Piping type for recirc lines

Do recirc lines require the copper pipes to be thicker? Type M or Type L ?

I was wondering if the constant movement of water would slowly corrode pipe faster.

pipe

It will, but mainly because the return circulation pipe is usually smaller so its velocity it higher, unless you install a valve BETWEEN the pump and heater, NOT BEFORE the pump, and close it until you have just enough flow to keep the line warm.

How quickly might one expect errosion to penetrate the wall of copper tubing? In my proposed re-circulating system I'll be using Type L, 3/4" return line in a gravity flow system.

You should be fine with the 3/4" type L.

I was just reading an ASPE article yesterday on recircing systems. The author recommends not going smaller than 3/4" due to velocity and using balancing valves to control the flow as hj suggested.

Curious, on plans I always call for Type L copper for the water lines (when specifying copper anyway), do you typical use Type K?