The use of electrochemical noise methods (ENM) to study thick, high impedance coatings

Thick, high impedance organic coatings are those class of coatings used to provide corrosion protection to naval vessels, pipelines, gasoline storage tanks, and other large structures such as bridges and plant structures. These coatings, especially the newest generations now being used in practice, can provide exceptional protection and lifetime of performance such that properly and accurately assessing and differentiating among competing coatings is a very difficult task. The standard protocol of salt fog testing (ASTM B117), immersion testing, and outdoor exposure in a corrosive environment with subjective evaluation of a coating's performance durings and after testing, does not adequatcly rank and predict coating lifetimes for new coating systems, especially for the environmentally compliant coating systems such as powder coatings (especially the thick, fusion bonded epoxy (FBE) coatings used for pipelines), two component epoxy and urethane coatings and waterborne coatings. New, objective test methods are desperately needed by users and manufacturers of coatings. A relatively new electrochemical test procedure, electrochemical noise methods (ENM), as developed by Skerry and Eden, has been shown in our laboratory to be very successful in the ranking and prediction of relative coating performance. We have used the method successfully on naval ship coatings, several pipeline coatings and other related systems, and Skerry has used them successfully on industrial maintenance coatings. We have used these methods in conjunction with electrochemical impedance spectroscopy, d.c. resistance measurements and cyclic salt fog testing of the Prohesion^TM type. In our studies of pipeline coatings, we needed to investigate thermal effects because of their extended range of use temperature. In these studies, we have discovered that electrochemical methods can be used for an in situ measurement of the T_g of coatings in electrolyte immersion. Further, the ‘plasticizing’ effect of aqueous electrolyte absorption as well as its relative irreversibility has been shown. For all coatings studied, ENM provided useful, objective, numerical data which rapidly ranks coatings and provides useful information on the relative lifetime prediction of coatings which may provide up to 30 years of service.     

The assessment of a smart anticorrosive coating by the electrochemical noise technique

Zinc phosphate and related compounds are convenient replacements for chromates. However, more eco-compatible pigments are being investigated. The objective of this research was to develop a modified zeolitic rock which is intended to replace phosphate pigments in anticorrosive paints. The modified zeolitic rock was obtained by grinding the rock followed with ionic exchange with molybdenyl ions. This “composite” has an intelligent behavior because molybdenum compounds are leached from the zeolite particle by corrodent species. The anticorrosive properties of this zeolitic rock were studied by electrochemical techniques, employing inhibitor suspensions, and formulating anticorrosive coatings. Coatings performance was evaluated by accelerated tests (humidity chamber and salt spray) and electrochemical noise measurements (ENM). Electrochemical noise data were analyzed in the time domain. The noise resistance (R_n) was compared, as far as possible, with the polarization resistance.     

A comparative study of the available electrochemical methods for the investigation of epoxy and fluoropolymer coatings deterioration

The corrosion behaviour of phosphatized galvanized steel coated with both epoxy films of different thickness and fluoropolymer films has been studied by means of a.c. impedance spectroscopy (EIS), break-point frequency, potentiodynamic measurements and faradaic distortion methods as well as by the salt spray test. It was observed that the degradation of protective films appears after a long-lasting initial period but once the process starts, the area of defects increases with exposure time. The rate of degradation depends both on type of polymer and of film thickness for the same type of polymer. The same behaviour can be observed from the decrease in pore resistance and charge-transfer resistance (EIS) and increase in double-layer capacitance (a.c. impedance measurements) and corrosion current (potentiodynamic measurements and harmonic analysis).     

Characterization of active pigments in damage of organic coatings on steel by means of electrochemical impedance spectroscopy

Active anticorrosive pigments are solid additives for primers which can give further protection for areas with coating damage in addition to their barrier effect. These pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damage (chemical protection). Electrochemical Impedance Spectroscopy (EIS) was applied to characterize the corrosion protection behaviour of alkyd primers containing different pigments. Impedance spectra were recorded in the frequency range 50 mHz f 50 kHz at the open-circuit potential as a function of the type of pigment and the exposure time in different corrosive media. In general, two different parts can be distinguished in the impedance diagrams. The higher frequency part is related to the insulating properties of the primer and the lower frequency part can be attributed to electrochemical processes taking place within the coating defects. The parameters derived from EIS results show that the low frequency data can be used for characterization of the protective properties of anticorrosive pigments in the presence of defects in organic coatings.     

