Removal Of Corrosion Products Research Articles

Investigation on the Dry-Type Electrochemical Polishing of 304 Stainless Steel with Ion-Exchange Resin Particles

Abstract Abstract: Dry-type electrochemical polishing (DECP) technology combines the mechanical action of resin particles with the electrochemical action of working solutions to enhance the results of existing treatments. Homogeneous polishing across the entire surface of the piece is the main advantage compared to mechanical polishing. This article utilizes this method to polish the surface of 304 stainless steel, resulting in a metal surface roughness (Ra) in the range of 1.23m to 98nm. This innovation expands the potential applications of 304 stainless steel in the medical and food industries. Energy dispersive X-ray spectroscopy (EDS) analysis cannot detect residual electrolyte components, and resin particles containing liquid electrolytes can effectively avoid the harmful substanc remain on the metal surface. Electrochemical analysis of polished sample shows that the Ecorr was -0.109V for DECP specific sample compared with -0.291V for initial sample, and the polishing process can be seen as the generation and removal of corrosion products.

El láser en la restauración de hierros arqueológicos intensamente mineralizados: criterios y protocolo de intervención en el caso de las armas de una necrópolis Vaccea (Valladolid, España)

The aim of this paper is to present the conservation treatment of a set of iron weapons from the Vaccean necropolis of the Ruedas de Pintia (Padilla de Duero, Valladolid) dated between the 4th century BCE and 2nd century CE. The cremation and burial conditions have been very detrimental to their state of conservation, generating an intense corrosion and their complete mineralization. This left the weapons in a very fragile and brittle state that made their restoration difficult, since it implied the removal of hard crusts that were much adhered to the surface and, during this process, the pieces were in danger of further fragmentation. Our proposal is the combined use of traditional abrasive cleaning techniques together with the 1064nm Nd:YAG laser cleaning that, in addition to removing corrosion products, transforms the surface into magnetite, a stable iron oxide.

Effect of Tungsten Inert Gas Remelting on Microstructure and Corrosion Resistance of Q450NQR1 High-Strength Weathering Steel-Welded Joints.

In this paper, the corrosion environment of a railway coal truck was simulated with 1.0%H2SO4 + 3%NaCl solution. The effect of weld toe Tungsten Inert Gas (TIG) remelting on the microstructure and corrosion resistance of welded joints of Q450NQR1 high-strength weathering steel was studied. The results show that the weld toe melts to form a remelting area after TIG remelting. After TIG remelting, the weld geometry was improved, and the stress concentration factor decreased from 1.17 to 1.06 at the weld toe, a decrease of 9.4%. TIG remelting refines the microstructure of the weld toe and improves the corrosion resistance of the welded joint. The surface of the TIG-remelted sample is uniformly corroded with no "deep and narrow" pits after the removal of corrosion products. The weight loss rate and corrosion rate of remelted welds are lower than those of unremelted welds. The structure of corrosion products is loose at the initial stage of corrosion, and the corrosion products are transformed into Fe3O4 and Fe2O3 protective rust layers with a dense structure after 480 h of corrosion. With the extension of corrosion time, the tensile strength and percentage elongation of the specimen decreased linearly. The decreasing rates of tensile strength of remelted and unremelted specimens were 0.09 and 0.11, respectively, and the decreasing rates of elongation after fracture were 0.0061 and 0.0076, respectively.

A Cu-Containing High-Entropy Alloy with High Corrosion Resistance and Low Surface Electrical Resistance

Cu-base alloys with high corrosion resistance and low surface electrical resistance in corrosive environments were studied for application to electrical connectors. To achieve both high corrosion resistance and low surface electrical resistance, duplex alloys were fabricated.Non-equiatomic CuSnZnAlNi high-entropy alloys were prepared by induction melting. As raw materials, Cu-Zn alloy and pure metals of Sn, Al, and Ni were used. After casting, heat treatment was performed at 500 ℃ for 5 h (water-quenching). After that, the specimen surfaces were polished with a diamond paste down to 1 µm. The microstructure of the specimens was observed using an optical microscope and a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy system (EDS). Electrochemical measurements were performed in synthetic seawater (pH 8.2) at 25 ℃. Dip-dry corrosion test was conducted. The specimens were dipped in synthetic seawater (pH 8.2) for 3.5 h and dried at 25 ℃ and 30 %RH for 0.5 h. Before and after the dip-dry corrosion test, surface electrical resistance was measured.Cu36Sn16Zn16Al16Ni16 was prepared, and the phases were identified by SEM/EDS. As a result, the alloys were separated into four phases. The corrosion resistance of each phase was compared by SEM/EDS between the specimens before polarization and the specimens after polarization and removal of corrosion products. 20 cycles of dip-dry corrosion test was conducted, with one cycle being the combination of immersion of the specimen in the solution and drying.

