Corrosion monitoring of galvanised coatings through electrochemical impedance spectroscopy

In this work, corrosion of industrial galvanised coatings is monitored through potentiodynamic and electrochemical impedance spectroscopy (EIS) methods and supported by real-time immersion tests. For such purpose, the corrosion behaviour is studied in different media (NaCl, NaOH and rain water), at different concentrations and varying immersion times. The results show that EIS allows to establish the interfacial reactions and the dissolution mechanisms occurring in three corrosive media, hence to foresee the protection conferred by these coatings. The impedance diagrams of the coated steel do not provide information on the slowest reactions, which only occur in natural rain water. Finally, each Zn/medium interface is characterised by a specific equivalent circuit giving a similar impedance response.     

Detection of corrosion degradation using electrochemical noise (EN): review of signal processing methods for identifying corrosion forms

Electrochemical noise (EN), as one of the most promising in situ electrochemical methods in corrosion and electrochemical science, has been developing rapidly in recent years with the advancements in instrumentation and signal processing methods. One advantage of EN is its application in long-term or early stage corrosion process monitoring because it instantly detects corrosion rate and corrosion forms. Investigators have applied various mathematical methods to extract characteristic parameters from EN. In this paper, identifying corrosion forms using parameters obtained from time domain, frequency domain and time–frequency domain is reviewed, and the correlation between parameters and corrosion forms is discussed. Finally, other in situ techniques are recommended to be employed synchronously with EN measurement in order to obtain reliable analyses.     

Assessment of intergranular corrosion (IGC) in 316(N) stainless steel using electrochemical noise (EN) technique

In the present work, an attempt was made to demonstrate the use of electrochemical noise (EN) technique in assessment of intergranular corrosion (IGC) in 316(N) stainless steel (SS). Degree of sensitization (DOS) in the specimens aged at 923 K for 24, 40, 50 and 100 h was determined using double loop electrochemical potentiodynamic reactivation (DLEPR) technique. Immediately after applying a prior preconditioning treatment, current and potential noise measurements were made. The DOS was determined from standard deviation of current noise (σ_I) versus time plot and it was found to bear a good correlation with the values obtained by DLEPR. Shot noise analysis of the EN data confirmed the above results.     

Corrosion of furnace wall materials in waste-wood fired power plant

Abstract

Corrosion tests were performed with four different materials exposed at the furnace wall in a power boiler burning recycled wood, with the aim of evaluating coatings to reduce the corrosion. The nickel base Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT had the lowest corrosion rates followed by the stainless steel 310S. The low alloy steel 16Mo3, from which the walls are constructed, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. Thermodynamic modelling showed that chlorine gas exists at extremely low levels under the prevailing conditions and the hydrated form is thermodynamically favoured.     

Corrosion behavior of iron thin film in deaerated phosphate solutions by an electrochemical quartz crystal microbalance

The corrosion rates of iron thin film in deaerated phosphate solutions were measured by an electrochemical quartz crystal microbalance as a function of solution pH or phosphorus concentration in solutions. The dependences of corrosion rate and corrosion potential on solution pH and phosphorus concentration have suggested that the corrosion mechanism changes in the vicinity of pH 5 and H₂PO₄⁻ contributes to both anodic dissolution of iron and hydrogen evolution. The corrosion mechanism which contains iron-phosphate-hydroxide complex ion as an adsorbed intermediate was proposed to explain the experimental results.     

Corrosion of carbon steels in high-temperature water studied by electrochemical techniques

Electrochemical methods were used to study the corrosion behavior of five Fe-Cr alloy steels and 304L stainless steel in high-temperature water. Passivity can be achieved on A-106 B carbon steel with a small content of chromium, which cannot be passivated at room temperature. The formation rate and the stability of the magnetite film increased with increasing Cr content in the steels. A mechanistic model was developed to describe the corrosion processes of steels in high-temperature water. The crack growth rate on steels was calculated from the maximum current of the repassivation curves according to the slip-oxidation model. There was a high crack growth rate on 304L stainless steel in high-temperature water. Of the four Fe-Cr alloys, the crack growth rate was lower on 0.236% Cr- and 0.33% Cr-steels than on 0.406% Cr steel and 2.5% Cr-1% Mo steel. The crack growth rate on 0.33% Cr steel was the smallest over the tested potential range.An increase in Cr content in the steel is predicted to reduce the corrosion rate of steel at high temperatures. However, this increase in Cr content is predicted not to reduce the susceptibility of steel to cracking at high temperatures.     

