I am not sure what you mean by your description " out packing aluminizing tech", but any attempt to fuse aluminum metal and Nickle metal will form a Raney Nickle alloy. This may be being activated by the HCl reacting with the dispersed Al to make AlCl3 vapors and electrons transfered via remaining Ni metal cluster, to to remove H2 from NH3, and nitriding the Al. At 1000C nitriding of Al surfaces Is probably best achieved by NH3 anyway,even without Ni.

Thanks a lot for you kind answer, I didn't make the question clear. I put activator NH4Cl and alloy FeAl into a closed vacuum champer and heat it to above 1000C, the specimen hang above the masteralloy and activator powder instead of packing in to them .

Is that the nitriding of FeAl surfaces best achieved at 1000C with N2 and NH3? Is it preventable by using only AlCl3 as activator?

I am not sure that one desires nitriding the Al in the alloy surface- it will be subject to hydrolysis by water, and will surely disrupt the surface during common uses. Whether the transition metal is Fe or Ni, these Raney alloys are prone to acid or base removal of Al under mild aqueous reactions, and the H2 formed is occluded to make a porous catalyst with Al( OH) x  supported structure.

Assuming that you desire the complete surface nitriding anyway, one would figure that you have already determined that the spacefilling model of the alloy and its nitrided form do not provide surface disruption via mismatch. Under the conditions for gas nitriding, 1000C either N2 or NH3 are effective, and I am puzzled at the need for an activator. Salt nitriding would be effected at milder temperatures than 1000C but since NH4+ salts readily dissociate to coresponding  Bronstead acid, and NH3,  the hot acid vapors would likely act in unwanted ways. A role for a Lewis Acid like AlCl3 instead is not readily apparent to me unless the reaction is being attempted to run under relatively milder conditions than 1000C. If the desire of using an activator is to remove surface oxides, then there may be a role for vapor phase Lewis Acid,especially and either anhydrous FeCl3 or AlCl3 might work under vacuum and mild heat ( 300- 600 C). N2 could be the bulk gas until the complete sweep of activator and products, then brought up to 1000C and NH3 introduced ( if the surfaces are not sufficiently nitrided).

Yes, you are right, I don't want to nitride the Al in the alloy surface. I just want to know how it formed so I can prevent it.

what I want is to form NiAl coating on the surface, so I use NH4Cl as an activator because it can decomposite into HCl and react with FeAl alloy to form AlCl3 which is the desired gas phase in the champer.

But because of the activator introduce the unwanted NH3 in the champer. surface of the specimens begain to from AlN instead of NiAl, it also poisoned the masteralloy FeAl and prevent the formation of AlCl3.

From the picture you can see the surface of the specimens form an undesirable porous NiAl coating stuctuere with AlN intrude at 1000C, and an continous AlN coaitng on surface at 1050C.  Seems like the formation of AlN is encouraged with the rising temperature.

I want to rising the temperature and diminish the formation of AlN at the same time. So I mildly vent the champer at about 600-800C to exhaust NH4 out of the champer, bacause NH4 is light tham HCl and AlCl3 so I can ust the up-pipe of the champer to do it.  But the loss of AlCl3 and HCl also happened so sometime I got neither NiAl nor AlN.