A team at a British Univ. says they have discovered why stainless corrodes. If true, this could lead to truly stainless steels.
http://www.ic.ac.uk/templates/text_3.asp?P=3197
It would be interesting to see what Roger Winiarski (sp?) of Bristol
Bronze thinks of this.
Rluby@aol.com
Stainless Corrosion Explained!
Moderator: Jim Walsh
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Tom
Fascinating link, Bob, thanks.
Fascinating link, Bob. Thanks. Now how do we keep our brass lamps and ports from needing polishing? Don't put the FIB microscope away.
TomCambria@mindspring.com
Bob Luby wrote: A team at a British Univ. says they have discovered why stainless corrodes. If true, this could lead to truly stainless steels.
http://www.ic.ac.uk/templates/text_3.asp?P=3197
It would be interesting to see what Roger Winiarski (sp?) of Bristol
Bronze thinks of this.
TomCambria@mindspring.com
-
Bristol Bronze
Re: Stainless Corrosion Explained!
Stainless Steel, All stainless steel, is a mixture of nickel, chrome and iron. Usually there are a few other things in there in small percentages but the nickel, chrome and iron are the major elements. It is the iron that rusts.Bob Luby wrote: A team at a British Univ. says they have discovered why stainless corrodes. If true, this could lead to truly stainless steels.
http://www.ic.ac.uk/templates/text_3.asp?P=3197
It would be interesting to see what Roger Winiarski (sp?) of Bristol
Bronze thinks of this.
The metals that make up stainless steel do not homogonize well and you will usually find a wide variation in the make up from spot to spot. Those spots with the highest iron content will corrode first.
Stainless steel gets its corrosion resistance from a clear oxide that builds up on its surface, Chromium Oxide. This particular oxide needs a constant supply of oxygen to maintain itself. In open air it is fine. Even under water, as long as the water is free flowing it can get all the oxygen it needs from the water. When the water gets trapped inside a small opening, such as a stuffing box, cutlas bearing, between the straps on a pintal or gudgeon and the hull, etc the chromium oxide will use up all the available oxygen from the water. When this happens and there is no more oxygen available the oxide breaks down and the stainless steel begins to corrode. The higher the water temperature the more active the molecules of the metal and the faster the corrosion takes place.
This type of corrosion is called "crevice corrosion" and can take place anywhere. Any place that provides a small crevice where the water can penetrate and then just sit is a prime place for this corrosion. I have seen chain plates that were not bedded properly and leaked a little almost rot through. Other factors such as contact with dis-similar metals or stray electrical current can enhance this problem. I replaced (with Bronze) the stainless steel pintals and gudgeons on a boat in New Rochelle, N.Y. a few years ago. The stainless ones were only ten years old and looked like swiss lace.
My own personal boat, the one on the home page of my web site, was built at Herreshoff Mfg. Co. in September of 1920. All her fittings are still fine. They are all made of Bronze. Anyone that uses Stainless Steel under water and expects it to last is crazy. Some of the newer low iron, low carbon stainless steels, such as 316L, are better than the common grades like 302 and 304 but they will all rust and corrode in time.
I would like anyone to show me pintals, gudgeons, a rudder post, or prop shaft that has been in continuous use since 1920, that are made of stainless steel, and that are still fine. Last Spring I replaced all the fittings on a coat boat that was made in 1892. The fittings were all original and still working. Over the years they had gotten scratched and dinged and the owner wanted new ones. My point is that the original ones were still working after 109 years.
Roger W.
Bristol Bronze
401-625-5224
rogerw@meganet.net
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Tom
And now, the rest of the story
Roger, Being traditional boat owners here, we're all bronze chauvinists, but to be fair there is another side to the bronze story that should probably be mentioned.
When I bought my CD she was only 5 years old at the time. The fellow in the yard recommended that all the bronze throughulls be replaced because they were showing pink spots - a sign of electrolytic degradation. Hmm, shot in 5 years rather than 109. Well, maybe the previous owner didn't keep the zincs up or something. Then I had a "bronze" valve installed for an overboard flush out for the head. This one made by Valpres in Italy, sold by West Marine, and advertised as all bronze. This past summer while hauled out I discovered that the "bronze" ball inside the valve no longer existed and thus the valve could not be shut off at all.
Apparently what happened is that the ball corroded away to the point that it was small enough to pass through the through hull and out. This was in a matter of 10 years or so. The boat has been hauled every year for the last 10 years and the zincs have never gone missing since I've owned her.
