A newly synthesized glycine derivative to control uniform and pitting corrosion processes of Al induced by SCN anions - Chemical, electrochemical and morphological studies

A newly synthesized glycine derivative (termed GlyD), 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride, was used to inhibit uniform and pitting corrosion processes of Al in 0.50 M KSCN solutions (pH 6.8) at 25 °C. For uniform corrosion inhibition study, Tafel extrapolation, linear polarization resistance and impedance methods were used, complemented with SEM examinations. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectrometry) was also used to test validity of corrosion rate measured by Tafel extrapolation method. GlyD inhibited uniform corrosion, even at low concentrations, reaching a value of inhibition efficiency up to 97% at a concentration of 5 × 10⁻³ M. Results obtained from the different corrosion evaluation techniques were in good agreement. This new synthesized glycine derivative was also used to control pit nucleation and growth on the pitted Al surface based on cyclic polarization, potentiostatic and galvanostatic measurements. The pitting potential (E_pit) and the repassivation potential (E_rp) increased by the addition of GlyD. Thus GlyD suppressed pit nucleation and propagation. Nucleation of pit was found to take place after an incubation time (t_i). The rate of pit nucleation and growth decreased with increase in inhibitor concentration. Morphology of pitting was also studied as a function of the applied anodic potential and solution temperature. Cross-sectional view of pitted surface revealed the formation of large distorted hemispherical and narrow deep pits. GlyD was much better than Gly in controlling uniform and pitting corrosion processes of Al in these solutions.     

Pit growth behaviour of aluminium under galvanostatic control

The pit growth process on (1 0 0) aluminium under anodic pulse current in a mixed solution of 1 M HCl and 0.1 M H₂SO₄ at 30 °C has been evaluated using potential transient measurements and pit size distributions obtained by scanning electron microscopy. Sustained pit growth is observed for all pits during the initial anodic potential rise before reaching a steady-state etch potential, whereas a substantial fraction of the pits passivate at the steady-state etch potential. The pit growth rate during the initial potential rise is 3.4 μm s⁻¹, which is similar to that at the steady-state etch potential. The growth rates of active pits are potential-independent.     

Study of the electrochemical behaviour of nanocrystalline zinc by statistical methods

Pulse electrodeposition was used to synthesize nanocrystalline (NC) zinc coatings from citric acid bath. The electrochemical behaviour of the NC zinc coatings was investigated by using potentiostatic and potentiodynamic polarization methods in 0.5 mol/L NaCl (pH = 12) solution and compared with that of cast zinc. Pitting corrosion behaviour was characterized by pitting potential, induction time and stable pit growth rate which were analyzed according to statistical method. The results showed that nanocrystallization increased the sensitivity of E_pit refer to potential sweep velocity, changed the type of the pit generation from B1 (parallel) to B2 (series), accelerated the pitting initiation process and inhibited the stable pit growth process of NC zinc.     

Environmental factors affecting the corrosion behavior of reinforcing steel III. Measurement of pitting corrosion currents of steel in Ca(OH)2 solutions under natural corrosion conditions

Using a simple electrolytic cell, the pitting corrosion current of reinforcing steel is measured in Ca(OH)₂ solutions in presence of chloride and sulfate as aggressive ions. Pitting corrosion current starts to flow after an induction period which depends on the concentration of both the aggressive and the passivating anions. The pitting corrosion current densities reach steady-state values which depend also on the type and concentration of the corrosive and passivating anions. The corrosive action of the aggressive species decreased in the order: SO₄²⁻ > Cl⁻. Corrosion of the steel is found to be governed by a single electron transfer reaction. Raising the temperature decreases the induction period associated with pit initiation and increases the corrosion current associated with pit propagation. From Arrhenius plots, the activation energies for both pit initiation and pit propagation in presence of chloride and sulfate ions are calculated.     

Explanation of the effect of high chloride concentration on the critical pitting temperature of stainless steel

We correlate the effect of high chloride concentration on the critical pitting temperature (CPT) of type 316L stainless steel with its effect on the critical pit solution chemistry as determined by the artificial pit technique. It is shown that the change in CPT with bulk chloride concentration (0.5-9 mol kg⁻¹) can be correlated with a change in the ratio of C^∗/C_s, where C^∗ is the critical dissolved alloy cation concentration to sustain pitting, and C_s is the solubility of FeCl₂ at the pit surface. A complicating factor is that natural pits can only grow with C^∗ = C_s at the lower chloride concentrations, but can grow without the salt film at very high chloride concentrations; this transition is believed to occur close to 5 or 6 m bulk chloride concentration. The dependence of C_s on bulk chloride concentration is given a new interpretation based on a common-ion effect operating within an altered local chemistry with complexation.     

