| Aimed at achieving cost reduction and corrosion resistance assurance simultaneously for gathering pipeline, the duplex stainless steel(DSS, UNS S32205) was selected as research object, corrosion resistance in related environment was investigated. Numerous convertional tests to characterize&evaluate its microstructure, mechanical properties and corrosion resistance. Based on this, differential scanning calorimetry(DSC) were applied to insight into its transformation kinetics first, which may result in structure&properties degradation. On the other hand, immersion corrosion tests in laboratory simulant gas field environment and scanning electrochemical microscope(SECM) were used to research its corrosion behavior in macroµ domain. The results are as following:The base metal is composed of ferrite&austinite, which shows a banded structure. Widmanstatten structures have been found in weld joint, which own poor plasticity. Then inclusions&a crack(more than500um) have been observed in root weld bead, which may be coused by2%over ASTM standard of Ni content. Volume fractions of ferrite determined by "Systematic Manual Point Count" shows that volume fractions of ferrite in HAZ are29%, lower than standard requirements.Superior resistance to various environmental cracking(EC) of UNS S32205in wet H2S-CI-environment has been demonstrated, while pitting resistance in Cl" containing environment is relatively poorer. Test of pitting corrosion resistance of stainless steels in the ferric chloride solution indicates corrosion rate of base metal is288.36～618.26mdd and corrosion rate of weld joint is0.00～1.63mdd. The polarization curves show that GB/T17899appiled to UNS S32205insn’t suitable. Cathodal curves indicate hydrogen evolution on UNS S32205has been inhibited in-0.5～1.0V in3.5wt.%NaCl solution, this range may have meaning to cothodic protection.DSC results show some second phases exist in raw material, which is on solution heat treatment(holding in1050～1080℃then quenching). Orthogonal experiments indicate that a endothermic transition of a metastable second phase is most fastest in1095～1235℃, which is a function of holding temperature&cooling rate in cooling process(holding temperature:1150～1250℃, cooling rate:10-50K/min). So it can be concluded that the heating treatment process to UNS S32205(holding in1050～1080℃then quenching) of Outokumpu is improper, liklihood of detrimental phases precipitation may be considerble in the holding temperature.Under test conditions, pitting initiate both in a&y of UNS S32205, one of them turns into main cathode for hydrogen evolution, while another phase almost doesn’t participate in reactions, but pitting current peaks on former phases are larger. Hydrogen evolution may happen on CRA elements enriched region both in a&γ. Except for individual positive current peaks&negetive current peaks, which correspond to CRA elements enriched region&CRA elements depleted regions(or scales), there are pairs(or groups) of positive current peaks&negetive current peaks. Anodic dissolution regions may correspond to two structures:CRA elements depleted regions or KCl scales(more likely to be), In the former case, according to the distance between CRA elements depleted&CRA elements depleted enrichment region. These cases in a may attribute to spinodal decomposition reaction in which the d-ferrite decomposes into iron-rich a and chromium-enriched a’, but these cases in y have still been unknown. With inductive current peaks’projections of all SECM, all shapes of these projections(rhombus, polygon, cone, geminai-shape or spindle-shape) share a common characteristic—their sharper corner orientate to another nearby projection. |
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