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Stainless Steel "rust" marks on paint
Hi,
I know that this possibly has a simple answer that everyone but me knows, but is there any way I can prevent the unslightly brown "rust" marks on my white hull paint from a welded fabricated stainless steel fitting that is bolted on with stainless bolts? I am about to spray on the white topcoat over the primer during a hull repaint job and I'd like it to stay looking good. I suspect that the drop ladder on the stern is not made entirely of 316. It has been 15 years since I fitted it, so that I can't recall whether the bolt is 316, 304 or (unlikely) something else. There are no 316 markings on the bolt heads. We use oxalic acid powder mixed with water to get rid of it but it always comes back. I am beginning to quote Lady MacBeth when I rub it away ("out foul spot") and it's embarrassing when passersby overhear. I know that there are several (expensive) "special" products to clean it with but I would appreciate some knight on a white charger offering me a permanent fix. It doesn't matter if he's young or old, handsome or otherwise - I'm not playing Sleeping Beauty - so long as he or she has the answer. I wonder whether it would be possible to electrically isolate it with say, a teflon washer? cheers Peter (who actually sails places and doesn't pretend to) |
Stainless Steel "rust" marks on paint
On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel
wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra |
Stainless Steel "rust" marks on paint
Peter Hendra wrote:
Hi, I know that this possibly has a simple answer that everyone but me knows, but is there any way I can prevent the unslightly brown "rust" marks on my white hull paint from a welded fabricated stainless steel fitting that is bolted on with stainless bolts? I am about to spray on the white topcoat over the primer during a hull repaint job and I'd like it to stay looking good. I suspect that the drop ladder on the stern is not made entirely of 316. It has been 15 years since I fitted it, so that I can't recall whether the bolt is 316, 304 or (unlikely) something else. There are no 316 markings on the bolt heads. We use oxalic acid powder mixed with water to get rid of it but it always comes back. I am beginning to quote Lady MacBeth when I rub it away ("out foul spot") and it's embarrassing when passersby overhear. I know that there are several (expensive) "special" products to clean it with but I would appreciate some knight on a white charger offering me a permanent fix. It doesn't matter if he's young or old, handsome or otherwise - I'm not playing Sleeping Beauty - so long as he or she has the answer. I wonder whether it would be possible to electrically isolate it with say, a teflon washer? cheers Peter (who actually sails places and doesn't pretend to) where is the rust comming from the weld or the mounting point or the bolts them selfs Shaun |
Stainless Steel "rust" marks on paint
On Fri, 23 Mar 2007 19:22:33 +1100, Peter Hendra
wrote: I know that this possibly has a simple answer that everyone but me knows, but is there any way I can prevent the unslightly brown "rust" marks on my white hull paint from a welded fabricated stainless steel fitting that is bolted on with stainless bolts? My first reaction would be to prime and paint the stainless but that might have its own issues. |
Stainless Steel "rust" marks on paint
Probability is that the welds are not ground flat and smooth and
crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . In article , Peter Hendra wrote: Hi, I know that this possibly has a simple answer that everyone but me knows, but is there any way I can prevent the unslightly brown "rust" marks on my white hull paint from a welded fabricated stainless steel fitting that is bolted on with stainless bolts? I am about to spray on the white topcoat over the primer during a hull repaint job and I'd like it to stay looking good. I suspect that the drop ladder on the stern is not made entirely of 316. It has been 15 years since I fitted it, so that I can't recall whether the bolt is 316, 304 or (unlikely) something else. There are no 316 markings on the bolt heads. We use oxalic acid powder mixed with water to get rid of it but it always comes back. I am beginning to quote Lady MacBeth when I rub it away ("out foul spot") and it's embarrassing when passersby overhear. I know that there are several (expensive) "special" products to clean it with but I would appreciate some knight on a white charger offering me a permanent fix. It doesn't matter if he's young or old, handsome or otherwise - I'm not playing Sleeping Beauty - so long as he or she has the answer. I wonder whether it would be possible to electrically isolate it with say, a teflon washer? cheers Peter (who actually sails places and doesn't pretend to) |
Stainless Steel "rust" marks on paint
On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman"
wrote: A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. Thanks for this Gordon, At the moment the offending articles are sheathed by masking tape as I am halfway through spray painting the hull. I would very much appreciate you asking your friend if it is not too much trouble. The more information about this, the better. When it was welded, it, like all other Stainless welds was brushed with that nitric acid gel solution that then is washed off with water, and which removes the discolouration from the tig/mig/arc welding process. Is this the same thing? I can't recall getting anything "passivated". Thanks and cheers Peter |
