Cathodic protection monitoring of buried carbon steel pipeline: measurement and interpretation of instant-off potential

The criterion used to verify the cathodic protection condition of a structure is based on structure-to-electrolyte potential measurement, which can include an ohmic drop contribution. Among the available techniques, the use of potential probes with internal reference electrode and of corrosion coupons with a so-called soil-access tube represents the most appropriate way to measure the IR-free potential, eliminating the ohmic drop contribution. An alternative is represented by on-off technique if equalising, galvanic or stray currents are not present. Laboratory tests have been carried out in soil-simulating solution on cathodically protected carbon steel in order to evaluate the effectiveness of off-potential for the assessment of cathodic protection. Instant-off potential is not reliable in overprotection condition; moreover, the accuracy of the measurement is strongly influenced by the presence of external current as galvanic or equalising currents. The effect of low-pass filter in the data acquisition system on the potential reading is discussed.     

Influence of irregularities in the electrolyte on the cathodic protection of steel: A numerical and experimental study

This paper shows how the last algebraic matricial form can be obtained when the finite element method is used to approximate the potential distribution of a cathodic protection system that includes low conductivity irregularities in the electrolyte away from, close to and directly on the cathode. In order to study the influence of the resistivity of these irregularities on the possibilities of steel protection, five conductivities were analysed. The numerical results, validated with COMSOL® Multiphysics, show the importance of considering irregularities in the domain in order to prevent systems from becoming unprotected. The experimental data agrees with the theoretical data.     

Corrosion behavior of carbon steel coated with a zinc-rich paint containing metallic compounds under wet and dry cyclic conditions

The corrosion behavior of carbon steel coated with a zinc-rich paint containing two metallic compounds, Al₂(SO₄)₃ and CaO, as anticorrosive additives was examined under wet and dry cyclic corrosion test conditions. The zinc-rich paint coating without the two metallic compounds formed a white corrosion product and red iron rust on the surface after the corrosion test, whereas the coating with the metallic compounds showed reduced surface corrosion products. The corrosion current density of the painted steel substrate decreased drastically due to the incorporation of metallic compounds in the paint. The zinc-rich paint coating modified with the metallic compounds contained dispersed simonkolleite (Zn₅(OH)₈Cl₂·H₂O) phase and possibly very fine CaSO₄ particles, which remarkably improved the protectiveness of the zinc-rich paint coating.     

Wet/dry accelerated laboratory test to simulate the formation of multilayered rust on carbon steel in marine atmospheres

The prolonged exposure of carbon steel in marine atmospheres with high chloride deposition rates and long times of wetness of the metallic surface leads to the formation of thick multilayered rust. The present work proposes an accelerated cyclic laboratory test based on immersion (4.2 min) in a 3.5% NaCl solution followed by drying (12 min) under infra-red lamps in the laboratory atmosphere. The carbon steel corrosion process is thus accelerated, giving rise to the generation of thick rust layers in relatively short times. The rust phases and the structure of the rust layers formed offer a good approximation to the multilayered rust formed in marine atmospheres. The study includes a gravimetric evaluation of the magnitude of corrosive attack and a characterisation of the rust phases and corrosion layers formed using XRD, optical microscopy and SEM.     

The beneficial secondary effects of conventional and pulse cathodic protection for reinforced concrete,evidenced by X‐ray and microscopic analysis of the steel surface and the steel/cement paste interface

The present study reports on the investigation of conventional and pulse cathodic protection (CP), in terms of steel surface analysis and investigation of the product layers at the steel/cement paste interface, after a long term (460 days) of conditioning and monitoring. The techniques used were: X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and Energy‐dispersive X‐ray (EDX). Wet chemical analysis was used as supportive evidence for ion concentrations in the vicinity of the steel re‐bars. Generally, CP promotes beneficial secondary effects i.e. enhanced OH^? concentration and reduced Cl^? concentration near the steel surface. Cathodic polarization also results in additional deposition of portlandite, which stabilizes the protective properties of the product layer on the steel surface. Consequently, the fundamental mechanisms, underlying the efficiency of CP techniques in reinforced concrete, are strongly related to beneficial secondary effects of CP, affecting the morphology and transformations of these product layers. Since the experimental evidences to support the aforementioned beneficial effects are rather limited, this study investigates the morphology and composition of the “naturally” formed steel surface layers, along with the properties of the steel/cement paste interface, on corroding and protected steel reinforcement (in comparison to reference, non‐corroding, non‐protected conditions) after 460 days of conditioning. For the corroding specimens, the formation and substantial deposition of voluminous corrosion products, with low adherence to the steel surface is relevant (low protective ability), whereas in the protected specimens, a compact and adherent product layer of more stable high valent iron oxides, or calcium substituted such, was observed. To this end, the present work provides the experimental evidence for the fundamental mechanisms, related to the otherwise recognized positive secondary effects of CP in reinforced concrete.     

