Evaluation of the scale integrity in the Al2O3/MA956 system at different polarisation by using EIS

Abstract

In this work anodic polarisation test and electrochemical impedance spectroscopy (EIS) are combined to evaluate the integrity of the alumina layer generated “in situ” at high temperature (1100°C/3 h) on MA956 alloy. The SEM observations did not show the presence of voids or porosity in the alumina layer and a good adherence was revealed with the metallic substrate. The electrochemical response of the scale/metal system has been evaluated at different points of the anodic polarisation curve. In general, a polarisation applied to the sample, below the breakdown potential, did not affect the EIS results, irrespective of the polarisation withstood by the system. In other cases, besides the slightly higher current density measured during the anodic polarisation test, a decrease in the impedance modulus was observed. This behaviour could be related to the presence of microdefects in the scale, not visible by SEM, which become increasingly and/or connected during the polarisation test. The very low corrosion current densities in the passive region, about 10−9 A/cm2, and the high polarisation that the alumina layer withstands till breakdown occurs confirm the excellent corrosion protection provided by the scale. Once breakdown of the alumina layer occurs, a good repassivation of the system is observed due to the high chromium content of the alloy.

Evaluation of different sealing methods for anodized aluminum‐silicon carbide (Al/SiC) composites using EIS and SEM techniques

“…5) were not obtained. In the case of pure aluminum the porous layer exhibits a hexagonal cell structure with a central pore in each cell, in which the pores are well spaced as parallel columns perpendicular to the surface [14,15,24]. For the Al/SiC composite, these pores probably are forced to go around the reinforcing particles causing high distortion and leaving defects in the columnar growth film structure.…”

“…Lightweight aluminum alloys matrices reinforced with silicon carbide (SiC) particles possess higher tensile strength and ductility at 300-400 8C and lower coefficient thermal expansion, but the use of these materials has been limited partially because they suffer from localized corrosion in the form of pitting in solutions containing chloride [6][7][8][9]. Aluminum alloys are usually chromated, painted, oxidized, or anodized in chromic, phosphoric, or sulfuric acid solutions to improve their corrosion resistance under a variety of aggressive atmospheric conditions [10][11][12][13][14].…”

“…In applications where the anodized aluminum is exposed to corrosive atmospheres sealing of the anodic oxide films is required because the porosity of the oxide layer decreases the corrosion resistance of the alloy. Industrially, this has traditionally been done by immersion in boiling deionized water, a method known as hot water sealing (HWS) [14]. This method becomes uneconomical due to the high temperature that uses substantial amounts of energy and is subject to health problems when chromates (Cr þ6 ) are involved [18,19].…”

Evaluation of liquid metal embrittlement susceptibility of oxide dispersion strengthened steel MA956

“…Although important in many other material properties of ODS alloys, the yttrium in the original Y 2 O 3 powders and subsequent Y-Al-O particles do not play a significant role in the corrosion resistance of ODS steels, and the aluminum concentration in these alloys (nominally 4.5-5.5 wt%) is sufficient to form both the Y-Al-O particles in the metal matrix and the protective Al 2 O 3 outer layer [1]. The general corrosion resistance of MA956 and other similar aluminum-containing ODS alloys has been demonstrated to be good in a wide variety of environments including air [2], super-critical water (SCW) [1,3], lead bismuth eutectic (LBE) [4][5][6][7], and molten salts [8][9][10]. In each of these environments, a thin, tightly adherent Al 2 O 3 protective oxide layer(s) formed on the surface protecting the alloy from corrosion and oxidation.…”

“…In each of these environments, a thin, tightly adherent Al 2 O 3 protective oxide layer(s) formed on the surface protecting the alloy from corrosion and oxidation. Results by several authors have shown that the oxide layer(s) formed on MA956 (and similar ODS alloys with aluminum concentrations of 3.5 wt% or greater) are thin and tightly adherent at a range of temperatures up to the processing temperature of the alloy (nominally 1100°C) for hundreds of hours [2,3,6,7,11,12]. The same protective oxide layers are not formed in ODS alloys without aluminum (such as MA957, 12/14 YWT, and 9/12 Cr-ODS), and the general corrosion resistance of these alloys has been shown to be inferior to that of aluminumcontaining ODS alloys [1,[5][6][7]13].…”