A cooler passivation process for a coolant cooler of a cooler device mounted in a motor vehicle A cooler device with a coolant cooler is provided, the latter having at least one aluminum cooling channel with a cooler surface provided with flux. The cooler device is provided in a motor vehicle and is then put into operation with at least one fuel cell of the motor vehicle. In order to form a passivation layer on the coolant cooler, according to the invention a previously provided aqueous passivation solution is formed from passivation layer forming material and fuel cell wastewater is applied to the cooler surface, provided with the flux, of the at least one aluminum cooling channel, the passivation solution reacting with the cooler surface, provided with the flux, of the at least one aluminum cooling channel while thermal energy provided by the coolant cooler is supplied, forming a passivation layer.