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.     

Environmental factors affecting the corrosion behaviour of reinforcing steel. V. Role of chloride and sulphate ions in the corrosion of reinforcing steel in saturated Ca(OH)2 solutions

The corrosion behaviour of reinforcing steel in saturated naturally aerated Ca(OH)₂ solutions in absence and presence of different concentrations of NaCl, NH₄Cl, Na₂SO₄ and (NH₄)₂SO₄ is followed by measuring of the open circuit potential complemented with SEM and EDS investigation. These salts cause breakdown of passivity and initiation of pitting corrosion. The rates of oxide film thickening by OH⁻ ions and oxide film destruction by the aggressive ions follow a direct logarithm law and depend on the concentration and type of aggressive salts anions and cations. The values of the activation energies for oxide film thickening are calculated and discussed.     

Environmental factors affecting the corrosion behavior of reinforcing steel. IV. Variation in the pitting corrosion current in relation to the concentration of the aggressive and the inhibitive anions

The pitting corrosion current of reinforcing steel is measured under natural corrosion conditions in Ca(OH)₂ solutions in presence of Cl⁻ as aggressive ions and as inhibiting anions. The corrosion current starts to flow after an induction period which depends on solution composition (concentration, pH and presence or absence of the aggressive and the inhibiting anions). The limiting corrosion currents increase with increasing the Cl⁻ ion concentration and decrease with increasing the pH and inhibiting ions concentration. The inhibition efficiency of the studied inhibiting ions increases in the following order: , and depends on the way by which the inhibitor is added to the solution. Injection of the inhibiting anions in solution causes repassivation of the pre-formed pits through competition with Cl⁻ ions for adsorption sites on metal oxide surface. The adsorbability constant and the free energy of repassivation of the inhibiting anions are calculated.     

EQCN study of anodic dissolution and surface oxide film formation at Au in the presence of Cl or Br ions: A model process for corrosion studies

The anodic dissolution of an Au electrode and associated thin-layer oxide film formation in aq. H₂SO₄ in the presence of Cl⁻ or Br⁻ ions at various concentrations provides a model process for metal corrosion. In the present work such processes were investigated using cyclic voltammetry and chronoamperometry, with complementary nanogravimetry measurements using the EQCN. The results clearly indicate that in 0.5 M H₂SO₄ electrolyte, containing 1 mM Br⁻ or Cl⁻, Au dissolves over the potential range 1.0 - 1.45 V(RHE) through a 3e oxidation process involving Au complex-ion formation that can be followed in situ by means of UV spectroscopy. The linear relationship between mass changes and reciprocal square-root of sweep-rate and between anodic currents in cyclic voltammetry at ca. 1.20 V for Br⁻ (1.39 V for Cl⁻) and square-root of sweep-rate/or electrode rotation rate indicated quantitatively that the dissolution process is diffusion-controlled. It was interesting to find that electrode rotation in the presence of Cl⁻ ions has little effect on the anodic formation of surface oxide, while, on the contrary, with Br⁻ ions present, currents for oxide film reduction are not observed at rotation rates > ca. 400 rpm.     

On the corrosion behaviour of phosphoric irons in simulated concrete pore solution

The corrosion behaviour of three phosphoric irons P₁ (Fe-0.11P-0.028C), P₂ (Fe-0.32P-0.026C) and P₃ (Fe-0.49P-0.022C) has been studied in simulated concrete pore solution (saturated Ca(OH)₂ solution) containing different chloride concentration. This has been compared with that of two commercial concrete reinforcement steels, a low carbon steel TN (Fe-0.148C-0.542Mn-0.128Si) and a microalloyed corrosion resistant steel CS (Fe-0.151C-0.088P-0.197Si-0.149Cr-0.417Cu). The beneficial aspect of phosphoric irons was revealed from potentiodynamic polarization experiments. The pitting potentials and pitting nucleation resistances for phosphoric irons and CS were higher than that for TN. Electrochemical impedance spectroscopy (EIS) studies revealed thickening and growth of passive film as a function of time in case of phosphoric irons and CS in saturated Ca(OH)₂ pore solutions without chloride and in the same solution with 0.05% Cl⁻ and 0.1% Cl⁻. In case of TN, breakdown of passive film resulted in active corrosion in simulated pore solution containing 0.1% Cl⁻. Linear polarization resistance measurements complemented EIS results. Visual observations indicated that phosphoric iron P₃ was immune to corrosion even after 125 days of immersion in saturated Ca(OH)₂ solution containing 5% NaCl. The good corrosion resistance of phosphoric irons in simulated concrete pore solution containing chloride ions has been related to the formation of phosphate, based on ultraviolet spectrophotometric analysis and Pourbaix diagram of phosphorus-water system.     

