Stainless steel

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Stainless steel is a group of alloys. It contains iron and a minimum of 10.5% chromium.^{[citation needed]}

Stainless steel is the material of choice for modern brewing vessels, and is the only metal in contact with wort and beer.

When buying stainless look at the alloy, there's a significance difference between 200 and 300 series. 200 series has much lower corrosion resistance and it's not very good for use with brewing. 304 and 316 alloys are best.

See here for a summary of corrosion mechanisms: https://astropak.com/wp-content/uploads/2014/07/the_passive_layer.pdf

Pitting corrosion arises where passivity is lost at localized spots on the metal surface, usually under the action of aggressive anions, among which chloride is the most commonly encountered aggressive anion causing pitting corrosion. Moreover, as well known, the evolution of corrosion pits on stainless steel immersed in chloride solution occurs in three distinct stages: nucleation, metastable growth and stable growth. Several studies have shown that pits can initiate from oxide inclusions and it has already been demonstrated that manganese sulfide inclusions play a critical role and pitting events are found to occur at, or close to, such second-phase particles. Chemical changes in and around sulfide inclusions have been postulated as a mechanism for pit initiation.

Beer is an excellent electrolyte and that if the brewer has mild steel contacting copper (e.g. a copper immersion chiller in a kettle), the steel will corrode. If the brewer has copper contacting passivated stainless steel, the copper will corrode. Brass fittings and silver solder are right in the middle with regard to potential, but fortunately the difference is small and corrosion rates will therefore be low.

Crevice corrosion: Let's say we have an electrolytic solution containing chlorine ions; bleach water, for example. These chlorides are caustic (alkaline) and deteriorate the protective oxide layer. If a stainless steel container is completely full of this electrolyte, every surface is at the same electrical potential and nothing happens. But what if the wall has a deep scratch in it, or a rubber gasket against the steel creates a crevice? These areas can become electrically different from the surrounding area, creating a galvanic cell. On a microscopic scale, the chlorides combine with the oxygen, both in the water and on the steel surface, to form chlorite ions, thus depleting that local area of oxygen. If the bleach water is still (not circulating), then that crevice becomes a tiny highly active site relative to the more passive stainless steel around it, and it corrodes. This is known as crevice corrosion.

Biofouling and beerstone scale (calcium oxylate) can also cause corrosion. The metal beneath the deposit becomes oxygen-depleted through biological or chemical means, and corrosion occurs. This is one reason why it is important to remove beerstone.

Concentration corrosion: A third way that chlorides can cause corrosion of stainless is by concentration. This mode is very similar to the crevice mode described above. By allowing chlorinated water to evaporate and dry on a steel surface, the chlorides become concentrated and change the electrical potential of the surface at that site. The next time the surface is wetted, corrosion will immediately take place, creating a shallow pit. The next time the surface is allowed to dry, that pit will probably be one of the last sites to evaporate, causing chloride concentration again. At some point in the life cycle of the keg, that site will become deep enough for crevice corrosion to take over, and the pit will corrode through.

Corrosion: https://www.bssa.org.uk/faq.php?id=9

Discovery of stainless steel

Stainless steel is considered to be the most appropriate material for the fabrication of plant and equipment in the food and beverage industry. The apparent ease by which the surface finish of the material can be kept hygienically clean is a key factor in favor of stainless steel over other materials and those with applied surfaces, as is its resistance to corrosion.

Stainless steel has long been a choice material for manufacturing various types of food processing equipment. It's popular for several reasons: • It is highly resistant to impacts, strain, wear, abrasion and erosion. • It is malleable, easy to weld and easy to machine. • It is highly resistant to corrosion and chemicals, when properly passivated. • It is resistant to extreme temperatures and thermal shocks. • Its smooth, nonporous surface prevents the adhesion of food and reduces the adherence of biofilms. • It does not contaminate food or alter its organoleptic properties, when properly passivated. • It has a nice appearance.

On the other hand, stainless steel is relatively easy to scratch. The easiest way to reduce the risk of scratching is by handling with care and by avoiding touching the surface of your equipment with anything that results in scratches. Common culprits of equipment scratches are cleaning pads, cleaning abrasives, and contact with other metal tools. Green scrubbing pads used to clean cookware can quickly ruin polished surfaces because these pads contain bits of mineral crystals bonded to the fibers of the pad causing scratches to the surface. Not only do green pads damage the beauty of uniform surfaces, they can also make cleaning progressively more difficult if repeatedly used. White-colored cleaning pads, Teflon meshes, and natural fiber pads can be used on polished equipment.

Do not use steel brushes or steel tools made of carbon steel. » Do not carry out shot blasting using carbon steel blasting materials or blasting materials that have been used for shot blasting carbon steels. » Hydrochloric acid, or cleaners containing chlorides, must not be used for cleaning stainless steels. » Do not use hydrochloric acid to remove cement or mortar residues from stainless steels. » Throughout storage, avoid contact between stainless steel and carbon steel. » When using forklifts, avoid direct contact between carbon steel forks and stainless steel. » At installation, use fasteners (e.g. nails, screws and bolts) made of stainless steel. » In areas exposed to moisture, avoid the risk of galva- nic corrosion between stainless steel components and plain carbon steel components (e.g. by providing electrical insulation). » Use clean tools that are free from residues of plain carbon steel (e.g. swarf and iron particles from previous work). » Remove the protective plastic film only when it is no longer needed, i.e. when the construction phase is over and the local environment is free of debris and dirt particles. Some plastic films deteriorate in sunlight and can become difficult to strip.

