3.5 Redox Thermodynamics, Deoxidation & Slag-Metal Refining

3.3 Oxidation-Reduction Kinetics, Slag Basicity & Flux Reactions

During arc welding with flux-shielded processes (SMAW, SAW, FCAW, ESW), the molten metal droplet and weld pool interact with surrounding gas and molten slag across brief reaction intervals ($0.1\text{ to }2.0\text{ seconds}$). The chemical reactions that take place during these high-temperature contact periods govern the weld metal's cleanliness, oxygen content, inclusion morphology, and mechanical properties.

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1. Oxidation-Reduction Thermodynamics in Arc Welding

Oxidation represents the loss of electrons (increase in oxidation state), while reduction represents the gain of electrons (decrease in oxidation state). In pyrometallurgical arc reactions, these processes occur at liquid metal-gas and liquid metal-slag interfaces:

$$\text{Oxidation: } M \longrightarrow M^{n+} + n e^- \qquad \text{Reduction: } \frac{1}{2} \text{O}_2 + 2 e^- \longrightarrow \text{O}^{2-}$$

Gibbs Free Energy and Ellingham Diagrams

The thermodynamic driving force for the oxidation of any metallic element is defined by the change in standard Gibbs free energy ($\Delta G^\circ$):

$$\Delta G^\circ = \Delta H^\circ - T \Delta S^\circ$$

where $\Delta H^\circ$ is enthalpy change and $\Delta S^\circ$ is entropy change.

Harold Ellingham plotted $\Delta G^\circ$ versus temperature $T$ for the oxidation reactions of various elements, normalized to the consumption of one mole of diatomic oxygen gas ($1\ \text{mol } \text{O}_2$):

$$\frac{2x}{y} M + \text{O}_2(g) \rightleftharpoons \frac{2}{y} M_x\text{O}_y(s/l)$$

Ellingham Line Characteristic       Thermodynamic Origin                             Welding Engineering Significance
Slope (dΔG°/dT) is Positive         Reaction consumes 1 mole of gas to form a solid/ Slopes are upward; oxide stability decreases
                                    liquid oxide; ΔS° is strongly negative           as temperature increases

Relative Vertical Position          More negative ΔG° indicates higher               Elements positioned lower on the diagram oxidize
                                    thermodynamic stability                          first and act as deoxidizers for higher elements

Change in Slope (Break in Line)     Phase transformation of metal or oxide           Melting or boiling changes the entropy of the system,
                                    (solid → liquid → gas)                           shifting the slope (+ΔS°)

Carbon-Oxygen Line Exception        2C + O₂(g) ⇌ 2CO(g) produces 2 moles of gas      Slope is NEGATIVE; carbon becomes a more potent
                                    from 1 mole of gas; ΔS° is POSITIVE              reducing agent at higher temperatures (> 1500°C)

Oxidation Reaction (at 1600°C / 1873 K)      ΔG° (kJ/mol O₂)   Deoxidation Potency   Primary Oxide Formed in Weldment
4/3 Al + O₂ ⇌ 2/3 Al₂O₃                      -710              Extremely High        Al₂O₃ (corundum), fine pinning inclusions
Ti + O₂ ⇌ TiO₂                               -610              Very High             TiO₂ / Ti₂O₃, nucleates acicular ferrite
Si + O₂ ⇌ SiO₂                               -570              High                  SiO₂ (silica), liquid manganese silicates
2 Mn + O₂ ⇌ 2 MnO                            -480              Moderate              MnO, combines with SiO₂ to form silicates
4/3 Cr + O₂ ⇌ 2/3 Cr₂O₃                      -430              Moderate              Cr₂O₃, forms passive film in stainless
2 Fe + O₂ ⇌ 2 FeO                            -280              Weak (Base Matrix)    FeO (wüstite), drives weld metal oxidation
2 Ni + O₂ ⇌ 2 NiO                            -170              Very Weak             Ni remains primarily in metallic solid solution

