420 Stainless Steel Ha!

[GotAir]

I just bought an BC knife. I went to Coz earlier this month and dove for a week.
as the sad no dive day came I thought it would be a good Idea to go thru and
clean-up and check all the equipment. so when I pulled the BC knife out of its
sheath it was rusted at the line cutter, along the knife's edge and on the serated
cutter/saw. what should I do ? take it back to my LDS or just bite the bullet and never buy that brand again.

GotAir

 

[DA Aquamaster]

It is generally bad form to ignore the knife for a week in salt water. It is not the knife's fault as there are some trade offs involved in stainless steel dive knives. You can get a 300 series stainless steel knife (306 is common) that will be comparatively rust resistant but will not hold an edge well. Or, you can get a 400 series stainless knife (420 is common) that will do a better job of holding an edge but does not hold an edge as well.

Personally, I like sharp knives. So..if it were me, I'd put an edge back on the knife, which will take care of the rust on the edge in the process, and coat it silicone grease to make it a bit more rust resistant.

A small wire brush attachment on a dremmel tool should do the trick on the rust on the serrated edge and line cutter. If you don't have one available, try brushing it with a small brass wire brush.

 

[Tom Smedley]

It is a misconception that stainless steel will not rust. It does form surface rust - it just doesn't pit. If you didn't take your knife out of the sheath and rinse it you do have rust.

Just clean the rust away with a brass or steel brush and coat the metal with lubricant. It should look just like new.

Make the knife part of your cleaning process after every dive. Also check the brass valves on your rescue sausage etc. The threads need to be rinsed, moved, and lubricated else the mechanism will not work when you need it.

 

[captain]

Cleaning stainless with a wire brush that is carbon steel will inbed microscopic particles of the carbon steel into the surface of the stainless which will then form rust. This is especially true if a power driven wire wheel is used. Use WD 40 and a Scotch Brite pad to remove surface rust.

Captain

 

[cool_hardware52]

There are many types of Stainless. Generally speaking the grades with the greatest resistance to corrosion cannot be hardened, and the hardenable grades offer much less corrosion resistance.

Cutting tools will not take and hold an edge if they aren't hardened.

If care is not used when heat treating the hardenable grades, and if proper passivation is not used corrosion resistance can be reduced, but even when all is done correctly most stainless cutting tools will rust if not rinced and oiled after use in salt water.


Regards,



Tobin George

 

[dtdiver]

what is passivation?

dt

 

[miketsp]

It is a misconception that stainless steel will not rust. It does form surface rust - it just doesn't pit.
..snip..
Just clean the rust away with a brass or steel brush and coat the metal with lubricant. It should look just like new.
..snip..

Cleaning stainless with a wire brush that is carbon steel will inbed microscopic particles of the carbon steel into the surface of the stainless which will then form rust. This is especially true if a power driven wire wheel is used. Use WD 40 and a Scotch Brite pad to remove surface rust.
Captain

As Captain correctly states, it is possible to cause pitting to stainless steel by using a power driven steel brush - I know, I did it with my 420 SS knife which now has some deep streaks of pitting.
Since I changed to a brass wire wheel I had no further pitting. I nowadays dismantle the whole knife, brush it with the brass wheel, resharpen, smear well with vaseline and reassemble.

 

[cool_hardware52]

what is passivation?

dt

Passivation removes "free iron" from the Stainless surface. As stainless products are produced, either at the mill by rolling, or in subsequent manufacturing processes like forming or drilling or milling or grinding etc. it is almost guarantteed that microscopic bits of iron will be imbeded in the surface of the part.

This free iron serves as a "point of origin" for rust, and this rust can continue beyond the original free iron.

Passivation is anything that removes this free iron. Most often this done with a heated nitric or citric acid bath. The acid removes the free iron.


Regards,


Tobin

 

[deadend]

Get a titanium knife and forget about it. This is the ONLY use for titanium in any diving application and well worth it due to its utter inability to corrode.

 

[Daryl Morse]

You can get a 300 series stainless steel knife (306 is common) that will be comparatively rust resistant but will not hold an edge well. Or, you can get a 400 series stainless knife (420 is common) that will do a better job of holding an edge but does not hold an edge as well.

I believe you meant to say that 400 SS holds an edge well, but is comparatively less rust resistant than 300 SS.

I have a UK Blue Tang which is 420 SS. I accidently left it overnight at the DS after using it for the first time. When I got it home, it was rusted. I thought it was quite bad, but it cleaned up nicely. Now I take it completely apart after every dive and spray it with WD40 before reassembly. I haven't had a spot of rust on it since.