Passivation spinels

Nickel aluminate, a spinel, has long been known to trap nickel. Metals like arsenic(19), antimony(20-21) and bismuth(20) are known to passivate transition elements and can be used to decrease and coke make. Sulfur is also a known inhibitor for nickel therefore, higher sulfur-containing crudes may be a little less sensitive to nickel poisoning. In our work we also found that nickel at low concentrations is actually a slight promoter of the cracking reaction when incorporated into a molecular sieve (Figure 17). 

Stainless steels, all of which contain at least 11% Cr, owe their usual passivity to an oxide layer that can be approximately formulated as FeCr204, or (if nickel is also present) as (Ni,Fe)Cr2C>4. These spinel-type oxides are similar to the mineral chromite (Section 4.6.2), which is extremely insoluble in aqueous media. Again, however, the oxide film may be weakened or lost under strongly reducing conditions, and the stainless steel may then become active. This problem is analyzed in electrochemical terms in Section 16.6. 

The formation of the passive layer on Co has been followed also in the time domain [58], During the first seconds, Co(II) forms predominantly, even in the range of secondary passivity, but disappears gradually within several minutes, whereas Co(III) increases at the same extent. Simultaneously, hydroxide is replaced by oxide, although finally it is still present up to 30 %. Again, the formation of Co(III) is accompanied by the appearance of the spinel signal. 

The presence of a hydrated film (FeOOH) was found to depend on the concentration of iron cations at the passive layer-solution interface [64]. In situ surface X-ray diffraction studies indicated that the film consists of a spinel crystal structure [65]. 

The TM ions usually used for passive g-switching of infrared solid-state lasers include Cr" " (940-1100 nm range), (1.30-1.35 pm), and Co " (1.3-1.6 pm). Although aU these ions can be doped in cubic materials, i.e., Cr and in garnets and Co " in spinels, only Cr YAG ceramics have been reported for ceramic SA g-switches [37, 42, 132, 133]. The SA materials are usually used as components together with the Nd laser-active materials in the cavity. In this case, if the laser materials and the g-switch materials have a same host, such as Nd YAG lasers passively g-switched with Cr" YAG, the difference in refractive index between them should be very small, so that there is no reflection at the interface when the two materials are intimately joined. For single crystals, they can be either diffusively bonded or mechanically clamped. In contrast, when one or both of them... 

Low-energy electron-diffraction (LEED) techniques are used to detect adsorbed films, including those responsible for passivity. Surface analytical techniques that can be used to study films on metals include Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and others [18,19]. Using these advanced techniques, evidence has been developed to show that the passive film formed on iron in borate solution is an anhydrous, crystalline spinel with a Y-Fc203/Fe304-like structure [19]. 

Normal steam cycle environments are not corrosive. They passivate the cycle component surfaces and promote formation of protective oxides such as magnetite, hematite, and spinel. Pure, low-oxygen hot water and steam are good passivating agents, and where impurities are present, alkahzmg agents such as ammonia, sodium phosphate, and other water treatment chemicals are added [4,5,11-14 ]. 

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