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Stainless Steel

What is Stainless Steel?

Stainless steel is a group of iron-based metal containing at least 10% chromium (alloy metals).

The chromium oxide "CrO" creates an invisible barrier ("passive film") to oxygen and moisture. Therefore the Chromium protects the iron against most corrosion or red-colored rust; thus the term “stainless” steel.

The layer is too thin to be visible, meaning the metal stays shiny. It is, however, impervious to water and air, protecting the metal beneath. Also, when the surface is scratched this layer quickly reforms.

The purpose of stainless steel is to provide hard steel material highly resistant to stain, rust and corrosion and resistance against:

- Adverse atmospheric conditions such as carbon dioxide, moisture, electrical fields, sulfur, salt, and chloride compounds
- Natural and artificially produced chemical (e.g. ozone)
- Extremes of weather such as cold temperatures

In 1821 the Frenchman named Berthier found that iron when alloyed with chromium was resistant to some acids. It is worth stating that all steel types exposed to water and oxygen will corrode. However it is accepted as an international standard, that if less than 0,1 mm of the surface of a stainless steel plate is corroded in a year, then it is durable and can be accepted as chosen quality for the application.


Environmental issues

Stainless steel is 100% recyclable. In fact, over 50% of new stainless steel is made from remelted scrap metal, rendering it an eco-friendly material. Product dusts from processing
may be classified as a hazardous waste, depending on various properties of the dust (e.g. toxicity, solubility, flammability).

Articles produced from solid product are not an ecological hazard. However finely divided product are hazardous, based on its components, to fish, animals, plants and the environment if released, the degree of which would depend on the particle size and quantity released.

The solid product is not expected to migrate easily into soil or groundwater based upon its insoluble form. Nevertheless if finely divided material becomes mobile in water and contaminate soil and groundwater. In fact, this material may persist in the environment for long periods, based upon its corrosion resistant, insoluble, and non-biodegradable properties.

The energy consumption based on the stainless steel production has decreased recently due to technological improvements and energy conservation measures.

Occupational health and safety issues

Any articles manufactured from these solid products would be generally classified as non-hazardous. However, some metallic elements contained in these products have been determined to be toxic and are subject to regulatory controls. These elements can be emitted as airborne contaminants under certain processing conditions such as burning, melting, cutting, sawing, brazing, grinding, milling, machining.

Certain materials and equipment utilized in processing of steel products (cutting/machining fluids, coatings, processing, lubricants, cleaning/pickling chemicals, welding fluxes, torch and plasma cutting systems) may constitute a health hazard and should be treated accordingly.

Types of stainless steel

There are over 150 grades of stainless steel, of which fifteen are most common.
The AISI (American Iron and Steel Institute) defines the following grades among others:

- 200 Series—austenitic iron-chromium-nickel-manganese alloys

- 300 Series—austenitic iron-chromium-nickel alloys
Type 301—highly ductile, for formed products. Also hardens rapidly
during mechanical working.
Type 303—free machining version of 304 via addition of sulfur
Type 304—the most common; the classic 18/8 stainless steel
Type 316—Alloy addition of molybdenum to prevent specific forms of
corrosion
- 400 Series—ferritic and martensitic alloys.

Austenitic means a metallic, non-magnetic solid solution of carbon and iron that exists in steel above the critical temperature of about 723°C. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel.

Ferritic and martensitic alloys are highly corrosion resistant, but far less durable than austenitic grades and cannot be hardened by heat treatment.

Steel types used in water of different salinity and temperature

Salinity/ temperature max 15 degrees max 20 degrees max 30 degrees
max 0.5% salinity AISI304 AISI304 AISI304
AISI316L AISI316L EN1.4462


More information about the elements of the periodic table

Other materials:

Monel

Teflon

Glass

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