Stainless Steel "rust" marks on paint
"Peter Hendra" wrote in message ... On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. |
Stainless Steel "rust" marks on paint
On Tue, 27 Mar 2007 16:53:07 +0700, Bruce
wrote: I've been reading some of your other posts -- do you reckon that your paint job is cheaper, or more expensive then it wold be in Malaysia? Bruce in Bangkok (brucepaigeatgmaildotcom) Hi Bruce, Thanks for the info. It looks as if I shall have to take my stern ladder off and polish it as you say. Yes, I always use Sikaflex. As to the relative cost of the paint job, haven't really done the sums. I have to get it done so I just spend what I need to. The paint is relatively the same - I am using Ameron - made by the people who own the yard. In Malaysia I used Jotan antifouling which I bought in a 20 litre drum from Singapore and had ferried up to Langkawi by a series of patrol boats (free of course). I can't recall the price but it seems rather expensive here. I am considering ABC3 but have to test it on a metre square patch as to whether it will adhere to the existing Jotan (I should post an enquiry regarding this). The other difference and increase in cost for me is that in Malaysia I had free use of the 30 ton travel lift at our Langkawi Marine Customs base as well as the large new shed to paint in. Here I am paying yard fees and small extras such as daily rental for planks and scaffolding. There is an extra daily charge for living on the boat whilst in the yard. I plan to take my mast down and renew the rigging and do a number of other things. Rental is also charged on the mast whilst it is on the ground. In Langkawi there is an excellent stainless workshop as well as experienced electrical and mechanical engineers - all very cheap. Here, prices for labour and services are much dearer, partly due to the large American and other customers who are happy to pay the somewhat cheaper cost than in the US. One price I do know that you should be able to trealte to was the cost of having two hatches re-teaked - US$75 per square foot. Also, here I have no car so have to purchase most groceries at the local minimarket which has marina pricing unless I want to take half a day off to buy only that which I can carry on a maxi taxi. In Langkawi I could have my meals at the Customs Base. Another difference, not factored in any financial calculations, is that here there are many like minded souls who are able to give advice regarding things boating. In Malaysia, by nature of my job and the close knit family style of Customs and because my wife and son were with me, I mainly socialised with locals and met few yachtees. Actually, many of the later disliked me before they had met me as many were annoyed that I had access to Customs bases and facilities throughout the country. One even officially approached the commander of the Penang base and asked why he was not able to tie up there, leave his dinghy there in security and get water etc. as I did. No amount of telling them that I worked for Customs seemed to satisfy them. Also, I was regarded with suspicion (sometimes in quite a hostile manner) by some as a lot of yachtees sailed up from places like the marina at Port Dixon to load their boats with duty free cigarettes and alcohol which they sold or shared with their neighbours. They expected me to do something about it and curb their activities but frankly I and we had other priorities and never worried much about yachts. We knew of it but let it pass as Customs is kept busy by contraband smugglers, the heroin traffic from Thailand, and human trafficking. One specially built GRP boat with three new Mercury 200HP outboards bringing 2.5 tons of cigarettes from Indonesia avoids $615,000 Ringit of duty (US$178,000). I have seen two of these apprehended in one night at the Port Klang base. Besides, many of these yachtees I had nothing in common with anyway and they had spent years drinking and socialising in the one place as a way of life. They won't leave and there are those that finance themselves by selling off vital equipment from their boats. There were others, however, who were great company such as Bob and Ely and their delightful two little boys who, at aged 4 and 5 could climb to the spreaders - no mast steps, just pull themselves up a halyard - and sit and eat their bag of potato chips. Bob, a retired military man, a rock conservative fundamentalist Christian from the Mid-West, who at age 75 is on his third marriage, will help anyone with their radio or electronics. He now is moored at a Marine base in Penang most of the time - a wonderful man. Hey, I apologise - I got carried away. Yes, thinking about it - it is cheaper in Malaysia but commercial haulout is much dearer there. cheers Peter |
Stainless Steel "rust" marks on paint
On Tue, 27 Mar 2007 20:51:00 +1200, Eric Stevens
wrote: On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman" wrote: "Peter Hendra" wrote in message . .. On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. It won't be Hydrofluoric Acid, which is one of the nastiest nasty chemicals on the face of this earth. It's much more likely to be a 50~50 mixture of Hydrochloric and Nitric acids plus, sometimes, a little Phosphoric acid. You should somewhere find an expert and read http://www.assda.asn.au/asp/index.asp?pgid=17982 Whatever you do, you don't want a strong brew and you should let time be your friend. Eric Stevens Thanks Eric, A great site and I love your phrase "let time be your friend" - poetic. May I have your permission to use it in future - yes, I'll cite you as the originator? I don't want to get flamed. cheers Peter |
