Myth Buster?

[High Hopes]

I have looked at a couple of physics and inorganic chemistry sites on the internet and discovered that Sodium Chloride (NaCl), the salt in salt water, does NOT precipitate out with higher temps. As a matter of fact, hot water will hold just slightly more NaCl. The solubility of NaCl in water is almost independent of the temperature. With many other salts, solubility INCREASES with temperature.

So, does anything precipitate out?

What happens if you bring Bay water to a boil in a pot?

Do hotter engines cooled with raw water really corrode / collect sludge at higher temperatures?

If so, then what is happening?

An inquiring mind wants to know.

[tenders]

I believe it is not "salt" that precipitates out...but "salts," not necessarily of the sodium variety. The stuff that builds up in an engine is calcium salts (perhaps calcium chloride and calcium phosphate?) which are soluble in vinegar and other acids.

[High Hopes]

Hi Tenders,

You were right. I read this at the following link.

http://www.accepta.com/industry_water_treatment/water-industrial-plant.asp

"Scale and sludge formation are the result of the precipitation of compounds that are no longer soluble under the conditions prevailing at the point of deposition. This typically occurs in the regions of highest temperature, normally heat exchange surfaces, and low flow areas, such as cooling tower ponds.

Every solid has a definite solubility in a particular solvent under specific conditions of temperature and pressure.

In general, as temperature increases so does the solubility. However, in some cases increasing temperature reduces solubility, such is the case with calcium carbonate."

I found a site that describes marine engine corrosion in great detail.

http://www.oldmarineengine.com/technical/corrosion_1.html

I am way over my head. I better save those pennies for a fresh water kit. Thanks for your response.

BTW, do you sell yacht tenders? I am curious about your nic.

[tenders]

I do not sell yacht tenders, or chicken tenders, or my own version of Lovin' Care (TLC)--at least, not to the public.

It's just my first initial and my last name.

[blackdove]

Hello,
The chlorine can join (high probability) the hydrogen in the water at a higher temperature as it goes to gas phase and creates a concentrated hydrochloric acid solution. This is one of the corroding factors The calcium salts also form calcium hydroxide, a caustic (base) solution, which also has corrosive properties. Independently they play havoc in chemical plants. When joined, the effect is not linearly cumulative, but not so good for metals, including high grade stainless steel. Not being a smart ***, I do this for a living:)

[HerbertFriedman]

These discussion are great, puts the whole business of corrosion in raw water cooled engines on a solid basis, especially the use of muriatic (hydrochloric) acid as a yearly flush.

But I still have one question: I flush my (raw water cooled) engine after every use with fresh water using a dock hose and flushing tee. My thought is that even a ambient temperature, some corrosion with sea water takes place and subsituting fresh water for sea water for the long periods in which the engine just sits at the dock, is doing some good. Sort of a poor man's alternative to a heat exchanger system. From a chemical corrosion point of view, am I doing anything worthwhile?? Or am I wasting my time??

[tenders]

One the one hand, the freshwater flush can't hurt.

On the other hand, the amount of corrosion wear incurred at ambient temperature may be very small compared to that incurred at operating temperature. The engine was designed to be raw-water cooled and I've been told the alloy in which it's cast has an unusually high nickel content for better corrosion control. The extent to which it's WORTH IT to flush for a few minutes after each use vs. spending two hours or so on an acid flush every 5-7 years that you may need to do anyway, is less clear to me. I'm on a mooring and don't freshwater flush, although if I were going to leave the boat for a month or more I probably would.

[HerbertFriedman]

I tend to agree with you but a clear graph of "corrosion" vesus temperature sure would be nice, but I cannot think of a good and easy method to measure the corrosion. My thinking is that even if the corrosion at ambient temperature is much lower than that at operating temperature, the amount of time spent at ambient temperature compared to operating temperature is huge. If you motor one hour a week, for say day sailing, the ratio of time hot to time cold is only 1%.

Your comment about flushing every 5-7 years intrigues me. I thought Don recommended a 15 minute acid flush once a year. Don, or anyone else care to comment. I am about to install another flushing valve in the thermostat circuit. Steve Markowski came up with a very nice arrangement using a three way valve to eliminate the need for the thermostat removal during that acid flush procedure. Since I usually cannot get the thermostat out without ruining at least one hose, and since I do the acid flush every year, I thought this was a worthwhile addition. But if the recommended acid flushing is 5-7 years, this addition might not be worth the effort.

[tenders]

Just to be clear, that isn't Don's recommendation, that's just my practice. I am (perhaps compulsively) concerned about overheating as to me it is far and away the most likely driver of a catastrophic engine problem. Thus I run my engine very cold with a blocked bypass, no thermostat, a high-temperature alarm, and plenty of Marvel in the fuel to keep the valves from sticking.

Another benefit from doing this is that scale is the least of my worries so flushing doesn't have to happen often. (And I would be surprised if even 1% of A4 owners do an annual acid flush of any length of time, active participants to this forum notwithstanding.)

This of course allows me to concentrate on the fun things in life, like fuel delivery problems (new electric pump and carb in the last several years), water pump seals, and the triple thrills of Moyer's grease-gun replacement of the grease cup, the water pump extension bolt, and Pertronix electronic ignition (which really is quite amazing).

[Don Moyer]

Both acid flushing and pressure flushing intervals should be established mostly with respect to local water quality, how often you use the boat, etc. As a starting point, the Cooling System Preventative Maintenance section in our MMI Service and Overhaul Manual on page 2-8 (within the context of acid flushing between pressure flushing) recommends an interval of "at least every six years". Earlier in the same Chapter of the manual (page 2-5) an interval of 3 to 5 years is suggested more within the context of acid flushing as a standalone procedure. But again, these are simply suggestions and should be tailored to your local water quality and use of your boat.

Don

[High Hopes]

Herb, I am planning to a rust inhibitor or antifreeze solution for flushing my raw water cooled A4 after each use.

Black Dove, what do you recommend for a flush?

Steve

[blackdove]

I agree with the statements that Steve has made regarding the inability of NaCl to