The authors have developed 2D, 3D and axisymmetric boundary-element-method (BEM) programs, and a BEM system based on these programs, in order to quantitatively estimate cathodic protection and macro-cell corrosion such as galvanic corrosion and differential-aeration-cell-corrosion. Because the knowledge of physical quantities (potential and current density) on the surface of corroding materials is of prime importance in corrosion problems, the authors have developed the BEM, which does not require discretization with internal elements. Whereas, conventional analysis techniques, e.g. finite-difference-method (FDM) and finite-element-method (FEM) require internal elements discretization which brings difficulties on the analysis of complicated 3D regions. In this paper, the mechanism of galvanic corrosion and cathodic protection, and the importance of the numerical analysis technique for the predictions of these corrosion problems are described firstly. Next, the history of development of the FDM, FEM and BEM are reviewed briefly. Lastly, the BEM and the BEM system are explained, and the usefulness of the BEM system is demonstrated by showing the application examples for corrosion protection design of seawater pumps.
The gold surface was contaminated with a small amount of salt particles and its effect on water adsorption was measured by means of quartz crystal microbalance. The salts employed were NaCl, MgCl2⋅6H2O and (NH4)2SO4 and the amounts of contamination were ranged from 0.01 to 1.0μg/cm2. The critical relative humidity where the water adsorption sharply increases, depends on the amount of contamination as well as the kinds of salts. The water adsorption was analyzed by Frenkel-Halsey-Hill (FHH) model and was classified into three stages according to the interaction parameter r in the FHH isotherm equation. The three stages can be attributed to the existence of different stages of absorbed water molecules. Stage I (r<0.8): Adsorbed water molecules exist as clusters and water molecules adsorbs on the clusters as relative humidity rises. Stage II (0.8<r): The transition stage between the stage I and II. The adsorbed water molecules coalesce and form an island. Stage III (2.0<r): The adsorbed water molecules grow from cluster into a form of island and begin to exhibit the liquid water characteristics. Gradually this leads to the formation of continuous water film on the surface as the amount of water adsorption increases.
Carbon steels which are used for such as water supply line, core spray line, and clean up heat exchanger in Boiling Water Reactor (BWR) Plant, are main structural materials as well as an austenic stainless steels, and Ni based alloys. It has been well known that carbon steels can become susceptible to stress-corrosion cracking (SCC) in BWR primary coolant water environments, i.e., the high-temperature, high-purity water containing dissolved oxygen. Nevertheless, their sensitivity of SCC appears to be markedly smaller compared to that of weld-sensitized Type 304 stainless steels, whole failure has often been observed. This paper examines the critical condition, especially effects of hardness, and temperature for the initiation of SCC by means of Slow Strain Rate Tensile (SSRT) test, and Creviced Bent Beam (CBB) test as laboratory accelerated tests. It has been shown that, (1) Intergranular stress-corrosion cracking (IGSCC) initiates over hardness of Hv 400 for single bead weld material, simulated corner weld; (2) in middle temperature domain around 160 to 190°C range, so many stress-corrosion cracks initiate, but each crack is not so deep, on the other hand, in high temperature domain, stress-corrosion cracks initiate few in number, but each cracks is developed so deep.
In order to elucidate the mechanism for the protective ability of final stable rust layer formed on a weathering steel by atmospheric corrosion, the ion selective property of rust layer was studied by the use of synthetic iron rust membranes. Anion selective property was increased in this order: Fe3O4<α-Fe2O3<α-FeOOH<γ-FeOOH<β-FeOOH. Fe3O4 has little effect on the ion selective property. Anion selective property decreased with increasing Cr content in α-(Fe1-x, Crx) OOH (Cr-substituted goethite), cation selective property came out when the Cr content exceeds 3.8 mass%. As the effect of adsorbed oxyanions, all of PO43-, MoO42- and SO42- reduced the anion selective property or changed to cation selective property. Among these anions, PO43- has significant effect on the ion selective property of rust membrane and suppresses the permeation of Cl- through rust layer. In acidic solution, the ion selective property of α-(Fe1-x, Crx) OOH changed from cation to anion selective one, though little effect for α-FeOOH. The anion selective property of α-(Fe1-x, Crx)OOH was reduced as the concentration of KCl solution increased under the constant ratio of KCl solutions (CI/CII). Furthermore, it was thought that protective rust layer formed on a weathering steel acts as a kind of bipolar membrane which was composed of outer γ-FeOOH layer of anion selective property and inner α-(Fe1-x, Crx) OOH layer of cation selective property.