Use of electrochemical noise method to investigate the anti-corrosive properties of a set of compliant coatings

This particular study has the aim of developing a new set of reduced VOC (also known as compliant) coatings. The reasoning for this is linked with the mass reduction scheme (MRS) and the requirement for environmentally friendly coatings. To this end a comparison needs to be made between these products and the higher solvent products which it is hoped that they will replace. Accelerated tests are required to get results in reasonably short times. Previous work has established the electrochemical noise method (ENM) as a useful technique for investigating the corrosion resistance of intact organic coatings on steel substrates. To date the most useful parameter obtained has been resistance noise (R_n) and results have shown that coatings that effectively protect the metal substrate demonstrate considerably higher R_n values and these remain high with time. In this work ENM has been used to monitor intact coatings under immersion conditions. Results will be given and compared with standard “salt spray” testing. The immersion test has also had a temperature cycle incorporated and this latter modification makes it effective at separating coatings within an even shorter time. The results so far look very promising and this test regime is helping the company to formulate the lower solvent replacements to be as effective from the anti-corrosion angle as the coatings which they will replace.     

Study of the anticorrosive properties of polypyrrole/polyaniline bilayer via electrochemical techniques

In this work, using electrochemical techniques the authors investigated the protective properties of a polypyrrole/polyaniline bilayer as a conductive polymer. A polypyrrole/polyaniline bilayer was deposited on carbon steel substrate by potentiostatic method. The electric capacitance and resistance of the films were monitored with the immersion time in a corrosive solution to investigate the water permeability of the films. Polypyrrole/polyaniline bilayer has a relatively low permeability and good catalytic behavior in passivation of carbon steel in longer periods. The results show that the bilayer has a better anticorrosive behavior compared to homopolymers (polypyrrole and polyaniline).     

Defect dimension evaluation in organic coated galvanized steel by electrochemical impedance spectroscopy

The protective effectiveness of organic coatings, in controlling corrosion processes by the barrier effect, is dominated by the absence of defects passing through the coating and reaching the substrate. It is, however, difficult in general to identify and quantify the presence of defects. This work is an effort to reach a more precise quantification of the size of defect in organic coatings by means of electrochemical impedance spectroscopy (EIS) measurements. Artificial defects with controlled dimensions between 60 and 200 m were produced on organic coated galvanized steel (coil coating). After optimization of the experimental procedure for EIS data acquisition, the parameters obtained, according to a classical electrical model, were correlated with the defect dimensions. The results show that the double layer capacitance (C _dl) values depend in linearly on the defects area, while this is not true for the pore resistance (R _p) values, as the electrolyte resistivity inside the defects is a function of the defect size. Further work is necessary to extend the results to smaller defects and different systems, that is, different organic coatings and substrates.     

