MEASUREMENT OF STEEL CORROSION IN CONCRETE

Transcription

1 Seminar in Hokkaido University MEASUREMENT OF STEEL CORROSION IN CONCRETE 2 March 2010 K.Y. Ann Concrete Materials, Mechanics and Engineering School of Civil and Environmental Engineering Yonsei Univ., Seoul , KOREA

2 Contents 1. Importance of corrosion-induced problematic issue 2. Mechanism of corrosion - Corrosion model - Onset of corrosion: corrosive/inhibitive - Corrosion propagation 3. Measurements - Visual examination - Half-cell potential - Polarisation resistance - Galvanic current - AC impedance - Mass loss 4. Electrochemical repair - Cathodic protection - Electrochemical chloride extraction - Realkalisation 5. Conclusion

3 Importance of corrosion in concrete bridges Heringsdorf Bridge, Germany (taken by K.Y. Ann, 2004) Concrete spalling out Severe rusting

4 Volume change of rusting Volume Corroded Fe FeO Fe 3 O 4 Fe 2 O 3 Blackfriar bridge UK, 2004 Fe(OH) 2 Fe(OH) 3 Fe(OH) 3 3H 2 O Mehta and Monteiro, Properties of concrete, 1993

5 Life of concrete structures exposed to marine Damage limit Abati ing Physical damage Corrosion initiation Chloride threshold Loss of steel/concrete properties Cover protection Seawater contamination Cl-free Time

6 Concrete quality Steel Ca + Na + K + OH - OH - OH - Ca + Na + Cl - Cl - Cl - Cl - Cl - K + Paste Steel Hydrations Hydrations Ann and Song Corros Sci 2007 Pores at the interface

7 Chloride binding vs buffering Solid precipitated hydrations Buffering a ph fall C 3 A Hydrations C 4 AF Cl - Cl - Cl - Cl - ph fall Steel Cl - Buffering a ph fall Solid precipitated hydrations: Ca(OH) 2 Pores at the interface Ann et al, Consec 07, 2007

8 Steel-concrete interface External chlorides Pores at the interface Hydration layer Cl - Cl - Cl - Cl - Cl - Steel Reou and Ann, Mag Concr Res 2008 Hydration layer

9 Corrosion initiation SALT CO 2 Concrete Passive Film Steel Fe 2+ Fe H 2 CO 2 Cl - Cl - Pit Nucleation Events Fe Cl 2+ - Fe 2+ +OH - ANODE Oxide Fe H 2+ 2 O HCl Film Fe Fe 2+ Fe 2+ Fe Fe Fe 2+ Fe 2 Stages of Cl - induced corrosion initiation Pit Nucleation Stable Pit Growth local ph reduction local chloride build up

10 Visual examination Blackfriar bridge UK 2004 Deck of Po-Hang harbour Korea 2007 Inconclusive

11 Half-cell potential Critical potential for corrosion (ASTM C 876) -350 mv vs CSE -275 mv vs SCE Qualitative information, but not quantitative for the corrosion rate (rust amount) Pourbaix, Corrosion, 1966

12 Half-cell potential Principle Passive Electrode Corrosive Corrosive Passive Voltmeter Passive Corrosive Half-cell potential mapping

13 Polarisation technique Principle Anodic polarisation Corrosion potential Cathodic Intercept E Anodic Concrete resistance Corrosion current I Ann et al Cem Concr Res 2006

14 Polarisation technique Anodic polarisation Tafel extrapolation Rp= E I Anodic activity (Ba) E (mv) -500 E (mv) Cathodic activity (Bc) I (ma) I (ma) I = B Rp B: 26 mv (corrosive) 52 mv (passive) Reou and Ann, Mater Chem Phy, 2008 BaBc I = 2.3Rp( Ba+ Bc) Quantitative, conclusive

15 AC Impedance Electric circuit for AC impedance Quantitative, but too sensitive to noise and wave -jz Polarisation resistance of steel Resistance of concrete Resistance of passivation Z

16 Galvanic current Principle 3 Corroded e- e- e- Galvanic current Uncorroded Galvan nic current (ma/m 2 ) 2 1 Corrosion initiation Time (days) Ann and Buenfeld, Mag Concr Res, 2007 Informative for detecting corrosion initiation, but less conclusive for its propagation

