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Old 11-02-2005, 09:21   #1
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Yanmar Engine Zincs

It seems odd that the fresh water cooled yanmar 3gm and 3ym engines don't have zincs. The older 3gms had places for them in the block if they were salt water cooled, but none were used in the fresh water versions.

Has anyone had any issues with the heat exchanger in the 3gm's? Any recommendation on the zincs?
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Old 12-02-2005, 20:47   #2
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Yanmar

There is a Yanmar help site. www.yanmarhelp.com I think.
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Old 12-02-2005, 21:41   #3
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It's only the Salt water circuits that need the anodes. The fresh water circuit, which is normally a closed circuit, needs another form of protection. A top product is made by Alfloc. The chemicals stop rust and cavitation erosion in Cast heads/blocks and electrolosys between the Alloy head and cast block,( for engines with an alloy head of course.)
There are several factors that stop the effectivness of anodes in a fresh water closed circuit system. The fresh water does not conduct electricity very well, so you do not have an electrolyte such as sea water presents to metals.
If you had the anode in a closed circuit, the Zinc would have no where to go and would end up on things you don't want coated in Zinc.
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Old 13-02-2005, 08:18   #4
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Actually I should have said saltwater/freshwater heat exchangers. I would have expected there to be zincs in the slatwater side of the Heat exchanger. There are none. In the 3GM versions there are provisions for a couple of zincs, but they are not used in the freshwater block versions. In the 3YM engines, they just completely elimnated them. When I asked the Yanmar dealer about this, he said there were provisions for zincs in the the 3GMs because some of them were entirely saltwater cooled. He did not seem to think it a problem. Maybe because he will be selling me new exchangers in 6 or 7 years??

I will cross post a question to Yanmarhelp. I did a search there, did not come up with any hits. NOT a very easily navigated site.

Thanks,

Keith
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Old 13-02-2005, 11:31   #5
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Please let us, or me at least, know the reply from Yanmar. I have a similar set up with my vessel. Different engine but same thin g, no Zinc in the saltwatetr side. I have been presuming that the main Hull Zinc takes care of this, but could be wrong, so would like to know for sure. By the way, this set up has been in the water for a few years now, so I would have thought a major would have occured by now, should the main Zinc not handle the protection.
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Old 13-02-2005, 15:02   #6
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As I understand galvanic corrosion, it occurs when two dissimilar metals are placed in an electrolytic solution and those two metals are electrically connected to one another. The less noble, anodic, metal will give up electrons to the more noble, cathodic, medal. Thus, the less noble medal being subject to corrosion. The accepted method of preventing this is by electrically attaching the metals to a very anodic metal. Zinc being an excellant candidate. Magnesium is less noble than zinc, so don't try this with any old VW engines! .

hmmm.... If the heat exchanger is galvaized, which is essentially plating iron or steel with zinc, that sould provide a sufficiently large anodic area to protect the underlying steel. in other words it does have a big zinc already bonded to it. Probably take a LONG time before that area wore completely off! Especially if the rest of the engine were made of a similar metal. Indeed, if they are the same alloy, there would be no galvanic corrosion.

Okay, I see how they get away without zincs now....

Keith
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Old 13-02-2005, 23:40   #7
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Ummm, first point, the two dissimilar metals in an electrolyte basicaly become a battery. Or more accurately, electrons flow from the cathod to any metal that presents itself as an Anode. But there is a stream of things going the other way. They are at an atomic level in size and are the atoms that make up the very metal itself. If the two opposing metals are close enough, the Anode will "plate" itself onto the other metal. But in most boat situations, the distances are too great for this to occur. So the anode, usually being Zinc, simply disappears.
So just to correct the "electrical connection" part, they are electrically connected in the salt solution, but not connected electrically any otherway. If you should connect a conductor from the anode to the cathode, then you short the "cell" of the battery and the electrons and thus the atoms can't flow. However, in our Sea environmet, things other than different metals on just the one boat can become the cathode. It maybe a neighbouring boat. It maybe the bolts in the marina piles, it could be the can somebody foolishly discarde overboard, it could be a poor dockside electrical system and so on. I have even seen stones Zinc plated, but that is a different storyu and situation.
Second point,sometimes it is not galvanising inside a heat exchanger, so becareful not to assume such. Sometimes it is Tin, or a similar alloy like Tin/Lead. Sometimes nothing at all, just the copper. But if it is Galv or Zinc, then that is the last thing you want to have dissapate away. It is just a thin plating and would be gone in days or maybe hours if an electical eroding problem was occuring.
The rest of the engine won't be made from Zinc or Galv or Magnesium, just plain old Cast iron. I am pretty sure all the Yanmar engines are cast in both block AND head.
Last point, the anode that you usually find in a heat exchanger is for protection of the heat exchanger itself. Normally, (yours may differ) they have copper tubes as a core. That core is then inside a single large diameter tube with two end caps. Sometimes the outer tube or casing can be copper, but sometimes they are cast aluminium or cast steel. Especially if they have some design shape to them. So the copper tubes inside have to be protected from any other metal that is different. A heat exchanger will seldom erode away to the engine itself. The link in the cicuit there is fresh water. But on the Saltwater side, the exchanger could find an electrical path to some other metal elsewhere on the boat.
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Old 15-02-2005, 06:47   #8
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Galvanic Corrosion

I didn’t know if I remember correctly, it has been a long time since I took chemistry. But I had been meaning the buy Nigel’s book anyway. So, I popped down to Barnes and Noble’s last night and picked up a copy. There is a whole chapter on corrosion and its prevention in boats. This is a passage from Nigel Calder’s “Boat owner’s Mechanical and Electrical Manual” Second Edition Page 134.

