Warranty on Franke Genesis sink declined

I installed a Franke Genesis sink a few years ago. The sink has rusted very badly. The Franke rep called (having had to travel a 300 km round trip) and said that the sink has rusted because we store cleaning chemicals under the sink which causes excessive corrosion. Most people store their cleaning Chemicals under the sink! The unit is supposed to have a 25 year warranty but after approximately 5 years it needs to be replaced.