Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

Effects of pH solution and chloride (Cl⁻) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.003 wt% and 5.5 wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting E_pit and corrosion E_cor potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH = 2) and alkaline chloride solutions (pH = 12) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH = 6).     

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.     

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.     

Environmental factors affecting the corrosion behavior of reinforcing steel II. Role of some anions in the initiation and inhibition of pitting corrosion of steel in Ca(OH)2 solutions

Potential-time curves are constructed for the steel electrode in naturally aerated Ca(OH)₂ solutions simulating the corrosion behavior in concrete. Cl⁻ and SO₄²⁻ ions cause the destruction of passivity and initiation of pitting corrosion. The rate of oxide film growth by Ca(OH)₂ and oxide film destruction by Cl⁻ and SO₄²⁻ ions follows a direct logarithmic law as evident from the linear relationships between the open-circuit potential and the logarithm of immersion time. Chromate, phosphate, nitrite, tungstate and molybdate ions inhibit the pitting corrosion of steel. The rate of oxide film healing and thickening increases with their concentrations. In presence of constant inhibitor concentration, the efficiency of pitting inhibition increases in the order: (weak) CrO₄²⁻ < HPO₄²⁻ < NO₂⁻ < WO₄²⁻ < MoO₄²⁻ (strong).     

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.     

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.     

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⁻.     

Pitting corrosion behaviour of austenitic stainless steels - combining effects of Mn and Mo additions

Mn and Mo were introduced in AISI 304 and 316 stainless steel composition to modify their pitting corrosion resistance in chloride-containing media. Corrosion behaviour was investigated using gravimetric tests in 6 wt.% FeCl₃, as well as potentiodynamic and potentiostatic polarization measurements in 3.5 wt.% NaCl. Additionally, the mechanism of the corrosion attack developed on the material surface was analysed by scanning electron microscopy (SEM), X-ray mapping and energy dispersive X-ray (EDX) analysis. The beneficial effect of Mo additions was assigned to Mo⁶⁺ presence within the passive film, rendering it more stable against breakdown caused by attack of aggressive Cl⁻ ions, and to the formation of Mo insoluble compounds in the aggressive pit environment facilitating the pit repassivation. Conversely, Mn additions exerted an opposite effect, mainly due to the presence of MnS inclusions which acted as pitting initiators.     

Effect of common water contaminants on the corrosion of aluminium alloys in ethylene glycol-water solution

The effects of common water contaminants of chloride (Cl⁻), cupric (Cu²⁺) and ferric (Fe³⁺) ions, in four different mixture combination of Fe³⁺ + Cu²⁺, Cl⁻ + Fe³⁺, Cl⁻ + Cu²⁺ and Cl⁻ + Fe³⁺ + Cu²⁺, were examined on the corrosion behaviour of aluminium alloys in ethylene glycol-water solution, using mass loss technique. The highest material losses were recorded for the two alloys in ethylene glycol solution containing the combination of the chloride and the two heavy metal ions. The corrosivity of the solution in the presence of the combination of ions was in the order of Cl⁻ + Fe³⁺ + Cu²⁺ > Cl⁻ + Cu²⁺ > Cl⁻ + Fe³⁺ > Fe³⁺ + Cu²⁺. The results gave first-order kinetics with respect to aluminium in ethylene glycol solution-ion systems. Alloy 3SR exhibits maximum corrosion in all the solutions. It is concluded that the two commercial alloys in the solution polluted with all the three ions would not be able to survive for reasonable period of time without corrosion inhibitor.     

Electrochemical behaviour of stainless steels in media containing iron-oxidizing bacteria (IOB) by corrosion process modeling

Localized corrosion mechanism of stainless steel (SS) types UNS S30403 and UNS 31603 in the presence of iron-oxidizing bacteria Sphaerotilus spp. isolated from rust deposits was studied electrochemically. OCP transient, cyclic anodic and cathodic potentiodynamic polarization curves were measured on steel electrodes through their exposure to 3% NaCl solution supplemented with Sphaerotilus culture. The exposure period was composed of three parts: (a) 5 days incubation of steel electrodes in sterile 3% NaCl solution; (b) addition of 3 days-old Sphaerotilus culture to 3% NaCl at 3:2 v/v ratio with subsequent electrodes exposure for 11 days up to complete sedimentation of ferric oxides and (c) subsequent exposure of electrodes for 14 days in upper and bottom (sediments layer) fractions of the experimental medium. The results revealed an instantaneous gradual shift of the transient potential of both steels towards negative potentials from steady-state value of −0.15 V to −0.35 to −0.42 V (SCE) during the whole exposure interval since IOB culture addition into sterile 3% NaCl solution.No evidence of pitting corrosion was found on SS samples subsequent to their exposure to sterile 3% NaCl solution, though in the presence of IOB culture, numerous pits were revealed on 304 L steels specimens exposed to iron hydroxides sediments layer. Electrochemical characteristics (OCP or corrosion potential - E_CORR, breakdown potential - E_BD, repassivation potential - E_RP, passivation current - i_PASS) periodically measured by cyclic polarization method, allowed monitoring the electrochemical behavior changes of experimental SS and to establish the initiation of pitting corrosion in the presence of IOB, resulting in crevice effect caused by biogenic ferric oxides deposits precipitated on steel surface. Overall, steel 316L demonstrated higher resistance to pitting corrosion compared to 304L.     

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.     

