In order to clarify the effect of environmental factors on the atmospheric corrosion of steel, novel model for predicting the reduction of atmospheric corrosion considering relative humidity and rain falls was developed. We conducted a one-year calculation simulation of atmospheric corrosion in Miyakojima City, Choshi City, and Tsukuba City using the developed model. Corrosion weight loss by the simulation could reproduce the measured value well. Corrosion weight loss was greatly affected by the amount of airborne sea salt, relative humidity, and rain falls.
A combined cycle corrosion test including the conditions simulating rainfall was conducted to investigate the effect of rainfall on the atmospheric corrosion behavior of carbon steel. Atmospheric corrosion sensors developed were used as the electrodes, and the corrosion rate during the corrosion test was monitored by an electrochemical impedance method. The corrosion rate was monitored by continuously measuring the impedance at low frequency (10 mHz) and high frequency (10 kHz) at 5-minute intervals. The corrosion rate of corrosion sensor exposed to the corrosive environment without a rainfall process showed a high value in a salt spray process, and dropped sharply after proceeding to the drying process. The salt deposited on the sensor surface absorb moisture in the wet process, resulting that the corrosion rate increased again. This change in corrosion rate was almost the same at the beginning of corrosion and after the formation of corrosion products. The corrosion sensors exposed to rainfall process gave very high corrosion rates after rainfall. Although The corrosion sensor with the rainfall process for 1 minute showed a higher corrosion rate in the early stage of corrosion, there was no difference in the corrosion rate due to rainfall time after the formation of corrosion products. Analysis of corrosion products showed that the attached salt was washed away by rainfall. However, in this study, the wetting due to rainfall greatly affected the corrosion rate of carbon steels.
Raman micro-spectroscopy was conducted for in-situ observation of atmospheric corrosion occurring on carbon steel surfaces, on which NaCl nanoparticles with a size of < 1 µm were deposited, under the controlled relative humidity at room temperature. Increasing relative humidity induced the formation of amorphous-FeOOH at a micro-droplet of NaCl solution where an NaCl nanoparticle was dissolved on the steel surface, and the initiation time of the corrosion generation decreased with increasing relative humidity. Drying treatment at 100ºC for 600 s resulted in modification of the surface condition of carbon steels and obstructing the corrosion initiation at high humidity conditions but reducing the initiation time at low humidity conditions.
Pitting corrosion behavior of SBHS500 steel in boric-borate buffer solutions containing chloride ions was investigated by macroscale and microscale polarization, immersion tests, optical microscopy, and scanning electron microscopy. Calcium sulfide inclusions (CaS) existed in the SBHS500 steel. When the specimen was immersed in a boric-borate buffer solution (pH 8.0) containing 10 mM NaCl for 24 h at 25°C, the steel matrix was not corroded. However, partial dissolution of the CaS inclusions was observed. Pitting occurred after the wet-dry corrosion test, and calcium and sulfur were detected near the center of the pit. From the results of the microscale polarization measurements, the pitting initiation sites for the SBHS500 steel were determined to be the CaS inclusions. No pitting was observed at the microscale electrode area without inclusions. In a boric-borate buffer solution containing 10 mM NaCl, the depassivation pH at the microscale electrode area without inclusions was 6.0. The depassivation at the microscale electrode area with the CaS inclusions occurred at approximately pH 6.6. The CaS inclusions in the SBHS500 steel were found to be a trigger of the depassivation of the steel matrix surrounding the inclusions.
One of the controlling factors for atmospheric corrosion is oxygen diffusion behavior in thin solution layer formed on metals. The oxygen diffusion limiting current in thin solution layer formed on Zn electrode was measured electrochemically to clarify the influence of solution layer thickness, temperature difference in the solution layer on the oxygen diffusion behavior. A solution of 10 mM NaCl was used as electrolyte to form solution layers. The solution layer thickness was controlled by masking adhesive tape with through holes. The environmental temperature, TE, was controlled by circulating temperature-controlled water and the sample temperature, TS, was controlled by a Peltier device. At TE = TS, the oxygen limiting current was independent of solution layer thickness. At TE < TS and the solution layer thickness thinner than 0.6 mm, the oxygen diffusion limiting current increased with thinning of the solution layer. The results suggest that TE and TS are also take into account as estimation of steel corrosion rate at snowy and cold region.
Mass gain rates of various steels during atmospheric corrosion under cyclic conditions of dry and controlled-humidity air were investigated. Fe, SBHS500 carbon steel and SMA490AW weathering steel were selected for the test materials. A droplet of MgCl2 solution was set on the plate steel specimen. The specimen was exposed in the humidity-controlled air for about 82.8 ks and in the dry air for 3.6 ks. After the process, an area as well as a volume of the corroded part, surface appearance and a mass of the specimen were recorded. The process was repeated over 5000 ks of an accumulated wet time. Selected values of relative humidity were 75, 43 and 33%. For the Fe specimen, the condition of RH75% induced Region(i), (ii) and (iii), where Region(i) is the period in which a corroded area was independent of time and a mass of the specimen linearly increased with time, Region(ii) is the period in which an area and a mass increased more rapidly, and Region(iii) is the period in which increments of an area and a mass relatively decrease. The conditions of RH43% induced only the Region(i) and RH33% provided Region(ii) in addition to (i). A mass gain rate in Region(i) was larger under RH75% than those under RH43 and 33%. For the condition of RH75%, any of the three specimens showed the three regions, and a mass gain rate in Region(i) was a maximum for SBHS500 steel and a minimum for Fe.
