Study On Corrosion Behavior Of Sherardized Carbon Steels In Seawater

Shearardizing is a chemical heat treatment process, which is used to form Zn-Fe alloy, namely shearardized coatings. They may act as sacrificial anode protecting carbon steel from the corrosion of chemical media. They have extensive industrial applications in marine environment due to the low consumption rate and better corrosion resistance. However, pitting corrosion is inclined to occur on shearardized carbon steel during their service in seawater heat exchanger.Solid powder embedding method was used to make thermal diffusion sherardizing penetrations to 20# carbon steel in this paper. The whole immersion corrosion experiment of sherardized carbon steels were carried out in sea water at 20℃,40℃,60℃, and corrosion products of sherardized carbon steels in seawater were analyzed, the results show that corrosion products of sherardized carbon steels in seawater at 20℃are in the form of zinc hydroxychloride, and the corrosion products in seawater at 40℃and 60℃are in the form of zinc oxide and zinc hydroxychloride.In order to prove the mechanism of corrosion of sherardized carbon steel in seawater, the galvanic corrosion experiments of sherardized carbon steel with carbon steel, carbon steel with carbon steel covered by zinc oxide film, sherardized carbon steel with sherardized carbon steel covered by zinc oxide film were carried out respectively, and variation law of galvanic current, galvanic potential and open circuit potential of galvanic couples in 20℃, 40℃,60℃seawater with immersion time increasing were monitored. The results show that the pitting corrosion of sherardized carbon steel in seawater is related to the formation of zinc oxide in high temperature, and the process of pitting corrosion is accelerated with seawater temperature increasing.Sherardized coating can serves as sacrificial anode to protect carbon steel substrate when it is uniform corrosion in 20℃seawater, but it is inclined to passivation for zinc oxide formed in corrosion products of sherardized carbon steels in seawater as temperature increasing, and it promotes the formation of macrogalvanic cell on the surface of sherardized carbon steel, and which results in failure of sherardized coating and pitting corrosion.