Galvanized Pipe and Engine Cooling Systems

My specialty is corrosion from electrical sources in marine environments. However since the "corrosion textbooks" rarely address the real world issues we encounter in ships and boats I'll give my take on this topic.

Cut threads remove the protective coating. Therfore you have a location with no protection at which any corrosion will be concentrated. Chances are good you will break of the galvanized pipe right at the edge of a threaded hole. Now you have a real challenge to remove the stub. Most of the cooling systems I have encountered are either fitted for hose barbs or in larger sizes, they have flanged couplings to facilitate removal in situ without having to dismantle the whole system. Your only other choice is to use a lot of unions. More potential leaks in joints etc.

The galvanized pipes I have seen in new condition does not look like it has a consistently thick coat of zinc on the inside. some do not have any zinc on th einside. This may lead to pinhole perforation as a premature failure.

Remember the galvanic table is based on a given temp and velocity of electrolyte ( read sea water). Pumped systems will have consistently faster velocity leading to accelerated corrosion.

The prevalent practice of using screw-in sacrifical zincs has proven to be a longer lasting solution. Easier to inspect and fix when zincs are gone.

Threaded connections should be avoided as being a weak point in the system which is most likely to break first. Flanged connections are welded. You should not weld galvanized pipe; so now you must first fabricate, then galvanize. In which case you might as well just powder coat the fabricated and welde pipe and fittings. Being electrostatic deposition you are more likely to get an even coating all over. Then insert a screw-in pencil zinc.

One reason not to use galvanised pipe in the engin cooling system is the following. Under normal circumstances Zinc is a sacrifical anode and iron is a cathode in galvanized pipe. By forcing iron to be the cathode, it does not corrode so long as there is any zinc left in the galvanized coating. Howerev, at high temperatures, such as might be found in the hot end of an engine cooling cirucit, the electromotive potential of the zinc-iron couple reverses and the iron becomes the anode. The reverse in polarity occures about the boiling point of water.

I actually experienced this while I was the manager of an ammonia plant. The air compressor third stage intercooler intercooler shell had had a corrosion problem for years that caused us to replace the carbon steel shell about every 3-4 years. The intercooler had tower water on the shell side and hot air on the tube side. The hot end ran close to the boiling point of water. We reasoned that galvanising the shell, inside and out would make it impervious to corrosion. Wrong! The corrosion rate could be measured in feet per year not mill per year. The shell corroded from the inside uot in less than 3 months.

It tool a while, but a senior metalurgist and corrosion engineer figured uot what was wrond and demonstrated it in the lab. Corrosion chemistry and the dependence of emf on temperature was the culprit.

Hope this helps.