From: mmathys@tic.bisman.com (Mark Mathys)
Newsgroups: rec.autos.tech
Subject: Re: Coolant  cavitation erosion
Date: Sun, 12 May 1996 12:00:16 -0500

In article <4n2pnt$uaq@news3.realtime.net>, jsummers@bga.com (James
Summers) wrote:

> A friend who has a Ford diesel says there is a special additive that he 
> was told to put in the coolant to prevent "cavitation erosion".  Does 
> anyone know what this is?  Are there other engines that have this 
> "problem"?

Cavitation erosion is a problem common in diesel engines. As the pistons
move up and down there is some movement that takes place with the cylinder
liner. The liner tends to move away from the surrounding coolant and the
result is cavitation. As you may know the cavitation results in the
formation of very small bubbles which implode and damage the surface on
the coolant side of the liner. The result is small holes than can
penetrate the liner. The "special additive" that your friend told you
about forms a protective coating on the surface of the liner. There are
several ways to do this, some systems uses a nitrite or borate salt.
Others such as the new long life coolants use a carboxcylic/fatty acid
type of inhibitor. The later lasts twice as long and does a better job of
corrosion/erosion control, it also acts as a lubricant which can help
extend water pump seal life. It's important that you use the correct
coolant for your diesel engine as not all coolants contain the correct
additives for diesel engines. 

Mark

From: mmathys@tic.bisman.com (Mark Mathys)
Newsgroups: rec.autos.tech
Subject: Re: Coolant  cavitation erosion
Date: Tue, 14 May 1996 23:05:34 -0500

In article <DrEJwt.H4H@orpheus.nsc.com>, alanh@rockie.nsc.com wrote:

> In article <mmathys-1205961200160001@sl11.tic.bismarck.nd.us>,
mmathys@tic.bisman.com (Mark Mathys) writes:
>
> >Cavitation erosion is a problem common in diesel engines. As the pistons
> >move up and down there is some movement that takes place with the cylinder
> >liner. The liner tends to move away from the surrounding coolant and the
> >result is cavitation. As you may know the cavitation results in the
> >formation of very small bubbles which implode and damage the surface on
> >the coolant side of the liner. The result is small holes than can
> >penetrate the liner. The "special additive" that your friend told you
> >about forms a protective coating on the surface of the liner. There are
> >several ways to do this, some systems uses a nitrite or borate salt.
> >Others such as the new long life coolants use a carboxcylic/fatty acid
> >type of inhibitor. The later lasts twice as long and does a better job of
> >corrosion/erosion control, it also acts as a lubricant which can help
> >extend water pump seal life. It's important that you use the correct
> >coolant for your diesel engine as not all coolants contain the correct
> >additives for diesel engines. 
> >

> An interesting description, but not entirely accurate.  If your cylinder
> liners are moving, you've got other problems.

Liners or cylinder wall commonly flex in high compression engines.

> Cavitation erosion occurs in both lined and unlined motors.
> The cause is casting imperfections in
> the water jacket, which allow small bubbles to form as the cooland moves
> through the jacket.

Why don't we usually see it gasoline engines? They have imperfections and
moving coolant?

> These bubbles form at any spot where there is an 
> imperfection, whether it's a high spot or a low spot.  When a cylinder
> fires, the shock wave travels through the cast iron and knocks the bubble
> off it's perch.  When this happens, the bubble takes a microscopic amount
> of iron with it.  Over time enough iron is removed to form a hole.

There is some disagreement about the actual mechanism, but it is generally
believed that the oxide coating from the SCA or conditioner is removed by
the implosion. Once unprotected, corrosion can occur at accelerated rates.

 
> The additives that are used are basically wetting agents that lower the
> surface tension of the coolant, reducing their propensity to form bubbles.

They are compounds that form protective coatings on the surface of the liner.

The main cause of cavitation erosion in diesel engines IS flexing of the
cylinder walls. If it was due to surface imperfections as you propose then
it would be common in all engines, which it is clearly not. Diesel
engines, because of their much high compression, have more flex or
deformation of the cylinder walls. I am not aware of this being a problem
is gasoline engines. Below is an explanation in more detail.

Erosion-corrosion is a combination of mechanical and chemical or
electrochemical action that causes corrosion. Cavitation is a particular
type of erosion- corrosion and a common cause of cylinder wall pitting.

Cavitation of the cylinder wall begins when air bubbles remove the wall's
protective oxide film. Flexing of the cylinder wall after the fuel mixture
explodes in the combustion chamber causes cylinder wall vibration and
creates air bubbles in the coolant. Concentration of air bubbles increases
when cooling system pressure is low or when the system leaks. Also,
increased vibration amplifies the quantity of air bubbles. Vibration
multiplies when the engine is run cold, because of increased
piston-to-cylinder clearance. Vibration also multiplies when the engine is
lugged.

These air bubbles form on the outside of the cylinder wall (perpendicular
to the wrist pin) and then explode inward, or implode. When air bubbles
continue to implode, sufficient energy is released to physically attack
the cylinder wall and remove the oxide film. Corrosion and pitting then
take place at a high rate.

Eventually, a pit can become deep enough to break through the cylinder
wall and allow coolant to leak into the cylinder. This coolant leak
contaminates the lubricating oil.

Supplemental coolant additives coat metal surfaces and control
cavitation-erosion and pitting. Unfortunately, small particles or ferrous
scale often shield the surfaces underneath from the protective action of
coolant additives. If this condition persists, pits can form. Keeping your
cooling system clean, along with regularly replenishing your coolant
additives, helps prevent pitting. However, if coolant additives are not
added at the proper intervals and in correct quantities, cavitation
erosion and pitting intensifies. Eventually, coolant can penetrate the
cylinder wall and cause major damage to the engine.

This is the explaination I received from Caterpillar, I think they know
how cooling systems work!

Mark Mathys