Free «Electrochemical Behavior of Biomedical Magnesium is SBF» Essay Paper

Introduction

Magnesium is the fourth well-spread cation in the body of a man and it is the second well-spread metal in the seawater after sodium. Therefore, it comparatively costs not much. Magnesium is also the lightest of the metals used in practice. Alloying magnesium has such characteristics as heat and creep resistance and strength.

Stainless steels, cobalt-based alloys and titanium alloys represent a significant class of materials for hard tissue replacements. These materials do not degrade after being implanted into the human body, therefore a surgical procedure may be required after recovering the tissues. The prices for repeated surgeries arise and a number of sick people increases. In this regard biodegradable magnesium-based alloys are used and bio consistency has been taken into consideration in medical and vitro evaluation. Pure magnesium and its alloys corrode very fast as they contain a great amount of chloride ions that is why they lose mechanical completeness before the tissues can recover enough. During the process of corrosion hydrogen gas is released very quickly to be sustained.

 

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Electrochemical techniques like electrochemical impedance spectroscopy (EIS) and open circuit potential evolution have investigated the electrode controlled processes. The atmosphere and the environment influence greatly on the corrosion behavior. For example, the physiological environment has lots of other ions such as sulfates, phosphates and carbonates as well as chlorides that have a feature to influence the materials so that the alloys may corrode in another way.

Most studies are concentrated on the degradation behavior in the physiological environment and correlation with body tissues and they do not touch the electrochemical behavior. They also do not investigate in detail information about electrode-controlled processes in the alloys and their variations.

In this research paper we are going to discuss the electrochemical behavior of magnesium alloys in simulated body fluids (SBF). It will be systematically investigated by open-circuit potential evolution and electrochemical impedance spectroscopy. We will also describe the corrosion mechanism and electrode controlled processes.

Experimental details

AZ91 Mg alloys provided by YiHo Corporation, Shenzen, China were used in the experiment. Their compositions are shown in Table 1.

Table 1. Composition of AZ91 magnesium alloys.

The dimensions of the samples used in the experiments were 15mm×15mm×3mm.

After taking out the samples from the test solution and exposing them to air, the surface got white and the morphology was also changed. The corrosion morphology was investigated with the help of the optical microscope. The micromorphologies after immersion in SBD were also examined with the help of the scanning electron microscope for 1 day, 4 days and 7 days.

The electrochemical corrosion behavior was examined by open-circuit potential evolution. The ion concentrations in SBF are shown in Table 2.

Table 2. Ion concentrations in simulated body fluids (SBF)

A three-electrode cell with the sample as the working electrode filled potassium chloride electrode as the reference electrode, and platinum electrode as the counter electrode was used. Changes in the open circuit potential were monitored and the data were recorded every 120sec. All electrochemical experiments were conducted at room temperature.

The hydrogen evolution rates and degradation rates were monitored as a function of immersion time. The average degradation rates after immersion in SBF were calculated for 7 days recording weight loss and hydrogen evolution. The hydrogen evolution volume was measured in the hydrogen evolution method, and the degradation rate was concluded from the following reaction:

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Mg + 2H₂O → Mg(OH)₂ ↑

In the weight loss method, the corroded samples were taken out from the test solution after immersion, cleaned with distilled water and dried. They were soaked in chromate acid to get rid of the corrosion products. Then the samples were cleaned with distilled water and dried again. The received samples were weighed, and the degradation rate was calculated according to the following formula:

Where DR refers to the degradation rate, W is the weight loss from the sample, and A and t represent the exposure area and exposure time in SBF, respectrively.

The AZ91 magnesium alloy is a typical two-phase material. The corrosion occurs on the whole surface with hydrogen evolution when the materials are poured in SBF. And because of the surface tension hydrogen bubbles remain on the surface. According to the Figure 1, intense corrosion stimulates form the boundary where the β phase contacts the surrounding matrix, that is the α phase.

But when there is a continuous β phase, the situation changes. Spreading of corrosion tends to be blocked. And after 2 hours of immersion the matrix around the fine β phase has completely corroded, whereas there is no corrosion on the adjacent β phase. Mg₁₇Al₁₂ shows more passive behavior over a wide pH range than either Al or Mg and it is turned to be inactive in chloride solutions in comparison with the Mg matrix. So it tends to be a corrosion barrier. But the role of the barrier is very restricted to precluding corrosion from spreading laterally or inserting deeply into the matrix.

On the other hand, the β phase can considerably increase the rate of the matrix corrosion with the help of the forming micro-galvanic couples between the β phase and the matrix. The filiform corrosion does not occur during the whole immersion duration.

SEM monitores microcorrosions after immersion in SBF for 1,4, and 7 days. EDS defines the composition of the corrosion products. All the surfaces turned out to have been corroded completely for three different time durations as we can observe cross-linked cracks everywhere on the investigated materials. The reason of the cracks can be explained by dehydration of the corroded layer after drying in warm air and in the SEM vacuum chamber. The corroded samples soaked in SBF are shown in Figure 2.

The corrosion potential (E_corr) shows that electrochemical reactions are carried out at the electrode-solution interface. We can use variations in the corrosion potential to study the reactions which take place at the electrode-solution interface. At first E_corr shifts gradually to the positive direction, and from about 50 ks, the potential becomes stable and we can not observe large fluctuations during subsequent immersion.

A byproduct produced in the cathodic reaction is OH^-, which can raise the localized pH value, increasing the formation and stabilization of the corrosion products, that is why pitting corrosion can be reduced gradually to some extent.

Nevertheless, it is not easy for the corrosion products to take part in the microcathode (β phase) area where hydrogen is evolved. Because odd effusion of OH^-, the corrosion products fall out mainly at the immediate proximity to the microanode (α phase).

In the AZ91 magnesium alloy, Mg₁₇ Al₁₂ has a higher standard voltage. And forms an electrolysis junction with surrounding matrix. Pits are often formed due to selective attack along the Mg₁₂ Al₁₇ networks. The corrosion product can reduce the galvanic effect between the two phases. After being soaked for significantly long time, the galvanic effect is reduced to some extent, and the corrosion mechanism changes from general corrosion to pitting corrosion. Hence, pitting corrosion is a typical behavior of magnesium alloys exposed to SBF.

Conclusion

We studied the corrosion behavior of AZ91 magnesium ally soaked in SBF. The methods of situ visual observation, SEM and electrochemical methods were used. The structure of the corrosion products was indicated by EDS and FTIR. With the help of the immersion tests we defined the average degradation rates and variations in the degradation rates with exposure time. It was also found out that the secondary phase, that is β phase, influences greatly on the corrosion. This phase can form galvanic couples and increase the corrosion rates. But the continuous β phase tends to block the corrosion spreading along the Mg matrix. During the immersion in SBF filiform corrosion does not occur. Magnesium and calcium phosphate and magnesium oxide fall out as the corrosion products. The phosphate contents increase and stabilize after about 4 days of immersion. Pitting corrosion is a typical behavior of AZ91 magnesium immersed in SBF. Very high corrosion rates slow down considerably at the beginning of immersion but then they become stable in approximately 2 days.

The volume of evolved hydrogen is connected to dissolution of magnesium and the reaction takes place in a watery corrosion medium. The corrosion products do not affect the relations between hydrogen evolution and magnesium dissolution. The hydrogen evolution method is more reliable, easy to implement and not inclined to errors attributable to weight loss measurements. By the way, this method has an advantage that variations in the degradation rates can be monitored by the hydrogen evolution rates. That fact allows the study of the degradation rate variation versus exposure time.