Zinc anodes: 
Maximum operating temperature for zinc anodes is 60 deg. C. (140 deg. F.) 
Temperatures higher than 60 deg c. will cause reversal of the anode, cathode, if the temperature exceeds 60 deg. C. the vessel may experience high rates of corrosion. 
Aluminum anodes: 
The foremost application for most oilfield operations is the aluminum anode. When temperatures are above 50 deg. C. (122 deg. F.) caution should be taken as the protection from the anode is reduced with higher temperatures. The aluminum anode is considered to be the most effective at an economical cost.
Magnesium anodes: 
The best protection from corrosion is provided by the magnesium anode; however, they have the shortest life and are considerably more expensive. If you have a new facility and are unsure of your conditions it is recommended to use magnesium anodes for you initial placement.