Third element effect in the surface zone of Fe-Cr-Al alloys

The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.

Figure 1

(Color online) Schematic figure of the surface of an Fe-Cr-Al alloy. The surface zone tends to deplete from Al and Cr due to oxidation. ${\varphi }_{\text{O}}$ and ${\varphi }_{\text{X}}$ are the fluxes of oxygen and metal atoms, respectively. The superscripts b and z refer to the flux from deep bulk to the surface zone and from surface zone to the oxide scale, respectively.

Figure 2

(Color online) The calculated chemical potential difference ${\mu }_{\text{Fe}}-{\mu }_{\text{Al}}$ for Fe-Cr-Al as a function of Cr and Al content. The curves are polynomial fits to the calculated data points shown in the figure. The zero level corresponds to –2059.919 Ry.

Figure 3

(Color online) Schematic plot of ${\mu }_{\text{Fe}}-{\mu }_{\text{Al}}$ as a function of Cr content with Al content as a parameter. The Al content increases from curve to curve downward. By the sequence of the arrows we illustrate both the changes in the concentrations and ${\mu }_{\text{Fe}}-{\mu }_{\text{Al}}$ within the surface zone during the Al and Cr depletion and the subsequent Al recovering of the surface zone (SZ). ${\mu }_{\text{Fe}}-{\mu }_{\text{Al}}$ in three different stages are: 1) initially in the SZ: orange circle; 2) Al- and Cr-depleted SZ: head of the red arrow (upward); 3) Al recovered SZ: head of the green arrow (downward). Second process in cascade: the dashed arrows.

Figure 4

(Color online) If SZ is depleted with Al and Cr, process 1 (upward arrow in Fig. 3), the driving force for $\text{Fe}\leftrightarrow \text{Al}$ exchange between deep bulk and SZ increases, process 2 (downward arrow in Fig. 3).