There have been a number of references to the tendency of stainless steel to corrode if it is not allowed exposure to oxygen. My understanding is that corrosion of metals is generally a process of oxidation, and that corrosion of steels is formation of iron oxides. My question is this: how can a metal oxidize if it is not exposed to the very element that is part of the oxidation process? If stainless is isolated from an air or liquid medium, how can it corrode? I understand that in a liquid electrolyte, such as sea water, corrosion is likely and almost inevitable, yet I have seen some stainless that has been under water for quite some time that is still shiny and uncorroded. There seem to be some contradictions here.
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Read this.
Just happened to receive the Jamestown Distributor catalog in the mail. And was reading the answer to that very question. Read down the page in the Related Link and see their answer.
Just happened to receive the Jamestown Distributor catalog in the mail. And was reading the answer to that very question. Read down the page in the Related Link and see their answer.
Stainless Steel 101
Most stainless steels are high -alloy steels that have superior corroision resistance than standard steel because they contail large amounts of chromium. Also there are two grades of stainless steels, Ferritic and austinitic. The austenitic grades are non-magnetic while the ferritic are almost always magnetic. The key thing to remember with stainless steel is that it always has the potential to corrode. This is due to the treatment whan it is manufactured. Also when stainless steel is welded or heated to high temps, the carbon leaches out, or comes to the surface. This must be removed chemically with a strong acid or the weld will corrode and weaken. This is called pickling, and uses H2SO4 as the primary ingredient. FYI if you use any stainless steel on your boat use 316 or 316L. These are low carbon stainless steels. 304 is the most common. NEVER USE A 400 grade stainless on your boat! It will corrode away! Get int the habit of carrying a small magnet with you when you shop for parts. Then you can test parts for their composition. If you send me your address I will send you one I use for testing. Hope this helps David L. Delich Beneteau 331 First Light
Most stainless steels are high -alloy steels that have superior corroision resistance than standard steel because they contail large amounts of chromium. Also there are two grades of stainless steels, Ferritic and austinitic. The austenitic grades are non-magnetic while the ferritic are almost always magnetic. The key thing to remember with stainless steel is that it always has the potential to corrode. This is due to the treatment whan it is manufactured. Also when stainless steel is welded or heated to high temps, the carbon leaches out, or comes to the surface. This must be removed chemically with a strong acid or the weld will corrode and weaken. This is called pickling, and uses H2SO4 as the primary ingredient. FYI if you use any stainless steel on your boat use 316 or 316L. These are low carbon stainless steels. 304 is the most common. NEVER USE A 400 grade stainless on your boat! It will corrode away! Get int the habit of carrying a small magnet with you when you shop for parts. Then you can test parts for their composition. If you send me your address I will send you one I use for testing. Hope this helps David L. Delich Beneteau 331 First Light
Stainless 101
David,, when testing stainless with a magnet, what do you look for? Should it be strongly attracted to the magnet or visa versa?
David,, when testing stainless with a magnet, what do you look for? Should it be strongly attracted to the magnet or visa versa?
It's magic
Hello, There are lots of texts on this topic, written by people far more qualified than I, but I believe the short form is that stainless corrodes slowly, not because it resists oxidation, but because it does indeed oxidize, forming an extremely thin oxode layer on its surface which prevents further oxidation. But under water, in the absence of sufficient oxygen, no such layer forms, and the metal molecules progressively drift away. More corrosion-resistant alloys will hold up better, above or below the water, as will ones with superior metallurgy and degree of polish. The other big variable is size, as the volume of most solids will go up much faster than the surface area; in a sphere, for instance, as you square the surface area, you cube the volume. This means that any damage to the surface has a proportionately smaller effect on the integrity of the piece. That's why stainless screws, which are very low volume, go to pieces under water much faster than propeller shafts. Of course, the corrosiveness of the medium also affects the rate of decay, with warm, salty water being worse than cold fresh water. If you have seen stainless that has remained uncorroded under seawater, you have probably seen excellent metal, well-made, and highly-polished. Then again, the corrosion might simply have been invisible to the naked eye, which is why we always, always, bring a pocket microscope to our rig surveys. Fair leads, Brion Toss
Hello, There are lots of texts on this topic, written by people far more qualified than I, but I believe the short form is that stainless corrodes slowly, not because it resists oxidation, but because it does indeed oxidize, forming an extremely thin oxode layer on its surface which prevents further oxidation. But under water, in the absence of sufficient oxygen, no such layer forms, and the metal molecules progressively drift away. More corrosion-resistant alloys will hold up better, above or below the water, as will ones with superior metallurgy and degree of polish. The other big variable is size, as the volume of most solids will go up much faster than the surface area; in a sphere, for instance, as you square the surface area, you cube the volume. This means that any damage to the surface has a proportionately smaller effect on the integrity of the piece. That's why stainless screws, which are very low volume, go to pieces under water much faster than propeller shafts. Of course, the corrosiveness of the medium also affects the rate of decay, with warm, salty water being worse than cold fresh water. If you have seen stainless that has remained uncorroded under seawater, you have probably seen excellent metal, well-made, and highly-polished. Then again, the corrosion might simply have been invisible to the naked eye, which is why we always, always, bring a pocket microscope to our rig surveys. Fair leads, Brion Toss
good ss is non magnetic
Jim, when checking for ferricity on ss, a magnet should have no attraction to it whatsoever. That is what you are looking for when shopping for any ss, including bolts, screws etc. Even in packages at you local harware store, a small magnet will detect the quality of ss.
Jim, when checking for ferricity on ss, a magnet should have no attraction to it whatsoever. That is what you are looking for when shopping for any ss, including bolts, screws etc. Even in packages at you local harware store, a small magnet will detect the quality of ss.
NEVER SAY “NEVER”
A couple of cautions about using a magnet to ascertain the quality of stainless steel: good ss is non magnetic - - - Take your magnet to a dollar store and put it next to a mixing bowl or colander made in India. I’m willing to bet that it’s non-magnetic, yet it’s some of the poorest quality stainless steel you’ll ever see. On the other hand, try the magnet on some known high quality 18/8 or 18/10 stainless steel flatware. It should be non-magnetic, BUT if you run the magnet around the edge of a spoon bowl, it may very well be slightly magnetic. The magnetism results from the “working” of the spoon bowl to a gauge thinner than the handle. The ONLY true way to know what you’re buying is to know AND trust the person you’re dealing with. Paul
A couple of cautions about using a magnet to ascertain the quality of stainless steel: good ss is non magnetic - - - Take your magnet to a dollar store and put it next to a mixing bowl or colander made in India. I’m willing to bet that it’s non-magnetic, yet it’s some of the poorest quality stainless steel you’ll ever see. On the other hand, try the magnet on some known high quality 18/8 or 18/10 stainless steel flatware. It should be non-magnetic, BUT if you run the magnet around the edge of a spoon bowl, it may very well be slightly magnetic. The magnetism results from the “working” of the spoon bowl to a gauge thinner than the handle. The ONLY true way to know what you’re buying is to know AND trust the person you’re dealing with. Paul
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