Abstract

Magnesium based alloys have been considered as a promising replacement to conventional implants owing to their compliant material properties, biocompatibility and most importantly, their biodegradability. Magnesium has properties similar to human bone, which avoids stress shielding effect, a major problem experienced with conventional implants. In addition, magnesium implants eliminate the need for successive surgery due to their biodegradable nature, which is inevitable in the case of conventional implants. Despite having numerous advantages, the major challenge in implementation is its high corrosion rate. The rapid in-vivo corrosion of magnesium implants result in instability and affect bone growth. In addition, corrosion results in hydrogen gas evolution, which is considered undesirable inside human body. It is evident from the literature that there are many methods developed with an aim to control their high corrosion rate and implement them as implants. Of these, Laser Shock Peening (LSP) is considered the most suitable method owing to its ability to provide precise control over the parameters that affect corrosion performance. However due to its prohibitive cost, its implementation is limited.
Recently a new method called High Repetition Laser Shock Peening (HRLSP) was developed to make peening more affordable and encourage widespread implementation. While some previous work on the effect of HRLSP on hardness, wettability and wear resistance have been studied, in this work, the effect of HRLSP on corrosion resistance and biocompatibility of magnesium is analyzed. In addition, the effect of peening process parameters such as % overlap and number of scans on corrosion rate are studied. The experiments are initiated with unpolarized techniques and the corrosion rate & pH variation results formed the basis to validate the effectiveness of HRLSP and determine the feasibility of peened samples as implants. In the subsequent phase, the corrosion rate is tested using a specially designed test bench maintaining physiological conditions. Using this study, the importance of circulation, for understanding dynamic corrosion was stressed and the difference between static and dynamic corrosion of both peened and unpeened magnesium are highlighted. Finally, the change in hardness, surface profile and wettability resulted due to peening are determined to summarize the overall effects.
Throughout the experiments, peening is confined to an area of 10 x 10 mm with laser density set at 1.4 GW/cm2. Hanks solution is used as the test medium and the results of peened samples are compared against unpeened sample to understand the effects of peening and its process parameters.
Under static conditions, the corrosion rate of peened samples reduced by at least 2 times compared to unpeened sample while under dynamic conditions it reduced by 6 times. The differences in results from both studies are compared to stress the influence of surrounding conditions on corrosion process. In addition, peening reduced the size of hydrogen bubbles due to refined grain layer, which is advantageous from biological aspect. The enhanced surface hardness and precise corrosion controlling ability improve the suitability of magnesium as implants.