Radiotracer study of embedding of SO4 and Cl ions into the surface film formed on Mn corroding in perchlorate solution

The adsorption of radiolabelled Cl⁻ and SO₄⁻ ions from aqueous perchlorate solutions onto powdered Mn metal was studied. It was found that the extent of the adsorption is determined by the continuous increase of the thickness of the surface oxide/hydroxide layer formed as a consequence of the slow corrosion of the metal in neutral or quasi-neutral media. At low pH values (pH < 6) the extent of the adsorption decrease significantly owing to the dissolution of the surface layer. At high pH values (pH > 10) the adsorption decreases presumably owing to the competitive adsorption of OH⁻ ions or the modification of the adsorption centres by some kind of deprotonation.     

Electrochemical corrosion behaviour of microcrystalline aluminium in acidic solutions

The electrochemical corrosion behaviour of microcrystalline pure aluminium coating, fabricated by a magnetron sputtering technique, has been investigated in both 0.5 mol/l NaCl and 0.5 mol/l Na₂SO₄ acidic (pH = 2) aqueous solutions. The corrosion resistance of the microcrystalline Al coating has deteriorated more compared with that of the cast pure Al in Na₂SO₄ acidic solution. However, its oxide film has a higher pitting resistance in the NaCl acidic solution. Chloride ions play a big role in the formation of the oxide film on the microcrystalline Al coating. The higher pitting resistance was attributed to the more acidic isoelectric point which the oxide film achieved.     

Effect of calcium ions on pitting corrosion and inhibition performance in CO2 corrosion of N80 steel

The electrochemical impedance spectroscopy (EIS) and polarization resistance technique were used to investigate the susceptibility of N80 steel to pitting corrosion under stagnant condition and the corrosion behavior under flowing condition with/without precorrosion in 3% NaCl solution, in 3% NaCl + 1.5% CaCl₂ solution and in 4.6% NaCl solution (equivalent Cl⁻ concentration to 3% NaCl + 1.5% CaCl₂ solution) saturated by CO₂ at 57 °C. The results showed that under stagnant condition the initiation period of pitting corrosion decreased with increasing Cl⁻ concentration, but at the same Cl⁻ concentration, Ca²⁺ could prolong the initiation period. Under flowing condition the corrosion rate without stagnant precorrosion was bigger than that with precorrosion, and the corrosion rate with precorrosion in 3% NaCl solution was bigger than that in 3% NaCl + 1.5% CaCl₂ solution. Inhibition of quaternary alkynoxymethyl amine (IMC-80-Q) under stagnant condition showed that the optimum inhibitor concentration significantly increased with the increase of Cl⁻ concentration from 3% NaCl solution to 4.6% NaCl solution, and for the solutions with the same Cl⁻ concentration, adding Ca²⁺ did not change the optimum inhibitor concentration.     

Pitting corrosion mechanism of Type 304 stainless steel under a droplet of chloride solutions

Pitting corrosion of Type 304 stainless steel under drops of MgCl₂ solution has been investigated to clarify the rusting mechanism in marine atmospheres. A pitting corrosion test was performed under the droplets with various combinations of the diameter and thickness (height) by exposure to a constant relative humidity. Probability of occurrence of pitting corrosion decreased with decreasing the diameter and thickness. Pitting corrosion progressed only when the [Cl⁻] exceeded 6 M (RH < 65%). In almost cases, there was a small hole (∼10 μm diameter) in the center of a single pit, which may be the trace of an inclusion particle like MnS dissolved out. The pitting corrosion mechanism of Type 304 under droplets containing chloride ions has been proposed.     

The inhibition activity of ascorbic acid towards corrosion of steel in alkaline media containing chloride ions

The activity of ascorbic acid towards steel corrosion in saturated Ca(OH)₂ solution containing chloride ions was investigated in this study. Concentration and time dependence of the protective properties of the passive film were acquired by electrochemical impedance spectroscopy. The best inhibitive performance, i.e. the longest pitting initiation time was obtained in the presence of 10⁻³ M ascorbic acid, while both lower and higher concentrations showed shortening of the pitting-free period. The overall behaviour of ascorbic acid was attributed to its ability to form chelates of various solubility having various metal/ligand ratios and oxidation states of the chelated iron. The assumption of ascorbic acid assisted reductive dissolution of the passive layer at higher inhibitor concentrations was confirmed by cyclic voltammetry and ATR FTIR spectroscopy. It is proposed that the overall inhibitive effect at lower concentrations is due to the formation of insoluble surface chelates and the effective blocking of the Cl⁻ adsorption at the surface of passive film. A pronounced inhibitive effect observed after the pitting had initiated was ascribed to the formation of a resistive film at the pitted area.     

