Magnetic property is not a valid test for determining the difference between
304 and 316 stainless. There are test kits but they cost more than your
hatches. The "official" test is to put a drop each of concentrated
hydrochloric and nitric acid on the sample. After 3 or 4 minutes move them
to a slide and add 2 drops of potassium thiocyanate. Add a little 25%
sodium thiosulfate and stir. If the final color is violet it contains
molybdenum and is 316. (Aren't you sorry you asked.) :-)

Both can become slightly magnetic when cold worked. If a stainless casting
is then cold forged to finished dimensions or milled it can gain a little
magnetic permeability. The only way you can be assured of quality is to be
sure of the manufacturer. The name brand people like Hood, Bomar, Lewmar
and New Found Metals can't afford to use anything less than they advertise.
The difference in price between 304 and 316 casting ingots is not enough
compared to the fabrication costs.

OTOH, magnetism induced during polishing is something I would think about.
That can be an indication that free iron molecules were left on the surface
and unless it is passivated or electropolished you can get some rust
staining. The stain will come off with a little work and usually does not
leave pitting

Welds are another place where the alloys can migrate and leave free iron
ions unless it is passivated after welding. That can cause real problems.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Bob" wrote in message
oups.com...
I received two 7x15 stainless ports today. When I ordered them I was
assured they were 316 ss. So today I thought I would see if a magnet
would stick on one. The magnet stuck. When I called the supplier about
another issue I mentioned that my magnet was sticking to the 316 ss
port. I was told the polishing process was to blame and the 316 had
"picked up a slight magnetic" charge.

Is that true?

Are there simple NDT methods to determine if a stainless alloy is 304
or 316?
Bob