Characterizing the Electrochemical Corrosion Behaviour of a Ni–28wt.%Al Composite Coating in 3.5% NaCl Solution

Abstract

The electrochemical corrosion behaviour of an electrodeposited Ni-28wt.%Al composite coating was characterized after 24 h and 72 h immersion periods in 3.5% NaCl solution, using electrochemical and surface probe techniques. Open circuit potential (OCP) and potentiodynamic polarization revealed that the Al particles modify the electrochemical corrosion behaviour of the Ni coating by shifting its E OCP more negatively and increasing its anodic dissolution current density, after 24 h immersion in 3.5% NaCl solution. Compared with the Ni coating, the composite can exhibit wellreduced anodic current density and slightly increased cathodic current with immersion up to 72 h. XPS characterization showed that a high rate of water adsorption and rapid formation of a continuous Ni(OH) 2 initially occurs on the composite surface which, however, readily thickens during prolonged immersion time and promotes the corrosion product enrichment with Al 2 O 3 . This greatly decreased the rate of corrosion and susceptibility to pitting for the Ni-28wt.%Al composite after 72 h immersion in 3.5% NaCl solution.

Ni Corrosion Product Layer During Immersion in a 3.5% NaCl Solution: Electrochemical and XPS Characterization

“…3(a) reveals that the surface of the Ni coating contained Ni species deconvoluted at 856.7 eV (most enriched), 857.7 eV and 858.6 eV (least enriched), with corresponding O 1s peaks at 532.4 eV and 533.5 eV. The binding energy at 856.7 eV is consistent with the formation of Ni(OH)2 [11,12,[19][20][21]. Although higher binding energies are commonly correlated with higher oxidation state species, the phenomenon for a Ni oxidation to Ni +3 was not observed during the polarization experiments.…”

“…The reduction in the maximum phase angle, and the noticeable shift of the high frequency loop towards lower frequency after 72 hr immersion, can be attributed to certain localized thinning of the Ni corrosion product layer [18]. In a previous work [11], SEM surface characterization also revealed that the corrosion products, which formed sparsely in the crevices between the large surface grains of a similar Ni coating, were highly prone to attack by chloride ions, leading to localized pitting corrosion after prolonged immersion up to 72 hr in a 3.5% NaCl solution. Our impedance observation differs markedly from the report of [6] for nanocrystaline Ni coating in a 3% NaCl solution without an immersion time experiment.…”

“…The substrates were prepared by mechanically abrading to final 800 grit size using SiC paper, followed by washing with distilled water, cleaning in acetone (with ultrasonic vibration) and drying with a mechanical dryer. The procedure for the Ni coating electrodeposition has been detailed in our earlier reports [11,12]. The as-deposited Ni coating samples were, thereafter, dried in an oven at 105 ºC for 2 hr and kept in an oxygen-free desiccator.…”

“…(533.5 eV). In the middle and inner layers, only NiO and unreacted Ni at 853.5 eV and 852.6 eV [11,12], respectively, were detected. These peaks corresponded with O 1s peak at 530 eV, which confirms the formation of O 2-compounds.…”

“…However, the corrosion product layer characterization focused mainly on a surface probe without due consideration to a depth profiling approach which could provide more detailed information about the distribution of the corrosion products, especially at the corrosion product layer-substrate interface. Recent reports based on electrochemical characterization, however, have shown that the corrosion resistance of the electrodeposited Ni coating decreased with prolonged exposure in a more aggressive 3.5% NaCl containing solution [11]. Therefore, despite establishing the constituents of the Ni corrosion product layer, characterizing their transformation mechanism over time, is highly imperative.…”