Abstract Over the past forty years a significant number of corroding and at risk reinforced concrete structures have been successfully managed using Impressed Current Cathodic Protection
Introduction
Thermodynamic Considerations of the Corrosion of Steel in Concrete
A discussion of the thermodynamic driving forces behind corrosion and how it is affected by environmental factors.
Kinetic Considerations of the Corrosion of Steel in Concrete
A discussion of the kinetics of corrosion and how interaction with the environment defines the overall corrosion rate.
Corrosion of Steel in Concrete in Marine Environment – Chloride Induced Corrosion
An introduction into the effects of chloride on the reaction kinetics and an exploration of the mechanisms behind these effects.
Exposure Zones in Marine Structures
A discussion of the different exposure zones which are present in marine infrastructure and of how these different levels of exposure affect the corrosion reactions.
Monitoring Corrosion of Steel in Concrete
A description of the industry standard testing and inspection techniques used to monitor and assess the corrosion of steel in concrete.
Impressed Current Cathodic Protection of Steel in Concrete
An introduction to the technique of impressed current cathodic protection, its underlying principles and constituent elements.
Monitoring the Effect of an ICCP System
A description of the currently accepted methods for monitoring an ICCP system.
Disruptive Technologies and Residual Protection
A discussion of alternate electrochemical protection techniques and the phenomena of residual protection
An explanation of the methodology employed including; how sites were selected, the method of monitoring the system during the study and how the results were examined
Chapter 3 Wharf Description and History
A brief structural and operational history of the two wharves and their ICCP systems
4.1 Transverse Beams Detailed Analysis of Operation to Date
A presentation of the calculated steel densities and total charge passed to the Transverse Beams during the operation of the ICCP system at the time of the study’s commencement
4.2 Fender Blocks Detailed Analysis of Operation to Date
A presentation of the calculated steel densities and total charge passed to the Fender Blocks during the operation of the ICCP system at the time of the study’s commencement
A presentation of the study results including the transverse beam’s protection status and passivity status
A presentation of the study results including the fender block’s protection status and passivity status
5.1 Wharf B Detailed Analysis of Operation to Date
A presentation of the calculated steel densities at the time the study’s commenced
A presentation of the study results including the elements protection status and passivity status
An exploration of the results gathered for the transverse beams and how these results relate to residual protection and the corrosion process occurring at the steel surface
An exploration of the results gathered for the fender blocks and how these results relate to residual protection and the corrosion process occurring at the steel surface
A discussion of the results gathered for wharf B and how these results relate to residual protection and the corrosion process occurring at the steel surface
A discussion of the overall results from the study sites
Chapter 9 Conclusion and Future Study
A brief summary of the main findings of the study
A summary of the limitations of the study and potential future works which may provide further insight into residual protection
Steel reinforced concrete is one of the most vital construction materials in modern infrastructure due to its low cost, high strength, and durability Its widespread use makes it crucial in various construction projects worldwide However, the corrosion of reinforced concrete structures poses a significant economic challenge, leading to high maintenance and repair costs globally A report by the U.S Federal Highway Administration highlights the importance of addressing corrosion issues to ensure the longevity and safety of reinforced concrete infrastructure.
The annual cost of reinforcement corrosion in American concrete bridges exceeds eight billion dollars, highlighting the importance of effective remediation and protection strategies Controlling steel corrosion in concrete requires understanding both the thermodynamics and kinetics of corrosion reactions; Pourbaix's 1966 research links steel stability to pH levels and electrochemical potential, indicating that increased hydroxide ions promote protective iron oxide and hydroxide films These films make steel passive by shielding it from electrolytes, significantly reducing corrosion rates Reinforced concrete is generally durable because high alkalinity maintains steel passivity; however, corrosion accelerates when this passive layer is compromised, especially due to chloride ingress Chloride-induced corrosion leads to rapid pitting, which is particularly problematic in marine environments, ultimately compromising structural integrity and reducing lifespan.
Impressed Current Cathodic Protection (ICCP) is a key electrochemical method used to control corrosion in steel reinforced concrete, especially in aggressive marine environments The technique involves supplying electrical charge to the steel reinforcement to negatively polarize its potential, which reduces the corrosion current density Achieving a potential below a critical threshold enables the steel to become thermodynamically stable, effectively arresting corrosion Additionally, secondary mechanisms such as hydroxyl ion production at the steel surface, chloride ion repulsion, and oxygen removal from the cathode further enhance the steel’s protection within concrete.
Research indicates that the secondary effects of ICCP systems on existing structures can have lasting consequences previously overlooked, with some steels remaining passive even after the current is turned off Initial studies focused on short-term interruptions, such as power outages or electrolyte supply issues, lasting hours to days in marine environments Recent research expands on this by examining structures with discontinued ICCP systems due to reaching their lifespan or unforeseen incidents, revealing that some can remain protected for up to three years post-cessation However, this residual protection is not universal; many structures experience rapid re-activation of corrosion within hours after ICCP is stopped.
