Why stainless steel rust and why
One, why stainless steel also rust?
In layman's terms, stainless steel is not easy to rust the steel, in fact, part of the
stainless steel, both stainless, and acid resistance (corrosion resistance). The stainless steel's
stainless and corrosion resistance is due to the formation of chromium-rich oxide film
(passivation film) on its surface. This kind of stainless steel and corrosion resistance is
relative. The results show that the corrosion resistance of steel in the weak medium such as
atmosphere, water and nitric acid increases with the increase of chromium content in steel. When
the chromium content reaches a certain percentage, the corrosion resistance of steel occurs
Mutation, that is, from easy to rust to rust, never corrosion to corrosion. When the surface of
stainless steel pipe appears brown rust (point) when people were surprised: that "stainless steel
is not rusty, rust is not stainless steel, steel may be a problem." In fact, this is a lack of
understanding of stainless steel a one-sided error view. Stainless steel under certain conditions
will be rusty. Stainless steel has the ability to resist atmospheric oxidation --- that is,
stainless steel, but also in the acid, alkali, salt medium is the ability to corrosion - that is,
corrosion resistance. But its corrosion resistance is the size of its steel chemical composition
itself, plus the state of each state, the use of conditions and environmental media type and
change. Such as 304 steel pipe, in the dry and clean atmosphere, there is absolutely excellent
resistance to corrosion, but it moved to the beach area, in a large amount of salt containing sea
fog, will soon rust; and 316 steel performance good. Therefore, not any kind of stainless steel,
in any environment can be resistant to corrosion, no rust.
Second, the reason for rusty stainless steel
Stainless steel is the surface of the formation of a thin layer of thin and strong and stable
stable chromium oxide film (protective film), to prevent the continued infiltration of oxygen
atoms, continue to oxidize, and get the ability to resist corrosion. Once for some reason, the
film has been constantly destroyed, the air or liquid oxygen atoms will continue to infiltrate
or metal iron atoms continue to precipitate out to form loose iron oxide, metal surface will
continue to rust. This surface film is damaged in the form of many, more common in daily life are
the following:
1. The surface of the stainless steel is stored with dust or heterogeneous metal particles
containing other metal elements. In the humid air, the condensed water between the attached
material and the stainless steel is connected into a micro-cell, which triggers an electrochemical
reaction , The protective film is damaged, called electrochemical corrosion.
2. Stainless steel surface adhesion of organic juice (such as melons, soup, sputum, etc.),
in the case of water and oxygen, constitute organic acids, long time the organic acid corrosion
of metal surface.
3. Stainless steel surface adhesion contains acid, alkali, salt substances (such as decorative
walls of alkaline water, lime water splash), causing local corrosion.
4. In contaminated air (such as containing a large amount of sulfide, carbon oxides, nitrogen
oxide atmosphere), in case of condensed water, the formation of sulfuric acid, nitric acid,
acetic acid liquid point, causing chemical corrosion.
In short, the above can cause damage to the surface of the stainless steel protective film caused
by corrosion. So, to ensure that the metal surface is permanently bright and not corroded, we
recommend you:
(1) must always clean the surface of the decorative stainless steel scrub, remove the attached
material, to eliminate the external factors that lead to modification.
(2). Beach area to use 316 stainless steel, 316 material can resist seawater corrosion.
3. Some chemical composition of the stainless steel pipe on the market can not meet the
corresponding national standards, up to 304 material requirements. So it will cause rust, which
requires users to seriously choose a reputable manufacturers of products.
Third, why will the stainless steel with magnetic
People often think that the magnet adsorption of stainless steel, to verify its advantages
and disadvantages and authenticity, do not smoke non-magnetic, that is good, genuine; suction
magnetic, is considered counterfeit fake. In fact, this is a very one-sided, unrealistic way to
identify the wrong.
A wide range of stainless steel, according to the organizational structure at room temperature
can be divided into several categories:
1. Austenitic type: such as 304,321,316,310,201 and so on;
2. Martensite or ferrite type: such as 430,420,410;
Austenitic type is non-magnetic or weak magnetic, martensite or ferrite is magnetic.
Most of the stainless steel commonly used as decorative tube sheet is austenitic 304 material,
generally magnetic or weak magnetic, but due to smelting caused by chemical composition
fluctuations or processing status may also appear magnetic, but this can not be considered Fake or
unqualified, what is the reason for this?
