Intermittent galvanostatic polarization measurements were carried out using iron and stainless steel electrodes. With consideration to the potentials indicated in the Pourbaix diagram of the Fe-H₂O binary system, and also the effects of pH and polarizing current density on the shape of the potential vs. time diagrams, conclusions were drawn concerning the corrosion products anticipated under the given conditions, and also the stability of passive films. The differences in character of the passive films on iron and stainless steel are reflected primarily in their degree of protectiveness and their susceptibility to breakdown.

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A tracing potentiostat was constructed, that records automatically the change in immersion potential and polarization resistance with time in the studies of corrosion or electroless plating. This potentiostat usually traces the immersion potential of test electrode. When a trigger signal operates, however, the sum of the immersion potential and the voltage of a triangular wave of amplitude ±8mV is fed to the potentiostat circuit as set-potential, so that a linear potential-sweep automatically starts from the immersion potential in any conditions. Polarization resistance is obtained from the slope of current-potential (time) curve at the immersion potential. The measurements by the tracing potentiostat were made on iron in acid and neutral solutions, nickel in molten nitrate salt, and cobalt in electroless cobalt plating bath. The polarization resistances obtained were compared with the rates of processes obtained by weight measurement. It was confirmed that the tracing potentiostat is successfully used to monitor the rates of corrosion and electroless plating.

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SCC tests have been carried out in CH₃OH+0.5% I₂ solution for high strength Ti-6 Al-4V alloy solution treated at 926°C, 940°C, 954°C, and 968°C, and aged at 538°C or 550°C after (α+β) processing. Fatigue precracked CT specimens with a thickness of 7mm were tested under constant load. The specimens were prepared so that the cracking direction was parallel (specimen code: L) or normal (specimen code: T) to the prior rolling direction. Crack growth was monitored by measuring crack opening displacement and Acoustic Emission (AE). Both crack growth rate and AE count rate showed three stages on the curves plotted versus stress intensity factor K, demonstrating a good correlation between these two parameters. The measurement of the plateau crack growth rate at K=120kg·mm^-3/2 in stage II showed that the SCC susceptibility of the specimens-L was higher than that of the specimens-T. The higher SCC susceptibility was attributed to the large prior β-phase grains elongated in the direction of the prior hot rolling at high temperature above β transus. The cracking morphology was shown to be the mixture of intergranular cracking along the prior β-phase grain boundaries or the (α-β) phase boundaries and transgranular cracking. The detailed observations of microstructure along the cracking path and the analysis of AE detected discontinuously during the crack growth revealed that the cracking was obviously discontinuous and occurred in jumps of some α-grain diameters on a small scale and of a prior β-phase grain diameter on a large scale. In this case grain boundaries often arrested the cracks, although examples of cracks accelerating on crossing a boundary were observed.

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Carbide precipitation behavior and intergranular stress corrosion cracking (IGSCC) of Ni-base Alloy 600 were studied under various aging treatments. IGSCC test was performed on stressed double U-bend specimens in non-deaerated water containing 500ppm Cl^- at 300°C. The relationship between the IGSCC sensitivity and intergranular corrosion (IGC) sensitivity in boiling 40% HNO₃ solution was also examined. The carbide precipitation during aging was much affected by the prior metallurgical conditions or the annealed conditions. Undissolved carbides in annealed material accelerated the carbide precipitation within the grains and thus acted to depress the grain boundary precipitation. The chromium carbides found in Alloy 600 were Cr₇C₃ and Cr₂₃C₆. The former existed as undissolved carbides in annealed material, and the latter precipitated at aging stage. The sensitivities to both IGSCC and IGC increased, reached the maximum and then decreased with aging time. This result supports that these intergranular attacks are caused by the formation of chromium depleted areas along grain boundaries. But the IGSCC sensitivity was much higher than the IGC sensitivity at early stage of aging duration. This difference in the sensitivity between IGSCC and IGC is also discussed.

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This report attempts to describe the main features of the present understanding of the mechanisms of corrosion under paint films. The resistance inhibition theory, in which high ionic resistance is responsible to protection action against corrosion, is examined. Emphasis is laid on the initiation process as well as the propagation process. New ideas of Funke are introduced to compensate for weak points of the resistance inhibition theory.

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