Assessing residual stresses in the surface layer of the ZK60 alloy after an exposure to corrosion solution

Magnesium alloys preliminary immersed in a corrosion solution suffer from embrittlement, referred to as pre-exposure stress corrosion cracking (PESCC). It was suggested that PESCC can be attributed to the corrosion product film-induced stress (CPFIS), which is known to be responsible for SCC in many alloys. However, the internal stress associated with the formation of the corrosion products (CP) layer on Mg alloys have not been investigated as yet. Thus, in the present study, the internal residual stresses of the first and second kinds were assessed in the alloy ZK60 exposed to the corrosion solution, using the deflection of the thin plate and by the X-ray diffraction technique, respectively. It is found that the deposition of CP on the surface of the alloy ZK60 creates the compressive internal stresses of both kinds — I and II. The macro residual stress of the kind I in the thin plate is found to be not exceeding 3 MPa, while the micro residual stress of the second type in the surface layer of 20 – 30 µm can be as high as 290 MPa and cause plastic deformation of the bare metal with the internal stress which cannot be relieved by the removal of CP.

Pitting corrosion of 2A12 aluminum alloy long-scale specimen in simulated seawater splash zone, dynamic waterline zone and full immersion zone

PurposeThe aim of this paper is to construct a device that simulated the seawater splash zone, dynamic waterline zone (splash zone) and full immersion zone. Localized corrosion of 2A12 aluminum alloy long-scale specimen was studied.Design/methodology/approachCorrosion morphologies before and after the corrosion product removal were used to identify corrosion intensity at full seawater immersion zone, dynamic waterline zone (splash zone) and atmospheric zone. The average depth and diameter of corrosion pits in the three zones were evaluated by three-dimensional optical microscopy. The impact of wetting time of the atmospheric zone on the localized corrosion was investigated.FindingsCorrosion pits were observed on the surface on day 4 for the wetted atmospheric zone (Case 1), and on the surface on day 8 for the alternant wet/dry atmospheric zone (Case 2). The corrosion product layer on the surface for Case 1 was partially broken down while the layer on the surface for Case 2 was intact. Average pitting depth and pitting diameters for Case 1 were more serious than that for Case 2.Originality/valueThe above findings revealed that the humidity of the atmospheric zone had great impact on the localized corrosion of aluminum alloy at the seawater splash zone.

Long-Term Performance of Ultrafiltration Membranes: Corrosion Fouling Aspect

The past decade has seen a rise in the importance of the ultrafiltration (UF) technique in the separation of various complex solutions. However, the fouling phenomenon is the main limitation to faster process development. To the best of the authors' knowledge, the present paper is the first to aim to identify the role of corrosion fouling in long-term UF. For this purpose, polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes were used. The investigations were carried out with the use of both pilot-scale and laboratory-scale units. Results obtained in the present study have clearly demonstrated that the oil concentration has a significant impact on the process performance. Indeed, it has been noted that a reduction in oil concentration from 160 to 100 mg/L resulted in an increase in the PVDF membrane flux from 57 to 77 L/m2h. In addition, it has been shown that the feed temperature has a significant influence on the UF performance. Importantly, it has been shown that corrosion fouling is of vital importance in UF membranes. Indeed, corrosion products such as iron oxides contaminated the membrane surface leading to an irreversible decrease in the UF process performance. In addition, it has been found that repeating the chemical cleaning of the membrane units significantly reduced the intensity of the fouling phenomenon. However, the complete elimination of its effects was not achieved. Therefore, it has been indicated that cleaning agents recommended by membrane manufacturers do not remove corrosion products deposited on the membrane surface. Undoubtedly, the obtained results can be used in the design of UF units leading to the extension of membrane installation lifetime.

