## 4 Discussion
In addition, it is possible that the quantity of aluminium reported in the weld metal did not fully participate in gas-based reactions. Therefore, one scenario was calculated to simulate this possibility of 3.25 g aluminium input. The calculated gas compositions and quantities are summarised in Table 7.  
The calculation speciation results confirmed that increased aluminium added into the arc cavity can maintain the cobalt in the metallic state via lowered gas-phase partial oxygen pressure, resulting in cobalt metal vaporisation rather than formation of CoF\({}_{2}\)(g). The gas composition volume% CoF\({}_{2}\)(g) values are low at 1%, 0.01%, and 0.002%, as calculated for zero, 3.25, and 6.50 g of added aluminium, respectively. Because somewhat larger gas volumes are formed with aluminium reactions, the cobalt mass balance numbers in the last column in Table 7 indicate that more cobalt is vaporised at higher aluminium additions, even though the %Co in the gas decrease with increased aluminium. The partial oxygen pressure values of the gas phase were calculated as 6.3 \(\times\) 10\({}^{-5}\) atm, 4.0 \(\times\) 10\({}^{-7}\) atm, and 8.5 \(\times\) 10\({}^{-8}\) atm for zero, 3.25, and 6.50 g of aluminium, respectively, illustrating that more effective reducing conditions prevailed in the arc cavity with increased addition of aluminium powder. From the results in Table 7, it appears that CaF\({}_{2}\)(g) and MgF\({}_{2}\)(g) are replaced by Mg(g), AlF(g), Mn(g), and SiO(g) due to the chemical interactions with added aluminium in the arc cavity. Although this calculation is a simplification of the overall process, the speciation outputs from the calculation are in agreement with the deductions made from the consideration of simplified reactions in Figures 4 and 5. In Figure 4 it can be seen that AlF\({}_{3}\)(g) formation from Al is thermodynamically most probable and CoF\({}_{2}\)(g) is thermodynamically least probable. This means that the forward reaction (9) for Al and reverse reaction (9) for Co sum to reaction (11), below, so that any CoF\({}_{2}\)(g) presnet in the gas phase will be easily transformed to cobalt vapour via the reaction with Al vapour. This reaction is in agreement with the calculated composition of the equilibrium gas phase shown in Table 7, showing cobalt vapour but little CoF\({}_{2}\)(g) in the gas, as enumerated in the text above. Although AlF\({}_{3}\)(g) is displayed in reaction (11), the same type of reaction is also favoured in the formation of AlF\({}_{2}\)(g) and AlF(g) because the standard Gibbs free energies for the formation of these compounds are lower than for formation of AlF\({}_{3}\)(g) from Al(g) and F\({}_{2}\)(g), as displayed in Figure 6:  
Figure 5.0  
3.25 & 7 & 8 & 16 & 1 & 5 & 16 & 7 & 3 & 5 & 9 & \(<\)0.5 & 8 & 9 & 16 \\ \hline
6.50 & 2 & 5 & 25 & 0.3 & 3 & 24 & 2 & 1 & 4 & 8 & \(<\)0.5 & 7 &Considering the behaviour of cobalt in the presence of aluminium in the gas-based reactions discussed above, the likely reaction sequence in the SAW process with the use of aluminium and cobalt powders is shown in Figure 5.1 Reaction steps A to E, as displayed in Figure 7, are as presented in previous works [4; 5; 6; 33; 34]. Reaction steps A to E represent oxygen transfer from the molten flux (slag) to the weld pool. Reaction A corresponds to the same type of reaction as equation (10), for the reaction of Al\({}_{2}\)O\({}_{3}\) with CaF\({}_{2}\)(g) to form AlF\({}_{3}\)(g) and CaO. This reaction has the lowest Gibbs free energy values, as displayed in Figure 5.2 Previous studies conclusively showed that the initial excessive quantity of oxygen added into the weld pool via the molten weld wire droplets from the arc cavity gas phase may be up to 2000-3000 ppm O [4; 5]. The word \"excessive\" means that the ppm O in the weld pool exceeds the solubility limit of oxygen at the cooling weld-pool temperature once the arc has passed and the weld pool cools from the high arc plasma temperatures, typically 2000 \({}^{\circ}\)C to 2500 \({}^{\circ}\)C [9; 10; 33].