(Invited) Corrosion Process of Metals below the Freezing Point

Abstract

Atmospheric corrosion has been intensively investigated using an acceleration test using a temperature/humidity cycling in a laboratory to investigate the corrosion mechanism and the outdoor exposure test to evaluate the actual corrosion progress in a real environment. Corrosion of metals in a cold district is recently paid attention because a considerable amount of anti-freezing agents are sparged to the driveway in the winter season and cause severer corrosion of vehicles and structures. Repetition of freezing/melting of water also induces physical stress to the material and influences, for example, the mass-transportation process related to the corrosion reaction. In this study, therefore, the corrosion behavior of metals in the freezing condition was investigated both the outdoor exposure test and the laboratory test. In the laboratory test, low-temperature cycling was applied to the galvanic couples of Zn-steel plate cold commercial (SPCC) pair. A pair was set in a sample holder with a gap distance of ca. 0.1 mm between two metal plates and placed in a small box to avoid the direct blowing in a temperature-controlled chamber. Basic temperature cycling was composed of 4-stages as (i) low temperature (T L = –20°C, 1 h), (ii) transition from T Lto T H (1 h) (iii) high temperature (e.g., T H = 0, 1 h), and (iv) transition from T H to T L (1 h),. Relative humidity (RH) in a chamber was not actively controlled. The T and RH presented in this paper were measured by using a T & RH sensor close to the sample. At T L, 0.1% NaCl solution of ca. 1 cm3 was dropped at the gap that was immediately frozen. Fig. 1(a) shows the typical galvanic current i g in a single T-cycle observed at the gap covered with ice. In this condition, the ice was not almost melted at T H. The i g increased with T but not dependent on RH because the current flowed mainly under or around the ice. The considerably large i g observed under the ice proposed that the ionic conductivity was maintained even at the freezing temperature as –20 °C. Probably a thin liquid layer of concentrated NaCl solution was formed at the interface between the metals and ice due to the exclusion of salt from the solid ice phase during the freezing process. The presence of the liquid phase was also investigated by the different experiments of the impedance measurement between two Pt electrodes in the freezing condition. After many T-cycles, T H was raised to 2 °C to accelerate the melting of ice and, thus, the evaporation of melted water. After the many cycling, the surface was almost dried and covered with a mixture of corrosion products and salt. Fig. 1(b) shows the i g in this condition. The i g flowed at T L where the RH was rather high at stage i, increased with increasing T at stage iv, and dropped with lowering RH at T H in stage iii. This behavior was similar to that observed in the ordinal wet, and dry test conducted higher than the melting point. From these results, the atmospheric corrosion process in the cold district includes the ordinal atmospheric corrosion on the dry area and wet-corrosion in the quasi-solution under the ice. Figure 1