Mechanism Of Cathodic Protection Research Articles

Cathodic protection mechanism of iron and steel in porous media

Cathodic protection was introduced two centuries ago and since has found widespread application in protecting structures such as pipelines, offshore installations, and bridges from corrosion. Despite its extensive use, the fundamental working mechanism of cathodic protection remains debated, particularly for metals in porous media such as soil. Here, we use in-situ and ex-situ characterisation techniques coupled with electrochemical measurements to characterise the spatio-temporal changes occurring at the steel-electrolyte interface. We show that upon cathodic protection, the interfacial electrolyte undergoes alkalinisation and deoxygenation, and that depending on polarisation conditions, an iron oxide film can simultaneously form on the steel surface. We further demonstrate that these changes in interfacial electrolyte chemistry and steel surface state result in altered anodic and cathodic reactions and their kinetics. We propose a mechanism of cathodic protection that integrates previous theories, based on both concentration and activation polarisation, complimentarily. We discuss the implications of this study in enhancing corrosion protection technologies and the safe, economical, and environmentally friendly operation of critical steel-based infrastructures.

Stable Ultra-thin Silver Films Grown by Soft Ion Beam-Enhanced Sputtering with an Aluminum Cap Layer.

Ultra-thin silver films are susceptible to ambient environments and form grayish layers in the silver mirroring process. The poor wettability together with the high diffusivity of surface atoms in the presence of oxygen accounts for the thermal instability of ultra-thin silver films in the air and at elevated temperatures. This work demonstrates an atomic-scale aluminum cap layer on the silver to enhance the thermal and environmental stabilities of ultra-thin silver films deposited by sputtering with the assistance of a soft ion beam reported in our previous work. The resulted film consists of an ion-beam-treated seed silver layer of ∼1 nm nominal thickness, a subsequent silver layer of ∼6 nm thickness produced by sputtering alone, and an aluminum cap layer of ∼0.2 nm nominal thickness. Although the aluminum cap is only one to two atomic layers and likely non-continuous, it significantly improved the thermal and ambient environmental stability of the ultra-thin silver films (∼7 nm thick) without affecting the film's optical and electrical properties. The improved environmental stability is attributed to the cathodic protection mechanism and reduced diffusivity of surface atoms. The improved thermal stability is attributed to the reduced mobility of surface atoms in the presence of aluminum atoms. Thermal treatment of the duplex film also improves the film's electrical conductivity and optical transmittance by enhancing its crystallinity. The annealed aluminum/silver duplex structure has exhibited the lowest electric resistivity among the reported ultra-thin silver films and high optical transmittance similar to the simulated theoretical results.

Research on the difference of characteristics at Steel/electrolyte interface under cathodic protection and in High-pH alkaline solution

• In situ spectroelectrochemistry is used to compare steel/electrolyte interfaces under CP to those at high-pH. • No protective passive film with high impedance formed under CP conditions. • Anodic excursions can induce unstable passivition on steel surfaces under high CP. In situ spectroelectrochemistry measurements, including electrochemical impedance spectroscopy (EIS) and UV–vis light reflecting spectroscopy, were conducted simultaneously to compare the characteristics at the steel/electrolyte interface under cathodic protection (CP) to that in high-pH alkaline solution. The results show that the electrochemical characteristics and optical light reflectance of the steel/electrolyte interface under CP are significantly different from those with a passive film formed in high-pH alkaline solution. These data indicate that no protective passive film is formed under the CP potential of −1.2V SSC , presumably because of the continuous supply of CP current to the cathodic interface and subsequent cathodic reduction reactions. The essence of the cathodic protection mechanism is to eliminate the anodic and cathodic zones by cathodic polarization, not to produce passive film on the steel surface. Another interesting phenomenon observed in the experiments was for the steel specimen under CP potentials equal to or more negative than −1.2 V SSC , for which the interruption of CP current and anodic excursions could induce passivation on the steel surface for a certain time. Analyses indicate that the occurrence of passivation in this case is related to the high pH environment produced by previous cathodic reactions and the positive potential shift caused by the anodic excursion; whereas the disappearance of passivation correlates with a decreasing pH and the subsequent electrochemical reactions, which could provide some insights for understanding the effect of dynamic stray current on cathodically protected steel.

