Surfaces aluminized

When alloy steels do not give adequate corrosion protection— particularly from sulfidic attack—steel with an aluminized surface coating can be used. A spray coating of aluminum on a steel is not likely to spall or flake, but the coating is usually not continuous and... 

The inner surface of an Incoloy 800 tube had been exposed to aluminum vapors at high temperatures aluminum diffused into the surface, resulting in an aluminized surface. 

Although more information is needed to determine details concerning factors that favor inactive coke formation, relatively high levels of surface sulfides probably promote formation of such coke. On the other hand, metal oxides on the surface likely favor production of active coke. Sulfiding the reactor tube immediately upon completion of the decoking step would form metal sulfides. An aluminized surface, such as provided by the alonized Incoloy 800 reactor, also has been found to be an effective way to prevent the production of active coke. Quite possibly, the initial type of coke formed on the just-cleaned tube would have an important effect on the length of time a reactor tube could be used in a commercial plant before decoking would be required. 

A second finding is that the morphology of the coke deposited on aluminized Incoloy 800 and Incoloy 800 surfaces often was quite different. As depicted in Figures 2B, 2D, 2F, 4D, 4F, 5B> 6B, and 8B> the predominant structure of the coke observed on aluminized surfaces tended to be either a film of tar or a globular coke deposit. Filamentous coke was found on numerous Incoloy 800 samples (Figures 2A, 2C> 2E, 3B> 5C-5F, 7A, and 70 whereas filamentous coke was detected at most in only small amounts on the aluminized surfaces of only three runs (Figures 2F, 7B, and 7D). Third, EDAX analyses indicated appreciable metal, generally mainly iron and nickel, in the coke formed on (or brushed off) the Incoloy 800 surfaces analyses of coke deposited on aluminized Incoloy 800 coupons in the same runs indicated trace amounts of aluminum but no detectable iron, nickel, or chromium. 

Surface heterogeneities described earlier often were important relative to coke formed or deposited on aluminized Incoloy 800 surfaces. Figures SB, 6B, and 10B show globular and cylindrical coke which resulted preferentially in the pitted areas of aluminized surfaces in several runs. Filamentous coke formed at 700 C on an aluminized Incoloy 800 coupon subjected to a 0.05 atm. acetylene feed is depicted in Figure 7D every filament observed was in or near a pitted area but interestingly not every pitted area contained filaments. 

Build-ups of coke in the coils of a pyrolysis furnace and in the transfer line exchanger necessitate shut-down of the unit for decoking purposes generally every one to six months. Many factors affect the rates of coke formation and collection on the surfaces of the coils and the transfer line exchangers. including the composition and roughness of the metal surfaces. For example. significantly less coke results on aluminized surfaces <1. 2) and on silica-rich or silicon-rich surfaces (3). Furthermore, more coke is normally formed on stainless steel surfaces that have been coked and decoked once as compared to new surfaces (2). 

Much more coke formed on aluminized surfaces that had been sulfided than on any other aluminized surface (see Table III). An appreciable number of coke filaments formed on both the polished and unpolished... 

The results of Wielers and co-workers have shown that Fe(II) is stabilized by some interaction with alumina. The stabilization may be due to the formation of FeAl204 or, more probably, to an interaction between ferrous oxide and an alumina or iron(II) aluminate surface. Kock and co-workers demonstrated the stabilization of Fe(II) by alumina using magnetic measurements. The authors studied the reduction of goethite (FeOOH), hematite (Fe203), a physical mix of FeOOH and alumina, and FeOOH deposited on alumina with carbon monoxide. They raised the temperature of their samples held in a flow of 5% carbon monoxide in helium, with a heating rate of 4.8 K min up to a temperature of 770 K and... 

---