Measurement of loss of steel from reinforcing bars in concrete using linear polarisation resistance measurements

The measured weight loss data for a number of mild steel bars contained in Portland Cement concrete, together with predicted weight loss values monitored using potentiostatically controlled linear polarisation resistance (LPR) measurements are reported. Three sets of reinforced concrete specimens, each containing electrically isolated mild steel bars, were subjected to either• chloride-induced corrosion,• carbonation-induced corrosion• a control nitrogen rich environment with minimal corrosion.Each set of specimens was initially exposed to a 22-hour dry, 2-hour wet controlled environment for a duration of between 1026 and 1085 days. This was later changed to a 6-day dry, 1 day wet cycle for the carbonation exposure specimens after the initial set of gravimetric testing. The weight loss for each bar due to corrosion was recorded. Instantaneous LPR measurements were also taken on each bar at regular intervals throughout the exposure period. These resistance measurements were then integrated over the exposure period to estimate total weight loss. The results show that weight loss evaluated from experimental LPR measurements gives a significant over-estimate of the weight losses measured gravimetrically.     

Measuring the corrosion rate of steel in concrete – effect of measurement technique,polarisation time and current

Both on‐site investigations and laboratory studies have shown that different corrosion rates are obtained when different commercially available corrosion rate instruments are used. The different electrochemical techniques and the measurement parameters used, i.e. polarisation current and time, are in some studies considered the main reasons for the variations. This paper presents an experimental study on the quantitative effect of polarisation time and current on the measured polarisation resistance – and thus the corrosion current density – of passively and actively corroding steel. Two electrochemical techniques often used in instruments for on‐site corrosion rate measurements are investigated. On passively corroding reinforcement the measured polarisation resistance was for both techniques found to be highly affected by the polarisation time and current and no plateaus at either short or long polarisation times, or low or high polarisation currents were identified. On actively corroding reinforcement a large effect of the polarisation time was also found, but only a minor effect of the polarisation current. The effect of the polarisation time was, however, practically independent of the corrosion rate for actively corroding steel. For both techniques guidelines for polarisation times and currents are given for (on‐site) non‐destructive corrosion rate measurements on reinforcement steel in concrete.     

Linear polarisation resistance measurements using a potentiostatically controlled guard ring

The use of a sensor controlled guard ring has been developed in recent years to enhance the accuracy of linear polarisation corrosion rate measurements on reinforced concrete structures. The established method of measurement uses a galvanostatically controlled guard ring device. In the method reported in this article the reinforcing steel is polarised potentiostatically by an inner auxiliary electrode and the real time plot of the current response is displayed on a laptop computer which controls the guard ring device. The area of steel polarised is confined by a current applied from an outer guard ring electrode which is controlled by two sensor electrodes positioned between the inner auxiliary and outer guard ring electrode. The potential between the two sensor electrodes is continually monitored, and the current output from the guard ring electrode varied to maintain a constant potential difference between the two sensor electrodes. The method has been validated on an electrical test circuit simulating active and passive reinforcement corrosion. Further testing has been conducted on a reinforced concrete specimen and the results compared with those of commercially available galvanostatically controlled equipment and those of standard unconfined linear polarisation resistance measurements.     

Condition survey with embedded sensors regarding reinforcement corrosion

Besides visual inspections, taking cores or samples and measurements at the concrete surface, the use of embedded sensors can be an effective tool for the assessment of the condition of concrete structures. For new structures sensors can be placed in positions without the possibility of access during use and for existing structures sensors can be used to follow the condition of the structure at relevant selected points. Actually different types of sensor systems are available based on different electrochemical or other methods.     

