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In situ investigation of crevice corrosion on UNS S32101 duplex stainless steel in sodium chloride

solution

Presented by- Pintu Kumar (13MT60R30)

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• Duplex stainless steels (DSSs) consist of ferrite phase and austenite phase,

Properties it offers• high strength and • resistance to localized corrosionlean DSS with lower nickel contents, have attracted a lot

of attention for its reduced resistance to such form of corrosion.

• DSS 2101 (UNS S32101) is such a grade of lean DSS

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• To investigate the mechanism of crevice corrosion on UNS S32101 duplex stainless steel in a spontaneous passive system

• To provide a better understanding of the diversity of crevice corrosion morphology

The aim of experiment

critical crevice solution mechanism

• Induction period of crevice corrosion

• Accumulation of aggressive ions within crevice

• Subsequent de passivation and active dissolution of base metal within crevice

• Migration of chloride and hyroxyl ions into crevice

• depletion of oxygen in the crevice and separation of anodic and cathodic reactions

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According to CCS theory

• The most severely attacked area-deepest regions of the crevice.

• crevice corrosion can not go into propagation stage directly to allow significant variation in crevice solution composition.

Observations made, however, contradicted the CCS theory

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IR mechanism

• It states, IR> ΔΦ criterion,for the onset of crevice corrosion where I is the ionic current flowing out of the crevice and R is the resistance of the crevice electrolyte,

• ΔΦ is the difference between the applied potential on crevice outer surface and the active/ passive transition potential.

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Experimental

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experimental

• The specimens -commercial UNS S32101 duplex stainless steel plates

• The chemical composition

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• plates as specimens were solution annealed at 1050 C for 0.5 h followed by water quenching. Samples were mounted in epoxy resin to expose an area of 4 cm2 to serve as working electrode

• Prior to tests, the working electrodes were ground with emery papers.

experimental

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• Followed by cleaning with acetone and methanol, washed in double-distilled water and derided in air thoroughly before use.

• The interfaces between the epoxy and sample were sealed to prevent unwanted crevice corrosion

• then, allowing the system to react for 0.5 hr in 0.1 M NaCl

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Fig. 1. Schematic diagram of the experimental setup for the in situ observation of crevice corrosion

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Electrochemical measurement

• a potentiostat -To perform the Electrochemical measurements

• platinum plate- counter electrode• Saturated calomel electrode- reference electrode• Cell arrangement is done to realize an artificial

crevice as shown before• Electrolyte-0.1 M NaCl solun, 0.1M HCl+ 0.1M NaCl• Applied potential- 0.1V, 0.3V, 0.5V• Potentiodynamic polarisation curve-(-0.7 to 0.8V)

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Optical and SEM/EDX characterization

• scanning electron microscope- to investigate microstructure and corrosion morphologies

• surface profiler- to find surface depth profile

• Camera-For recording the changes on the crevice wall during crevice corrosion

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Results and discussion

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Potentiodynamic polarization curves

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Potentiodynamic polarization curves

• In acidic-chloride media, As the crevice solution becomes more concentrated in

aggressive ions, the active peak becomes larger and active/ passive transition potential increases and ΔΦ decreases making IR> ΔΦ, as reqd for active dissolution of base metal.

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In 0.1 M NaCl with crevice• corrosion potential lower-due to restricted area of crevice

hindering reduction of oxygen gas• Higher passive current density-larger net anodic current

density• Passive region-current density independent of potential

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Potentiostatic polarization to initiate crevice corrosion

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Potentiostatic polarization to initiate crevice corrosion

• At 0.1V(applied potential), Cell current low, crevice corrosion in induction stage.• At 0.3 V,A delayed crevice corrosion (induction + propagation). • At 0.5V,Immediate crevice corrosion-Measured cell current

increased more rapidly than the one at 0.3V.

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In situ visual inspection of crevice corrosion

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In situ visual inspection of crevice corrosion

• During the early stage of crevice corrosion, the corrosion products increase resistance of crevice corrosion making IR> ΔΦ, reqd for onset of reaction

• Corrosion products on parts of the wall reduce active current lowering local IR voltage turning part of the crevice wall into passive state hence relocation of active dissolution region occurs.

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Ex situ morphology analysis

Region-I(centre of the crevice)

Region-IISevere corrosion

Boundary Between II & III

Region-IIIFerrite phase slightly etched

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A,C-most severe corrosion ( crevice mouth)B- lightly corroded(centre of crevice)

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Fig. a- pot. Distribution on crevice wall in the induction stage of delayed corrosion

Fig. b- Pot. Distribution in the propagation stage

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Induction stage• As the aggressive ions accumulate in the crevice

solution, the passive films become unstable.• The generated current fluxes result in large IR drop

with which the HER potential was reached in the crevice.

• The evolution of H2 could further increase the crevice solution resistance R.

• Consequently, a significant larger IR voltage is produced by both larger current I and R that the IR > ΔΦ criterion would be met. The induction stage ends and propagation stage starts

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Propagation stage

• a net anodic polarization curve with active peak will inhabit on the crevice wall.

• The crevice wall is subsequently attacked and the corroded area expands towards crevice opening with the moving passive/active boundary

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Conclusions

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Experimental results based on IR mechanism

1. Delayed and immediate crevice corrosion can be initiated by potentiostatic polarization at EAPP = 0.30 VSCE and EAPP = 0.50 VSCE, respectively in neutral 0.1 mol/L NaCl solutions at room temperature.

2. The transition from induction stage to propagation stage of the delayed crevice corrosion was explained by IR mechanism with variation of the crevice electrolyte composition.

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3. Diversity of crevice corrosion morphology –due to the relocation of active dissolution region on the crevice wall that occured as a result of the effects of corrosion products.

4. Reason for the immediate crevice corrosion -The current fluxes caused by passive/active transition of passive films on the crevice wall

conclusion

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References

• Yang Y. Z., Jiang Y. M., Li J. , in situ investigation of crevice corrosion on UNS S32101 duplex stainless steel in sodium chloride solution, corrosion science 76 (2013) 163-169

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Thank you