Stainless Steel "rust" marks on paint
On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman"
wrote: "Peter Hendra" wrote in message .. . On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. It won't be Hydrofluoric Acid, which is one of the nastiest nasty chemicals on the face of this earth. It's much more likely to be a 50~50 mixture of Hydrochloric and Nitric acids plus, sometimes, a little Phosphoric acid. You should somewhere find an expert and read http://www.assda.asn.au/asp/index.asp?pgid=17982 Whatever you do, you don't want a strong brew and you should let time be your friend. Eric Stevens |
Stainless Steel "rust" marks on paint
On Mon, 26 Mar 2007 17:50:22 +1000, Peter Hendra
wrote: On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman" wrote: A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. Thanks for this Gordon, At the moment the offending articles are sheathed by masking tape as I am halfway through spray painting the hull. I would very much appreciate you asking your friend if it is not too much trouble. The more information about this, the better. When it was welded, it, like all other Stainless welds was brushed with that nitric acid gel solution that then is washed off with water, and which removes the discolouration from the tig/mig/arc welding process. Is this the same thing? I can't recall getting anything "passivated". Thanks and cheers Peter The acid treatment was "passivating". My experience is that unless the stainless is polished to a mirror bright finish you will get staining. All the bits have to be polished; Assuming that it is a bolt on fitting the bolts and washers that are exposed to sea water need to be polished. I've been making things out of stainless and putting them on boats for quite a number of years and I have found that if the part has all the welds ground smooth (so the welding ripples don't show), no pin holes or voids in the weld and then polished bright I seldom get staining. If I don't do this I always get staining. A couple of years ago I built a new stitch and glue dinghy. Welded up a couple of plates with a lifting eye; dosed them with a 20% acid bath until all the discoloration was cleaned off; gave them a lick with a sanding disk on a high speed grinder; mounted them and gave them two coats of epoxy primer and two coats of two part poly urethane (spelling?). Got "rust" stains. Should have polished them. By the way, I get my stainless polished at the local chrome shop, Cheaper then the "stainless shop". I also take care to bed everything with either 3M or Sikaflex. Much better then household "silicone" sealant. I've been reading some of your other posts -- do you reckon that your paint job is cheaper, or more expensive then it wold be in Malaysia? Bruce in Bangkok (brucepaigeatgmaildotcom) -- Posted via a free Usenet account from http://www.teranews.com |
Stainless Steel "rust" marks on paint
Bruce wrote: The acid treatment was "passivating". My experience is that unless the stainless is polished to a mirror bright finish you will get staining. All the bits have to be polished; Assuming that it is a bolt on fitting the bolts and washers that are exposed to sea water need to be polished. I've been making things out of stainless and putting them on boats for quite a number of years and I have found that if the part has all the welds ground smooth (so the welding ripples don't show), no pin holes or voids in the weld and then polished bright I seldom get staining. If I don't do this I always get staining. snip... Bruce in Bangkok (brucepaigeatgmaildotcom) Hi Bruce, I've just recently started building stainless parts for my two sailboats. I'm using 316 SS and 316L SS exclusively and passivating with Citrisurf which is a strong citric acid. It can also be used for electropolishing, and I intend to try using it for that in the future. Google "Citrisurf". It is supposed to be better and more environmentally friendly than nitric acid paste. Several of the welders that post to sci.engr.joining.welding have used it with good results. I've only started using it about six months ago, so the stuff I've built has not been out in the salt spray long enough to tell how good the passivation was. Don W. |
Stainless Steel "rust" marks on paint
On Tue, 27 Mar 2007 06:52:12 +1000, Peter Hendra
wrote: On Tue, 27 Mar 2007 20:51:00 +1200, Eric Stevens wrote: On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman" wrote: "Peter Hendra" wrote in message ... On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. It won't be Hydrofluoric Acid, which is one of the nastiest nasty chemicals on the face of this earth. It's much more likely to be a 50~50 mixture of Hydrochloric and Nitric acids plus, sometimes, a little Phosphoric acid. You should somewhere find an expert and read http://www.assda.asn.au/asp/index.asp?pgid=17982 Whatever you do, you don't want a strong brew and you should let time be your friend. Eric Stevens Thanks Eric, A great site and I love your phrase "let time be your friend" - poetic. May I have your permission to use it in future - yes, I'll cite you as the originator? I don't want to get flamed. Feel free to use it. :-) I was really trying to say use a longer exposure to a weak brew and don't use a concentrated mix just to get a quick result. Eric Stevens |
