Hoe Kxx got put 5 years warranty, so should be OK ?

Chrome products will oxidized and rust eventually after the outer layer of protection is damaged. For SS it will not rust be oxidation will takes place. It will looked rusty but it can be polished away however be reminded the layer of protection, nickel will reduce and tends to increase the oxidation process. For owners leaving near the sea, SS will corrode. As Salt in its own state is acidic. Mixing acid with moisture and air it will corrode the SS. It is possible to prevent this only by getting marine grade SS which is defintely very expensive. Not currently used in any household products only for commercial so far. Another way to reduce the effect of rusting is by Hot dip galvanized process, this will prolong the lifespan of the products from getting corrosion. Another way you can try getting it anodized. A process quite similiar to electroplating also the layer of protection must not be scratched or damaged.

There are different grades of stainless steel. 18/20 is what's used for kitchen sinks most of the time. Ask for the grading.

Hmm... there is only 18/8, 18/10 stainless steel... 18/20 is not heard of.... or you meant 18 to 20% Chromium?