Polished aluminized Incoloy

Treatment Aluminized Incoloy 800 Polished Aluminized Incoloy 800... 

Metal coupons were inserted at various positions in the horizontal tubular reactor. The coupons had two types of surfaces an Incoloy 800 surface and an aluminized Incoloy 800 surface. To prepare these coupons, flat pieces of Incoloy 800 were aluminized (or alonized) by the Alon Processing, Inc. of Tarentum, PA. In this process, gaseous aluminum was contacted with the Incoloy 800, and aluminum diffused into the surface. The alonized samples as received from Alon Processing were cut in small coupons so as to expose an Incoloy 800 surface which was cleaned and polished before use. 

The tubing containing the coupon samples were placed inside a Vycor glass tube (2.2 cm. I. D. and 85 cm.long) that was positioned horizontally in an electrical furnace controlled at either 700 or 900°C. For inorganic gas treatments Incoloy 800 and polished Incoloy 800 coupons were positioned side by side 4. 5 cm. upstream of the reactor midpoint polished and unpolished aluminized Incoloy 800 coupons were likewise positioned side by side except 2.0 cm. upstream... 

Treatments of aluminized (or alonized> Incoloy 800 coupons with helium, oxygen, or steam at 700°C in general resulted in only rather small changes in appearance, as shown in both Figures 3 and 4 for the unpolished and the polished coupons, respective The findings with the polished aluminized coupons must however be considered less reliable. The objective had been to remove only the outer layer that in places was almost pure aluminum and polish until the depth was reached at which aluminum had alloyed with the Incoloy 800. For some coupons, too much or perhaps too little metal was removed, such as for the rough portion of the steam-treated coupon shown in Figure 4C. 

An appreciable amount of sulfur (probably as metal su1 f ides> was incorporated in the surface of all coupons exposed to hydrogen sulfide at 700°C as shown in Tables I and II. For Incoloy 800 couponsi there was a very large decrease in the chromium content at the surface. On a sulfur—free basis the iron content increased greatly for polished Incoloy 800 but the nickel content increased for the regular Incoloy 800 coupon. For aluminized Incoloy 800 coupons there was little change in the metal content on a sulfur-free basis as a result of hydrogen sulfide treatments as shown in Table II. 

The results of this investigation greatly clarify several factors that affect coking in pyrolysis coils and transfer line exchangers. In particular, the role of the surface is better defined since significant differences in results were obtained when directly comparing Incoloy 800 versus aluminized Incoloy 800 and polished versus unpolished surfaces. 

Incoloy 800 surfaces acts to produce surfaces with more desirable compositions these new surfaces are more resistant to oxidizing. sulfiding. or surface coking reactions. Of interest. Qregg and Leach (11) had found that less coke was formed on electropolished nickel surfaces as compared to unpolished nickel surfaces. In the case of the aluminized Incoloy BOO. care must be taken not to remove too much of the surface since the aluminum has penetrated only short distances into the surface. The results of the present investigation imply that the method or degree of polishing is important. It is not clear why polished coupons in some cases collected considerable coke near the corners of the coupons. 

The results of this investigation help clarify why machining is beneficial relative to coke formation rough areas on the surface act as excellent collection sites for coke or tar droplets formed in the gas phase. Such tar droplets eventually become globular coke. Smoother surfaces hence minimize such collection or surface deposits. In addition. smoothing or polishing of both Incoloy 800 and aluminized... 

---