Characterization of steel corrosion in mortar by various electrochemical and physical techniques

In the study characterization of steel corrosion in concrete at the macro- and micro-level was performed. Physical (electrical-resistance probes) and electrochemical techniques (coupled multi-electrode arrays) were implemented in order to upgrade the general information that conventional electrochemical techniques can provide. Measurements were performed in mortar exposed to periodic wetting and drying. Steel corrosion damage was assessed by micro X-ray computer tomography (CT) and SEM. The results were compared and interpreted. By combined use of micro-CT and electrochemical methods, new insights into the corrosion mechanisms of steel in concrete were obtained.     

Potentiodynamic polarization behaviour of AISI type 316 stainless steel in NaCl solution

Potentiodynamic polarization behaviour of AISI type 316 SS in NaCl solution was investigated in terms of the potential scan rate effect. The critical pitting potential, E_pit, of the stainless steel appeared to be strongly dependent on the potential scan rate. A cumulative anodic electric charge density of the steel was defined as the total charge density from the open circuit potential, E_ocp, and calculated using the potentiodynamic polarization curves. It was found that, plotted as a function of the polarization time, the values of the cumulative charge density consisted of two lines with different slopes. It was confirmed that the deflection of the cumulative charge density vs. time plots corresponded to E_pit and the values of the cumulative charge density at the deflection were little dependent on the applied scan rate. The cumulative charge density at the deflection was defined as a critical electric charge density for the stable pitting. Also, it was suggested that this electric charge density should be associated with the critical condition for the stable pitting and the critical electric charge for stable pitting should be a representative parameter for the pitting resistance of a material.     

Investigation of AISI 1040 steel corrosion in H2S solution containing chloride ions by digital image processing coupled with electrochemical techniques

This paper presents a study of AISI 1040 steel corrosion in aqueous electrolyte of acetic acid buffer containing 3.1 and 31 × 10⁻³ mol dm⁻³ of Na₂S in both the presence and absence of 3.5 wt.% NaCl. This investigation of steel corrosion was carried out using potential polarization, and open-circuit and in situ optical microscopy. The morphological analysis and classification of types of surface corrosion damage by digital image processing reveals grain boundary corrosion and shows a non-uniform sulfide film growth, which occurs preferentially over pearlitic grains through successive formation and dissolution of the film.     

Influence of ultrasound on pitting corrosion and crevice corrosion of SUS304 stainless steel in chloride sodium aqueous solution

The change of polarization curves and surface morphologies of SUS304 stainless steel was investigated in 3.5 mass% NaCl solution with or without the application of ultrasound (US). As the result, both the pitting corrosion and the crevice corrosion were largely suppressed by the application of US. The reason is attributed to the decrease in the concentration of hydrogen and chloride ions in pits or in the crevice by removing the corrosion product and stirring the liquid there.     

Novel images of the evolution of stress corrosion cracks from corrosion pits

SCC in turbine disc steels exposed to simulated steam-condensate tends to nucleate preferentially from corrosion pit precursors. The evolution of these cracks is not straightforward and not well understood. In this work, unique three-dimensional X-ray microtomographic images have confirmed that cracks develop predominantly at the shoulder of the pit, near the pit/surface interface, for specimens stressed to 50–90% σ_0.2. In support of this observation, FEA of model pits indicate that strain is a maximum on the pit wall just below the pit mouth. Implications of these observations for the pit-to-crack transition and predictive-modelling of crack nucleation and growth are discussed.     

Employing electrochemical frequency modulation for pitting corrosion

By using the electrochemical frequency modulation (EFM) technique, the non-linear behavior of a corroding system is measured. This non-linear behavior is likely to be different for a system undergoing uniform or pitting corrosion. The implementation of the EFM technique to detect pitting corrosion has been investigated by observing the fluctuations in the so-called causality factors. These causality factors, resulting from an EFM test and in the ideal case having values of 2 and 3, respectively, are normally used for quality and data validation purposes. While investigating pitting corrosion, they show different behavior leading to the CPT (critical pitting temperature) detection.     

Influence of input power to vibrator and vibrator-to-specimen distance of ultrasound on pitting corrosion of SUS304 stainless steel in 3.5% chloride sodium aqueous solution

The pitting corrosion of SUSU304 steel can be suppressed by the application of a 19.5 kHz ultrasound (US) in 3.5% NaCl aqueous solution. At a constant vibrator-to-specimen distance of d = 76 mm equal to the wavelength, the suppression effect increased with the input power to vibrator and the largest effect was obtained at the power of I = 8. At constant input powers of I = 2 or 8, the suppression effect decreased with the increase in the vibrator-to-specimen distance, but the largest effect was obtained at d = 68 mm in each case of I = 2 and 8.     

