AA2024 aluminium alloy is widely used in the aerospace industry. However, it is known to be highly susceptible to localised corrosion, which is related to its complex microstructure, mainly the presence of numerous Cu-rich intermetallic particles. One of the effective solutions to increase their corrosion resistance relies on the formation of layered double hydroxide (LDH) based conversion coatings (CC). This investigation aims to understand how the conditions of Li-Al LDH-CO/OH CC growth affect their further protective ability. For that purpose, concentration of reactants (0.05–0.15 M LiCO), pH of the electrolyte (10–12), temperature (30–50 °C) and treatment time (15 min-30 h) were systematically varied. Additionally to that, NHOH was applied as a chelating agent for the synthesis of Li-Al LDH-CO/OH as well as for the pH control. The obtained protective coatings were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES), Raman spectroscopy, salt spray test (SST) and electrochemical impedance spectroscopy (EIS). Among all varied parameters, involvement of chelating agent was found to be the most efficient for the formation of a coating with the highest level of corrosion protection. This is attributed to the formation of soluble complexes between NHOH and Cu, resulting in dissolution of the intermetallics. It also prevents further redeposition of Cu species and consequently, the formation of new highly active Cu cathodes on the AA2024 alloy surface.

中文翻译：

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在 AA2024 合金上形成 Li-Al LDH 转化层以防止腐蚀

AA2024铝合金广泛应用于航空航天工业。然而，众所周知，它非常容易受到局部腐蚀，这与其复杂的微观结构有关，主要是存在大量富铜金属间颗粒。提高其耐腐蚀性的有效解决方案之一是形成基于层状双氢氧化物（LDH）的转化涂层（CC）。本研究旨在了解 Li-Al LDH-CO/OH CC 的生长条件如何影响其进一步的保护能力。为此，系统地改变了反应物浓度（0.05-0.15 M LiCO）、电解质 pH 值（10-12）、温度（30-50 °C）和处理时间（15 分钟-30 小时）。除此之外，NH4OH 还用作 Li-Al LDH-CO/OH 合成以及 pH 控制的螯合剂。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、辉光放电发射光谱(GDOES)、拉曼光谱、盐雾试验(SST)和电化学阻抗谱(EIS)对所得保护涂层进行了评价。在所有不同的参数中，发现螯合剂的参与对于形成具有最高腐蚀防护水平的涂层是最有效的。这是由于 NH4OH 和 Cu 之间形成可溶性络合物，导致金属间化合物溶解。它还可以防止铜物质进一步再沉积，从而防止在 AA2024 合金表面形成新的高活性铜阴极。