The water made by RO is missing an ion (cannot remememer whether positive or negative) Anyway it attempts to grab back this ion on the discharge piping (ie corrosion). Best practice is to make your discharge piping of PVC.
RO product water should always be considered corrosive however RO reject water can be either corrosive or scale-forming. The reject water contains all the ions being removed from the feed, in concentrated form. If your primary foulants are scale-forming or hardness releated you can get by with non-composite piping. If your foulants tend to be corrosive (chlorides and such) you should stick to composites.
You will find that the material of choice is ABS for plants in SE Asia and Oceania. This is manufcatured to AS 3518 by Eurapipe in Austrlai, Malaysia and Indonesia. It has been used by Seimens (US Filter( Memtec)) for decades for RO services. The latest Veolia plants in Australia ( Gold Coast & Sydney 125 & 250M ML/day) use ABS for the low pressure services up to DN400. Kranji and Changi in Singapore also used the material
It has high strength and has a higher fracture toughness than PVC. Also the solvent cement joint integral bonds the two ABS surfaces. Whereas the "glue" used for PVC can be seen when you section a joint.
Eurapipe is a Tyco compnay represented in most countries in this region. ABS is being used in major projects in the Middle East.