Because aluminum, titanium, silicon, and manganese lie significantly lower on the Ellingham diagram than iron, their oxides have much more negative $\Delta G^\circ$ values. Consequently, when present in the weld puddle, these elements preferentially scavenge dissolved oxygen from molten iron, protecting the base metal from oxidation and reducing iron oxide:

$$\text{Si} + 2\text{FeO} \rightleftharpoons \text{SiO}_2 + 2\text{Fe} \qquad \Delta G^\circ_{1600^\circ\text{C}} \approx -290\ \text{kJ}$$

The Carbon-Oxygen Anomaly & The Carbon Boil

The oxidation of carbon to carbon monoxide exhibits a unique thermodynamic trend:

$$2\text{C} + \text{O}_2(g) \rightleftharpoons 2\text{CO}(g) \qquad \Delta S^\circ > 0 \implies \frac{d\Delta G^\circ}{dT} < 0$$

Because one mole of gaseous reactant yields two moles of gaseous product, entropy increases ($\Delta S^\circ > 0$). This gives the $\text{C}/\text{CO}$ equilibrium line a negative slope on the Ellingham diagram. At room temperature, iron oxide is more stable than carbon monoxide. However, above $\sim 720^\circ\text{C}$, the carbon monoxide line plunges below the iron line, and at typical arc welding temperatures ($1600\text{--}2000^\circ\text{C}$), it crosses below manganese and approaches silicon.

This behavior drives the carbon boil reaction in liquid steel:

$$[\text{C}] + [\text{O}] \rightleftharpoons \text{CO}(g) \qquad K_{\text{CO}} = \frac{P_{\text{CO}}}{a_{\text{C}} \cdot a_{\text{O}}}$$

During weld pool solidification, carbon and oxygen are rejected by advancing dendrites into the remaining liquid. When their local concentration product ($[\%\text{C}] \cdot [\%\text{O}]$) exceeds the equilibrium threshold for $P_{\text{CO}} > 1.0\text{ atm}$, carbon monoxide nucleates violently. If the gas cannot escape before the freezing front overtakes it, it forms severe carbon-monoxide wormhole porosity.

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2. Deoxidation Mechanisms: Synergistic Mn-Si Systems

To prevent the carbon boil, welding consumables introduce intentional deoxidizing elements that lower dissolved oxygen ($a_{\text{O}}$) below the critical threshold for $\text{CO}$ bubble nucleation.

Deoxidizer System    Chemical Reactions in Weld Puddle             Activity of Reaction Products    Resulting Dissolved [O]   Weld Metal Cleanliness
Silicon Alone        [Si] + 2[O] ⇌ SiO₂(s)                        a_SiO2 = 1.0 (pure solid)        ~250–350 ppm              Solid silica inclusions,
                                                                                                                             poor flotation
Manganese Alone      [Mn] + [O] ⇌ MnO(s)                          a_MnO = 1.0 (pure solid)         ~400–600 ppm              MnO inclusions, incomplete
                                                                                                                             deoxidation
Combined Mn + Si     x MnO + y SiO₂ ⇌ (MnO)_x·(SiO₂)_y (liquid)    a_SiO2 ≪ 1.0, a_MnO ≪ 1.0        ~150–250 ppm              Liquid silicates, rapid
                                                                                                                             coalescence & flotation
Triple Deoxidized    [Al] + [Ti] + [Zr] + [Si] + [Mn] + [O]       Forms complex oxy-nitride spinels < 100–180 ppm             High density of fine
(Al, Ti, Zr)         ⇌ Al₂O₃·TiO₂·ZrO₂ (solid/liquid)                                                                        acicular ferrite nucleants

The Thermodynamics of Synergistic Mn-Si Deoxidation

When silicon is used alone, it forms solid silica ($\text{SiO}_2$) with unit thermodynamic activity ($a_{\text{SiO}_2} = 1.0$). At $1600^\circ\text{C}$, this equilibrium leaves approximately $250\text{ to }350\text{ ppm}$ of dissolved oxygen in the steel—often insufficient to suppress $\text{CO}$ formation in high-carbon steels or under pure $\text{CO}_2$ shielding.

