Stainless steel is a material that’s easy to fall in love with. It is sleek, shiny, strong, doesn’t flake or wear-off and has a nice smooth feel to it. And stainless steel’s beauty is long-lasting, which it owes to its most notable characteristic – it doesn’t rust. Stainless steel has brought such vast changes to industries as automotive, aviation, food, machinery and medicine that it can easily be called the metal of the 21st century.
Iron and steel production go back thousands of years B.C. In contrast, stainless steel has been discovered less than a hundred years ago. Harry Brearley (1871-1948) is mostly credited with its invention. In 1912, Brearley was working at the Brown-Firth Research Labratory in Sheffield, England, and in search for a corrosion-resistant steel for gun barrels, when he noticed that a combination of chromium and iron led to desired result.
The word about Brearley’s invention spread fast in Sheffield, a town known for it’s fine cutlery since the 16th century. The cutlery industry highly embraced the new alloy. Up to that time kitchen utensils were mainly made out of carbon steel, which starts to corrode rapidly when in contact with food. Silver, then the only ‘affordable’ metal which was corrosion resistant, was too expensive for most people. A Sheffield cutlery manufacturer also coined the term “stainless steel”, hoping for a positive marketing effect (until then it was called ‘rustless steel’).
I’m not sure how much of stainless steel’s success can be credited to the name, but it soon paved the way for modern technology. It was used in car parts, airplane engines, toasters, vacuum cleaners, trains, kitchen equipment, tools, surgical instruments and jewelry. Stainless steel became so vital to the war industry that England banned its production for anything else during WWI, in 1917.
The sign of modernity which stainless represented may be best embodied in the Chrysler building. When built in 1928, it was not only the city’s tallest building, but the top arches were clad in shining 302 grade stainless steel. A recent inspection of the building showed how well the material was suited for the job – no signs of corrosion or deterioration were found.
Corrosion is a big deal. It is estimated to cost the US 276,000,000,000 dollars every year (4.2% of the GNP). An average 8% of our electricity bill is due to corrosion, and in some industrial countries it is the reason that 30% of the water never makes it from the water plant to the consumers.
In general, all metals except gold, platinum, and palladium corrode. Also stainless can rust when it loses it’s ‘corrosion shield’. This protection is a thin film (one ten thousandth of a human hair) of chromium oxide, which keeps the iron in the steel from turning into rust. The film is automatically formed when a minimum of 10.5% chromium is added to iron and it immediately repairs itself when scratched.
When looking for reasons why stainless steel corrodes under certain circumstances, you have to first look for things that destroy the chromium oxide film. Because only when the film is destroyed, the usual environmental influences that lead to corrosion take effect.
Chloride, acid and absence of oxygen are the biggest dangers to the chromium oxide film. Regular water has too low levels of chloride and enough oxygen to react with the chromium and no effect on stainless steel. But when the water has a very low PH (high acid), or the chloride is very high (as in some swimming pools or sea water), or there is very little oxygen (as sometimes in sea water with a large amount of algae), stainless steel will corrode. Also the acids in food, or contact with other metals (e.g. lower grade alloy screws) can lead to corrosion. But usually a long-term exposure is required for the corrosion to start taking effect.
IN THE KITCHEN
Today, stainless steel is the standard in commercial food processing, storage, transportation and preparation as well as for equipment and surfaces in modern restaurant kitchens. And for good reason. Stainless steel’s surface has no pores or cracks to harbor dirt, grime or bacteria. It doesn’t change the color or taste of food. It releases small amounts of iron and chromium, which are healthy. Stainless steel doesn’t chip, need painting or surface finishes. It is fingerprint resistant and doesn’t require aggressive cleaners.
For the home chef, a wide array of stainless steel products are available – knives, silverware, utensils, measuring cups, pots, containers and cookware, and others. With proper care, stainless steel has almost unlimited life expectancy. Proper care means keeping the chromium oxide film intact, so substances with chlorine (as in some cleaners) and or salt should not be exposed to stainless steel for extended periods. Also food, whose acids can destroy the film should not be stored or exposed to in some stainless steels for a longer time.