Thus I don't think we want to give people the impression that all you have to do is install bronze everything and you're good for 100 years. Metallurgy is a complicated subject and one still has to maintain vigilance and stay up on things. As you know, so called "Tobin Bronze", was developed by Lt. Tobin for the U.S. Navy many years ago and is a common alloy used for prop shafts. "Tobin" bronze contains in the neighborhood of 40 percent zinc. Zinc is that sacrificial metal that erodes away in salt water. Beats me why it works at all, but I think it is suspect at best.
Now there may be some alloys called bronze that contain a lot of phosphor or some other metal and will last 100 years, but the metallurgy textbooks tend to indicate that "bronze" is a largely meaningless term that is loosely applied to any alloy of brass. Some of them are good against saltwater corrosion and some are not so good. They generally recommend that one speak in terms of the job to be done and find an alloy of brass that suits that purpose rather than think in terms of "bronze" as if it were a well defined substance. Without taking up a lot of space here there is a textbook titled "Metallurgy for Engineers" that is particularly good on the subject of "bronze".
This is not to put "bronze" down in any way. It's a fine material under saltwater - or at least certain "bronzes" are. I thought Bob's article was excellent because it tried to explain why so called "stainless" steel rusts and is subject to crevice corrosion especially under saltwater. As with most things there is no magic bullet whether it be "bronze" or "stainless" steel. One must always be diligent even as we struggle to understand why and how things occur and what might be done about them. We shouldn't be lured into a false sense of security just because something is labeled as being "bronze". All bronze things are bronze, but some bronze is more bronze than others. Even so with stainless steel, some is more stainless than others. That's my two cents worth.
TomCambria@mindspring.com
When I bought my CD she was only 5 years old at the time. The fellow in the yard recommended that all the bronze throughulls be replaced because they were showing pink spots - a sign of electrolytic degradation. Hmm, shot in 5 years rather than 109. Well, maybe the previous owner didn't keep the zincs up or something. Then I had a "bronze" valve installed for an overboard flush out for the head. This one made by Valpres in Italy, sold by West Marine, and advertised as all bronze. This past summer while hauled out I discovered that the "bronze" ball inside the valve no longer existed and thus the valve could not be shut off at all.
Apparently what happened is that the ball corroded away to the point that it was small enough to pass through the through hull and out. This was in a matter of 10 years or so. The boat has been hauled every year for the last 10 years and the zincs have never gone missing since I've owned her.
Thus I don't think we want to give people the impression that all you have to do is install bronze everything and you're good for 100 years. Metallurgy is a complicated subject and one still has to maintain vigilance and stay up on things. As you know, so called "Tobin Bronze", was developed by Lt. Tobin for the U.S. Navy many years ago and is a common alloy used for prop shafts. "Tobin" bronze contains in the neighborhood of 40 percent zinc. Zinc is that sacrificial metal that erodes away in salt water. Beats me why it works at all, but I think it is suspect at best.
Now there may be some alloys called bronze that contain a lot of phosphor or some other metal and will last 100 years, but the metallurgy textbooks tend to indicate that "bronze" is a largely meaningless term that is loosely applied to any alloy of brass. Some of them are good against saltwater corrosion and some are not so good. They generally recommend that one speak in terms of the job to be done and find an alloy of brass that suits that purpose rather than think in terms of "bronze" as if it were a well defined substance. Without taking up a lot of space here there is a textbook titled "Metallurgy for Engineers" that is particularly good on the subject of "bronze".
This is not to put "bronze" down in any way. It's a fine material under saltwater - or at least certain "bronzes" are. I thought Bob's article was excellent because it tried to explain why so called "stainless" steel rusts and is subject to crevice corrosion especially under saltwater. As with most things there is no magic bullet whether it be "bronze" or "stainless" steel. One must always be diligent even as we struggle to understand why and how things occur and what might be done about them. We shouldn't be lured into a false sense of security just because something is labeled as being "bronze". All bronze things are bronze, but some bronze is more bronze than others. Even so with stainless steel, some is more stainless than others. That's my two cents worth.
Bob Luby wrote: A team at a British Univ. says they have discovered why stainless corrodes. If true, this could lead to truly stainless steels.
http://www.ic.ac.uk/templates/text_3.asp?P=3197
It would be interesting to see what Roger Winiarski (sp?) of Bristol
Bronze thinks of this.
