EVALUATION OF PITTING CORROSION BEHAVIOR OF 316L AND 304 STAINLESS STEELS EXPOSED IN INDUSTRIAL-MARINE-URBAN ENVIRONMEN

1. EVALUATION OF PITTING CORROSION BEHAVIOR OF 316L AND 304 STAINLESS STEELS EXPOSED IN INDUSTRIAL-MARINE-URBAN ENVIRONMENT: FIELD STUDY By P. DHAIVEEGAN Under the Guidance of Dr. N. RAJENDRAN Professor Department of Chemistry Anna University Chennai-600025

2. Atmospheric Corrosion Strategy • Global studies- Overall cost of corrosion loss is 4-5 % of GDP • 20-25 % of this cost could be avoided by using appropriate corrosion control technology • Atmospheric corrosion makes major contribution to this cost. • The aggressiveness of the atmospheric constituents can be assessed – measuring climatic & pollution factors or measuring corrosion rate of exposed metals • Factors Affecting the Atmospheric Corrosion • Relative humidity (RH) • Temperature (T) • Sulfur content (SO2) • Salinity (Chloride) • Exposure time (t) • Pollutants

3. Suffering Industries of Atmospheric Corrosion

4. Corrosion Failures on Infrastructure Corrosion in collapsed bridge at North Carolina speedway (2000) Corrosion Failure on bridges A Concrete bridge failure 4

5. Effect of ATM corrosion in Different Environments

6. Atmospheric Corrosion Studies in Chennai • Atmospheric corrosion studies in India, have been limited in number. • From the above discussion we concluded that the Atmospheric Corrosion is very high in Chennai compared with other States. • In Chennai most of these studies were aimed at establishing pollution levels and none concentrated on the marine environment. • Deterioration of these materials along the coastal regions of Chennai has been a major problem probably due to marine atmosphere.

7. Atmospheric Corrosion Studies in India • In India, atmospheric corrosion data for 26 field exposure stations were published (1970). • The rate of corrosion was found to be vary from region to region. • The intensity of attack was more in industrial area & along the seacoast. • Corrosion map of India (Natesan et al.) indicated that the rate of corrosion is spot specific and not region specific owing to insufficient data to evaluate atmospheric corrosivity. Four Types Of Environments (A) Humid–saline, (B) Humid–saline–urban (C) Humid–industrial And (D) Plain Dry–urban Environments Chennai is the zone where all the four above environments namely coastal + urban + industrial + plain dry pollutions exit.

8. Introduction • 316L and 304 SS have caught the attention in modern technology in a wide range of mechanical and corrosion resistant properties in the wide atmosphere conditions. • 140 tons of 304 stainless steel was used as the construction material to build a historic building of guildhall in London estimating it to have life span of 750 years • It is generally accepted that high corrosion resistance of stainless steel due to its passive film formation on its surface and it offers the protective ability in the corrosive environments • Over the decades, the Chennai City has emerged as an important Centre for economical, historical, and cultural and trades development in the State. • This region includes thermal power plants, petrochemical plants, ennore port, refineries, pharmaceuticals companies and residential buildings. • Region is classified as marine-industrial-urban environment.

9. Objectives • To understand the synergistic effect of chloride and sulfur dioxide in corrosion rate of steels at in Industrial-Marine-Urban Environment were studied in Chennai region. • To determine the amount of corrosive agents viz., Cl-, SO2 in the atmosphere from wet candle and sulphation plate method. • To study the surface morphology of the atmospheric corroded steels by polarized optical microscopy (POM), atomic force microscopy (AFM) and scanning electron microscopy equipped with energy dispersive X-ray analysis (SEM-EDAX). • To study the mechanical stability of the exposed samples by the Vickers microhardness test • To employ FT-Raman spectroscopy to investigate and to identify the corrosion products which could not be obtained from XRD analysis. • Corrosion behavior of exposed samples were monitored under open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarisation.

10. Experimental Procedure Exposure of specimen Specimen size - 100x50x3 cm Exposure period - 3 Years Exposure area - Industrial- Marine- Urban Oil refinery Power plant Urban area Exposure site

11. Characterization Techniques Sulphation plate Sample exposure Wet candle Correct

12. Meteorological Corrosion Parameters Spring Season Spring Season Spring Season Spring Season Rainy Rainy Rainy Rainy Summer and Fall Summer and Fall Summer and Fall Summer and Fall Figure 1 Average monthly values of (a) Temperature, (b) Relative humidity (RH), (c) Rain fall, (d) Wind speed in industrial-marine-urban environment.

13. Cl- and SO2 Content in the IMU Atmosphere Spring Rainy Summer Figure 2 (a) Seasonal variations of Cl- and SO2 obtained in the industrial-marine-urban environment.

14. Corrosion Rate of SS Summer and Fall Rainy Spring Spring Rainy Summer and Fall Figure 3 Change of weight loss of 316L and 304 SS during the exposure in the industrial-marine-urban environment.