Despite acknowledgment of the phenomenon, the underlying causes remain poorly understood, especially why certain structures exhibit it while others do not A deeper understanding of residual protection could highlight the significance of secondary effects associated with Impressed Current Cathodic Protection (ICCP), leading to improved design and monitoring strategies Currently, ICCP systems rely partly on empirical knowledge and experience, but advancing understanding could enable more precise system optimization This could reduce the required applied currents, minimizing overprotection drawbacks and decreasing initial protection costs.
To fully realize the benefits of residual protection in ICCP systems, enhancing knowledge of passivity and residual protection principles is essential This research aims to identify critical factors influencing passivity and offer ongoing guidance for effective management of existing ICCP systems Additionally, it will deepen the understanding of electrochemical processes and operational parameters that govern residual protection, ensuring optimal system performance and durability.
To explore residual protection on operational ICCP systems in marine environments
This study aims to systematically explore residual protection by intentionally suspending an ICCP system for a designated period and monitoring its effects, providing a more structured approach compared to previous studies that collected such data in a less organized manner.
To study a variety of structures with variations in the amount of injected charge, operating parameters, and physical environments
To study the differences between regions that show residual protection and those that do not in order to understand what is driving the presence of residual protection
1 What determines whether a structure will exhibit residual protection?
2 Why does residual protection vary between structures and within a structure?
3 Can the current required to protect steel from corrosion within a concrete ICCP system be refined?
4 Are there potential optimisation strategies for existing ICCP systems?
Steel corrosion in concrete occurs due to the formation of macro and micro corrosion cells on the reinforcing steel surface These cells develop because of impurities in the steel and variations in the concrete chemistry, leading to anode-cathode pairs either co-located (microcell corrosion) or separated by several meters (macrocell corrosion) Anodic reactions at the anode cause atomic dissolution and generate electrons, which are consumed by cathodic reactions at the cathode, with the reinforcement facilitating electron flow known as the corrosion current The concrete acts as an electrolyte, allowing ionic current to flow and completing the electrical circuit that drives the corrosion process.
Steel comes in a wide variety of types, each with unique chemical compositions and properties suited for industrial applications Iron is the primary constituent element in all steels, influencing their behavior and performance The corrosion of steel occurs mainly through the loss of iron atoms from its atomic structure, leading to material degradation When steel is embedded in concrete, electrochemical reactions—specifically anodic and cathodic processes—occur on its surface, driving the corrosion process and compromising structural integrity.
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Steel in acidic conditions tends to become active, leading to corrosion as iron ions are released from the steel matrix These ions react with chemicals in the electrolyte to form rust, which includes iron oxides and hydroxides Rust formation causes volume expansion, creating stresses within concrete that can lead to cracking, delamination, and spalling This deterioration exposes the steel to the external environment, accelerating the corrosion process and compromising structural integrity.
1.2Thermodynamic Considerations of the Corrosion of Steel in Concrete
Steel in concrete exists in various thermodynamic states influenced by concrete chemistry and the steel's electrochemical potential The electrochemical potential of reinforcement is measured as the voltage difference between steel and a saturated hydrogen electrode; however, due to practical limitations outside laboratories, it is commonly assessed using copper/copper sulfate or silver/silver chloride electrodes In 1966, Marcel Pourbaix investigated how steel's potential relates to its chemical stability and environmental chemistry, providing crucial insights into corrosion behavior in concrete structures.
This article discusses eight experiments involving steel in water systems, where Pourbaix modified both the water's pH and the steel's electrical potential He observed the dominant iron oxide and hydroxide species formed under various conditions These findings were depicted in a Pourbaix diagram for the Fe-H2O system, illustrating the stability regions of different iron compounds across different pH levels and potentials.
Figure 1 Pourbaix diagram of Fe - H2O [11]
Pourbaix discovered that increasing hydroxyl ion concentration transforms the stable form of iron from Fe²⁺ ions, which cause steel corrosion, to stable iron oxide and hydroxide species These protective layers form on the steel surface, acting like a coating that shields the metal from the electrolyte, thereby slowing the corrosion process This surface layer, known as a passive film, reduces the active surface area exposed to corrosive elements When the corrosion rate drops to two microns per year or less, the steel is considered to be in a passive state, indicating negligible material loss due to corrosion.
It is also possible for the iron to exist in a thermodynamically stable state known as an immune state
Steel becomes immune to corrosion when its potential drops below a critical threshold known as the immune potential (EImmune) At potentials below EImmune, the attractive forces between the steel matrix and iron atoms are strong enough to prevent atomic dissolution, effectively inhibiting corrosion.