A mechanistic study of initial atmospheric corrosion kinetics using electrical resistance sensors

This paper describes a novel experimental approach to the study of atmospheric corrosion of iron and zinc, utilising electrical resistance sensors that are sensitive to corrosion losses of the order of one atomic monolayer. Using such devices, a mechanistic study of the initial stages in the atmospheric corrosion of iron and zinc was performed in a rectangular flow cell using controlled relative humidity (RH), temperature and gas flow rate. Additionally, the effects of SO₂ contamination in the gas phase and prior NaCl contamination of the metal surface were studied. It was found that the initial corrosion kinetics of iron and zinc are, not unexpectedly, dominated by the development of surface corrosion product films, but that the growth kinetics vary with metal, humidity, etc. Specifically, in the presence of gas-phase SO₂, activation energies and kinetic and chemical rate orders were consistent with control of the atmospheric corrosion process by solution-phase oxidation of sulphite–sulphate ion. For iron, this implies that the well-known sulphate-nest theory is inoperative at least during the early stages of atmospheric corrosion. In contrast, for chloride–contaminated zinc, the data were consistent with a rate-controlled diffusion of a species, probably water vapour or oxygen, through a thickening corrosion product film. Finally, the kinetic and chemical rate orders for corrosion of chloride–contaminated iron precluded a diffusion-controlled mechanism, but were consistent with a rate-controlling process involving some regeneration of chloride: e.g. as in metal–ion hydrolysis in a pit or similar localised corrosion events.     

Effect of surface condition on the initial corrosion of galvanized reinforcing steel embedded in concrete

The present work was aimed at determining the effect of coating surface condition on the initial corrosion of hot-dip galvanized reinforcing steel bar (HDG rebar) in ordinary Portland cement (OPC) concrete. During zinc corrosion in OPC concrete, calcium hydroxyzincate (CHZ) formed on untreated HDG steel provided sufficient protection against corrosion. Therefore, it is concluded that treating HDG rebar with dilute chromic acid is unnecessary as a method of passivating zinc. A layer of zinc oxide and zinc carbonate formed, through weathering, on HDG bars increased the initial corrosion rate and passivation time compared with the non-weathered rebar exposed to concrete. HDG steel with an alloyed coating, i.e. containing only of Fe-Zn intermetallic phases, required a longer time to passivate than those with a pure zinc surface layer. The lower zinc content of the surface limited the rate of CHZ formation; hence, delayed passivation. Regardless of the surface condition, the coating depth loss after two days of embedment in ordinary Portland cement concrete was insignificant.     

The study on corrosion resistance of high-strength spring steel

The corrosion resistance in medium carbon high-strength spring steel was investigated in neutral salt spray of 5% NaCl solution and potentiodynamic polarisation measurements. The results showed that the corrosion process in the salt spray environment could be divided into two stages. In the initial corrosion stage, the corrosion rates of tested steels were high and rapidly decreased with increasing corrosion time. At later stages where rust layers formed and the corrosion rates of the tested steels all tended to be stable, corrosion resistance was reinforced with the increase of alloying elements, which is just new finding about formation and evolution of corrosion products of spring steel to improve corrosion resistance further in this research. The observation of corrosion products of the tested steels by energy dispersive spectroscopy and electron probe micro-analysis indicated that both Cr and V which had an obvious synergy concentrated mainly in the inner region of the rust layers, and the corrosion medium Cl^– was mainly distributed in the outer region of the rust layers.     

On the study of elastic wave scattering and Rayleigh wave velocity measurement of concrete with steel bar

This paper presents results on a study of the Rayleigh wave scattering and Rayleigh wave velocity measurement in concrete with a steel bar using transient elastic waves. To study the characteristics of the scattered waves induced by a steel bar in concrete, a three-dimensional heterogeneous finite difference formulation with staggered grids was adopted. The cases for both elastic wave propagation parallel and perpendicular to the steel bar are discussed. The effect of the cover thickness and steel bar spacing on the Rayleigh wave velocity measurement was studied. To confirm the numerical investigations, a concrete specimen containing steel bar was made and the corresponding transient elastic wave experiments were conducted. The numerical results are in good accordance with those of the measured. We note that the result of this study can serve as an important reference in the Rayleigh wave velocity measurement of concrete with steel bar.     