Ionic liquid enhanced electrochemical characterization of organic coatings

Our laboratory recently began work on the use of room temperature ionic liquids ((RTIL's) to enhance our capabilities for the electrochemical characterization of organic coatings [A.M. Simões, D. Tallman, G.P. Bierwagen, The use of ionic liquids for the electrochemical characterization of water transport in organic coatings, Electrochem. Solid-State Lett. 8 (2005) 60]. The RTIL's are electrically conductive liquids consisting of large molecules that can be used to investigate the electrochemical properties of coatings in a non-aqueous medium. The enhancement of coating characterization comes from the fact that RTIL's have sufficient conductivity to be an immersion medium for electrochemical measurements, but they do not directly penetrate and effect organic coatings as do aqueous electrolyte solutions. This allows the separate examination of the effects of water on coatings in immersion or cyclic exposure. Indeed, our initial studies showed that a hydrophilic RTIL could be used to electrochemically characterize the drying of a coating after immersion, a process which heretofore had not been followed electrochemically. Thus, electrochemical measurements of coatings based on aqueous electrolyte immersion can be enhanced by the use of RTIL's and the effects of water on the coatings under study isolated and analyzed separately, especially the diffusion of water out of coatings during drying processes. Recent papers from our group have introduced the methodology whereby RTIL's in conjunction with capacitance monitoring via electrochemical impedance spectroscopy (EIS) can be used to determine the diffusion coefficient of water out of a non-pigmented, additive free coating [A.M. Simões, D. Tallman, G.P. Bierwagen, The use of ionic liquids for the electrochemical characterization of water transport in organic coatings, Electrochem. Solid-State Lett. 8 (2005) 60; K. Allahar, B. Hinderliter, A. Simoes, D. Tallman, G. Bierwagen, S. Croll, Simulation of wet–dry cycling of organic coatings using ionic liquids, J. Electrochem. Soc. 154 (2007) 177–185; B. Hinderliter, K. Allahar, O. Stafford, S. Croll, Using Ionic Liquids to Measure Coating Properties via Electrochemical Impedance Spectroscopy, Presented the 2006 International Coatings Exposition, Federation of Societies for Coatings Technology, New Orleans, LA, 2006 Oct.; B.R. Hinderliter, K.N. Allahar, G.P. Bierwagen, D.E. Tallman, S.G. Croll, Thermal cycling of epoxy coatings using room temperature ionic liquids, J. Electrochem. Soc. 155 (3) (2008) 1]. The technique has been extended to several types of coatings as well as the study of the cyclic wetting and drying of coatings [K. Allahar, B. Hinderliter, A. Simoes, D. Tallman, G. Bierwagen, S. Croll, Simulation of wet–dry cycling of organic coatings using ionic liquids, J. Electrochem. Soc. 154 (2007) 177–185]. This latter set of processes is one of the key set of events in exterior exposure that causes the failure of exterior protective coatings. Recently, RTIL's have been used to simulate the alternate wetting and drying of a Zn-rich epoxy coating system. EIS experiments were conducted on the Zn-rich epoxy under constant immersion in 0.05 M NaCl and RTIL. The experimental results were analyzed to determine the dielectric response and changes due to Zn oxidation within the Zn-rich system.     

Developments of the electrochemical noise method (ENM) for more practical assessment of anti-corrosion coatings

The electrochemical noise method (ENM) has particular attractions because of its non-intrusive nature, quickness in gathering data and ease of interpretation. The electrode arrangement for the standard (“Bridge”) method of conducting ENM requires two separate working electrodes, e.g. two painted Q panels and a reference electrode. Although satisfactory for laboratory use, it is not so suitable for monitoring or quality control. An improved experimental configuration is the single substrate (SS) method but this still requires the metal to be connected to the measuring instrument. This is avoided in the most recent development which needs no connection to substrate (NOCS). Results will be given for immersed low VOC samples monitored using the ENM NOCS arrangement and compared with the standard (“Bridge”) method and DC resistance. Results will also be presented for work done using several different electrodes (platinum, calomel and silver/silver chloride). It is accepted that, because of the very small voltages and currents involved, ENM data can sometimes be affected by extraneous signals (although normally the results are changed by only a factor of two or less) and it may be that NOCS is more sensitive to interference of this type than the standard bridge arrangement. A simple data analysis package checking on the Gaussian nature of data enables the operator to have confidence in the R_n value. This has been applied to NOCS data. Further work is required to make ENM attractive enough to be employed as the electrochemical method of choice by users, specifiers and producers of organic anti-corrosive paints.     

