Wet-dry Cyclic Condition Research Articles

Electrochemical noise study on 2024-T3 Aluminum alloy corrosion in simulated acid rain under cyclic wet–dry condition

Potential noise records have been collected for 2024-T3 aluminum alloy, which was exposed to simulated acid rain with different pH value for 15 wet–dry cycles. Meanwhile, Potentiodynamic polarization and SEM techniques were also used as assistant measurements. Three mathematic methods including average, standard deviation and wavelet transformation have been employed to analyze the records. The results showed that each single wet–dry cycle can be divided into three regions with respect to the change of the cathodic reaction rate, and with the increase of pH value the main cathodic reaction changes from the reduction of protons to that of oxygen molecules. The analysis of the EDP versus time evolution clearly indicates that the whole corrosion process can be divided into three segments for the case of pH 3.5 and only one for the cases of pH 4.5 and 6.0, which have been theoretically interpreted according to the corrosion theory and experimentally proved by SEM. The results also showed that the corrosion in the case of pH 3.5 was much more rigorous than that in the cases of pH 4.5 and 6.0. It may due to synergistic effects of that, the characteristic of hydrogen ions which is much more active than that of oxygen molecules, the high diffusion/migration rate of hydrogen ions in solution or through surface films and the lower stability of surface passive film at low pH value system.

EIS study on 2024-T3 aluminum alloy corrosion in simulated acid rain under cyclic wet-dry conditions

The wet-dry cycle corrosion processes of 2024-T3 aluminum alloys under different pH environments were investigated using electrochemical impedance (EIS) technique in conjunction with SEM method. The results show that, during the dry period of each cycle, the electrode potential was relatively stable and the corrosion processes can be studied using EIS technique. In the case of pH 3.5 and during the entire corrosion process, the Nyquist plot was composed of a high-frequency capacitive arc due to corrosion reaction and a low-frequency capacitive are due to the formation of corrosion products layer, and the corrosion rate during the same cycle was almost the same. In the case of pH 4.5 and during the first several cycles, the Nyquist plot was composed of nearly only one capacitive arc due to the corrosion reaction, and the corrosion rate during the same cycle increased with time. Additionally, the EIS features of the entire corrosion processes of 2024-T3 aluminum alloy were interpreted theoretically.

Corrosion Behavior of 55mass%Al-1.6mass%Si-Zn Alloy in Wet-Dry Cyclic Environment

The corrosion of 55mass%Al-1.6mass%Si-Zn (GL) alloy has been tested under a wet-dry cyclic condition using an AC impedance technique to clarify its corrosion behavior. The results have been compared with that of a zinc sample exposed under identical condition. The sample was exposed to alternate conditions of immersion for 1 h in a 0.05 kmol/m 3 NaCI solution and drying for 7 h at 298 K and 60 % RH, and the corrosion rate was monitored for 10 d (30 cycles) using a two-electrode type probe. Simultaneously, the corrosion potential was continuously monitored using a special arrangement in combination with a Luggin probe tip. From the monitoring results of the corrosion rate and corrosion potential and the polarization curve, it was found that the corrosion rate of GL was decreased by inhibition of the cathodic reaction. The impedance response was evaluated by using an equivalent circuit considering the charge and mass transfer consequences of the coupled corrosion processes. It was found from the time constant of the diffusion impedance that the corrosion product layer formed on GL alloy acted more effectively as a diffusion barrier against oxygen transport compared to zinc.

Electrochemical impedance study on galvanized steel corrosion under cyclic wet–dry conditions––influence of time of wetness

Electrochemical impedance technique has been applied to study the corrosion behavior of galvanized steel under wet–dry cyclic conditions with various drying periods. The wet–dry cycles were carried out for the period of 336 h by exposure to alternate conditions of 1 h immersion in a 0.5 M NaCl solution and drying for various time periods (11, 7 and 3 h) at 298 K and 60% RH. During the wet–dry cycles, the polarization resistance, R p, and solution resistance, R s, were continuously monitored. The instantaneous corrosion rate of the coating was estimated from the obtained R p −1 and time of wetness was determined from the R s values. The corrosion potential, E corr, was also measured only during the immersion period of each wet–dry cycle. In all cases, the corrosion was accelerated by the wet–dry cycles in the early stage, and started to decrease at a certain cycle and finally became similar to that at the initial cycle. The underlying steel corrosion commenced after the corrosion rate started to decrease. The shorter drying period in each cycle led to higher amount of corrosion of the coating because the surface was under wet conditions for longer periods. On the other hand, time to red rust appearance due to occurrence of the underlying steel corrosion became shorter as the drying period increased, although the total amount of corrosion was smaller. The corrosion mechanism of substrate steel under various drying conditions has been discussed, the galvanic coupling effect being taken into account.