The effect of different exposure conditions on the biofilm/copper interface

The effects of the different exposure conditions on the electrochemical behavior of copper were evaluated in a growth medium containing Shewanella oneidensis MR-1. Impedance spectra were recorded at the corrosion potential (E_corr) in three different cells for one week of exposure followed by cyclic voltammetry. A second time constant was observed in the impedance spectra of copper that was partially immersed in the test cell, where the electrode was in contact with an air/liquid interface (cell B). These spectra resembled those usually observed for metals covered with a polymer coating. Complete immersion of copper in the electrolyte (no air/liquid interface) or deaeration of cell B resulted in one-time-constant spectra that are typical of those found for passive metals. Excellent corrosion protection was provided by MR-1 regardless of exposure condition. E_corr increased with time for the partially immersed Cu electrode exposed to the aerated solution in cell B, while it decreased for the other two exposure conditions. Cathodic polarization curves recorded after exposure for 7 days showed two reduction peaks for copper tested in cell B, while no reduction peaks were observed for the other cases. Similar results were obtained using cyclic voltammetry.     

Evaluation of metastable pitting on titanium by charge integration of current transients

The metastable pitting of titanium has been studied under potentiostatic control in solutions containing chloride ions. An approach based on the charge integration of current transients was proposed for a quantitative determination of metastable pitting. A pit density (d_mpit) was defined as the number of metastable pits per unit area per unit time (cm⁻² h⁻¹) with a typical size, instead of a size distribution. The calculated d_mpit of titanium at 0.5 V_SCE in 0.6 M NaCl was about 1.0 × 10³ cm⁻² h⁻¹ with a typical radius of 0.12 μm. An exponential potential dependence of d_mpit was obtained through the integration approach.     

Initial stages of corrosion pits on AISI 1040 steel in sulfide solution analyzed by temporal series micrographs coupled with electrochemical techniques

This work presents a study of the initial instants in the pitting corrosion of AISI 1040 steel, analyzed by temporal series micrographs coupled to an open circuit potential (E_oc) and polarization curves. During the E_oc measurement, the pit induction time and the initial pit growth in MnS inclusions was detected in alkaline sulfide solution. The pit area behavior has two distinct rate of area changes in specific regions directly associated to current slope changes. Finally, it was possible to create a three-dimensional model of the pit depth evolution on the metal, using Faraday’s law and the bullet-shaped geometry.     

Effect of copper on metal dusting of austenitic stainless steels

Three steels, 304SS, 310SS and 800H, were alloyed with 5%, 10%, and 20% (by weight) copper, and then exposed to 68%CO-31%H₂-1%H₂O gas at 680 °C (a_C = 19 and p_O2=5.4×10^-25 atm) under thermal cycling conditions. Kinetic measurements showed that copper-free alloys all dusted, with 304SS experiencing the greatest metal wastage. Copper additions did not have any effect on metal wastage of 304SS, but reduced the attack on 310SS and 800H markedly at levels of 5% and 10%. However, increasing the copper content to 20% produced large copper-rich precipitates which accelerated dusting by promoting internal graphitisation.Dusting was associated with surface coking. When pitting occurred, on copper-free alloys and on copper containing 304SS, large coke structures grew above the pits. Internal grain boundary carburisation always took place, and intragranular carbides also precipitated when dusting occurred. A lamellar surface layer of internally precipitated spinel and austenite also developed in association with dusting. The copper effect is discussed in terms of its alloy solubility and its known beneficial effect in Ni-Cu binaries.     

Cathodic reduction of the duplex oxide films formed on copper in air with high relative humidity at 60 °C

The cathodic reduction of duplex air-formed oxide film on copper was performed at a constant current density of i_c = −50 μA cm⁻² in deaerated 0.1 M KCl solution to investigate the sequence of cathodic reduction of each oxide layer and its mechanism. The single-phase thick CuO film on copper was also cathodically reduced at i_c = −50 μA cm⁻² or −2.5 mA cm⁻². The surface characterizations of the air-formed oxide film and single-phase CuO film before cathodic reduction and after partial or complete cathodic reduction were performed by XPS and X-ray diffraction, respectively.The two plateau regions appeared in the potential vs. time curve during cathodic reduction of the duplex air-formed oxide film on copper, while one plateau region was observed in the potential-time curve during cathodic reduction of the single-phase CuO film on copper. The potential in the first plateau region for the air-formed film coincided with that in the plateau region for the CuO film. The results of XPS and X-ray diffraction suggested that in the first plateau region, the outer CuO layer is directly reduced to metallic Cu, while in the second plateau region, the inner Cu₂O layer is reduced to metallic Cu.     