Interfacial Schottky junction of Ti3C2Tx MXene/g-C3N4 for promoting spatial charge separation in photoelectrochemical cathodic protection of steel

Photoelectrochemical (PEC) cathodic protection was an ecofriendly and promising method for anti-corrosion of metal. In this work, the Ti3C2 MXene-based photoanode was the first time to apply in the PEC anti-corrosion process. The interfacial Schottky junction in TC-x/CN composites was designed by the electrostatic self-assembly method. Different content of Ti3C2 was to investigate the effect on the PEC performance in TC-x/CN composites. Specifically, the TC-1.0/CN showed the highest PEC cathodic protection performance for 304 SS, which revealed low content of Ti3C2 was beneficial for spatial charge separation. Various characterization techniques analysis further verified the Schottky junction successfully formed at the interface between Ti3C2 and g-C3N4 and significantly promote the spatial charge separation. Furthermore, this work proposed the possible electron transfer pathway and deepened understanding of the PEC cathodic protection mechanism.

Protective primers with reduced zinc powder content caused by the addition of core-shell micro talc/polyaniline pigment

This study aims to verify the possibility of replacing a portion of zinc (up to 30 wt.%) with a newly designed micro talc/polyaniline (PANI) core-shell pigment that can be used in the protective primer compositions for metals. Measurements of the electrode potential of painted steel and the electrical capacity of the steel/coating/electrolyte system showed that synthesized pigment bears two functions. Firstly, its presence in the epoxy-amine coatings provides a cathodic protection mechanism for steel with the lower zinc content. Secondly, it allows long-term preservation of the sacrificial metal activity and passive state of the substrate due to the autocatalytic redox cycle based on the reversible conversion of PANI from its emeraldine to leucoemeraldine form. The corresponding mechanism that allows the retaining of sacrificial metal activity and the passive state of the substrate is proposed.

Using the photoinduced volt-ampere curves to study the p/n types of the corrosion products with semiconducting properties

This article specifically addresses the issue that there is no suitable method to determine the semiconductor types of non-uniformly distributed corrosion products. The photoinduced volt-ampere (i-V) curve is proved to be able to accurately measure the p/n type of corrosion products, in addition, it can also be used to study the transfer direction of the photogenerated electrons and quantitatively measure the photoelectrochemical effects of the corrosion products with semiconducting properties. This work will help to promote the study of the influencing mechanism of light illumination on corrosion process of metals and the photoelectrochemical cathodic protection mechanism.

The cathodic protection potential criteria: Evaluation of the evidence

AbstractRecent publications have ascribed the mechanism of cathodic protection (CP) in soils to the development of passivity at the steel surface. This view has, in turn, prompted discussion on the need to re‐evaluate the long‐established criteria for protection. This paper presents a contribution to that discussion. It advances, or rather it restates, the proposition that neither immunity nor passivity is necessarily relevant. It argues that protection is simply a consequence of a potential‐driven lowering of the anodic dissolution rate. However, it also emerges that elucidating any mechanism convincingly requires dependable field or laboratory data on the very low corrosion rates occurring on cathodically polarised specimens. A critical review of published data highlights a lack of reliability in this area.

On the Mechanism of Cathodic Protection and Its Implications on Criteria Including AC and DC Interference Conditions

In recent years, a wide agreement with respect to the processes associated with cathodic protection (CP) has been reached within the CP industry. The increase of the pH value at the steel surface and the generation of passivating conditions as a result of the CP current are widely accepted as the relevant underlying mechanism. Based on this understanding it was possible to identify the relevant processes with respect to AC and DC interference and explain the empirical observations. This led to the development of ISO 18086 and has significantly influenced the work on ISO/DIS 21857. This paper summarizes present state of knowledge, the more recent developments, and their implications with respect to the CP criteria. It summarizes the relevant aspects in association with interference conditions and highlights possible future approaches with respect to the assessment of the effectiveness of CP.

