Marine grade stainless --- What is it?

[Mark42]

I often buy stainless steel hardware for boat projects, such as cleats, transom eyes, and basic nuts & bolts. The description of a product usually states "marine grade stainless steel" or specifies a number, usually 18-8, 304, 308 or 316.

I realize that the SS hardware from the local Home Depot is only 304. Also some "marine grade" hinges are 304, unless they specify a grade number.

So I am wondering if the term "Maring grade SS" actually infers 316,or if it is a generic term meaning the product is rust resistant?

I also read that 316 has the highest corrosion resistance of the common 300 series ss grades. Crevice corrosion seems to be the big worry, and 316 can hold it off submerged continually in salt water for a year or so.

So, if 316 is the real "marine grade" ss, why are other parts made of 304, like hinges, latches, etc? Is it because these ss parts are not continually in contact with salt water, or continually submerged in salt water? (I noticed most through hull fittings are 316 and they are near or below the water line all the time)

All the fasteners and deck hardware (deck fills, cleats, latches, etc) have been holding up fine for years. Only the screws that came with the Smart Tabs that are wet the whole time the boat is being used show some corrosion. But everything else, the tank vent, bilge through hull, gas deck plate, all are in like new condition. They all are doing their jobs well.

Just wondering about the whole SS grade and what terms like "marine grade ss" really mean.

 

[Woodonglass]

Re: Marine grade stainless --- What is it?

Here's what my research tells me...

The AISI number grade is 316 and refers to the chromium content.
The salinity contained with the oceans acts as a catalyst to the rusting process.
Type 316 is the main stainless used in the marine environment, with the exception of fasteners and other items where strength and wear resistance are needed, then Type 304 (18-8) is typically used