Wet-dry Cyclic Condition Research Articles

Effect of temperature on initial corrosion of AZ91 magnesium alloy under cyclic wet–dry conditions

The effect of temperature on the initial corrosion of an as cast AZ91 magnesium alloy with surface deposition of salt particles in a water–vapour atmosphere under cyclic wet–dry conditions was studied using an environmental scanning electron microscope (ESEM). The results showed that the corrosion on the surface accelerated with increase in temperature and wetting time. The corrosion rates at various temperatures were a function of time, which expressed as with A &SetFont Typeface="44";representing the initial corrosion rate and n, usually in a range of 0 to 0·5, the exponent for the protective ability of corrosion products.

Arsenic species in weathering mine tailings and biogenic solids at the Lava Cap Mine Superfund Site, Nevada City, CA

BackgroundA realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived from historic mining operations is a major challenge to redevelopment of California's famed "Mother Lode" region. Arsenic, a known carcinogen, occurs in multiple solid forms that vary in bioaccessibility. X-ray absorption fine-structure spectroscopy (XAFS) was used to identify and quantify the forms of As in mine wastes and biogenic solids at the Lava Cap Mine Superfund (LCMS) site, a historic "Mother Lode" gold mine. Principal component analysis (PCA) was used to assess variance within water chemistry, solids chemistry, and XAFS spectral datasets. Linear combination, least-squares fits constrained in part by PCA results were then used to quantify arsenic speciation in XAFS spectra of tailings and biogenic solids.ResultsThe highest dissolved arsenic concentrations were found in Lost Lake porewater and in a groundwater-fed pond in the tailings deposition area. Iron, dissolved oxygen, alkalinity, specific conductivity, and As were the major variables in the water chemistry PCA. Arsenic was, on average, 14 times more concentrated in biologically-produced iron (hydr)oxide than in mine tailings. Phosphorous, manganese, calcium, aluminum, and As were the major variables in the solids chemistry PCA. Linear combination fits to XAFS spectra indicate that arsenopyrite (FeAsS), the dominant form of As in ore material, remains abundant (average: 65%) in minimally-weathered ore samples and water-saturated tailings at the bottom of Lost Lake. However, tailings that underwent drying and wetting cycles contain an average of only 30% arsenopyrite. The predominant products of arsenopyrite weathering were identified by XAFS to be As-bearing Fe (hydr)oxide and arseniosiderite (Ca2Fe(AsO4)3O3•3H2O). Existence of the former species is not in question, but the presence of the latter species was not confirmed by additional measurements, so its identification is less certain. The linear combination, least-squares fits totals of several samples deviate by more than ± 20% from 100%, suggesting that additional phases may be present that were not identified or evaluated in this study.ConclusionsSub- to anoxic conditions minimize dissolution of arsenopyrite at the LCMS site, but may accelerate the dissolution of As-bearing secondary iron phases such as Fe3+-oxyhydroxides and arseniosiderite, if sufficient organic matter is present to spur anaerobic microbial activity. Oxidizing, dry conditions favor the stabilization of secondary phases, while promoting oxidative breakdown of the primary sulfides. The stability of both primary and secondary As phases is likely to be at a minimum under cyclic wet-dry conditions. Biogenic iron (hydr)oxide flocs can sequester significant amounts of arsenic; this property may be useful for treatment of perpetual sources of As such as mine adit water, but the fate of As associated with natural accumulations of floc material needs to be assessed.

Comparative Studies on the Deterioration Process of Organic Coatings under Immersed and Cyclic Wet-Dry Conditions by EIS

Comparative Studies on the Deterioration Process of Organic Coatings under Immersed and Cyclic Wet-Dry Conditions by EIS

Electrochemical methods study on corrosion of 5% Al–Zn alloy‐coated steel under thin electrolyte layers

AbstractThe corrosion behavior of a 5% Al–Zn alloy (GF) coated steel was investigated under cyclic wet–dry condition using electrochemical techniques. The wet–dry cycle was conducted by exposure to alternate condition of 1 h immersion in seawater and 7 h drying at ambient temperature. The polarization resistance, Rp of the coating was monitored during the wet–dry cycles by two points AC impedance method and the corrosion potential, Ecorr was measured only when the coating was immersed in seawater. Simultaneously, the electrochemical impedance spectroscopy (EIS) of the coating was obtained after it was immersed in different cycles of wet–dry condition. The results obtained by two points AC impedance method had good agreement with those achieved from EIS technique, which proved that the two points AC impedance method was correct and an effective method for atmospheric corrosion study. The monitoring results indicated that the corrosion rate of GF coating firstly increased, then decreased slowly with time, and at last reached a relative steady state with local corrosion under the cyclic wet–dry alternate condition.

