Initial Corrosion Behavior and Local Corrosion Origin of 9%Cr Alloy Steel in Cl-Containing Environment
The South China Sea is a marine atmosphere environment with high humidity,high salt content,and strong radiation.Traditional weathering steel and 3Ni advanced weathering steel cannot meet the service requirements in the South China Sea environment,necessitating the development of steel with improved corrosion resistance.Alloy steels with Cr of 2.5%-10%(mass fraction)provide a mar-ginal gain in corrosion performance at a low cost and have great potential for marine atmospheric applica-tion.A 9%Cr alloy steel was designed to obtain higher corrosion resistance,and the relevant results can offer a reference for developing novel corrosion-resistant steels for the marine atmospheric environment.The initial corrosion behavior of 9%Cr alloy steel in a Cl-containing environment was investigated using dry-wet cycle test,SEM,TEM,XRD,and electrochemical approaches,and the effects of composite inclu-sions(Mg,Si,Al)O-MnS and Cr-rich M23C6 on its local corrosion behavior were discussed.The findings demonstrate that the initial corrosion resistance of alloy steel was more than 12 times that of 09CuPCrNi,and local corrosion occurred during the 360-h dry-wet cycle.Pits' depth below the rust layer followed the lognormal distribution,and the pits' maximum depth(Dmax)and average depth(Dave)with time(t)were in line with the power functions Dmax = 8.4844×t 0.65717 and Dave = 7.3181×t 0.53866,respectively.The rust layer' s compactness and the α/γ* ratio increased over time,but the addition of high Cr delayed the corrosion.Thus,the rust layer did not entirely cover the surface and only provided limited protection,and an expo-nent value obtained by fitting the weight loss according to the power function was greater than 1.(Mg,Si,Al)O-MnS caused metastable pitting corrosion through a preferential dissolution of MnS or MgO regions,but its immersion in 2%NaCl solution for 300 min did not induce surrounding matrix's dissolution.The Cr consumption caused by Cr-rich M23C6's precipitation was the primary reason for preferentially inducing lo-cal corrosion.
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