Corrosion resistance of mild steel (hull plate) in sea water in the presence of a coating of an oil extract of plant materials

Mild steel can be used as hull plates in ship industry.Mild steel has to be in contact with sea water which contains aggressive sodium chloride ions to the extent of 3.5%.This leads to severe corrosion problems.To mitigate this corrosion problem an oil coating containing extracts of natural products has been used.The corrosion protection nature of this oil coating has been evaluated by weight loss method which reveals that the oil coating offers 99% inhibition efficiency to mild steel immersed in natural sea water.The polarization study reveals that the coating acts as a barrier film controlling the anodic reaction predominantly.The corrosion potential is shifted from -0.875 V SCE to -0.611 V SCE.The linear polarization value increases from 1630 Ohm•cm 2 to 9238 Ohm•cm 2 (IE = 82.35%).The corrosion current decreases from 1.918×10 -5 A/cm 2 to 0.3538×10 -5 A/cm 2 (IE = 81.55%).These factors confirm that the oil coating controls the corrosion of hull plates in sea water.The AC impedance spectra confirm that the protective coating is very stable as revealed by the fact that in the presence of coating on mild steel, its charge transfer resistance increases, impedance increases, whereas corrosion current decreases to a great extent.Vickers hardness study confirms that in the presence of sea water, the hardness of mild steel decreases whereas in the presence of coating the hardness of mild steel increases.SEM images of various metal surfaces reveal that in the presence of sea water alone, pits are noticed on mild steel whereas in the presence of oil coating the surface appears to be smooth, when immersed in sea water.When mild steel is immersed in sea water, the contact angle decreases when compared with polished metal.But for oil coated mild steel the contact angle increases, hydrophobicity increases and hence corrosion protection increases.It is suggested that this oil coating containing extracts of plant material may be coated on hull plates made of mild steel to protect them from severe corrosion due to the aggressive ions present in sea water.