Stainless steel is a corrosion resistant alloy of iron, chromium up to 10% to 11% and in some cases nickel and other metals like molybdenum, manganese.
What makes stainless steel stand out from all is its corrosion resistance property. Its stainless property is because of the chromium which reacts with oxygen in the air that forms a protective layer that makes stainless steel highly resistant to corrosion and rust. The higher the contents of chromium the less corrosion would be. Stainless steel is formed when raw materials of nickel, iron ore, chromium, molybdenum, manganese are fused together forming an alloy.
Resistant of corrosion can be obtained through:
- Increasing chromium content.
- Adding of nickel upto 8%
- Addition of molybdenum
- Addition of nitrogen which also increases its strength.
Varying with the amount of chromium and nickel content in the alloy stainless steel are made accordingly to suit the environment it is to be used or endured. Requirements of low maintenance and luster make stainless steel an ideal material for many applications which require corrosion resistance and strength.
Some usage of stainless steel are:
- Surgical instruments
- Major appliances for vehicles
- Large building construction materials
- Equipment for industries like paper mills, chemical plants, water treatment storage.
- Storage tank for chemical
- Food products storage,etc.
The invention of stainless steel followed a series of scientific developments starting in 1798 when chromium was first shown to the French Academy. In the 1800s British Scientists observed the resistance of chromium-iron alloys to oxidizing agents. The scientists were James Stoddart, Micheal Faraday and Robert Mallet. Then in 1861 Robert Forester Mushet, a British metallurgist and businessman, took out a patent on chromium steel in Britain. The product patent for the product was issued in 1896. The first production of chromium containing steel in America was done by J. Baur of the Chrome steel works of Brooklyn for the construction of bridges. Later John T Wood and John Clark pursued the commercial value for ” Weather Resistant Alloys”. It was during 1904-1911, Leon Guillet of France prepared alloys that are considered stainless steel today among several researchers. In 1912 while seeking a corrosion resistant alloy for gun barrels, Harry Brearley discovered and industrialized a martensitic steel alloy that can be hardened and tempered through multiple ways of treatment with heat.
There are 5 families of stainless steel classified primarily by their crystalline structure:
- Austenitic stainless steel: It is the largest contributing about two-third of the stainless steel production. It is achieved by alloying steel with nickel or manganese and nitrogen with sufficient amount to maintain an austenitic microstructure at all temperature ranges up to melting point so austenitic stainless steel cannot be hardened by heat treatment.
> 200 series: chromium-manganese-nickel alloys with maximum use of manganese and nitrogen and minimum use of nickel.
chromium-nickel alloy, achieved by nickel alloying.
- Ferritic stainless steel: It possesses ferrite microstructure like carbon containing 10%-26% chromium with very little nickel. Ferritic stainless steel is magnetic and less expensive than austenitic stainless steel due to the near absence of nickel.
Presence in product like
– automobile exhaust pipe
– architectural application
– chimney duct in building
– roofing, etc
– power plates operating at 700°C.
- Martensitic stainless steel: It is used in stainless engineering steel, stainless tools, etc. It is also magnetic but not as corrosion resistant as ferritic stainless steel and austenitic stainless steel due low chromium content.
- Subdivided into:
– iron-chromium alloy C grade
– iron-chromium-nickel alloy C grade
– Precipitation hardened alloy is the best grade among martensitic stainless steel.
– creep resistant alloy.
- Duplex stainless steel: It has 50-50 mixed microstructure of austenitic stainless steel and ferritic stainless steel property having high chromium content up to 32% and molybdenum up to 5% and lower nickel content than austenitic stainless steel.
- Based on corrosion resistant it is of three subgroup:
– lean duplex
– standard duplex
– super duplex
Properties of stainless steel:
- Stainless steel is a relatively poor conductor of electricity.
- Martensitic stainless steel and ferritic stainless steel are magnetic. The central atom of these two types of stainless steel is responsible for magnetic properties. Austenitic stainless steel when bent or cut along the edges magnetism occurs as the crystal structure rearranges itself.
- Galling, a process sometimes known as cold welding which is a form of severe adhesive wear that occurs when the surface of two metals are in relative motion and under pressure is likely to occur as thread galling in austenitic stainless steel.
Recycle, reuse and sustainability of stainless steel:
Stainless steels are cent percent recyclable. By a study it was found that an average per capita stock of stainless steel in society for use is 50-150 kg in developed countries and up to nearly 16 kg in developing and less developed countries. It also mentioned that high availability of stainless steel is preventing the rate of recycling of stainless steel.
Cycle of stainless steel steps are:
- It starts with carbon steel scrap, primary metals and slag.