Влияние скорости деформирования на механические свойства и характер разрушения сплава AZ31 и технически чистого магния, предварительно выдержанных в коррозионной среде

Magnesium alloys are promising materials for aviation, automotive engineering, and medicine, however, due to the low resistance to stress corrosion cracking (SCC), their wide application is limited. To create alloys with high resistance to SCC, a comprehensive study of this phenomenon nature is required. Previously, it was suggested that diffusible hydrogen and corrosion products formed on the magnesium surface can play an important role in the SCC mechanism. However, the contribution of each of these factors to the SCC-induced embrittlement of magnesium and its alloys is understudied. Since the influence of diffusible hydrogen on the mechanical properties of metals increases with the strain rate decrease, the study of the strain rate sensitivity of the SCC-susceptibility of magnesium alloys is a critical task. In this work, the authors studied the effect of the strain rate in the range from 5·10−6 to 5·10−4 s−1 on the mechanical properties, the state of the side and fracture surfaces of the as-cast commercially pure magnesium and the AZ31 alloy before and after exposure to a corrosive environment and after removal of corrosion products. The study identified that the preliminary exposure to a corrosive medium leads to the AZ31 alloy embrittlement, but does not affect the mechanical properties and the fracture mode of pure magnesium. The authors found that the AZ31 alloy embrittlement caused by the preliminary exposure to a corrosive medium appears extensively only at the low strain rate and only if the layer of corrosion products is present on the specimens’ surface. The study shows that a change in the strain rate has little effect on the mechanical properties of pure magnesium. The authors concluded that the main cause of the AZ31 alloy embrittlement after soaking in a corrosive medium is the corrosion products layer, which presumably contains the embrittling agents such as hydrogen and residual corrosive medium.

Скоростная чувствительность механических свойств сплава ZK60 с высокой степенью коррозионных повреждений

According to the stable opinion, hydrogen absorbed by magnesium alloys during corrosion can cause their stress corrosion cracking. One of the characteristic markers indicating the involvement of diffusible hydrogen into the fracture mechanism of metals is the negative strain rate dependence of the embrittlement degree. Recent studies show that the loss of ductility of the ZK60 alloy specimens subjected to a short-term (1.5 h) pre-exposure in a corrosive medium actually decreases with the increasing strain rate. However, after the removal of corrosion products from the surface of specimens, the strain rate dependence of the ductility loss becomes positive, which indicates the absence of hydrogen in the bulk of a metal. At a short-term exposure in a corrosive environment, the deep penetration of hydrogen into a metal could be limited due to the insufficient time for hydrogen diffusion. The paper studies the mechanical behavior of the ZK60 alloy subjected to a longer (12 h) pre-exposure in a corrosive medium followed by tensile testing in air at various strain rates. The authors consider the effect of strain rate, long-term pre-exposure in a corrosive medium, and subsequent removal of corrosion products on the strength, ductility, stages of work hardening, and localized deformation, as well as on the state of the side and fracture surfaces of specimens. The study identified that the ductility loss of specimens pre-exposed in a corrosive medium for 12 h decreases with the increasing strain rate, regardless of whether the corrosion products have been removed from their surface or not. It is shown that in this case, the negative strain rate dependence of the ductility loss is associated not with hydrogen dissolved in the bulk of a metal but with the presence of severe corrosion damage of the specimens’ surface. The authors proposed an explanation for the effect of corrosion damage on the mechanical properties and their strain rate sensitivity.

Laser cleaning on severely corroded steel members: Engineering attempt and cleanliness assessment

A highly effective, environmentally friendly, high-power continuous-wave laser (CWL) transmitter was newly developed to remove corrosion products from severely corroded steel structures. In this regard, treatment conditions of dry laser cleaning (DLC) and wet laser cleaning (WLC) of the drench, soak, and rinse methods are adopted, and the cleanliness effect is compared with that of conventional abrasive blasting treatment. The morphologies and chemical compositions of untreated and treated surfaces are analyzed using three-dimensional digital microscopy, laser confocal microscopy, and scanning electron microscopy–energy dispersive X-ray spectrometry. The results show that laser cleaning can efficiently remove corrosion products and salts that severely corrode steel members, as well as prolong the re-corrosion time of the cleaned surfaces. WLC can achieve excellent microscopic cleaning and reduce the effect of high-temperature inputs on the surface via laser irradiation. Further mechanistic analysis indicates that ablation, melting, and evaporation during laser cleaning primarily contribute to the removal of corrosion products and salts. As a result, the WLC drench method yields the best surface cleanliness and is suitable for practical engineering applications. This study provides a basis for maintaining severely corroded structures via laser-cleaning techniques.