Initiation and Propagation of Atmospheric Corrosion on Aluminum Alloys under a Cyclic Wet-dry Condition with a Constant Dew Point

To clarify the mechanism of atmospheric corrosion of aluminum alloys in practical environments, a cyclic wet-dry corrosion test, in which the dew point of the circulating air was maintained at a constant level, has been conducted. The atmospheric corrosion of AA1100-O and AA6061-T6 aluminum alloys was randomly initiated around dispersed intermetallics due to the galvanic coupling between the aluminum matrix and intermetallics. The corrosion rate of aluminum alloys in the first cycle was higher than in the subsequent cycles due to the initiation of localized corrosion. As the number of wet-dry cycles increased, the increase of the amount of corrosion loss on aluminum alloys gradually tapered off, which was mainly attributed to the propagation of localized corrosion initiated in the first cycle. In the wet-dry conditions employed in our test, once localized corrosion was initiated in the first wetting period, no new sites of corrosion occurred in the subsequent cycles.

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Electrochemical study on cut edge corrosion of prepainted Zn coated (GI) and 55%Al–Zn coated (GL) steels has been performed in wet–dry cyclic conditions in order to clarify the difference of their degradation mechanisms. In addition, the role of chromate in the primer coating to the cut edge corrosion has been also investigated. Maximum width of delaminated polymer coating from the cut edge for GI and GL specimens was evaluated under wet–dry cyclic conditions. The cyclic tests were carried out for 1000 h by change of relative humidity, where the salt of NaCl or MgCl2 was deposited on the specimen every 48 h. The cut edge corrosion test under NaCl deposit indicated that the delamination of the GL specimen progressed at a higher rate than the GI. This is attributed to difference of corrosion morphology of their coatings from cut edge. The delamination was inhibited under MgCl2 deposit, especially for the GI specimen. The chromate in the primer coating inhibited the delamination for the GL specimen. The electrochemical corrosion monitoring was also performed under condition of alternate exposure to immersion in NaCl solution and drying at 60% RH and 25°C. On the basis of the results of the delamination tests and electrochemical measurements, the mechanism of cut edge corrosion for GI and GL and role of chromate in the primer coating were discussed.

Initial corrosion behavior of AZ91 magnesium alloy in simulating acid rain under wet-dry cyclic condition

Corrosion behavior of AZ91 magnesium alloy in simulating acid rain under wet-dry cyclic condition was investigated. The results show that corrosion potential shifts positively and the corrosion current density decreases at low wet-dry cyclic time. Further increase of the cyclic time results in the negative movement of corrosion potential and the increase of current density. SEM observation indicates that corrosion occurs only in α phase, β phase is inert in corrosive medium, and the corrosion of AZ91 magnesium appears in uniform characteristic. XPS analysis suggests that the corrosion product is mainly composed of oxide and hydroxide of magnesium and aluminum, and a small amount of sulfate is also contained in the film.

Hydrogen permeation and corrosion behaviour of high strength steel 35CrMo under cyclic wet–dry conditions

Hydrogen permeation behaviours of high strength steel 35CrMo under different cyclic wet–dry conditions have been investigated by using Devanathan–Stachurski's technique. Four electrolytes were used: distilled water, seawater, seawater containing 1500 ppm H2 S and seawater containing 0 · 03 mol L2 1 SO2. The corrosion weight loss of 35CrMo in the wet–dry cycles was measured simultaneously. The experimental results show that hydrogen can be detected at the surface opposite to the corroding side of the specimen during wet–dry cycles and the permeation current density during a wet–dry cycle showed a maximum during the drying process. The hydrogen permeation was obviously promoted by Cl2 ions, H2 S and SO2. The hydrogen permeation in the real marine atmosphere has also been investigated. There is a clear correlation between the amount of hydrogen permeated and the corrosion weight losses. Results show the importance of hydrogen permeation that merits further investigation.

