It is generally observed that the susceptibility to localized corrosion of stainless steels in natural seawater is higher than that of stainless steels in synthetic seawater. The higher susceptibility to corrosion in natural seawater is considered to be caused by microorganisms in the biofilm, and therefore it is attributed to MIC (Microbially Influenced Corrosion). MIC of stainless steels in some plant failure cases in fresh water environments were often observed at welded joints, therefore, it is also important to understand the MIC mechanism for welds of stainless steels. The contents of this article are as follows ; (1) Corrosion potential ennoblement in natural seawater : It is considered that the corrosion potential ennoblement in natural seawater is caused by the peroxide generated by the metabolism of aerobic bacteria in the biofilm. (2) Reproduction of corrosion potential ennoblement by laboratory method : The corrosion potential ennoblement can be reproduced in synthetic seawater by the addition of GOD (Glucose Oxidase) with glucose, due to the peroxide generated by the GOD catalytic reaction. (3) Effect of the bacterial adhesion on MIC occurrence at welded joint : Bacterial adhesion corresponded with the occurrence of pitting corrosion in weldmetal, therefore, the weld bead shape considered to have significant influence on bacterial adhesion and MIC occurrence in stainless steel.
Colloidal particles of α-, β, γ-FeOOH, Fe3O4 and poorly crystallized iron oxide, the components of steel rusts, were prepared by hydrolysis and oxidation of aqueous solutions of Fe(II) or Fe(III) containing Ti(IV), Cr(III), Cu(II) and Ni(II) which enhance resistance to corrosion. The obtained rust particles were characterized by ICP, TEM, XRD, EXAFS, Mössbauer spectroscopy and measurement of specific surface area. Except for several cases the addition of these metal ions reduced the crystallinity and particle size of the rusts. The decrease in particle size of β-FeOOH rust by Ti(IV) addition and the reduction of crystallinity of α-FeOOH rust by Cu(II) addition were especially marked. The influences of metal ions depended on the kinds of metal ions and rusts. No metal ion effectively reducing particle sizes over all the rusts was found out in the metal ions under investigation. These results suggest that suitable alloying elements should be selected based on the corrosion environment and multiple addition of the alloying elements is required to form dense rust layer.
Piping tubes of pure copper are often used for a household plumbing or a coil for cooling water and so on, since it is excellent in corrosion resistance as well as in thermal conductivity. Leakage accidents, however, due to corrosion are sometimes reported. The cause of them may be a so-called erosion-corrosion or flow velocity difference corrosion, which are together categorized in flow-induced localized corrosion. So-called erosion-corrosion is originated through the turbulence in fluid flow, which breaks the protective film on the metal surface so that the area exposed to the fluid flow of higher velocity is possibly damaged. In contrast, the surface area exposed to the fluid flow of lower velocity is corroded by a flow velocity difference corrosion, which seemingly occurs due to differential aeration. In this study, corrosion tests were carried out on pure copper specimen in a 3% NaCl solution using a jet-in-slit apparatus that has two test cells of the same structure. In a test cell, so-called erosion-corrosion was easily reproduced. In another cell, flow velocity difference corrosion was successfully reproduced on the specimen surface by simply reversing the flow direction of test liquid. Polarization curves as well as cyclic voltammograms were measured to find that the character of protective film determines the type of flow-induced localized corrosion.
A jet-in-slit testing apparatus is suitable for evaluating so-called erosion-corrosion resistance of pure copper. The followings are primary features. (1) Strong turbulence, caused by the rapid slowdown in flow velocity, occurs in the fluid flow over the specimen surface. (2) The location of maximum turbulence is different from the position where maximum shear stress occurs. Eventually, it was found that the inlet tube attack of copper alloys was caused not by shear stress, but rather by turbulence. In this study, the apparatus was modified to examine the effect of flow velocity difference on corrosion in the absence of turbulence. At a lower flow rate in a 1% CuCl2(II) solution, specimens showed the exactly expected morphology : the damage depth was shallower in a higher velocity region, and deeper in a lower velocity region. At a higher flow rate, however, a deep pit was observed in the center of the specimen where the velocity is higher. Visualization of the flow conditions near the center of the specimen revealed the presence of a vortex at this location. Polarization curve measurements indicated that the deep pit under the fixed vortex occurred by the same mechanism as that of differential aeration cell corrosion. This proposed mechanism was confirmed by macro cell current measurements.
Chloride stress corrosion cracking (SCC) of Type 304 and 304L austenitic stainless steels (SS) was studied by a constant load method in air at a temperature of 353K with relative humidity (RH) of 35%. Chlorides simulating sea salt particles were put on the gage section of the SS as droplets of synthetic sea water. The following stress[σap(kgf/mm2)]-minimum rupture time [ζ(h)] relationships were obtained for Type 304 and 304L SS ; σap=-51log(ζ)+153 (304 SS), σap=-141log(ζ)+386 (304L SS). The threshold stress of SCC was as low as 1/2 of the 0.2% proof stress for solution annealed Type 304 and 304L SS, and smaller than 1/4 of the 0.2% proof stress for sensitized Type 304 SS. This result suggests the difficulty of suppressing SCC by reducing the residual tensile stress of a SS structure. SCC test using NaCl and MgCl2 as chlorides suggested that MgCl2 was responsible for cracking in the test condition of 353 K with RH=35%.