In order to improve the corrosion resistance of ASTM A 669 (1.5Si-19Cr-5Ni-2.7Mo duplex stainless steel), the effect of nitrogen addition of about 0.1mass% were studied. The resistances of base metal and welded joints of duplex stainless steels with and without nitrogen to crevice corrosion and stress corrosion cracking in chloride environments were evaluated in comparison to those of SUS 304 and 316L as reference materials. The effects of nitrogen addition, restraint stress and filler metal on microstructure, especially formation of austenite phase in the welding heat affected zone were also investigated in detail. The addition of nitrogen was clarified to improve extremely the corrosion resistance and to accelerate the precipitation of austenite phase in the welding heat affected zone of the duplex stainless steel. Although austenite free zone, which might cause intergranular attack, tended to form in the heat affected zone by welding under restraint stress even in nitrogen containing steel, the selection of proper filler wires, such as 0.01C-23Cr-7Ni-3Mo-0.14 Nsteel, made it possible to produce the sound welded joint with adequate amount of austenite phase to prevent the intergranular attack.

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Modulated reflection spectra of passive films on Alloy 600 have been measured in 1kmol·m^-3 Na₂SO₄ (pH 2.0 and 6.0), 0.15kmol·m^-3 H₃BO₃+0.0375kmol·m^-3 Na₂B₄O₇ (pH 8.45) and 0.1kmol·m^-3 LiOH (pH 12.8) solutions, respectively. Quantitative relations between the characteristic of spectra and the composition of the films have been derived from the spectra for thin composite films of NiO-Cr₂O₃-Fe₂O₃ prepared on Pt by MO-CVD technique. From these relations, cationic mass fractions of Ni²⁺, Cr³⁺ and Fe³⁺ ions, X_Ni, X_Cr and X_Fe, of passive films on the alloy were determined. It was found that the cationic mass fractions of passive films on Alloy 600 changed remarkably with potential; for the films formed at pH 2.0, the value of X_Cr decreased from 0.7 at 0.0V to 0.1 at 1.0V and, at the same time, the value of X_Fe increased from 0.05 to 0.3 with increasing potential. This indicates that Cr³⁺ ions are substantially enriched in the passive films and Fe³⁺ ions are in the transpassive films. The value of X_Cr for the passive films gradually decreased with increasing pH, while that of X_Fe was almost independent of pH.

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The initial repassivation processes of anodic reaction on newly created surfaces of high nickel alloys containing various chromium contents were investigated by a rapid straining electrode technique in a 0.4% NaOH solution at 553K at constant anodic potentials. The results obtained are as follows: (1) The decrease in current density with time after rapid straining was stopped follows the formula of i∝t^-n (i: current density, t: time, n: constant) over a time range from about 20ms to at least 5000ms. (2) The effects of chromium contents on the repassivation rate were small in the range of about 0-15 mass% Cr, whereas were large in the concentration above about 15 mass% Cr. This behavior seems to correspond to the effect of chromium contents on the caustic stress corrosion resistance of high nickel alloys.

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Effects of P, Si, Mn and Cu on the repassivation potential (E_R) for the metal/metal-crevice were studied with high purity 18Cr-14Ni steels with systematic variations in alloying elements in 3×10^-3-3% NaCl solutions at 80°C. The depassivation pH, pH_d, for the steels were also measured in a deaerated 3% NaCl solution at 25°C. It is found that P has a strong effect on both E_R and pH_d values. Decreasing P content makes E_R less noble. Increasing P content is beneficial to lower pH_d in line with a general tendency that an element which raises E_R lowers pH_d. Si, Mn and Cu give no significant change in E_R and pH_d values.

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Key points on design of cathodic protection system particularly in sea water environments were reviewed along with an explanation of fundamental way of thinking for design procedures in reference to Technical Standards and Recommended Practices published by Lloyd's Register of Shipping, England: Det Norske Veritas, Norway: NACE, U. S. A: Harbour Bureau of the Ministry of Transport and the Coastal Development Institute of Technology, Japan: etc.

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The electrochemical noise analysis for corrosion systems is reviewed. Origins of the noise, methods for the measurement and for the analysis, and examples of the application are described. The electrochemical noise analysis is found to be a useful technique for evaluating localized corrosion.

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