1. Galvanic corrosion (bimetallic corrosion) behavior of titanium-coupled brass and carbon steel in 3.5 wt% NaCl solution Wen-Ta Tsai*1, Kadek Trisna Surya Hariyantha1 and Szu-Jung Pan2   1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 2Ocean Energy Research Center, Tainan Hydraulics Laboratory, National Cheng Kung University, Tainan, Taiwan

2. Material Selection and Material Characteristics
Carbon steels
Titanium (Ti) and Titanium alloys (e.g., Ti-6Al-4V, etc.)
Active / passive
Copper (Cu) and Copper alloys (e.g., brass, naval brass, aluminum bronze, etc.)
Noble than carbon steels, Aluminum alloys and most metallic materials
Structural Integrity and long service life
-- mechanical strength and corrosion resistance

3. Cost of corrosion US DOD report (2009):
Naval ships: 2.6 billion (23.2 % maintenance cost)
Aviation vehicles: 2.2 billion (30.7 % maintenance cost)
Infrastructures: 1.6 billion (20.4 % maintenance cost)

4. Types of Corrosion General (Uniform) corrosion Localized corrosion
Pitting corrosion
Crevice corrosion
Stress corrosion cracking
Galvanic (bimetallic, dissimilar metal ) corrosion, etc.

5. Galvanic corrosion examples
Stainless steel
Cadmium plated steel
Corrosion of copper pipe in a stainless-steel hot water cylinder.
Francis, R., Galvanic Corrosion: A Practical Guide for Engineers; NACE: Houston, TX, 2001.
H. R. In Corrosion Resistance of Metals and Alloys, 2nd ed.; LaQue, F. L.,
Copson, H. R., Eds.; Reinhold Publishing: New York, NY, 1963; p 36, Chapter 1.

6. Galvanic corrosion in heat exchangers
Schematic corresponding shell and tube oil cooler heat exchangers
The oil cooler tube sheets in contact with copper tubes and aluminum gaskets exposed to galvanic corrosion: (a) rear-end side of the oil cooler and (b) front-end side of the oil cooler.
R. Taherzadeh Mousavian , E. Hajjari, D. Ghasemi, M. Kojouri Manesh, K. Ranjbar, Engineering Failure Analysis 18 (2011) 202–211

7. Principle of galvanic corrosion – Potential difference
ZRA Cu Fe
Fe2+ O2
Electrolyte solution: Seawater
Corrosion
ZRA
Current Cu Fe
Fe2+ O2
Electrolyte solution: Seawater
Electrons
Galvanic corrosion (accelerated rate, Ig)
Electrochemical nature of corrosion
Anodic reaction (corrosion): M  Mn+ + n e-
Cathodic reaction: O2 + 2H2O + 4 e-  4OH-

8. Important factors in galvanic corrosion behavior
Cathode
Anode
Potentials difference
“Mansfeld et al., ASTM, (1976) 20-47”
A general increase of the galvanic corrosion current density with increasing potential difference
Cathode/anode area ratio
“Mansfeld et al., Corrosion Science, 15 (1975) ”
Galvanic corrosion current density is linearly proportional to cathode/anode area ratio. IgA = iO2L AC/AA
Passive film
“Davis et al., Corrosion-NACE, 42 (1986) 6”
In the case of titanium-hastelloy alloy B-2 couple in boiling 10% H2SO4, the galvanic current density decreased as the cathode/anode area ratio increased. This due to active-passive transition of anode titanium.

9. The changes of polarity of the galvanic couples are seldom reported.
Literature review of Ti-metal couples
Topic
Measurements
Focus
Ref
Corrosion and galvanic corrosion of steels I: In Aqueous NaCl (32.7 g/l NaCl solutions)
Eg , Ig, Corrosion rate
Titanium-Carbon steel couple:
Temperature effect
Different carbon content of carbon steels
(1)
Galvanic coupling of Ti with Cu and Al alloys in chloride media (32.7 g/l NaCl solutions)
Eg , Ig, cathodic and anodic polarization
Titanium coupled to admiralty brass, Al-brass and Cu alloys
Flow of O2
Temperature
(2)
Electrochemical, galvanic, and mechanical responses of grade 2 titanium in 6% sodium chloride solution
Ecor, cathodic reaction rate, critical potential of hybride formation
Titanium coupled with the copper-nickel alloy, naval brass, type 316 SS, alloy 600, HY80 steel, five-nines aluminum, Al 6061, and zinc. pH
Applied potentials
(3)
The changes of polarity of the galvanic couples are seldom reported.
Eg = Galvanic potential, Ig = Galvanic current, Ecorr = Corrosion potential.
(1) S. H. Sanad et al., Surface technology, 16 (1982) 49-55, (2) L. A. Shalaby, Corrosion Science, 11 (1971) , (3)Z. F. Wang et al., Corrosion-NACE, (1999)