When manganese and silicon are added together in appropriate ratios (typically $\text{Mn}:\text{Si}$ between $2:1\text{ and }3.5:1$), they react with dissolved oxygen to form liquid manganese silicates:

$$x\text{MnO}(l) + y\text{SiO}_2(l) \rightleftharpoons (\text{MnO})_x\cdot(\text{SiO}_2)_y(l) \quad (\text{rhodonite } \text{MnSiO}_3, \text{tephroite } \text{Mn}_2\text{SiO}_4)$$

Because the reaction products dissolve into each other as a liquid solution, their chemical activities are reduced well below unity ($a_{\text{SiO}_2} \approx 0.1\text{ to }0.3$, $a_{\text{MnO}} \approx 0.1\text{ to }0.2$). In accordance with Le Chatelier's principle, lowering the activity of the reaction products shifts the equilibrium forward:

$$K_{\text{Si}} = \frac{a_{\text{SiO}_2}}{[\%\text{Si}] \cdot [\%\text{O}]^2} \implies [\%\text{O}] = \sqrt{\frac{a_{\text{SiO}_2}}{K_{\text{Si}} \cdot [\%\text{Si}]}}$$

Reducing $a_{\text{SiO}_2}$ from $1.0$ down to $0.1$ lowers the equilibrium dissolved oxygen concentration by more than half, bringing $[\text{O}]$ down to $150\text{--}200\text{ ppm}$. Furthermore, these liquid silicates have low melting points ($1250\text{--}1350^\circ\text{C}$, well below the solidification point of steel at $1530^\circ\text{C}$) and low interfacial energy with liquid iron. This encourages them to coalesce into spherical droplets that float rapidly into the covering slag via Stokes' law:

$$v_{\text{flotation}} = \frac{2 g r^2 (\rho_{\text{metal}} - \rho_{\text{inclusion}})}{9 \mu}$$

AWS Filler Metal Deoxidation Designations

• AWS A5.18 ER70S-3: Contains moderate deoxidizers ($0.45\text{--}0.75\%\ \text{Si}$, $0.90\text{--}1.40\%\ \text{Mn}$). Intended for clean, rust-free base metal under argon-rich shielding gas.
• AWS A5.18 ER70S-6: Formulated with high deoxidizer levels ($0.80\text{--}1.15\%\ \text{Si}$, $1.40\text{--}1.85\%\ \text{Mn}$). Designed for welding over mill scale, light rust, or when using $100\%\ \text{CO}_2$ shielding, where substantial deoxidizer loss occurs across the arc.
• AWS A5.18 ER70S-2: Triple-deoxidized wire containing aluminum ($0.05\text{--}0.15\%$), titanium ($0.05\text{--}0.15\%$), and zirconium ($0.02\text{--}0.12\%$) in addition to $\text{Mn}$ and $\text{Si}$. Designed for out-of-position root passes and contaminated joints where zero porosity is permissible.

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3. Slag-Metal Equilibrium: Desulfurization & Dephosphorization

Sulfur ($S$) and phosphorus ($P$) are detrimental impurities in structural weldments. Sulfur forms low-melting iron sulfide films ($\text{Fe}-\text{FeS}$ eutectic, $T_m = 988^\circ\text{C}$) along solidifying grain boundaries, causing hot solidification cracking. Phosphorus segregates to boundaries, promoting temper embrittlement and hot tearing.

