## 3 Results and Discussion
### Corrosion Inhibition Mechanism
#### 3.5.2 Corrosion Inhibitors
In ambient conditions, they coordinate in hypercrystallic structures and minerals through tetrahedra or octahedra (or connected trioctahedra by others [111], linking by corners or edge sharing that form chains, clusters, and sheets [112, 113, 114]. In the aquatic system of our study, equally to the increase in local electronegativity and valence of the cations, phosphates tend to form the most thermodynamically stable hypercrystallic structures, with an ascending affinity to cyclic or/and branched complexes, with the latter to be achieved either through electrostatic attraction to small cations, as Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), CO\({}_{3}\)\({}^{2-}\) or with ligand exchange, as with solid crystalline phases and metastable iron aqua ions [indicatively FeOH2\({}^{+}\), Fe(OH)4\({}^{-}\), Fe(OH)2\({}^{+}\), and Fe(H\({}_{2}\)O)6\({}^{2+}\)], which were formed through the hydration, olation, or oxolation of ferrous and ferric species [113, 115, 116, 109]. It is established that transition metals can exchange a mere electron, whereas non-metal reactive species, as dioxygen and radical species, are restricted by Pauli's principle, reacting in a slower way [109]. Specifically, between 5 and 9 pH values, the instantly forming ferric salts hydrolyze immediately, while ferrous salts, in the absence of oxidizing substances, react with water, resulting in aqua ions generation, as Fe(H\({}_{2}\)O)6\({}^{2+}\). Generally, ferric complexes tend to condense from very acidic (near pH 1, room temperature) up to near neutral solutions, conferring interstitial oligomeric or/and polymeric Fe(III) compounds that are named as ferrihydrites [117], while ferrous complexes condense only above pH 6 and in oxygen deprivation onto the hardly accessible metal surface, where they are hydrolyzed, to precipitate as Fe(OH)\({}_{2}\). A chain of reactions occur in the meanwhile, as the olation of the dimeric [Fe\({}_{2}\)(OH)8(H\({}_{2}\)O)8]2\({}^{+}\) complex to planar tetrameric [Fe\({}_{4}\)(OH)8(H\({}_{2}\)O)8]0, and the following rapid nucleation of the mentioned tetrahedral and/or octahedral interstitial formations, simultaneously at many sites of each emerging plane, initiating a process of crystalline epitaxial growth for the generation of the layered double hydroxides. The ferrous phases, either in solution or in solid state, have expressed a propensity to oxidation, from which mixed ferric-ferrous phases are considered to be formed, known as green rust, magnetite, goethite and lepidocrocite. In the literature, two physicochemical pathways for phosphorus bonding are distinguished. A fraction of it is rapidly adsorbed and triggers the conversion process of mild steel surface, whereas most of the phosphorus is gradually diffusing into the crystalline iron oxide and hydroxide compounds of the inner layer, as in case of goethite, either into the pores or into the crystal lattice [118]. It was reported that at pH 7, the transformation of Fe(OH)\({}_{2}\) to lepidocrocite is a characteristic reaction, in conjunction with a noted elevation of the potential [109], whereas the hydrolysis and then the aggregation of ferrihydrites often results in the crystallization of goethite and hematite, which is a process chiefly depending on the oxolation of metastable [Fe,OH,O] species [107, 117, 119]. Phosphate anions are considered to have a unique coordination effect in the corrosion process by executing selective adsorption in different phases [107, 21]. Ascending to the microscale, phosphates seem to play a binary role in ferrous corrosion products, either by being adsorbed into iron oxides and oxyhydroxides, as into goethite and lepidocrocite crystal lattices, with the creation of bidentate surface complexes by replacing two hydroxyls--their main ion competitor in aquatic solutions [118, 120, 121, 122, 123]--or by binding on mild steel surface molecules and forming phosphate and iron phosphate crystalline structures, creating new active sites for further phosphorus adsorption [122].