- Production of hot rolled and cold finished steel products or directly reused in melting shops.
- Components manufacturing.
- Collection for recycling of stainless steel such as kitchenware, pulp and paper plants and automobile plants.
Health effects of stainless steel:
- Welding of stainless steel produces fumes which being inhaled indicate possible risk of cancer specially lung cancer as per many researchers done on.
- Nickel and chromium that leach out of new stainless steel cookware cooking highly acidic foods can contribute to lung cancer and nasal cancer.
Advantages of stainless steel:
- Corrosion resistance
- Readily available
Disadvantages of stainless steel:
- Can’t be mold at any shape easily
- As steel is conductive in nature it had to be treated for electric resistivity.
Hard anodizing is the process of anodizing which means an electrolytic process for producing thick oxide specially aluminium in a special cold electrolyte. The cold electrolyte is an acid solution that cools down to the freezing point of water. It is the process which is also known as hard coat anodizing. In simpler words, it is the process in which aluminium is placed in an acid bath and given an electric charge. These electro- chemical processes combined the aluminium with oxygen to produce aluminium oxide. This coating is a non-metallic conversion coating which is very thin.
The thickness of hard oxide film of aluminium and aluminium alloy depends on the temperature, concentration of electrolyte current density and raw materials composition. Hard anodized are low cost, lightweight and enhanced appearance and performance. The thickness of hard anodizing coating on aluminium surfaces is usually 20-150 um. Aluminium oxide coated on aluminium surface by hard anodizing increases the hardness and makes the aluminium alloy more durable, corrosion resistant, abrasion resistant and wear resistant.
Properties of hard anodizing aluminium products:
- Hard anodized aluminium is thicker than standard anodized in regards to its thickness and appearance. The surface of hard anodized aluminium is more abrasion resistant, corrosion resistant and durable and also smoother and the surface is more uniform .
- When the thickness of the coating of hard anodized aluminium is more than 25 microns, low temperature and high current density are enough to produce the hard anodized coating.
- Hard anodized uses sulphuric acid solution and organic acid added sulfuric acid such as oxalic acid, aminosulfonic acid, etc.
- During the process of conversion to aluminium oxide hard anodized aluminium does not need to be sealed to close the pore as in standard anodized aluminium because its thicker oxide layer is more wear resistant.
Application of hard anodized aluminium:
- The products of hard anodized aluminum are more suitable for heavy duty or wear resistant and harsh environments. Such as exterior of buildings, cookware, hydraulic, pistons, etc.
- Hard anodized aluminium is found more in industrial and commercial applications. Those for architecture, automobile industries and aerospace are not hard anodized aluminium but standard anodized aluminium.
The hard anodized non-stick aluminium cookware:
In most kitchens nowadays, non-stick cookware which are hard anodized aluminium have become a very popular must have cookware.
The hard anodized cookware are those categories of cookware like pans and pots, or saucepan and stock pots that have been manufactured to be resistant to sticking and durable. These categories are expensive in most of the product but people still invest in it for its quality of durability as once brought with proper care and maintenance, would not have to worry of breaking or holes if its the good quality product.
Cookware made of hard anodizing is extremely durable, it is twice as durable as most of the stainless steel products available in the market as stainless steel cookware, and also abrasion resistant and durability also.
Hard anodized aluminium hold up much better than the standard aluminium products, as the atomic configuration of aluminium and hard anodized aluminium is same and the products are able to easily bind to anodized one as easy as aluminium but hard anodized aluminium are more durable, resistant to abrasion and corrosion.
Health effects of hard anodized aluminium:
- Many researchers have linked Alzheimer and some certain cancers to hard anodized non-stick cookware.
- Certain non-stick coating when heated to very high temperature can emit toxic fumes potentially.poor quality and inexpensive hard anodized coated can disintegrate over time which can lead to flaxing so top quality products have been recommended to avoid such problems.
Stainless steel vs hard anodized:
Points that can be noted down after discussion of stainless steel and hard anodized are:
- Hard anodized cookware is harder than stainless steel cookware.
- Hard anodized is durable than stainless steel, but also abrasion resistant, wear resistant along with corrosion resistance.
- Hard anodized cookware is less expensive and conducts heat more uniformly than stainless steel. As for browning ingredients, stainless steel is better than coated non-stick ones.
- Stainless steel is safer and healthier in the case of cooking acidic foods. In case of using oil, hard anodized cookware needs less oil as it has been made non-stick whereas the foods might stick to stainless steel cookware.
- Stainless steel is easier for maintenance than hard anodized.
- Stainless steel is more versatile.