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Corrosive Sulfur in Transformer Oil

By Anzar Hasan – Chief Inspector

Since about 1996, the electric power industry has seen a rash of failures of large power transformers, shunt reactors, load tap changers and bushings due to conditions caused by corrosive sulfur compounds in electrical insulating oil.

Corrosive sulfur is not unique to transformer mineral oils. The oil often used contains sulfur compounds, some of which may be corrosive. This includes hoses, gaskets, some water-based adhesives, copper and paper insulation. Dibenzyl Disulfide is a sulfur compound often found in transformer oil that is known to react with the copper (such as windings), creating byproducts that attack the insulation. Eventually, the transformer may fail.

In recent surveys of large transformers at the customer locations surveyed by BPC, large HV transformers have been found to have corrosive sulfur present in the transformer oils where mitigations actions had to be taken to remove the DBDS.

The standards normally applied to test the oil for corrosive sulfur are:

  1. ASTM – D1275 A & B
  2. ASTM – D130 A & B

ASTM – D1275 A & B and ASTM – D130 A & B are methods that utilize a copper strip that is impregnated with oil and color observed at various temperatures and time period. The evaluation standards are provided in the table below:

Covered conductor deposition or CCD tests

The purpose of the CCD test was to determine if copper sulfide deposition would occur either on the conductor or in the paper insulation surrounding it. There are several variations of which two are briefly stated below:


Doble CCD Test


Due to limitation of space for this article, only pertinent information relevant to Equipment Breakdown surveys is provided.

ASTM D – 1275 Copper Strip Classification
Classification Description
Non Corrosive Orange, red, lavender, multicolored with blue or silver, or both, overlaid on claret red, brassy or gold, magneta overcast on brassy strip, multicolored with red and green showing (peacock) but no gray.
Corrosive Transparent black, dark grey or dark brown, graphite or lusterless black, glossy or jet black, any degree of flaking
ASTM D – 130 Copper Strip Tarnish Level Classifications
Classification Designation Level Description
Freshly polished strip Unable to reproduce upon aging so no description is provided
1 Slight tarnish a Light orange, almost the same as freshly polished strip
b Dark Orange
2 Moderate tarnish a Claret Red
b Lavender
c Multicolored with lavender blue, or both, overlaid on claret red
d Silvery
e Brassy or gold
3 Dark tarnish a Magenta overcast with brassy strip
b Multicolored with red and green showing (peacock) with no gray
4 Corrosion a Transparent black, dark gray or brown with (peacock) green brassy showing
b Graphite or luster black
c Glossy or jet black

Pass/Fail Criteria for CCD Tests

Copper D 130 Tarnish Level Result
Non-corrosive 1a through 3b Pass
Corrosive 4a, 4b, 4c Fail


Paper Deposition Result
No deposition Pass
Heavy deposition, dull in color Pass
Moderate deposition, dull in color Pass
Light deposition, dull in color Pass
Heavy deposition, metallic sheen Fail
Moderate deposition, metallic sheen Fail
Light deposition, metallic sheen Fail

Corrosive Sulfur Removal Process:

  1. Addition and treating with Passivators: The class of metal passivators consists of compounds on benzotriazole basis, which can react with the copper surface, thus protecting it. This reaction, however, can be reversible under certain conditions. The most widely used metal passivator with respect to the copper corrosion protection is considered to be Irgamet 39 (Ciba).
  2. Changing oil – Another possible approach is the oil change. It must be guaranteed, however, that the mixtures of “old“ and “new“ oils are not corrosive. Oil change will also not be able to recover corroded copper and remove deposits on paper.
  3. Safe Transformer Reclaiming Process: This is propriety, on-site removal process developed by Siemens in Europe. They process utilizes a special filtration process that removes the corrosive sulfur. This process is widely used in Europe.

Recommendation:  Equipment breakdown surveys of large high-voltage oil-filled transformers in steel mills, utilities, etc., especially those that have external cooling system (heat exchangers) and circulating pumps, should include discussion of corrosive sulfur testing and recommendation forwarded if the testing is not part of the electrical maintenance and testing program.


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