanced weathering steel to atmospheric corrosion was significantly improved in a simulated tropical marine environment.

Atmospheric Corrosion and Stress Corrosion Cracking of Corrosion Resistant Alloys Used for Dry Storage of Spent Nuclear Fuel

Spent nuclear fuel (SNF) in the United States is stored onsite at the commercial reactors where it is generated. The dry storage systems consist of welded 304/316 stainless steel canisters enclosed in passively-ventilated concrete overpacks. The ambient cooling air comes from the surrounding environment and contains typical atmospheric aerosols, including chloride-rich salts (e.g. Cl-), which accumulate on the storage canister surface over time. When the canister surface cools sufficiently, surface salts can absorb water from the air and form corrosive surface electrolyte droplets. Over time, these droplets may interact with weld regions that have susceptible microstructures and high through-thickness tensile stresses to promote stress corrosion cracking (SCC). A study is underway to understand atmospheric corrosion mechanisms and morphology that leads to SCC initiation on metals currently used for SNF dry storage canisters and on other corrosion-resistant alloys (CRA) that could be used for future storage. Austenitic SS304H (both annealed and sensitized), PH13-8 Mo SS, and Ni-based Alloy 600 corrosion test coupons have been coated with finely disseminated synthetic sea salt deposits using an ink-jet printer, simulating deposition of naturally occurring sea-salt aerosols, and exposed to expected atmospheric conditions with the aim of studying the resulting corrosion morphology, surface electrolyte chemistry, and SCC crack initiation behavior. Corrosion morphology (pit size distribution, pit number density, and shape) will be characterized as a function of salt surface load density, relative humidity, temperature, and exposure time using optical profilometry, FIB-SEM, microXCT, and robomet (a robotic cross-sectional metallographic examination technique). The surface electrolyte and corrosion products will be characterized by SEM-EDS, TOF-SIMS, and microRaman spectroscopy. Identical salt loading and atmospheric conditions will be applied to dog-bone style tensile samples used for SCC studies where intermittent high R ripple-style fatigue loading will be applied to mark the progression of fracture after crack initiation has occurred. These marks on the fracture surface will be used to identify the critical crack initiating features. Ultimately, the results of this study will be used for modeling initiatives aimed at designing new CRA alloys for next generation SNF tanks and to aid in lifetime assessments for current SNF temporary storage canisters. This work is supported at The Ohio State University by the Center for Performance and Design of Nuclear Waste Forms and Containers (WastePD), and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. The work conducted under WastePD is all stress corrosion cracking analysis and corrosion morphology investigations utilizing optical profilometry. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Improving the Corrosion Behavior of Magnesium-based Metal Matrix Composites

Magnesium-based metal matrix composites (Mg-based MMCs) are promising lightweight materials with many applications in the automotive and aerospace industries. Mg-based MMCs combine the high specific strength and good castability of Mg alloys with the excellent properties of the reinforcement. The reinforcing material is typically a ceramic, which exhibit tempting properties such as high melting point, excellent wear resistance, and high stiffness. The main driving force behind the development of Mg-based MMCs is that conventional Mg alloys show insufficient yield strength, damping capacity, wear, fatigue, thermal shock and creep resistances for many high-end applications, and thus, MMCs usually outperform their monolithic counterparts in this respect. However, it has been shown that Mg-based MMCs corrode faster than their monolithic counterparts. To date, attempts to produce corrosion-resistant Mg-based MMCs have failed, largely due to a lack of fundamental understanding of the corrosion phenomena occurring in this class of materials. In this paper, we studied the atmospheric corrosion behavior of silicon carbide (SiC)-reinforced Mg-based MMCs produced via a semi-solid casting method. We investigated the MMCs’ microstructure using the scanning transmission electron microscope (STEM/EDX) and electron energy loss spectroscopy (EELS). It was shown that the corrosion resistance of Mg-based MMCs can be improved via changing the casting parameters and pre-treatment of the reinforcements. A modified Mg-based MMCs exhibit favorable microstructural characteristics e.g., a lower fraction of pores and less numbers of ''undesirable'' intermetallic particles. We discuss, in detail, the atmospheric corrosion mechanism of Mg-based MMCs and formulate a working hypothesis for producing cast Mg-based MMCs that show enhanced atmospheric corrosion behavior.