Corrosion Assessment of Zn-Rich Epoxy Primers With Carbon Nanotube Additions in an Electrolyte With a Bacteria Consortium

Multifunctional coatings for corrosive environments have been developed to include two basic principle protection mechanisms: a barrier mechanism acting as a mass transfer process blocker, and cathodic protection mechanism acting as a charge transfer process promoter. Both mechanisms were assessed for Zinc-Rich Epoxy (ZRE) coatings in the presence of Carbon Nanotubes (CNTs) and exposure to a bioelectrolyte in order to study the evolution process during microbial corrosion conditions. The purpose of this study is to characterize in a comprehensive experimental platform the electrochemical response of a dual-protection zinc epoxy coating with different ratios of additive carbon nanotubes to active zinc particles upon exposure to a sulfate reducing consortium. Carbon nanotubes addition was found to affect both the prevailing mechanism at the coating interfaces and the formation of a biofilm at the coating surface that influenced the relatively dominance of the barrier protection mechanism. These multifunctional coatings with active particles could help to balance the charge transfer efficiency in terms of the sacrificial of zinc and barrier mechanisms, which influence biofilm formation and have potential consequences for biocorrosion on carbon steel.

Study of the physical and electrochemical properties of hybrid paint system based on zinc-rich primer for mild steel protection

PurposeThis paper aims to study the corrosion performance and physical properties of hybrid paint systems based on zinc-rich primer (ZRP) and to determine the optimum modification approach that guarantees the most overall performance enhancement for the developed coating films that have been fabricated based on the usage of ZRP.Design/methodology/approachFour different approaches were applied to enhance the corrosion protection performance and the physical properties of ZRP-based paint systems, namely, incorporation with TiO2 pigment, introducing SiO2 nanoparticles, usage of polyamide curing agent and application of epoxy base coating as a second layer. The physico-mechanical properties were examined using pull-off test, glossiness test, pencil hardness test and cross-cut adhesion tape test. Moreover, the contact angle measurement was used to study the wettability of the developed coated surfaces and the corrosion protection performances were evaluated by using electrochemical impedance spectroscopy and salt spray test.FindingsThe obtained results revealed the ability of a certain approach to enhance the physical and corrosion protection properties of the ZRP paint system. Moreover, developing an intact hydrophobic nanocomposite paint system based on the usage of ZRP as the host matrix and SiO2 nanoparticles as the reinforcing agent was confirmed without altering the cathodic protection mechanism or the other desired characteristics of ZRP paint system.Originality/valueThe innovation of this work can be clearly observed by the ability to enhance the physical and corrosion protection properties of ZRP with four different approaches.

Corrosion Protection of Graphene-Modified Zinc-Rich Epoxy Coatings in Dilute NaCl Solution

In order to hold the excellent corrosion resistance of zinc-rich epoxy coatings with reduced zinc content, five types of graphene modified zinc-rich epoxy coatings (G-ZRCs) were prepared. The effect of zinc content on the corrosion protection behaviors of G-ZRCs was investigated using electrochemical impedance spectroscopy (EIS). Open circuit potential (OCP) was also utilized to deepen the understanding of the anticorrosion mechanism of the as-prepared composite coatings. The results confirmed that addition of 0.3 wt % graphene could make the coating with 40 wt % (G-40ZRC) and 55 wt % zinc (G-55ZRC) provide cathodic protection for a short period of time. The EIS and OCP results showed a dominant physical barrier protection mechanism for the coatings without zinc (G-ZRC), while there was a mixed mechanism for G-40ZRC and G-55ZRC and a dominant cathodic protection mechanism for the coatings with 70 and 85 wt % zinc (G-70ZRC and G-85ZRC). Furthermore, the graphene modified zinc-rich coatings and corresponding corrosion products immersed in a 3.5% NaCl solution for 100 days were also characterized, which could further reveal anticorrosion mechanisms of the graphene modified zinc-rich coatings and the passivation effect of polyaniline (PANI).