Effect of cobalt on the corrosion resistance of low alloy steel in sulfuric acid solution

This study examines the effect of cobalt addition on the corrosion resistance of low alloy steel in a 10 wt.% sulfuric acid solution at room temperature. All specimens show passive behaviors over the potential range, 0.5–1.55 V_SCE, and the passive current density decreased with increasing Co addition. The Co-containing steels also show higher polarization resistance and lower corrosion rate. The beneficial effect of Co is attributed to the formation of a uniform and continuous rust layer due to an interaction between Co and other elements such as Fe, O and S.     

Microstructure and corrosion behaviour of pulsed plasma-nitrided AISI H13 tool steel

The effect of pulsed plasma nitriding temperature and time on the pitting corrosion behaviour of AISI H13 tool steel in 0.9% NaCl solutions was investigated by cyclic polarization. The pitting potential (E_pit) was found to be dependent on the composition, microstructure and morphology of the surface layers, whose properties were determined by X-ray diffraction and scanning electron microscopy techniques. The best corrosion protection was observed for samples nitrided at 480 °C and 520 °C. Under such experimental conditions the E_pit-values shifted up to 1.25 V in the positive direction.     

Experimental and theoretical studies of thiazoles as corrosion inhibitors for mild steel in sulphuric acid solution

The inhibition effects of 2-amino-5-mercapto-1,3,4-thiadiazole (2A5MT) and 2-mercaptothiazoline (2MT) on mild steel corrosion in 1.0 M H₂SO₄ were studied with potentiodynamic polarization, linear polarization resistance and electrochemical impedance spectroscopy techniques. It was shown that both 2A5MT and 2MT act as good corrosion inhibitors for mild steel protection. The high inhibition efficiencies were attributed to the simple blocking effect by adsorption of inhibitor molecules on the steel surface. The effects of the presence of extra NH₂ group and N atom in 2A5MT on the ability to act as corrosion inhibitors were investigated by theoretical calculations.     

Effect of niobium on the corrosion behaviour of low alloy steel in sulfuric acid solution

This study examines the effect of niobium (Nb) addition on the electrochemical properties of low alloy steel using electrochemical techniques in a 10 wt.% sulfuric acid solution as well as surface analysis techniques. The potentiodynamic test reveals the passive behaviour of all specimens and a decrease in the passive currents with increasing Nb content. Electrochemical impedance spectroscopy (EIS) shows that the Nb-containing steels have higher passive and charge transfer resistance than the control samples. These results suggest that the interaction of elements improves the corrosion resistance of low alloy steel due to the formation of Nb, C, S, P, and Fe products on the surface.     

Pitting corrosion of cold rolled solution treated 17-4 PH stainless steel

In the present paper the effects the of cold rolling on pitting corrosion of 17-4 precipitation hardening stainless steel in 3.5 wt% NaCl solution was investigated. In order to clarify the effect of cold rolling the metastable pitting has been examined in more details. The results presented show that cold rolling increases the dissolution rate of metastable pitting in a manner which facilitates the transition from metastable to stable pitting. On the other hand, the frequency of occurrence of metastable pits decreases with cold working. In overall, cold rolling has no significant effect on pitting potential.     

Recent advances in local probe techniques in corrosion research - Analysis of the role of stress on pitting sensitivity

Local probe techniques have been largely applied to predict the spatial distribution of the electrochemical reactions and to quantify the corrosion rate. These local probe techniques are now being developed to capture the temporal information, in-situ and in real-time. Nevertheless only few works have been devoted to investigate with these local probes the role of stress on pitting sensitivity on passive materials. In this paper, a part of the most recent studies based on combination of local electrochemical measurements with finite element mapping of surface stress gradients are reported illustrating the role of inclusions under straining and the mechanical effect of surface preparation on pitting sensitivity of stainless steels.     

Repassivation of the damage generated by cavitation on UNS N08031 in a LiBr solution by means of electrochemical techniques and Confocal Laser Scanning Microscopy

The objective of this work is to study the influence of cavitation on the corrosion behaviour of Alloy 31, a highly-alloyed austenitic stainless steel (UNS N08031), in a LiBr heavy brine solution (992 g/L) at 25 °C. The presence of cavitation shifted the OCP value towards the active direction by 708 mV_Ag/AgCl, increased anodic current densities and passivation current density, i_p, and reduced the pitting potential, E_p.Repassivation behaviour of Alloy 31 has been investigated by using potentiostatic tests at different potentials. The current density transient obtained after interrupting cavitation was used to obtain the repassivation index, n, provided by the slope of the log i(t) vs. log t representation. The value of n decreased as the applied potential was increased, reaching values near zero for potentials close to the pitting potential. The damage generated during the potentiostatic tests has been quantified by means of Confocal Laser Scanning Microscopy.     