Steels are widely used for structural material of bridges. Some of bridges have been used for more than half century, and degradation of bridges become severe problems, due to the atmospheric corrosion of metallic materials. Especially, atmospheric corrosion of steel is caused by cycles of wet and dry conditions, and accelerated by evaporation of raindrops containing Cl- ions. In this study, corrosion of pure iron and steels is investigated under wet-dry cycling condition with NaCl solution by scanning electron microscopy (SEM) and 3D-optical microscopy (3D-OM). Initially 20 mm3 of 0.02 M-NaCl solution was dropped on pure Fe, SM490Y, and SMA490AW specimens, and then, droplets of water were dropped at the same position of the specimens at 9 min intervals for 150 cycles.
All specimens were covered with white, black, and reddish corrosion products at the water-dropped position, and the reddish ones became major with increasing water-dropping cycles. SEM images after corrosion product removal showed pitting corrosion on all specimens, and the corrosion was more severe at the edge areas of water-dropped position than at the center areas. 3D-OM obtained after 150 cycles showed that the deepest pit produced at the edge areas were in the order of Fe >> SM490Y = SMA490AW, and that the total volume loss at the edge areas by corrosion were in the order of Fe > SMA490AW > SM490Y. The corrosion mechanism can be explained by higher rates of O2 supply at the edge areas and denser corrosion products on steel than on Fe.
The test sample composed of 100 iron wires of 1 mmφ in diameter in the arrangement of 10 × 10 matrics was exposed to the temperature cycling between 0 and -20 °C. 0.1 w% NaCl solution was dropped on the surface to form an ice droplet, and the coupling current of each iron electrode against the other 99 electrodes was sequentially measured to obtain a coupling current map. The averaged coupling current of 100 electrodes fluctuated with temperature cycling. In the absence of an ice droplet, the coupling current increased at the relative humidity higher than ca. 65%, which was similar with atmospheric corrosion at the temperature higher than the freezing point. When an ice droplet exists, the coupling current increased with increasing temperature rather than the relative humidity. This behavior was interpreted that the thin solution layer of concentrated NaCl solution was formed at the interface between the electrode surface and ice due to the exclusion of NaCl from the growing ice crystal of pure water. In the coupling current map, an inner area of iron electrodes beneath the ice droplet tended to be a cathode, while an outer and a surrounding area tended to be an anode. An open circuit potential map was also measured using a quasi-Ag/AgCl electrode placed on the sample surface. The potential of the inner area was less noble against the outer and surrounding area and shifted with temperature cycling. The ice droplet shrank in the temperature cycling and left rust on the surface.
Effects of metal cations on the corrosion behavior of carbon steel SM490Y in 10 mM Cl‐ model fresh water were investigated by immersion tests, electrochemical tests, and surface analysis. Immersion tests and electrochemical tests results showed that Zn2+ had a significant inhibition effect on corrosion of SM490Y. The results of EDS and XPS showed that Zn2+ was incorporated in the oxide films by making a strong bond and formed zinc hydroxide or zinc oxide which can cover the surface of carbon steel. It is suggested that Zn2+ forms a protective film which can efficiently prevent break down of surface film by Cl‐ and inhibit the corrosion.
Eight thin iron wires were embedded in a cement test block to evaluate the corrosion behavior during the carbonization of cement by the CO2 gas absorption. The gas supplied to the chamber containing the cement test block was subsequently changed as air, CO2, and air. Electric resistance measurement of thin wires revealed that the iron kept passivated during supplying of air and uncorroded during the CO2 supply to promote carbonization and began to be corroded from the surface side when the air was resupplied after carbonization due to the existence of oxygen in the air as an oxidizing agent. A series of two-electrode impedance between the selected two iron wires were measured as a function of depth from the surface. The impedance of all electrodes dropped considerably several hours after the CO2 supply. That was caused by the difference in the immersion potential of iron wires as a function of depth, i.e., pH transition from alkaline to neutral range. As a result, iron wires were polarized when electrically connected to each other during the progress of carbonization. When the carbonization was completed, each iron wire showed the impedance response corresponding to the corrosion condition depending on the depth.
This paper presents an introduction of the relationship between the electrochemical properties of microstructures and pitting corrosion resistance of carbon steels in chloride-containing near-neutral pH environments. Recent investigations by micro-scale electrochemical measurements have been demonstrated that the pitting corrosion resistance of typical microstructures was ordered as follows: (high) as-quenched martensite > primary ferrite > pearlite (low). In the case of pearlite, it has been reported that pits proceeded along the lamellar structure consisted of Fe3C and ferrite. On the other hand, in the case of martensite, according to the studies based on the first-principles calculations, it has been proposed that the superior corrosion resistance was related with the electronic interaction between Fe and interstitial C. It was reported that the electronic density of states of Fe around the Fermi level decreased by the presence of interstitial C.