Quantitative evaluation of general corrosion of Type 304 stainless steel in subcritical and supercritical aqueous solutions via electrochemical noise analysis

Electrochemical noise (EN) sensors have been developed to measure the corrosion rate of Type 304 stainless steel (SS) in subcritical and supercritical environments. The EN sensors were tested in flowing aqueous solutions containing NaCl and HCl at temperatures from 150°C to 390°C, a pressure of 25 MPa, and flow rates from 0.375 to 1.00 ml/min. The potential and coupling current noise were recorded simultaneously and the noise resistance (R_n) was calculated from the standard deviations in the potential and current records. We found that the inverse noise resistance correlated very well with the corrosion rate evaluated from separate mass loss experiments, and that both the inverse noise resistance and the average corrosion rate were functions of temperature and flow rate. In the temperature range from 200°C to 390°C, the corrosion rate was found to be proportional to the inverse noise resistance and hence the Stern-Geary relationship can be used to evaluate the corrosion rate. However, at 150°C, the relation between inverse noise resistance and corrosion rate significantly deviated from the Stern-Geary relationship. It was found that the deviation was related to the low corrosion rate of Type 304 SS and 150°C.     

A measurement system based on electrochemical frequency modulation technique for monitoring the early corrosion of mild steel in seawater

The electrochemical frequency modulation (EFM) technique, featuring of obtaining the Tafel constants directly, small polarization and short measuring time, offers a novel way for electrochemical corrosion monitoring. According to current research, the EFM approach should be mostly limited to those corrosion systems with fairly high corrosion rate, such as mild steel or iron in acidic environment. In this paper, a system for EFM measurement was set up with a potentiostat, a laptop, a NI DAQ card and applications developed with LabVIEW 7.0. It was then applied to the mild steel/seawater system. The early corrosion rate for mild steel in Zhoushan seawater was determined in laboratory with EFM technique and another two general electrochemical techniques as well-analysis of the polarization curve in the vicinity of the corrosion potential and electrochemical impedance spectroscopy. The EFM measurements were performed with 0.02 and 0.05 Hz potential perturbation frequencies, and the EFM data were analyzed with activation-controlled equation. It was shown that the corrosion rates determined with EFM technique were higher than the real values, especially at longer exposed times when diffusion-controlled effect became obvious. But they were still at the same order of magnitude as those obtained with polarization curve analysis or weight loss method.     

Corrosion of the component phases presents in high copper dental amalgams. Application of electrochemical impedance spectroscopy and electrochemical noise analysis

The corrosion resistance of three of the constituent phases in high copper dental amalgams has been investigated by electrochemical methods in 0.9% NaCl solution. Polarization curves show corrosion potentials most positive for γ₁-Ag₂Hg₃, followed by Ag-Cu, and γ-Ag₃Sn in agreement with the order of corrosion resistance deduced from the corrosion currents. Complex plane impedance plots at the open circuit potential showed distorted semicircles with diffusional components at low frequency for Ag-Hg and Ag-Cu, while for γ-Ag₃Sn a layer of corrosion products is formed, partially or completely covering the surface of the electrode. Impedance and noise spectra have been compared in the frequency domain, and show good agreement.     

基于微弧氧化和电化学沉积在纯Ti基体表面制备FHAP/MAO复合涂层的腐蚀行为和机械性能研究

本论文采用微弧氧化和电化学沉积（ED）技术在工业纯钛（CPTi）表面沉积氟掺杂羟基磷灰石（FHAP）/微弧氧化（MAO）复合涂层。并在Hsnk"s溶液中对未涂覆的CPTi基材和涂覆的样品进行电化学耐腐蚀性测试。本文研究了MAO界面层对涂层微观结构，力学性能和电化学性能的影响。结果表明，HAP / Ti，FHAP / Ti和FHAP / MAO / Ti复合涂层样品在模拟Hank"s溶液中显着提高了CPTi基体的耐腐蚀性能。然而，力学性能测试表明，与具有MAO界面层的FHAP / Ti涂层的结合强度（18.1MPa）相比，FHAP / Ti涂层的结合强度较差（10.7MPa）。此外，FHAP / MAO / Ti涂层与去离子水的接触角约为35.8°，这更有利于促进细胞附着和增殖。     