Slag-Metal Desulfurization Thermodynamics

Desulfurization requires transferring sulfur from the liquid metal into the molten slag via an exchange reaction with basic oxide ions ($\text{O}^{2-}$):

$$[\text{FeS}] + (\text{CaO})_{\text{slag}} \rightleftharpoons (\text{CaS})_{\text{slag}} + (\text{FeO})_{\text{slag}}$$

In ionic form:

$$[\text{S}] + (\text{O}^{2-})_{\text{slag}} \rightleftharpoons (\text{S}^{2-})_{\text{slag}} + [\text{O}]$$

The sulfur partition ratio $L_{\text{S}}$ is defined as:

$$L_{\text{S}} = \frac{(\%\text{S})_{\text{slag}}}{[\%\text{S}]_{\text{metal}}} = K_{\text{S}} \cdot \frac{a_{\text{O}^{2-}}}{a_{\text{O}}}$$

To drive desulfurization forward ($L_{\text{S}} \gg 1$):

1. High Slag Basicity: High concentrations of basic oxides ($\text{CaO}, \text{MgO}$) supply free $\text{O}^{2-}$ ions, elevating $a_{\text{O}^{2-}}$.
2. Reducing Conditions (Low Oxygen Potential): Low dissolved oxygen in the metal ($a_{\text{O}}$) and low iron oxide in the slag ($(\%\text{FeO}) < 5\%$) prevent the backward reaction.
3. High Reaction Temperature: Desulfurization is endothermic ($\Delta H^\circ > 0$); high arc temperatures favor sulfur transfer into the slag.

Slag-Metal Dephosphorization Thermodynamics

Dephosphorization requires oxidizing phosphorus into phosphate ions, which are then bound by calcium ions into stable calcium phosphate complexes:

$$2[\text{P}] + 5(\text{FeO})_{\text{slag}} + 3(\text{CaO})_{\text{slag}} \rightleftharpoons (3\text{CaO}\cdot\text{P}_2\text{O}_5)_{\text{slag}} + 5[\text{Fe}]$$

Thermodynamic requirements for dephosphorization:

1. High Slag Basicity: Free $\text{CaO}$ is required to neutralize and stabilize acidic $\text{P}_2\text{O}_5$ as $3\text{CaO}\cdot\text{P}_2\text{O}_5$.
2. Strongly Oxidizing Slag: High $(\%\text{FeO})$ activity is required to oxidize metallic phosphorus.
3. Lower Reaction Temperatures: Dephosphorization is strongly exothermic ($\Delta H^\circ \ll 0$). Elevated temperatures shift the equilibrium to the left, driving phosphorus back into the liquid steel.

Because the arc welding pool operates at very high temperatures ($> 1700^\circ\text{C}$), dephosphorization is thermodynamically inefficient in arc welding. Consequently, phosphorus cannot be reliably removed by flux reactions; it must be kept strictly limited in the base metal and raw consumable wire ($[\text{P}] < 0.015\text{ wt}\%$).

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4. Flux Chemistry, Optical Basicity & The Tuliani Basicity Index

Welding fluxes consist of complex oxide-halide mixtures formulated to regulate slag viscosity, melting range, arc stability, and weld metal oxygen content.

Network Formers vs. Network Modifiers

• Acidic Oxides (Network Formers): $\text{SiO}_2, \text{TiO}_2, \text{Al}_2\text{O}_3$. In molten slags, $\text{Si}^{4+}$ coordinates tetrahedrally with four oxygen atoms to build extended, three-dimensional $[\text{SiO}_4]^{4-}$ polymeric networks. These bridged networks significantly increase slag viscosity, retard chemical reaction kinetics, and promote high weld metal oxygen content.
• Basic Oxides (Network Modifiers / Breakers): $\text{CaO}, \text{MgO}, \text{BaO}, \text{Na}_2\text{O}, \text{K}_2\text{O}, \text{MnO}, \text{FeO}$. These oxides dissociate to donate free oxygen anions ($\text{O}^{2-}$): $$\text{CaO} \longrightarrow \text{Ca}^{2+} + \text{O}^{2-}$$ The free $\text{O}^{2-}$ ions break bridging oxygen bonds ($\equiv\!\text{Si}-\text{O}-\text{Si}\!\equiv$) in the silicate network, depolymerizing the melt into smaller orthosilicate units ($[\text{SiO}_4]^{4-}$). This lowers viscosity, elevates chemical reactivity, and promotes desulfurization.
• Neutral Halides: $\text{CaF}_2$ (calcium fluoride / fluorspar). Fluoride ions ($\text{F}^-$) cleave silicate chains without introducing additional oxygen ions. $\text{CaF}_2$ lowers slag melting point and viscosity while maintaining a neutral oxygen potential.

