## 4 Discussion
This initial excessive oxygen quantity in the metal droplets is due to the decomposition of less stable oxides at the high temperatures prevailing in the arc plasma. Therefore, flux chemistry is used to manage oxygen transfer to the weld metal from the arc plasma [7]. Considering the established hierarchy of oxide stability in the arc plasma, the MnO, SiO\({}_{2}\), MgO, and Al\({}_{2}\)O\({}_{3}\) contained in the aluminate basic flux used in this work may all have dissociated in the arc plasma to release oxygen in the arc cavity. This oxygen was then adsorbed onto the molten weld wire droplets for transfer to the molten weld pool via the arc-plasma-weld-pool interface; see reactions A to C in Figure 7[7]. Previous works have shown that the excessive quantity of oxygen transferred from the arc cavity reacts with the molten steel at the arc-plasma-weld-pool interface to form FeO; see reaction E in Figure 5.3 This FeO is incorporated into the slag. The correlation of increased FeO in the molten flux with increased weld-metal ppm O has been well illustrated in previous works [34; 35; 36; 37; 38; 39; 40]. In this study, the oxygen potential prevailing at the flux-weld-pool interface, as represented by the quantity of FeO in the slag, was reduced by adding aluminium powder. In the same type of reaction as reaction G in Figure 7, namely reduction of MnO from the molten flux by reaction with aluminium, the FeO formed from excess oxygen in the weld pool may also be reduced by aluminium; see reaction (4) in the text. These aluminium-based reduction reactions, as displayed in Equations (2)-(4) in the text, are exothermic and therefore release chemical energy into the weld pool in the form of  
Figure 5.4 Standard Gibbs free energy for metal vapour and F\({}_{2}\) gas reactions of Al and Co as calculated in FactSage 7.3 [31]. heat. This effect was quantified in Section 3.2 above. The extra added heat can be used to melt and dissolve cobalt metal powder into the weld pool. The reduced oxygen potential at the flux-veld-pool interface prevented oxidation of cobalt powder to CoO, and so prevented loss of cobalt into the slag. Instead, the cobalt powder melted and dissolved into the weld pool; see reaction I. Since there an excess of Al was added, some of the Al also dissolved into the weld pool; see reaction H. Based on the Gibbs free energy values displayed in Figures 4 and 5 it appears that cobalt loss in SAW with metal powder alloying occured due to cobalt vaporisation and/or subsequent reaction of cobalt vapour with fluorine gas to form CoF\({}_{2}\)(g), and reaction of CoF\({}_{2}\)(g) with aluminium vapour according to the reaction displayed in Equation (11). This reaction sequence is marked in Figure 7 in blue text, as reactions J, K, and L, and in orange text for reaction with aluminium vapour, reaction M. Cobalt can be vapourised at the arc-plasma-weld-pool interface from the weld pool and from the unconstrained cobalt powder, before it is dissolved into the weld pool. This conclusion is confirmed from the FactSage Equilib gas-slag-metal powder calculations summarised in Figure 5.5 Formation of F\({}_{2}\)(g) from the dissociation of CaF\({}_{2}\)(g) in the arc plasma appears possible, since the Ca and F were analysed in the arc cavity gas phase when a CaF\({}_{2}\)-based flux was used in SAW [1, 41]. The added aluminium powder played a role in gas phase reactions by shifting cobalt to vapour instead of oxidation to CoO, as discussed in the above thermodynamic analysis. However, the final arbiter of the oxidation state of cobalt is the partial oxygen pressure at the flux-weld-pool interface, because the final influencing reactions takes place at this interface as the weld pool cools from the high arc plasma temperatures to its solidus temperature.