Stray current-induced corrosion in cathodic protection installations of steel-reinforced concrete structures: FEM study of the critical parameters

A wide range of parameters was investigated by numerical calculations concerning their impact on DC stray current corrosion of reinforced concrete (RC) structures. A simplified model geometry was used to extract the relevant parameters and their interaction in terms of stray current-affected structures. This study mainly focuses on RC structures that are fitted with cathodic protection installations. The findings reveal a complex interaction between the investigated parameters. The possible relevance of further parameters, which is not the subject of this study, was emphasised.     

Corrosion behaviour at the interface of steel bars embedded in cement slurries: Effect of phenol polymer coatings

Cyclic voltammetry has been employed to investigate the behaviour of carbon steel and stainless steel electrodes in solutions obtained by filtering of calcium aluminate cement and portland cement slurries. Electro-polymerized phenol coating on steel electrodes has also been studied in carbonate medium. The phenol electro-polymerization occurs on a passivated surface and leads to adherent and stable polymeric film exhibiting a partial protection against corrosion. The Fourier transform infrared–attenuated total reflection spectroscopy shows that this film maintains the aromatic character and contains ether-linked rings.     

Corrosion behaviour at the interface of steel bars embedded in cement slurries: Effect of phenol polymer coatings

Cyclic voltammetry has been employed to investigate the behaviour of carbon steel and stainless steel electrodes in solutions obtained by filtering of calcium aluminate cement and portland cement slurries. Electro-polymerized phenol coating on steel electrodes has also been studied in carbonate medium. The phenol electro-polymerization occurs on a passivated surface and leads to adherent and stable polymeric film exhibiting a partial protection against corrosion. The Fourier transform infrared-attenuated total reflection spectroscopy shows that this film maintains the aromatic character and contains ether-linked rings.     

Novel zinc‐rich epoxy paint coatings with hydrated alumina and carbon nanotubes supported polypyrrole for corrosion protection of low carbon steel: Part II: Corrosion prevention behavior of the hybrid paint coatings

Utilization of various types of multi‐walled carbon nanotubes (MWCNTs) in zinc‐rich paints (ZRPs) is presented addressing percolation and porosity related phenomena of traditional ZRPs. Hybrid paint coatings were formulated with 3.21 wt% polypyrrole (PPy) deposited alumina‐MWCNT inhibitor particles (PDAMIPs) and 70 wt% zinc contents. Corrosion protection behavior of the hybrid coatings was investigated by electrochemical impedance spectroscopy (EIS), glow‐discharge optical emission spectroscopy (GD OES), X‐ray photoelectron spectroscopy (XPS), and FT‐Raman spectroscopy. Immersion and salt‐spray chamber tests gave evidence of improved galvanic protection and barrier nature of the hybrid coatings over the conventional ZRPs, whereas inhibited zinc corrosion and ignorable steel corrosion took place besides lower degradation of the binder. Zinc‐rich hybrid paints with either high relative amount of polyelectrolyte‐modified or low proportion of functionalized MWCNTs afforded enhanced corrosion prevention. This result is partly attributed to the nanotube volume fractions around the threshold of infinite cluster formation contributing to electrical percolation and galvanic action of the hybrids. Experimental results are discussed in a broader context on the basis of structure related findings of the PDAMIPs (described in Part I) and in the light of recent literature data. From the newly developed inhibitor particles, some of them are respected as worthy additives for application in hybrid coatings featuring high performance corrosion prevention functionality.     

TiC硬质合金/碳钢爆炸焊接复合板界面微观组织

用透射电镜、扫描电镜和X射线能谱仪对TiC硬质合金/碳钢爆炸焊接复合板界面微观组织和相组成进行分析.结果表明,界面上有一断续的熔合层,层厚约10μm,层内为尺寸位于几十个～几百个纳米之间的纳米或亚微米超细晶粒,组成相为铁素体、奥氏体和少量TiC.在界面附近碳钢侧可以看到明显的流线状组织特征,铁索体具有板条状马氏体的结构特征,珠光体层片间距减小,呈流线分布.焊接过程中Ti向钢中扩散15μm左右.     

