Samenvatting

Mild steel is the main material of alloys in use. Approximately 20% of corrosion damage of metals is induced by bio corrosion. The aim of this research was to study the corrosion performance of mild steel and stainless steel when immersed in natural seawater. In order to verify the effect of the microorganisms compared to the results with the ones obtained with artificial seawater and sodium chloride solution. The chloride ions in salty water are some of the most aggressive elements in seawater. The maximum oxygen concentration is reached at 3,5% of sodium chloride.
The samples are immersed in seawater, artificial seawater and 3,5 % Nacl solution. The surface area of the samples was 5cm2. Open circuit potential (OCP), potentiodynamic polarization and weight loss methods have been used to understand the corrosion behavior of the two alloys. Comparing the OCP of mild steel and stainless steel, it was clearly seen that the OCP of stainless steel is in the higher range approximately in the range of -200 mV while the mild steel is in the range of ~ -500 mV. The OCP of stainless steel in seawater is ~100 mV nobler than the OCP in artificial seawater and NaCl solution. The corrosion behavior of stainless steel in seawater showed ennoblement when exposed to seawater.The corrosion rate of mild steel in seawater was 0,097 mm/year, in artificial seawater was 0,109 mm/year and in NaCl solution was 0,088 mm/year. No significant difference in corrosion rate of mild steel in different solution is observed. The average corrosion rate of mild steel in natural seawater, artificial seawater and NaCl solution was 0,075, 0,089 and 0,082 mm/year respectively. There was no significant difference in corrosion rate of mild steel in natural seawater, artificial seawater and NaCl solution. It is observed that the corrosion rate from weight loss method and polarisation method are similar and have little variation. The average corrosion rate of stainless steel in seawater, artificial seawater and NaCl was 0,0016, 0,0042 and 0,0017 respectively. There was no significant difference in corrosion rate of stainless steel in different solutions. However it has to be noted that, the deviation of data between different measurements was high. This is because the polarisation of stainless steel results in small active region and large passive region afterwards with increase in potential. It is observed that the corrosion rate calculated from two different methods are comparable and not significantly different. The corrosion rate of stainless steel is very less approximately 10 times less compared to the mild steel.
Microscopy surface analysis showed, that there are different sort of corrosion products on the surface of the mild steel after immersing in the three salt solutions. The reason for this is the different combinations of Fe and O. The mild steel samples immersed in seawater and NaCl had the same green rust, this color is obtained from the Fe and O combination. After cleaning the mild steel in seawater and NaCl solution had all lot of little pits on the surface, mild steel in artificial seawater had less pits but the pits were bigger. The stainless steel samples immersed in all the three salt solutions had had pits on the surface. However the size of the pits on samples immersed in seawater and artificial seawater is large compared to the size of pits formed on the surface immersed in NaCl solution. For the future research more focus should be put on the effect of the biofilm in a corrosion process.