On the novel approach of corrosion protection of carbon steel by TiO2 coating under illumination, photoeffect of TiO2/C-steel system is affected by the structural and electronic properties of the interfacial Ti-Fe oxide. In the present work, Ti-Fe oxide with a wide range of composition, from TiO2 to α-Fe2O3, was prepared by sol-gel method. By studying the characteristics of the Ti-Fe oxides and their influences on the photoeffect of TiO2/C-steel system, a multi-layer coating, expressed as from outer coating layer to substrate: TiO2 (amorphous)/TiO2 (anatase)/Ti-Fe oxide/α-Fe2O3/C-steel, was proposed for more effective photoelectrochemical cathodic protection of carbon steel. TiO2 (anatase) layer without Fe contamination exhibits a good photoeffect and behaves like the source of photo-excited electrons. The inner α-Fe2O3 layer acts as both a barrier to retard the diffusion of Fe into TiO2 and a way for the transfer of photo-excited electrons from the TiO2 (anatase) coating to the substrate. The outer layer of amorphous TiO2 inhibits the oxygen reduction and reduces the photopotential to a more less noble value. The intermediate layer of amorphous Ti-Fe oxide provides deep levels for Fe2+/Fe3+ redox couple, for which Fe3+ is reduced to Fe2+ by the photo-excited electrons under illumination. The state of Fe2+ left after stopping illumination contributes to phenomenon that the electrode potential is maintained at considerably less noble value until all the Fe2+ is re-oxidized by oxygen reduction.
The investigation of surface fatigue crack growth behavior in sea water was made for low carbon steel wire SWRM 22K, and also high carbon steel wires SWRH 32 and SWRH 42A in comparison with the previous test results of low carbon steel wire SWRM 10 at 0.33, 0.25, 0.17, 0.09 and 0.05Hz by the use of the measured crack growth rates and the examined fractured surface of specimens. The rates of crack growth for tested steels in air were nearly equal to each other in relatively low ΔK regime. At 0.33Hz, the rate of crack growth in sea water increased with increase of tensile strength (σB) of tested steels in order of SWRM 10, SWRM 22K, SWRH 32, and SWRH 42A. The environmental acceleration factor, (dl/dN)CF/(dl/dN)air was 1.6 for SWRN 10, 1.7 for SWRM 22K, 2.0 for SWRH 32, and 2.2 for SWRH 42A respectively at ΔK of 5.5MN/m3/2. When testing at lower frequencies, there appeared a critical frequency, at or below which the rate of crack growth in sea water slowed and finally stopped for each steel. This critical frequency decreased with increase of σB of each tested steel in order of 0.25Hz for SWRM 10, 0.17Hz for SWRM 22K, 0.09Hz for SWRH 32, and 0.05Hz for SWRH 42A. These observations can be explained reasonably by crack tip blunting caused by dissolution in sea water.
The influence of heat treatment conditions on intergranular corrosion (IGC) was investigated for the weldments of the Co-based alloy, Stellite by gas welding. The Stellite has IGC susceptibility when it is heated at temperatures ranging from 693 to 973K after welding. It is clarified that this IGC susceptibility of weldments which are subjected to post-weld heat treatment is due to the formation of the grainboundary Cr depletion zone which resulted from precipitation of the M7C3 type Cr carbides. It is also clarified that the grainboundary precipitation of the secondary complex M23C6 carbides consisting of W-Co-Cr-C which are induced by heating above around 1000K suppresses the IGC in Stellite equivalent to production because of mitigation of Cr-depleted zones formation.