Stainless Steel is a preferred material for obtaining different designs. Stainless steel is preferred due to its beautiful appearance, corrosion resistance.
Stainless steel is an alloy that contains at least 50% iron and 10% chromium, the chromium slows down the corrosion process and plays a key part in defining stainless steel, generally the higher the chromium percentage (up to 30%), the more corrosion resistance. Other elements are also added to enhance the properties of a particular grade and type of stainless steel.
|Austenitic Stainless Steels|
|301, 301L, 301LN||High strength for roll formed structural components|
|302HQ||Low work hardening rate grade for cold heading fasteners|
|303, 303Se||Free-machining bar grades|
|304, 304L, 304H||Standard 18/8 grades|
|310, 310S, 310H||High temperature resistant grades|
|316, 316L, 316H||Improved resistance to pitting corrosion in chloride environments|
|321, 321H, 347||Stabilised grades for heavy section welding and high temperature applications|
|253MA||High temperature resistant grade|
|904L||High resistance to general corrosion, pitting and stress corrosion cracking|
|Ferritic Stainless Steels|
|AtlasCR12||Utility steel resistant to wet abrasion and mild corrosion|
|AtlasCR12Ti||Utility steel resistant to wet abrasion and mild corrosion – weld stabilised|
|409||Automotive exhaust grade – weld stabilised|
|430, 430F||Resistant to mildly corrosive environments|
|F18S||Resistant to mildly corrosive environments – weld stabilised|
|F18MS / 444||A ferritic alternative to grade 316 / 316 L – Weld stabilised|
|F20S||A ferritic alternative to grade 304 / 304L – Weld stabilised|
|Duplex Stainless Steels|
|2101||Lean duplex for tanks and structural applications|
|2304||Duplex alternative to grade 316|
|2205||Standard duplex stainless steel – high resistance to pitting and stress corrosion|
|2507||Super duplex with very high resistance to pitting and stress corrosion|
|2507Cu||Super duplex with very high resistance to pitting and stress corrosion|
|Martensitic Stainless Steels|
|410||Standard martensitic grade for low-duty hardened applications|
|416||Free-machining bar grade|
|420||Higher hardness martensitic grade for cutlery, cutting tools and dies|
|431||High hardness and toughness grade, primarily for shafting|
|440A, 440B, 440C||Very high hardness grades used in cutting tools|
|Precipitation Hardening Stainless Steel|
|630||(17-4PH) High strength shafting grade|
The chromium oxide formed on the stainless steel surface has the ability to self-repair. However, if the damage is too intense, corrosion occurs. Types of corrosion that need attention.
The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack. The pitting corrosion resistance can be assessed using the Pitting Resistance Equivalent Number calculated from the alloy content.
Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices, it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.
Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and sulphuric acid at some concentrations are particular aggressive towards stainless steel.
This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. Typical applications where SCC can occur are hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.
This is now quite a rare form of corrosion. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitisation and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur. It is avoided by choosing a low carbon grade the so-called ‘L’ grades or by using a steel with Titanium or Niobium which preferentially combines with Carbon.
If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. This is rather like a battery and can accelerate corrosion of the less ‘noble’ metal. It can avoided by separating the metals with a non-metallic insulator such as rubber.
Poor design and fabrication can more serious corrosion of stainless steels. In the design stage, it is advisable to avoid crevices such as intermittent welds and areas where water can collect, it is also imperative to avoid carbon steel contamination.
Rough surfaces promote corrosion. The main cause of this problem is the salt deposited on the stainless steel surface, the smoother the finish the better the corrosion resistance. To reduce the risks of corrosion, choosing or creating a smooth surface are ideal combatants. Smooth surface finishes stay cleaner between washes and don’t have deep surface grooves where chlorides and other contaminants can collect and concentrate.
A process called pickling may be used to remove surface contaminants but may dull the surface. Alternatively, electropolishing may be used which has the added benefit of brightening the surface.
Even smooth stainless steel finishes in external applications may show corrosion if not washed regularly. Rain washing the stainless steel surface can help reduce corrosion, and should therefore be an important project design consideration. If this is not possible then best results are achieved by washing with soap or mild detergent and warm water followed by ringsing with clean cold water. Surface appearance may be further enhanced by wiping dry the washed surface and treating the stainless steel with an approved polish and protective coating spray.
The outstanding properties of stainless steel were gradually realized over the years, and below is a short timeline outlining some of the key moments in the history of stainless steel.
Over the last 100 years, about 100 grades of stainless steel have been discovered and made commercially available. These grades fall under four main family groups, namely, martenistic, ferritic, austentic, and duplex. Ferritic and martensitic stainless steels are magnetic while austenitic stainless steels are non-magnetic. These have varying quantities of other alloying metals like nickel, titanium, and copper, added to them. Carbon and nitrogen are also added to enhance the overall characteristics of stainless steel.
Today, China is the largest producer of stainless steel in the world. One of the leading stainless steel producers and distributors is Outokumpu, a group of companies headquartered in Espoo, Finland.
Stainless steel has found a myriad of applications from the tiniest structural parts in artificial heart valves to the largest architectural structures. Several world famous monuments, such as the Cloud Gate sculpture in Chicago, have been constructed using stainless steel.
With the growing awareness about the importance of reducing carbon emissions, many countries are keen on promoting stainless steel which is resistant to corrosion and has prolonged service life, therefore not requiring frequent replacements and repairs. Additionally, stainless steel is also 100% recyclable and does not degrade when reprocessed, thus allowing for multiple life cycles. The sustainability of stainless steel is unmatched compared to other metals.
The growth of stainless steel is likely to increase as the sustainability benefits of stainless steel become more widely known. Researchers have even found that coating stainless steel with certain bio-inspired adhesives makes it anti-bacterial, thus adding to the long list of benefits of stainless steel.
This innovative material is therefore a material that is very likely to extend its use well into the next century and beyond.
Stainless steel is 100% recyclable. An average stainless steel object is composed of about 60% recycled material of which approximately 40% originates from end-of-life products and about 60% comes from manufacturing processes. According to the International Resource Panel’s Metal Stocks in Society report, the per capita stock of stainless steel in use in society is 80–180 kg in more developed countries and 15 kg in less-developed countries.
There is a secondary market that recycles usable scrap for many stainless steel markets. The product is mostly coil, sheet, and blanks. This material is purchased at a less-than-prime price and sold to commercial quality stampers and sheet metal houses. The material may have scratches, pits, and dents but is made to the current specifications.