Initiation and Propagation of Atmospheric Corrosion on Aluminum Alloys under a Cyclic Wet-dry Condition with a Constant Dew Point

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Corrosion Monitoring Characteristics of Plasma RF Sputtered Mg Thin Films under Cyclic Wet-Dry Conditions

In general, magnesium metal is not used as uncoated material on account of its corrosion characteristic. This work was done by preparing 99.99% pure magnesium coating films by RF magnetron sputtering onto SPCC substrates [1]. The individual preparation conditions of gas pressures and bias voltages were 1×10-2 ~ 1×10-3 Torr and 0V ~ -300V, respectively. The analysis about crystal orientation, morphology and electrochemical characteristic of Mg thin films was performed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), electrochemical impedance spectroscopy (EIS) etc.. It is as a results of experiment that the morphology of the sputtered films changed from a columnar structure to a granular structure when Ar gas pressure increased or bias voltage decreased. Also, the prepared Mg thin film’s morphology did not have defects and pinholes, and the corrosion resistance was improved by the formation of a homogeneous passive layer.

In situ observation of crack initiation and propagation of the charged magnesium alloy under cyclic wet–dry conditions

The process of crack initiation and propagation of the charged MgAl9Zn1 alloy in an atmosphere of water vapor under cyclic wet–dry conditions was in situ observed using environmental scanning electron microscope (ESEM). The results showed that crack initiated at scratches after 6 wet–dry cycles, indicating that residual stress caused by scratches promoted crack initiation. With the increase of wetting time, the fracture interval of the cracking β phase enhanced and the crack length increased, implying that hydrogen produced by corrosion reaction participated in the cracking process.

Evaluation of impedance spectra of zinc and galvanised steel corroding under atmospheric environments

The evaluation of impedance spectra of zinc and galvanised steel corroded under wet–dry cyclic condition has been performed. The wet–dry cyclic test was performed by exposing samples to alternate conditions of 1 h immersion in 0·05M NaCl solution and 7 h drying at 298 K and 60% relative humidity (RH). During the corrosion test, the impedance responses of zinc and galvanised steel samples were recorded once per day for 10 days. The impedance spectra were evaluated on the basis of an equivalent circuit considering the charge and mass transfer consequences of the coupled corrosion processes and the porous oxide layer. After initial exposure, both samples showed the evolution of two capacitive loops. However, a sharp decrease in characteristic frequency, characterised by an increase in double layer capacitance in the range of several mF cm−2 as corrosion progressed, was observed. This was explained by taking into account the coupling of characteristic frequencies of the two capacitive loops due to formation of porous rust on iron. The parameters for the diffusion impedance of the cathodic process were estimated from the low frequency capacitive loops attributed to a finite Warburg diffusion impedance. A sharp increase in impedance after several days of corrosion in the case of galvanised steel was attributed to the dissolution of Fe–Zn alloy layer that is formed during galvanising. The results are supported by cross-sectional analysis of corrosion products using FESEM-EDX.

Pitting and galvanic corrosion behavior of stainless steel with weld in wet-dry environment containing CI −

Accelerated corrosion test of stainless steel with weld was carried out to investigate the corrosion behavior under the wetdry cyclic condition in the atmosphere containing Cl −. In the surface morphology, corrosion products were analyzed by metallographic observation, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that the damage to stainless steel with weld in the atmosphere containing Cl − is due to localized corrosion, especially pitting and galvanic corrosion. Weld acts as the anode, whereas matrix acts as the cathode in the corrosion process. The pitting corrosion, including the nucleation and growth of a stable pit, is promoted by the presence of wet-dry cycles, especially during the drying stage. Pits centralizing in weld are found to be grouped together like colonies, with a number of smaller pits surrounding a larger pit. The composition of the corrosion products is Fe 2O 3, Cr 2O 3, Fe 3O 4, NiCrO 4, etc.

Hydrogen permeation behavior and corrosion monitoring of steel in cyclic wet–dry atmospheric environment

AbstractHydrogen permeation of 16Mn steel under a cyclic wet–dry condition was investigated by Devanathan–Stachurski's electrolytic cell with a membrane covered on the exit side by a nickel layer and the weight loss was measured for each wet–dry cycle. The results show that hydrogen permeation current change with different atmospheric environment: distilled water, seawater, and seawater containing 100 ppm H2S. The results show that seawater can induce an increase in the hydrogen permeation current due to the hydrolyzation reaction. And after the increase, equilibrium is reached due to the equilibrium of hydrolyzation reaction effect and the block of the rust layer. On the other hand, H2S contamination also can induce an increase in the maximum hydrogen permeation current due to the hydrolyzation reaction. And H2S contamination delays the time that hydrogen permeation is detected because of the formation of the FeS(1−x) film. The FeS(1−x) film can block the absorption of hydrogen onto the specimen surface. The surface potential change and the pH change of the metal surface control the hydrogen permeation current. And a clear linear correlation exists between the quantities of hydrogen permeated through the 16Mn steel and the weight loss. Based on the linear correlation, we monitored the corrosion rate by monitoring the hydrogen permeation current by a sensor outside. Good coherences were shown between results in laboratory and outside.

