Corrosion Performance Investigations Of High Strength Low Alloy Weathering Steel

The high strength low alloy weathering steel is an important direction of research and application development of high strength low alloy steel(HSLA),the research and development of low alloy high strength weathering steel with good comprehensive mechanical properties and excellent corrosion resistance has important and urgent significance.The present research reports the influence of different heat treatments and Cr?Ni elements on microstructure and properties of weathering steel and its corrosion behaviors in 3.5% NaCl solution with optical microscope(OM),scanning electron microscope(SEM)with its energy dispersive spectrometer(EDS),X-ray diffractometer(XRD),tensile testing,immersion test and electrochemical test,the main conclusions are as follows:The microstructure and corrosion properties of as-rolled,annealed and normalized Q500 H weathering steels were analyzed.The results showed that the three kinds of specimens were composed of pearlite and ferrite.The grain size of as-rolled steel was the largest,followed by annealed and normalized.The corrosion current density of bare steels in as-rolled,annealed and normalized states measured by polarization curves were 0.1287×10-5A·cm-2,0.4418×10-5A·cm-2,0.7364×10-5A·cm-2 and 1.064×10-5A·cm-2,respectively.In addition,the charge transfer resistance of steels measured by electrochemical impedance spectroscopy was decreased in turn.After five month,the surface of as-rolled steel was the smoothest after removing the corrosion products.The composition of rust layer in the as-rolled steel was ?-FeOOH,?-FeOOH,?-FeOOH and Fe3O4.The rust layer of as-rolled steel was densest and thinest,the normalized steel was loosest and thickest.The annual corrosion rate of as-rolled steel was the smallest,second was annealed steel and the normalized steel was the largest by weight loss test.The microstructure and corrosion properties of hot rolled Q500 H and Q700 H low alloy weathering steels were analyzed.The results showed that the two kinds of specimens were composed of pearlite and ferrite.The Cr element was distributed in the two phases uniformly,and the content of Ni is more in the ferrite.The corrosion rate of Q700 H steel with higher content of Cr/Ni was 0.1068 mm/y,which was reduced by 4.4% compared with Q500 H steel.After immersion for 60 days,the current density of samples increased.Compared to Q500 H weathering steel,Q700 H weathering steel has higher value of charge transfer resistance and smaller value of current density.The corrosion pits in surface of Q700 H steel was smaller after 5 months immersion.The composition of the rust layers of two kinds of hot-rolled weathering steel were the same,the increase of Cr,Ni promoted the formation of ?-FeOOH and Fe3O4.After 5 months,Ni and Cr elements were enriched in the rust layer,Cr was concentrated in the inner layer,and the content of Ni at the interface between the substrate and the rust layer was more.The more the content of Cr and Ni was,the more obvious the enrichment was,so the rust layer was more dense.The microstructure and corrosion properties of thermo-mechanical controlled processing Q500 H and Q700 H low alloy high strength weathering steels were analyzed.With the increasing of Cr and Ni content,the tensile strength of weathering steels were improved significantly,the yield strength increased from 585 MPa of Q500 H steel to 749 MPa of Q700 H steel.Compared to Q500 H weathering steel,the Q700 H bare steel with more content of Cr?Ni had smaller value of current density and higher value of charge transfer resistance.The composition of the rust layers of two kinds of weathering steel were the same,the increase of Cr,Ni promoted the formation of stable phase Fe3O4.The increase of Cr,Ni promoted the continuity of rust layer in Q700 H steel,and enhanced the protective ability of rust layer in ten days.Ni and Cr elements in Q700 H steel which has higher content of alloying enriched significantly in the interface of the rust layer and the matrix after 5 months,thus enhancing the adhesion of rust layer and substrate,the annual corrosion rate decreased.