Numerical simulation is gathering many expectations in the field of corrosion engineering with the development of computer. In this review the two example topics are introduced.
The first topic is “Numerical method for time-dependent localized corrosion analysis with moving boundaries”. The numerical method for time-dependent localized corrosion analysis is presented. Electromigration, mass diffusion, chemical reactions, and moving boundaries are considered in the numerical simulation of localized corrosion of engineering alloys in an underwater environment. The method combines the finite volume method （FVM）and the voxel method.
In the second topic, the forecasting system of the current from sacrificial anodes using data assimilation was introduced.
Some numerical verification example analysis are shown to demonstrate the effectiveness of the methods.

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The service environment of equipment such as combination of processes and materials and works are very complicated in chemical plants. The best material selection or solution of corrosion cause in the service environment is required large number of corrosion tests. for this reason the corrosion studies are required a lot of time and cost. Therefore, as for reduction of time, the electrochemical noise analysis（ENA）was introduced instead of standard mass loss method and as for cost reduction, numbers of measurement points were concentrated using a corrosion simulation program（CSP）. As the results of the studies, the corrosion study period is reduced less than 10％ of the conventional period and corrosion test cost is greatly reduced.

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Process simulation and heat exchanger simulation are applied as design tools for oil refining plants. Calculation accuracy has been enhanced and these techniques have been generally used. On the other hand, electrolyte thermodynamic model has been practically used to predict corrosion environment recently. Many case studies using this model are reported. Corrosion caused by ammonium salts such as NH₄Cl or NH₄HS is one of the serious issues in oil refining plants because corrosion rate can reach several mm per year in the concentrated ammonium salts solutions and it is difficult to predict corrosion rate. This is because it is quite difficult to predict localized corrosive environment in the plant. Thus, employment of electrolyte thermodynamic model is helpful to evaluate localized corrosion environment. This paper describes some examples that simulation technique was applied to study ammonium salts corrosion.

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The authors have developed 2D, 3D and axisymmetric boundary-element-method（BEM）and 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 internal elements discretization with internal element. Whereas, conventional analysis techniques, e.g. finite-difference-method（FDM）and finite-element-method （FEM）require internal element discretization which brings difficulties on the analysis of complicated 3D regions.
In this paper, the history of development of corrosion analysis are reviewed briefly, Next, the BEM and BEM system are explained, and validity and usefulness of the BEM system are demonstrated by showing verification tests and application examples for cathodic protection.

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Current status of corrosion simulation in plant engineering is reviewed. As a result, it is shown that corrosion simulation is used for evaluation of corrosion resistance of materials, study of corrosiveness by change in operation parameter and identification of highly corrosive locations.
In addition, an example of application of corrosion simulator for gas production facility which handles corrosive media such as hydrogen sulfide and carbon dioxide is introduced. As are result, it is indicated that calculated results are different between simulators due to difference in input parameters or calculation algorithm.
Through the study, it is suggested that corrosion simulation is a powerful tool but knowledge and special consideration on corrosion phenomenon are required for engineers who use it.

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About copper localized corrosion water factor, clear-cut decisions was conventionally difficult in addition to corrosion specialists. The hole which can be judged from a calculation result to anyone now - the number of forefingers is shown. Furthermore, it is possible by using Nakajima Diagram to judge the corrosion Risk of water quality, the propriety of past water quality, etc. also by non-experts of corrosion. This was confirmed employing copper I type pitting corrosion 4 example and healthy 2 examples in a cooling-water system. Furthermore, the proposal and efficacy of improvements index in water quality： WQII（improvements index in water quality）which can judge the propriety of the present water quality management simple were shown.

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In the clean seawater without chlorination, the protective film of copper alloy tube is biofilm. The biofilm is damaged by low chlorination, therefore the biofilm is necessary to reinforce by Fe²⁺ injection. The protective film can be evaluated by measuring polarization resistance. According to the polarization resistance value, we regulated the concentration of Fe²⁺. Here we report a result for about four years. With this method, good corrosion control and good cleanliness have been maintained.

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Involvement of microorganisms to corrosion refers to microbiological influenced corrosion（MIC）. Systematic methods for MIC diagnosis are not established, because relationship between microbial community and progression of corrosion has not yet been clarified. Here we conducted the lab-scale corrosion test and monitored the corrosion rate and microbial community in order to investigate a relationship between microbial community and progression of corrosion, which will be basic for the MIC diagnosis. First, we collected bottom water from three different oil-storage tanks in Japan. Microbial communities in these samples differed with each other. Next, corrosion rate of these samples was estimated by measuring release rate of Fe ions from metallic Fe. An accelerated corrosion was only observed in the sample supplied with inorganic salts. DGGE analysis showed that microbial community in this sample was changed from that in starting water sample. These results indicate that the inorganic salts added, which offers a model situation of environmental water quality change, may influence the rate of microbial growth. We propose that periodical monitoring of microbial community and water quality should be carried out in order to establish useful methods for MIC diagnosis.

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Silica scale was formed on copper tubes by immersing in test solutions containing silica and chloride ion for 7 days. The film formed on copper tubes immersed in 20 ppm Cl^－ solution without silica composed mainly cuprous oxide（Cu₂O）. Silica in test water inhibits the formation of Cu₂O on copper tubes. The corrosion potential of the copper tube with silica scale was less-nobler than that of the copper tube without silica scale. The galvanic monitoring was made for a pair consisting of the copper tubes with and without silica scale. The galvanic current flew from the copper tube without silica scale to the copper tube with silica scale. This result indicates that the copper tube with silica scale acted as the anode, whereas the copper tube without silica scale acted as the cathode.

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Constant i_abs or δ^≠ has been determined at eight electrolyte film salt concentration c, on basis of the following equation ; 1/i_lim＝i_abs/（1＋δ/δ^≠）, where i_lim is the cathodic limitting current of oxygen reduction, δ is the thickness of electrolyte thin film formed on cathode, and i_abs or δ^≠ is a constant inherent in c.
Provided that the wet corrosion rate of steel under drying process is proportional to the limitting reduction current due to oxygen in atmosphere, i_lim has been simulated as a function of c which is the good indicator of drying process state on corroding steel surface.
The i_lim simulation obtained from ten c agrees fairly well with the corrosion rate of steel in wet&dry repeat corrosion test measured by electrochemical impedance method reported by Nishikata et al.

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