The repassivation response from single cycle anodic polarization: The case study of a sensitized Al-Mg alloy

Abstract

The repassivation behaviour of artificially aged 5083-H111 Al-Mg alloy in near neutral NaCl solutions was investigated by means of single cycle anodic polarization. Artificial aging was carried out at 150°C during different times up to 360 h. The characterization of the microstructure and composition by XRD, SEM/EDS and metallographic analyses indicated most favoured grain-boundary β(Al 3 Mg 2) phase precipitation for 168 h of isothermal treatment time. Nitric acid mass loss test (NAMLT) and microhardness measurements indicated the highest degree of sensitization (DoS) and developed strength for 168 h as well. For all the aging conditions, the potential drop at high currents during the reverse scan of the cyclic polarization was detected at the pit transition potential E ptp with similar but higher values than the corrosion potential of β phase (E corr ≈ −900 mV vs SCE). The associated current density i ptp and the steepness of the potential drop increase as the amount and contiguity of β phase precipitates along grain boundaries. The onset and sustenance of metastable conditions that limit the simultaneous repassivation of all the corroded surfaces are driven by the anodic dissolution of β(Al 3 Mg 2). Metastable processes that limit the hydration of metal ions while producing high local hydrogen concentration are more likely to involve the formation/decomposition of reactive hydride intermediates. E ptp corresponds to the mixed electrode potential at which both metal dissolution and hydrogen evolution occur beyond some distance into the cavity, while i ptp evaluates the catalytic activity of the corroding surfaces. The effective anodic charge transfer coefficient α eff determined from the steepness of the potential drop estimates the contribution of Cl − electromigration in response to local electrodissolution processes.

Inhibition of Q235 Carbon Steel by Calcium Lignosulfonate and Sodium Molybdate in Carbonated Concrete Pore Solution

“…In other words, molybdate ions could block the active sites on the surface [42] and affect the nucleation of pits by deactivating or reducing the number of sites, leading to decreased pitting corrosion susceptibility [43]. As shown in Figure 3, on the reverse scan, a potential step, the pit transition potential ( E ptp ) [39,44,45], is detected for steel in the test solution with molybdate. The E ptp is a characteristic potential that is correlated with repassivation at the pit bottom [44,45], which might lead to the concentration gradients for mass transport and promote further pit nucleation.…”

“…As shown in Figure 3, on the reverse scan, a potential step, the pit transition potential ( E ptp ) [39,44,45], is detected for steel in the test solution with molybdate. The E ptp is a characteristic potential that is correlated with repassivation at the pit bottom [44,45], which might lead to the concentration gradients for mass transport and promote further pit nucleation. In the pit environment, a series of chemical reactions involving hydrolysis and polymerization of molybdates may occur as the pH value decreases [29,46]:7MoO 4 2− +8H + →Mo 7 O 24 6− +4H 2 O…”

“…The inhibition efficiency (IE%) was calculated using Equation:IE false(%false)=iboldnormalcorr0−iboldnormalcorriboldnormalcorr0×100 where i 0 corr and i corr are the corrosion current density values in the test solutions without and with the inhibitors, respectively. In addition, the pitting potential ( E b ) is the critical potential above which pitting corrosion starts on steel surface [36], the repassivation potential ( E pp ) is the lowest potential for localized corrosion to propagate [38], and the pit transition potential ( E ptp ) is a characteristic potential that is correlated with the repassivation at the pit bottom [39,44,45].…”

Effects of Aging Treatment on Corrosion Behavior of a Tensile Deformed Al-Cu-Mn-Fe-Zr Alloy in 3.5% NaCl Solution

“…OM images of the longitudinal section of the alloy are shown in Figure 8 . Generally, the main corrosion types of Al-Cu alloys are intergranular and pitting corrosion, and the corrosion type is affected by the microstructures [ 59 , 60 , 61 , 62 , 63 ]. In Figure 8 a, the main type of corrosion is intergranular corrosion for the alloy aged at 155 °C for 6 h. The corrosion area and the corrosion depth are large-, and the maximum corrosion depth is 196 .…”

Study on Synergistic Corrosion Inhibition Effect between Calcium Lignosulfonate (CLS) and Inorganic Inhibitors on Q235 Carbon Steel in Alkaline Environment with Cl−

“…The increase of E rep indicates the increase of the re-passivation ability of carbon steel. When pits appear on the steel surface, the pH value in the corrosion pit environment is decreasing, which will lead to the concentration gradients for mass transport and promote further pit nucleation [ 38 ]. It is well known that, in the low pH environment, MO 4 2− and WO 4 2− condense into various polymolybdate and polytungstate [ 7 , 11 ], respectively.…”