Experiences with high temperature corrosion at straw‐firing power plants in Denmark

By the end of 2009, there will be eight biomass and five biomass co‐firing plants in Denmark. Due to the steep increase of corrosion rate with respect to temperature in biomass plants, it is not viable to have similar steam data as fossil fuel plants. Thus for the newer plants, Maribo Sakskøbing, Avedøre 2 biomass boiler, Fyn 8 and Amager 1 (Fyn 8 and Amager 1 are under commissioning), the steam temperature of the final superheaters are approximately 540 °C and the steel type used is an 18–10 stainless steel, (TP347H). However there is still a need to monitor corrosion rates, and to collate data to enable better lifetime prediction of vulnerable components in straw‐firing plants since the corrosion rates are so much faster than in coal firing plants. Therefore, there are continued investigations in recently commissioned plants with test tubes installed into actual superheaters. In addition temperature is measured on the specific tube loops where there are test tube sections. Thus a corrosion rate can be coupled to a temperature histogram. This is important since although a superheater has a defined steam outlet temperature, there is variation in the tube bundle due to variations of heat flux from the flue gas. This paper will describe the corrosion investigations for tube sections removed from Maribo Sakskøbing and Avedøre 2 biomass boiler which have been exposed for up to 30 000 h. In addition to monitoring the corrosion rates of actual components, there is a need to measure corrosion rates at higher temperatures to assess if there is a possibility to increase the outlet temperature of the plant, thus making the plant more cost effective. For this purpose Avedøre 2 biomass boiler has a test superheater loop fabricated in TP347H FG (the same material as the final superheaters). Some results from this test superheater will also be described. Effects of flue gas temperature and flue gas direction on corrosion rates are also discussed.     

The application of electrochemical noise resistance to evaluate the corrosion resistance of AISI type 304 SS in nitric acid

The paper presents the application of noise resistance to evaluate the corrosion behaviour of sensitized AISI type 304 SS in nitric acid of varying concentration (4 N, 12 N, 16 N) and temperature (298 K, 323 K, 348 K). Electrochemical noise data was acquired from a three identical electrode configuration in the required conditions at open circuit potential. The noise resistance was evaluated as the ratio of the standard deviation of the potential to that of the current noise after removing the DC component. The inverse relationship between noise resistance and corrosion rate was exploited to qualitatively assess the corrosion behaviour of AISI type 304 SS in nitric acid. Noise resistance decreased with increase in concentration implying an increase in corrosion rate with increase in nitric acid concentration. An increase in temperature from 298 K to 323 K and 348 K decreased the noise resistance in 4 N and 12 N nitric acid implying higher corrosion rates at higher temperatures. The corrosion rates were similar at 323 K and 348 K in these concentrations. The simultaneous measurement of current and potential noise facilitated the evaluation of the frequency dependence of the noise data to determine the spectral noise resistance (R_sn) and the DC limit of the spectral noise resistance . The results from R_sn and also indicated higher corrosion rates at higher concentration and temperature. Also R_n and correlated well in 4 N and 12 N nitric acid at 323 K and 348 K while disparity was observed at room temperature in 4 N and 12 N nitric acid.     

Corrosion and electrochemical evaluation of an Al–Si–Cu aluminum alloy in ethanol solutions

The corrosion of aluminum alloy AlSi8Cu3Fe(Zn) in ethanol and ethanol solutions containing 10 vol.% water and 10 vol.% acetic acid, respectively, was investigated by means of electrochemical impedance spectroscopy (EIS), polarization curve, immersion, optical microscopy, scanning electron microscopy and element mapping. The Al alloy in the ethanol and its solutions exhibited a capacitive loop in the measured Nyquist EIS spectra at high frequencies, which can be attributed to the ethanol’s dielectric response. Addition of 10 vol.% acetic acid increased the ethanol corrosivity more significantly than the same amount of water addition. The Al–Si–Cu–Mg precipitated zones in the alloy were susceptible to corrosion attack due to the micro-galvanic effect by the Cu-containing precipitates.     

