Two kinds of steel rusts were characterized by N2 adsorption. The artificial rusts of α-, β-, γ-FeOOH, Fe3O4 and poorly crystallized iron oxide were synthesized from aqueous solutions of iron salts dissolving different metal ions such as Cu(II), Ni(II), Cr(III) and Ti(IV). Another series of rusts were obtained by exposing five kinds of weathering steels in various environments for 17 years. The specific surface area of these rust particles reflecting the particle size was obtained from the adsorption isotherms of N2. The particle size of synthesized rusts was increased by the addition of the metal ions, depending on the kinds of steels and metal ions. The particle size of weathering steels was affected by the exposure environment but not by the kind of weathering steels. Irrespective of the kind of steels, the atmospheric NaCl increased the size of rust particles and the particle size increased with the increase of corroded amount. It was shown that the gas adsorption method gives useful information on the steel rusts.
Although it is the problem which makes atmospheric corrosion familiar and has many subjects on the occasion of practical use of materials, conventional electrochemical measurement is difficult to apply to the evaluation. Many measurements have been tried to evaluation of the atmospheric corrosion. In this description, the monitoring technology and the surface observing method in evaluation of atmospheric corrosion was introduced mainly. The effectivity and application limits of exposure tests, ACM sensor method, AC impedance method and QCM were explained as the monitoring technique. The ion penetration resistance measurement and the ion selective permeability evaluation of the rust film on the steel used for atmospheric corrosion evaluation of steel materials was introduced from sample fabrications as experimental method to the analysis. The importance of characteristic of a corrosion product was also explained. Furthermore, the development of surface visualization technology and expectation of application to evaluation of the atmospheric corrosion were indicated by explaining the effectivity of the surface potential measurement using the Kelvin probe method and the AFM as a surface observing method, and the surface pH distribution measurement.
An ACM (Atmospheric Corrosion Monitor) type corrosion sensor, consisting of a Fe-Ag galvanic couple was developed and applied for the evaluation of corrosivity of atmospheric environments. The sensor was designed considering mass-production and good reproducibility of results, making it convenient for long-term corrosion data acquisition. Besides the sensor output, I, temperature, relative humidity, RH, were also recorded by a microcomputer. By analyzing the magnitude and time variation of I, the occurrence and duration of rain, dew and dry periods, Train, Tdew and Tdry, respectively, could be distinguished and determined. And by referencing to the empirical I-RH calibrating curve, the amount of deposited sea salt, Ws, could also be estimated. It was also found that the corrosion loss could be estimated in the indoor site by analyzing sensor output. Corrosivities of some kinds of exposure sites were evaluated by using the ACM sensor. It was also demonstrated that this ACM sensor could be applicable to the evaluation of corrosivity of atmosphere for stainless steels.
In order to understand the atmospheric corrosion behavior of structural steels at semi-closed space such as box girder of steel bridge, the temperature and relative humidity were measured with thermo-hygrometers, and corrosion rate of structural steel was estimated using steel coupons with various deposited salt amount on the surface. Besides, Fe-Ag galvanic type ACM (Atmospheric Corrosion Monitor) sensors were employed to evaluate the influences of both the deposited salt amount and relative humidity on the corrosion of steels. It was found that the relative humidity inside box girders was lower than that outside, but it may be as high as 100% in some cases. Although the pollutant deposition is possible near the entrance of box girder, the airborne pollutant hardly invade into the interior of the box girder. The corrosion rate of structural steel without pollutant deposition on the surface is extremely low inside the box girder. The pollutant deposited on the surface before exposure promotes the corrosion of steel, but the effect of pollutant on corrosion rate of steel becomes weak with time. It can be suggested that low corrosion rate of structural steel inside box girder results mainly from no invasion of airborne pollutant.
Atmospheric corrosion for carbon steel was discussed with taking notice of the relation of corrosion rate, CR, to the thickness of adsorbed water layer, d, onto deposited sea salt. Amount of water adsorbed by sea salt was weighed under various relative humidity, RH. Deduced concentration of the solution film was compared with that calculated thermodynamically, and the limitation for the application of thermodynamics was determined as a window of the amount of deposited sea salt, Ws, and RH. Within the limitation, d was evaluated from density of sea-salt solution and its concentration. Corrosion amount of carbon steel specimens exposed for a month under various Ws and RH was measured. Then, CR was obtained and d was calculated for each condition. The relation of CR to d showed a maximum CR=0.07mgm-2s-1 located adjacent to d=50μm.
We applied poly (ethylene terephthalate) (PET) homopolymer, in stead of PET copolymer, to powder coating on galvanized steel. Despite the concern over internal stress originated from its rigid molecular structure, pin-hole-free thick PET overcoating was formed with good adhesion to the substrate. This coating coupled with the sacrificial corrosion of zinc undercoating provided an excellent corrosion protection capability against salt-water spray (6000h) and seaside weathering (5 years) tests. The coating had enough strength to withstand scratches and impacts that can be inflicted during the constructi on and use. These properties are equivalent to those of the PET copolymer coating, which has been successfully used for outdoor telecommunications plants over 15 years. The introduction of PET homopolymer reduces the cost for raw material, and recycled PET can be employed.