10. Oxygen in Seawater
The Sea contains a much higher percentage of oxygen (34 %) than the atmosphere (ca. 21 %). Most of the oxygen is of course bound to hydrogen in water molecules. A smaller part is bound in molecules of other substances (like calcium carbonate), and only part of the oxygen is freely available for respiration as oxygen gas dissolved in the water. Oxygen occurs as a by-product of photosynthesis in plants (plankton in the open sea). Oxygen is also dissolved at the interface between the sea surface and the atmosphere. Most of the oxygen-rich ocean water and the animal life which uses oxygen for respiration is therefore found near the surface. As one descends into the depths, the amount of dissolved oxygen in the water drops rapidly, and so does animal life. In a zone occurring at depths of about 200 to 1,000 metres, depending on local circumstances, oxygen saturation in seawater in the ocean is at its lowest. This zone is called the Oxygen Minimum Zone (sometime referred to as the shadow zone). From the oxygen minimum layer downward the amount of dissolved oxygen increases initially, and another decrease occurs near the bottom. Please note that oxygen profiles like the one shown here vary from location to location. Just now I would like to stress that the occurrence as such of a minimum oxygen zone is a natural phenomenon due to destruction of dissolved oxygen by respiration (something more or less like this CH2O + O2 = CO2 + H2O). I’ll come back to the degree of oxygen depletion later.

11. Main scopes of this study
To understand the changes of polarity and galvanic behavior based on main influence factors such as:
Effect of dissolved oxygen
Effect of the cathode/anode area ratio
Effect of anodic oxide film

12. Experimental procedures
Materials:
Commercially Pure (CP) titanium
Carbon steel (0.05 wt% C)
Brass (65Cu-35Zn)
Test solution:
3.5 wt% NaCl solution, either de-aerated by purging nitrogen or air exposed
The temperature of the solution was at room temperature
Open circuit potential (OCP):
The CE electrode and reference electrode were a platinum sheet and a saturated calomel electrode (SCE), respectively.
The measurement of OCP of each electrode until steady-state values were established
Anodizing titanium
0.5 M H2SO4 solution at room temperature
Current density = 1080 A/m2
Variation cells potentials =
The area exposed:
0.9 cm2

13. Experimental procedures Galvanic corrosion test
Titanium coupled to carbon steel:
WE1: Titanium
WE2: Carbon steel
Surface area of WE1 to WE2:
0.1 : 1 (0.09 cm2 : 0.9 cm2 )
: 1 (0.9 cm2 : 0.9 cm2 )
: 1 ( cm2 : 0.9 cm2 )
Current
ZRA
WE1
WE2
solution
Electrons
Titanium coupled to brass:
WE1: Brass
WE2: Titanium
Surface area of WE1 to WE2:
0.1 : 1 (0.09 cm2 : 0.9 cm2 )
: 1 (0.9 cm2 : 0.9 cm2 )
: 1 ( cm2 : 0.9 cm2 )
If the galvanic current sign on positive value, the current flow from WE1 to WE2 and vise versa.
Anodized titanium coupled to brass:
WE1: Brass
WE2: Anodized Titanium
Surface area of WE1 to WE2:
: 1 (0.9 cm2 : 0.9 cm2 )
Separation between coupled specimens was about 4 inches.

14. Part I – Effect of dissolved oxygen
Galvanic corrosion behavior in two different conditions:
(1) De-aerated (purged with Ar gas)
(2) Air-exposed

15. Effect of dissolved oxygen Open circuit potential (OCP)
With the presence of dissolved oxygen the open circuit potential toward the more positive direction.
Brass had the highest open circuit potential in 3.5 wt% NaCl solution with and without deaeration.

