One may suppose that stainless steels do not stain. Yet, you will be mistaken if you think so.
Stainless steels do not stain as easily as other iron metals. However, they are not actually stainless. Similar to standard steel, it is possible to mark out stainless steel through fingerprints and oil. They lose their color and get rusted. The matter is about elasticity, endurance to overuse prior to emerging of any wear-out sign.
Any steel is basically composed of iron and carbon. Stainless steels consist of a certain amount of chrome. This creates the composition which makes stainless steel stain-resistant.
At this point, we get into confusion. There exist varieties of stainless steel. Every variety has got a slight difference in composition, hence, it is characterized by varied features.
Stainless steels are comprised of no less than 10.5 % chrome. Some grades might have more quantities of chrome, other admixtures, for instance, molybdenum, nickel, titanium, aluminum, copper, nitrogen, phosphor and selenium.
304 and 316 grades seem the most widespread varieties. 316 variety has molybdenum, whereas 304 has none. Molybdenum turns the steel more stain-resistant, particularly for an environment of higher saline and chloride exposure.
For outer furnishing, for example, rails or bollards, stainless steels are perfectly suitable. They can resist long abuse provided that their grades meet the needs of the environment. 304 mostly seems the best option for its economy and practicality, yet it is not chloride-resistant like 316. Thus, the high costs of 316 can be compensated by this feature, particularly for waterside and saline pavements. Applications for these materials dictate certain requirements. Each variety of stainless steel should meet the needs of the application.
Corrosion is something that occurs naturally. Each pure element comes into the reaction with surroundings. This is the reason any pure element in nature is so scarce. The same refers to iron.
In terms of humidity, iron gets into the reaction with oxygen forming iron-oxide. The latter is seen as rust. Reddish flaked oxide corrupts naturally, corroding even more portions of metal. Iron and any steel of standard carbon can easily be exposed to such staining.
Stainless steels are inherently capable of forming an inactive coat to prevent staining. How can this be explained?
Chromium
Chrome contained in stainless steel gets into reaction with oxygen, like iron. At this point, a very thin coating can get organized (in general just-few-molecule thick). In contrast to flaked, unsteady iron-oxide, chrome is long-lasting and resistant to the reaction. It sticks to the outer side of steels and does not come in reaction with any other material. Chrome has the capability of getting renewed. In case of its removal or damage, even more, chrome gets into reaction with oxygen to create the hurdle. The higher the chrome quantities, the quicker the hurdle recovers itself.
After oxidation and passivization, these materials generally get rusted very slowly, approximately 0.002 inches each year. If provided ideal conditions, stainless steels will surely offer a shiny and smooth surface suitable for any constructional and scenery design.
Stainless Steels of 304 Variety
This type has a wide application worldwide owing to the perfect stain-resistant feature it has. Its composition includes 16-24 % chrome, less than 35 % nickel, a little carbon, and manganese.
The 18/8 stainless steel of 304 grade consisting of 18 % chrome and 8% nickel has the vastest application. These metals are able to resist oxidizing substances. They are durable to sanitation, thus, suitable to be applied in kitchens. It can often be used in building, furnishing, and decoration.
However, 304 has got one drawback: it is sensitive to staining brought about by chloride solution and saline environment. Ions of chloride produce secluded zones of staining, known as “pits”, spreading underneath the chrome hurdles of protection and endangering the inner architectonic layers.
Any solution containing at least 25 ppm sodium chloride has corroding effects.
Stainless Steels of 316 Variety
The mentioned above material comes 2nd in the application of these metals. It shares the basic characteristics with the 304 series comprising the same makeup content. 316 differs from 304 with its component of 2-3 % molybdenum. This makes the steel more resistant, in particular to chloride and any solvent of industrial type.
Stainless steels of 316 have a wide application in industry, including chemical processing, and surroundings containing significant amounts of saline, for example, coast and open zones, where salt deicing is quite common. Since steels of 316 get into reaction less easily, they are intensively applicable in the production of the medical apparatus.
Optional 300-range contains no more than 7 % molybdenum. These steels are even more resistant to chlorides, yet, such high levels of resistance are required for strict exposures.
Universal Scopes of Application
Certain quantities of nickel are added into 304, 316 and 300 grades to keep the austenite makeup at a low temperature. Steels of austenite provide an equal distribution of force, machinability and stain withstanding, which makes them suitable for open structural solutions, surgical instruments, and apparatus for food industries.
Stainless steels (mostly 316 grade) are vastly applicable in the food and drink industries. Stainless steels are applicable not just in the commercial cookery but also in food factories:
- They are readily shaped and manufactured in the forms required for the production of various equipment and mechanisms, like a kitchen table, ventilation hood, tank, and hopper.
- They are ideal for a stream of applications in decoration and polishing.
- They are resistant to shocks and abrasion in kitchens and food factories.
- They are easy to clean and can repeatedly be washed with chemical and detergent stuff to meet the sanitary needs.
- They do not get into reaction with alkalis and acid present in milk, prepared food, vegetables, and food additions.
Stainless steels are advantageous, due to their longer service life, which provides a fine and neat appearance. These materials require low maintenance costs if looked after and cleaned suitably.