Why is stainless steel reducers rusty?

The biggest advantage of ASTM B16.9 4'' Sch 10 Seamless Pipe Fitting Stainless Steel Reducer is not rust, so what is the reason for stainless steel reducers not rust?

　　Now many users are suffering from pipe rust, so these users should choose the size of stainless steel reducers such as never rust products, its main material is stainless steel, this material has good corrosion resistance, is one of the strongest metal materials. Because of the hardness of stainless steel, it enables structural components to permanently retain the integrity of engineering design. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, expanding the corrosion and eventually forming voids. Carbon steel surfaces can be electroplated with paint or oxidation-resistant metals, but, as is known, this protection is only a thin film. All metal boxes react with oxygen in the atmosphere and form oxide films on the surface.