What do you need protection from?

Electroless nickel plating can give you a very shiny surface to a surface that looks like a grain structure (very nice in my opinion). It is also chip resistant.  You can vary the hardness from around RC35? to RC65 by changing the phosphorus content and heat treatement.  It can provide good wear resistance and lubricity.  However, it will not provide good corrosion resistance in an agressive environment, although thicker coatings are supposed to offer better corrosion resistance.  But if a corrosive species like chlorides reach the aluiminum it's all over.

Nickel plated aluiminum is very cool because it is lightweight but doesn't look like aluminum, so unless you know about nickel plating it appears to be unobtainium.  If you can live with shiny or grainy and are not in a corrosive environment it is a very good option.  But I'm not sure how it would look on a net cast part.

I don't see much advantage to chrome over electroless nickel.

You didn't mention why you can't use a hard anodize or a decorative anodize.  If you were considering hard anodize I assume you need wear resistance?

I can't remember whats in 308 aluminum and I don't know if it causes any particular plating or anodizing challenges.  Maybe that's why you are looking for something other than anodize?

Beware of platers and anodizers. In my experience getting high, consistant quality is difficult (bordering on impossible).  If you are pushing the capabilities of the coating this becomes a big issue.

Do a search on this forum using "nickel plating" or "electroless nickel".

Sorry, I don't know 0f anything black except anodize.

Thanks for the advice.  I do not know much about the anodizing process myself, but an earlier internet search found the same repeated phrase that you cannot anodize 308 well b/c of its specific alloying characteristics being detrimental to the coating.

Essentially we are looking for wear resistance, and we have had good success with a hard coat anodize on higher grade aluminum (6061 i believe), but we would like to be able to use the lower cost 308.

Thanks for the help.

By alloy 308, do you mean the former SAE 308 which is now known as 380? I have some experience with hardcoated 380 components and you can't build much of a film.Plus, cosmetically it looks poor. With 6061, you can readily build .001" or more hardcoat film, whereas with 380 you are lucky if you get .0003". You could possibly dye the anodic film black if you want a black finish. As for wear resistance, you'll just have to test the hardcoated 380 to determine if is acceptable. Another alternative you might consider if you are looking at die castings would be alloy 390. This is a hypereutic with enhanced wear resitance due to large primary silicon particles.For a black finish, you would probably want to go with a chromate conversion, such as Alodine or Irridite, dyed black.

Hard electroless nickel offers great wear resistance.  I believe it is the best wear resistant coating you can put on aluminum (or at least the best economical coating).  It is less abrasive to the bearing and has better lubricity than hard anodize.  Very good for bushing applications. I don't know of any way to make it black, but it never hurts to ask.  I have seen some teflon impregnated samples which had a darker color (still shiny but not blinding bright).

There are some exotic ceramic coatings available which are applied by thermal spray techniques.  I suspect they are very pricey.

Also, you might run a test to see if Type II anodize would give you enough wear resistance.  I believe it's basically the same stuff but not as dense.  Type II is usually more flexible as to the substrate.  

I would be wary of electroless nickel over aluminum if the component will be exposed to a corrosive environment. I have seen aluminum components that had electroless nickel that eventually developed pinholes. One the nickel was breached, massive pits developed in the aluminum. Nickel over aluminum is very bad from a galvanic standpoint.