Stainless Steel "rust" marks on paint
On Tue, 27 Mar 2007 17:35:18 GMT, Don W
wrote: Bruce wrote: The acid treatment was "passivating". My experience is that unless the stainless is polished to a mirror bright finish you will get staining. All the bits have to be polished; Assuming that it is a bolt on fitting the bolts and washers that are exposed to sea water need to be polished. I've been making things out of stainless and putting them on boats for quite a number of years and I have found that if the part has all the welds ground smooth (so the welding ripples don't show), no pin holes or voids in the weld and then polished bright I seldom get staining. If I don't do this I always get staining. snip... Bruce in Bangkok (brucepaigeatgmaildotcom) Hi Bruce, I've just recently started building stainless parts for my two sailboats. I'm using 316 SS and 316L SS exclusively and passivating with Citrisurf which is a strong citric acid. It can also be used for electropolishing, and I intend to try using it for that in the future. Google "Citrisurf". It is supposed to be better and more environmentally friendly than nitric acid paste. Several of the welders that post to sci.engr.joining.welding have used it with good results. I've only started using it about six months ago, so the stuff I've built has not been out in the salt spray long enough to tell how good the passivation was. Don W. Don, I've tried things ranging from purpose made passivating stuff to vinegar. At the moment I am using a tile cleaner sold, here in Thailand in every supermarket, that is 20% hydrochloric acid. One comment I probably should make is that all the passivation chemicals work by dissolving (eating) some material from the surface of the piece being worked on. Of course this is how it works, removing all the oxides and other rubbish from the just welded surfaces. BUT, it also etches the nice shiny surface of the expensive piece of stainless you just welded. My experience is that if you don't polish the etched surface back to a mirror finish you still get staining. In another message I mentioned some dinghy lifting brackets I made that stained. They had been left in acid overnight and were perfectly clean but with a matt finish when I installed them -- thought the matt finish would hold the paint better -- wrong. Bruce in Bangkok (brucepaigeatgmaildotcom) -- Posted via a free Usenet account from http://www.teranews.com |
Stainless Steel "rust" marks on paint
On Tue, 27 Mar 2007 06:52:12 +1000, Peter Hendra
wrote: On Tue, 27 Mar 2007 20:51:00 +1200, Eric Stevens wrote: On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman" wrote: "Peter Hendra" wrote in message ... On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. It won't be Hydrofluoric Acid, which is one of the nastiest nasty chemicals on the face of this earth. It's much more likely to be a 50~50 mixture of Hydrochloric and Nitric acids plus, sometimes, a little Phosphoric acid. You should somewhere find an expert and read http://www.assda.asn.au/asp/index.asp?pgid=17982 Whatever you do, you don't want a strong brew and you should let time be your friend. Eric Stevens Thanks Eric, A great site and I love your phrase "let time be your friend" - poetic. May I have your permission to use it in future - yes, I'll cite you as the originator? I don't want to get flamed. I take back most of what I have written. I have just found a little booklet about stainless steels I bought some years ago from New Zealand's Department of Scientific & Industrial Research. Here is their advice: _____________________________ Pickling and passivation Acid pickling effectively removes all surface blemishes of low corrosion resistance, such as iron contamination, oxide scale, and heat tints. The process involves using an etchant (hydrofluoric acid) in the presence of an oxidising agent (nitric acid) which inhibits the etching action. A typical formulation consists of 15% nitric acid and 1-3% hydrofluoric acid, which will remove most scales on immersion of the stainless steel component for 5-30 min at ambient temperature. As mentioned previously, passivation treatment involves the use of 15-30% nitric acid on its own, to restore a passive film which may have been damaged by heat treatment or contact with harmful chemicals. The disadvantages of using concentrated chemical etchants, apart from the handling of hazardous materials, include the risk of excessive pickling at grain boundaries and hydrogen embrittlement of certain grades of stainless steels. Weld dressing and descaling are essential to enhance the corrosion resistance of welds. If immersion of the welded component in an acid mixture is not practicable, a weld-descaling paste may be used. This is an inert base material containing nitric acid and hydrofluoric acid, or, if a passivation treatment only is required, just nitric acid. These weld-dressing pastes are applied over the weld in a thin layer and then left for about 1 hour before the residues are cleaned off. All pickling and passivation chemicals applied to welds must be thoroughly rinsed off with fresh water after the appropriate residence time for the chemical cleaning is completed. Electropolishing is another batch process for post-fabrication shop cleaning and passivation of stainless steel components and smaller equipment. This technique removes heat tints on welds and surface impurities such as iron contamination, and it also imparts a highly corrosion-resistant finish with a pleasant dull-grey appearance to the treated surface. Electropolishing must be carried out by trained staff under carefully controlled conditions, because, as with the anodising process on aluminium, it is possible to produce a defective finish if the process goes out of control. ______________________________ What I said previously about hydrofluoric acid stands. Its a very nast material. Treat it with very great respect, even in the diluted form. See http://ptcl.chem.ox.ac.uk/MSDS/HY/hy...oric_acid.html and http://adm.monash.edu/ohse/assets/do...acid-draft.pdf Eric Stevens |