“What happens can crudely be described as follows: Given any two metals immersed in an electrolyte, when an electrical connection is made between them the current flows from the higher-voltage metal to the lower-voltage metal. The effect of this is to raise the voltage of the lower-voltage metal above its natural potential. In an effort to reestablish equilibrium, the lower-voltage metal discharges a current into the electrolyte. This current flows through the electrolyte to the higher-voltage metal, completing the electrical circuit between the two pieces of metal. Unfortunately, the current flowing through the electrolyte is generated by an electro-chemical reaction that steadily consumes the lower-voltage metal – a process known as galvanic corrosion (Figure 4-5B).

As long as the external circuit remains in place between the two metals, the current being electrochemically generated at the lower-voltage metal flows back around the circuit through the external connection, preventing the lower-voltage metal from reestablishing equilibrium at its natural potential, and so causing the reaction to continue. We have what is known as a galvanic cell or couple. Eventually the lower-voltage metal will be entirely consumed, while the higher-voltage metal will remain intact. The metal that is feeding the current into the electrolyte is known as the anode; the metal that is receiving the current from the electrolyte, and discharging it into the external circuit, is known as the cathode.

The most commonly known galvanic cell is the lowly flashlight battery, which contains two dissimilar metals and an electrolyte, with the cathode forming the positive terminal and the anode forming the negative terminal. When an external circuit is made (for example, through a flashlight bulb), the anode is steadily consumed until it is destroyed, at which point the battery is dead.

Since the cathode in a galvanic cell does not dissolve. It is also referred to as the most noble, or passive, metal in this relationship, while the anode is referred to as the least noble, or active, metal. It should be noted that this relationship is entirely relative: Depending on its position in the galvanic series table, a metal will be cathodic with respect to metals that have a lower electrical potential, but anodic with respect to those with a higher potential.”

On page 135 Calder goes on to say:

“Preventing galvanic corrosion.” What we have seen so far is that for galvanic corrosion to occur, three preconditions must be met:

1. There must be two dissimilar metals, or dissimilarities within one piece of metal.
2. The dissimilar metals must be in contact with an electrolyte (an electrically conductive fluid).
3. There must be an electrical connection between the metals (other than the path provided by the electrolyte).”

If these three preconditions are met a galvanic cell will form in which one metal (the anode) will corrode while the other (the cathode) will be unharmed.

…Steel can be given a substantial amount of extra protection with a coating of zinc-galvanizing- prior to painting. Since zinc is more anodic than steel, any time the paint layer is damaged, allowing moisture access to the metal and so establishing a localized galvanic cell, the zinc will corrode, protecting the steel and in fact plating out on the steel to “heal” the scratch. Proper coatings are not so important with aluminum (which will develop its own relatively inert oxide skin) or many bronze fittings (which are naturally resistant to corrosion; however, note that manganese bronze unlike other bronzes, has a high percentage of zinc and as a result is susceptible to dezincification- it should be more properly be called a brass). Stainless steel should not be painted since the exclusion of oxygen from the surface of the metal may cause its protective oxide film to break down.”
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Old 15-02-2005, 10:21   #9
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Solution dawns!

Chuckle! The metelurgical discussion is/was great, but nothing like taking something apart a seeing what they did. Since I have an extra engines just sitting here, I decided to take the end cap off the heat exchanger to see what Yanmar had done. On the 3GM30 engine, the saltwater is routed through tubes that flow through the center of the radiator. The antifreeze actually flows around the outside of these tubes. So the saltwater/electrolyte only touches one metal. Thus, no galvanic curcuit and no galvanic corrosion.

Actually, the tubes appear to be in good shape. There is a light film of salt residue, but no pitting that I could see.

Now I see why they don't have zincs. The question now is. Why do other engines!?

Keith
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Old 15-02-2005, 11:55   #10
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Thanks for transposing that info from the book. It is for that very fact that there are two very valid trains of thought out there. One for the argument of bonding everything and one for bonding nothing.
For me, it is bond everthing. But the reason is just as involved as the one you listed above. However, to condense it, with everything bonded, the anode (Zinc) becomes the lesser "sacrifical" metal to everything else that has been bonded. While the anode is in good shape, everything else that is bonded is seen at the same potential when related to the anode. Once the anode is gone, or close to and thus no longer doing it's job, the danger lies in the fact that the very next metal on the scale becomes the anode to the rest of the bonded circuit.
The next point is that the bonding system, the electrical wire, is seen a a circuit with the least resistance. It brings all metals to the same potential difference, thus electrically making them look as one and the same. You have to remember that the salt water, although it is an electrolyte and as an electrolyte will produce a voltage through chemical reaction, it is also a conductor. It has greater resistance than a copper wire, but it still conducts. Thus bonding the metals creates an electrical path of least resistance when observed against the salt water conductive path.
I am going to stop there, as it is becoming a novel and is off the original subject. However, hopefully it explains why some engines have Zinc's and others don't. It is a train of thought as to the approach of solving the problem, It is design, it is what metals are being used, it is how the manufacturer has described how there engine should be bonded to the rest of the vessel, it is where the heat exchanger is placed in the system, i.e. is it part of the motor or seperate. It just depends.
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