Adhesion characterization of protective organic coatings by electrochemical impedance spectroscopy

Adhesion is considered in many situations to be a very important property of organic coatings for corrosion protection and much scientific work is devoted both to the study of the mechanism involved in polymer-metal adhesion and to the ways of measuring this property. The large number of experimental methods in existence to obtain information on coating adhesion is an indication of both the scientific and the technological interests in this material science and engineering area, but it is also a consequence of the difficulty in measuring adhesion in a general sense. As a partial alternative to the traditional adhesion measurement approaches for organic coatings, the evaluation of adhesion by electrochemical techniques such as electrochemical impedance spectroscopy (EIS) is discussed for different examples. The influences on adhesion of different pretreatments or organic coatings are discussed, considering aluminium, galvanized steel, and stainless steel substrates, and we have shown that the information obtained by using an electrochemical approach can be used for adhesion evaluation, with particular attention to the monitoring of adhesion in an aqueous environment, which is the most detrimental for protective organic coatings.     

Choice and measurement of crucial aircraft coatings system properties

All components of an aerospace coatings system must be functioning properly for it to provide all of the elements of physical protection and optical performance that it was designed to provide. To insure proper coatings system functions over its desired lifetime, one must have the capability to define, measure and evaluate those system properties that determine these functions in use. For military aircraft, the desired properties can be summarized as mechanical/chemical protection of the airframe with controlled electromagnetic emission and reflection properties. This is easily enough said, but difficult to achieve in practice. There are many possible properties one can measure on an aerospace coatings system and the cohort of designers and users of these systems are uncertain of which is the absolute correct set of measurements to choose. They seek the set that gives the maximum amount of pertinent information at the least amount of effort and expense. The sine qua non of such properties has yet to be determined, but the current “best practice” thinking among the coatings science community focuses on system flexibility, adhesion, solvent resistance, corrosion protection and camouflage optics as determined over the coatings lifetime while taking into account the effects of UV exposure, hot and cold organic fluids, sea water, humidity and temperature cycling, mechanical stressing and abrasion. All of these must be considered in choosing the tests/results protocol that the coatings/pretreatment must undergo to be “fit for use.” We will discuss the current thinking on the measurements/exposures for aerospace coatings as well as the accuracy/validity that they provide in practice. We will concentrate on spectral, optical and electrochemical testing of coatings system properties and their changes in simulated and field exposure. The differences between test performance and real use performance will be considered, especially with respect to film application uniformity and film damage.     

Embedded electrode electrochemical noise monitoring of the corrosion beneath organic coatings induced by ac-dc-ac conditions

The degradation of urethane topcoat/epoxy primer systems used for military aircraft and industrial applications were monitored using the electrochemical noise method and measured with embedded electrodes. The degradation was induced by the ac-dc-ac method. The trend in the noise resistance parameter was consistent with the trend in the low frequency impedance modulus obtained from electrochemical impedance spectroscopy experiments. The localization index was not informative about the corrosion mechanism. However, the two shot noise parameters, average charge of event and event frequency, were able to characterize the corrosion process beneath the coating. There was a change from uniform corrosion to localized corrosion on the metal substrate associated with the aircraft coating as indicated by the decrease in frequency event as this coating failed. There was an increase in the charge per corrosion event of the substrate of the industrial coating indicating that the uniform corrosion occurring became more severe as the coating failed.     

Continuing work to enable electrochemical methods to be used to monitor the performance of organic coatings in the field

Although the most commonly used electrochemical method to assess anti-corrosive coatings in the laboratory is Electrochemical Impedance Spectroscopy (EIS), the Electrochemical Noise Method (ENM) is finding increasing use. EIS has also been used occasionally to assess coatings on metals in the field. However despite ENM's advantages (non-intrusive, quickness in gathering data, etc., ease of interpretation) rarely has ENM been used for the latter application even though Resistance Noise (R_n) has been shown to quantifiably relate to DC resistance and hence to protection afforded. However to obtain R_n requires two contemporaneous measurements, one of the current noise and one of the voltage noise and hence two separate working electrodes are required. This is difficult to achieve in most practical situations. To overcome this in previous work there were validated two novel experimental arrangements of ENM, viz. single substrate (SS) and no connection to the substrate (NOCS). The current paper builds on this work. It describes methods of dealing with the practical considerations involved in making a measurement in the field, for example, dismountable non-marking cells acting as temporary “connectors” to the substrate, light battery operated equipment and experiments designed to minimize the length of time that the measurement takes to make. The “time to settle” experiments indicate that for most coatings a time of about 30–45 min is likely to be needed. The results obtained from changing the noise gathering parameter suggest that a frequency of 10 Hz may be suitable, enabling measurements to be made in one fifth of the time.     

