Compact Oxides Formed on Zinc during Exposure to a Single Sea-Water Droplet

Abstract

Compact oxide layers predominantly consisting of zinc oxide naturally grow on zinc upon exposure to a single seawater droplet. SEM and TEM reveal the existence of these oxides beneath the surface, close to the metal surface. EIS demonstrates that these oxides serve as an electrochemical barrier on zinc. The compact oxide immediately above the metal surface is 20-70 nm thick and is undulating in appearance making it difficult to identify crystalline features within. However, the oxide layer above this compact oxide shows a dispersion of a large number of fine-sized voids that cluster in certain areas to form larger voids.Atmospheric corrosion of zinc occurs by contact with deposited aerosols or a thin film of moisture. 1 This results in the formation/precipitation of a number corrosion products (zinc compounds) constituting the zinc patina. 2-7 Droplet experiments have been carried out to understand atmospheric corrosion of zinc by deposited aerosols. [8][9][10][11][12] Electrochemically, droplet tests are significantly different from conventional (bulk) electrochemical tests, as droplets in contact with the metal form micro-electrochemical cells with (i) higher oxygen diffusion to the metal surface, 13,14 ii) low volume of electrolyte contacting the metal, which favors precipitation of zinc corrosion products, and iii) smaller anode-cathode separation, due to a small area of metal exposed and also possibility of cathodic reactions on the corrosion product itself. [15][16][17][18][19] Previous cross-sectional analysis of the Zn patina formed on exposure to a single sea-water droplet using FIB-SEM, revealed that the patina vertically arranges itself as a multi-layered structure. 8,13 The lower layers of the patina especially those in contact with the metal surface were found to become more compact with time.The morphology of the oxide formed on metals, especially passive films, plays an important role in it being an effective barrier against corrosion. Inter-granular sites in passive oxides have been reported to promote passivity breakdown and pitting initiation in metals such as Fe, In those works the passive films were mostly developed by electrochemical polarization in the passive region of the corresponding metal, [22][23][24][25] and not under free-corrosion conditions, which is a more realistic case to test the existence, morphology and impact of the passive layers. Furthermore, the analysis of passive films has been made based on lateral views encompassing only the top surface of these oxides. A sectional view enables visualization/detection of the oxide layers beneath the surface, which may play a more pronounced role in protecting the metal. In bulk electrochemical cells, super-saturated solutions to favor precipitation of corrosion products are rarely achieved, and consequently the role of these corrosion products in protecting the metal is not clearly discernible.In the present study, the oxides formed on zinc after exposure to a 1 μL seawater droplet are analyzed across their cross-section using FIB-SE...

Understanding corrosion mechanism of Sn–Zn alloys in NaCl solution via corrosion products characterization

“…The cracks and pores could easily provide channels for the penetration of corrosive medium (e.g., Cl À ), revealing the lack of protective properties of the films formed on Sn-Zn alloys. This type of structure is similar to that reported on the study of pure Zn [25][26][27], and can be connected to the poor corrosion performance of Sn-Zn alloys. The pores may either be due to coalescence of crystal defects (e.g., metal or oxygen vacancies) or just open spaces left out during the inter-dendritic growth of corrosion products [25].…”

“…This type of structure is similar to that reported on the study of pure Zn [25][26][27], and can be connected to the poor corrosion performance of Sn-Zn alloys. The pores may either be due to coalescence of crystal defects (e.g., metal or oxygen vacancies) or just open spaces left out during the inter-dendritic growth of corrosion products [25]. Figure 8 shows the typical cross-sectional SEM micrographs and EDX mapping analysis of Sn-9Zn alloy after potentiodynamic polarization in 0.5 M NaCl solution.…”

Corrosion Behavior of Field-Exposed Zinc in a Tropical Marine Atmosphere

“…38,[44][45][46] As can be seen in Figures 3 through 8, corrosion products present different characters after exposure for different periods. Therefore, EIS is utilized to evaluate the barrier effect of the product layer in this section.…”

“…44 Generally, it is accepted that the protectiveness of the corrosion products depends on their chemical composition, conductivity, compactness, thickness, and morphology. 22,[44][45][46] The following section will discuss that which controls the protectiveness of the corrosion products formed on zinc during the exposure test.…”

Oxygen Reduction Investigation on Sputtered ZnO Layers with Nano‐granular Structure

“…ORR activity reduction on steel substrate was also evidenced by Thébault et al while investigating ZnO deposits at the cut-edge of galvanized steel samples using the electrochemical microcapillary cell technique. [26] The patinas exhibited a significant gradient of porosity with an open structure on the electrolyte side and a compact one close to the zinc substrate. [23,24] In a scanning Kelvin probe study, Nazarov et al demonstrated an increase of surface potential of "thick" ZnO films (ca.…”