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Why does stainless steel also rust?
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Why does stainless steel also rust?

Views: 17     Author: Site Editor     Publish Time: 2018-07-03      Origin: www.fuchun-casting.com

Why does stainless steel also rust?

When the rust spots appear on the surface of the stainless steel tube, people are very surprised that "stainless steel is not rusty, rust is not stainless steel, and it may be a problem of steel." In fact, this is a one-sided misconception about the lack of stainless steel. Stainless steel is also rusty under certain conditions.


Stainless steel has the ability to resist atmospheric oxidation, that is, rustless property, and also has the ability of corrosion resistance in acid, alkali and salt medium, that is, corrosion resistance. However, its corrosion resistance is changed with the chemical composition, protection state, use condition and environmental medium type of the steel itself. For example, 304 steel pipes, in the dry and clean atmosphere, have excellent corrosion resistance, but move it to the seaside area, and in the sea fog containing a large amount of salt, it will soon rust; and 316 steel pipes are good. Therefore, not any stainless steel can be corroded and rust free under any environment.


rust spots appear on the surface of the stainless steel tube

Stainless steel is a thin, sturdy and stable chromium oxide film (protective film) formed on its surface to prevent the continuous infiltration and oxidation of oxygen atoms, which has the ability to resist corrosion.


Once for some reason, the film is constantly destroyed, the oxygen atoms in the air or liquid are constantly infiltrated, or the iron atoms in the metal are constantly separated out to form a loose iron oxide, and the metal surface is constantly corroded. There are many forms of destruction of the surface film.


1. Stainless steel surface deposits other metal elements of dust or heterogeneous metal particles attachment.In wet air, the condensate between the attachments and the stainless steel will connect the two to a micro battery, which causes the electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.


2. The surface of stainless steel adheres to organic juice (such as melon, soup, phlegm, etc.), which forms organic acids in the presence of water and oxygen, and the corrosion of organic acids to the metal surface for a long time.


3. The surface of stainless steel adheres to acid, alkali and salt substances, such as alkaline water and lime water splashing, which cause localized corrosion.


4. In the polluted air (such as the atmosphere containing a large amount of sulphide, carbon oxide and nitrogen oxide), condensate water can form sulfuric acid, nitric acid and acetic acid solution, causing chemical corrosion.


To ensure that the metal surface is permanently bright and not corroded, we suggest:


1) The surface of decorative stainless steel must be cleaned and scrubbed regularly, removing attachments and eliminating external factors that cause modification.


2) 316 stainless steel should be used in the seaside area, and 316 can resist seawater corrosion.


3) The chemical composition of some stainless steel pipes in the market can not meet the corresponding national standards, less than 304 of the material requirements. Therefore, it will also cause rust. This requires users to choose products of reputable manufacturers seriously.



1. 316 material stainless steel (18Cr-12Ni-2.5Mo), because of the addition of Mo, its corrosion resistance, atmospheric corrosion resistance and high temperature strength are very good, which can be used under harsh conditions, processing hardening excellent (no magnetic). Applied in the production of equipment, chemical, dyestuff, paper, oxalic acid, fertilizer and other production equipment in the sea water; image, food industry, coastal area facilities, rope, CD rod, bolt, nut.


2. 304 material (18Cr-8Ni), as a kind of widely used steel, has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; punching, bending and other heat processing properties, no heat treatment hardening (no magnetic, then temperature -196 to 800 C). Applied to household items (1 and 2 types of tableware, cupboards, indoor pipelines, water heaters, boilers, bathtubs), auto parts, medical appliances, building materials, chemistry, food industry, agriculture, andship parts

stainless steel auto parts



  • What is 'multiple certification'?

    This is where a batch of steel meets more than one specification or grade. It is a way of allowing melting shops to produce stainless steel more efficiently by restricting the number of different types of steel. The chemical composition and mechanical properties of the steel can meet more than one grade within the same standard or across a number of standards. This also allows stockholders to minimise stock levels.

    For example, it is common for 1.4401 and 1.4404 (316 and 316L) to be dual certified - that is the carbon content is less than 0.030%. Steel certified to both European and US standards is also common.

  • What surface finishes are available on stainless steels?

    There are many different types of surface finish on stainless steel. Some of these originate from the mill but many are applied later during processing, for example polished, brushed, blasted, etched and coloured finishes.

    The importance of surface finish in determining the corrosion resistance of the stainless steel surface cannot be overemphasised. A rough surface finish can effectively lower the corrosion resistance to that of a lower grade of stainless steel.

  • Can I use stainless steel at high temperatures?

    Various types of stainless steel are used across the whole temperature range from ambient to 1100 deg C. The choice of grade depends on several factors:

    1. Maximum temperature of operation
    2. Time at temperature, cyclic nature of process
    3. Type of atmosphere, oxidising , reducing, sulphidising, carburising.
    4. Strength requirement

    In the European standards, a distinction is made between stainless steels and heat-resisting steels. However, this distinction is often blurred and it is useful to consider them as one range of steels.

    Increasing amounts of Chromium and silicon impart greater oxidation resistance. Increasing amounts of Nickel impart greater carburisation resistance.

  • Can I use stainless steel at low temperatures?

    Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.

    Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.

    Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.

    In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.

    Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite. 

  • Is stainless steel non-magnetic?

    It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1. 

    It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

    In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

    Martensitic, ferritic, duplex and precipitation hardening steels are magnetic.

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