Study On The Corrosion Behaviors And Protection Of Hull Steels In Marine Environment

Seawater is recognized to be very corrosive for metal. Especially, with the pollution aggravated, the marine environment becomes more complex which makes metal corrosion more serious. Therefore, the study of corrosion behaviors and protection of the structural steels in the marine environment is of great significance. In order to study the corrosion behaviors and protection of the structural steels, 1~#, 2~# and 3~# hull steels′corrosion behaviors in sea water of deferent temperature , pH, dissolved oxygen, salinity and N, P nutrient content were studied by electrochemical test, coupon test and ESEM observation. Then the marine environmental factors′influence to corrosion rate was analyzed. Meanwhile, the synergistic effect between anticorrosion coating and cathode protection were evaluated by immersion and electrochemical test, and the minimum cathodic protection potential of the three hull steels were determined. Conclusions are summarized as follows:(1) 1~# steel, 2~# steel, and 3~# steel were manifested as uniform corrosion in seawater, and their corrosion rates had little difference.(2) The relationship between corrosion rate of hull steels and main environmental factors in seawater were studied. The results showed that: with increasing temperature in the scale of 5℃~35℃, the corrosion potentials of 1~# steel ,2~# steel, and 3~# steel decreased and corrosion rates increased; the corrosion potentials of 1~# steel, 2~# steel, and 3~# steel were slightly higher at the salinity of 1.6%, their corrosion rates increased at first, and then decreased when the salinity increased to 4.0%; the corrosion potentials and corrosion rates of 1~# steel, 2~# steel, and 3~# steel increased with increasing dissolved oxygen concentration from 0.0mg/L to 4.8mg/L, and their corrosion resistance decreased; the corrosion potential of 1~# steel, 2~# steel, and 3~# steel were a little higher at the pH of 9.5 with the pH in the scale of 6.5~9.5 , and their corrosion rates decreases almost linearly with increasing pH; N, P nutrient contents had little influence to corrosion behaviors of the three hull steels, the corrosion was more serious in the seawater containing N and P than that of the one without N and P.(3)The corrosion rates of coated specimens without cathodic protection had approached or exceeded the bare steels′when the damage rate reached 5%. The minimum cathodic protection potential of the three hull steels with 2% and 5% damage rate coated specimen should be -0.80V and it should be -0.85V for the coated specimen with 10% damage rate.