Acceleration and quantitative evaluation of degradation for corrosion protective coatings on buried pipeline: Part I. Development of electrochemical test methods

The electrochemical degradation of polyethylene coated onto SS400 was examined in synthetic groundwater. Electrochemical techniques (electrochemical impedance spectroscopy, potentiodynamic and potentiostatic polarization tests) and surface analysis (scanning electron microscopy) were used to accelerate and evaluate the coating degradation. The pulsed potentiostatic polarization test accelerating both the cathodic reduction and anodic oxidation reactions was applied to reproduce the coating degradation mechanisms of cathodic disbondment and oxide lifting. The applied potentials were determined to be ±300 mV_SCE versus open-circuit potential from the analysis of the anodic and cathodic polarization data. Results from the EIS confirmed that coating degradation is accelerated effectively by the pulsed potentiostatic polarization testing.     

Corrosion resistance properties of electroless nickel composite coatings

Electroless nickel (EN) composite coatings incorporated with PTFE and/or SiC particles demonstrated significantly improved mechanical and tribological properties as well as low surface energy which are desired for anti-sticking and wear resistant applications. The corrosion resistance of these composite coatings, however, has not been systematically studied and compared. This work aimed to investigate the corrosion characteristics of EN composite coatings using electrochemical measurements which include open circuit potential (OCP), electrochemical impedance spectroscopy and potentiodynamic test. The effects of the co-deposited particles on corrosion behavior of the coatings in 1.0 N H₂SO₄ and 3% NaCl media were investigated. The surface autocatalytic properties and the post-heat-treatment on coating corrosion resistance were also discussed. The results showed that both EN and EN composite coatings demonstrated significant improvement of corrosion resistance in both acidic and salty atmosphere. Ni striking substantially enhanced the corrosion resistance due to the improvement of the surface autocatalytic properties and homogeneity. Proper post-heat-treatment significantly improves the coating density and structure, giving rise to enhanced corrosion resistance.     

Effect of electroless nickel interlayer on the electrochemical behavior of single layer CrN, TiN, TiAlN coatings and nanolayered TiAlN/CrN multilayer coatings prepared by reactive dc magnetron sputtering

The electrochemical behavior of single layer TiN, CrN, TiAlN and multilayer TiAlN/CrN coatings, deposited on steel substrates using a multi-target reactive direct current (dc) magnetron sputtering process, was studied in 3.5% NaCl solution. The total thickness of the coatings was about 1.5 μm. About 0.5 μm thick chromium interlayer was used to improve adhesion of the coatings. With an aim to improve the corrosion resistance, an additional interlayer of approximately 5 μm thick electroless nickel (EN) was deposited on the substrate. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to study the corrosion behavior of the coatings. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the corroded samples. The potentiodynamic polarization tests showed lower corrosion current density and higher polarization resistance (R_p) for the coatings with EN interlayer. For example, the corrosion current density of TiN coated steel was decreased by a factor of 10 by incorporating 5 μm thick EN interlayer. Similarly, multilayer coatings of TiAlN/CrN with EN interlayer showed about 30 times improved corrosion resistance as compared to the multilayers without EN interlayer. The porosity values were calculated from the potentiodynamic polarization data. The Nyquist and the Bode plots obtained from the EIS data were fitted by appropriate equivalent circuits. The pore resistance (R_pore), the charge transfer resistance (R_ct), the coating capacitance (Q_coat) and the double layer capacitance (Q_dl) of the coatings were obtained from the equivalent circuit. Multilayer coatings showed higher R_pore and R_ct values as compared to the single layer coatings. Similarly, the Q_coat and Q_dl values decreased from uncoated substrate to the multilayer coatings, indicating a decrease in the defect density by the addition of EN interlayer. These studies were confirmed by examining the corroded samples under scanning electron microscopy.     

