Types of stainless steel
There are many types and grades of stainless steel: when nickel is added to iron, for instance, the austenite structure, a metallic non-magnetic phase of iron, is stabilized. This crystal structure phase makes such steels non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. Sometimes significant quantities of manganese are used in stainless steel compositions. Manganese preserves an austenitic structure in the steel, like nickel, but at a lower cost.
Stainless steels are also classified by their crystalline structure:
Austenitic (300 series) stainless steels are roughly 70% of total stainless steel produced. They are made up of about 0.15% carbon, a 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from cryogenic to melting point of the alloy.
Superaustenitic A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless is often used in flatware. 18/0 and 18/8 is also available as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high Molybdenum content (>6%), nitrogen additions, and a higher nickel content ensures better resistance to stress-corrosion cracking. The higher alloy content of "Superaustenitic" steels results in a fearsomely expensive product and similar performance is often achieved using duplex steels at a lower cost.
Ferritic stainless steels are highly corrosion resistant, but less durable than austenitic grades. They contain between 10.5% and 27% chromium and very little, if any, nickel. Usually they are comprised of molybdenum with some, aluminum or titanium. Common ferritic grades are 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.
Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely strong, tough, and machineable. Martensitic stainless steel can also be hardened thru heat treatment. Martensitic stainless steels are comprised of chromium (12-14%), molybdenum (0.2-1%), zero to less than 2% nickel, and about 0.1-1% carbon (giving it more hardness but making the material a bit more brittle). It is magnetic and is often referred to as "series-00" steel.
Precipitation-hardening Martensitic stainless steels offer corrosion resistance comparable with austenitic varieties, and it can be precipitation hardened to even higher strengths than the previously described martensitic grade. The most common, 17-4PH, is comprised of about 17% chromium and 4% nickel.
Duplex stainless steels are a mix of austenite and ferrite structures, the aim being to produce a 50:50 mix (although in commercial alloys the mix may be closer to 40:60). Duplex stainless steels have improved strength over austenitic stainless steels and also improved resistance to localized corrosion particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19-28%) and molybdenum (up to 5%) and lower nickel contents than the austenitic stainless steels.
Now that you know more than you probably ever wanted to know about stainless steel makeup lets talk about what you're most likely to run into with brewery tanks.
300 series Austenitic Stainless Steel
Amongst the austenitic stainless steels, Grade 304 is the most commonly used. The second most common is Grade 316, the standard molybdenum-bearing grade. The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments. It has excellent forming and welding characteristics. The austenitic structure gives these grades excellent toughness, even down to cryogenic temperatures. Other 300-series metals should be avoided for brewery use, especially 303. The 303 variety has much less corrosion resistance and is much more difficult to weld.
Both 304 and 316 stainless steel offer excellent corrosive resistance in a range of atmospheric environments and many corrosive media - but 316 is generally more resistant than 304 which is subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60░C. For vessels that border or exceed the limits of 316, 2205 the most widely used duplex (ferritic/austenitic) stainless steel grade is the knowledgeable standard in the U.S.
Possible alternative grades to 316 stainless steels
Why it might have been chosen instead of standard 316
Better resistance to temperatures of around 600-900░C was needed.
Higher strength than standard 316.
Higher resistance to chlorides than 316L, but with similar resistance to stress corrosion cracking.
Much higher resistance to chlorides at elevated temperatures, with good formability
Much higher resistance to chlorides at elevated temperatures, and higher strength than 316
2205 Duplex Stainless Steel
2205 is the most widely used duplex (ferritic/austenitic) stainless steel grade. It finds applications due to both excellent corrosion resistance and high strength. 2205 is not generally suitable for use at temperatures above 300░C as it suffers from precipitation of brittle micro-constituents, nor below -50░C because of its ductile-to-brittle-transition. Corrosion Resistance - Excellent general corrosion resistance; superior to Grade 316 in most environments. Excellent resistance to localized corrosion including intergranular, pitting and crevice corrosion. The grade is also resistant to chloride stress corrosion cracking (SCC) at temperatures of up to about 150░C. Grade 2205 will often perform well in environments which cause premature failure of austenitic grades.
Possible alternative grades to grade 2205 stainless steels
Why it might have been chosen instead of standard 2205
Better formability is needed, with similar corrosion resistance and lower strength.
High resistance to corrosion is required, eg resistance to higher temperature seawater.
Higher corrosion resistance is required, but with lower strength and better formability.
The high corrosion resistance and strength of 2205 are not neededů 316L is lower cost.