Inhibition efficiencies of polar organic compounds for the corrosion of various metals in acid solutions are related to the hard and soft acids and bases principle (HSAB principle). Since a bulk metal is classified to a soft acid, the compound acting as a soft base is more readily chemisorbed on surface of the metal than that of a hard base, resulting in the higher inhibition efficiency. In this paper, effect of the polar organic compounds on current densities of Fe electrode in active and passive region of anodic polarization curves and amount of the compounds adsorbed on γ-Fe₂O₃ powder are discussed on the basis of the HSAB principle. The compounds, (n-C₄H₉)₃X (X=N, P, As, and Sb) were used as the inhibitors. The polatization measurements were carried out on 99.99% Fe in deaerated 3.0M HClO₄ and boric-borate buffer (pH 8.45) at 30°C. The surface coverages of the compounds were obtained on the powder of 99.9% γ-Fe₂O₃ in methanol by HPLC. The inhibitor acting as the soft base was effective for the dissolution of Fe in HClO₄ than that of the hard base, indicating the agreement with the previous results. The passive current density of Fe electrode in the buffer solution decreased by the addition of R₃N and R₃P while increased by the addition of R₃As and R₃Sb. This result suggested that the compound classified to the hard base was adsorbed on the oxidized Fe surface acting as the hard acid to more extent than that classified to the soft base. The compound of the hard base was readily chemisorbed on the surface of γ-Fe₂O₃ powder acting as the hard acid. The adsorption behavior of the polar organic compounds on the oxidized Fe surface was concluded to be closely related to the HSAB principle.

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The influences of temperature ranged from 278K to 343K on cyclic SCC crack growth under a low frequency varying load have been investigated on a 7N01 high-strength aluminum alloy in 3.5% NaCl solution which is sensitive to active path corrosion type SCC. The threshold stress intensity factor, K_FSCC, at T≤318K is independent of temperature, provided that testing duration is comparably equal. However, the K_FSCC values increase in the cases at T=343K and longer testing duration at T=278K. This increase is caused by the corrosion products-induced wedge effects. In Region II, however, crack growth rates increase with increasing temperature in terms of ΔK_eff, not in terms of ΔK (K_max). The apparent activation energies in Region II for cyclic SCC are about 8-10kJ/mol, which are smaller than those for static and dynamic SCC. Applying the summation model based upon fracture area fractions to cyclic SCC crack growth rates, the intergranular crack growth term, (da/dN)_AD, and the transgranular hydrogen embrittlement one, (da/dN)_HE, have been obtained. At lower temperature, (da/dN)_HE is much larger than (da/dN)_AD compared with those for dynamic SCC. This is the reason that activation energies for cyclic SCC are so small compared with those of static and dynamic SCC.

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NO₂ is an air pollutant which accelerates the atmospheric corrosion of metals, since it transforms into corrosive nitrous and nitric acids, when dissolved in thin water-films on metals. The concentration of SO₂ in the atmosphere, which is also a common air pollutant, has been decreasing recently, whereas the concentration of NO₂ has not been decreasing, and its effect on metals has become relatively serious.
In this study, corrosion behaviours of common metals, Zinc, Aluminum, Iron, nickel, and copper in NO₂-containing atmospheres are investigated.
The results are as follows:
(1) NO₂ does accelarate the corrosion of metals, but its effect is not so serious as SO₂. (2) The corrosion rate of each metal in NO₂-containing atmosphere is nearly equal value. (3) The corrosion rate of metals in NO₂-containg atmosphere depends on relative humidity, as well as SO₂-containing atmosphere. (4) Nitrate is formed as a corrosion product on the metal surface exposed to NO₂-containing atmosphere. (5) NO₂ added to SO₂-containing atmosphere has the effect of decreasing the corrosion rate of metals. (6) The corrrosion product formed on Zinc, Iron, and Nickel surface in NO₂-containing atmosphere does not accelarate the corrosion of each metal.

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Corrosion inhibiting effect of benzothiazole derivatives on copper in an aqueous solution containing 0.01M NaOH, 0.0057M NaHCO₃ and 0.0036M Na₂SO₄ was investigated by means of polarization measurement with a rotating disk electrode. The inhibiting effects of 2-mercaptoben-zothiazole, 3-[2-(benzothiazolyl) thio] propionic acid and 2-(benzothiazolyl) thio acetic acid were increased by substitution of methyl group for hydrogen atom of benzene ring. The inhibiting effects of 2-[2-(benzothiazolyl) thio] propionic acid and 2-[2-(benzothiazolyl) thio] butyric acid were as excellent as the effect of 2-mercaptoben-zothiazole. The inhibition mechanism of these inhibitros was discussed.

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