Stainless Steel "rust" marks on paint
On Wed, 28 Mar 2007 07:44:50 +0700, Bruce
wrote: I've tried things ranging from purpose made passivating stuff to vinegar. At the moment I am using a tile cleaner sold, here in Thailand in every supermarket, that is 20% hydrochloric acid. One comment I probably should make is that all the passivation chemicals work by dissolving (eating) some material from the surface of the piece being worked on. Of course this is how it works, removing all the oxides and other rubbish from the just welded surfaces. BUT, it also etches the nice shiny surface of the expensive piece of stainless you just welded. My experience is that if you don't polish the etched surface back to a mirror finish you still get staining. In another message I mentioned some dinghy lifting brackets I made that stained. They had been left in acid overnight and were perfectly clean but with a matt finish when I installed them -- thought the matt finish would hold the paint better -- wrong. Bruce in Bangkok (brucepaigeatgmaildotcom) Thanks again Bruce, There is just so much to learn Peter |
Stainless Steel "rust" marks on paint
On Wed, 28 Mar 2007 13:29:59 +1200, Eric Stevens
wrote: On Tue, 27 Mar 2007 06:52:12 +1000, Peter Hendra wrote: On Tue, 27 Mar 2007 20:51:00 +1200, Eric Stevens wrote: On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman" wrote: "Peter Hendra" wrote in message m... On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. It won't be Hydrofluoric Acid, which is one of the nastiest nasty chemicals on the face of this earth. It's much more likely to be a 50~50 mixture of Hydrochloric and Nitric acids plus, sometimes, a little Phosphoric acid. You should somewhere find an expert and read http://www.assda.asn.au/asp/index.asp?pgid=17982 Whatever you do, you don't want a strong brew and you should let time be your friend. Eric Stevens Thanks Eric, A great site and I love your phrase "let time be your friend" - poetic. May I have your permission to use it in future - yes, I'll cite you as the originator? I don't want to get flamed. I take back most of what I have written. I have just found a little booklet about stainless steels I bought some years ago from New Zealand's Department of Scientific & Industrial Research. Here is their advice: _____________________________ Pickling and passivation Acid pickling effectively removes all surface blemishes of low corrosion resistance, such as iron contamination, oxide scale, and heat tints. The process involves using an etchant (hydrofluoric acid) in the presence of an oxidising agent (nitric acid) which inhibits the etching action. A typical formulation consists of 15% nitric acid and 1-3% hydrofluoric acid, which will remove most scales on immersion of the stainless steel component for 5-30 min at ambient temperature. As mentioned previously, passivation treatment involves the use of 15-30% nitric acid on its own, to restore a passive film which may have been damaged by heat treatment or contact with harmful chemicals. The disadvantages of using concentrated chemical etchants, apart from the handling of hazardous materials, include the risk of excessive pickling at grain boundaries and hydrogen embrittlement of certain grades of stainless steels. Weld dressing and descaling are essential to enhance the corrosion resistance of welds. If immersion of the welded component in an acid mixture is not practicable, a weld-descaling paste may be used. This is an inert base material containing nitric acid and hydrofluoric acid, or, if a passivation treatment only is required, just nitric acid. These weld-dressing pastes are applied over the weld in a thin layer and then left for about 1 hour before the residues are cleaned off. All pickling and passivation chemicals applied to welds must be thoroughly rinsed off with fresh water after the appropriate residence time for the chemical cleaning is completed. Electropolishing is another batch process for post-fabrication shop cleaning and passivation of stainless steel components and smaller equipment. This technique removes heat tints on welds and surface impurities such as iron contamination, and it also imparts a highly corrosion-resistant finish with a pleasant dull-grey appearance to the treated surface. Electropolishing must be carried out by trained staff under carefully controlled conditions, because, as with the anodising process on aluminium, it is possible to produce a defective finish if the process goes out of control. ______________________________ What I said previously about hydrofluoric acid stands. Its a very nast material. Treat it with very great respect, even in the diluted form. See http://ptcl.chem.ox.ac.uk/MSDS/HY/hy...oric_acid.html and http://adm.monash.edu/ohse/assets/do...acid-draft.pdf Eric Stevens Thanks for quote. I knew that stainless reacted with oxygen to form a non-corrosive surface and that unless you had a flow of water the lack of dissolved oxygen might result in crevice corrosion but somehow had never related it to pasivating. I'll now use at least come nitric acid as an oxidizing agent. Bruce in Bangkok (brucepaigeatgmaildotcom) -- Posted via a free Usenet account from http://www.teranews.com |
