Couple of ways
1) contract out cathodic protection scope to third party. Depending on the soil conditions, water level, salinity,grounding; they will propose sacrificial anodes or hook up to an above ground active corrosion protection system.
2) Depending where you are, add 3 mm (1/8") corrosion allowance (in my case, Northern Alberta). We ground the pilecap reinforcement to one of the pile in the group.
Rate of corrosion varies depending on several factors including soil properties. See "NBS Monograph 58, Corrosion of Steel Pilings in Soils" which you can download from this page of my website
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