The reasons are as follows:
Stainless steel surface deposits containing other metal elements of dust or dissimilar metal particles of the adherents, in the humid air, the adherents and stainless steel condensate, will be two connected to a microcell, triggering an electrochemical reaction, the protective film is damaged, known as electrochemical corrosion.
(PS: Stainless steel has the ability to resist atmospheric oxidation - that is, corrosion resistance. The size of the corrosion resistance of stainless steel is with the steel itself chemical composition, processing state, use conditions and the type of environmental media and change)
Expanded information
340 stainless steel application characteristics
304 stainless steel is the most widely used one kind of chromium - nickel stainless steel.
1. As a widely used steel, it has good corrosion resistance, heat resistance, low-temperature strength and mechanical properties;
2. It has good hot workability such as stamping, bending, etc., and has no heat-treatment hardening phenomenon (using temperature -196 ℃ ~ 800 ℃).
Resistant to corrosion in the atmosphere, if it is an industrial atmosphere or heavily polluted area, it needs to be cleaned in time to avoid corrosion. Suitable for food processing, storage and transportation, with good processing performance and weldability.
Reference:
Baidu Encyclopedia: 304 Stainless Steel