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Understanding Corrosive Sulfur in Transformer Oil

17 Jun 2025

Expert Oil Diagnostic Services

What is Corrosive Sulfur?

Corrosive sulfur refers to certain reactive sulfur compounds in transformer oil that can chemically attack metal components, especially copper and silver. These compounds can form metal sulfides, which are often conductive and can lead to insulation failure. This issue has been recognized for decades, with early reports dating back to 1948.


What is The Impact to Transformer?

Corrosive sulfur can lead to the formation of copper sulfide on transformer windings and silver sulfide on selector switch which compromises the insulation and can cause electrical failures. The extent of the damage depends on the concentration of reactive sulfur compounds and the operating conditions of the transformer.


How to Detect Corrosive Sulfur?

  • ASTM D1275B : Standard Test Method for Corrosive Sulfur in Electrical Insulating Liquids

  • IEC 62535 : Insulating liquids - Test method for detection of potentially corrosive sulphur in used and unused insulating oil

  • DIN 51353 : Testing of insulating oils - Detection of corrosive sulfur - Silver strip test


How to mitigate the effect of corrosive sulfur?

  • Oil Replacement

  • Putting metal passivator

  • Oil’s Chemical Treatment




Copper Corrosive Phenomenon



Silver Corrosive Phenomenon




For more info on our insulating oil analysis services, please don't hesitate to contact our team members:


Shaily Hatun Mohamed Saleh (shaily.hatun@tnb.com.my)

Muhammad Hadi Ahmad Fauzi (hadi.fauzi@tnb.com.my)

Roshaniza Ismail (roshaniza.ismail@tnb.com.my)

Nur Syahida Akryl Azam (nursyahida.azam@tnb.com.my)







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