Stainless Steel "rust" marks on paint
Bruce wrote:
snip--Eric's good primer on pickling and passivation of austentetic stainless steels Pickling and passivation Thanks for quote. I knew that stainless reacted with oxygen to form a non-corrosive surface and that unless you had a flow of water the lack of dissolved oxygen might result in crevice corrosion but somehow had never related it to pasivating. I'll now use at least come nitric acid as an oxidizing agent. Bruce in Bangkok (brucepaigeatgmaildotcom) Bruce, I'll try to steer you to looking into stainless steel passivation using strong citric acid (ie Citrisurf, etc.) once more. I've not time to provide the relevant links at the moment. Use nitric acid if you like. It works also, although not as well as citric acid according to various studies. It is also much more hazardous to transport and dispose of. Citric acid can be diluted and legally flushed down the drain. Citric acid passivation for SS is fairly new technology, and Nitric acid is the traditional way of doing it. Citric acid can also be used with a power supply to electropolish stainless steel. The people who turned me on to the citric acid passivation do a lot of industrial and food grade stainless welding and have switched over. Good luck, Don W. |
Stainless Steel "rust" marks on paint
On Wed, 28 Mar 2007 15:03:32 -0500, Don W
wrote: Bruce wrote: snip--Eric's good primer on pickling and passivation of austentetic stainless steels Pickling and passivation Thanks for quote. I knew that stainless reacted with oxygen to form a non-corrosive surface and that unless you had a flow of water the lack of dissolved oxygen might result in crevice corrosion but somehow had never related it to pasivating. I'll now use at least come nitric acid as an oxidizing agent. Bruce in Bangkok (brucepaigeatgmaildotcom) Bruce, I'll try to steer you to looking into stainless steel passivation using strong citric acid (ie Citrisurf, etc.) once more. I've not time to provide the relevant links at the moment. Use nitric acid if you like. It works also, although not as well as citric acid according to various studies. It is also much more hazardous to transport and dispose of. Citric acid can be diluted and legally flushed down the drain. Citric acid passivation for SS is fairly new technology, and Nitric acid is the traditional way of doing it. Citric acid can also be used with a power supply to electropolish stainless steel. The people who turned me on to the citric acid passivation do a lot of industrial and food grade stainless welding and have switched over. Good luck, Don W. Don't get me wrong, I really appreciate your information. My reply was supposed to indicate that while I knew that stainless needed a certain amount of oxygen to be "stainless" I had never related that information to the passivation process. Citric acid may prove a problem though as I live in Thailand thousand and some things are just not available. Add to this the (sometimes) problem with translating technical terms and life sometimes becomes very frustrating. However, when I return to Bangkok I'll stop by my chemical supply place and see what is available. One nice thing about Thailand is that it is still primitive enough that people actually want to sell you things. You go to a large chemical supply house and say, "I need a liter of a 6-1/3% solution of nitric acid and distilled water and they never blink an eye. Just apologize for not being able to hand it to you straight away and ask you to come back tomorrow morning, about 09:00. Bruce in Bangkok (brucepaigeatgmaildotcom) -- Posted via a free Usenet account from http://www.teranews.com |
Stainless Steel "rust" marks on paint
Don, Bruce,
While nitric acid is the historical method of passivation, citric acid had been the standard for *well* over a decade - not new technology at all. For any given temperature and contact time, nitric works better. Citric acid is used since it's far less dangerous and environmentally unfriendly. Phosphoric acid is also frequently used for ambient temp passivation. Hydrochloric acid is NOT used for passivation of stainless - ever. Neither is HF, unless you're pickling (i.e. removing significant material - etching). The Citrisurf material looks OK, but I have little faith in combination products that both clean and passivate. Far better to remove all oils *first* with a heavy duty surfactant (e.g. TSP), then passivate with citric (or other) acid. As has been previously discussed, mechanical polishing of non-orbital welds (prior to passivation) is still a pre-requisite for prevention of oxidation. Keith Hughes Don W wrote: Bruce wrote: snip--Eric's good primer on pickling and passivation of austentetic stainless steels Pickling and passivation Thanks for quote. I knew that stainless reacted with oxygen to form a non-corrosive surface and that unless you had a flow of water the lack of dissolved oxygen might result in crevice corrosion but somehow had never related it to pasivating. I'll now use at least come nitric acid as an oxidizing