Corrosion after only three months

I had an espresso maker for 4 cups of another manufacturer. The also has 4 cups of espresso cooked. This provided for six (!) Much larger cups of Bialetti stove provides only about 3 cups of espresso, because obviously a relatively large amount of water a) remains in the water tank and b) evaporating. I'd still be tolerated due to the favorable purchase price. In addition, the manual showed the way for a brand-name product from Italy to a remarkable number of spelling mistakes. Unfortunately rusts now after only zwölfmaligem use in three months the inside bottom of the water tank (lt. Product Description stainless steel?), So I have to take the warranty service.