Ali Soleimani-Dorcheh

Oxidation-Nitridation of Chromium at High Temperatures and its Mitigation by Alloying


Front	Back

ISBN:

978-3-8440-5078-3

Series:

Schriftenreihe des DECHEMA-Forschungsinstituts
Herausgeber: DECHEMA-Forschungsinstitut
Frankfurt am Main

Volume:

Keywords:

Chromium; Nitridation; Oxidation; High Temperature; Cr-Si; Cr-based alloys

Type of publication:

Thesis

Language:

English

Pages:

132 pages

Figures:

58 figures

Weight:

186 g

Format:

21 x 14,8 cm

Bindung:

Paperback

Price:

45,80 € / 57,30 SFr

Published:

February 2017

Buy:

Download:

Available PDF-Files for this title:

You need the Adobe Reader, to open the files. Here you get help and information, for the download.

These files are not printable.


	Document		Document
	Type		PDF
	Costs		11,45 EUR
	Action		Purchase in obligation and display of file - 26,0 MB (27335548 Byte)
	Action		Purchase in obligation and download of file - 26,0 MB (27335548 Byte)


	Document		Table of contents
	Type		PDF
	Costs		free
	Action		Diplay of file - 106 kB (108912 Byte)
	Action		Download of file - 106 kB (108912 Byte)


	Document		Abstract
	Type		PDF
	Costs		free
	Action		Diplay of file - 232 kB (237193 Byte)
	Action		Download of file - 232 kB (237193 Byte)

User settings for registered users

You can change your address here or download your paid documents again.

User:	Not logged in.
Actions:	Login / Register Forgotten your password?

Recommendation:

You want to recommend this title?

Review copy

Here you can order a review copy.

Link

You want to link this page? Click here.

Abstract

This work investigates oxidation and nitridation in the binary Cr-Si system. Kinetics and thermodynamics of oxidation in this alloy system have been investigated with a systematic approach. Oxidation and nitridation behavior of each individual phase (solid-solution phase, Crss and silicide phase, A15) were studied separately by investigating the oxidation and nitridation kinetics for short and long term exposures, thermodynamic stability, and post-exposure characterization of the scale, and subscale morphologies. Results revealed that in both phases chromium is the predominant element in the oxidation process which made such alloys major chromia formers. It was found, however, that the role of localized SiO₂ formation in reducing oxidation kinetics is significant. The oxidation rate of chromium was reduced by more than an order of magnitude upon the addition of only 3 at.% Si. The impact of SiO₂ increased further with increasing Si content. The A15 Cr₃Si silicide phase produced a continuous SiO₂ layer at long term exposures. When combined in a two-phase eutectic alloy, both solid-solution and A15 silicide phases oxidized cooperatively via primary depletion of chromium which led to the dissolution of the Crss phase and formation of an A15 layer at the alloy-scale interface. The influence of nitrogen as an important oxidant in air was investigated for pure chromium and the binary Cr-Si alloys and it was shown that a chromium subnitride layer exclusively grew via inward diffusion of nitrogen. The positive role of Si in hindering nitridation was significant. It was revealed that the A15 silicide is stable at any nitrogen pressure at high temperatures and showed almost no solubility for nitrogen. As a continuous barrier in the subsurface region of the two phase eutectic Cr-Cr₃Si alloy, it offered a self-protecting character against internal nitridation. Further development of this alloy system was conducted after exploring the ternary Cr-Ge-Si system in the high chromium range (Cr > 80 at.%). Si and Ge showed interchangeable solubility in both solid-solution Crss and A15 phases, and addition of Ge stabilized the A15 phase by supporting the peritectic reaction in the Cr-Cr₃Ge system. The microstructure of the eutectic alloy remained fine-lamellar when up to 2 at.% Ge was added. Using this approach, nitridation, as the most important challenge in the development of chromium alloys, was significantly improved as the alloy microstructure remained unaffected from internal nitridation for a period of 1000 hours oxidation at 1200°C in air. Finally, the optimized oxidation behavior of binary and ternary alloys was discussed with regards to the morphology and adhesion of the chromia scale.