The effects of various formwork surfaces on the corrosion performance of reinforcing steel in concrete

In this study, the effects of various formwork surfaces on the corrosion performance of reinforcing steel in concrete were examined. For this purpose, seven formwork surfaces from populus nigra, pinus silvestris, steel sheet, and four of plywoods were prepared. Three of the plywood formworks were covered with different geotextile liners and drainage channel and holes were drilled on their surfaces. One of the plywood formworks having no processes on its surface was for control. Reinforcing steels were first embedded in formworks and then the concrete was poured. The samples were exposed to corrosion in a way of settling them in 5% NaCl solution. We tested the corrosion potential and bonding strength of the samples. Later, the mass loss and tensile strength were measured on reinforcing bars that were pulled out from the concrete. The chloride content and pH values were also tested on concrete powders. The results indicated that drained‐lined formwork (F5 with TB50 + F613 geotextile liner) compared to steel sheet (F4 undrained and unlined) gave 16% lower corrosion and 68% higher bonding strength on reinforcing bars embedded samples, 73% lower mass loss and 4% higher tensile strength on reinforcing bars, and 70% lower chloride content and 4% higher pH on concrete powders. In addition, it was found that the corrosion strength of reinforcing steels in concrete could be increased if drained‐lined formworks were utilized.     

Investigations on the re-passivation of carbon steel in chloride containing concrete in consequence of cathodic polarisation

Although the complex changes at the steel–concrete interface due to cathodic polarisation are widely acknowledged to have a beneficial influence concerning the cathodic protection (CP) of steel in concrete, some questions concerning the repassivation of carbon steel in consequence of cathodic polarisation are still not satisfactorily clarified. In the recent literature, some indications are presented that repassivation occurs after a certain time of polarisation. Therefore, the investigations discussed in this paper aim to clarify, to what extent the re-passivation of carbon steel due to cathodic polarisation occurs, and if the ennoblement of OCP is a sufficient indication for repassivation. In a first step, the corrosion state of five nominal equal test specimens was determined by electrochemical impedance spectroscopy (EIS). After determining the initial corrosion state by evaluating the charge transfer resistance and the polarisation resistance, respectively, the specimens were polarised cathodically. Impedance data were recorded before, during and after polarisation. The impedance data were evaluated by equivalent circuit fitting with special attention to charge transfer resistances and the impact of diffusion on the corrosion and polarisation behaviour. The results indicate that the reduction of oxides and oxygen diffusion during cathodic polarisation has strong impact on the systems behaviour and that repassivation effects occur after switching off the polarisation current and during depolarisation, respectively.     

Polarisation behaviour of steel bar samples in concrete in seawater. Part 1: Experimental measurement of polarisation curves of steel in concrete

The most widely used technique for the investigation of corrosion of reinforcing steel in concrete is the linear polarisation resistance (LPR) method, which however needs to assume the Tafel slopes or the B constant for calculation of the corrosion rate. This paper aims to explore the use of a polarisation curve technique to study the polarisation behaviour and to evaluate simultaneously the corrosion rate and Tafel slopes of steel samples in concrete. As the first part of the study, this paper reviews the relevant literature, and examines the effect of the experimental technique on the measured polarisation curves of steel in concrete in seawater. In particular, differences in the results obtained by two different test procedures are examined. A comparison of the experimental polarisation curves with the theoretical curves based on charge transfer reactions is made.     

Detection and imaging of surface corrosion on steel reinforcing bars using a phase-sensitive inductive sensor intended for use with concrete

A new inductive sensor is described that exploits the principle of frequency shift and phase sensitive detection to identify and image corrosion on the surface of steel reinforcing bars intended for embedment within concrete. A search coil radiating a time-varying magnetic field experiences impedance changes when conductive and/or permeable targets are brought within its vicinity. Depending on the electrical properties of the target, the impedance changes are manifest predominantly as a shift in the Q-factor of the coil, or as a change in its inductance. It is the latter condition that is the primary effect with corrosion product. Under normal circumstances the change in inductance L is very small, but can be detected by configuring the coil as part of a free-running tuned oscillator, whose resonant frequency is governed by L. A shift in the sensor signal frequency is detected by comparing its phase to that of a signal produced by a stable reference oscillator (heterodyning), and producing a voltage proportional to the difference. Such phase sensitive detection is widely employed in metal detector instrumentation, but in this case the signal is used to produce images of corrosion. Significantly, experiments also show that the system is capable of detecting varying amounts of corrosion product, something that is beyond the capabilities of ultrasonic, X-ray or microwave sensing systems. At the present time, the sensor is capable of detecting and imaging a 2 mm thick layer of corrosion on a 20 mm diameter steel bar, located 30 mm below the surface of ceramic material whose conductivity and relative permeability is almost identical to that of concrete.     