Corrosion behaviour of aluminium in copper containing environment

Corrosion behaviour of pure aluminium galvanically connected to metallic copper or in the presence of Cu²⁺ ions was investigated by electrochemical measurements in Na₂SO₄ and Na₂SO₄ + NaCl test solutions. It has been found that in aerated Cl⁻ ion containing solutions pitting corrosion of aluminium emerged immediately, while in the absence of oxygen this process was less violent. Effect of passivating pre-treatment of aluminium surface on corrosion behaviour Cu-Al bimetallic system is also demonstrated.     

On the pitting corrosion currents of zinc by chloride anions

The changes in the pitting corrosion current density with time on zinc electrode concerning the concentration of both the passivating borate and the aggressive chloride anions were followed using a simple electrolytic cell. The pitting corrosion currents started to flow after an induction period, τ. This period is found to be a function of the concentration of Cl⁻ anion, according to the relation logτ=β−γlogC_Cl⁻. The pitting corrosion currents finally reached a steady-state value, which depended on the concentration of both B₄O₇²⁻ and Cl⁻ anions. At a constant B₄O₇²⁻ anion concentration, the pitting corrosion current varied with the concentration of Cl⁻ anion according to the relation logi_pit=a₁+b₁logC_Cl⁻. It also varies at constant Cl⁻ anion concentration and various B₄O₇²⁻ anion concentration according to the relation logi_pit=a₂−b₂logC_B4O7²⁻. The susceptibility of the passivating zinc to pitting corrosion was found to be increasing as the temperature and pH of the solution increases. Results are discussed on the basis of adsorption of the aggressive anion on the passivating film, followed by penetration through the film and incorporation in it. This undermines the oxide film and causes pitting corrosion.     

The effect of fluoride on corrosion of reinforcing steel in alkaline solutions

The influence of fluoride content of alkaline solutions on the corrosion of reinforcements has been studied, by means of electrochemical techniques, in saturated solutions of Ca(OH)₂ and same pH NaOH solutions with additions of NaF, thus simulating the liquid phase of concrete. Fluoride can be present as a minor component in low-energy cements. The joint presence of chlorides and fluorides in the corrosion process of steel has also been analysed. It has been established that fluoride anions are able to produce pitting of reinforced steel in alkaline media of high pH. In Ca(OH)₂ saturated solution, the precipitation of CaF₂ lowers F⁻ concentration below the minimum that promotes a pitting corrosion process; if there is not enough Ca(OH)₂ to precipitate all F⁻ ions, the presence of fluoride in these media causes an increase in passive film dissolution and corrosion rates.     

Molybdate/Al(III) composite films on steel and zinc-plated steel by chemical conversion

A molybdate(VI)-Al(III) chemical conversion process was developed as an alternative to the chromate processes. Steel and zinc-plated steel specimens were treated in the solutions of 0.16 mol l⁻¹ ammonium alum (AlNH₄(SO₄)₂ · 12H₂O) with small amounts of ammonium molybdate(VI) (0.002-0.016 mol l⁻¹ (NH₄)₆Mo₇O₂₄ · 4H₂O) at 60 °C for 10-30 min in an ultrasonic rinsing apparatus. The formed films were composed of oxyhydroxides containing Mo(V,VI), Al(III), Fe(II,III), and sulfate ions (and Zn(II) ions in the case of zinc-plated steel), and showed good corrosion resistance in an aerated 0.5 mol l⁻¹ NaCl-0.15 mol l⁻¹ H₃BO₃ solution (pH=7). The films macerated during the corrosion test, but they did not detach and functioned as a protective layer. This process may be useful in forming undercoats for paints and polymer coatings on steel and zinc-plated steel.     

The initial stage of pitting corrosion on coated steels investigated by photon rupture in chloride containing solutions

A photon rupture method, film removal by a focused pulse of pulsed Nd-YAG laser beam irradiation, has been developed to enable oxide film stripping at extremely high rates without contamination from the film removal tools. In the present study, Zn-55mass%Al alloy and Al-9mass%Si alloy-coated steel specimens covered with protective nitrocellulose film were irradiated with a focused pulse of a pulsed Nd-YAG laser beam at a constant potential in 0.5 kmol m⁻³ H₃BO₃-0.05 kmol m⁻³ Na₂B₄O₇ (pH = 7.4) with 0.01 kmol m⁻³ of chloride ions to investigate the initial stage of localized corrosion. At low potentials, oxide films on both coated alloys were reformed after the nitrocellulose films were removed by this method. The oxide film formation kinetics follows an inverse logarithmic law, in agreement with Cabrera-Mott theory. However, at high potentials, localized corrosion producing corrosion products occurs at the area where nitrocellulose film was removed. Nevertheless, when the applied potential is less noble, the dissolution current of the Zn-55mass%Al-coated steel samples is higher than that of Al-9mass%Si-coated samples.     