Microstructural evolution and localized corrosion resistance of an aged superduplex stainless steel

The present investigation was undertaken to analyze the effects of isothermal ageing treatments, carried out between 700 and 900 °C for a variety of times up to two weeks and followed by water quenching, on the microstructure and on the localized corrosion resistance of a superduplex stainless steel, SAF 2507.The quantitative metallography coupled with X-ray diffraction techniques was adopted to follow the microstructural evolution, together with SEM microscopy.Electrochemical potentiodynamic tests, as cyclic polarization curves recorded in sodium chloride solutions, together with weight loss measurements were employed to evaluate the susceptibility of the aged specimens to pitting corrosion.The influence of the transformation of ferrite into secondary austenite and sigma phase and of other microstructural variations, as chromium nitrides precipitation, on the stability of the passive film is shown. The susceptibility of the aged alloy to pitting corrosion phenomena, is related to sigma phase precipitation in association to the secondary austenite formation, which lead to a noticeable Cr depletion at grain boundaries.     

Effects of solution heat-treatment and nitrogen in shielding gas on the resistance to pitting corrosion of hyper duplex stainless steel welds

The effects of solution heat-treatment and shielding gas on the pitting corrosion of hyper duplex stainless steel (HDSS) welds were investigated in highly concentrated chloride environments. The pitting resistance of a solution heat-treated HDSS after welding with an Ar shielding gas supplemented with N₂ was greatly increased due to the dissolution of Cr₂N in α-phase, which followed the diffusion of N atoms from the α-phase to the γ-phase and an increase of the γ-phase in the weld metal and heat affected zone. It was also attributed to a decrease of the pitting resistance equivalent number difference between the two phases.     

Effect of the micro-plasma arc welding technique on the microstructure and pitting corrosion of AISI 316L stainless steels in heavy LiBr brines

The effects of the micro-plasma arc welding technique on the microstructure and pitting corrosion of different zones of an AISI 316L stainless steel were studied using different microscopy and electrochemical techniques. Galvanodynamic measurements and laser scanning confocal microscope were used to evaluate the corrosion evolution in situ. Results show, in general, the worst corrosion behaviour for the heat affected zone. Furthermore, there is a relation between the effects of the micro-plasma arc welding process on the materials microstructure and their pitting corrosion resistance. The weld zone was always in the cathodic position of the possible galvanic pairs.     

Validity of dynamic electrochemical impedance spectra of some amine corrosion inhibitors in petroleum/water corrosive mixtures by Kramers-Kronig transformation

In our previous works, the corrosion inhibition properties of propylamine (PA), iso-propylamine (i-PA), butylamine (BA), tert-butylamine (t-BA), hexylamine (HA), ethylenediamine (EDA), diethanolamine (DEA), 3-amino-1-propanol (3-AP), 2-dimethylethanolamine (2-DEA), cyclohexylamine (CHA), N-methylcyclohexylamine (N-MCA) and dicyclohexylamine (DCHA) have been investigated by means of electrochemical impedance spectroscopy (EIS) with a carbon steel rotating disc electrode in some petroleum/water corrosive mixtures containing acetic acid and NaCl at 25 °C. In this work, the Kramers-Kronig transformations (KKTs) were applied to evaluate the validity of the impedance data of these amine compounds in hydrodynamic condition. The results obtained showed that experimental impedance data did not satisfy in K-K relations completely viz. indicated some errors. These errors were related to stray capacitances, external transmission line effects, pits formation and change in interface during data acquisition at high and low frequencies regions. In addition, the selection of a suitable equivalent circuit strongly emphasized to better interpret of EIS data which in turn cause better resulted KKTs spectra. With considering obtained transformations, more effective inhibitor was selected with regard to its charge transfer resistance, the corrosion capacitance and well satisfying in K-K relations.     

Effect of biofilm on ennoblement and localized corrosion of stainless steel in fresh dam-water

AISI 304 stainless steel specimens were exposed in-situ to fresh dam-water for nearly 2 years. Open circuit potential (OCP) of the specimens becomes remarkably ennobled after exposure for about 40 days. The ennobled OCP of coupons is greatly affected by both immersion depth and sunlight. It is found that ammonium, nitrate, nitrite and chloride ions are enriched in the biofilm. It is suggested that the reduction of nitrate may be partly responsible for the OCP ennoblement. Localized corrosions at welds and crevices of specimens are attributed to both the OCP ennoblement of specimens and the chloride ion enrichment in the biofilm.