The electrochemical noises obtained from pure aluminium in an alkaline solution have been analysed as a function of sulphate, nitrate or sodium sulphide concentration. A stochastic theory has been applied to the experimentally obtained electrochemical noises based upon the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, the present analysis method permitted us to distinguish between the dissolution of Al and the stochastic process concerning the series of nucleation, growth and detachment of hydrogen bubbles in an engineering sense. Moreover, the conditional event generation rate and the average charge in each event were quantitatively determined for the stochastic process of hydrogen bubbles as well as the dissolution of Al. Effects of sulphate, nitrate and sodium sulphide additives on the corrosion of pure aluminium in an alkaline solution have been discussed in terms of the conditional event generation rate and the average charge in each event.