Electrochemical polymerization of o-anisidine on low carbon steel from aqueous salicylate solution: Corrosion protection study

The electrochemical polymerization of o-anisidine was carried out on low carbon steel from an aqueous salicylate medium using cyclic voltammetry. The resulting poly(o-anisidine) coatings were uniform and adherent to the steel substrates. These coatings were characterized by cyclic voltammetry, UV-visible absorption spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction measurements and scanning electron microscopy. The ability of the poly(o-anisidine) coatings to protect low carbon steel in an aqueous 3% NaCl was evaluated by the potentiodynamic polarization measurements. The potentiodynamic polarization measurement reveals that the poly(o-anisidine) coating increases the corrosion potential and reduces the corrosion rate of low carbon steel almost by a factor of 15.     

Microstructural properties of the bulk matrix and the steel/cement paste interface in reinforced concrete,maintained in conditions of corrosion and cathodic protection

Although rarely considered, especially within the investigation of steel corrosion phenomena or electrochemical protection techniques in reinforced concrete structures, the concrete bulk matrix has a significant contribution in the global performance of the system “reinforced concrete.” This is especially the case when chloride‐induced corrosion or electrical current flow [as within impressed current cathodic protection (CP)] are involved. In the latter cases, the concrete bulk matrix undergoes significant alterations in chemical composition, electrical properties, and microstructures, thus influencing the overall performance of the system. This work reports on the microstructural investigation of the bulk concrete matrix and the steel/cement paste interface in reinforced concrete, previously subjected to corrosion and CP for 460 days. The emphasis hereby is to evaluate the altered structural properties, i.e., porosity, pore size, permeability of the bulk cement matrix, and the steel/cement paste interface (translated to bond strength) as a result of chloride‐induced corrosion and two types of CP (conventional and pulse), compared to control (non‐corroding, non‐protected) conditions. The research revealed a major contribution and close dependence of all microlevel interfaces on the global performance of reinforced concrete. The electrical current flow (as in CP applications) was found to bring about unfavorable modifications to the material structure, both in the bulk matrix (reducing porosity) and at the steel/cement paste interface (enlarging interfacial gaps). The derived microstructural parameters show that the conventional CP leads to a higher level of structural heterogeneity, whereas the pulse CP exerts minimal or no effects, maintaining the material properties close to the reference (control) conditions, the underlying mechanism being a more homogeneous material microstructure.     

The corrosion behavior of zinc-rich paints on steel: Influence of simulated salts deposition in an offshore atmosphere at the steel/paint interface

The corrosion behavior of an epoxy zinc-rich paint on interface-contaminated carbon manganese-silicon steel was studied. SEM observation on the cross-section of the paint indicates that the zinc corrosion products grew from the surface to the inner of the paint and salts contamination promoted the growth at locations close to the steel/paint interface. The results of electrochemical impedance spectroscopy show that the corrosion resistance of the contaminated paint was significantly influenced by diffusion of zinc corrosion products during the initial stage of immersion, and diffusion of iron corrosion products at the end of immersion. Three transmission line models were applied to account for the corrosion process of the uncontaminated and contaminated zinc-rich paints.     

Streaming potential measurements on stainless steels surfaces: evidence of a gel-like layer at the steel/electrolyte interface

Electric charges at the surface of a passive stainless steel are generally considered as concentrated either in the passive film itself, or at the metal/passive film interface, or in the electrical double layer at the film solution interface. Rest potential time dependence after immersion of a passive surface in aqueous electrolytes suggests however that slow processes occur in the onset of the surface charge. Specific experiments, such as streaming potential measurements and electrochemical impedance spectroscopy in a thin electrolyte cell, were carried out for understanding better this phenomenon. An AISI 304 type austenitic stainless steel with polished or bright annealed surface finishes was immersed in NaCl aqueous solutions with various pH and chloride concentrations. The streaming potential time evolution shows two steps: a first rapid one (∼2 min) is attributed to the onset of the surface charge. The second step is much slower (approximately several hours) and possibly due to an interphase layer between the passive film and the solution. Following this idea, the whole kinetics is controlled by cation migration across the interphase when the pH is larger than the isoelectric pH (pH_iep), while chloride ions are incorporated in the interphase when pH < pH_iep. Impedance measurements allow determining both the kinetics of charge transport and the thin cell conductivity. When glass is used as reference material for the cell walls instead of stainless steel, the Nyquist plots show a high-frequency response. For stainless steel cell walls, a low-frequency response is observed, attributed to a slow charge reorganisation inside the interphase layer. The charge distribution at metal/electrolyte interface is discussed in terms of a gel-like layer which possibly takes place at the passive film/electrolyte interface.