Monitoring of water uptake in organic coatings under cyclic wet–dry condition

The absorption of water in the double-layer coatings, comprising epoxy and urethane layers, was monitored under a cyclic wet–dry condition using AC impedance method. Four types of double-layer coatings were prepared according to coating order, namely, epoxy–epoxy (EE), urethane–epoxy (UE), epoxy–urethane (EU), and urethane–urethane (UU) double-layer coatings. The coated specimens were exposed to the alternate conditions of 4 h immersion in a 0.5 M NaCl solution and 8 h drying at 25 °C and 60% RH. The capacitances of coatings were monitored during exposure and the amounts of water absorbed in coatings were estimated from the obtained capacitance. The mechanism of corrosion beneath organic coatings under cyclic wet–dry exposure in chloride-containing environments was discussed on the monitoring results of water uptake.

Influence of Copper on Iron Corrosion in Weakly Alkaline Environment Containing Chloride Ions

The influence of copper content on the corrosion of iron has been investigated to make possible the use of scrap as a source material for reinforcing steel in concrete structures. An AC impedance technique was used to monitor the corrosion of iron, Fe-0.4 mass%Cu and Fe-3 mass%Cu undergoing cyclic wet-dry conditions with a 1 hour immersion in a pH 10 solution of Ca(OH) 2 containing 0.01 M NaCl followed by a 3 hour drying at 298 K and 50% RH. The corrosion rate of iron is greatly accelerated by wet-dry cycles. This is because active FeOOH species produced by the oxidation of Fe(II,III)oxide in air during drying act as very strong oxidants that promote corrosion in the wet condition. In each cycle, shortly before the surface dried out, a large increase in the corrosion rate was observed. This can be explained by the acceleration of oxygen transport through the thin electrolyte layer. Fe-0.4 mass%Cu showed a similar trend to iron. On the other hand, Fe-3 mass%Cu showed much lower corrosion rates and the corrosion was not accelerated by wet-dry cycling. Monitoring showed that the addition of 0.4 mass%Cu does not diminish the corrosion resistance of iron and the addition of 3 mass%Cu improves it under these wet-dry conditions. Energy Dispersive Analysis of X-ray (EDX) results indicated enrichment of copper in an inner rust layer after exposure to 56 wet-dry cycles.

Electrochemical corrosion monitoring of galvanized steel under cyclic wet–dry conditions

The corrosion behavior of a galvanized steel was investigated under cyclic wet–dry environments using electrochemical techniques. The wet–dry cyclic was conducted by exposure to alternate conditions of 1 h-immersion in a 0.05 M NaCl solution (or Na 2SO 4 solution) and 7 h-drying at 60% RH. The polarization resistance R p of the galvanized steel was monitored during the wet–dry cycles by AC impedance method. Simultaneously, the corrosion potential E corr was measured only when the specimen was immersed in the solution. The corrosion current density i corr of the zinc coating was estimated from the monitored R p using Stern–Geary equation. The corrosion mass loss Δ M, which was obtained from the i corr vs time curve, was plotted vs the wet–dry cycle number. The average corrosion rate per cycle starts to decrease and the E corr shifts in the noble direction, immediately before the red rust (FeOOH) appears on the coating surface. It seems that the underlying steel corrosion commences when the zinc coating does not act as sacrificial anode due to accumulation of zinc corrosion products on the coating surface. The electrochemical methods, such as impedance and corrosion potential measurements, are an effective tool for monitoring the degradation of metallic coating under atmospheric environments.

Corrosion Monitoring of Zn and Zn–Al Coated Steels under Wet-Dry Cyclic Conditions Using AC Impedance Method

AC impedance method has been applied to evaluate the performance of Zn, Zn-5%Al and Zn-55%Al coated steels under wet-dry cycles. The wet-dry cycles were carried out by exposure to the alternate conditions of 1 h-immersion in a 0.5 or a 0.05 kmol/m3 NaCl solution and drying for various times at 298 K and 60%RH. The impedances at two frequency points, a high frequency (10 kHz) and a low frequency (10 mHz), were monitored during the cyclic wet-dry exposure. The reciprocals of the high frequency and low frequency impedances were taken as the solution conductance of water layer on the coating surface and the corrosion rate of the coated steels, respectively. The impedance monitoring results showed that the Zn-55%Al coating exhibited the most excellent corrosion resistance under the given conditions. This is attributed to the formation of aluminum oxide on the specimen surface. On the other hand, the corrosion resistances of Zn coated and Zn-5%Al coated steels were almost the same under relatively wet and highly chloride-concentrated conditions. An increase in the chloride concentration of the test solution employed during the wet-dry cyclic test decreased the average corrosion rate in all cases. The average corrosion rate of the Zn-55%Al coating under wet-dry cyclic condition in 0.5 kmol/m3 NaCl solution was lower than that in 0.05 kmol/m3 NaCl by one order of magnitude. The combined wet-dry cyclic exposure test and AC impedance corrosion monitoring is very useful for the mechanistic study of the corrosion of coated steels, as well as the evaluation of their corrosion performance, in marine atmospheric environments.

