1. 課 程:高等物理冶金 課堂教師:戴子堯 副教授 報 告 者:蔡中銘
Effect of grain size on pitting corrosion of 304L austenitic stainless steel
不同的尺寸晶粒對於不鏽鋼304L的點蝕
課 程:高等物理冶金
課堂教師:戴子堯 副教授
報 告 者:蔡中銘

2. Table of Contents I. Introduction II. Experimental procedure
III. Experimental results
IV. Conclusions

3. Introduction
Pitting corrosion is characterised by two consequent stages comprising pit initiation and stable pit growth. Since pit initiation and transition to stability are considered to be the key factors in pitting corrosion phenomenon,evaluation of metastable pitting gives valuable information about pitting corrosion. It is known that the environment, the chemical composition, heat treatment, and the microstructure affect the pitting potential.

4. Introduction
Grain size variation could be achieved using a number of different processing including rolling, extrusion, and electrodeposition. In the present work, the effect of grain size on pitting corrosion of 304L stainless steel was investigated by evaluation of pitting probability using a series of potentiodynamic experiments.

5. Experimental procedure
Chemical composition of experimented 304L SS (wt.%).

6. Experimental procedure
AISI304L
1050 oC/1hr
70% of thickness.

7. Experimental procedure
Influence of grain size on pitting corrosion resistance
Time
3 min
15 min
30 min
1050 oC

8. Experimental procedure
Electrochemical measurements
3.5 wt.% NaCl
All potentials quoted in this work refer to SCE.
ambient temperatures
SCE:飽和甘汞電極（實際指氯化鉀達飽和）於25℃時的電極電位約為0.244V。

9. Experimental procedure
scan rate30 mV min-1
sweeping the potential from 50 mV below the rest potential
Potentiodynamic polarisation measurements
current density exceeded 300μA cm2
current density exceeded 100μA cm2
pitting potential

10. Experimental procedure
Sample
Test
Electro-etched in 60% HNO3
Ultrasonic
SEM & EDS

11. Experimental results
Sample optical micrographs of 304L SS after recrystallisation annealing at
1050 C for: (a) 3 min, (b) 15 min, and (c) 30 min.

12. Experimental results
shows the average grain diameters versus recrystallisation annealing time.

13. Experimental results
An SEM image of metastable pits in sample with 28m grain.
High content of the manganese and sulphur indicate that pits nucleate on the MnS inclusions sites.

14. Experimental results
shows the distribution of inclusions was not changed by recrystallisation annealing time.

15. Experimental results
Typical potentiodynamic curves of 304L SS with various grain sizes.
The cumulative probability of pitting potential distribution of 304 stainless steel with different grain sizes obtained from potentiodynamic polarisation conducted in 3.5 wt.% NaCl.

16. Experimental results
Metastable pit frequencies.

17. Experimental results
Calculated radius of metastable pits for 304L SS with various grain sizes plotted as the cumulative distribution using the probability function

18. Conclusions
Potentiodynamic measurements conducted in 3.5 wt.% sodium chloride solution revealed that grain size variation has no considerable effect on pitting potential of 304L austenitic stainless steel.
Obtained results from potentiostatic tests revealed that grain refinement decreases metastable pit frequency.

19. Thank you for your attention & Q&A