Corrosion Behavior of S420 Steel in Different Marine Zones
The effect of diverse marine zones on the corrosion behavior of low alloy steels exhibits unique environmental characteristics,leading to significant differences in the corrosion rate of steels,as well as the composition and structure of rust scales.In this paper,the corrosion behavior of S420 steel,exposed in four different marine zones,i.e.atmospheric,splash,tidal,and immersion zones in the sea ar-ea of Qingdao by 36° 06'N and 120° 25'E for one year,was assessed by means of mass loss measure-ment,macroscopic and microscopic morphology observation,three-dimensional morphology detection,and corrosion product analysis.The results show that S420 steel exhibits the highest corrosion rate in the tidal zone and the lowest corrosion rate in marine atmosphere.Which may be attributed to the water-hold-ing ability of the rust scale,thus providing sufficient electrolyte for the cathodic reaction.On the other hand,in the tidal zone,the wet-dry cycle results in the increase of the Cl-concentration,accelerating the anodic reaction.The formed corrosion products are mainly composed of γ-Fe2O3,Fe3O4 and α/γ-FeOOH.The persistent presence of the electrolyte film may facilitate the formation of γ-FeOOH,making it domi-nant in the rust scales formed in tidal zone and full immersion zone.In the splash zone,the production of Fe3O4 may be promoted due to the synergist of adequate oxygen supply and wet-dry cycle,thus Fe3O4 is dominant in the formed rust scale.In the marine atmosphere,the thickness of the formed rust scale is the smallest,and the value of α/γ* is the largest,which has protective effect against further corrosion of the steel substrate to certain extent.In the tidal zone,the thickness of the formed rust scale is the highest and the value of α/γ* is the lowest,which is loose and porous,and the number and width of cracks within the rust scale are larger and wider,resulting in worst protective effect for the substrate.In other words,the S420 steel exhibits obvious localized corrosion characteristics with the maximum depth and high vol-ume of pits in the tidal zone.
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