Passive film aging

Nickel and nickel alloys possess good resistance to sea-water in conditions where the protective properties of the passive film are fully maintained. As pointed out above, Ni-30 Cu Alloy 400, in contrast to its behaviour in acidic solution, normally forms a protective film in neutral and alkaline environments, including sea-water this alloy and its age hardening variant... 

Wieczorek, S.K. and Todd, C.D., Inhibition and facilitation of bryozoan and ascidian settlement by natural multi-species biofilms effects of film age and the roles of active and passive larval settlement, Mar. Biol., 128, 463, 1997. 

The electrochemical properties of passive layers lead to the question of their structure on a mesoscopic scale and at atomic resolution. Their barrier character with respect to metal corrosion postulates a dense, poreless film their electronic properties, in some cases, crystalline structures. The change of their properties with film aging, as in e.g. film-breakdown phenomena, support the existence of many defects that may heal with time. In many cases an amorphous structure is assumed. Some ex situ... 

Ih of passivation [17]. In this study, it was observed that after 1 h of passivation the amount of disorder increased sharply for a Cr content of the alloy between 14.7 and 16.5%. The effect of aging was tested only in ex-situ conditions and it was observed that it favors the (re)crystallization of the passive film for 18 and 21% Cr content [15]. The structural order observed in the passive film was consistent with the (0001) orientation of a-Cr203. In the case of pure Fe sputter-deposited thin films polarized at high anodic potentials in borate buffer solution, the same group reported long-range crystalline order in both ex-situ and in-situ conditions of examination [18]. 

It appears firom these studies that both the Cr content of the alloy and the conditions of aging of the film are critical ctors ruling the crystallization of the passive film. 

Figure 7. STM images of the passive film formed on Fe-22Cr(l 10) in 0.5M HjSO at +500 mV/SHE and aged 63 hours under polarization. Reproduced with permission from reference (4).

The major role of dehydration (deprotonation or dehydroxylation) was first emphasized by Okamoto (1973). Recent results by XPS and STM (Maurice et al., 1994a, 1996,1998 a) have provided direct evidence of this process which occurs during the aging of passive films (see Sec. 3.7.2). 

Structural Aspects of Chromium Enrichment in Passive Films on Stainless Steels in Aqueous Solution Aging Effects... 

Studies of chemisorption of gaseous molecules on oxides may provide new insights into the various processes involved in the formation, aging, and degradation of passive films. We will restrict ourselves to a brief description of the main features of the chemisorption behavior on oxides. We will focus on results obtained on well-defined surfaces of single crystals with O2 and H2O as adsorbates, hr the past 10 years, precise data on the structure and the electronic properties of surfaces of single crystals of oxide have been obtained by LEED and surface-sensitive spectroscopies (UPS, XPS, HEELS). Excellent introductions to this subject can be found in Refs. Ill and 112. A large part of the data presented here is extracted from these reviews. 

The improvement of the pitting resistance by aging the passive surface in the corrosive medium itself but in conditions under which no pitting occurs is also a matter of practical evidence on a much larger time scale (of the order of some months). It is believed that this improvement is due to some passive film modifications. This question forms the subject of some current work [3d]. 

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