The above mentioned austenite is non-magnetic or weak magnetic, and martensite or ferrite is
magnetic, due to smelting component segregation or improper heat treatment, will cause austenitic
304 stainless steel in a small amount of martensite or iron Body tissue. In this way,
304 stainless steel will be with a weak magnetic.
In addition, 304 stainless steel after cold processing, the structure will be transformed
into martensite, cold forming the greater the degree of deformation, the greater the
transformation of martensite, steel, the greater the magnetic. As a batch of steel, production
Φ76 tube, no obvious magnetic induction, production Φ9.5 tube. Due to Ling bending deformation
of the larger magnetic sense is obvious, the production side of the rectangular tube deformation
than the tube, especially the corner part of the deformation is more intense magnetic more obvious
.
In order to completely eliminate the above-mentioned causes of the 304 steel magnetic, can be
recovered by high temperature solution to stabilize the austenite structure, thereby eliminating
the magnetic.
In particular, it is due to the above reasons caused by the 304 stainless steel magnetic, with
other materials of stainless steel, such as 430, carbon steel is not the same level of magnetic,
that is, 304 steel magnetism always shows the weak magnetic.
This tells us that if the stainless steel with weak magnetic or completely without magnetic,
should be judged as 304 or 316 material; if the magnetic properties of carbon steel, showing
strong magnetic, because the judge is not 304 material.
We recommend that the purchase of stainless steel products should be reputable manufacturers
of products, do not be cheap, beware of fooled.
Fourth, the physical and chemical properties of stainless steel
The physical properties of stainless steel mainly with the following aspects to express:
①. Coefficient of thermal expansion: a change in the quality of the element due to
temperature changes. The expansion coefficient is the slope of the expansion-temperature curve,
the instantaneous expansion coefficient is the slope at a particular temperature, and the average
slope between the two specified temperatures is the average coefficient of thermal expansion.
The expansion coefficient can be expressed in terms of volume or length, usually expressed in
length.
②. Density: The density of the substance is the mass per unit volume of the substance in
kg / m3 or 1b / in3.
③. Elastic Modulus: The force required per unit area is called the elastic modulus when the
force is applied at both ends of the unit length to cause the unit to change in length. The unit
is 1b / in3 or N / m3.
④. Resistivity: The resistance measured between two opposite sides of a unit length cube
material, expressed in Ω • m, μΩ • cm or (discarded) Ω / (circular mil.ft).
⑤. Permeability: no dimension coefficient, indicating the degree of material susceptibility
to magnetization, magnetic induction and magnetic field strength ratio.
⑥. Melting temperature range: Determines the temperature at which the alloy begins to
solidify and solidify.
⑦. Specific heat: the mass of material per unit temperature changes the heat required by 1
degree. In both the imperial and CGs systems, the specific heat is the same as the heat, since
the unit of heat (Biu or cal) depends on the amount of water that is raised by 1 degree per unit
of water. The value of specific heat in the international unit system is different from that of
the imperial or CGS system because the unit of energy (J) is defined by a different definition.
The specific heat units are Btu (1b • 0F) and J / (kg • k).
⑧. Thermal conductivity: a measure of the rate of matter thermal conductivity. When the
temperature gradient of 1 degree per unit length is established on the unit cross-sectional area,
then the thermal conductivity is defined as the heat conduction per unit time. The unit of
thermal conductivity is Btu / (h • ft • 0F) or w / (m • K)
⑨. Thermal diffusivity: is a property that determines the temperature forward temperature of
the material. It is the ratio of thermal conductivity to the ratio of thermal and density
products. The thermal diffusivity is expressed in Btu / (h • ft • 0F) or w / (m • k ).
Fifth, the concept of steel and iron and that:
1, the concept of steel and iron
Strictly speaking, steel and iron are not necessarily a certain standard, the main components
are iron, (element symbol: Fe)
According to the provisions of the World Industry Association, to the number of carbon in the
pig iron to divide. Generally, the carbon content is between 0.03% and 2.00%, and the carbon
content is between 2.00% and 4.30%, and iron is mainly contained in the iron. less. Iron is the
main raw material of steelmaking, the process of steelmaking, one is to remove too much iron in
the iron and some dregs, and then add some other ingredients. At the same time, steel has a
variety of uses, different uses of steel, which will add some special additives, such as
stainless steel to add chromium-based elements, manganese steel, the main addition of manganese.
But in general, the main chemical composition is the same, have a certain magnetic.
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Eric Xie
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