Study of the photoelectrochemical cathodic protection mechanism for steel based on the SrTiO3-TiO2 composite

Up to now the mechanism of the photoelectrochemical (PEC) cathodic protection of the semiconductor materials is still unclear. In this work, the conduction band (CB) potential of the SrTiO3-TiO2 composite was regulated by controlling the deposition amount of SrTiO3 on the surface of TiO2 nanotube arrays. When the hydrothermal treating time was 1 h, the prepared SrTiO3-TiO2 photoanode (denoted as SrTiO3-TiO2-1) exhibited the most excellent PEC cathodic protection performance for 304 stainless steel and Q235 carbon steel in NaCl electrolyte. Further analysis indicates that the PEC cathodic protection performance of the photoanode is determined by the CB potential of the n-type semiconductor material, the separation efficiency of the photogenerated electron-hole pairs and the redox potential of the electrolyte. Among them, sufficient negative CB potential is the prerequisite for providing the PEC cathodic protection for the coupled metals. Under this premise, the higher the separation efficiency of the photogenerated electron-hole pairs and the more negative redox potential of the electrolyte are, a more negative potential of the coupled metal can be cathodically polarized to under the illumination of incident light with suitable wavelength.

Cathodic protection of soil buried steel pipelines - a critical discussion of protection criteria and threshold values

This paper represents the consensus reached during a panel discussion held at the CEOCOR International Congress in Stockholm, Sweden, June 2–5, 2015. It reviews the state-of-the-art about the generally accepted understanding of the working mechanism of cathodic protection of ground buried steel structures. Based on this, limitations of existing protection criteria are discussed, particularly the difficulties related to measuring and interpreting current-applied potentials, instant-off potentials, and IR-free potentials. A set of protection criteria and their interpretation in accordance with the understanding of the working mechanism of cathodic protection is proposed. The main idea is that it is distinguished between the following two situations: 1) conditions that permit an increase in pH at the steel surface as a result of the applied protection current (good bedding conditions inhibiting convection) and thus provide corrosion protection by achieving passivity; and 2) situations where this is not possible (e.g. in streaming soft water) or where it is unknown if this is possible, and thus effective corrosion protection can only be achieved by polarizing the steel into the immunity zone.

Corrosion protection mechanisms of carbon nanotube and zinc-rich epoxy primers on carbon steel in simulated concrete pore solutions in the presence of chloride ions

This study investigates the electrochemical characterization of carbon nanotube and zinc-rich epoxy primers (CNT-ZRPs) on carbon steel in simulated concrete pore (SCP) solutions in the presence of chloride ions. The mechanistic performance of CNT-ZRPs was characterized by adding different zinc content. The electrochemical results indicated a dominant barrier protection effect for the coating with 60wt% Zn while there was a mixed corrosion protection mechanism for the coating with 70wt% Zn and a dominant cathodic protection mechanism for coatings with higher zinc content (80wt% and 90wt% Zn). These barrier and cathodic protection control mechanisms were characterized quantitatively by electrochemical and high-resolution techniques.

Transient numerical model for crevice corrosion of pipelines under disbonded coating with cathodic protection

Cathodic protection can strongly affect the chemical and electrochemical characteristics of the solution under a disbonded coating. In this work, a transient mathematical model was developed to evaluate the effectiveness of cathodic protection on the pipeline steel under a disbonded coating, and the mechanism of cathodic protection for preventing crevice corrosion was discussed. The model includes the temporal evolution of the potential of the steel, pH, oxygen concentration, and conductivity of the solution within a crevice. The implicit finite difference method was used to solve the complex set of mass-transfer equations that describe the system. The chemical and electrochemical processes inside the crevice were numerically analyzed, and the results were compared with those obtained from the experimental data. The results showed that the dissolved oxygen inside the crevice is rapidly consumed, thereby inhibiting oxygen corrosion; the alkaline environment, which is generated by oxygen reduction, results in the formation of a passivation environment. The cathodic current gradually flows into the crevice by overcoming the solution resistance, and the steel is completely electrochemically protected. As the local-solution environment changes as a function of time and distance, various mechanisms may synergistically act to protect the metal inside the crevice from corrosion.