PH-Responsive Semi-Interpenetrated Polymer Networks of pHEMA/PAA for the Capture of Copper Ions and Corrosion Removal.

Bronze artifacts constitute a fundamental portion of Cultural Heritage, but effective methodologies for the removal of corrosion layers, such as those produced by the “bronze disease”, are currently missing. We propose the formulation and application of novel poly(2-hydroxyethyl methacrylate) (pHEMA) networks semi-interpenetrated (SIPN) with poly(acrylic acid) (PAA) to achieve enhanced capture of copper ions and removal of corrosion products. The pHEMA/PAA SIPNs were designed to improve previous pHEMA/poly(vinylpyrrolidone) (PVP) networks, taking advantage of the chelating ability of pH-responsive carboxylic groups in PAA. Increasing the pH ionizes carboxyls, increases the porosity in pHEMA/PAA, and leads to the co-presence of enol and enolate forms of vinylpyrrolidone (VP), changing the macroporosity and decreasing the mesh size in pHEMA/PVP. The ion–matrix interaction is stronger in pHEMA/PAA, where the process occurs through an initial diffusion-limited step followed by diffusion in smaller pores or adsorption by less available sites. In pHEMA/PVP, the uptake is probably controlled by adsorption as expected, considering the porogen role of PVP in the network. Upon application of the SIPNs loaded with tetraethylenpentamine (TEPA) onto corroded bronze, copper oxychlorides dissolve and migrate inside the gels, where Cu(II) ions form ternary complexes with TEPA and carboxylates in PAA or carbonyls in PVP. The removal of oxychlorides is more effective and faster for pHEMA/PAA than its /PVP counterpart. The selective action of the gels preserved the cuprite layers that are needed to passivate bronze against corrosion, and the pH-responsive behavior of pHEMA/PAA allows full control of the uptake and release of the Cu(II)–TEPA complex, making these systems appealing in several fields even beyond Cultural Heritage conservation (e.g., drug delivery, wastewater treatment, agricultural industry, and food chemistry).

Effect of Zn content on corrosion behavior of Mg-Y-Zn alloys

The microstructure, corrosion behavior, and electrochemical behavior of as-cast Mg-4Y-xZn (x=1,2,3,4 wt.%) are studied by SEM, weight loss and electrochemical tests. Mg12YZn (X), Mg3Y2Zn3 (W) and Mg24Y5 constitute the phase composition system of the alloy. When Zn content is 1 wt.%, all tests reveal that alloy has the optimal corrosion performance. The second phase in these alloys, due to their nobler nature than ?-Mg, exists as a cathode during the corrosion process, so that ?-Mg preferentially occurs corrosion to accelerate the formation of corrosion pits. After soaking in 3.5 wt.% NaCl solution for some time, the stability of the W phase changed, and gradually dissolved, which was finally removed by chromic acid used for removal of corrosion products. In addition, X phase can be used as an anode in the micro-galvanic cells formed with W phase to reduce the corrosion rate of ?-Mg and thus improve the corrosion performance of the alloy.

Electrochemical Study of Clean and Pre-Corroded Reinforcements Embedded in Mortar Samples with Variable Amounts of Chloride Ions.

The present study investigates the possibility of re-surfacing previously corroded reinforcements and the suitability of the two electrochemical techniques that are widely used to determine the state of corrosion of steel (the corrosion potential Ecorr and the corrosion rate icorr). In order to test this, 32 pre-corroded B500SD reinforcing steel bars have been used for one year, where half of the bars have been cleaned to eliminate corrosion products. The other half have been maintained with the generated corrosion products. Subsequently, the bars have been embedded in cement mortar samples with variable amounts of chloride ion, and Ecorr and icorr have been measured for 250 days. The results showed that it is not possible to rework the reinforcement without removing corrosion products and that it is not possible to predict the passive or active state of steel by measuring Ecorr only.