Improvement of the electrochemical behavior of steel surfaces using a TiN[BCN/BN]n/c-BN multilayer system

The aim of this work is to improve the electrochemical behavior of AISI 4140 steel substrates by using a TiN[BCN/BN]_n/c-BN multilayer system as a protective coating. We grew TiN[BCN/BN]_n/c-BN multilayers via reactive r.f. magnetron sputtering technique, systematically varying the length period (Λ) and the bilayer number (n), maintaining constant the total thickness of the coating and all other growth parameters. The coatings were characterized by FTIR spectroscopy that showed bands associated to h-BN bonds, and c-BN stretching vibrations centered at 1385 cm^− 1 and 1005 cm^− 1, respectively. Film composition was studied via X-ray photoelectron spectroscopy where typical signals for C1s, N1s and B1s are shown. The electrochemical properties were studied by electrochemical impedance spectroscopy and Tafel curves. In this work, the maximum corrosion resistance for the coating with (Λ) equal to 80 nm was obtained, corresponding to n = 25 bilayers. The polarization resistance and corrosion rate were around 10.1 kOhm cm² and 0.22 mm/year; these values were 83 and 15 times higher, respectively, than uncoated AISI 4140 steel substrate (0.66 kOhm cm² and 18.51 mm/year). Optical microscopy was used for surface analysis after corrosive attack. The improvement of the electrochemical behavior of the AISI 4140 coated with this TiN[BCN/BN]_n/c-BN multilayer system can be attributed to the presence of several interfaces that offer resistance to diffusion of Cl⁻ of the electrolyte toward the steel surface.     

Tuning interfacial chemistry and electrochemical properties of solid oxide cells via cation interdiffusion

Electrochemical performance of solid oxide cells is sensitive to electrode/electrolyte interfacial chemistry. Herein, taking SmBaCoCuO_5+δ (SBCC) oxygen electrode and Gd_0.1Ce_0.9O_1.95 (GDC) electrolyte as an example, the role of cation interdiffusion in tuning interfacial chemistry and electrochemical properties of solid oxide cells was studied. An increase in cation diffusion between SBCC and GDC layers results in bigger and better-connected SBCC particles with slight shrinkage in unit cell and denser GDC layer with distorted crystal structure. Cation diffusion is also responsible for abnormal deviation in electrochemical impedance spectroscopy that can be used to characterize catalytic properties of the electrode for oxygen redox reactions in SBCC/GDC/SBCC symmetric cell. The internal mechanism that contributes to this phenomenon was revealed through a series of well-designed tests and theoretical modeling. Single cell, SBCC/GDC/GDC-NiO, has high cation interdiffusion, works well in both solid oxide fuel cell and solid oxide electrolysis cell model; it also exhibits decreased catalytic activity and open-circuit voltage. This work provides helpful guides for the design of advanced solid oxide cells and other related devices.     

Evaluation of thick organic coatings degradation in seawater using cathodic protection and thermally accelerated tests

Organic coatings are often associated with cathodic protection to fight against the corrosion of metallic structures when immersed in seawater. However, cathodic protection leads to the generation of a strong alkalinity at the metal/coating interface, which causes the degradation of the coating. It is then necessary to develop a reliable method to evaluate the compatibility between organic coatings and the application of cathodic protection.

On one hand, cathodic disbonding tests (ASTM G-8 and G-80) can be driven with an artificial defect but this defect is mainly responsible for the electrochemical response. In addition, calcareous deposit rapidly forms onto the defect zone when cathodic protection is applied which can make difficult the evaluation of coating delamination. On the other hand, immersion of defect-free specimens requires very long testing periods (several months or even years) in order to detect the coating degradation.

In this work, an attempt to accelerate the coatings degradation by imposing a high temperature and thermal cycles were made in order to decrease the test-time duration. The influence of the applied cathodic potential was also investigated. The coating degradation was evaluated by EIS, considering the defect-free zone of coatings. It was shown that the coating degradation is faster in the presence of a defect and for high temperature (45 °C). Moreover, thermal cycles allow to greatly accelerate the degradation of defect-free coatings and then to compare the compatibility of both coatings with cathodic protection.