Measurements of pitting corrosion currents of zinc in near neutral media

Using a simple model cell the susceptibility of the zinc electrode to pitting corrosion by SO₄²⁻, SO₃²⁻, S₂O₃²⁻ and S²⁻ anions were examined in naturally aerated carbonate solutions. It was found that, pitting started after an induction period, τ, which depended on the type and concentration of the aggressive and passivating anions. The pitting corrosion current increased with time until steady state values were attained. These values depended on both the type and the concentration of the passivating and pitting anions. For the same concentration of the passivating anions, the corrosion current varied with the concentration of the aggressive anion according to the relation: logi_pit.=a₁+b₁logC_agg. At a constant concentration of the aggressive anion, the corrosion current varied with the concentration of the passivating anions according to: logi_pit.=a₂−b₂logC_pass. The constants a₁ (a₂) and b₁ (b₂) were determined for all the systems studied. From the values of a₁ the corrosivity of the sulphur-containing anions is found to decrease in the order SO₄²⁻>SO₃²⁻>S₂O₃²⁻>S²⁻.     

Deterioration in critical pitting temperature of 904L stainless steel by addition of sulfate ions

This paper deals with the effect of adding sulfate on the critical pitting temperature (CPT) of highly alloyed austenitic stainless steel. A large number of potentiodynamic CPT measurements and potentiostatic current-time curves were obtained in 1 M NaCl containing 0, 0.2, 0.5 and 0.75 M Na₂SO₄. Provided the CPT is defined as the first temperature where stable pitting occurs at intermediate potentials, such as 600 mV (Ag/AgCl), addition of sulfate is shown to have the unexpected effect of lowering the CPT. The growing pits formed in sulfate-containing solution passivate anodically as the potential is increased, perhaps via salt precipitation. The effect of sulfate on pitting kinetics was studied using 50 μm-dia. 302SS wire in 1 M NaCl and 1 M NaCl + 0.5 M Na₂SO₄ at 40 °C. Sulfate increases the critical concentration of metal salt in the pit, expressed as a fraction of the saturation concentration, that is required to sustain pit dissolution. Provided this fraction does not exceed 100% of saturation, passivation is enhanced just inside the pit rim, allowing earlier undercutting of the metal surface and a finer pore structure in the lacy metal cover over the pit. The pitting potential measured above the CPT is increased by sulfate addition, but the CPT itself is lowered. Related examples are cited where pitting shows an unusual dependence on some variable such as anion concentration or temperature.     

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.     

Stress corrosion cracking of B13, a new high strength aluminium lithium alloy

The present paper focuses on the study of SCC behaviour of a new Al–Cu–Li alloy. For this purpose, two conventional media – NaCl and NaCl + H₂O₂ – were used for comparison with commercial alloys 7075 and 8090. This new alloy shows lower susceptibility to SCC than conventional alloys as it does not undergo environmentally-induced embrittlement in NaCl solutions and in 1 M NaCl + 0.3% H₂O₂ in which the 7075 and 8090 alloys, respectively, undergo environmentally-induced fracture.Solution composition was modified in order to determine the environmental conditions and strain rates under which this new alloy will crack due to a stress corrosion cracking phenomenon. The addition of 0.6 M sulphates to 1 M NaCl + 0.3% H₂O₂ solution allows the definition of a range of strain rate (between 10⁻⁷ and 10⁻⁶ s⁻¹) in which this new alloy undergoes stress corrosion cracking.     

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 μg/cm⁻² NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg₅ (CO₃)₄ (OH)₂4H₂O) and nesquehonite (MgCO₃ 3H₂O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH₂)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO₂ and O₂ to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na⁺ and Cl⁻ ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.     

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni shape memory alloy

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni (mass%) shape memory alloy at 25 °C in 0.5 M H₂SO₄ and 3.5% NaCl solutions has been studied using potentiodynamic polarization and electrochemical impedance techniques. Three different microstructures viz., single-phase γ, γ-δ and γ-Fe₅Ni₃Si₂, were produced by heat-treating the alloy in different equilibrium phase fields. The corrosion behaviour in 0.5 M H₂SO₄ solution is almost same for all three microstructures, barring a slight difference in the passivation range. Although, the passivation current in 0.5 M H₂SO₄, is in the same range as that of SS 304, the critical current required for onset of passivation is almost three orders higher and the passivation range is much shorter. In 3.5% NaCl solution the corrosion behaviour of all three microstructures of the Fe-15Mn-7Si-9Cr-5Ni shape memory alloy was that of general dissolution without passivity or localized attack (pitting). The best corrosion resistance in both H₂SO₄ and NaCl solutions is shown by the single-phase γ microstructure.     

Electrochemical pitting corrosion behaviour of α-brass in LiBr containing solutions

The potentiodynamic anodic polarization curve of α-brass (70% Cu-30% Zn) in 1 M LiBr solution showed an initial active region of the alloy dissolution followed by two well defined anodic current peaks then a narrow passivation region before the pitting potential (E_pit) is reached. The initial active anodic region exhibited Tafel slope with 90 mV dec⁻¹ attributed to the formation of CuBr₂⁻ complexes. The anodic current peaks were attributed to the formation of CuBr and Cu²⁺ ions, respectively. The change of pH values of LiBr solution did not affect the anodic polarization curves of α-brass. Increasing the solution temperature from 30 to 90 °C changed the corrosion type from pitting to general one. The addition of 10⁻² M benzotriazole (BTAH) to 1 M LiBr solution is completely inhibited the pitting corrosion at 30 °C while it did not inhibit the pitting at 90 °C. The inhibition effect was attributed to the adsorption of BTAH molecules on the alloy surface, which obeys Langmuir isotherm. The presence or absence of pitting corrosion was confirmed by using SEM.     

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 carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.