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#1
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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 |
#2
posted to rec.boats.cruising
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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 |
#3
posted to rec.boats.cruising
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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 |
#4
posted to rec.boats.cruising
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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 |
#5
posted to rec.boats.cruising
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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. |
#6
posted to rec.boats.cruising
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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 |
#7
posted to rec.boats.cruising
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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. |
#8
posted to rec.boats.cruising
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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. |
#9
posted to rec.boats.cruising
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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 |
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