This paper aimed to evaluate the polyurethane (pu) and polyurethane/polyvinyl chloride (90 wt.%/10 wt.%) as organic coating of carbon steel substrate against marine environment (3.5 wt.%) as a severe corrosion environment. The electrochemical impedance spectroscopy (EIS) and fitting impedance data by ZsimpWin 3.22 software were used to estimate the physical barrier of the samples for different exposure times. Different equivalent electrical circuits were taken for the physical barrier at different immersion times to get appropriate fitting. Both pu and pu/pvc coatings showed excellent corrosion protection for steel .The pu/pvc coating showed better protection and stability than pu coating against aggressive salt solution. The relation between dielectric properties (impedance spectrum) of coating film with their behavior as physical barrier protection gives useful information about electrochemical process and predicts the barrier protective behavior in natural environment.

The tension test was used for studying the effect of stress variance on stress corrosion cracking susceptibility of low carbon steel immersed in 0.1 M of the sulphuric acid solution. Three unequal values of stress applied on three similar specimens after preparing and grinding them in three metal frames that were manufactured for this purpose were used for 30 days of exposure. The stress increased and the acidic solution effect caused more corrosion and cracking so, the weight loss was more. The relation between stress and weight loss variance was approximately linearly dependent. The corrosion rate was calculated using the weight loss method, and its value was also increased because of the increase of weight loss. The sulphuric acid solution was a good corrosive medium for low carbon steel because of the high percentage of Iron (Fe) which is approximately 98, and a low percentage of Cr and Mo minerals.