15. Surface Appearance of Atmospheric Exposed SS 24 Month 36 Month 3 Month 12 Month 6 Month 9 Month 316L SS 304 SS Figure 4 Macroscopic images of surface appearance of the 316L and 304 SS specimens exposed for 3 years at industrial-marine-urban environment.

16. Hardness Analysis Figure 5 Evolution of surface hardness of 316L SS and 304 SS in the industrial-marine-urban environment.

17. XRD Analysis Figure 6 XRD patterns of 316L SS during the atmospheric corrosion exposure.

18. Raman Analysis 304 SS 316L SS Figure 7 Comparison of Raman spectra of (a) 316L and (b) 304 SS during atmospheric corrosion process.

19. SEM Analysis 6 Month 9 Month 3 Month 12 Month 36 Month 24 Month Figure 8 SEM images of 316L SS after atmospheric exposure in IMU environment as a function of exposure durations.

20. SEM Analysis 6 Month 9 Month 3 Month 12 Month 36 Month 24 Month Figure 9 SEM images of 316L SS after atmospheric exposure in IMU environment as a function of exposure durations.

21. Pit Depth Analysis of 304 SS 6 Month 9 Month 3 Month 12 Month 24 Month 36 Month Figure 10 SEM images of 316L SS after atmospheric exposure in IMU environment as a function of exposure durations.

22. Pit Depth Analysis of 316L SS 6 Month 9 Month 3 Month 12 Month 24 Month 36 Month Figure 11 SEM images of 316L SS after atmospheric exposure in IMU environment as a function of exposure durations.

23. AFM Analysis 6 Month 3 Month 9 Month 12 Month 24 Month 36 Month Figure 12AFM images of 316L SS after atmospheric exposure in IMU environment as a function of exposure durations.

24. AFM Analysis 3 Month 9 Month 6 Month 9 Month 6 Month 3 Month 12 Month 24 Month 36 Month 12 Month 24 Month 36 Month Figure 12AFM images of 316L SS after atmospheric exposure in IMU environment as a function of exposure durations.

25. Schematic Diagram of Mechanism of Pitting Corrosion

26. Pit depth and Surface Roughness Analysis Figure 13 Pit depth and surface roughness of the exposed 316L and 304 SS after the exposing in IMU environment.

27. XRF Analysis Figure 14 XRF analysis of elemental composition of exogenous particle deposited on the SS surface after 3 years of atmospheric exposure

28. Potentiodynamic Polarization Curves Figure 15 Potentiodynamic polarization curves of SS in 3.5% NaCl solution before and after 3 years of atmospheric exposure

29. EIS-Bode Plot Curves Figure 16 EIS curves of SS in 3.5% NaCl solution before and after 3 years of atmospheric exposure

30. Bode Phase Angle Analysis Figure 17 Bode phase angle curves of SS in 3.5% NaCl solution before and after 3 years of atmospheric exposure

31. Equivalent Circuit Diagram Cpass (b) Rs (a) Cpass Cdl Rs Rct Rct Rct w Passive Film Solution Steel Figure 18 Equivalent circuits for fitting the EIS data. (a) is for bare SS: (b) is for atmospheric corrosion exposed in IMU environment.

32. Conclusions • High alloy steels viz., 316L and 304 were field exposed for the period of 3 years (2012-2015). • Surface appearance through photographic images of macroscopic morphology of the Exposed Stainless Steels confirms the localized corrosion (pitting corrosion) processes has taken place with the noticeable pits. • 304 SS showed more weight loss and corrosion rates compare to 316L SS. The corrosive behavior of SS was found to be maximum during the winter season due to the moisture content of their water holding capacity ability for higher time period. • The XRF analysis gave the reliable quantitative information about the elemental chemical composition of deposited Cl- and SO2on the SS surface after the exposure period in IMU environment. During the initial exposure the XRF analysis does not showed the appreciable deposition of Cl- and SO2 on the both SS surface due to the low concentration on the steel surface.

33. After 6 months of exposure the presence of high amount of corrosive agents was detected in XRF studies along with drastic increases in the elemental composition of Na, Ca, Si, K. During the rainy season remarkable amount of N present in the both steel surface were confirmed. • AFM analysis of exposed 316L SS surface areas confirms the presence passive film formed of small and large grains with different sizes of large grains. The measured roughness using AFM analysis was found to be 20 to 40 nm. • The Raman spectra obtained is in proof of the corrosion processes and the formation of corrosion products as described in Stratman model. The formation of stable goethite and iron oxides were confirmed with the sharp peaks obtained in Raman spectra. Also Raman spectra confirmed the existence of Mixture of goethite in higher amounts and with lower amount of super paramagnetic maghemite . • Polarisation studies confirms the higher corrosion resistance through lower passive current of 316L in comparison to 304 steel. The higher RP value in EIS spectra confirms the good corrosion resistance nature of exposed samples.