A fundamental study is reported on the initiation of crack growth from a pre-existing delamination; growth is due to the diffusion of a corrosive species from the side face of a sandwich layer. The corrodent diffuses along the delamination and simultaneously through the sandwich layer. It is envisaged that a chemical reaction occurs on the intact interface ahead of the delamination tip, at a rate that scales with the local concentration of corrodent. Debonding initiates at the tip of the pre-existing delamination when a critical quantity of corrodent per unit area has reacted at the interface immediately ahead of the tip. Diffusion theory is used to predict the duration of the initial transient prior to the establishment of a steady-state value of reaction rate at the interface, directly ahead of the delamination. Once steady state has been attained, the Laplace equation is solved for the corrodent concentration within the sandwich layer and delamination zone. The reaction rate at the delamination tip and the time to initiate debonding of the interface are determined. Maps are constructed to show regimes of behaviour, with axes that make use of the sandwich layer geometry and the relative diffusivity of corrodent within the delamination crack and within the sandwich layer. A number of asymptotic solutions shed light on the general numerical case. The analysis is motivated by the practical problem of delamination of adhesive joints employed in ship construction, but has much wider applicability.