The Study On Corrosion Behavior Of Low Alloy Steel Weided Joints In Seawater

When low alloy steels are used to manufacture ships，the welding zone is oftensubject to serious corrosion. Shielded metal arc welding (SMAW), submerged arcwelding (SAW) and carbon dioxide shielded gas welding (CO2welding) are the mainwelding methods to manufacture the ships. In order to study the performance of lowalloy steel welded joints in practical applications, in this paper, the microstructure,corrosion behavior and stress corrosion cracking (SCC) susceptibility of the three kindsof welded joints made by SMAW, SAW and CO2welding were investigated. The opencircuit potential (OCP) and electrochemical noise (EN) technologies were utilized todetect the SCC process. The research can provide theoretical support and data referencefor low alloy steel weld assembly applications in the marine environment.The microstructures and the corrosion behavior in seawater of the three kinds ofwelded joints were studied using optical microscopy (OM), scanning electronmicroscopy (SEM), electrochemical test and slow strain rate test. By means ofmetallographic observation found that the weld metal was composed of fine acicularferrite and fine granular bainite, heat affect zone main contain granular bainite, temperedsorbite and fine acicular ferrite, grain size nonuniform, the microstructure of the basemetal contain fine acicular ferrite and tempered sorbite. Welded joints corrosionresistance in seawater were studied by electrochemical impedance spectroscopy andpolarization curve. The results shown that the corrosion resistance of base metal wasworst, the corrosion resistance of weld metal was best. The corrosion rate of submergedarc welded joint maximum（0.04mA/cm2）, and the corrosion rate of gas shielded weldedjoint minimum（0.025mA/cm2）.Galvanic corrosion tendency of welded joints were studied by galvanic corrosiontest. Galvanic current fluctuated near0μA/cm2, which appeared several times cathodeand anode conversion. Corrosion resistance of welded joints difference regions wereapproximate, galvanic corrosion tendency of welded joints lower.The SCC sensitivity of SMAW, SAW and CO2welding were studied by slowstrain rate test. The results indicated that shielded metal arc welding(SMAW),submerged-arc welding(SAW), carbon dioxide shielded gas welding (CO2welding) samples have higher resistance to SCC in seawater, in which submerged arc weldingsample stress corrosion susceptibility maximum, shielded metal arc welding(SMAW)sample stress corrosion susceptibility minimum. The fracture position of SMAW andGMAW specimen was in base metal zone, but SAW specimen occurred in the heataffected zone.Open circuit potential were used to study typical characteristics of stresscorrosion cracking process of low alloy steel welded joints. The results shown that opencircuit potential would occur mutation in the process of yield and fracture, while crackinitiation and propagation wouldn’t be detected during stress corrosion cracking.SCC process of welded joints in slow strain rate test was studied by Zeroresistance galvanometer (ZRA) mode electrochemical noise. By the time domain analysis,the characteristic parameters of frequency domain analysis studied the electrochemicalnoise of welded joints of low alloy steel. It obtained potential and current ENcharacteristics and explored the change of Rn and WLin SCC process. Thus, ENcharacteristic were obtained the judgment to distinguish stress corrosion crack initiationand propagation．...