## 4 Discussion  
There was steep drop in corrosion rates of tested alloys calculated by hydrogen volume (Figure 4). This was due to the formation of corrosion products, which formed a layer on alloy surfaces, protecting alloys from further corrosion [34; 35; 36]. Pure Zn had significant lower initial corrosion rate calculated by hydrogen evolution (Figure 4) indicating that pure Zn had better initial corrosion resistance. The corrosion rates calculated by hydrogen evolution was real-time with multiple time points, however, the overall corrosion rates were calculated by mass loss and volume loss in this study.  
By the volume loss and mass loss methods, the corrosion rates of pure Mg are higher than that of pure Zn calculated by volume loss and mass loss. This result is consistent with published data [37]. Corrosion rates of pure Zn and Mg were relatively lower than tested alloys. This was due to possible impurity and secondary intermetallic phase of alloying elements (Zn, Ca, and Zr), which may induce galvanic corrosion [38; 39; 40]. The volume loss and mass loss data are consistent to calculate the corrosion rates of the six biodegradable samples, since there is no significant difference between volume loss and mass loss in corrosion rates calculation of all tested alloys. A lot of researchers measured the corrosion rates in various conditions by volume loss and mass loss [25]. Corrosion rates are calculated by volume loss when: (1) the tested samples are too small to measure mass loss (e.g., Mg wire [31]); (2) the change of mass is not measurable due to the short corrosion duration [30; 31]; or (3) the corrosion product is not accessible to remove (e.g., in vivo [25]). Micro-CT can provide high resolution of three-dimensional images of small and light samples, and the volume changed can be measured by the software and corrosion rates can be calculated by Equation (1). However, equipment access is often limited. Not every lab can afford to maintain micro-CT equipment. Mass loss method provides the most accessible way to calculate the corrosion rates. By measuring the weight of the intact sample material before and after corrosion (after corrosion product removal), the corrosion rate can be calculated according to Equation (2).  
The audiometer system allowed real-time record of hydrogen evolution volume and corrosion rate changing over time. The initial burst of corrosion rate (Figure 4) indicates the importance of initial corrosion resistance of biodegradable material. The difficulty of using hydrogen volume to calculate corrosion rate is that the audiometer has to be calibrated and well-sealed, since dihydrogen is the smallest molecule [41]. A audiometer is recommended when the experiment duration is long and there are multiple measurements of time points. By a well-sealed audiometer system, the real-time hydrogen evolution results can facilitate monitoring the process of corrosion without touching the samples. However, to measure volume and mass in different time points in a long-term experiment, the samples must be handled to measure the mass loss or volume loss, and there is a risk of damage to the sample to measure the mass or volume multiple times.  
Figure 6: Corrosion rates of tested alloys calculated by mass loss and volume loss in the audiometer system.