Machining of non-conducting materials using electrochemical discharge phenomenon—an overview

Machining with electrochemical discharges is an unconventional technology able to machine several electrically non-conductive materials like glass or some ceramics. After almost 40 years of its first mention in literature, this technology remains an academic application and was never applied in industrial context. The knowledge about machining of non-conducting materials using electrochemical discharge phenomenon is reviewed up to this date with some particular attention to the electrochemical point of view. Some main limiting factors are highlighted and possible solutions are discussed.     

Effect of inorganic inhibitors on the corrosion behavior of 1018 carbon steel in the LiBr + ethylene glycol + H2O mixture

The effect of inorganic inhibitors on the corrosion behavior of 1018 carbon steel in the mixture LiBr (55%) + ethylene glycol + H₂O at room temperature has been evaluated. Used inhibitors included LiNO₃ (Lithium Nitrate), Li₂MoO₄ (Lithium Molybdate) and Li₂CrO₄ (Lithium Chromate) at concentrations of 5, 20 and 50 ppm. Electrochemical techniques included potentiodynamic polarization curves, electrochemical noise resistance (EN) and electrochemical impedance spectroscopy (EIS) measurements. Additionally, adsorption isotherms were calculated. The results obtained showed that both, the corrosion rate and the passive current density decreased with inhibitors, and, in general terms, inhibitors efficiency increased with inhibitor concentration, except in the case of Li₂CrO_4, where the highest efficiency was obtained with 20 ppm of inhibitor. Pitting potential with 5 ppm of inhibitor, regardless its chemical composition, was more active than in absence of inhibitor, increased at 20 ppm, especially with Li₂CrO₄, and remained unaltered with 50 ppm. EN measurements showed that at 5 ppm of inhibitor, the number of film rupture/repassivation events was higher than that obtained at 20 or 50 ppm. Adsorption isotherms suggested a different adsorption mechanism for each inhibitor, whereas EIS results suggested that the corrosion process when nitrates were added was under charge transfer control, but in the case of molybdates or chromates was under diffusion control.     

Exfoliation corrosion and electrochemical impedance spectroscopy of an Al‐Li alloy in EXCO solution

The exfoliation corrosion and electrochemical impedance spectroscopy (EIS) of an Al‐2.8%Cu‐1.5%Li‐0.3%Mg‐0.3%Zn‐0.3%Mn‐0.15%Zr alloy with various aging states in EXCO solution were investigated. The equilibrium precipitates at grain boundaries are anodic to the alloy base at their adjacent periphery. With prolonging aging time, the amount and the size of the equilibrium precipitates at grain boundaries are increased, resulting in an enhanced susceptibility to exfoliation corrosion. At the beginning of immersion in EXCO solution, the EIS plot of the alloys is composed of a capacitive arc in the high frequency range and an inductive loop in the low frequency range. As immersion time is increased, two capacitive arcs appear in the high‐mediate and mediate‐low frequency ranges respectively and the appearance time of two capacitive arcs could be an indication of the speed of localized corrosion development in EXCO solution. The longer appearance time of two capacitive arcs of the under‐aged alloy indicates its slower localized corrosion development.     

Nitridation of grate in a biomass‐fired boiler

After a straw‐fired power plant in Denmark had been in operation for 16 years with 93,815 hr operation, there was a need for overlay welding repair of the grate due to a decrease in wall thickness from erosion/corrosion. The grate consisted of 15Mo3 (Werkstoff nr. 1.5415) tubes where water/steam flows through, which were welded together with fins, and where one side faced the combustion zone coming into contact with the fuel (the side that required weld overlay). The water/steam temperature was 320°C and the surface metal temperature was estimated to be 350°C. However, there were difficulties when trying to overlay weld the grate as the weld pool was unstable. The microhardness measurements on cross‐sections of the tubes revealed a higher hardness of the tube facing the combustion zone compared to opposite the combustion zone. In addition, there was higher hardness adjacent to the combustion side than the steam/water side. Analysis of etched cross‐sections with light optical and scanning electron microscopy revealed the presence of acicular structures at the convex surface. Further analysis has been conducted with X‐ray diffraction and a LECO nitrogen analyser indicating the presence of iron nitrides. Reasons for their evolution and implications for the repair welding are discussed.