Study on Under-film Corrosion Simulation Method Based on Cellular Automaton (2st Report: Corrosion Simulation of Free Edges in Water Ballast Tanks)

Abstract

A simulation method to examine under-film corrosion of epoxy coated free edges in ship's Water Ballast Tanks (WBT) has been developed, and the corrosion behavior on the longitudinal member's free edges made of conventional steel and Corrosion Resistant Steel (CRS) in VLOC's WBTs, which was measured by Shiotani and Tachibana (2016), are simulated. The simulations are performed by using the two-dimensional cellular automaton developed in the previous report (Osawa et al., 2016). In order to emulate the point coating defect's corrosion incubation on edges, a 'spattering' model is developed. The onboard exposure tests of the scribed coated steel panels examined in the previous report were performed in the WBTs in which the edge corrosion was measured. The cellular automaton parameters determined in the previous report are adopted for intact coated cells. The determination techniques of automaton parameters for thin film regions (edge) and point coating defects are developed. Those parameters are determined from the measured stochastic characteristics of corroded spot length on free edges. The effectiveness of the proposed spattering model and the parameter determination technique are examined by comparing simulated and measured stochastic characteristics of corroded spot length. Based upon these simulation results, CRS's resistance improvement mechanisms are discussed.