Effect of strain rate and corrosion products on pre-exposure stress corrosion cracking in the ZK60 magnesium alloy

Several recent studies indicated that the layer of corrosion products deposited on the surface of Mg alloys being immersed in a corrosive solution can be the key reason of the subsequent alloy's embrittlement in air. This phenomenon has been referred to as pre-exposure stress-corrosion cracking (PESCC). To clarify this suggestion and get a deeper insight into PESCC, in the present study, we investigate the effect of strain rate on the mechanical properties and fractographic features of the alloy ZK60 before and after pre-exposure to corrosive media as well as after pre-exposure and removal of corrosion products. It is found that the ductility of pre-exposed specimens increased with strain rate in contrast to that of the reference unexposed specimens. This confirms that the mechanism responsible for PESCC should be time-dependent. Furthermore, it is concluded that this mechanism is associated with corrosion products layer containing sealed corrosive media rather than with diffusible hydrogen, which has been long believed to be the root cause of PESCC. The presence of retained corrosive media sealed within the corrosion products layer is confirmed by the manifestation of corrosion products on the edge of the fracture surface as well as inside the secondary cracks of the pre-exposed specimens tensile tested in air. Mechanisms of the crack growth responsible for PESCC of the alloy ZK60 are discussed comprehensively.

Optimization of the taper angle of the jetter confuser used for reversible blast cleaning of propellers from corrosion

It is shown that it is very effective to remove corrosion products from various surfaces, including the metal surface of the propeller, using the new technology of reverse-jet cleaning (RJC) developed by the authors. The RJS technology is based on the physical principle that the jet of the working fluid (pulp based on river sand or bentonite clay), upon impact with the surface to be cleaned, turns 180°, which leads to an increase in the jet effect on the surface to be cleaned by 1.5–2 times due to the occurrence of the reactive component. To ensure the marked turn of the jet, an original design of the body was developed, which is distinguished by a patent novelty. One of the main elements in this design is a jet-forming device in the form of a confuser. The presented theoretical studies of the pressure loss of the working fluid in the confuser channel, based on the study of the pressure loss function to the extremum, which made it possible to obtain a dependence for calculating the optimal taper angle in a wide range of Reynolds numbers characterizing the turbulent mode of motion taking into account the influence of the working fluid density and its dynamic viscosity, the average speed of movement of the working fluid, the radius of the confuser, as well as the coefficient of equivalent roughness, i.e. from gradual wear of the confuser channel. The resulting dependence can be recommended for calculation in the design of jet cleaning devices and other installations of jet technology.

Influence of the Transients and Water Chemistry Parameters on Corrosion Product Behavior in the Primary System of Naval Reactor Plants

This technical note describes the influence of the transients and corrective additives on the distribution of loosely bonded forms of solid-phase corrosion products between the equipment surfaces and the primary coolant of a naval reactor plant. It is shown that the concentration of loosely bonded corrosion products increases by tenfold during the transient, and the feeding of corrective additives allows a several-fold reduction in the rate of their resedimentation on the internal surfaces of equipment, and as a consequence, improvement of corrosion product removal efficiency by cleanup filters. The proposed solutions allow removal of up to 70% of loosely bonded corrosion products from the coolant using standard filters.

On the role of pre-exposure time and corrosion products in stress-corrosion cracking of ZK60 and AZ31 magnesium alloys

Abstract Pre-exposure of Mg alloys to corrosive media causes their embrittlement, which has been long associated with diffusible hydrogen penetrating into the metal. The time of the pre-exposure was suggested to promote embrittlement due to increasing both the corrosion damage and hydrogen concentration. However, recently it was shown that removal of corrosion products from the pre-exposed specimens results in full elimination of the all harmful effects induced by pre-exposure. This observation casts doubts about the significance of diffusible hydrogen in pre-exposure embrittlement. In view of those new findings, the effect of pre-exposure time on the embrittlement phenomenon is revisited in the present study. The specimens of the ZK60 and AZ31 alloys were pre-exposed to 4% NaCl + 4% K2Cr2O7 corrosive media for different durations and then slow-strain rate tensile tested in air either as are or after removal of corrosion products. It is found that the pre-exposure embrittlement of both alloys increases with the increasing pre-exposure time and is caused by the combined effect of the corrosion damage and the brittle cracking mechanism. The latter is (i) activated by the deposition of the essentially thick corrosion products layer on the specimen surface during pre-exposure, (ii) promoted by the increasing amount of corrosion products and (iii) can be fully inhibited by the removal of corrosion products from the specimen surface. The corrosion damage produces the irreversible loss in ductility and strength, which becomes more pronounced with the increasing pre-exposure time, but it does not alternate the fracture mode. No evidence is found for the increasing concentration of diffusible hydrogen in Mg alloys with the increasing pre-exposure time.