The best way to keep stainless steel intact is immediate cleaning with water, mild soap and a soft cloth. To keep it shiny, you should never use abrasive powders or materials on stainless.
GRADES: HARDNESS, CORROSION RESISTANCE & MACHINABILITY
Not all stainless steels are the same. In fact there are more than 180 different steel alloys that fall under the stainless steel category (containing a minimum of 10.5% chromium). Nickel is most commonly added, but also molybdenum, copper, carbon, titanium, silicon, aluminum, vanadium, nitrogen, sulfur and others. The addition of these metals and non-metals influence stainless steel’s properties, most importantly corrosion resistance, hardness, machinability and production costs.
But there are other factors that influence these qualities. Finishes, which vary in smoothness and shininess, affect the corrosion – the smoother the finish, the better the corrosion resistance. Stainless steel can be hardened through a series of temperature changes (heat treatment); and rolling, hammering, or stretching at low temperature (cold working). Heat hardening achieves better results, but not all stainless steels can be subjected to it. Hardness can be measured in ‘Rockwell‘ or ‘Brinell’.
You can generally speak of a quality difference (which also relates to the price) of stainless steel, but cheaper stainless steel work better in some applications than higher priced ones, because they might have properties that higher priced stainless steels don’t possess (e.g. machinability).
The most commonly used rating for stainless steel is SAE grade. It consists of three to four digit numbers, sometimes with the addition of he letter “L” or “H”. “L” is an indicator for low amounts of carbon, and “H” for high amounts of carbon. Carbon makes stainless steel harder but also more sensitive to corrosion, and vice versa. The best quality stainless steel knife blades have a high carbon content, and usually have molybdenum and vanadium in their composition.
Stainless steels are also classified in four different types (see also chart at bottom) –
AUSTENITIC (SAE 300series)
They make up 70% of total stainless steel production. Austenitic stainless steels have a 4-22% content of nickel, which makes them generally best in corrosion resistance (especially to food acids). However, since they cannot be thermal hardened, they are referred to as low-grade for knives and some kitchen equipment. Austenitic stainless steels can be bent in shape easily, without fracturing (for example in kitchen sinks). They are excellent to weld and are superior for uses in very low-temperature environments.
The second-largest class of stainless steel, constituting of approximately 25% of stainless steel production. Ferritic stainless steels contain no nickel. This makes them less corrosion resistant and less strong, but cheap and well suited for high temperatures. They are used in automotive trim, mufflers, interior architectural trim, and hot water tanks. Due to their lower corrosion resistance, kitchen products made out of ferritic stainless steel should not be put in dishwashers (no matter what the label says).
Martensitic stainless steels contain no nickel, but a high amount of carbon. Carbon makes them especially hard, and through thermal treatment their hardness can be further increased. They are the preferred steel for knives. In order to keep a good corrosion resistance, the chromium needs to be increased in relation, when adding of carbon. The complex production process as well as the high nickel content makes martensitic steels most expensive. However, they are not suited for all purposes, since they can’t be formed into complex shapes.
Duplex stainless steels were developed to achieve a balance between the corrosion resistance of austenitic, and the lower price of ferritic stainless steels. They are excellent in resisting chlorine, which makes them especially well suited for the offshore oil and gas industry
Stainless steel is 100% ‘truly’ recyclable. Truly, because there is no loss in quality no matter how many times it’s being processed (other than for example in plastics where re-processing usually goes along with a downgrade in quality). Approximately 60% of all stainless steel comes from recycled steel, and 90% of all stainless steel is being recycled.
As in all steel production, a lot of energy is needed for its production (also when recycled), but this may be justified due to it’s long lasting qualities. With proper care, stainless steel products last hundreds of years.