THE RELATIONSHIP BETWEEN PITTING SUSCEPTIBILITY AND INCLUSIONS IN CARBON STEELS IN ARTIFICIAL SEAWATER

The pittings of five carbon steels have been studied by using anodic polarization test and microscope observation. The results show that pitting susceptibilities are related to the types and shapes of inclusions which are the sites of pitting initiation. The pitting initiating at inclusion needs a potential which is defined as pitting potential and the pitting potential is determined by the type and shape of inclusion. The influence of oxygen content in steel on pitting potential is also discussed.

Influence of sulfur on inclusion and pitting resistance of 316L stainless steel

The effect of the sulfur content on the microstructure and inclusions of 316 L stainless steel was studied by optical microscopy,scanning electron microscopy,and image analysis,and the effect of sulfur on the pitting corrosion resistance of 316 L stainless steel was studied by conducting ferric chloride immersion test and plotting the electrochemical polarization curves.The results show that the added sulfur is mainly in the form of manganese sulfide inclusions in 316 L stainless steel.With increases in the sulfur content,the grade and percentage of the sulfide in the steel gradually increased,and its distribution became increasingly dense.When the sulfur content exceeded0.1%,the number of sulfide inclusions in the sample increased sharply.When the sulfur content reached 0.199%,the sulfides in the steel were primarily in spindle form,and a large number of spindles were found to refine the grain size of 316 L stainless steel.The pitting corrosion weight loss rate of 316 L stainless steel increased with increases in the sulfur content,while the pitting potential gradually decreased.However,the pitting potential of 316 L stainless steel rebounded when the sulfur content reached 0.199%,which may be related to the grain refinement of the test steel and requires further study.

Electrochemical Method for Evaluating the Passivated State of Medical StainlessSteel 317L

The electrochemical behavior of the medical Stainless steel 317L which is used as in vivo fixation materials has been investigated at different passivated states in several candidate-testing solutions. The potentiodynamic scanning polarizaion technique was employed to measure the polarizaion curves of 317L in 0.9%NaCl solution at 37℃. The results showed that the electrochemical behaviorin 0.9% NaCl solution at 37℃ and in 2% NaCl solotion at 30℃ can be better used to detect and evaluate passivated states and corrosionresistance of 317L. In addition,the pitting potential Eb can be used as a criterion and itS lower limit could be 0.85 V(SEC) for this system.

Passivation method of 317L stainless steel used in medicine

The electrochemical performance of 317L stainless steel used in medicineunder different conditions of passivation (different contents of HNO_3 solution, differentpassivation time and different passivation temperatures) was studied. The results show that thepitting potential of 317L stainless steel used in medicine can reach about 1.0 V (SCE) whenelectrochemically tested in 0.9% NaCl solution after the steel was passivated in 30% HNO_3 solutionat 35℃ for 6 h, which indicates that the passivation film has a relatively strong resistance tocorrosion. The results also show that the corrosion resistance of the passivation film on thesurface of 317SS can be increased after suitable amount of K_2Cr_2O_7 is added into HNO_3passivation solution.

Influence of cold deformation on microstructure,mechanical properties,and corrosion resistance of high-sulfur free-cutting 316LS stainless steel

In this study,the influence of plastic deformation produced by cold rolling at reduction ratios ranging from 10% to 80% on the microstructure,mechanical properties,and pitting corrosion behavior of high-sulfur freecutting 316 LS austenitic stainless steel was investigated. The results indicate that slipping is the predominant effect and that sulfide inclusions extend along the rolling direction during the cold deformation of 316 LS. The strong austenite stability of 316 LS results in the formation of only a small quantity of deformation-induced martensite. The experimental results reveal that the strength,hardness,and yield ratio increased with increases in the reduction ratio,mainly due to work hardening,whereas the elongation decreased drastically,due to the combined effect of the work hardening and brittleness caused by the numerous sulfide inclusions. Electrochemically active sites on the surface of316 LS increased with an increased reduction ratio,which caused an increased current fluctuation in the passive zone. This also caused the breakdown potential（ E_b） near the pitting zone to exhibit a gradual increase in the zigzag current shift to the left on the polarization curves. The E_b of 316 LS decreased with increases in the reduction ratio,mainly due to the extended sulfide inclusions,the increased dislocation density,and the deformation-induced martensite content.

Effects of chloride ion concentration and pH values on the corrosion behavio of Cr12Ni3Co12Mo4W ultra-high-strength martensitic stainless steel

The effects of Cl ion concentration and pH values on the corrosion behavior of Cr12Ni3Co12Mo4W ultra-high-strength martensitic stainless steel（UHSMSS） were investigated by a series of electrochemical tests combined with observations by stereology microscopy and scanning electron microscopy. A critical Cl- ion concentration was found to exist（approximately 0.1wt%）, above which pitting occurred. The pitting potential decreased with increasing Cl- ion concentration. A UHSMSS specimen tempered at 600°C exhibited a better pitting corrosion resistance than the one tempered at 400°C. The corrosion current density and passive current density of the UHSMSS tempered at 600°C decreased with increasing pH values of the corrosion solution. The pits developed a shallower dish geometry with increasing polarization potential. A lacy cover on the pits of the UHSMSS tempered at 400°C accelerated pitting, whereas corrosion products deposited in the pits of the UHSMSS tempered at 600°C hindered pitting.

A New Resource-Saving,High Chromium and Manganese Super Duplex Stainless Steels 29Cr-12Mn-2Ni-1Mo-xN

A new family of resource saving, high chromium and manganese super duplex stainless steels （DSSs）, with a composition in mass percent, % of Cr 0.29, Mn 0.12, Ni 2.0, Mo i. 0, and N 0. 51-0. 68, has been developed by examining the effect of N on the microstructure, mechanical properties and corrosion properties. The results show that these alloys have a balanced ferrite-austenite relation. The austenite volume fraction decreases with the solution treatment temperature, but it increases with an increase in N content. The increases in nitrogen enhance the ultimate tensile strength （UTS） and reduce the ductility of the material slightly. The pitting corrosion potential increases first and then decreases with an increase in nitrogen content when the amount of N arrives to 0.68%. The yield stress and ultimate tensile strength of solution treated samples were more than 680 and 900 MPa, the elongation of experimental alloys are higher than 30%, respectively, what is more, the pitting potentials were beyond 1 100 mV.