H2S and O2 influence on the corrosion of carbon steel immersed in a solution containing 3 M diethanolamine

Aqueous solutions with 3 mol L⁻¹ (M) diethanolamine (DEA) concentration are extensively used in the gas processing industry to remove acid gases. However, the degradation of the DEA and the formation of heat-stable salts (HSS) lead to severe corrosion problems. Even worse, equipment corrosion can be magnified by the unavoidable presence of sulphide acid and dissolved oxygen as a result of hydrocarbon (natural gases and crude oil) processing. The aim of this work is to study the combined corrosion effects of DEA, sulphide acid and oxygen on carbon steel. Electrochemical methods revealed that in the 3 M DEA medium without oxygen, corrosion processes are modulated by adsorbed DEA film formation. Furthermore, it was shown that the addition of oxygen and 15 × 10⁻³ mol L⁻¹ (15 mM) H₂S produced the formation of an adherent film on the carbon steel surface. Chemical analyses by EDAX revealed a homogeneous film of corrosion products composed of iron oxide and sulphide formed in DEA solution containing O₂ and H₂S, respectively. Equivalent circuits were used to estimate the parameters associated with ion diffusion through the formed corrosion films. The results showed that the presence of H₂S induced the formation of thin iron sulphide films that provide protective properties to the metal. It is concluded that the presence of oxygen in a sweetening plant should be avoided as DEA degradation can be produced with the subsequent decrease in chelating process efficiency and the increase in corrosion problems.     

Chronoamperometric studies of pitting corrosion of Al and (Al-Si) alloys by halide ions in neutral sulphate solutions

The pitting corrosion of Al, (Al + 6%Si), and (Al + 18%Si) alloys in neutral 0.50 M Na₂SO₄ solution in the absence and presence of NaCl, NaBr and NaI under the influence of various experimental variables has been studied by using potentiodynamic and chronoamperometric techniques. The results showed that the pitting corrosion resistance of the three Al samples decreases in the order: (Al + 18%Si) > (Al + 6%Si) > Al. The current/time measurements showed that the overall process can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involving pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increasing halide concentration, temperature, and applied potential. The pit growth current density (j_pit) increases linearly with t^1/2, indicating that the pit growth can be described in terms of an instantaneous three dimensional growth under diffusion control. The effect of Cr₂O₇²⁻, CrO₄²⁻, WO₄²⁻, MoO₄²⁻, NO₂⁻ and NO₃⁻ as inorganic inhibitors on the pitting corrosion inhibition of pure Al and its alloys in (0.5 M Na₂SO₄ + 0.2 M NaCl) solution has also been studied. The presence of these anions (except NO₃⁻) results in an increase in the incubation time and a decrease in the pit growth current density of the three samples to an extent depending on the nature and the concentration of the inorganic inhibitors and the composition of the Al samples. The inhibition efficiency of these inhibitors decreases in the order: Cr₂O₇²⁻ > CrO₄²⁻ > WO₄²⁻ > MoO₄²⁻ > NO₂⁻.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

Galvanostatic study of the breakdown of Zn passivity by sulphate anions

The anodic behavior of Zn electrode in Na₂SO₄ solutions was studied by galvanostatic polarization technique. The polarization curves are characterized by a distinct arrest corresponding to ZnO formation, after which the potential rises linearly with time up to a well-defined value, the breakdown potential, at which the potential drops with time, down to more negative direction. This denotes the destruction of the passive film and initiation of pitting corrosion. It was found that, the breakdown potential, the time consumed till the breakdown potential and the rate of potential rise with time depend on the sulphate anions concentration, solution temperature and magnitude of the imposed current density. Addition of increasing concentration of phosphate, molybdate, tungstate or chromate anions causes a shift of the breakdown potential into the noble direction, indicating the inhibitive effect of these anions. The inhibitive effect of these inhibitors decreases in the order: CrO₄²⁻ > WO₄²⁻ > MoO₄²⁻ > HPO₄²⁻.     

Role of alloyed copper on corrosion resistance of austenitic stainless steel in H2S–Cl− environment

Corrosion test, surface analysis and thermodynamic calculation were carried out in the H₂S–Cl⁻ environments to clarify the role of alloyed Cu on the corrosion resistance of austenitic alloys. The alloyed Cu improved pitting corrosion resistance in the H₂S–Cl⁻ environment. The surface film of Cu-containing alloy indicated double layer consists of copper sulfide and chromium oxide, and the copper sulfide was able to exist stably compared to iron sulfide and nickel sulfide. It is concluded that the copper sulfide would enhance the formation of chromium oxide film which improve the pitting corrosion resistance in the H₂S–Cl⁻ environment.