**1. Introduction**

Aluminum and its alloys are considered promising materials for a number of applications due to their high strength to weight ratio and relatively high thermal and electrical conductivities, along with their abundance and low price. In addition, aluminum forms a passive oxide layer on its surface, which increases its corrosion resistance. On exposure to an acidic or alkaline medium, especially chloride-incorporating media, the protective oxide layer is damaged and aluminum becomes susceptive to corrosion. Various series of aluminum alloys have been developed to improve the properties of aluminum with respect to their usage applications. AA6xxx is relatively a new class of aluminum alloys which contain mainly additions of manganese, magnesium, silicon, and a small amount of copper [1–4]. Although these alloys show relatively high general corrosion resistance, they are susceptible to forms of localized corrosion, especially pitting and intergranular corrosion [5,6]. Therefore it has become indispensable to develop an efficient, environmentally friendly protective coating on aluminum and its alloys.

Chromate-containing conversion coatings were employed for a long time to protect aluminum alloys but since those coatings contain hexavalent chromate compounds, which have been proved toxic for the environment and hazardous to health [7,8], efforts have been made to develop suitable alternatives. A number of approaches have been reported, including conversion coatings [9,10], magnetron sputtering [11,12] anodizing [13,14], sol-gel synthesis [15,16], and self-assembly [17,18], and polymer coatings [19,20] to develop non-chromate based corrosion resistance coatings. Layered double hydroxide, a promising type of chemical conversion coatings and also known as hydrotalcite-like compounds or anionic clays, have had prominent attention in the fields of biomedical science, applied chemistry, and environmental purifications and recently as corrosion resistant coatings for metals [21,22]. Generally, two different methods have been employed to fabricate layered double hydroxide, the single step in situ approach [23,24] and the other a colloidal assembly technique [25,26]. Coatings fabricated using the two-step method improved the corrosion resistance of their substrates; however, the poor adhesion of the film to the substrate was reported and also the fabrication process itself appears slightly complicated [27]. Numerous works have been reported to fabricate different types of layered double hydroxide to protect the light metals alloys [27–30]. The growth rate of crystals and their size and distribution can be controlled by adjusting the crystallization time and the reaction temperature. The properties and structural characteristics of MgAl–layered double hydroxide (LDH) actively depend upon the fabrication method, operating parameters, and conditions used for the fabrication [31–34]. Therefore, the optimization of the synthesis parameters plays an important role in developing a suitable structure for numerous applications. In our previous work, we reported the effect of different salt concentrations on LDH structural growth to obtain various distinct LDH morphologies as well as their effect on corrosion resistance properties [35]. However, the combined effect of extended reaction temperature and the aging time on in situ growth MgAl–LDH structural growth rate without using any complexation agents (used to promote specific LDH structural growth) to further their impact on their corresponding corrosive resistance behavior has not ye<sup>t</sup> been thoroughly investigated. In this work we succeeded to synthesize a series of MgAl–NO3 LDH film on the surface of AA6082 by using magnesium salt only, and developed a range of balanced combinations of reaction temperature and aging time, at constant initial cationic concentration (Mg2+) to investigate in detail the impact of the above-mentioned parameters on LDH geometry, structural growth, morphology, and their effect on the corresponding corrosion resistance properties. In particular, in this study, the one-step in situ growth method was used to develop MgAl–LDH coatings on AA6082 at different combinations of extended reaction temperature and crystallization time without using any surfactants or complexation reagents. This was done in order to understand the effect of the mentioned parameters on the geometry of LDH crystallites, film growth, and on the deposition rate to explain the relationship of LDH structural variations with its anticorrosion behavior. This work provides insight into the corrosion resistance properties of MgAl–LDH, and into the correlation between the electrochemical response of the coatings and their structural properties.
