Introduction
Corrosion resistance, along with the non-toxicity requirements stand the most important criteria when designing metallic biomaterials. By alloying with elements including Chromium (Cr), Nickel (Ni) and Molybdenum (Mo), medical-grade stainless steels have good corrosion resistance. However, the alloying method fails to prevent the material from stress corrosion cracking (SCC), which is initiated by the co-effect of the tensile stress and corrosive medium. With the assistance of corrosion, cracking can occur at a stress much lower than tensile stress limit. SCC has been found to occur at implants with residual stresses. Also the alloying elements including Ni and Cr are responsible for allergy.
Titanium (Ti)-based biomaterials demonstrate the best corrosion resistance and biocompatibility among the metallic biomaterials. However, stress shielding effect remains a vital problem in their application as implant materials as a result of the incoherency in mechanical properties with human bone tissue.
The biodegradable magnesium (Mg)-based biomaterials appear to be a good solution for the stress shielding effect, attributed to their similar density and Young’s modulus with human bone tissue. Whilst, owing to their biocompatibility and degradability, Mg-based biomaterials allow native tissue to integrate with the implant and finally replace it. However, the hydrogen evolution during the degradation process severely hinders the application of such materials.
This practical session aims to introduce the topic of in vitro metallic corrosion by means of practically exploring various key observations of corrosion and relating those to thermodynamic and kinetic (electrochemical) principles.
Simple exposure tests and polarisation tests will make up the bulk of the experiment. There are several demonstrations that will occur. Students should take notes during the demonstrations (even take pictures if you like). The data from polarisation tests will be described during the session, and students will be given a copy of the data for their report.
Experiments
Exposure tests
1. Measure the pH value of the prepared MEM solution.
2. Expose stainless steel, Ti and Mg in MEM solution and observe what happens
3. Upon removal from solutions at the end of the session, take brief notes on observations. Feel free to take pictures.
Electrochemical tests
1. Electrochemical testing will be carried out with aid of the demonstrator.
2. Collect a polarisation curve on stainless steel Ti and Mg. Observe the form of the data, and be prepared to discuss what is observed in terms of how such data relates to corrosion and electrochemical reaction kinetics.
Report
Following contents are suggested in your lab report:
1. Introduction (introduce the materials used in the practice and explain the significance or importance of this lab practice) (20 marks)
2. Experimental procedures (Exposure and Electrochemical test procedures) (20 marks)
You are required to pay attention to the experimental procedures during the practice, and record as much as possible. If unclear, feel free to ask the demonstrator.
3. Results
(1) Descriptions to the exposure tests.
(2) Polarisation curves of the 3 materials in MEM solution.
(3) Estimate the corrosion potential and corrosion current density for all materials from polarisation curves. Ensure they are presented in appropriate units.
4. Discussion
(1) Describe the anodic branch of the polarisation curves of the three materials.
(2) Describe the differences in polarisation curves of the three materials.
(3) Of the materials tested electrochemically, which displays the most rapid anodic reaction kinetics?
(4) How you envisage the controllable corrosion rate can be achieved for biodegradable Mg-based biomaterials.
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