This chapter discusses rational determination techniques of automaton parameters for thin film regions and point coating defects are developed. There are few reports on the simulation technique for under-film corrosion. S. Sakashita et al. performed onboard investigations of under film corrosion behavior of epoxy coated steel panels exposed in the un-immersed part of Water Ballast Tanks (WBT). N. Osawa et al. developed a coating degradation-metal corrosion coupling simulation method for epoxy coated steel panels in ship's WBT. The onboard exposure tests of the scribed coated steel panels examined in the previous report were performed in the WBTs in which Shiotani and Tachibana measured the edge corrosion, and the test panel's coating specification is the same as that of the WBT's longitudinal members. The corrosive environment and coating specifications of the WBT in which Corrosion Resistant Steel was adopted are the same as those of the WBT in which the conventional steel was adopted.
Osawa et al. (2016) developed a two-dimensional cellular automaton (CA) for under-film corrosion analysis and succeeded in consistent analysis of coating film deterioration and corrosion depletion. In this CA analysis, the CA parameters for coating performance and metallic corrosion are given by random number fields without spatial covariance structures. In this report, a corrosion test of coated steel panels with double scribes was performed, showing non-uniformity in corrosion surface profiles. The under-film corrosion simulations of double scribed coated panels were carried out by using the improved CA in which CA parameters were given by lognormal random number fields with spatial covariance structures. As the result, the probability that the non-symmetric corrosion surface profile in the experiment could be reproduced by simulation was improved by 10.5 times by considering the covariance structures. Based on this result, the effectiveness of the developed analysis method was verified.
Coating deterioration behavior of water ballast tanks in a very large ore carrier using a corrosion resistant steel (CRS) was investigated by analyzing the photographs which the same parts were taken in 4.8 and 7.3 years after shipbuilding. The investigated components were upper decks and edges of upper deck longitudinal members. Coating deterioration from 4.8 to 7.3 years was classified into the growth of single deterioration, combination and new generation. The ratio of new generation on the edge in 7.3 years was less than that on the upper deck, and decreased very much compared to 4.8 years. Deterioration area on the upper deck and deterioration length on the edge of the longitudinal member using the CRS were suppressed to about 40% and 70% of those using a conventional steel, respectively. One dimensional coating deterioration rates of the CRS were reduced to about 70% of those of the conventional steel on the edge part of the longitudinal member and the upper deck. This ratio of the CRS to the conventional steel in the hull components agrees with those in a laboratory corrosion test and an onboard doubling plate test. From the change of coating deterioration length on the edge until 7.3 years, the average recoating life with the CRS is considered to be 25 years or more.
A simulation method for under-film corrosion has been developed for epoxy coated steel panels within a ship’s Water Ballast Tank (WBT) environment. The incubation and extension of coating failure is simulated by using two-dimensional cellular automaton, and the steel diminution is simulated by modifying IACS CSR-H’s 3-phases probabilistic model. Analysis parameters are determined by using the results of onboard exposure and cyclic corrosion tests performed by Shiotani et al. (2012, 2015). The change in corroded surface shape of epoxy coated scribed steel panels made of conventional steel and corrosion resistant steel (CRS) exposed in an ore carrier’s WBT for 4.8 years is simulated. The simulated coating deterioration (blister) area and the corroded surface profile agree well with those measured. This demonstrates the effectiveness of the developed simulation method and the determined parameters. The differences in analysis parameters between conventional steel and CRS suggest that CRS can reduce the harmful effect of the active corrosion region on the remaining coating life at the blister’s frontline and the corrosion under the blister.
The influence of the particle size of Fe 3 O 4 on the surface of steel on the corrosivity and electrochemical properties under wet/dry cyclic conditions was investigated. The results showed that Fe 3 O 4 on the steel surface increases the cathodic current and corrosivity of the steel, but this effect is minor in the case of fine Fe 3 O 4 particles. The diffusion mobility of cathodic reactants on the surface of fine Fe 3 O 4 was evaluated by a molecular dynamics simulation, and the influence of the Fe 3 O 4 rust particle size on the corrosion behavior of steel is discussed. It is probable that the diffusion mobility of cathodic reactants (H 2 O and O 2 ) is low in between fine Fe 3 O 4 particles, which significantly retards the cathodic reaction on Fe 3 O 4 and the steel surface.
