Electrochemical corrosion behavior of alloys either used in industry or as implants in human body
The main problem in our life is the corrosion of many types of alloys either industrial or biological. Corrosion behavior had been studied using open circuit potential, electrochemical impedance (EIS) spectroscopy, Potentiondynamic polarization, cyclic voltammetry measurements. Many electrodes have been studied as Titanium and its alloys as one of the best engineering materials for use in industrial applications . Titanium is widely used in the equipment of chemical enterprises, petroleum-refining industries and food industry. A study to investigate the stability of the Titanium and Ti-6Al-4V in aqueous solutions of oxalic acid has been done. Oxalic acid is a relatively strong organic acid used as purifying agent in pharmaceutical industry, it is a metabolite of the catalytic oxidation of phenol  and other aromatic substrates like, for instance, coumaric acid (a by-product of olive oil manufacturing). It was found that the corrosion rate increase with either increasing oxalic acid concentration or temperature. EIS results of titanium and its alloy in 0.01 M oxalic acid containing either SO 4 2 −or Cl − ions showed that oxide film resistance decreases with the increase of the sulphate ion concentration. However, for Cl − ion, it decreases from 0.001 till 1 mM, then increases at higher concentration for both Ti and its alloy .
The titanium alloys have been increasingly used in aerospace, biomedical, and chemical industries due to their very high strength to weight ratio, biocompatibility, and high corrosion resistance due to the formation of titania on its surface. The nature, composition and thickness of the protective oxide scales depend on environmental conditions . It was previously reported  that titanium has a high resistance to corrosion in physiological saline and artificial saliva. Metallic materials are being increasingly used in medical applications as implants to restore lost functions or replace organs functioning below acceptable levels. Titanium alloys are among the most used metallic biomaterials, particularly for orthopaedic applications . Magnesium alloys as potential biodegradable material provides both biocompatibility and suitable mechanical properties . Mg 2+ is an essential element and present in large amounts in the human body. The presence of magnesium in the bone system is beneficial to bone strength and growth. Magnesium alloys have specific density (1.74–2 g/cm 3) and Young’s modulus (41–45 GPa) most close to those (1.8–2.1 g/cm 3, 3–20 GPa) of human body’s bone. Therefore, in orthopedic and bone repairing or replacement applications magnesium alloys are particularly superior to any other metallic or polymer implants in terms of physical and mechanical properties, as the dissimilarity in Young’s modulus between an implant and natural bone can result in stress shielding effects, leading to concentration of stress at the interface between the bone and implant reducing stimulation of new bone growth and decreasing implant stability .
The electrochemical behavior of AZ91D and Ti–6Al–4V alloys was investigated in simulated body fluid (SBF) at 37 ◦C. The study is aimed to improve knowledge for the nature of the corrosion films in those systems by comparing the behavior of AZ91D alloy (biodegradable material for temporary implant) and Ti–6Al–4V alloy (passive alloy for permanent human use) and tried to correlate the electrochemical behavior to their structure, using conventional electrochemical techniques complemented with surface examination. Very low current density was obtained for Ti–6Al–4V alloy compared to that of AZ91D alloy, indicating a typical passive behavior for Ti alloy. EIS results exhibited high corrosion resistance indicating a highly stable film on titanium alloy compared to magnesium alloy in SBF .