**1. Introduction**

Corrosion is an unavoidable natural phenomenon. It is the destruction or deterioration over time of metals and alloys and the alteration of their composition and their physical properties caused by a reaction with the surrounding environment [1]. The tendency of metals to corrode depends on several factors; involving the reactivity of the metal, the presence of impurities, the presence of air, moisture, gases such as sulfur dioxide and carbon dioxide, and the presence of corrosive electrolytes [1, 2]. In addition, corrosion process is defined as the spontaneous tendency of the material to revert back to its original state as found in nature which is more thermodynamically stable form. For this reason, corrosion is also called the reverse of extractive metallurgy [3]. Corrosion processes are a constant and continuous problem that develop rapidly and cause significant damage to society as they deteriorate structural innovations and engineering materials, etc. Corrosion is expensive due to the loss of materials or their properties [4, 5]. Meanwhile, the environmental consequences of corrosion are both severe and complex. They generally extend far beyond the immediate issue of resource depletion. In some cases may be toxic and cause injury [5]. Therefore, corrosion is a subject of great importance due to its economic and safety concerns. This is why metals and alloys

require protection in various process industries. Nevertheless, by implementation of corrosion prevention strategies in an appropriate manner, the metal degradation cost can be reduced. At the present time, corrosion control comprises an array of available treatments and approaches that have been developed, such as the isolation of the structure from aggressive media using coatings or the compensation for the loss of electrons from the corroded structure (e.g. cathodic protection by impressed current or by using active sacrificial anodes), or the use of corrosion inhibitors, etc. [6]. Among them, corrosion inhibitors have proved themselves effective in protecting metals against corrosion with obvious advantages regarding availability, strong adaptability, economic efficiency, and high protection efficiency. A corrosion inhibitor is defined as a chemical substance that is added in small concentration to the corrosion medium, which leads to a decrease in the corrosion rate of the metal. It fights against corrosion without directly treating the metal, but intervenes through the medium. Its effectiveness depends on its ability to react with the surface of the metal to form a protective film, thereby reducing or providing protection against corrosion [7–9].

However, the choice of corrosion inhibitors must also be consistent with non-toxicity criteria, since most traditional corrosion inhibitors have been considered highly toxic to living systems and have negative environmental impacts [9, 10]. Hazards arising from the toxicity of regular inhibitors have created the need to develop and explore highly effective and non-toxic inhibitors called "green inhibitors." It is based on natural products or plant extracts, oral medicines, food spices, rare earths, etc. The concept of green corrosion inhibitors has gained popularity as environmental awareness has increased dramatically. In this respect, this chapter is intended to present at first regular corrosion inhibitors and show a detailed description of their toxicity. Then, it provides an in-depth view at the contemporary studies on non-toxic natural product inhibitors with their sources, as well as a brief description of their mechanisms for preventing corrosion.
