I read with interest the comments by Tom and Glen. Tobin Bronze was invented in the late 1890's by a Navy Officer, LT. Tobin, for use on Naval Vessels. It does an excellent job in salt water. The chain plates, rudder post, and keel straps on my own boat are in Tobin Bronze. My boat was build at Herreshoff Mfg. on 23 Sept. 1920. Tobin Bronze is still available in most forms if you know where to look.
The problem with the propeller shaft in the cutlass bearing and stuffing box is that the shaft is not turning all the time. If it were there would be no problem with a supply of oxygen. However many boats sit on a mooring for days on end. The available oxygen in the non-flowing water will be used up in a few days. After that..... In addition, since most propellers are of Bronze there will be galvanic corrosion between the stainless shaft and the propeller. Since Bronze is the more noble metal the shaft will corrode.
rogerw@bristolbronze.com
propeller shafts
Moderator: Jim Walsh
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Matt Cawthorne
Stainless steel and the galvanic series.
Roger,
Be careful when quoting where Stainless steel is on the galvanic series. All of the common Cr-Ni stainless steels exist in two places in the galvanic series. In their passivated state they are more noble than any bronze or brass. By passivated essentially I am talking about being in the presence of oxygen. In their active state they are all less noble than brass or bronze. To get it into the active state you have to break the oxygen-chromium rich coating on the surface and keep oxygen away. (see "Elements of Materials Science and Engineering" by L.H. Van Vlack) Pitting is a problem with stainless steel under water and deprived of oxygen because of the disparity in the voltage levels of the active bottom of the pit and the passive material at the top of the pit. Not only are they electrically at very different voltages, but due to the close proximity there is a large electrical gradient which helps speed the process.(see 'Corrosion Engineering' by M.G. Fontana) The presence of a brass or bronze propeller on a steel shaft is a moot point in terms of corrosion of the shaft. If the shaft has oxygen available the prop will protect the shaft. If a pit gets started the passive layer is more noble than the prop and the corrosion damage will be done by the passive matieral on the shaft rather than the bronze on the prop.
One year I did a bad job of attaching the single screw that holds the zinc on my steel shaft. It fell off, and I believe in the beginning of the season. The shaft was fine at the end of the year. No pits or corrosion of any kind in the area of the cutlass bearing. The prop was free of barnacles indicating that it was corroding and the oxides of copper were killing the marine growth. The problem is that the 'bronze' propeller was disolving and the zinc was leaving faster than any of the other components. The propeller looks like a leopard now due to the dezincification. De-zincified brass (or bronze if it contains zinc) decreases it's strength. I would rather that not happen to the shaft.
Be careful about the term bronze. Brasses contain zinc. Bronze was originally a tin-copper alloy which did not preferentially corrode like brass. Sometime in the past the line got blurred when term 'bronze' started being used for alloys containing significant amounts of zinc. These zinc containing alloys can be subject to the same weakening due to the preferential loss of zinc. This is not a problem if the part is protected by a zinc. I forget the total list of the zinc containing bronzes, but Tobin bronze is one of them. Make certain that you always keep a zinc on it. (see "Metal Corrosion in Boats" by Nigel Warren)
More on keeping barnacles off your prop to follow in a week or so.
Matt.
mcawthor@bellatlantic.net
Be careful when quoting where Stainless steel is on the galvanic series. All of the common Cr-Ni stainless steels exist in two places in the galvanic series. In their passivated state they are more noble than any bronze or brass. By passivated essentially I am talking about being in the presence of oxygen. In their active state they are all less noble than brass or bronze. To get it into the active state you have to break the oxygen-chromium rich coating on the surface and keep oxygen away. (see "Elements of Materials Science and Engineering" by L.H. Van Vlack) Pitting is a problem with stainless steel under water and deprived of oxygen because of the disparity in the voltage levels of the active bottom of the pit and the passive material at the top of the pit. Not only are they electrically at very different voltages, but due to the close proximity there is a large electrical gradient which helps speed the process.(see 'Corrosion Engineering' by M.G. Fontana) The presence of a brass or bronze propeller on a steel shaft is a moot point in terms of corrosion of the shaft. If the shaft has oxygen available the prop will protect the shaft. If a pit gets started the passive layer is more noble than the prop and the corrosion damage will be done by the passive matieral on the shaft rather than the bronze on the prop.
One year I did a bad job of attaching the single screw that holds the zinc on my steel shaft. It fell off, and I believe in the beginning of the season. The shaft was fine at the end of the year. No pits or corrosion of any kind in the area of the cutlass bearing. The prop was free of barnacles indicating that it was corroding and the oxides of copper were killing the marine growth. The problem is that the 'bronze' propeller was disolving and the zinc was leaving faster than any of the other components. The propeller looks like a leopard now due to the dezincification. De-zincified brass (or bronze if it contains zinc) decreases it's strength. I would rather that not happen to the shaft.
Be careful about the term bronze. Brasses contain zinc. Bronze was originally a tin-copper alloy which did not preferentially corrode like brass. Sometime in the past the line got blurred when term 'bronze' started being used for alloys containing significant amounts of zinc. These zinc containing alloys can be subject to the same weakening due to the preferential loss of zinc. This is not a problem if the part is protected by a zinc. I forget the total list of the zinc containing bronzes, but Tobin bronze is one of them. Make certain that you always keep a zinc on it. (see "Metal Corrosion in Boats" by Nigel Warren)
More on keeping barnacles off your prop to follow in a week or so.
Matt.
mcawthor@bellatlantic.net
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D. Stump, Hanalei
Re: Barnacle free prop....STOVE POLISH!!!
Matt,
Excellent comments about corrosion, good job! As far as keeping the barnacles off of the prop, I used common stove polish on the prop for the first time this year and had essentially no barnacle growth when she was pulled, once half way through the season, and also at the end of the season. Don't know why it worked, it is just carbon black, but it did work!
Dave Stump
Captain Commanding
s/v Hanalei CD-30
Excellent comments about corrosion, good job! As far as keeping the barnacles off of the prop, I used common stove polish on the prop for the first time this year and had essentially no barnacle growth when she was pulled, once half way through the season, and also at the end of the season. Don't know why it worked, it is just carbon black, but it did work!
Dave Stump
Captain Commanding
s/v Hanalei CD-30
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M. R. Bober
Re: Stainless steel and the galvanic series. Me too.
In the eventually to be immortal words of Jack Holloway, "If they had made stainless steel before bronze. They wouldn't have continued to search for bronze." [Ed. note: Memorial markers aside.]
In my collection of broken & damaged shafts, albeit small, both are bronze.
Mitchell Bober
RESPITE
CD330
In my collection of broken & damaged shafts, albeit small, both are bronze.
Mitchell Bober
RESPITE
CD330