Surface passivation pretreatments

Etch primers partially fulfil the roles of both pretreatment and primer. They contain phosphoric acid for surface passivation and are based on polyvinyl butyral ... 

The sputtering time required for the atomic composition of the surface layer to reach that of the unetched, i.e., virgin PBTMSS depended on the passivation pretreatment process. The time increased in the order SME > high bias > barrel reactor reflecting the differences in thickness of the oxide layer. These results are also in qualitative agreement with the IR results presented in Figure 6 and Table II. 

The cleanliness of the surface produced by emulsifiable cleaners is rarely of a very high standard, and additional cleaning may well be necessary before further finishing operations. Success has been achieved, however, in the use of these products prior to some immersion phosphating operations, where the crystal growth can be quite refined due to the absence of the passivation effect often encountered with some heavy-duty alkali cleaners. The supplier of the phosphating solution should be asked to advise on the suitability of any particular cleaning/pretreatment combination. 

A1 is thermodynamically unstable, with an oxidation potential at 1.39 V. Its stability in various applications comes from the formation of a native passivation film, which is composed of AI2O3 or oxyhydroxide and hydroxide.This protective layer, with a thickness of 50 nm, not only stabilizes A1 in various nonaqueous electrolytes at high potentials but also renders the A1 surface coating-friendly by enabling excellent adhesion of the electrode materials. It has been reported that with the native film intact A1 could maintain anodic stability up to 5.0 V even in Lilm-based electrolytes. Similar stability has also been observed with A1 pretreated at 480 °C in air, which remains corrosion-free in LiC104/EC/ DME up to 4.2 However, since mechanical... 

A distinctive dissimilarity between the passivation of Al and the formation of an SEI on a graphitic anode is the effectiveness of surface pre-formation while it has been reported in numerous studies that an SEI formed in one electrolyte can continue to protect the graphite structure when the anode is transferred to another exfoliating electrolyte (such as p i xhat has been pretreated in LiPFe-... 

Comparison of the selectivity toward alcohols in the FT reaction of W03, WC, W2C and the XPS and CO adsorption results provide information on the specificity of active sites on each catalyst. W2C favors CO dissociation and formation of hydrocarbons with excess methane and C02 without oxygenate production. It has a more pronounced metallic character than WC perhaps because its metal atoms are in a close-packed arrangement. After pretreatment in flowing H2 at 673 K before FT reaction, part of the surface oxygens (abundant after passivation) probably reacts with carbidic carbon and are removed from the uppermost surface layer. Moreover,... 

The nature of the film formed by y-APS on steel is strongly dependent on the pretreatment of the steel surface. This in itself is not surprising and has been documented in the literature. However, the TOFSIMS delta indicate that methanol-cleaned, mechanically polished steel is passivated because it has formed iron soaps by reaction with fatty acids in the polishing process. 

Offline passivation involves treatment of equipment currently out of service. Treatment levels are typically higher consequently, passivation is completed more quickly. Passivation of nonchromate treatment generally uses either a polyphosphate, zinc, molybdate or other nonchromate-based inhibitor in combination with various surface-active cleaning agents. The passivation solution should be disposed of after the pretreatment stage, rather than dumped back into the cooling system where the potential for fouling can exist due to the precipitation of pretreatment compounds such as zinc or phosphate. Table 8.1 outlines both online and offline pretreatment procedures. 

The activity of cathodes may differ also for different activation procedures applied to the surface. In other words, electrodes as prepared may be inactive, or insufficiently active, and a sort of etching is needed before they exhibit the expected activity. The activation usually involves the removal of a passivating layer formed during the preparation. This is the case of Ti or Zr containing intermetallics [89, 152]. The surface is initially passive due to a TiOz or Zr02 overlayer, and can be activated by means of HF. The duration and the procedure of the pretreatment will influence the final activity by producing a surface of different composition. 

Prevention of General Corrosion. Proper selection of materials. In design, a metal or alloy that forms a stable passive film should be recommended. A surface pretreatment in oxidized solutions has been adopted for stainless steels and is recommended in many circumstances. The most popular process a 300 min immersion in a 20 vol% nitric acid at 50°C is recommended for some types of stainless steels.21 The environment can be modified in the bulk and should be effective at the interface in adding oxidizing agents, such as nitrite or strong nitric acid, that maintain the passive state on some metals and alloys.8... 

For all metal reactors, the yields of ethylene and total coke were often found to vary significantly with the past history or pretreatment of the reactor. Pretreatments used were with either oxygen, steam, or hydrogen sulfide. Total coke is defined here as the sum of the CO, C02, and net coke it is postulated that CO and C02 were formed by oxidation of part of the coke which formed and that net coke is the amount of coke left on the reactor walls at the end of the run. The results for runs M01 and D44 are one example of the large differences in yields that can be obtained in reactors of the same material of construction (see Table I). Run M01 was made after a stainless steel 304 reactor had been treated with hydrogen sulfide. Hydrogen sulfide results in the formation of metal sulfides (8) and acts in most cases to passivate the surface so that coke formation is minimized. D44 was made using a new stainless steel 304 reactor, whose surface became very active when it was treated with steam. 

These experiments demonstrated that the passivating qualities of steam pretreatment may be partially associated with the generation of a silica-rich oxide. However, brief attempts to increase the surface concentration to approach a continuous silica layer excluding all the alloying metals (by variations in preoxidation conditions) met with failure, exhibiting bulk inhomogeneity and oxide spallation problems. Further, many ESC plants are constructed with radiant coil alloys low in silicon content, and so preoxidation can only be considered a partial palliative. Hence, attention turned to the prospect of coating the internal surfaces of radiant coils with a thin, continuous layer of silica. 

The influence of the acid pretreatment of BEA on ifs acfivity in the model reaction between 2-MN and AAN was analyzed in depth/ The contribution of the inner and outer surfaces of the catalyst was examined by considering the selectivity with respect to the bulky product 28 and the linear product 29, which are assumed to be formed on fhe outer and on both the inner and outer surfaces of fhe catalyst, respectively/ " It is shown that the production of EFAL species located in the micropores of BEA subjected to high heating rate calcination provokes the increase in the selectivity of the less hindered 29 because the formation of the bulky 28 is sterically hampered. Indeed, when the external surface of zeolite BEA is passivated by coating with amorphous silica, a significant increase in the selectivity of 29 is observed, and fhis resulf is a clear example of shape-selective acylation with zeolite catalyst. On the contrary, acid treatment increases the catalytic activity of fhe oufer surface due to the extraction of the catalytically active EFAL species out of the micropores, leading to the preferential formation of fhe bulky 28. 

---