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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.One of the best alloys regarding corrosion resistance at high temperature is Fe-Al. To maintain good oxidation resistance both the oxygen penetration through the oxide layer to the alloy and iron diffusion to the surface should be kept blocked. However, this blocking to be effective the Al concentration in bulk should be at least 10-15 at. %. 1 Unfortunately, this amount of Al makes the alloy very brittle consequently limiting the usability of Fe-Al in applications requiring ductile materials. However, the brittleness problem can be solved by adding, e.g., chromium to the alloy. Cr considerably improves the formation of the protective Al 2 O 3 scale so that the amount of Al can be reduced to an acceptable level ensuring reasonable mechanical properties for the alloy.Chromium addition to Fe-Al is an example of a more general so-called third element effect ͑TEE͒ frequently discussed in literature. 2 Adding 10 at. % Cr to Fe-Al, for instance, allows to reduce the Al concentration to 3 at. % without weakening the protecting layer at the surface. 2 Several phenomena have been proposed to be responsible for the observed TEE in Fe-Cr-Al alloys. 3 Early theories associated this effect to a transition from internal to external oxidation. 4Later, it was argued that Cr can inhibit the external oxidation of Fe. 2 Most recently, Cr 2 O 3 was considered as nucleation centers for the ␣-Al 2 O 3 , which may improve corrosion protection in contrast to the other transient aluminas. 5 At low temperatures Cr improves the oxidation resistance of FeCr-Al due to the fast formation of Cr 2 O 3 compared to that of ␣-Al 2 O 3 . 6,7 In this Brief Report, using a first-principles quantum-mechanical approach, we shed light on another possible mechanism responsible for the TEE in Fe-Cr-Al system. The proposed mechanism takes place under the surface through the interface between two different bulk regions. The first one is the deep bulk and the other one is the surface zone where the concentrations differ from those of the deep bulk due to, e.g., surface oxidation. However, it is worth to note that the proposed phenomenon is in principle quite general expected to appear in many ternary alloys whenever suitable conditions prevail.Our computer modeling is based on the density-functional theory 8 in combination with the generalized gradient approximation. 9 The Kohn-Sham equations were solved using the exact muffin-tin orbitals ͑EMTO͒ method. [10][11][12][13] The EMTO method ...