Stainless Steel...

[merttan]

Hi everyone,
Another disturbing question from me
As an architect the grade of the steel is important on our design criteria for loads and size...
I was, however, wondering how to determine the so called stainless steel we use on our boats. I mean there are different grades for stainless steel like 316, which is good, and 304, which is not really "stainless" steel.
The importers of these materials also seem to be not sure what grade they are actually selling, thanks to our production line in China!

Example #1: All "stainless steel" bolts and screws I just changed last summer are all rusted out already!!! I bought them from a marine store that claimed they were "good" stainless steel. Some of those bolts and screws were installed in the cabin some out of the cabin... Yet they are rusted doesn't matter where they were used...

Example#2: The irony is that over 30 year-old original stainless steel parts of my boat are still shining and showing no signs of rusting... Yet, fairly new installed railings (apparently, the previous owner installed them about 3-4 years ago) are point rusting.

I do know you'll get what you pay for, however, where to get the right stuff is the question. What were your experiences with brands you used on your boat? Where did you get them and how satisfied are you?
I can use some information before changing the bolts and screws again this season...

[RAGTIMEDON]

For marine use, it is not the strength that matters as much as rust resistance. Plain carbon steel, even "soft" free machining steel, will make a screw that will hold parts together. But unless they are immersed in oil, rust will destroy 304 stainless in salt air. They call it stainless, but check it with a strong magnet before you buy! If a magnet has no attraction to it, it will probably last; if the magnet will pick up a screw, it probably will rust. That's a heck of a lot cheaper and more convenient than spectrographic analysis!

[merttan]

Originally Posted by RAGTIMEDON

That's a heck of a lot cheaper and more convenient than spectrographic analysis!

Oh yeah! It's a lot cheaper I'll try the magnet trick...

[DrB]

for quality SS is McMaster-Carr. I have had good luck with them. The magnet trick is a good technique.

McMaster-Carr Link

DrB

[TheFrog]

One often overlooked detail in stainless parts is the surface treatment. Yes, 316 is more resistant to rust than 304. But 316 can rust (at least surface stain) as well if the surface is not "passivated". Believe me, doctors are not happy if you give their instruments rust after just a couple cleaning/sterilization cycles. Passivation helps dramatically in reducing this. Of course, the chandler will probably not know what is being sold. Even in medical instruments you cannot tell and many vendors don't know, but this can be one of the big differences between high quality instruments and the cheap imitations from Asia.

For those who are curious passivation is an acid bath process that removes the iron oxides from the surface of the stainless. Every piece of stainless has some iron oxide on the surface after it is machined/formed. Removing these iron oxide seeds reduces the locations for rust to start.

[SVAuspicious]

Read this:

Stainless Steel Information Knowledge alloys 316 304

[sailingdog]

BTW, 304 and 316 are both austentitic forms of marine grade stainless and non-magnetic...so the magnet test does little to tell you which it is. The cheaper, non-marine grade martenistic stainless steels are magnetic...and really should be avoided on a boat.

While 304 is a bit stronger (about 15% or so IIRC) than 316, the fact that it is far less corrosion resistant and prone to chloride ion stress cracking makes 316 a much better choice on a boat.

[merttan]

Will it be company privilege abuse if I get the metal things casted and cut in our titanium medical factory in Turkey? Nevermind for this boat, for the next one maybe
I'll see if I can get some manufacturers' production method information. Acid bath you say... I wonder if... No I shouldn't try it at home...

[pfatyol]

If your really interested in passivation, it is covered in ASTM A967. Many different methods, but if you realy like your stainless to shine send it out for electro polishing. There a good number of firms that will either passivate or electro polish parts and hardware out there. Passivation does not remove the free iron from the surface of Stainless steel.

[merttan]

Originally Posted by SVAuspicious

Read this:

Stainless Steel Information Knowledge alloys 316 304

I think that I installed 304 on which I used cleaning solvents including but not limited to; bleach, oxyclean, citric acid based cleaners... And additionally, the rain we receive may have somewhat acidic characteristics...
I appreciate the link...
I guess, I'd better test the screws with solvents applied outdoor conditions for awhile before installing them.