## 4 Discussion
Therefore, the reaction of NiO with CaF\({}_{2}\)(g) is less likely than the reaction of Ni(g) with F\({}_{2}\)(g) for NaF\({}_{2}\)(g). Therefore, the likely reaction sequence for the application of aluminium and nickel powders in an unconstrained format in SAW is shown in Figure 6. Reaction steps A to E are as presented in previous works [6; 31; 32; 33; 34]. It was conclusively shown that an excessive quantity of oxygen is initially added to the molten weld wire droplets from the arc cavity gas phase, up to 2000-3000 ppm O [31; 32]. 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 to lower temperatures from the high arc plasma temperatures, typically 2000-2500 \({}^{\circ}\)C [2; 3; 33]. This initial excessive oxygen  
Figure 4: Standard Gibbs free energy for metal vapour and F\({}_{2}\) gas reactions as calculated in Fact-Sage 7.3 [30]. Figure 5: Standard Gibbs free energy for metal oxide and CaF\({}_{2}\) gas reactions as calculated in FactSage 7.3 [30]. quantity in the metal droplets is sourced from the decomposition of less stable oxides at the high temperatures prevailing in the arc plasma, and so flux chemistry is used to manage oxygen transfer to the weld metal from the arc plasma [6]. Based on the arc plasma stability hierarchy of oxides, MnO, SiO\({}_{2}\), MgO, and Al\({}_{2}\)O\({}_{3}\) contained in the flux, as presented in Table 2, may all dissociate in the arc plasma to release oxygen in the arc cavity to adsorb onto the molten weld wire droplets, reactions A to C in Figure 6[6]. It is clear from previous work 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, reaction E. This FeO is incorporated into the slag. The correlation of increased FeO in the molten flux with increased weld metal ppm O is well known [17; 35; 36; 37; 38; 39]. The oxygen potential prevailing at the molten flux-weld pool interface, as presented by the quantity of FeO in the slag, is reduced in this work by adding Al powder. Similar to reaction G for the reduction of MnO from the molten flux, FeO is also reduced by aluminium, see reaction (4) in the text. These aluminium based reduction reactions, the reactions as displayed in Equations (2)-(4) in the text, are exothermic and therefore release chemical energy in the form of heat into the weld pool. This extra added heat can be used to melt and dissolve nickel powder into the weld pool. The reduced oxygen potential at the molten flux-weld pool interface prevents oxidation of Ni powder to NiO, and so prevents loss of Ni to the slag. Instead, the Ni powder is melted and dissolved into the weld pool, see reaction I. Since there is an excess of Al added, some of the Al also dissolves into the weld pool, see reaction H. Based on the standard Gibbs free energy values displayed in Figures 4 and 5 it appears that Ni loss in SAW with metal powder alloying occurs due to Ni vaporisation and/or subsequent reaction of Ni vapour with Fluorine gas to form NiF\({}_{2}\)(g). This reaction sequence is marked in red text as reactions J, K, and L in Figure 6. Nickel can be vapourised at the arc plasma-weld pool interface from the weld pool and from the unconstrained Ni powder, before it is dissolved into the weld pool. The F\({}_{2}\)(g) formation from the dissociation of CaF\({}_{2}\)(g) in the arc plasma  
Figure 6: SAW reaction flow diagram with Al and Ni powder additions. 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; 34]. This reaction sequence may explain why Ni element transfer in SAW as reported in the literature is lower when Ni is alloyed in the form of pre-alloyed Cr containing powders. Cr or Al de-oxidisers added with Ni powder or pre-alloyed powder ensure that the Ni remains as Ni metal to vaporise easily in the arc cavity.