Predicting Minimum Al Concentrations for Protective Scale Formation on Ni‐Base Alloys: I . Isothermal Oxidation

Ternary A-B-C alloys, where A is the most-noble and C the most-reactive component, exposed to oxygen pressures above the stability of the least stable oxide AO (high-oxidant pressures) may present two different types of internal oxidation, i.e. either a coupled internal oxidation of both B and C beneath an external AO scale or a single internal oxidation of C beneath an external scale of the oxide of B. This paper examines the conditions required to avoid the coupled internal oxidation of B plus C beneath external AO scales, considering both the case of formation of a single and a double front of internal oxidation. The analysis, based on an extension to ternary alloys of the criterion defined by Wagner for the transition between the internal and external oxidation of the most-reactive component of binary alloys, shows that the addition of B to binary A-C alloys is very effective in reducing the C content needed for this transition in comparison with binary A-C alloys. This results provides a basis for a possible explanation of the third-element effect. However, the actual possibility of its occurrence depends also on the ability to avoid other oxidation modes, not examined here.

Section: Single Front Of Internal Oxidationmentioning

confidence: 99%

mentioning

confidence: 99%

The Internal Oxidation of Ternary Alloys. VIII: The Transition from the Internal to the External Oxidation of the Two Most-Reactive Components Under High-Oxidant Pressures

Niu

Gesmundo

2006

“…While Eqs. (18) and (19) have been selected to fulfill the above criterion requiring k = 1 when r = 0 and r = 1, their form is completely arbitrary and is not expected to correspond accurately to the behavior of real systems exhibiting the presence of two iof. These equations have been chosen only to highlight the possible effects of the presence of two iof as compared to the single iof solution.…”

Section: Double Front Of Internal Oxidationmentioning

confidence: 99%

“…Furthermore, the critical concentration of each reactive component for this transition decreases continuously as the corresponding concentration of the other reactive component increases, even though the precise form of this relation This section calculates the critical values for the present transition in the oxidation of ternary Ni-Cr-Al alloys at 1200 o C, for which experimental results are apparently not available. The relevant data are taken from various sources, but are mostly those quoted in a paper concerning a prediction of the critical-Al content needed to form external alumina scales on ternary Ni-Cr-Al alloys by Guan and Smeltzer 18 or in a paper on the same subject by Nesbitt,19 i.e., D O in Ni = 7.6 Â 10 )8 cm 2 s )1 ; 18,20 D Al in Ni = 5.0 Â 10 )10 cm 2 s )1 . 18,21 and D Cr in Ni = 3.9 Â 10 )10 cm 2 s )1 .…”

Section: Double Front Of Internal Oxidationmentioning

confidence: 99%

The Internal Oxidation of Ternary Alloys. VII: The Transition from the Internal to the External Oxidation of the Two Most-Reactive Components Under Intermediate Oxidant Pressures

Gesmundo

Niu

2006

The conditions for the transition between the coupled internal oxidation of two most-reactive components and the formation of external scales in the scaling of ternary alloys under oxidant pressures below the stability of the oxide of the most-noble component, denoted as a situation of intermediate oxidant pressures, are examined under a number of simplifying conditions which allow to develop an approximate analytical treatment. If the precipitation of the two oxides occurs at the same front of internal oxidation, the kinetics of internal oxidation as well as the critical B and C contents needed for the transition have a single solution under fixed conditions of all the parameters involved. On the contrary, in the presence of two different fronts, when the most-stable oxide forms at the innermost front, a whole range of possible solutions is predicted. In both cases, the critical-C content needed to avoid the simultaneous internal oxidation of B plus C is progressively reduced by the addition of B. This behavior provides the basis for a possible interpretation of the ''third-element effect''. However, the existence and the magnitude of this effect are complicated by the occurrence of other modes of oxidation for these systems. Thus, a

“…The classical interpretation of the third-element effect is based on the mechanism of secondary gettering [1,12,13]. For the Fe-Cr-Al system, the inclusion of Cr in the initially-formed transient oxide scale is proposed to reduce penetration of oxygen into the alloy by lowering the oxygen partial pressure at the alloy/scale interface such that external rather than internal Al 2 O 3 formation is favored at lower Al concentrations than those observed in binary Fe-Al [1,14].…”

mentioning

confidence: 99%

Sulfidation–Oxidation Behavior of FeCrAl and TiCrAl and the Third-Element Effect

Brady

Tortorelli

et al. 2010

Short-term sulfidation-oxidation exposures were conducted under high pS 2 and low pO 2 conditions for TiCrAl and FeCrAl alloys at 600 and 800°C. Low mass gains and submicron Al-and Ti-rich oxide scales were formed on TiCrAl at 600°C, while high mass gains and FeS-based scale formation were observed for FeCrAl. Based on the good behavior of TiCrAl, third-element effect additions of Cr are not inherently detrimental under sulfidation-oxidation conditions. Rather, differences in the mechanistic action of the third-element addition of Cr between FeCrAl and TiCrAl alloys and its relevance to low oxygen potential sulfidationoxidation environments were the key factors in determining whether or not a protective alumina scale was established.