I apologize if this is not proper form, but in my searches for the answer to this question I found several threads with conflicting information, and I felt there should be a post with accumulated/accurate information. A goal of mine (and I imagine others searching for this subject) is to obtain some stainless steel 15.5 gallon kegs, and convert them into brew kettles. Please reply if you have more information, or you see incorrect information. (maybe this could get stickied? 
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I read a few people saying that most kegs today are stainless steel, but I see others deny this. Does anyone know if one is more common? I'm not sure how to verify this other than someone who is knowledgeable on this subject.
Accurate Tests
Possibly Unreliable Tests
Sometimes kegs have information pressed onto them near the bottom skirt or the top. Like when they were made, material, capacity, etc.
If you match a number on your keg to one of the following steel grades, then you've got a stainless steel keg.
Society of Automotive Engineers (SAE) Steel Grades
Three Major Classes of Stainless Steel
Source:http://www.fantes.com/stainless-steel.html
)I read a few people saying that most kegs today are stainless steel, but I see others deny this. Does anyone know if one is more common? I'm not sure how to verify this other than someone who is knowledgeable on this subject.
Accurate Tests
- Scratch: aluminum scratches easier
- Dent: Aluminum dents easier
- Grinder: hit inconspicuous spot with angle grinder, if it sparks it's stainless
- File: If you want to RUN FOR EAR PLUGS its stainless
Possibly Unreliable Tests
- Weight: Aluminum MIGHT weigh less (unless it's thicker?)
- Magnet: stronger magnets (hard drive/neodymium) sometimes stick to SS, but never to AL
Sometimes kegs have information pressed onto them near the bottom skirt or the top. Like when they were made, material, capacity, etc.
If you match a number on your keg to one of the following steel grades, then you've got a stainless steel keg.
Society of Automotive Engineers (SAE) Steel Grades
- http://en.wikipedia.org/wiki/SAE_steel_grades#Stainless_steel
- http://en.wikipedia.org/wiki/Stainless_steel#Comparison_of_standardized_steels
- http://www.bssa.org.uk/topics.php?article=606&featured=1
Three Major Classes of Stainless Steel
Source:http://www.fantes.com/stainless-steel.html
- Austenitic: Chromium-nickel-iron alloys with 16-26% chromium, 6-22% nickel (Ni), and low carbon content, with non-magnetic properties (if annealed - working it at low temperatures, then heated and cooled). Nickel increases corrosion resistance. Hardenable by cold-working (worked at low temperatures) as well as tempering (heated then cooled). Type 304 (S30400) or "18/8" (18% chromium 8% nickel), is the most commonly used grade or composition.
- Martensitic: Chromium-iron alloys with 10.5-17% chromium and carefully controlled carbon content, hardenable by quenching (quickly cooled in water or oil) and tempering (heated then cooled). It has magnetic properties. Commonly used in knives. Martensitic grades are strong and hard, but are brittle and difficult to form and weld. Type 420 (S42000) is a typical example.
- Ferritic: Chromium-iron alloys with 17-27% chromium and low carbon content, with magnetic properties. Cooking utensils made of this type contain the higher chromium levels. Type 430 is the most commonly used ferritic.
- Two additional classes worth mentioning include Duplex(with austenitic and ferritic structures), and Precipitation Hardening stainless steel, used in certain extreme conditions.