agent. Bruce in Bangkok (brucepaigeatgmaildotcom) Bruce, I'll try to steer you to looking into stainless steel passivation using strong citric acid (ie Citrisurf, etc.) once more. I've not time to provide the relevant links at the moment. Use nitric acid if you like. It works also, although not as well as citric acid according to various studies. It is also much more hazardous to transport and dispose of. Citric acid can be diluted and legally flushed down the drain. Citric acid passivation for SS is fairly new technology, and Nitric acid is the traditional way of doing it. Citric acid can also be used with a power supply to electropolish stainless steel. The people who turned me on to the citric acid passivation do a lot of industrial and food grade stainless welding and have switched over. Good luck, Don W. |
Stainless Steel "rust" marks on paint
Hi Keith,
Keith Hughes wrote: Don, Bruce, While nitric acid is the historical method of passivation, citric acid had been the standard for *well* over a decade - not new technology at all. Well, my use of the term "new" is relative. A decade takes us back to 1997, and I still think of that as relatively "new". For any given temperature and contact time, nitric works better. I'm not sure that this is true... I read an independent technical lab report on the quality of passivation films formed by nitric acid and citric acid when I was researching the Citrisurf. It compared the CrO2? films at a microscopic level for different passivation techniques, and concluded that the quality of passivation using citric acid was slightly better than nitric acid. IIRC the thickness of the passivation film using any method is limited to some maximum because eventually the reaction stops due to complete oxidation of the surface. The report is available on the web somewhere. I did not bookmark it because I was only trying to decide what technique to use for myself. Citric acid is used since it's far less dangerous and environmentally unfriendly. Phosphoric acid is also frequently used for ambient temp passivation. Hydrochloric acid is NOT used for passivation of stainless - ever. Neither is HF, unless you're pickling (i.e. removing significant material - etching). I use Phosphoric for pre-cleaning, rust removal and etch of regular (non-stainless) steel before painting. It works well. Hydrochloric acid (Battery acid) can be used with a DC power supply for surface passivation of aluminum. The process is called anodizing ;-) The Citrisurf material looks OK, but I have little faith in combination products that both clean and passivate. Far better to remove all oils *first* with a heavy duty surfactant (e.g. TSP), then passivate with citric (or other) acid. AFAIK Citrisurf does not claim to clean. You are supposed to do a cleaning step (cleaner/degreaser) before passivation. At least that is what I do. As has been previously discussed, mechanical polishing of non-orbital welds (prior to passivation) is still a pre-requisite for prevention of oxidation. Agreed. Don W. |
Stainless Steel "rust" marks on paint
Hi Don,
While nitric acid is the historical method of passivation, citric acid had been the standard for *well* over a decade - not new technology at all. Well, my use of the term "new" is relative. A decade takes us back to 1997, and I still think of that as relatively "new". Notice I said well over a decade. At least a couple of the larger passivation firms (Calchem and Astropak) began using citric acid based passivation solutions in the later '80s (I'm just not sure of the dates - it's that old age thing :-) For any given temperature and contact time, nitric works better. I'm not sure that this is true... Yeah, I worded that poorly. What I meant was that Nitric works faster for any given temperature. I read an independent technical lab report on the quality of passivation films formed by nitric acid and citric acid when I was researching the Citrisurf. It compared the CrO2? films at a microscopic level for different passivation techniques, and concluded that the quality of passivation using citric acid was slightly better than nitric acid. IIRC the thickness of the passivation film using any method is limited to some maximum because eventually the reaction stops due to complete oxidation of the surface. Other processes come into play as well. When you're talking about wiping on versus dipping/soaking, you're at a significant disadvantage. With a dip, or circulation process for piping, you can add chelants/sequestrants like EDTA and suspension agents like polysorbate so that all (virtually) of the iron removed is chelated/sequestered/suspended and can't get redeposited on the surface being passivated. The report is available on the web somewhere. I did not bookmark it because I was only trying to decide what technique to use for myself. Citric acid is used since it's far less dangerous and environmentally unfriendly. Phosphoric acid is also frequently used for ambient temp passivation. Hydrochloric acid is NOT used for passivation of stainless - ever. Neither is HF, unless you're pickling (i.e. removing significant material - etching). I use Phosphoric for pre-cleaning, rust removal and etch of regular (non-stainless) steel before painting. It works well. Hydrochloric acid (Battery acid) can be used with a DC power supply for surface passivation of aluminum. The process is called anodizing ;-) The Citrisurf material looks OK, but I have little faith in combination products that both clean and passivate. Far better to remove all oils *first* with a heavy duty surfactant (e.g. TSP), then passivate with citric (or other) acid. AFAIK Citrisurf does not claim to clean. You are supposed to do a cleaning step (cleaner/degreaser) before passivation. At least that is what I do. I just briefly glanced at their site, and it appeared that their products were mostly one-step clean/passivate products. I could certainly be mistaken about that though. Keith Hughes |