合金钢与碳钢渗氮层的耐蚀性对比

在不同温度下对碳钢Q235和42CrMo钢进行气体渗氮,利用光学显微镜、扫描电镜、能谱仪及X射线衍射仪对比分析了Q235和42CrMo钢渗氮层的微观组织及相结构,并用电化学方法测试了样品的耐腐蚀性。经过渗氮处理后的样品的耐腐蚀性都较原材有大幅提高,而相同温度渗氮后42CrMo钢的自腐蚀电位较Q235钢的低,42CrMo钢渗氮层电阻小于Q235钢,碳钢渗氮层的耐蚀性优于合金钢。采用热力学和第一性原理计算探讨了42CrMo钢的合金元素对渗氮层的抗腐蚀性的影响。结果表明合金元素Cr、Mo通过降低Fe-N化合物稳定性从而降低渗氮层的抗腐蚀性。     

Corrosion of reinforcing steel in neutral and acid solutions simulating the electrolytic environments in the micropores of concrete in the propagation period

Steel embedded in concrete passivates due to the alkaline nature of the concrete pore solution. Reinforcement corrosion develops when the alkalinity is neutralized. Corrosion, in turn, induces acidification of the surrounding pore solution. In the present paper, corrosion rate of corrugated steel bars is studied and measured at different pH values in solutions simulating chloride environments. The media considered consists of saturated calcium hydroxide solutions containing Na and K⁺, neutralized with ferrous chloride. This latter substance is the soluble compound produced during the corrosion of steel after chloride attack. Hydrochloric acid solutions of different pHs were prepared in order to compare the steel corrosion rates in these solutions with those observed in ferrous chloride solutions of the same pH. A comparison of polarization resistance measurements (R_p) with gravimetrically weight loss determined is presented. Tafel slopes results are also included. Additionally, a comparison is made between measurements of AC impedance with those of the R_p method. The results indicate that the corrosion rate in the studied media follows the general trend found in other media of similar pH values: corrosion increases in acidic solutions, remains rather stable for pH range 3-11 and decreases significantly in highly alkaline solutions.     

A guide to polarisation curve interpretation: deconstruction of experimental curves typical of the Fe/H2O/H/O2 corrosion system

Experimental DC polarisation curves are the sum of the cathodic and anodic components and can be difficult to interpret. Schematic representations of ‘typical’ curves (together with their anodic and cathodic components) are available in the literature for comparison purposes. A better approach to curve analysis is to generate mathematically the experimental curve which is then deconstructed into its components. Unfortunately the appropriate computer programmes are not readily available. We have considered it useful to revisit the collected curve concept replacing schematic representations with experimental curves. Using an up-dated programme we have accurately analysed curves representative of the Fe/H₂O/H⁺/O₂ corrosion system.     

Sn对耐候钢高湿热海洋大气环境下耐蚀性能的影响

采用周浸腐蚀试验机、扫描电镜（SEM）、电化学工作站等,研究了含锡耐候钢和传统耐候钢在模拟高湿热海洋大气环境中的腐蚀行为规律,探讨了Sn元素对传统耐候钢耐高湿热海洋大气腐蚀性能的影响。结果表明：锡元素加入耐候钢中可以降低腐蚀速率,让锈层更均匀地生成,提高锈层的自腐蚀电位,降低锈层自腐蚀电流密度,对耐候钢阳极溶解起抑制作用,同时使锈层电阻Rr及与基体结合处的反应电阻Rt升高,增强锈层对钢基体的保护,有利于耐候钢耐海洋大气腐蚀性能的提升。     

DFG‐research‐group: Modelling reinforcement corrosion – overview of the project

A strong interest for the durability of reinforced concrete structures currently exists in industry and research [1]. Against the background of immense costs for maintaining reinforced concrete structures and repairing damage caused by corroding reinforcement steel, this interest lead to a German joint research project. The aim of this network‐based (www.bam.de/dfg537.htm) research group is, to deliver the basic knowledge of the corrosion propagation and to make a probabilistic tool available for engineers so that a complete design for durability, concerning reinforcement corrosion, will be possible.