TomCambria@mindspring.com
-
Bristol Bronze
Re: And now, the rest of the story
Dear Tom,Tom wrote: Roger, Being traditional boat owners here, we're all bronze chauvinists, but to be fair there is another side to the bronze story that should probably be mentioned.
When I bought my CD she was only 5 years old at the time. The fellow in the yard recommended that all the bronze throughulls be replaced because they were showing pink spots - a sign of electrolytic degradation. Hmm, shot in 5 years rather than 109. Well, maybe the previous owner didn't keep the zincs up or something. Then I had a "bronze" valve installed for an overboard flush out for the head. This one made by Valpres in Italy, sold by West Marine, and advertised as all bronze. This past summer while hauled out I discovered that the "bronze" ball inside the valve no longer existed and thus the valve could not be shut off at all.
Apparently what happened is that the ball corroded away to the point that it was small enough to pass through the through hull and out. This was in a matter of 10 years or so. The boat has been hauled every year for the last 10 years and the zincs have never gone missing since I've owned her.
Thus I don't think we want to give people the impression that all you have to do is install bronze everything and you're good for 100 years. Metallurgy is a complicated subject and one still has to maintain vigilance and stay up on things. As you know, so called "Tobin Bronze", was developed by Lt. Tobin for the U.S. Navy many years ago and is a common alloy used for prop shafts. "Tobin" bronze contains in the neighborhood of 40 percent zinc. Zinc is that sacrificial metal that erodes away in salt water. Beats me why it works at all, but I think it is suspect at best.
Now there may be some alloys called bronze that contain a lot of phosphor or some other metal and will last 100 years, but the metallurgy textbooks tend to indicate that "bronze" is a largely meaningless term that is loosely applied to any alloy of brass. Some of them are good against saltwater corrosion and some are not so good. They generally recommend that one speak in terms of the job to be done and find an alloy of brass that suits that purpose rather than think in terms of "bronze" as if it were a well defined substance. Without taking up a lot of space here there is a textbook titled "Metallurgy for Engineers" that is particularly good on the subject of "bronze".
This is not to put "bronze" down in any way. It's a fine material under saltwater - or at least certain "bronzes" are. I thought Bob's article was excellent because it tried to explain why so called "stainless" steel rusts and is subject to crevice corrosion especially under saltwater. As with most things there is no magic bullet whether it be "bronze" or "stainless" steel. One must always be diligent even as we struggle to understand why and how things occur and what might be done about them. We shouldn't be lured into a false sense of security just because something is labeled as being "bronze". All bronze things are bronze, but some bronze is more bronze than others. Even so with stainless steel, some is more stainless than others. That's my two cents worth.
Bob Luby wrote: A team at a British Univ. says they have discovered why stainless corrodes. If true, this could lead to truly stainless steels.
http://www.ic.ac.uk/templates/text_3.asp?P=3197
It would be interesting to see what Roger Winiarski (sp?) of Bristol
Bronze thinks of this.
You have made the same mistake that many people do. Just because something is advertised as Bronze does not mean that it is Bronze. There is a long way between the people in manufacturing and the people in marketing and they frequently have different goals.
Bronze is an alloy of copper and tin. Brass is an alloy of copper and zinc and uaually lead. For most of my life I worked as a sales engineer for one or the other of the Nation's largest brass and copper mills. Over the years I have worked for Chase Brass, Scott Brass, Bridgeport Brass and Extruded Metals. That last company is the second largest manufacturer of brass rod. In a normal month I would sell two million pounds. A good year for me was on the plus side of twenty million pounds.
Included in my customer base were many many valve manufacturers. I used to average well over a a hundred thousand pounds a month to them and to their sub-contractors. Usually the balls in ball valves are made on screw machines. With all the metal that I have sold to this industry I have yet to sell a single pound of Bronze for use as a ball in a ball valve. The balls are made of brass.
The brass that is used is alloy 360 Free Cutting Brass. It has a machinability of 100%. The Silicon Bronze that we use here to make marine fitting rates at 30%. Therefore it would take over three times as long to make a ball from bronze than from brass. When you think of the hundres of thousands of balls that are made per month times the difference of several dollars per ball you are looking at some serious money. If the ball in your valve had been made from Bronze you would probably be still using it.
When buying a metal product for your boat you should question what metal the various components are made of. Usually the balls and valve stems in valves are brass while the bodys are cast in bronze. Recently some manufacturere have even started making the bodys of brass.
If you have any questions I can be reached at 401-625-5224.
Roger W.
Bristol Bronze
rogerw@meganet.net