## 4 Discussion  
A thermodynamic analysis was applied to elucidate the chemical interaction between cobalt and aluminium, and its effects on cobalt yield to the weld pool. During SAW, a variety of complex reactions take place in the arc cavity, and these reactions may contribute to cobalt loss. For example, some of the cobalt is expected to vaporise, cobalt may react with F\({}_{2}\)(g) and/or CaF\({}_{2}\)(g) to form CoF\({}_{2}\)(g), and cobalt may also oxidise to CoO. As a starting point, simple thermodynamic calculations for typically expected chemical reactions were performed to investigate the relative likelihood of possible reactions involving CoF\({}_{2}\)(g) formation, relative to similar reactions for different compounds contained in the flux, metal powders, and steel. The first reactions considered for metal fluoride formation were of the type in Equation (9), with the reactions' standard Gibbs free energy values displayed in Figure 4. The second type of reaction considered for metal fluoride formation were those displayed in Equation (10), with the Gibbs free energy values displayed in Figure 5. The values in Figures 4 and 5 were calculated using FactSage 7.3 thermochemical software [31]. A calculation temperature interval of 1600 \({}^{\circ}\)C to 2500 \({}^{\circ}\)C was selected to include temperatures applicable to the main reactions in SAW, as identified in previous works, namely 2000 \({}^{\circ}\)C as the effective slag-metal equilibrium reaction temperature, 2500 \({}^{\circ}\)C at the plasma-veld-pool interface, down to 1600 \({}^{\circ}\)C which is close to the liquidus temperature of steel [9, 10, 33]. \[[\text{Co}]+[\text{F}_{2}]=[\text{CoF}_{2}] \tag{9}\]  
\[(CoO)+[CaF2]=[CoF2]+<CaO>():liquid;[]:gas;<>:solid. \tag{10}\]  
Figure 3: Solidification curves for BC and MP7 weld metal compositions as calculated in FactSage 7.3 [31]. The following input values were applied to calculate the standard Gibbs free energy values displayed in Figure 4: 1 atm for each gas, namely metal vapour, F\({}_{2}\)(g), and the product metal fluoride gas. The input values applied for the Gibbs free energy value calculations displayed in Figure 5 were: 1 atm CaF\({}_{2}\)(g) unit activity for the molten metal oxides, 0.10 atm for the metal fluoride gas formed, and 0.01 activity for the CaO formed from reaction (10). From the standard Gibbs free energy values displayed in Figure 4, the formation of CoF\({}_{2}\)(g) was found to be least likely among the metal vapour reactions with F\({}_{2}\)(g). However, in comparison to the Gibbs free energy values in Figure 5, all reactions of type (9) were more likely to occur than reactions of type (10). Therefore, the reaction of CoO with CaF\({}_{2}\)(g) was less likely than the reaction of Co(g) with F\({}_{2}\)(g) to form CoF\({}_{2}\)(g). A multitude of reactions are possible at the high temperatures prevailing in the arc cavity, which is the case for the metal vapor. Figure 4: Standard Gibbs free energy for metal vapour and F\({}_{2}\) gas reactions as calculated in FactSage 7.3 [31]. Figure 5: Gibbs free energy for metal oxide and CaF\({}_{2}\) gas reactions as calculated in FactSage 7.3 [31]. beyond the simple reactions of types (9) and (10) discussed above, and the following thermochemical simulation calculations were made. The Equilib module in FactSage 7.3 was used to calculate the gas-slag-metal-powder equilibrium between the molten flux (slag) and the alloy powders. The FToxid, FStel, and FactPS databases were selected, with the inclusion of plasma species [31]. The flux input composition was that shown in Table 2, and the flux mass input was equal to slag mass measured for the post-weld slag. The cobalt input mass was 7 g as feed into the weld run. The aluminium input mass was varied between zero and the maximum of 6.5 g aluminium, since the mass balance calculations showed that 0.50 g of the 7 g of aluminium input was consumed in reactions (2) and (3).