Blistering of resin-coated steel is accelerated by a temperature difference across the resin. The time for blistering of epoxy-coated steels was measured under three temperature different conditions. The investigated factors were temperature difference, thickness of epoxy coating, quantity of chromate coating under epoxy coating. The time for blistering shortened with an increase in temperature difference, and lengthened with increases in film thickness and quantity of chromate coating. An experimental equation was derived from the the view points; (1) temperature difference and epoxy film thickness affect the rate of water permeation, (2) chromate coating affects the strength of the bonding of the epoxy coating to the steel. The derived equation shows that the time for blistering is a function of temperature gradient (=temperature difference/film thickness) and quantity of chromate coating. The time for blistering can be estimated by this equation under temperature different conditions.
The ACM (Atmospheric Corrosion Monitor) sensors each of which consists of a Fe/Ag-galvanic couple were exposed to marine atmosphere for one year, for the purpose of evaluating the corrosivity of the site. The outputs of the sensors set up at Shimizu in Shizuoka Prefecture, which is regarded as the most moderate of the four exposure sites, were obtained every 10 minutes with temperature and relative humidity, RH. The sensor outputs were analyzed by distinguishing the time for the durations of dew, rain and dry periods in a day. The time durations for dew period and rain period at Shimizu were found to be 3454h and 1705h respectively throughout the year from 1992.7 to 1993.6. Critical humidity for dew, RH*, decreased exclusively from 80% to 50% with the increase of amount of deposited sea salt, Ws. It is thus suggested that the ISO recommendation method is not adequate by which the time of wetness is defined as the time when RH value is at and above 80%. The frequency data of dew period and dry+dew period against RH measured by the ACM sensor will be available to study the rusting for stainless steels. It is considered that corrosivity of each site can be estimated effectively by analyzing the above mentioned factors, especially amount of deposited sea salt, obtained by this ACM sensor.
A three-dimensional boundary element method (3D-BEM) was developed to estimate quantitatively catholic protection and macro-cell corrosion. To confirm the validity and usefulness of the BEM for analysis of fluid machines handling seawater with complex 3D fields, experiments and analyses were performed. A cast iron vertical pump with Zn anodes for cathodic protection, was submerged in seawater and operated. Potential distributions inside the pump and anodic currents on the Zn anodes were measured. The polarization curves of the pump material were measured as functions of flow rate, time and temperature, and the polarization characteristics were applied as boundary conditions in performing the BEM analysis. Through the analyses and experimental works, the following conclusions were obtained. By means of appropriate modelling that takes account of the symmetry of the object being analyzed, it is possible to apply the BEM effectively to corrosion problems of machines with complex 3D fields. Furthermore, extremely high accurate analysis on potential and current density distributions can be performed for fluid machines handling seawater, by exactly ascertaining the dependency of the polarization curves on flow rate, time and temperature, and reflecting these dependencies in the boundary conditions.
Copper tubes suffer from severe localized attack during storage after rinse and de-greasing process in organochlorine solvents. This type of corrosion is triggered by condensed organic acid vapor, e.g. acetic and formic acids that are decomposition products of the solvents used for rinsing and de-greasing of copper tubes. Because of morphological features of tunneling and branching of pits, it is often referred to “ant-nest” corrosion. Laboratory simulation has been carried out for the decomposition of solvents and condensation of acids. The so-called ant-nest corrosion has been reproduced in copper tube samples exposed to wet atmosphere containing formic and/or acetic acid vapor. A mechanism is proposed for the generation of this type of corrosion.
Copper corrosion in a simulated acid rain has been investigated by the electrochemical impedance analysis, the galvanostatic catholic-polarization, FT-IR and ICP-AES. Effects of pH and the Cl- concentration on the rate and the mechanism of the corrosion were studied adding H2SO4 (or NaOH) and NaCl to the simulated acid rain respectively. The corrosion rate increased with decreasing pH, and with increasing Cl- concentration. Warburg impedance resulting from a diffusion process for dissolved oxygen was observed at pH below about 3. The rate determining step of corrosion was found to be a film formation/dissolution process at pH≥3.5, and to be a diffusion process for dissolved oxygen at pH≤2.9. The film thickness increased with Cl- addition to the simulated acid rain, but the film formed at the Cl- concentration of 1000mg·dm-3 was thinner than that of 100mg·dm-3. This decrease in the film thickness at the Cl- concentration of 1000mg·dm-3 may result from the formation of CuCl whose solubility is much greater than that of Cu2O which is one of the major components of the film at low Cl- concentrations. The corrosion rate is thought to be controlled by CuCl2- diffusion when the Cl- concentration is high.
The ionic electrode potential is defined by the real potential of potential-determining ion in the electrode. i.e. the real free energy for the ion transfer from the point at the outer potential of aqueous solution to the point inside the eletrode. Provided that the electrode has both electronic and ionic energy levels, the ionic and the electronic electrode potentials are in the same scale with different zero levels, and hence either can be applied to ion transfer electrodes. However, only the ionic potential can be applicable to the electrodes that have no electronic energy level in the range of practical interest, such as ionic solid electrodes and electronically nonconductive polymer membrane electrodes. The equilibrium electrode potential of oxide-covered metal electrodes corresponds either to the oxide fomation potential of the metal electrode with ion transfer equilibrium at the metal/oxide interface, or to the flat band potential of the oxide electrode with electron transfer equilibrium at the metal/oxide interface. Two-layered oxide film on metal electrodes, consisting of lower and higher valence oxides, involves an irreversible potential difference in the inner layer of lower valence oxide.