Rusted stainless steel
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The 304 stainless steel tube I just bought has a very smooth outer surface, while its inner surface is almost completely covered with red rust. It is non-magnetic; no pink color appears when tested with stainless steel test solution, so 201 stainless steel can be ruled out. I had only used stainless steel plates before; I had never encountered 304 stainless steel that was covered in rust like this. I would like to ask the experts why this happens.
Stainless steel is not necessarily rust-proof either; it depends on the type of chemicals used. The most typical forms of 304 corrosion are acid corrosion and halide ion corrosion. Check against it to see if such chemicals exist
Just bought it; haven’t used it yet. The supplier refused to provide any information.
I suspect it’s second-hand, polished on the surface before being sold. I’m not sure if there are any precedents for this in the market.
It’s basically caused by the acid cleaning not being thorough enough; just do the acid cleaning again and it will be fine
The steel pipe is new and there’s no problem with it; the red color is simply due to the acid washing process not having been carried out thoroughly inside the pipe. This is a common issue among some stainless steel pipe manufacturers (many in Zhejiang province) – they are just being careless during the acid washing process. It generally doesn’t affect usability, so it’s no concern