Corrosion Behaviors And Evolution Of Marine Engineering E690 Steel In Marine Splash Zones

Marine splash zone is the place where ocean engineering low-alloy steel corrodes most severly.Research has focused on the corrosion types and environmental impact on the corrosion behavior of low-alloy steel in marine splash zone.Still,there is no effective attention to the typical marine splash zone and the rust layer performance of low-alloy steel in the splash zone.China has a vast territory,and the marine environment is extremely different from north to south.At the same time,rust layer is an important factor in determining the weatherability of low-alloy steel.Therefore,it is of great significance to study the corrosion behavior of low-alloy steels in different marine splash zones in China.In this work,corrosion behavior of E690 low-alloy steel for marine engineering in Qingdao and Sanya marine splash zone,the corrosion behavior of the rust layers,the effect of alloy compositions,the development of broad pitting corrosion under the rust layers and the stress corrosion cracking（SCC）behavior under ion concentration were studied.The control factors,corrosion behavior and evolution of low-alloy steel in the splash zone of Qingdao and Sanya were systematically studied and the following conclusions were obtained:（1）The non-iniform cation selective rust layers were formed in Qingdao marine splash zone.The inhibition effect of the rust layer to anions gradually decreases,and the corrosion rate gradually increases with a power index coefficient of 1.34.Rust layers formed in Sanya have a uniform inhibition to anions which increases with time,and the corrosion rate gradually decreases with a the power index coefficient of 1.01.（2）Dry-wet alternaty drives the anions overcome the cation selective property of the rust layer.The Cl^-concentrated in the rust layer of Sanya.and the concentration gradually increase.Cl^-concentration is constant in Qingdao,while the content of S-containing anions gradually increases.The anions in the rust layer has a competitive mechanism.In the SO₂-containing environment.the S-containing anions dominates the process.（3）The Cr,Ni,Mo and P alloying elements in low-alloy steels achieve good effects on inhibiting the diffusion of seawater electrolytes at the initial stage of diffusion control.The Mn.Cu and Ti alloying elements achieve good effects on inhibiting the diffusion of seawater electrolytes at diffusion control stage.The Fe,C and S contents reduce the ability ofinhibiting diffusion,but the total content of alloying elements should not exceed to 5%.（4）Ion concentration leads to severe pitting corrosion of low-alloy steel in both Qingdao and Sanya marine splash zones.The pitting is more sever in Sanya than Qingdao,but the pitting corrosion in Qingdao is gradually getting worse.The enrichment of Cl^-in Sanya environment destroyed the rust layer of the metal surface,increasing the surface roughness of the metal,promoting the anodic dissolution,and then pittings in anodic dissolution areas are promoted.The dense rust layers are produced in SO₂-containing environment,making the surface of metals have lower roughness.The regeneration mechanism of sulfuric acid promotes the development of pitting corrosion in Qingdao.（5）The SCC susceptibility of E690 low-alloy steel is less affected by Cl^-concentration.The SCC susceptibility of E690 low-alloy steel is higher when the Cl^-concentration is 14.5%or saturated.Under the synergistic effect of Cl^-and SO₂ concentration,the SCC susceptibility of E690 low-alloy steel in the highest when NaHSO3 concentration is low.In the SO₂ and Cl^-containing electrolyte,the low concentration of SO₂ weakens the uniform deformation capability of E690 steel.