Can ethyl mercaptan cause corrosion?

The question of whether or not ethyl mercaptan can cause corrosion arose in a recent explosion we investigated. The answer to that question is somewhat complex.

The underground copper line had been corroded at a point where a protective sleeve around the copper tubing had ended underground. The soil in the area was primarily clay, and the working assumption was that the soil retained a lot of moisture and may be highly acidic, and may contain high levels of chlorides, sulfates or ammonia, all known to cause copper tubing to corrode.

The line had been installed many years before the loss, and the installer of the line could not be sued as the statute of repose had passed.

The statute of repose is a rule that prevents a lawsuit from being brought after a certain number of years against a certain group. In this example, it would be for an improvement to real property. Most states will bar lawsuits for improvements to real property if that work was done 10 or sometimes 12 years ago, or more.

The installation of the copper tubing was somewhat peculiar. The protective rubber/plastic sleeve that surrounded the copper tubing extended from the riser at the tank to about 3 ft. underground and then extended from the riser to the second stage regulator at the house to about 3 ft. underground. It did not cover the entire length of copper tubing underground and left perhaps 30 ft. or more of buried copper tubing exposed to the surrounding soil. Any observer would reasonably assume the protective sleeve covered the entire length of the copper tubing.

Here, the odorant in the propane tank was being tested, and it had relatively high concentrations of ethyl mercaptan. That is common in older tanks as propane is known as a heavy end. The odorant vaporizes at a lower rate than the propane itself. As a result, over time, the odorant levels rise.

▶ Metallurgic testing

To test this theory, we needed to conduct metallurgic testing of the subject copper tubing that had been recovered from the site of the loss. That metallurgic testing involved cutting the copper tubing in half so the inside could be examined under a variety of scientific equipment. That work demonstrated that there was no corrosion occurring from inside the copper tubing out. Instead, all the corrosion seen in the various testing was from the outside of the copper tubing in.

This finding is consistent with what science tells us about corrosion occurring in copper tubing. For corrosion to occur in copper tubing over an extended period of time, it needs to have high concentrations of the odorant and a good amount of moisture and heat.

The fact that the odorant increases in the liquid phase of the tank also means that it is not vaporizing at substantially higher concentrations. The higher concentrations are only in the liquid phase.

▶ What bills of lading show

There is no practice in the industry to test the level of odorant in tanks after confirming that the bills of lading show the propane is properly odorized by reference to the bill of lading and by conducting a sniff test. Bills of lading commonly show that about 1.5 pounds per 10,000 gallons is added to commercial propane.

Each incident presents unique puzzles to solve. In the many years I have investigated and defended propane companies, this particular question was a first. We learned that corrosion of copper tubing from the odorant ethyl mercaptan in the vapor phase is not something that occurs under normal circumstances.

John V. McCoy is with McCoy, Leavitt, Laskey LLC. His firm represents industry members nationally. He can be reached at jmccoy@MLLlaw.com or at 262-522-7007.

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