"Although dry chlorine (gas or liquid) normally does not react with or corrode some metals such as copper or carbon steel, it is strongly reactive (strongly corrosive) when moisture is present."
excerpt taken from the ChlorineInstitute.org
http://www.chlorineinstitute.org/abo...ItemNumber=855

Jandy/Laars heater warranty states:
"8. Chemical contamination of combustion air or improper use of sanitizing chemicals, such as introducing sanitizing chemicals upstream of the heater and cleaner hose or through the skimmer."
http://www.jandy-downloads.com/pdfs/LXandLT_H02140B.pdf (page 32)

Pentair's Heat Pump troubleshooting section states:
"Corrosive pool water, chemical damage to water tubing inside heater. Chlorinator is not isolated from the heater, chlorine migration."
http://www.pentairpool.com/misc/owne...ThermalFlo.pdf (Page 35)

It's hard to say what level of chlorine may cause damage. There are a slew of pool owners that typically maintain higher than 10 ppm, which in my opinion is WAY too high, and have not complained of corrosion on their handrails, ladders, light rings, or heat exchangers.
However, manufacturers seem to have issues with chemical/chlorine corrosion on their products.