tcm
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i'm having a moderate run-in with a new boat supplier about rusty stainless steel. They said "it's 316" and I said "ah but..." and having explained to theme the rudiments of the problems, they're gong to sort it out.
Essentially, stainless steel is steel with additives, and these aditives are such that a layer of chroium oxide forms on the surface - hence "stainless". Once the surface layer has formed, the steel is known as 316(passive). Otherwise, its 316(active).
Technically, 316 shouldn't rust - if it does, the passive layer has broken down.
Breaking down the passive layer does two things :
firstly, the material is less resistant to rust: so the ends of a gurdaril wire covered in plastic rubs away at the s/s underneath, removes the passive layer and hence rusty ends of guardrail wires, rough/sawn stainless going rusty at the end, a light sprinkling of rust over a whole component, and so on.
Secondly, it's a diferent material, so with an active 316 and a passive 316 touching each other, then together with seawater as electrolyte, there'll be galvanic corrossion - you'll see this as perhaps one stainless steel nut and bolt going rusty when everything else is fine.
the passive layer isd broken down by
1) poor manufacturing- by introducing carbon impurities. drilling holes with high-carbon drills, cleaning with steel wool, using tools that have previously been working on mild steel, especially bending dies and so on. Big problem for any factory producing mild steel and stainless steel components.
2) fretting at the passive layer whilst in use; anything that rubs on the stainless steel may remove that passive layer. So stainless steel conectors/fasteners should always be tight, to avoid movement, and stainless steel chains or chained connectors may not resist rust for long in the presence of seawater. If possible, two pieces of stainless steel should be isolated with plastic washers, but movemnt (eg under o-rings) can remove that passive layer.
3) poor assembly: engineers have tendency to clean all parts before assembly - but should not do so with stainless steel. Once passive, the parts should be greased up and isolated with plastic washers, clingfilm if out of signt, and so on.
4) poor maintenance: of course, if a problem has not occurred as above, then maintenance should be minimal. But any maintenance and aftercare must avoid use of carbon steel: removing rust from stainless steel should be with a plastic or fabric but NOT wire wool or mild steel brushes which will make rust return even faster/worse. Use ceramics whwever possible. If you use a hackssaw to cut stainless steel, expect the sawn araea to be active, and remove steel deposits from the blade by grinding down, polishing as below.
Banishing rust from stainless steel 316:
1. The surface layer of impurities must be heavily ground off, with a polishing/buffing wheel and paste, using a series of new polishing wheel covers and repeated washdowns, returning a high polish to the surface.
2. Diluted nitric acid will dissolve anyremaining carbon impurities and allow the passive layer to re-form.
3. Light oils/wd40 etc will protect the general surface
4. Expect occassional small dings and bashes from tools to remove the passive layer: re-treat the area as above
Please respond if you have usefull additions and experiences
Essentially, stainless steel is steel with additives, and these aditives are such that a layer of chroium oxide forms on the surface - hence "stainless". Once the surface layer has formed, the steel is known as 316(passive). Otherwise, its 316(active).
Technically, 316 shouldn't rust - if it does, the passive layer has broken down.
Breaking down the passive layer does two things :
firstly, the material is less resistant to rust: so the ends of a gurdaril wire covered in plastic rubs away at the s/s underneath, removes the passive layer and hence rusty ends of guardrail wires, rough/sawn stainless going rusty at the end, a light sprinkling of rust over a whole component, and so on.
Secondly, it's a diferent material, so with an active 316 and a passive 316 touching each other, then together with seawater as electrolyte, there'll be galvanic corrossion - you'll see this as perhaps one stainless steel nut and bolt going rusty when everything else is fine.
the passive layer isd broken down by
1) poor manufacturing- by introducing carbon impurities. drilling holes with high-carbon drills, cleaning with steel wool, using tools that have previously been working on mild steel, especially bending dies and so on. Big problem for any factory producing mild steel and stainless steel components.
2) fretting at the passive layer whilst in use; anything that rubs on the stainless steel may remove that passive layer. So stainless steel conectors/fasteners should always be tight, to avoid movement, and stainless steel chains or chained connectors may not resist rust for long in the presence of seawater. If possible, two pieces of stainless steel should be isolated with plastic washers, but movemnt (eg under o-rings) can remove that passive layer.
3) poor assembly: engineers have tendency to clean all parts before assembly - but should not do so with stainless steel. Once passive, the parts should be greased up and isolated with plastic washers, clingfilm if out of signt, and so on.
4) poor maintenance: of course, if a problem has not occurred as above, then maintenance should be minimal. But any maintenance and aftercare must avoid use of carbon steel: removing rust from stainless steel should be with a plastic or fabric but NOT wire wool or mild steel brushes which will make rust return even faster/worse. Use ceramics whwever possible. If you use a hackssaw to cut stainless steel, expect the sawn araea to be active, and remove steel deposits from the blade by grinding down, polishing as below.
Banishing rust from stainless steel 316:
1. The surface layer of impurities must be heavily ground off, with a polishing/buffing wheel and paste, using a series of new polishing wheel covers and repeated washdowns, returning a high polish to the surface.
2. Diluted nitric acid will dissolve anyremaining carbon impurities and allow the passive layer to re-form.
3. Light oils/wd40 etc will protect the general surface
4. Expect occassional small dings and bashes from tools to remove the passive layer: re-treat the area as above
Please respond if you have usefull additions and experiences
