Stainless steel contains iron, chromium, manganese, silicon, carbon and, in many cases, significant amounts of nickel and molybdenum. These elements react with oxygen from water and air to form a very thin, stable film that consists of such corrosion products as metal oxides and hydroxides.
Chromium plays a dominant role in reacting with oxygen to form this corrosion product film. In fact, all stainless steels by definition contain at least 10 percent chromium. The presence of the stable film prevents additional corrosion by acting as a barrier that limits oxygen and water access to the underlying metal surface. Because the film forms so readily and tightly, even only a few atomic layers reduce the rate of corrosion to very low levels.
The fact that the film is much thinner than the wavelength of light makes it difficult to see without the aid of modern instruments. Thus, although the steel is corroded on the atomic level, it appears stainless. As such, this film, otherwise known as rust, achieves sufficient thickness to make it easily observable soon after exposure to water and air. In summary, stainless steel does not rust because it is sufficiently reactive to protect itself from further attack by forming a passive corrosion product layer.
Once stainless steel processing equipment has been passivized and placed in service, the passive layer can be damaged by being abraded or through expansion or contraction caused by heating and cooling.
If enough oxygen is present to combine with the chromium in the alloy and other conditions are right , the passive layer will "heal itself," which is one of stainless steel's major benefits.
Suppose your processes use certain chemicals under certain circumstances such as high temperatures. In that case, you won't be able to rely on the passive layer to protect your equipment investment because corrosion will be inevitable.
Processes that are more demanding due to chemicals involved, processing conditions, or both mean you should look at an upgrade from stainless steel to corrosion-resistant alloys. These additions provide a significant boost to corrosion resistance and are designed to withstand very challenging environments that involve chlorides , reducing acids, and salts. Even where a processing system's passive layer is successfully standing up to daily demand, regular system re-passivation is advised as part of routine maintenance.
Some processors re-passivate once a year, but others might do it more often because their products, such as those made from tomatoes, are high in chlorides and corrosive acids.
Some water used in processing is naturally high in chlorides and hard on the passive layer. There are test kits available from chemical supply firms that will test for free surface iron. If a high level is found, it could be time to passivate. Pickling is a process that's often confused with passivation, but they serve different purposes. They're both aimed at improving the corrosion resistance of stainless steel and other alloys by promoting an effective chromium oxide passive layer.
While underheating during welding can cause poor weld penetration, overheating can negatively affect the physical properties and chemistry of stainless steels and other alloys. It can oxidize the component metals, which causes the metal to take on a range of colors from yellow to brown to blue depending on the temperature they were exposed to and how thick the oxidized layer is.
This discoloration is called "heat tint. In stainless steels and other alloys where chromium has the central role in corrosion resistance, a heat tint area means the chromium has been depleted from the metal's surface and is unavailable to form the passive layer.
Therefore, the damaged, oxidized layer must be removed to re-expose the alloy in its original , corrosion-resistant form. Like passivation, pickling is done with chemicals - usually nitric or hydrofluoric acid solutions - and the acids are much more aggressive. Pickling can also be done by electropolishing when metal is immersed in a solution carrying an electrical current that removes a very thin layer of the metal's surface or by mechanical removal, which may leave small contaminating particles.
ASTM is an international organization that creates quality and practice standards for industrial materials, products, services, and processes. There are currently around 12, standards. New ones are created when stakeholders, including trade associations, government agencies, professional societies, manufacturers, and consumer groups, make requests for new ones to be developed as needs arise. New standards are developed by one of over technical committees devoted to specific areas of expertise.
ASTM Technical Committee G01 on Corrosion of Metals promotes knowledge and research, collects data, and develops standard test methods, practices, guides, classifications, specifications, and terminology relating to corrosion and methods for corrosion protection of metals. The difference is as it relates to cast or wrought steels, stainless steels and related alloys and ferrous alloys, and this is the committee that developed the two standards related to passivation:.
It sets out proper process steps, what the passivated surface should look like, and tests that should be conducted to show that the passivation was successful. Care should always be taken when deciding what material to use for your hygienic processing system. Removal of Excess Carbon. The molten material placed into a vacuum oxygen decarburization VOD or argon oxygen decarburization AOD system to remove excess carbon. Depending on how much carbon is removed, this process could result in a standard or a low-carbon variant of the alloy—for example, versus L stainless steel.
This can affect the tensile strength and hardness of the final product. Tuning or Stirring. This helps to ensure that the stainless steel is of uniform quality and will meet the specifications required by end users like Marlin Steel.
Forming the Metal. As the stainless steel begins to cool, it is put through a variety of forming processes—starting with hot rolling while the steel is still above its crystallization temperature.
Hot rolling helps get the steel into a rough shape, and is often used to create billets or blooms of metal. To create metal blooms or billets of precise dimensions, the stainless steel may be cold rolled.
Cutting and Shaping. After the annealing process, stainless steel is put through a variety of cutting and shaping processes to create an ideal final product for the application. For example, the steel may be cut mechanically with large metal shears if making thick metal plates. Meanwhile, CNC punch or laser cutting machines may be used to cut shapes out of thinner metal sheets. Applying Surface Finishes.
The stainless steel manufacturer may apply different surface finishes to their stainless steel billets, blooms, or wires before shipping them to other manufacturers. Quality Control Before finishing a stainless steel basket or product, the engineers at Marlin Steel run finite element analyses on each and every design. Stainless Steel Manufacturing at Marlin Steel Marlin Steel does not manufacture stainless steel wires and sheet metal in-house.
Author: Marlin Steel.
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