The Tuliani Basicity Index Formula

The standard formula used in welding engineering to quantify flux basicity was developed by Tuliani, Boniszewski, and Eaton (often referred to as the IIW or Tuliani Basicity Index, $BI$):

$$BI = \frac{\text{CaO} + \text{MgO} + \text{BaO} + \text{SrO} + \text{Na}_2\text{O} + \text{K}_2\text{O} + \text{Li}_2\text{O} + \frac{1}{2}(\text{MnO} + \text{FeO})}{\text{SiO}_2 + \frac{1}{2}(\text{Al}_2\text{O}_3 + \text{TiO}_2 + \text{ZrO}_2)}$$

(Note: All oxide concentrations are inserted directly as weight percentages, $\text{wt}\%$.)

Flux Classification   Tuliani Basicity Index (BI)   Weld Metal Oxygen Level   Slag Detachability & Bead Appearance   Toughness Potential (CVN)
Acidic Fluxes         BI < 1.0                      600 – 1000 ppm            Superb detachability, smooth wash,      Poor; high inclusion density,
                                                                              high speed, cosmetically clean          CVN often < 27 J at 0°C
Neutral / Semibasic   1.0 ≤ BI ≤ 1.2                400 – 550 ppm             Good detachability, moderate speed,     Moderate; general structural
                                                                              balanced operational envelope           fabrication
Basic Fluxes          1.2 < BI < 1.5                300 – 450 ppm             Moderate detachability, more convex     High; meets -20°C to -30°C
                                                                              bead contour                            impact toughness limits
Highly Basic Fluxes   BI > 1.5 (up to 3.0+)         200 – 350 ppm             Stiffer slag, harder detachment in      Exceptional; promotes acicular
                                                                              tight joints, convex bead profile       ferrite, meets -40°C to -60°C

Why CaF2 is Omitted from the Tuliani Formula

A common exam question centers on the role of $\text{CaF}_2$. In the Tuliani equation, $\text{CaF}_2$ appears in neither the numerator nor the denominator. Calcium fluoride is a halide, not an oxide; it supplies $\text{F}^-$ ions rather than basic $\text{O}^{2-}$ ions. While it lowers the melting temperature and dynamic viscosity of the molten slag pool, it does not alter the thermodynamic ratio of basic oxide donors to acidic network formers.

Optical Basicity ($\Lambda$)

Optical basicity, formulated by Duffy and Ingram, measures the electron-donating power of oxide slag systems relative to pure calcium oxide (defined as $\Lambda_{\text{CaO}} = 1.00$):

$$\Lambda = \frac{\sum x_i n_i \Lambda_i}{\sum x_i n_i}$$

where $x_i$ is the mole fraction of oxide $i$, $n_i$ is the number of oxygen atoms in the oxide formula, and $\Lambda_i$ is the individual component's optical basicity (e.g., $\Lambda_{\text{CaO}} = 1.00$, $\Lambda_{\text{MgO}} = 0.78$, $\Lambda_{\text{Al}_2\text{O}_3} = 0.60$, $\Lambda_{\text{SiO}_2} = 0.48$). Slags with $\Lambda > 0.70$ are basic and promote low weld metal oxygen content.

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