needing to replace the runners on trailer

i have an aluminum 20' lowe pontoon. i need the runners to be 18' but as big as i can find are 16, i do not want a splice. My question is should i use treated lumber green or yellow ? i heard that the chemical that they use in the treated wood deteriorated the aluminum.

As long as the runners are carpeted I doubt they would be an issue with corrosion. You could always just get exterior grade lumber and the coat with fiberglass resin thinned with a bit of acetone. Even untreated exterior grade lumber will last a long time as long as you leave a bit of an opening in the carpet on the bottom of the runner for water to drain out.