HEDP and AMP are the primary phosphonates used in cooling water systems. Before their use as corrosion inhibitors, phosphonates were recognized as excellent scale control agents and metal oxide sequestrants.

Whereas phosphate esters are characterized by carbon to oxygen to phosphorous bonding (COP), the phosphonates have a carbon to phosphorous to oxygen bonding (CPO). Because CPO bonding is more resistant to hydrolysis or cleavage than the COP bond, the potential for phosphate fouling is less with the phosphonates than it is with phosphate esters. 

Phosphonates provide excellent control of hydrated ferric oxide deposits at comparatively low concentrations. They reduce attractive forces between individual iron particles by adsorption of the phosphonate at the particle surfaces. They are also excellent for the control of hardness salts, and are capable of better threshold inhibition than polyphosphates and phosphate esters.

Phosphonates can also function as chelants or sequestrants for control of metallic-oxide (1e. Iron, manganese and zinc) deposits. Their control over such materials is superior even to that of traditional chelants, although they are somewhat less effective on calcium and magnesium.