The effects of molybdate and dichromate anions on pit propagation of mild steel in bicarbonate solution containing Cl

The effects of molybdate and dichromate anions on pit propagation of mild steel in bicarbonate solution containing Cl⁻ were investigated by electrochemical measurements. MoO₄²⁻ ion suppressed both pit nucleation and propagation. Cr₂O₇²⁻ ion suppressed pit nucleation, but stimulated pit growth. The different effects of the two anions on pit propagation were explained by the opposite effects on pH value within pits. The pH value in molybdate-containing solution increased as a result of polymerization of MoO₄²⁻, while in dichromate-containing solution, pH value decreased due to hydrolysis of Fe³⁺ and Cr³⁺ which are the products of oxido-reducing reactions between Cr₂O₇²⁻ and Fe²⁺ ions.     

Changes in mass and stress during anodic oxidation and cathodic reduction of the Cu/Cu2O multilayer film

The anodic oxidation and cathodic reduction processes of the Cu/Cu₂O multilayer film and pure Cu film in pH 8.4 borate buffer solution were analyzed by electrochemical quartz crystal microbalance (EQCM) for gravimetry and bending beam method (BBM) for stress measurement. The mass loss of the multilayer film during anodic oxidation at 0.8 V (SHE) in the passive region was less than that of the pure Cu film. The comparison between current transients and mass changes during anodic oxidation has succeeded in separating the anodic current density into two partial current densities of oxide film growth, i_O^2-, and of Cu²⁺ dissolution through the passive film, i_Cu²⁺. As a result, in the case of the pure Cu film, the anodic current density was mainly due to i_Cu²⁺, while in the case of the multilayer film, i_Cu²⁺ was almost equal to i_O^2-. The compressive stress for the multilayer film was generated during anodic oxidation, while the tensile stress for the pure Cu film was generated.The mass loss of the multilayer film during cathodic reduction at a constant current density (i_c = −20 μA cm⁻²) was significantly less than that estimated from coulometry, suggesting that H₂O produced by cathodic reduction remained in the multilayer film. The compressive stress was generated during cathodic reduction of the multilayer film, which was ascribed to H₂O remained in the multilayer film.     

Effect of metallurgical variables on the stress corrosion crack growth behaviour of AISI type 316LN stainless steel

Mill-annealed AISI type 316LN stainless steels, received from two different sources (one indigenous (SS-2) and the other foreign (SS-1)), were tested for stress corrosion cracking (SCC) resistance in a boiling acidified environment of NaCl. SCC results indicated a remarkably lower value of plateau crack growth rate (PCGR) and higher values of K_ISCC and J_ISCC for SS-2, which was attributed to the lower effective grain boundary energy resulting from a higher amount of copper in it. Cold working reduced K_ISCC and PCGR; while thermal aging and welding decreased K_ISCC and increased PCGR vis-à-vis the annealed material.     

The valence state of copper in anodic films formed on Al-1at.% Cu alloy

During anodising of Al-Cu alloys, copper species are incorporated into the anodic alumina film, where they migrate outward faster than Al³⁺ ions. In the present study of an Al-1at.% Cu alloy, the valence state of the incorporated copper species was investigated by X-ray photoelectron spectroscopy, revealing the presence of Cu²⁺ ions within the amorphous alumina film. However, extended X-ray irradiation led to reduction of units of CuO to Cu₂O, probably due mainly to interactions with electrons from the X-ray window of the instrument and photoelectrons from the specimen. The XPS analysis employed films formed on thin sputtering-deposited alloy/electropolished aluminium specimens. Such an approach enables sufficient concentrations of copper species to be developed in the anodic film for their ready detection.     