Monitoring of Corrosion Rates of Carbon Steel in Mortar under a Wet-Dry Cyclic Condition

The corrosion behavior of metal covered with mortar under a wet-dry cyclic condition were investigated to apply for the measurement of corrosion rates of reinforcing steel in concrete structure. The carbon steel in mortar having t=3 mm cover thickness was exposed to the alternate condition of 6 h immersion in chloride containing solution and 18 h drying at <TEX>$25^{\circ}C$</TEX> and 50%RH. The electrochemical phenomena of a carbon steel and mortar interface was explained by an equivalent circuit consisting of a solution resistance, a charge transfer resistance and a CPE(Constant Phase Element). The corrosion rates were monitored continuously during exposure using an AC impedance technique. Simultaneously, the current distribution over the working electrode during impedance measurement was analyzed from the phase shift, <TEX>$\theta$</TEX>, in an intermediate frequency. The result showed that corrosion rate monitoring using an AC impedance method is suitable under the given exposure conditions even during the drying period when the metal is covered with the wetted mortar.

In Situ Observation of Initial Corrosion of MgAl9Zn1 Magnesium Alloy in Cyclic Wet-Dry Conditions Using Environmental Scanning Electron Microscopy

Abstract The corrosion of a MgAl9Zn1 (UNS M11918, AZ91) alloy in an atmosphere of water vapor with cyclic wet-dry conditions was studied using environmental scanning electron microscopy (ESEM). The results showed that the oxygen concentration on the surface generally increased with the wetting time. At short wetting times the corrosion was localized at the droplets and mainly appeared on the α-Mg areas. The corrosion rate increased with the wetting time. At long wetting times a water film covered the surface and general corrosion occurred. The rate of corrosion decreased with increased time of wetting. The magnitude of oxygen concentration at AlMn-segregated areas was similar to that on the matrix area. The segregation of the AlMn phase contributed little to the average corrosion on the surface but contributed significantly to the localized corrosion.

Effect of Fe–Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe–Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe–Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn 13, FeZn 7 and Fe 3Zn 10 intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet–dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn 2+ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.

In Situ Observation of Initial Corrosion of AZ91 Alloy with Residual Stress in Wet-Dry Conditions Using Environmental Scanning Electron Microscope

The corrosion of AZ91 magnesium alloy with residual stress was observed in-situ in an atmosphere of water vapor with cyclic wet-dry conditions using environmental scanning electron microscope (ESEM). The results showed that the oxygen concentration on the surface generally increased with the wetting time. And the average corrosion rate of AZ91 alloy with residual stress (RS) was much larger than that without RS, indicating RS accelerated corrosion. The corrosion localized at primary α-Mg area at short wetting time, but with the increase of wetting time spread to the eutectic α-Mg region. Small-size as-cast defects (&lt;50μm) contributed little to the average corrosion rate while large-size as-cast defects (&gt;100μm) influenced the distribution of water film and corrosion on the surface.

Analysis of deterioration process of organic protective coating using EIS assisted by SOM network

Under cyclic wet–dry conditions, the deterioration process of the organic coating on carbon steel surface has been studied using electrochemical impedance spectroscopy (EIS) assisted by self-organizing feature map (SOM) network. According to the EIS characteristics, changing rate of impedance and the classification results by SOM network, the entire deterioration process can be divided into three main stages shown as follows. Stage I is the medium penetration into coatings, which is a slow process. Stage II is the corrosion initiation under coatings, which is a relatively fast transition period. Stage III is the corrosion extension which causes coating delamination and makes the coatings lose its corrosion protection eventually. Besides, the results indicate that SOM network is a very simple and effective technique for analyzing the deterioration process of organic coating.

In situ observation of formation and spreading of micro-droplets on magnesium and its alloy under cyclic wet–dry conditions

The formation and spreading of micro-droplets on pure magnesium and its alloy in an atmosphere of water vapor under cyclic wet–dry conditions was studied using an environmental scanning electron microscope (ESEM). The in situ observations indicated that the corrosion product stimulated the formation and spreading of micro-droplets, and the grain boundaries for pure magnesium and β phase for AZ91 magnesium alloy had no influences on this process. The oxygen concentration on the surface generally increased with the wetting time. The corrosion products decreased the corrosion rate.