Quantification of pitting corrosion from thermography using deep neural networks.

Pipelines serve as the major infrastructure for transmission and distribution in oil and gas and water industries. The health condition of a metallic pipe is determined mainly by the extent to which it is corroded. Thus, the quantification of pitting corrosion in terms of metal loss is required for the understanding of pipe condition. There are different ways to quantify corrosion pit geometry. Direct methods measure the pit depth of pipe samples, which are sand/grit blasted to remove corrosion products, and are often adopted in the laboratory. Indirect methods employ non-destructive inspection techniques to detect and quantify the corrosion without sandblasting, which is preferred for a field test. In this study, pulsed thermography is considered for the quantification of pitting corrosion in a metallic pipe. Thermography testing can generate a sequence of infrared images, which reflect the diffusion process of heat through the pipe wall. This paper proposes a new deep neural network-based approach to quantify the pitting corrosion damages from the acquired thermography images. Through extensive experimental tests, the quantified results demonstrate a reasonably good linear relationship with metal loss of pipe, as the quantification accuracy was better than 98%.

Conductive Polymer/SiO2 Composite as an Anticorrosive Coating Against Carbon Dioxide Corrosion of Mild Steel. A Simulation Study

In this work corrosion of mild steel affected by carbon dioxide was studied using a simulation model developed by Nordsveen M. and Nesic S. Using this comprehensive model of the uniform corrosion made possible to predict of corrosion rate of steel in the carbonic acid medium and the influence of different conditions on the anticorrosive property of coated electrode has been investigated. 1D model of corrosion process includes Butler-Volmer and Tafel equations and takes into account both the kinetics of anodic dissolution of an iron and electrochemical discharge of carbonic acid, water and hydrogen ions. The model has been created in COMSOL Multiphysics software and further improvement of this model allowed studying the influence of parameters such as solution composition, the partial pressure of CO2, temperature and flow velocity of the solution on the corrosion rate of the steel. The results of numerical simulation demonstrate that the use of conductive polymerpolypyrrole/ SiO2 composite as an anti-corrosive resin coating reduces the corrosion rate of mild steel by 7 times or more, depending on pH, temperature and flow rate. Furthermore, increasing of flow velocity from 0.1 to 10 m/s affects to the removal of corrosion products from the surface of mild steel and as a result corrosion rate raises from 0.3 to 0.45 mm/year at a temperature of 80 °C and pH=4.

Inhibiting stress corrosion cracking by removing corrosion products from the Mg-Zn-Zr alloy pre-exposed to corrosion solutions

Magnesium and its alloys are susceptible to stress-corrosion cracking (SCC), which can manifest itself during the slow-strain rate tensile (SSRT) testing in air if the specimens were pre-exposed to corrosive media. This phenomenon is generally associated with hydrogen embrittlement (HE) which is believed to be related to diffusible hydrogen penetrating into the metal during the pre-exposure. In the present study, we show that the corrosion product layer deposited on the surface of the pre-exposed specimens is crucial in the SCC mechanism. The specimens of the alloy ZK60 were SSRT tested in air, in corrosive media, in air after pre-exposure to corrosive media as well as after pre-exposure and removal of corrosion products. To vary the severity of SCC, four NaCl-based corrosion solutions were utilised. The embrittlement resulted in a marked decrease in ductility and the concurrent appearance of multiple side-surface cracks as well as brittle fragments on the fracture surface. The most striking finding of the present study is that the removal of corrosion products from the surface of the pre-exposed specimens results in complete recovery of the mechanical response and in the elimination of all harmful embrittling features regardless of the corrosive solution used. This effect is found to be inconsistent with the common viewpoint where the SCC mechanism is governed entirely by diffusible hydrogen. The obtained results shed new light on the role of diffusible hydrogen, corrosion products and irreversible corrosion damage in the SCC mechanism of Mg alloys pre-exposed to corrosive media.