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Metal Rot

By Tom Neale - Published March 04, 2013 - Viewed 636 times

You'd think that manufacturers of boat equipment would "get it” a little more than they do with some issues. Like the issue of metal rot, for example.  When you put dissimilar metals together, or perhaps even close to each other, in a salty environment, look out for trouble. There's nothing new about this.  You're creating a sort of battery, in essence, and one metal erodes to the other. One place where I've noticed this a lot is on engines. If they're going to do something stupid, why don't they do it on a cheap pocket knife or fishing lure.  But no, that would be too easy on the lowly boat owner who keeps them in business.

Tom checking for other traffic
Aluminum metal rot in heat exchange on old diesel.

Often you see metal rot on an engine where aluminum is involved. I've got nothing against aluminum, I like it. But salt water likes it even better. It can eat it alive. You see aluminum and you think, "Great! None of the cast iron that rusts from just looking at it.” But aluminum can be even worse. Because typically the aluminum is in close juxtaposition with some part made of bronze. For example, it amazes me to see the number of good engines which have bronze end caps over aluminum heat exchanger bodies. The heat exchanger bodies house the bundle of tubes through and around which raw (salty unless you're in a lake) and fresh (with anti-freeze) water flows. Typically the salt water flows through the tubes and the end caps contain that water.

In theory there's going to be no problem if you maintain the assembly exactly like the engine manufacturer employee sitting back in a cubicle writing manuals says you should. But I often get the feeling that some of these people have probably seldom if ever been involved with the real world of boating. Few people have the money and/or time and/or the access around the engine to do everything just right. In this instance, instructions generally tell you (among lots of other things) to drain the water and remove the end caps each year, clean the sealing surfaces, and install new gaskets and O rings. If you do this, what I'm about to describe is far less likely to happen. But, as simple as the job sounds, it's usually a real time consuming pain in the neck. If you don't do this, take a look at the picture accompanying this article and see what can happen.

Tom's Tips
Tom's Tips About Fixing Metal

1. It helps to have the right tools.

2. A set of hardened punches is invaluable. They may be used to make an indentation to avoid a drill bit "walking" to knocking out stuck pieces.

3. A hardened pick, such as a larch straightened fishhook will come in handy.

4. A Dremel tool is invaluable. The attachments with industrial diamond will cut even case hardened steel. Be patient and don't let the fitting get to hot. Withdraw if from the work frequently (probably every second or so) to avoid overheating and damage to the cutting attachment. Dremel tools work with speed, not pressure.

5. A set of stainless wire brushes, each with bristles of varying stiffness helps to clean metal.

6. Always follow instructions from the manufacturers when you use these or other products. Success and, more importantly, safety require it.

Click Here for More Tips

Salt water slowly began seeping past the gasket which was originally separating the bronze end cap and the aluminum body. The combination of the salt and the bronze began deteriorating the aluminum. At first this would probably not have been noticeable because it would be covered by the end cap. Eventually tell-tale signs would have appeared. This is usually the appearance of a white granule type of ring around all or part of the joint between the end cap and the body. It's a sign that the aluminum is being attacked. If you take action right away, the damage usually isn't too bad. If some of the aluminum has been corroded you can usually simply augment the new gasket that you install with Permatex gasket sealant or a similar product. The cleaning is essential because this makes whatever gasket material you use seat and seal better. If the pitting in the surface is too deep (and it probably won't be at an early stage) you can fill it with a product like JB Weld or StarBrite Epoxy/Aluminum Putty stick. Clean it first, removing not only dirt, but any loose material. Then wipe it with a solvent such as Acetone, and apply the product, making it as smooth as possible while it's still soft. After it's hardened, sand if necessary so that the repaired surface is flush with the old. I've seen this done various times on different boats and the fix is usually successful.

But boat engine problems, being what they are, have a tendency to sneak up on you in the most perverted of ways. First of all, many of us don't have the bucks to hire a mechanic to come aboard every year (like the manual would suggest) to do this work. Many of us are able to do it ourselves, but this usually is a bit time consuming and ...well ... just not very fun. So often the project isn't done every year. Also, as noted earlier, the initial phases of the corrosion usually aren't visible. And often the advanced phases of the problem aren't visible because we're talking about an engine shoe horned into a miniscule space on a boat. Frequently a heat exchanger is on the far side and the end plate area is very difficult to see.

In the accompanying photo of an old engine, the problem developed slowly at first, got much worse, and remained unseen and untreated for a long time. The photo shows the aluminum of the cast aluminum body having been turned into essentially crud. Left much longer, water will start leaking out when the engine is running. Often when this happens the crud expands first, doing damage to the end plate by warping it. In this instance, as is so often the case, the engine manufacturer really did a number by combining the heat exchanger with the exhaust manifold. This means that a replacement part would cost thousands of dollars in addition to requiring a lot of labor. An engine manufacturer's comment would assumedly be: Well, you should have followed the maintenance manual. A boater's response may be, "I did as much as I or any other real-world human could."

But, as bad as it looks, one can often repair this type of damage with JB Weld or Star brite's Epoxy/Aluminum Putty Stick or similar products and a little old fashioned ingenuity. The trick is to remove the end plate, clean the affected area, remove all loose material, clean with a solvent, and carefully rebuild the destroyed aluminum. Depending on how bad the problem is, several applications may be required, with careful sanding and solvent cleaning between each one. A new gasket and/or O ring will be needed, but it probably won't seal completely because the repair probably won't be that perfect. So Permatex or a similar product is used to augment the seal. I've seen this type of repair last many years. We did it on one of our engines some time ago. It had suffered through many previous owners who apparently hadn't given issues such as this much thought. The engine was old and not worth buying a new manifold/heat exchanger for many thousands of dollars. The job lasted the rest of the life of the engine, which was around 9 years. The engine died for other unrelated reasons, taking the repair, still holding well, with it to its grave.

Never try this if there's any question of flooding or fumes should the project fail. That's not remotely worth the risk. Also, be sensitive to any heat that may be too much for the product. But, in the right circumstances, this is one more example of what you may be able to do to fix things without going broke.


Boating and water sports involve risk. Any comments herein should be followed at your own risk. You assume all responsibility for risk or injury to yourself or others. Any person or entity that uses this information in any way, as a condition of that use, agrees to waive and does waive and also hold authors harmless from any and all claims which may arise from or be related to that use.

See www.tomneale.com





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