Influence of alloy grain growth during oxidation on the scaling behaviours of Fe-18Cr and Fe-18Cr-0.6Si was investigated by weight gain vs time measurements in 760 Torr oxygen at 1000° and 1100°C, and by subsequent examination of the scales. In the case of Fe-18Cr, oxidation rate increased rapidly, as the alloy grain began to grow, and nodular oxides containing α-Fe₂O₃ and Fe, Cr spinel type oxide could be found immediately after the breakaway. when an Fe-18Cr-0.6Si alloy was oxidized, abnormal oxidation was not observed and its surface was covered with a protective Cr₂O₃ rich oxide layer. The restraint upon the breakaway associated with the grain growth may be attributed to the effect of Si incorporated into the initial oxide scale.

View full abstract

In the previous paper, the protection potential for pitting growth of 1100 in fresh water has been reported to be -0.35V (vs. SCE) in flowing tapwater and -0.45V in flowing artificial water. In this paper, such potential for Al alloy (5052) in fresh water has been estimated by measuring the time dependence of current at various potential in artificial water and tapwater, the weight loss of Al alloy, the amount of Al³⁺ ion and Mg²⁺ ion dissolved, and surface observation. The artificial water contained 10ppm of Cl^- and 20ppm of SO₄^2-. As a result, the protection potential for pitting growth of 5052 in fresh water was determined to be -0.60V in tapwater and -0.55V in artificial water. The composition of the substance inside a pit was elucidated by use of a scanning electron microscope and an X-ray microanalyzer. It was ascertained that a large amount of iron and a small amount of sillicon existed in a marked projection of the center of pit. That is to say, the compound Al-Fe-Si which was cathodic against aluminum was a macroscopic nucleus of pitting corrosion of aluminum.

View full abstract

Mean protective current densities of cathodically protected steel structures including wharfs and piers have been measured at seventeen harbors in industrial districts in Japan. Dissolved oxygen, and sulfide- and hydrogen-ion concentrations were chosen as variables representing sea water pollution. A definition of “pollution index” was given as a sum of these concentrations each multiplied by a constant. Corrosion rates and mean protective current densities have been evaluated from the pollution indices in the following four zones of aquatic pollution: 1) Repurifled zone. 2) Zone of mild pollution 3) Zone of pollution 4) Zone of foul pollution. a) Repurified zone and zone of mild pollution are so-called clean zones. In these zones, corrosion rates and mean protective current densities are less than 0.1mm/yr and 50-55mA/m₂, respectively. b) In zone of pollution, corrosion rates and mean protective current densities are 0.1-0.2mm/yr and 75-85mA/m₂, respectively. c) In zone of foul pollution, dissolved oxygen is low throughout a year, and hydrogen sulfide, is detected even at sea surface in summer. Therefore, corrosion rates and mean protective current densities are higher in summer than that in winter, and their values are several times as large as those in clean sea water.

View full abstract

Instrumental analytical methods of rusts are reviewed. Diffraction techniques are one of the most powerful tools to characterize the corrosion products. Initial products of corrosion are, however, apt to be amorphous or poorly crystallized, preventing direct identification of observed diffraction patterns with standard data such as ASTM cards. This difficulty is surmounted if radial distribution function (RDF) technique is introduced. A comparison of RDF deduced from observed data and well defined crystal data of related compounds which come upon by chance as corrosion products at a given environment leads to determine short range structures of rusts.
Polycrystals with an uniaxial anisotropy which are sometimes found in corrosion products tend to align their well developped planes parallel to the plane of a base metal or a matrix, or of sample holder for electron and X-ray diffractions. This makes it impossible to identify the diffraction patterns obtained from the polycrystals with standard one, which is, however, overcome with the aid of a texture pattern technique. With this method, the identification of a sulfide on iron, and oxides on iron, nickel and copper was carried out successfully.
Applications of spectroscopies such as characteristic X-ray, ESCA and Auger to corrosion products are also described with several examples.

View full abstract

The mechanism, environmental and metallic factors, testing methods and protection methods of localized corrosion, pitting and crevice corrosion, are discussed from the standpoint of newly developed nucleation and growth theories. Importance of the geometrical factor for the localized corrosion is especially emphasized and made an attempt at its rating.

View full abstract