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Re: Rusty water on startup,
Rust on startup from any marine engine is the result of 'not using the engine enough' or that engine T'stat is not allowing the engine, when running, to get as 'hot as possible'.
Cast iron of the internal engine (and cast iron components of the heat exchanger, if the engine has one) rusts in two forms of Iron oxide, depending on the temperature at which the engine is run.
1. Ferrous rust is black rust or blue black rust that forms when the cooling water is at near 180 degrees F and the engine is heat soaked for relatively LONG periods of time when running. The longer the run time and the hotter the water, the deeper this protective form of rust is formed.
2. Ferric or 'red' rust is destructive and is promoted in marine engines as follows:
A. using a thermostat that opens at below 150 degrees
B. Letting the engine sit for long periods. The protective ferrous black rust slowly coverts to ferric red rust .... if you need to store your boat for long periods, you should fill the raw water cooling circuits with antifreeze with 'anti-rust' compounds
C. draining the raw water circuit and engine (air drying) without adding 'rust inhibitors'- severe red rust formation.
Its a common problem with cast iron and the reason that naval vessels with cast iron cooling water components are kept running (to keep the ferrous rust from converting back to ferric rust).
In marine engines it usually takes 25-30+ years for such internal rusting to become 'significant', the engines are usually built with sufficient 'corrosion allowance' so that the inevitable rusting will take a long time to become a serious issue.
However, one item that is extremely sensitive and 'most subjective' to ferric rusting is the engine's (cast iron) exhaust manifold ... doesn't have as much 'extra beef' of corrosion allowance (wall thickness) because of its 'cooling job'. Because of the thin wall construction and the way that thin wall cast iron 'stratifies' (like an onion) when it cools in its mold when made, when ferric (red) rust begins in earnest it starts to 'push' the casting stratifications apart and large visible 'platelets' of 'slabs' or large particles of rust begin to form .... and once you see or notice such 'particles' forming, then the exhaust manifold becomes very prone to leak between the 'gas side' and the 'water side' and if not monitored carefully - if such a pin-hole develops, the water from the cooling side can easily 'back drain' into a cylinder. So, once an exhaust manifold starts to develop red 'slab rust' then normal maintenance should be to remove it and periodically 'pressure test' the integrity of the internal passages to validate that the manifold has NOT developed 'pin holes'.
Rusting of cast iron marine engines is inevitable, the design normally includes many years of 'rusting' by adequate 'corrosion allowance' .... the longer and hotter you run the engine the least amount of rusting will occur. Marine engines usually never 'wear out', they rot away from the inside'. Once you denote 'particles' of rust coming out the exhaust, begin to consider to 'evaluate', starting with the most easily 'rot-able' exhaust manifold as that usually indicates rusting thats a bit more than 'normal' and can be the first signs of 'slab rust' which usually starts in the ex. manifold first.
Run the engine long and hard to lessen 'rusting'; when long term (weeks, etc.) inactivity is expected ... fill the raw water side with anti-freeze with rust inhibiters. Never drain a marine engine and store it 'dry'.
Last edited by RichH; 11-07-2012 at 07:50 PM.