Stainless steel, as the name suggests, NEVER RUST why in our life we see many stainless steel products being rusted?
Stainless Steel refers to the materials
which show the weak corrosive medium such as air, steam and water or the
chemical corrosive medium such as acid, alkali and salt, also known as
stainless acid resistant steel. But in application, some stainless steel products
are accidentally rusted.
When brown rust spots appear on the surface of
stainless steel pipes, people are surprised: they think that "stainless
steel is not rusty, rust is not stainless steel, and there must be a problem
with the steel". In fact, this is lack of understanding of stainless
steel. Stainless steel will rust under certain conditions. Stainless steel has
the ability to resist atmospheric oxidation, i.e. rust resistance, but also has
the ability to corrode in the medium containing acid, alkali and salt, i.e.
corrosion resistance. However, the corrosion resistance of the steel varies
with its chemical composition, additive state, service condition and
environmental medium type. For example, 304 steel pipe has absolutely excellent
corrosion resistance in dry and clean atmosphere, but when it is moved to the
coastal area, it will soon rust in the sea for containing a lot of salt, while
316 steel pipe performs well.
Therefore, not all kind of stainless steel can
resist corrosion and rust in any environment. Stainless steel is thin but firm
and stable chromium rich oxide film (protective film) formed on its surface,
which can prevent the continuous infiltration and oxidation of oxygen atoms and
obtain the ability of corrosion resistance. Once the film is constantly
damaged, oxygen atoms in the air or liquid will continue to penetrate, or iron
atoms in the metal will continue to separate out, forming loose iron oxide, therefore
the metal surface will be constantly corroded.
There are many forms of destruction of the surface
film.
First, Inherent Intergranular Corrosion
The material without Ti and Nb has the tendency of
intergranular corrosion. The addition of titanium and niobium, together with
stabilization treatment, can reduce intergranular corrosion. Stainless steel is
a kind of high alloy steel which can resist corrosion in the air or chemical
corrosion medium. It has beautiful surface and good corrosion resistance. It
does not need to go through surface treatment such as color plating, but its
inherent surface properties. It is used in many aspects of steel. It is usually
called stainless steel. The representative properties are 13 chromium steel,
18-8 chromium nickel steel and other high alloy steel. From the point of view
of metallography, because stainless steel contains chromium, a very thin
chromium film is formed on the surface of stainless steel, which can isolate
the oxygen invading into the steel for corrosion resistance. In order to
maintain the inherent corrosion resistance of stainless steel, steel must
contain more than 12% chromium. Used for welding. The lower carbon content
makes the precipitation of carbides in the heat affected zone near the weld
minimum, and the precipitation of carbides may lead to intergranular corrosion
of stainless steel in some environments.
Second, NO Solution Treatment
The alloy elements do not dissolve into the
matrix, resulting in low alloy content and poor corrosion resistance.
Third, there are chloride ions in the environment.
Chloride ions exist widely, such as salt / sweat /
seawater / sea breeze / soil, etc. The corrosion of stainless steel in the
presence of chloride ion is very fast, even more than that of ordinary low
carbon steel. Therefore, the use of stainless steel need to pay attention to environment,
and need to be cleaned, remove dust, keep clean and dry（ In this
way, we can give him a "misuse" There is an example in the United
States: An enterprise uses an oak container to hold a solution containing
chloride ions. The container has been used for nearly a hundred years, and it
was planned to be replaced in the 1990s. Because the oak material is not modern
enough, the container leaked due to corrosion just 16 days after it was
replaced by stainless steel.
: 
