The mysteries of electrolysis
. Many years ago my header tank body just simply disintegrated blowing salt water
over everything. Charming. The headertank/heat exchanger was bolted onto the engine
. It transpired that the body was an aluminium alloy casting while the core
was Cu Ni. It had worked well for many years because – according to the workshop chaps - the rubber rings that constrained the Cu Ni core
to electrically isolate the core from the body, were doing their job (As well as sealing the end caps – which were cast iron!). This circuit became conductive due to “deposits” or aging of the engine coolant
, thus completing the electrolysis
circuit past the rubber rings and then rapid corrosion
of the aluminium alloy ensued. There were no zinc blocks and it was pointed out to me that they would have done little in this case. Complicated, poor cheap
engineering, prone to failure. Eh? “Maybe some poor maintenance
as well!” My wife’s crystal clear remonstration rings sharply in my ears.
I replaced the entire header tank/heat exchanger with a Perkins
version from a much larger engine that would not bolt onto my engine and so mounted this on the engine room wall thus probably electrically isolating it all from the engine. (A plumbing
model has a bronze
body with a Cu NI core. No zinc blocks fitted. The Cu Ni core makes physical and electrical
contact with the header tank body. This year the headertank/heat exchanger was sent off for servicing. All was OK. The workshop guys advised against adding zinc blocks into the heat exchanger
as they would just rapidly disappear and achieve little except create worry, but did recommend I install a good earth strap to the engine and install a zinc block back at the bronze
seawater intake filter and also ensure that the filter body was earth strapped to the engine.
So far this year there have been no problems here.
It is also really important to use the correct engine coolant
. There are a few variants particularly amongst the type A species. Usually the basic cheapest type B is spot on, but check with your local marine diesel
expert. These coolants are often forgotten about but should be replaced every two years or whatever is recommended. Do an engine flush at the same time. The coolant type and chemical composition can affect electrolysis issues.
I also flush fresh water
through the cooling
system (just after the raw water
When in the marina, I connect a garden hose from the jetty supply. At my intake filter I have a 12 mm bleeder valve to allow sea water
in when relaunching the boat
. From this valve there is a clear plastic hose rising to over 30 cm above the LWL so I can see if the raw sea water has flooded in. At the top end of this hose is where I have situated the fresh water hose snap on connection.
But the particular fittings used will depend on your particular plumbing
arrangement but once installed it is all passive. If you do this, unsure you leave the sea cock open so you don’t expose the engine system to the mains supply water pressure. This flushing
will also back wash the intake filter and change the salinity back to the intake valve thus probably reducing algae growth. With the fresh water flowing fast, run the engine up to temperature and then close down, engine first, then the fresh water. However this gratuitous advice
is not of much use for your immediate concern, as the fresh water flow is usually through the heat exchanger core rather than the space between where the engine coolant resides.
The bottom line would seem that if the manufacture did not install zinc blocks then they are probably superfluous. If they did install them then they are presumably absolutely necessary and consulting the engine manual will put you on the right path. I think the real issue was related to the poor soldering of the fitting. Solder/brazing alloys are another land of the mysterious and can induce all sorts of problems in the presence of salt water
. The Cu Ni heat exchanger cores are usually soldered at the ends and this is why you should at least have the core part checked. Any leaks
here spells disaster as the engine coolant and the sea water then mix.