Black streaks/spots corrosion on stainless steel
Posted: Sun Jan 24, 2016 6:02 pm
Hi guys, I bought some stainless steel reflux gear which I thought mostly looked great out of the box on casual detection (there was a poorly welded thermowel which had obvious heat damage on it but it is tiny and easy to fix). However during cleaning I noticed something black coming off on the rags and paper towels I was using to clean. I assumed it was some kind of oil used in manufacturing/polishing and continued on. So first I washed with a mild dish detergent in warm water and scrubbed a bit with a cloth rag, then rinsed the pieces and placed them in a 5% sodium hydroxide bath for a day. At this point I was like "Super cleanl!!!" and was nearly going to start a cleaning run with it but decide instead to do a final acetone clean with paper towels on all pieces just to be safe. And this is where I first had my suspicions something was awry.
So when cleaning with acetone I was noticing that there were some "black spots" in places on the pieces, most of them were tiny and hard to notice due to how shiny all the pieces were. Most of the spots I saw were on the outside of the pieces, on welds and on the ferrules. Wiping them hard with an acetone paper towel caused them to disappear and the black stuff to disappear, so I thought, maybe its just really really tough grease. Wiping inside some of the pieces with acetone caused a black smudgy appearance to appear on the paper towels but inside it was hard to see that there was anything "black" to begin with. But on closer inspection, with a magnifying glass initially, I saw some streaks/lines of black discoloration. Anyhow I managed to get them all perfect with the acetone and paper towels, no spots anymore and left it for the next day.
So the next day I rinsed them with some distilled water and noticed these black spots reappear. At this stage I'm like this is corrosion, but I'm not really used to seeing corrosion like this. I'm assuming the black spots are nickel oxide but maybe it's a mixture of nickel and iron oxide I don't know. I sanded with fine grit sandpaper one of the pieces and then put it in a 4% citric acid bath for 30 minutes to test my suspicion it was corrosion and lo and behold the piece is now perfect and the spots no longer reappear. Upon closer inspect of the shotgun condensors I have, the inner pipes where the gas/condensate flows all have heat affected colors and stains on them. And inside the 2inch and 3inch long pipe extensions I have I can clearly see the actual heat pattern of the welds. I haven't even cleaned/touched the 3 inch pipes I have and I can see the same thing (just to throw out the idea it is my cleaning method). It seems they just polished over the heat tint and welding errors which mostly makes it invisible unless you are looking at it in certain ways.
So my questions. Does having a heat tint on stainless pass as a sanitary weld? I'm assuming not as it is corroding and also does not look good. And is there a better way to get rid of this type of corrosion than sanding it back and acid bathing it to passivate it? I've never really treated sanitary stainless which has had corrosion before and maybe there are better/easier/quicker ways.
So when cleaning with acetone I was noticing that there were some "black spots" in places on the pieces, most of them were tiny and hard to notice due to how shiny all the pieces were. Most of the spots I saw were on the outside of the pieces, on welds and on the ferrules. Wiping them hard with an acetone paper towel caused them to disappear and the black stuff to disappear, so I thought, maybe its just really really tough grease. Wiping inside some of the pieces with acetone caused a black smudgy appearance to appear on the paper towels but inside it was hard to see that there was anything "black" to begin with. But on closer inspection, with a magnifying glass initially, I saw some streaks/lines of black discoloration. Anyhow I managed to get them all perfect with the acetone and paper towels, no spots anymore and left it for the next day.
So the next day I rinsed them with some distilled water and noticed these black spots reappear. At this stage I'm like this is corrosion, but I'm not really used to seeing corrosion like this. I'm assuming the black spots are nickel oxide but maybe it's a mixture of nickel and iron oxide I don't know. I sanded with fine grit sandpaper one of the pieces and then put it in a 4% citric acid bath for 30 minutes to test my suspicion it was corrosion and lo and behold the piece is now perfect and the spots no longer reappear. Upon closer inspect of the shotgun condensors I have, the inner pipes where the gas/condensate flows all have heat affected colors and stains on them. And inside the 2inch and 3inch long pipe extensions I have I can clearly see the actual heat pattern of the welds. I haven't even cleaned/touched the 3 inch pipes I have and I can see the same thing (just to throw out the idea it is my cleaning method). It seems they just polished over the heat tint and welding errors which mostly makes it invisible unless you are looking at it in certain ways.
So my questions. Does having a heat tint on stainless pass as a sanitary weld? I'm assuming not as it is corroding and also does not look good. And is there a better way to get rid of this type of corrosion than sanding it back and acid bathing it to passivate it? I've never really treated sanitary stainless which has had corrosion before and maybe there are better/easier/quicker ways.