Aluminium Corrosion in Engine Parts

[Ancient.Mariner]

Hi,

I've been having overheating problems with my engine, in an attempt to resolve the issue I have stripped off the heat exchanger, water pump, thermostat housing and reservoir end caps to inspect the the engine innards...

The thermostat housing and expansion tank are made from aluminum, the expansion tank surrounds the exhaust manifold and is an integral unit.

When I removed the end cover from the expansion tank (has the exhaust elbow attached) to inspect the inside and make sure it wasn't blocked I found that the internal aluminum casting seems to be badly corroded around the exhaust section. The exhaust elbow seems to be a home made affair put together with various mild steel pipe fittings welded together, the external cover also has pitting around the the elbow.

The thermostat housing has also suffered badly from corrosion with a pin hole developing when I removed it.

Ive always understood that aluminum and steel should not be connected together, there should be a insulating layer between them.

The zinc in the water side of the reservoir was broken off from the fitting but seems hardly worn.

I have two questions:-

1) As the company that marinised the engine are no longer in business and I cant get a new one, can the reservoir/exhaust manifold be repaired and how?

2) What would be causing the corrosion

Thanks

[dannobee]

Fortunately, cast aluminum can be welded and holds up well. Remove the part and send it to someone who can weld and machine aluminum. Most cylinder head shops can handle a task like that.

As for what caused it, does that part see raw water or coolant? If coolant, chances are that it wasn't changed often enough. You can use a digital volt meter to check for voltage in the system. It should be less than 0.1v, ideally. If it's raw water cooled, you'll pretty much have to live with it happening again.

Testing For Electrolysis

[GordMay]

Originally Posted by dannobee

...
As for what caused it, does that part see raw water or coolant? If coolant, chances are that it wasn't changed often enough. You can use a digital volt meter to check for voltage in the system. It should be less than 0.1v 0.01V, ideally. If it's raw water cooled, you'll pretty much have to live with it happening again.

Testing For Electrolysis

According to your link, "Generally, a reading of hundredths (.01-.09v) is below the action level, and will not result in aluminum failure."

[dannobee]

Originally Posted by GordMay

According to your link, "Generally, a reading of hundredths (.01-.09v) is below the action level, and will not result in aluminum failure."

On a old cooling system with lots of aluminum, he might be chasing rainbows trying to get it to 0.01v, but yes, lower is better. And this exercise demonstrates the need to check the cooling system and change the coolant more often than most of us do.

[Ancient.Mariner]

Hello,

Thanks for the reply's.

I've managed to get the expansion tank off, only two broken bolts but working on the side of the engine in a confined space it took a while.

I took the tank to a local automotive head specialist but he is dubious that it can be repaired, but is willing to have a go. When the exhaust deposit was wiped away the aluminum was only wafer thin, it would almost have certainly failed in the near future allowing the reservoir to drain into the head.

The corrosion seems to be limited to the exhaust side of the casting (the water side seems to be generally in decent condition) and I think I know why, the mild steel section that the exhaust is connected to is in turn welded to a stainless steel fitting which is threaded into the aluminum housing. From what I've read on the web aluminum shouldnt used with stainless, they are too dissimilar..

If the welding is not successful it looks like the only other option is going to be to get a one off casting made, expensive but not as expensive as a new engine.

[gjordan]

what is the basic engine block? The original modifier may not be available, but unless it is a real oddball engine, there is probably another modifier that could sell you a new marinizing kit at much less than a new engine. Pricy YES, but still cheaper than a new engine.____Just my 2 cents worth._____Grant.