Because field studies seldom recover coupons less than 6 months from immersion they provide no information about early corrosion behaviour. Linear or power law functional behaviour for corrosion loss with time is often assumed. New field studies performed on the Australian east coast and described herein using very closely spaced recoveries of coupons show that the corrosion loss-time behaviour is initially highly non-linear and then almost linear until corrosion product formation begins to control the rate of corrosion. The reasons for this behaviour are discussed in terms of the recently introduced multi-phase phenomenological model for marine corrosion. Conventional oxygen diffusion arguments are then used to provide a mathematical model to describe the main part of this behaviour. The possible influences of seawater temperature and oxygen concentration are discussed.