17 Galvanic current Ladder system Steel Cl - Concrete Cl - Cl - Cl - Reference steel bars Noble metal (uncorrosive)

18 Mass loss Principle Mass loss (%) 1.0 y = 0.455x R 2 = M = I corr ta nf w 0.5 M : Mass (g) I corr : Corrosion rate t : Time A w : Atomic weight n : Valency F : Faraday constant % Chlorides in cast (%, cement) Reou and Ann, Chem Phy, 2008, unpublished the most tangible information, but destructive

19 Corrosion values at a given condition Corrosion rate Half-cell potential Corrosion potential Corrosion rate (ma/m 2 ) Galvanic current Linear polarisation Tafel's extrapolation Corrosion potential (mv, SCE) Chlorides in cast (%, cement) Chlorides in cast (%, cement) Corrosion values at corrosion Half cell potential Galvanic current Anodic polarisation Tafel extrapolation -343 mv 0.63 ma/m ma/m ma/m 2

20 Principle of electrochemical treatment Mechanism Application of in-situ e- Reduction Reactions - O 2 + H 2 O Oxygen OH - Reduction H 2 O H 2 + OH - DC Power Supply + Solution electrolyte (Current conduction via ions) Hydrogen Evolution OH - Cl - CO 3 2- Ca 2+ Fe 2+ Na + H + e- Oxidation Reactions Oxygen H 2 O Evolution Chloride Evolution H + + O 2 Cl - Cl 2 Control pipe Ionic Conduction Fe 3+ Ion Migration Fe Iron oxide Iron Reduction Oxidation Fe 2+ Fe 2+ Sacrificial anode Glass 1986, Corros St. Kinston bridge, Canada, 2003

21 Cathodic protection Potential Current Ann PhD thesis, Imperial College, Sacrificial anode: corrosive metal provides electrons to the steel (Eg. zinc). Attachment of anode on concrete is a key factor 2. Impressed current: electrons are provided by a DC power supply directly to the steel. Conductive paint serves as an anode

22 Chloride extraction Steel rebar H 2 gas Titanium mesh Sealing Frame Bracket Chloride extraction Shutter Electrolyte Setting-up of titanium mesh ( Tampa, US 2002) Blocker Cork Titanium mesh Plastic sheet Power supply Supplementary benefits - Increasing OH- at the steel -Densifying calcium hydroxide - Supply of electrons to the steel then to repassivate

23 Realkalisation Steel rebar H 2 gas Titanium mesh Sealing Frame Bracket Alkali ion injection Shutter Electrolyte Jubilee line extension UK, 2001 Banfill 1997 Const Build Mater Long term monitoring results concerning the effect of realkalisation have not been confirmed due to its short history

24 Cathodic prevention e - Power supply e - Calcium hydroxide layer OH - generated at the steel Ca + forced to move to steel Rusting Deposition of hydration products Concrete block Steel rebar Conductive Glass and Buenfeld, Corros Sci, 2001 At present, only investigation has undergone with no application to in-situ

25 Application to in-situ Cathodic protection Chloride extraction Realkalisation Cathodic prevention Lower the potential to enhance passivity Remove chlorides from the concrete Increase the alkalinity of concrete at the steel depth to counter carbonation Inhibit corrosion by enhancing passivity 1-20 ma/m A/m 2 5 A/m ma/m 2 (1 A/m 2 to concrete) Permanent after application 6-10 weeks 1-3 weeks Permanent after application Steel-bond reduction Hydrogen embrittlement Steel-bond reduction Steel-bond reduction Potential ASR None reported Ann, PhD thesis, Imperial College, 2005

26 Recommendations 1. Corrosion of steel in concrete is subjected to both corrosiveness of acidification and inhibition effect of cement matrix. 2. Half-cell potential is easily applied to in-situ, but its results are only restricted to qualitative determination whether or not corrosion starts. 3. Polarisation technique may be the best option to detect the state of steel in terms of corrosion, as being the quantitative in determining corrosion. 4. Galvanic current provide information on the onset of corrosion, but no more conclusive data for the rate of corrosion propagation. 5. AC impedance technique provides very convincible information on corrosion, but less applicable to field due to its sensitivity. 6. Electrochemical treatment is a noble option to cure corroded steel in concrete.

27 Acknowledgement The author would like to thank for helpful comments and advice to: Broomfield JP, Corrosion Doctors, UK Buenfeld NR, Imperial College, UK Glass GK, Boston Consulting, US Head MK, Leeds University, UK Syprounse S, ATCK Constructions, Greece Kim JH, Yonsei University, Korea Price WF, Lafarge Cement, UK Zhang J-Z, Lafarge Cement, France