Stainless steel precision castings have corrosion resistance. But its corrosion resistance will change with the chemical composition of its steel and the type of environmental medium. The surface of stainless steel precision castings has a very thin but dense and sturdy protective film (rich chromium oxide film), which can effectively prevent the infiltration of oxygen atoms and prevent further oxidation of the castings. So what are the reasons why stainless steel precision castings rust?
1. Air pollution
There are large amounts of sulfides, carbon oxides, nitrogen oxides, and other substances in the polluted air. These substances condense into water when cooled, forming liquid points such as sulfuric acid, nitric acid, and acetic acid, thereby causing chemical corrosion of castings.
2. Electrochemical corrosion
Dust containing other metal elements or attachments of dissimilar metal particles accumulate on the surface of stainless steel. In humid air, the attachment and the condensate between the attachment and stainless steel form a micro battery, triggering an electrochemical reaction and damaging the protective film, leading to the phenomenon of "rusting" on the casting.
3. The presence of surface attachments
During the production process of stainless steel castings, substances containing acids, alkalis, and salts may adhere to the surface, causing localized corrosion on the surface of stainless steel castings.
Once this thin film is damaged, oxygen atoms in the air and metal liquid will continue to penetrate, or iron atoms in the metal will separate, forming loose iron oxide. In this way, the metal surface will be corroded, and the casting will appear yellow spots, which is commonly known as "upper rust".