Rough surfaces are more prone to corrosion.

Stainless steel equipment should never be in contact with bleach solutions for any length of time. It can cause significant pitting.

Austenitic Stainless Steel This group of stainless steels (the 300 Series) is by far the most commonly used. The relatively high Nickel (Ni) content, whilst ensuring a fully austenitic microstructure, also serves to enhance the corrosion resistance. Additions of tertiary elements such as

• Molybdenum (Mo) improves the resistance to localised corrosion such as Crevice and Pitting Corrosion by making the passive film more stable.
• Titanium (Ti) prevents sensitisation and Intergranular Corrosion.

They possess excellent corrosion resistance to a wide range of aggressive environments. The response to cold working of the Austenitic Stainless Steels gives them excellent fabrication properties. They have very good Ductility. They are not hardenable by heat treatment, although their mechanical properties can be improved to varying degrees by cold working (such as in temper rolling or cold stretching techniques). They have excellent weldability. In the annealed condition they are essentially non-magnetic.

Ferritic Stainless Steels This group (of the 400 Series) consists of Plain Chromium Stainless Steels that cannot be hardened by heat treatment, but can be moderately hardened by cold working. The response of the Ferritcs to cold work is low compared to the Austenitics. They have good Ductility, are magnetic and have good resistance to high temperature oxidation and moderately corrosive environments.

The poor weldability of the Standard Ferritic Stainless Steels limits the thickness of the material that can be used in the welded condition to thinner gauges. However, the development of modified compositions, such as 3CR12, has resulted in Ferritic grades which are welderable up to 30mm thick whilst still retaining useful mechanical properties.

Martensitic Stainless Steels These are also Plain Chromium Stainless Steels but, because of a high Carbon (C) content, can be hardened by heat treatment to very high Tensile Strength and Hardness, although Ductility and Toughness diminish with increasing strength. The Martensitic Stainless Steels (included in the 400 series) are magnetic and have adequate corrosion resistance in mildly corrosive environments.

Duplex Stainless Steels These Stainless Steels contain high Chromium (Cr) but with insufficient Nickel (Ni) to produce a microstructure that is fully Austenitic, i.e. a mixed structure (usually 50:50) of Austenite and Ferrite exists. The Duplex Stainless Steels have excellent corrosion resistance (especially with regard to Stress Corrosion Cracking) as well as high Tensile and Yield strength when compared with either Ferritic or Austenitic Stainless Steels.

Precipitation Hardening Stainless Steels These are Chromium Nickel Stainless Steels containing other tertiary elements such as Aluminium (Al) or Copper (Cu). High mechanical properties can be developed associated with good fabrication properties. Hardening is achieved by solution treating the steel at 1100ºC, quenching (rapid cooling) and then ageing at temperatures in the region of 600ºC to achieve high strengths. The precipitation hardening grades have a moderate corrosion resistance to aggressive environments. A big advantage with these stainless steels is that mechanical and cold forming can be done in the annealed, softened state with subsequent hardening by heat treatment to the required level carried out as a final step after all forming or machining has been completed.

See Passivation

Lower grade austenitic stainless steel alloys (e.g., AISI 100 and 200 Series) are generally not recommended for use in food equipment.

Although not hardenable by heating, 300 Series austenitic stainless steel can be hardened by a process known as "work hardening," or "cold working" the material. This is accomplished mechanically by cold rolling down to lighter gauges or by drawing through a die or similar device.

Being composed of an iron-based alloy, nearly all common grades of stainless steel are prone to corrosion by removal of the passive film through continued exposure to incompatible cleaners, abrasive cleaners, abrasive cleaning pads, or chlorine and oxidizing sanitizers.

To prevent the problems of corrosion, it is imperative that an adequate preventative maintenance program is implemented for all food equipment. This program should include routine inspection of food contact surfaces for signs of corrosion and assurance that conditions that may induce corrosion (e.g., inadequate draining of solutions) are being avoided. In addition, an adequate cleaning and sanitizing program must be in place, which includes an appropriate frequency as well as validation. Adequate cleaning and sanitizing removes biological materials that can attack the surface with time and serve as a seed point for corrosion. Adequate cleaning also removes mild corrosion. However, strong cleaning and sanitizing chemicals, if used at improperly high concentrations or on surfaces that are not well maintained, could increase the potential for corrosion. Extended contact with lower concentrations of chlorine sanitizers and similar chemicals can also increase the likelihood of corrosion of stainless steel surfaces.

More: https://www.foodprotection.org/files/food-protection-trends/Oct-12-Schmidt.pdf

• https://www.bssa.org.uk/faq.php?id=10 types of steel
• https://www.bssa.org.uk/faq.php?id=24 magnetic?

Whether stainless steel is magnetic had nothing to do with how "cheap" it is. The main difference in price is from the cost of labor (e.g. when it's from China). Magnetism comes from cold working.