Raypak pool heater heat exchanger corrosion problems, what causes the corrosion

Atmospheric heaters are equipped with a thermal cut off device to prevent flame roll-out in the event the heat exchanger becomes blocked. This is a single- use type fusible link or thermal fuse that must be replaced when disabled by an over-temperature condition caused by excessive restriction in the heat exchanger flue passage.
Look to see what has blocked the flue.

I assume you are referring to the heat exchanger. Usually when these fail the heater is replaced. Reason being that this is the biggest money part of the unit. The part alone will run pretty deep into the price of a new heater.
Then you still have the old burner tray, old gas valve, old thermostat and old internal ignition device. Each of these components run several hundred dollars apiece. It doesn't make sense to throw good money after bad.
If this exchager was leaking from a salt water pool (common these days) it would be advisable to replace the heater with a Sta-Rite MaxiTherm with the cupro-nickle exchanger. This part is coiled which allows for better water flow and less corrosion than a standard copper exchanger.
To replace the exchanger is not all that tricky if you are handy with a screwdriver, wrenches and plumbing. If you have any doubts get a pro over. Remember you are literally playing with fire.

roll out fuse is open. for starters you have to replace that safety. then find out what caused the over temp situation. clogged venting or heat exchanger, negative down drafts, inadequate combustion/ventilation air, improper air/gas mixtures

DON'T KNOW WHY YOU HAVE A RAY PAK ON THE CAR WASH SINCE SO FAR I KNOW IS DESIGN FOR POOL AND SPA ONLY,THE HEATER NEED CONSTANT WATER FLOW BUT HERE WE GO, CHECK YOU BURNER FOR FIRE MARKS, CHECK THE HEAT EXCHANGER FOR SUIT BUILD UP (GREEN STUF) IF EVERYTHING OK THAN THE CONTROL BOARD NEED TO BE REPLACE.

thats is related acid left on the pool corroded i service so many salt pool with this heater and not problems at all the awser was the PH WAS balance on the past or now but the problem wont stop there will be future damage to the heat exchanger. are you add tabs to the skimmer? what happend to the heater is because low PH. SORRY not easy solution but to balance the chems and replace any corroded part