AC impedance monitoring of pitting corrosion of stainless steel under a wet-dry cyclic condition in chloride-containing environment

A study of the corrosion of stainless steels under a wet-dry cyclic condition has been carried out using the AC impedance method. Three types of stainless steel were exposed to the alternate conditions of 1 h immersion in chloride containing solution and 5 h drying at 25 °C and 60% RH. The corrosion rates were monitored continuously during exposure using an AC impedance monitoring technique. Simultaneously, the current distribution over the working electrode during impedance measurement was analysed from the phase shift Θ in an intermediate frequency. The results showed that corrosion rate monitoring using an AC impedance method is possible under the given exposure conditions even during the drying period when the metal is covered with a thin electrolyte layer. The different processes of passivation, pitting corrosion and repassivation could be distinguished from the monitored impedance data. It was found that in the wet-dry cyclic condition in the chloride-containing environment, pit generation and growth occur preferably just before the perfect drying of the metal surface, and repassivation occurs preferably when the surface dries out and is re-immersed in the solution.

An electrochemical impedance study on atmospheric corrosion of steels in a cyclic wet-dry condition

The corrosion rates of steels with different nickel contents (0, 2.5, 10 and 20 wt%) were monitored by the AC impedance method under a cyclic wet-dry condition, which was conducted by exposure to alternate conditions of 1 h immersion in a 0.05 M NaCl solution and 7 h drying at 60% RH and 25 °C. The corrosion rates of the ordinary carbon steel and the 2.5% Ni containing steel were greatly accelerated by the wet-dry cycles, while those of steels containing at least 5% Ni were only slightly affected. AC impedance tests indicated that the addition of 5% Ni greatly reduced the corrosion rates of the steel exposed to the cyclic wet-dry environments containing chloride ions, in good agreement with one year exposure tests in the atmospheric marine environments. Within the wet-dry cycle for the carbon steel, a gradual increase in the corrosion rate and a shift of the corrosion potential to negative values were observed at the initial stage of the drying period. These indicated that the anodic metal dissolution rate was accelerated by a slight increase in the chloride ion concentration. At the intermediate stage of drying, the corrosion rate rapidly increased and the corrosion potential remained constant. This can be attributed to the acceleration of both the anodic metal dissolution process and the cathodic oxygen reduction process which is usually controlled by the rates of O 2 transport through the thin electrolyte film. At the final stage, just before the surface dried out, the corrosion rate rapidly decreased, shifting the corrosion potential to the noble direction. At this stage, the anodic process was greatly inhibited. The corrosion testing system employed in this study is very useful for the rapid evaluation of the corrosion performance of metals in atmospheric marine environments and for mechanistic study.

Electrochemical Impedance Response of Zn and Galvanized Steel Corroding under Marine Atmospheric Environments

Impedance response of zinc and galvanized steel samples exposed in wet-dry cyclic condition using 0.05 M NaCl solution has been analyzed in order to evaluate their corrosion behavior. The wet-dry cyclic test was performed by exposing sample to alternate conditions of 1 h immersion in 0.05 M NaCl solution and 7 h drying at 298 K with relative humidity (RH) of 60% for 10 days consisting of 30 cycles. After initial exposure to corrosion test, samples showed the evolution of two capacitive loops. However, a sharp decrease of characteristic frequency, characterized by an increase of double layer capacitance in the rage of several mF/cm2, with the progress of corrosion was observed. This was explained by taking into account the coupling of characteristic frequencies of the two capacitive loops due to deposition of thick layer of porous rust. The parameters of diffusion impedance for the cathodic process were estimated from the low frequency capacitive loops attributed to finite Warburg diffusion impedance. A sharp increase of impedance after several days of corrosion in the case of galvanized steel was attributed to the dissolution of Fe-Zn alloy layer that is formed during galvanizing process.Keywords: galvanized steel, zinc, atmospheric corrosion, wet-dry cyclic test.DOI: 10.3126/jncs.v23i0.2094J. Nepal Chem. Soc., Vol. 23, 2008/2009 page: 33-42

Corrosion Monitoring of Galvanized Steel in Simulated Wet-Dry Cyclic Condition in 0.05 M NaCl Solution

The corrosion monitoring of galvanized steel in 1 h wet and 7 h dry cyclic condition in 0.05 M NaCl has been carried out using AC impedance technique. During the wet-dry cycles, the polarization resistance, Rp, and solution resistance, Rs, were continuously monitored. The instantaneous corrosion rate of the coating was estimated from the obtained Rp-1 and time of wetness (TOW) was determined from the Rs values. The corrosion potential, Ecorr, was measured twice a day during the immersion period of wet-dry cycle. The corrosion was accelerated by the wet-dry cycles in the early stage followed by a decrease in the final stage of wet-dry cycle. The corrosion potential shifted to more negative direction for the first 8 cycles due to dissolution of air formed oxide. A gradual shift in the corrosion potential with the progress of corrosion in the later stage of the wet-dry cycle was observed due to deposition of corrosion products. The corrosion mechanism of galvanized steel is discussed based on the results of corrosion monitoring and corrosion potential variation. Key words: Atmospheric corrosion; Galvanized steel; AC Impedance; Corrosion rate. DOI: 10.3126/sw.v8i8.3855 Scientific World Vol.8(8) 2010 pp.76-80