Atsushi Nishikata

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

Abstract 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, 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 was determined from the Rs values. The corrosion potential, Ecorr, 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.

An application of electrochemical impedance spectroscopy to atmospheric corrosion study

Abstract An electrochemical impedance spectroscopy (EIS) technique has been applied to estimate the corrosion rates of metals covered with a thin electrolyte layer. A two electrode cell system, which consists of a pair of identical metal electrodes embedded in parallel in epoxy resin, was used for measuring the corrosion rates. The impedance measurements for Type 304 stainless steels covered with an NaCl solution layer and ordinary carbon steels with an H 2 SO 4 solution layer, 10–1000 μm in thickness, were carried out in the frequency range of 10 mHz–10 kHz to determine the equivalent circuit of a metal-thin electrolyte layer interface and the influence of current (potential) distribution over the electrode surface on the EIS data. It was found that the obtained EIS data can be described by a transmission line (TML) model, in which the current distribution over the electrode surface is considered. The analysis of current distribution based on a TML model demonstrated that EIS data give information on the current distribution under the application of AC voltage. If the phase shift θ goes further than −45 ° on a plot of θ vs logf(f—frequency) the current distribution becomes uniform at least in the low frequency limit, and consequently an accurate corrosion rate can be determined from the obtained polarization resistance. The impedance of copper covered with an Na 2 SO 4 solution was measured to investigate the influence of the thickness of the electrolyte layer on the atmospheric corrosion rate. For copper covered with a neutral Na 2 SO 4 solution of pH 6, the corrosion rate was independent of the thickness at least down to the thickness of 10 μm. whereas the corrosion rate of copper covered with acidic solutions of pH 3 indicated a maximum at a thickness of several 10s of micrometers.

Pitting corrosion mechanism of stainless steels under wet-dry exposure in chloride-containing environments

Abstract Two types of stainless steel were exposed to the alternate conditions of 1 h immersion in chloride-containing solution and 7 h drying at 30 °C and 67% RH. The corrosion rates and corrosion potentials were monitored simultaneously and continuously during exposure by using combined AC impedance and corrosion potential monitoring techniques in order to study the mechanism of pitting corrosion of stainless steels under a marine atmospheric environment. The results show that pitting is initiated preferentially during the dry cycle after a critical chloride concentration is reached. Pit initiation is followed by a shift of the corrosion potential to about −400 mV (SSE) as growth commences. Afterwards, as the specimen surface dries up, repassivation commences, and it may continue towards the next wet cycle if perfect drying within the active pit is not permitted. Furthermore, it was found that the presence of thin solution layers in atmospheric corrosion contributes to the enhancement of pit initiation and growth. A mechanism was proposed to explain the pitting corrosion of stainless steel in a marine atmospheric environment.

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 O2 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.

Degradation mechanism of galvanized steel in wet–dry cyclic environment containing chloride ions

Abstract The wet–dry cyclic test of a galvanized steel (GI) and pure zinc (ZN), which simulates marine atmospheric environment, has been conducted to clarify the degradation mechanism of galvanized steel. The samples were exposed to alternate conditions of 1 h-immersion in a 0.05 M NaCl solution and 7 h-drying at 25 °C and 60%RH, and the corrosion was monitored for 10 days (30 cycles) using a two-electrode type probe. Simultaneously, the corrosion potential was measured every three cycles only during the immersed conditions. The reciprocal of polarization resistance R p −1 was taken as an index of the corrosion rate. Several sample plates of GI and ZN were exposed, together with the monitoring probes. They were removed from the test chamber at the end of 1st, 3rd, 9th, 18th, and 30th cycles of exposure and were analyzed for the corrosion products with XRD and laser Raman spectroscopy. Further, their cross sections were analyzed with FESEM–EDS. The FESEM photographs and elemental analysis of cross sections confirmed that the R p −1 value commences to decrease when the corrosion front reaches Zn–Fe alloy layers (boundary layers of zinc coating and steel substrate) due to localized nature of attack. A schematic model of degradation mechanism and the role of galvanic protection have been discussed.

Electrochemical impedance spectroscopy of metals covered with a thin electrolyte layer

The impedance behavior of metals covered with a thin electrolyte layer has been investigated. A transmission line (TML) circuit type behavior, which was attributed to an uneven current distribution profile over a working electrode, was observed in a wider frequency range as the thickness and the concentration of the electrolyte were decreased. It was found from the theoretical analysis based on the TML model that the current distribution profile becomes uniform at least in the low frequency limit when the phase shift θ goes further than −45 ° on a plot of θ vs. log f(f = frequency). In such a condition, an accurate corrosion rate can be determined from the obtained polarization resistance Rp. The EIS has been applied to investigate the dependence of the atmospheric corrosion of stainless steel and of copper on the thickness of the electrolyte layer. The corrosion rate of stainless steel covered with an NaCl solution layer was independent of the thickness at least down to the thickness of 10 μm because it was controlled by the anodic process. The corrosion rate of copper covered with a neutral Na2SO4 solution was independent of the thickness at least down to 10 μm, whereas the copper covered with an acidic solution of pH 3 showed a maximum at a thickness of several tens of micrometers.

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

Abstract 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.

AC impedance study on corrosion of 55%Al–Zn alloy-coated steel under thin electrolyte layers

Abstract The corrosion behavior of 55%Al–Zn alloy-coated steel under a chloride-containing electrolyte layer ranging from 15 to 888 μm in thickness has been investigated using electrochemical impedance spectroscopy (EIS). The interface of the coating-electrolyte layer can be represented by an equivalent circuit consisting of a solution resistance, a charge transfer resistance, a double layer capacitance and a Warburg element. The corrosion current density estimated from the polarization resistance significantly decreases as the thickness of electrolyte layer increases between 15 and 100 μm and is almost independent of the thickness up to 888 μm. Furthermore, corrosion monitoring of 55%Al–Zn alloy-coated steel has been performed under alternate conditions of 1 h-immersion in solution of 0.05 M NaCl or 0.05 M Na 2 SO 4 and 7 h-drying at 298 K and 60% RH. During the wet–dry cycles, the instantaneous corrosion rate was monitored by AC impedance method together with the corrosion potential. The corrosion rate increases at the initial stage of the wet–dry cycles because the native oxide on the coating surface is dissolved into the solution and converted to hydrated zinc and aluminum oxides, and then decreases slowly owing to the accumulation of the corrosion products over the coating surface. The corrosion mechanism of 55%Al–Zn coating is discussed on the basis of the monitoring results.

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

Abstract 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.

Initial Stage of Pitting Corrosion of Type 304 Stainless Steel under Thin Electrolyte Layers Containing Chloride Ions

The initiation of pitting corrosion of type 304 stainless steel under a thin electrolyte layer containing chloride ions has been investigated. 1 M MgCl 2 solution layers with four different thicknesses of 1000, 500, 125, and 25 μm were placed on the stainless steel electrode surface and then dried at different rates at 300 K. During drying, the corrosion potential E c o m was monitored to determine the time when the pitting corrosion commenced. A solution resistance R s of the thin electrolyte layer was simultaneously measured to monitor the concentration of chloride ion. From both monitoring data of E c o r r and R s , critical concentration of chloride ion [Cl - ] p i t for pit growth under thin chloride solution layers during drying was determined. The pitting corrosion mechanism of type 304 stainless steel during the drying process is discussed on the basis of these results.