Chemical oxidation of lithium salts

2024,08,27

In the 1990s, researchers found that lithium-aluminum conversion films could be prepared using alkaline lithium salt solutions to protect aluminum substrates.

Zheng Fu-yang et al. used lithium salt instead of chromate for chemical oxidation treatment of aluminum alloy. The results found that: lithium ion is conducive to the passivation of aluminum in alkaline carbonate solution, film-forming ions co-deposited on the surface of the aluminum substrate to form a lithium salt conversion film, which in essence is a composite salt composed of ions such as Al3+, Li+, OH- and CO32-. The results of immersion corrosion tests in 3.5% NaCl solution show that chemical oxidation treatment with alkaline lithium salt solution can significantly improve the corrosion resistance of the substrate.

MARCOEN et al. used lithium salt as a corrosion inhibitor for the organic primer coating of 2024-T3 aluminum alloy, used artificial scribing to damage the surface of the aluminum alloy, and exposed it to neutral salt spray, and then investigated the composition and growth pattern of this protective layer by time-of-flight secondary ion mass spectrometry (ToF-SIMS), and analyzed the mechanism of formation of lithium-based protective layer.ToF-SIMS imaging technique successfully monitored the lateral diffusion of leached lithium salts from the artificially 1 mm scribed area: when the scribed area was exposed to neutral salt spray, the lithium salts leached out into the coating defects and induced the growth of the lithium-containing hydrated alumina surface layer (pseudo-burrmuckite layer) on the 2024-T3 aluminum alloy substrate. As the exposure time in neutral salt spray increases, the lithium salt content in the defects as well as the local pH increases, the surface layer starts to undergo partial degradation through anodic dissolution of aluminum, while at the same time, the Al-Li layered double hydroxide layer starts to grow and eventually covers the surface of the 2024-T3 aluminum alloy to protect it. Based on the results obtained by ToF-SIMS, they divided the formation of the lithium-based protective layer into five stages, as shown in Fig. 1.

Figure 1 Schematic diagram of the growth stages of the protective layer

Stage I:

The scribing zone is first exposed to neutral salt spray, the aqueous electrolyte causes lithium salts to leach from the organic coating into the scribing zone, the leaching of the lithium salts results in an increase in the pH of the scribing zone to alkaline, which causes anodic dissolution of the aluminum in the presence of chlorides.

Stage II:

A protective layer adjacent to the organic coating begins to form with a localized pH of 9~10 near the metal surface, this alkaline condition leads to the formation of a pseudobombsite layer and with a small amount of lithium doped in it, a lithium-pseudobombsite layer (L-PB) begins to form.

Stage III:

The L-PB layer grows laterally towards the center of the scribed area as the lithium salt is further immersed into the scribed area, and this process is relatively fast. When the lithium salt content exceeds a certain threshold near the organic coating, the layered double hydroxide (LDH layer) begins to form.

Stage IV:

After a longer exposure to neutral salt spray, the lithium ion content in the center of the scribe zone increases, which allows the LDH layer to grow in the center of the scribe as well.

Stage V:

The cylindrical LDH layer grows laterally on the surface at the expense of the L-PB barrier layer below until the scratch is completely covered by the LDH layer.

Author:

Mr. Richard

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