The mass transfer given by an electrochemical reaction towards a rotating disk in a solution of surfactant having drag reducing abilities exhibits a first order transition. Beyond this transition marked by a significant slump of the mass transfer rate, the flow becomes unsteady and is characterised by an hysteresis cycle. This phenomenon was earlier observed for viscoelastic dilute solutions of linear polymers of high molecular weight and is induced by the elongational strain prevailing in the flow around a rotating disk.