What is the Corrosive Sulfur Test? - Part 1

In Part 1 of this three-part series on the Corrosive Sulfur test, we will define corrosive sulfur, briefly review the history of the issue, and describe how corrosive sulfur can cause a transformer to fail.

The Corrosive Sulfur test is a laboratory test performed on electrical insulating liquids, which detects the presence of corrosive sulfur in the sample. Corrosive sulfur can be in the form of elemental sulfur or thermally unstable sulfur compounds. Elemental sulfur is immediately detrimental to a transformer, while thermally unstable sulfur compounds are potentially detrimental. These forms of sulfur cause or can cause corrosion of metals such as copper and silver.

This corrosive sulfur issue has received much attention in the industry. The reason for the increased attention is that starting around the year 2000, several large and expensive newer transformers in various parts of the world suddenly failed unexpectedly. Subsequent internal inspections revealed fine deposits and corrosion on some of the windings and fine deposits imbedded in some of the paper insulation. These deposits were identified as copper sulfide (Cu₂S), an electrically conductive compound. Its presence in the paper insulation formed a conductive path, lowering the dielectric strength of the paper, resulting in subsequent failure. This copper sulfide was created when corrosive sulfur compounds in the oil reacted with the copper conductors.

Let’s take a closer look at the steps to failure from corrosive sulfur:

• When a transformer operates at sufficiently high temperatures, unstable sulfur compounds in the oil break down into corrosive sulfur compounds.
• The corrosive sulfur compounds attack the copper conductor, causing the formation of copper sulfide, some of which dissolves in the oil.
• Copper sulfide has an affinity for the paper insulation and “plates out” on and in the paper.
• The copper sulfide ruins the insulating characteristics of the paper by forming a conductive path that results in eventual failure.

Investigation of early transformer failures (before 2006) from corrosive sulfur showed that in many cases these transformers contained oil that passed the standard test for corrosive sulfur in oil, ASTM D1275, as it existed at the time. It was eventually shown that the conditions under which ASTM D1275 was then being performed were often insufficient to identify oil as corrosive.

The ASTM test was then revised in 2006 to contain two methods: the original method became method A, while a new method with more severe conditions became method B. Method B is performed at a higher temperature than method A (150°C vs. 140°C), and method B also runs longer than method A (48 hours vs. 19 hours). These more severe conditions make method B more sensitive to corrosive sulfur, and thus method B has the ability to identify some oils as Corrosive, when method A might not.

The ASTM test was revised again in 2015. In this latest version, method A was dropped, and method B became the only method for copper corrosion. However, since there is now only one method for copper corrosion, it is no longer called method B. SDMyers has used this more severe method from the time it first came out in 2006. The test will be discussed in more detail in Part 3 of this series. The other difference in the 2015 version of the test is that there is now an additional optional method for silver corrosion. While silver is slightly more sensitive to sulfur corrosion than copper, copper results are easier to interpret and less prone to error.

In Part 2, we will present guidelines regarding when the Corrosive Sulfur test should be performed.