The method in a recovery boiler comprises estimating the first melting temperature T0 of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) content of the fly ash; measuring or estimating the temperature Tss of superheated steam; evaluating a temperature difference TD1 between the first melting temperature T0 and the temperature Tss of the superheated steam, the temperature difference TD1 providing an estimate of the risk of corrosion; and selecting a control action for influencing the temperature difference TD1. Alternatively or additionally, the method comprises estimating the sticky temperature TSTK of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) and chlorine (Cl) contents of the fly ash; measuring or estimating the temperature TFG of the flue gases; evaluating a temperature difference TD2 between the sticky temperature TSTK and the temperature TFG of the flue gases; the temperature difference TD2 providing an estimate of the risk of plugging; and selecting a control action for influencing the temperature difference TD2.