16. Effect of dissolved oxygen (Titanium-carbon steel couples)
Galvanic corrosion behavior in 3.5 wt% NaCl
Titanium-carbon steel couples:
Galvanic current : Ig [O2]
Galvanic potential : Eg [O2]

17. Effect of dissolved oxygen (Titanium-brass couples)
Galvanic corrosion behavior in 3.5 wt% NaCl
The galvanic current increased as the potential difference increased.
Ig [O2] for titanium-carbon steel couples.
Ig [O2] for titanium-brass couples.
The galvanic potential:
Titanium-carbon steel couples, Eg [O2]
Titanium-brass couples, Eg [O2]

18. Part II – Effect of area ratio
The changes of polarity and galvanic behavior are explained:
Galvanic corrosion behavior
Corrosion product or oxide film formed on the surface

19. Effect of area ratio (Ti-Carbon steel couples)
Galvanic corrosion behavior in air exposed 3.5 wt% NaCl
“Catchment Principle”
Ig= k log AC/AA

20. Titanium/carbon steel area ratio
Effect of area ratio (Ti-Carbon steel couples)
Surface morphology observation on the carbon steel
Titanium/carbon steel area ratio
0.1:1
10:1 OM
SEM
0.75 mm
0.75 mm
100 µm
100 µm

21. Schematic diagram for Titanium coupled to carbon steel
Effect of area ratio
Schematic diagram for Titanium coupled to carbon steel
ZRA
ZRA
ZRA
Current
Electrons
Current
Electrons
Fe2+
Fe2+ Ti CS CS Ti O2 O2
3.5 wt% NaCl solution
3.5 wt% NaCl solution
Area ratio 0.1:1
Area ratio 10:1

22. Effect of area ratio (Ti-Brass couples)
Galvanic behavior in air exposed 3.5 wt% NaCl
After 830 seconds
Polarity changes
Titanium/brass area ratio:
1:0.1 and 1:1  Titanium (anode) , Brass (cathode)
1:  Titanium (cathode), Brass (anode)

23. Titanium/brass area ratio
Effect of area ratio (Ti-brass couples)
Surface morphology observation on the brass
Titanium/brass area ratio
1:0.1
1:10 OM
SEM
0.75 mm
7.5 mm
20 µm
20 µm

24. Titanium/brass area ratio
Effect of area ratio (Ti-brass couples)
Surface morphology observation on the titanium
Titanium/brass area ratio
1:0.1
1:10
SEM
No corrosion product observed on the surface titanium.
No significant changes of surface morphology after galvanic test with different area ratio effect.

25. Schematic diagram for Titanium coupled to Brass
Mechanism changes of polarity
1st stage
2nd stage
Latter stage
Fast dissolution of Ti
Rapid build up of Tin+
Rapid build up of Ti oxide
Continuous of Ti oxide
Passivation of Ti
Effect of area ratio:
Titanium-carbon steel couples:
Titanium-brass couples:
For ATi/ABrass > 1:
For ATi/ABrass < 1:
Initial Ti was the anode
Latter Ti was the cathode

26. Part III – Effect of anodic oxide film
Influence of anodic oxide film on the galvanic corrosion behavior
Natural oxide film vs. artificial oxide film
Anodized titanium coupled to brass

27. Surface morphology of anodized titanium
The anodized titanium showed the porous oxide formation with µm.
µm-sized porous were distributed homogeneously over the sample surfaces.

28. Effect of anodic oxide film Open circuit potential (OCP)
With the presence of anodic oxide film the OCP of titanium toward to more positive direction and became higher than brass

29. Effect of anodic oxide film Galvanic corrosion behavior
Oxide film of anodized titanium
Effect of anodic oxide film
Galvanic corrosion behavior
The formation of an oxide film made the anodized titanium acting as cathode when coupled with brass

30. Conclusions
Nature of the coupling metals in 3.5 wt% NaCl solution, with area ratio equal to 1:
Titanium (cathode) - Carbon steel (anode)
Titanium ( anode) - Brass (cathode)
Effect of dissolved oxygen (area ratio 1:1):
Ig [O2] for titanium - carbon steel couples
Ig [O2] for titanium - brass couples
Effect of area ratio:
Titanium-carbon steel couples:
Titanium-brass couples:
For ATi/ABrass > 1:
For ATi/ABrass < 1:
Initial Ti was the anode
Latter Ti was the cathode
Effect of anodic oxide film:
Anodized titanium (cathode) - brass (anode)

31. THANK YOU FOR YOUR ATTENTION
Acknowledgement
The authors would like to thank Bureau of Energy, Ministry of Economic Affairs of the Republic of China for supporting this study under contract no. 102-D0618.
The travel grant provided by NCKU Research and Development Foundation under contract no. 102S045 is also acknowledged.
THANK YOU FOR YOUR ATTENTION