A. C. impedance of the potentiostatically anodized zirconium was measured in ammonium nitrate solution. The frequency range covered widely from 10^-2 to 10⁴Hz by adopting an unified procedure. Sinusoidal A. C. voltage with fixed amplitude less than 5mV was applied in the potential setting branch of the potentiostat circuit and the A. C. signals of the current flowing between the specimen and the auxiliary electrode, as well as that of the output voltage of oscillator, were picked up with a 2ch. wave memory unit. Displayed data with a tape puncher was processed by computerized curve-fitting and calculation procedures.
At the potential region less noble than 1V, in which the electrode process was only the growth of anodic oxide film, the impedance was almost linearly propotional to inverse frequency. In this potential region, the phase angle was almost 90° and the impedance had capacitive nature. The capacitance changed slightly with frequency in general, but a small dispersion was found nearby frequency of several or several tens Hz. The dispersion which showed a distributed relaxation-time spectrum was explained with the ionic relaxation mechanism. In the potential region nearby 1.4V, the impedance became resistive at lower frequency and it was attributed to the Faradaic process of the oxygen evolution.

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The mechanical properties of diffusion-coating layer of SUS 304 diffusion-coated with Al and Al-10% Ti were investigated. Specimens were obtained by powder pack process under the treatment conditions, 1050°C, 3hr.
Stress corrosion cracking (S. C. C.) tests in boiling 42% MgCl₂ solution for these specimens prestrained under various stress were also carried out in order to study the effect of loaded stress on the resistance to S. C. C. of these specimens. Specimens for S. C. C. tests were also obtained by powder pack process under some various conditions.
The results obtained were as follows;
1) Tensile strength of Al and Al-10% Ti diffusion-coating layer under that treatment conditions were estimated to be 12.7-16.5kg/mm² and 9.1-12.5kg/mm² respectively.
2) Al diffusion-coating treatment improved the resistance to S. C. C. of SUS 304 remarkably.
3) Specimens diffusion-coated with Al-10% Ti showed almost the same resistance to S. C. C. as those with Al.
4) The resistance to S. C. C. of specimens diffusion-coated with Al was influenced remarkably by loaded stress. Since cracks grew in accordance with the increase of loaded stress, they were corroded preferentially.

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The present investigation has been carried out to clarify the potential dependence of stress corrosion cracking (SCC) susceptibility of austenitic stainless steels deformed at high strain rate. The specimens were prepared from SUS 301 stainless steel sheets with unstable austenite, deformed at low or high strain rate. Then the potential dependence of SCC susceptibility by constant potential method was estimated in boiling 35% MgCl₂ solution under the constant applied stress.
The main results obtained are summarized as follows:
(1) Effects of pre-plastic strain and strain rate on SCC susceptibility were recognized at the SCC critical potential. Its critical potential of these specimens deformed at high strain rate shifted monotonously to more noble values with increasing pre-plastic strain, while at low strain rate shifted to most noble values within the certain pre-plastic strain range.
(2) The potential ranges in which SCC, pitting and general corrosion occured were recognized with these specimens deformed at low and high strain rate. It was observed that the time variation of current density proper to each corrosion mode.
(3) The level of passivation current density showed by the anodic polarization curves was one of cause for the difference in SCC susceptibility of these specimens deformed at low and high strain rate.
(4) Effect of applied potential on SCC fracture surface mode was recognized at annealed specimens. It was observed that the rate of transgranular fracture for intergranular fracture increased as applied potential shifted to more noble values.

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The study of metal components that have fractured in corrosive environment and the case study of accidents caused by corrosion provide the information that can be of major importance in preventing future accidents.
The accident prevention will be effectively doned if the cooperation between chemical engineers, designers, maintenance crews and safety engineers is systematically performed.

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In order to mininze pollution of the land and sea and hazards to personnel, leakages in storage tanks used to store crude oil and refined products must be presented.
Leaks are usually caused by corrosion in the buttom plate of tanks but can also occur as a result of fracture of pipe connections and general corrosion of shell plate.
The internal surface of bottom plates holding keroshine, light oil or gasoline will corrode more rapidly in the presence of water-drain. Furthermore, external corrosion on the bottom plate will caused electro-chemical corrosion at each point of water concentration.
This paper discusses the observations gained from inspection of corrosion conditions in oil storage tanks.

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