Role of dissolved oxygen diffusion in coating defect protection by emeraldine base

The corrosion protection of emeraldine base (EB) on mild steel in the artificial defect exposed to 1 wt% NaCl solution was studied using in situ optical microscope and scanning electron microscopy (SEM). The EB coating displayed good corrosion protection on the exposed bare steel, which was difficult to understand by traditional anodic protection mechanism or inhibitory protection mechanism, or cathodic protection mechanism, since steel usually cannot be passivated in brine, and EB coating cannot release inhibitory anions to protect exposed bare steel, and further, the galvanic coupling measurement demonstrates that EB cannot provide cathodic protection to exposed bare steel. The importance of dissolved oxygen diffusion caused by EB in corrosion protection was disclosed in this paper, a related protection model was therefore proposed, which was qualitatively verified by corresponding electrochemical measurements.

Cathodic protection of steel in concrete using magnesium alloy anode

Corrosion of steel embedded in concrete structures and bridges is prevented using cathodic protection. Majority of the structures protected employ impressed current system. Use of sacrificial system for the protection of steel in concrete is not as widely employed. The use of magnesium anodes for the above purpose is very limited. This study has been carried out with a view to analyse the use of magnesium alloy anode for the cathodic protection of steel embedded in concrete. Magnesium alloy anode, designed for three years life, was installed at the center of reinforced concrete slab, containing 3.5% sodium chloride with respect to weight of cement, for cathodic protection. Potential of the embedded steel and the current flowing between the anode and the steel were monitored, plotted and analyzed. Chloride concentration of concrete at different locations, for different timings, were also determined and analyzed. The magnesium anode was found to shift the potential of the steel to more negative potentials initially, at all distances and later towards less negative potentials. The chloride concentration was found to decrease at all the locations with increase in time. The mechanism of cathodic protection with the sacrificial anode could be correlated to the removal of corrosive ions such as chloride from the vicinity of steel.

SVET and SECM imaging of cathodic protection of aluminium by a Mg-rich coating

The mechanism of cathodic protection of an aluminium substrate by a Mg-rich coating was investigated using localized techniques. Both scanning vibrating electrode techniques (SVET) and scanning electrochemical microscopy (SECM) were used to investigate the processes occurring at the surface of exposed metal when electrically connected or disconnected in a galvanic couple with the Mg-rich coating. The SVET has shown the evolution of the pit activity with time under conditions of sacrificial protection, whereas the SECM allowed indirect sensing of the cathodic activity above the electrodes. It was shown that the cathodic protection provided by magnesium to aluminium substrates acts by both preventing pit nucleation and inhibiting the growth of the pre-existing pits.

A study on cathodic protection against crevice corrosion in dilute NaCl solutions

Potential and current distributions in a cathodically protected crevice between a simulated coating and segmented mild steel electrodes were measured in dilute NaCl solutions. The distributions became more uniform with time due to an increase in solution conductivity and depletion of dissolved oxygen in the crevice. Generally, a negative shift of control potential and an increase in initial solution conductivity and crevice thickness resulted in a higher polarization level on the steel. However, if the control potential is too negative, the polarization level may be lower than that under a suitable control potential because of hydrogen evolution. On the basis of these results, a mechanism of cathodic protection against crevice corrosion in high-resistivity environments was proposed.

Corrosion Resistance of 2014 Aluminium Matrix Composites Reinforced with Atomised Ni3Al

The corrosion resistance of 2014 aluminium matrix composites has been evaluated. The base alloy was manufactured through mechanical alloying, and reinforced with Ni3Al (manufactured through atomisation). Composite materials were manufactured according to the following procedure: mixing, cold uniaxial compacting, and hot extrusion. All materials were tested as extruded and heat treated (T6 condition). Corrosion tests were carried out following ASTM standard methods G69 (measurement of corrosion potentials) and G110 (evaluation of intergranular corrosion). The influence that the intermetallic and T6 heat treatment have on the corrosion resistance of 2014 aluminium alloy has been studied. Results show that atomised Ni3Al improves the intergranular corrosion resistance of base aluminium by a mechanism of cathodic protection. T6 heat treatment is also favourable.