Saw an advertisement in the new edition of Trailer Boats on aluminum manifolds and risers. Wonder why these are not widely used since the biggest killer of IOs seems to be rusting manifolds & risers??
I don't know much about boats, but maybe electrolisis is a concern. I have an uncle that is a lobsterman and often heard him refer to hot-boats. A hot-boat being one that has alot of electrolosis associated with it and eats through anti-corrosion plates really fast. I know that when dissimiliar metals are connected it creats electrolosis which increases corrosion in plumbing or heating systems. We use dielectric unions to gaurd against this sometimes. Just a thought!
true aluminum does not rust but it does corrode. Add heat with salt water and I'm thinking very rapid oxidation. I could be wrong but I live in fl. ,and I weld for a living and see almost every day what the salt does to aluminum outdrives, fuel tanks, etc., and it isn't pretty.
4.3 chevy by kodiak with 12 years on aluminum manifolds and still going strong mostly in salt.my buddy ran a set of ****son logs on his 350 chev for 20 years but had to store them because he couldnt make them work with wet exhausted chevy zz4 fast burn. had to switch to kodiak aluminum with modifications to keep from sucking in water due to cam overlap. weboth launch and recover in saltwater and only about2% of time in fresh. 80 mile run to nearest river and waves were always to big to get there last summer so all 150 hours salt. niether of us ever flush raw water after going out.
inside waterjacket no mixed metals. inside exhaust runners no water until last inch. that is where the most corrosion is. aluminum corrosion forms a protective barrier protecting from further attack. rust seems to promote more rust.if exhaust gas and saltwater so harmful to aluminum why dont outboard legs fall apart every season? paint on aluminum bad if salt gets under it and we have found it is best to remove it for saltwater application. i dont type so dont say more than i need to usually.
I know that when dissimiliar metals are connected it creats electrolosis which increases corrosion in plumbing or heating systems. We use dielectric unions to gaurd against this sometimes. Just a thought! 