Stainless Steel "rust" marks on paint
Hi Keith,
Keith Hughes wrote: Hi Don, While nitric acid is the historical method of passivation, citric acid had been the standard for *well* over a decade - not new technology at all. Well, my use of the term "new" is relative. A decade takes us back to 1997, and I still think of that as relatively "new". Notice I said well over a decade. At least a couple of the larger passivation firms (Calchem and Astropak) began using citric acid based passivation solutions in the later '80s (I'm just not sure of the dates - it's that old age thing :-) Yeah, I know what you mean. Even twenty year old stuff still seems pretty new to me now-- and I'm _not_ that old. Ah well... I guess I mean I'm not _that_ old ;-) For any given temperature and contact time, nitric works better. I'm not sure that this is true... Yeah, I worded that poorly. What I meant was that Nitric works faster for any given temperature. That may very well be true, however the immersion times with the citric acid are not excessive. They recommend 2-5 minutes at 120F-160F, and that seems to be giving me good results. Ask me in another two years ;-) I read an independent technical lab report on the quality of passivation films formed by nitric acid and citric acid when I was researching the Citrisurf. It compared the CrO2? films at a microscopic level for different passivation techniques, and concluded that the quality of passivation using citric acid was slightly better than nitric acid. IIRC the thickness of the passivation film using any method is limited to some maximum because eventually the reaction stops due to complete oxidation of the surface. Other processes come into play as well. When you're talking about wiping on versus dipping/soaking, you're at a significant disadvantage. Yes. I always try to dip my parts in a temp controlled bath with some sort of agitation. Sometimes this cannot be done because the part is too large for my available containers. I'd like to find a process that would work well for re-passivating SS parts on the boat without removing them. With a dip, or circulation process for piping, you can add chelants/sequestrants like EDTA and suspension agents like polysorbate so that all (virtually) of the iron removed is chelated/sequestered/suspended and can't get redeposited on the surface being passivated. I had to think about what you were saying there for a moment. I don't use any of these additives at the moment, however they may be included in the Citrisurf and I just don't know it. The Citrisurf material looks OK, but I have little faith in combination products that both clean and passivate. Far better to remove all oils *first* with a heavy duty surfactant (e.g. TSP), then passivate with citric (or other) acid. AFAIK Citrisurf does not claim to clean. You are supposed to do a cleaning step (cleaner/degreaser) before passivation. At least that is what I do. I just briefly glanced at their site, and it appeared that their products were mostly one-step clean/passivate products. I could certainly be mistaken about that though. Quoting from the Citrisurf Product Information I have on file: "Description - CitriSurf 2050 is a high quality blend of chemicals formulated for the cleaning and passivation of stainless steel products. CitriSurft 2050 is a solution of chemicals specifically designed to provide low cost and efficient removal of contaminants and all free iron from the surface of stainless steel." "Application Procedure - To assure best results, the parts to be passivated should be thoroughly cleaned with an appropriate cleaning solution prior to treatment with CitriSurf solution..." So I see where you got the idea that it was a one-step. The description seems to imply that it is, however the directions make clear that you are supposed to clean and degrease the parts before passivation. Don W. |
Stainless Steel "rust" marks on paint
Hey Don,
Yeah, I worded that poorly. What I meant was that Nitric works faster for any given temperature. That may very well be true, however the immersion times with the citric acid are not excessive. They recommend 2-5 minutes at 120F-160F, and that seems to be giving me good results. Ask me in another two years ;-) That should be fine. Most folks don't have temp baths to work with, so I assumed you would be working at lower temps. There are also off-the-shelf ferroxyl test kits available for testing the surface passivity if you want. Yes. I always try to dip my parts in a temp controlled bath with some sort of agitation. Sometimes this cannot be done because the part is too large for my available containers. A buddy of mine has a business specializing in sanitary piping and equipment, custom s/s fab work, and mechanical / electropolishing / passivation. I can hook you up with him if you ever need a larger piece polished and passivated. He's in Tempe AZ (a suburb of Phoenix - but don't tell them that). Keith |
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