**4. Conclusion**

As a conclusion, both experimental and theoretical techniques prove that phosphonates and phosphonic acids are promising corrosion inhibitors and their high inhibitory activity is directly linked to the presence of phosphonic groups (O]Pd (OH)2 or O]Pd(OR)2) in their molecular structure. Also, the mechanism of action of this type of inhibitors consists to form an adsorbed layer of the inhibitive molecules on the metal surface. On the other hand, the electrochemical techniques show that phosphonates and phosphonic acids act as mixed type of inhibitor by controlling both anodic and cathodic reactions. Moreover, the microscopic techniques such as SEM and AFM confirm the formation of a protective thin layer of the adsorbed molecules on the metal surface, and this phenomenon is confirmed by the decrease in the values of roughness of metal surface after the addition of inhibitors to aggressive solution. Finely, the theoretical methods such as DFT and MDS demonstrate that the active sites responsible for the adsorption of these molecules are located on the oxygen atoms of the phosphonic functional groups.
