The differences between the general corrosion and microbiologically influenced corrosion (MIC) of steels were investigated in terms of its electrochemical behavior and surface phenomena. The corrosion potential of steels in the absence of SRB (sulfate-reducing bacteria) shifted to a negative value with the immersion time. However, the potential of the presence of SRB shifted to a positive value after 30 days' incubation, indicating the growth of SRB biofilms on the test metal specimens and the formation of corrosion products. In addition, the color of a medium inoculated with SRB changed from gray to black. The change in color appeared to be caused by the formation of pyrites (FeS) as a corrosion product, while no significant change in color was observed in a medium without SRB inoculation. Moreover, corrosion rates of various steels tested for MIC were higher compared to those of steels in the absence of SRB. In particular, the corrosion current density of TMCP steels in the presence of SRB was larger than that of other steels. Pitting corrosion was also observed at the surface of all steels in the SRB-inoculated medium. The pitting corrosion likely occurred due to SRB that was associated with the increasing corrosion rates through increasing cathodic reactions, which caused a reduction of sulfate to sulfide as well as the formation of an oxygen concentration cell.
The diesel engine in the merchant ship is being operated in severe environment more and more because temperature of exhaust gas of combustion chamber is getting higher and higher with increasing of using the heavy oil of low quality as the oil price is significantly being jumped for recent some years. As a result, the degree of wear and corrosion in between spindle and seat ring of exhaust valve are more particularly serious compared to the other parts of the engine. Thus the repair weld to the spindle and the seat ring is a unique method to prolong the life of the exhaust valve in an economical point of view. In this study, corrosion property of both weld metal and base metal was investigated with some electrochemical methods in natural sea water solution, in case of being welded with some welding methods and welding materials to the exhaust valve specimen as the base metal. In all cases, corrosion resistance of the weld metal was considerably superior to the base metal. In particular the weld metal of A2F(AC Shielded Metal Arc Welding with two pass by foreign electrode) showed a relatively good corrosion resistance compared to the other weld metals Furthermore pitting was somewhat observed at the corroded surface of the base metal while there was no its phenomenon on the surface of the weld metal.
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