The effect of small additions of ruthenium on the pitting corrosion resistance of LDX2101 duplex stainless steel

摘要

This dissertation is a study of the effect of small additions of ruthenium on pitting corrosionudresistance of LDX2101 duplex stainless steel. Four stainless steel alloys with incrementaludruthenium (wt %) as per Table I were produced from pieces cut from commercial LDX2101udduplex stainless steel plate with the manufacturer’s composition of 0.03C, 0.22N, 21.5Cr,ud1.5Ni, 0.3Mo and 5.0Mn plus, pressed ruthenium powder with purity of 99.8%. Afterudsolution annealing the samples, the actual chemical composition was analysed using XRFudanalysis and then, ASTM A923 (01.03) Test method A – Sodium Hydroxide etch test forudclassification of etch structures of duplex stainless steel was used to analyse theirudmicrostructure.udTable I: Chemical composition (wt %) of alloys which were produced.udAlloy Designation Targeted Composition (wt %)udA 0.03C, 0.22N, 22.26Cr, 1.58Ni, 0.25Mo and 4.99Mn + 0.13RuudB 0.03C, 0.22N, 22.46Cr, 1.50Ni, 0.29Mo and 5.14Mn + 0.20RuudC 0.03C, 0.22N, 22.31Cr, 1.60Ni, 0.26Mo and 4.84Mn + 0.31RuudD 0.03C, 0.22N, 20.10Cr, 1.37Ni, 0.30Mo and 4.32Mn + 0.66RuudNOTE: C and N shown in italics were not measured.udCorrosion potentials and pitting potentials of these samples were evaluated using audpotentiodynamic polarisation technique and the results were compared to corrosion potentialsudand pitting potentials of control alloys: LDX2101, 304L and 904L. The tests for bothudproduced and control alloys were carried out in naturally aerated 3.56% sodium chlorideud(NaCl) aqueous solution at 25oC±2oC. The results indicated that small additions of rutheniumudslightly improved the corrosion potential of the resulting alloys. However, there was audsignificant improvement on the pitting potential of the resulting alloys compared to LDX2101udand 304L stainless steels. Exposed to the same experimental environment, 904L stainlessudsteel did not experience pitting corrosion. Potentiodynamic polarisation evaluation ofudLDX2101 with 0.66%Ru samples in de-aerated 3.56% NaCl showed a decrease in bothudpitting and corrosion potentials with the decrease in oxygen content in the NaCl aqueousudsolution. Microstructural analysis results indicated that ruthenium addition has no detrimentalududeffect to the microstructure of the resulting alloys. However, alloys containing rutheniumudwere not commercially viable as the pitting corrosion resistance benefit ruthenium brought did not offset the cost of adding the ruthenium to LDX2101 stainless steel.