Pit to crack transition in stress corrosion cracking of a steam turbine disc steel

Long term exposure tests have been carried out on a 3 NiCrMoV steam turbine disc steel in the form of cylindrical tensile test specimens self-loaded to 90% of σ_0.2 and exposed to three environmental conditions, viz. deaerated pure water, aerated pure water, and aerated water containing 1.5 ppm of chloride ion. Pitting occurred in all environments but the density and depth of pits in the chloride-containing medium was markedly greater. No cracking was observed in deaerated pure water but cracks initiated in aerated water between 13 and 19 months and in less than 7 months in aerated 1.5 ppm Cl⁻ solution. The probability of a crack initiating from a pit of specific depth in aerated solution could be described well by a Weibull function. Profiling of pits and cracks in the disc steel tested in aerated 1.5 ppm Cl⁻ solution showed that there while there were many cracks with a depth greater than that of the corresponding pit the depth of some cracks was smaller than that of the corresponding pit, suggesting that cracks do not necessarily initiate from the bottom of the pits. The growth rate of short cracks emerging from pits appeared greater than that of long cracks in fracture mechanics specimens.     

The role and effect of residual stress on pore generation during anodization of aluminium thin films

The role and effect of residual stress on pore generation of anodized aluminium oxide (AAO) have been investigated into anodizing the various-residual-stresses aluminium films. The plane stresses were characterised by X-ray diffraction with sin²ψ method. The pore density roughly linearly increased with residual stress from 64.6 (−132.5 MPa) to 90.5 pores/μm² (135.9 MPa). However, the average pore size around 40 nm was not changed significantly except for the rougher film. The tensile residual stress lessened the compressive oxide growth stress to reduce AAO plastic deformation for higher pore density. The findings provide new foundations for realizing AAO films on silicon.     

Evaluation of the effect of dosage, pH and contact time on high-dose phosphate inhibition for copper corrosion control using response surface methodology (RSM)

The response surface methodology (RSM), particularly Box-Behnken design model, was used in this study to obtain the optimum operating conditions for reduction of copper corrosion by-product release using high-dose polyphosphate inhibition. Furthermore, the RSM was also applied to study the main and interactive effects of the parameters investigated. Both analysis of variance (ANOVA) and coefficient of determination (R²) showed that the RSM approach was appropriate for the optimization of high-dose polyphosphate inhibition. The main effects of polyphosphate dosage and pH were found significant in reducing copper release using high-dose polyphosphate whereas the effect of contact time was less significant. In terms of interactions between the effects, the relation between polyphosphate dosage and the pH was the most significant. The optimal polyphosphate dosage, pH and contact time were found to be 22-28 mg/L of polyphosphate as P, 9-10, and 36 h, respectively. The highest total copper release reduction was estimated as 95.86% under the optimum condition. Surface analysis using both SEM-EDX and XRD discovered that cupric phosphate was present on the copper surface treated with high-dose polyphosphate inhibition. This implied that cupric phosphate could be responsible for the reduction of copper release by forming a protective layer.     

Effects of blending on surface characteristics of copper corrosion products in drinking water distribution systems

Copper scales formed over 6-months during exposure to ground, surface and saline waters were characterized by EDS, XRD and XPS. Scale color and hardness were light red-brown-black/hard for high alkalinity and blue-green/soft for high SO₄ or Cl waters. Cl was present in surface or saline copper scales. The Cu/Cu₂O ratio decreased with time indicating an e transfer copper corrosion mechanism. Cu₂O, CuO, and Cu(OH)₂ dominated the top 0.5-1 A° scale indicating continuous corrosion. Cu₂O oxidation to CuO increased with alkalinity, and depended on time and pH. Total copper release was predicted using a Cu(OH)₂ model.     

Surface characterization and corrosion behavior of a novel gold-imitation copper alloy with high tarnish resistance in salt spray environment

A novel gold-imitation copper alloy (CuZnAlNiSnBRe) was designed and its corrosion behavior in salt spray environment was investigated. The new alloy has better tarnish resistance and corrosion resistance than the current coinage alloy used in China (H7211). A multi-layer film formed on the surface of the new alloy after a period of exposure to salt spray was responsible for the good resistance of the alloy. The corrosion products were a mixture of CuO, Cu₂O, ZnO, Al₂O₃ and Al(OH)₃, with the transition from Cu₂O to CuO occurring during the corrosion process.     

Phosphate ions used as green inhibitor against copper corrosion in tap water

Three dosages of ions are evaluated as corrosion inhibitors of copper in artificial tap water by measuring the corrosion potential, polarization resistance, electrochemical impedance and reflectance spectra. The water is moderately hard, highly carbonated and chloride-rich. The results show that the surface film is composed by Cu₂O. When the inhibiting agent is added, the film becomes thicker, denser and more compact. This behaviour is attributed to CuO incorporating into the passive layer. The optimal dosage of inhibitor is 10 mg l⁻¹ P when the polarization resistance increases three times. The inhibitor retards the pit initiation, without hindering pit growth.