Visual difference between 304 and 316 SS tubing 1

[Cirrus9]

I need to determine visually if a tubing installed on a metering skid is 304 SS or 316.
I understood that you can differenciate between SS 304 and 316 (or 316L) by looking at the tubing. 316 looks shinier.
Do you have to spray the tubing with water to start some surface corrosion?
Anybody knows more?

 

[metalonis]

There is no way to tell the difference between 304 and 316 stainless steels just by looking at them. A chemical test for the presence of molybdenum would be required.

 

[PacificSteve]

Confirming previous reply,
my SS fabricator indicates no way (sight, sound, etc) to tell short of a chemical test. It the perfect world, the tube may be inked with the material designation, or there could be some project specifications or project material submittals around that could provide information (although not a sure bet that what was specified or submitted was actually installed).

PacificSteve

 

[Rich2001]

According to ASTM STP 550, available from the ASTM

http://www.ASTM.org

, the test for molybdenum in 300 series stainless is to

a) Place equal drops of concentrated hydrochloric acid and concentrated nitric acid on the metal surface. Allow 3 to 4 minutes for reaction and transfer 2 drops to a spot plate.

b) Add 2 drops of 10% potassion thiocyanate.

c) Add 25% sodium thiosulfate dropwise stirring until the red color begins to disappear. A violet color will appear if molybdenum is present.

However, I think your best course of action is to get a chemical spot test kit from some company that makes them. A quick web search under "chemical spot test" should show manufacturers.

 

[mcguire]

As someone who has produced stainless steel for twenty years, I promise there is no visual difference between 304 and 316. You need elemental analysis by some means to find the molybdenum which distinguishes the two.

 

[quark]

Graphite tip and electrolyte test is handy. These kits are very cheap too. You can determine whether molybdenum is present or not. The test takes less than one minutes time.

Regards,

 

[Cirrus9]

Thank you all for the answers.

 

[PUMPDESIGNER]

A very useful but partial test is to simply use a magnet.
Place a good magnet against the 316 and you will feel no attraction when pulling away. This of course does not prove you have 316.
However,
If the magnet is attracted to the tube, then you definitely DO NOT HAVE 316. Richard Neff
Irrigation Craft