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Last week in Part 2 of this series we discussed when the Corrosive Sulfur test should be performed. In Part 3 we will describe how the Corrosive Sulfur test is performed, the possible results, and our recommendation if the test result is Corrosive.
As noted previously in this series, ASTM method D1275, Standard Test Method for Corrosive Sulfur in Electrical Insulating Oils, contains two procedures: Method A and Method B. Method A is the older method, which uses conditions of 19 hours at 140 oC. Method B is the newer method, which uses conditions of 48 hours at 150 oC. S. D. Myers, Inc. recommends ASTM D 1275B. A general overview of method B follows:
- About 250 mL of the oil sample is put in a glass flask.
- A copper strip of particular purity is cut to the proper size. The strip is polished to remove all existing tarnish, and then bent into a V shape.
- The prepared copper strip is placed on its edge on the bottom of the flask that contains the oil.
- Nitrogen gas is then bubbled through the oil for five minutes to drive off any oxygen.
- The flask is quickly sealed and then put in an oven for 48 hours at 150 C (Method B).
- After 48 hours, the flask is taken out of the oven, the copper strip is removed, the strip is cleaned of oil, and then the strip’s color (tarnish) is observed.
- The observed color of the copper strip is used together with the following table to classify the oil as Corrosive or Noncorrosive, and to determine the tarnish level:
Classification |
Tarnish Level |
Description |
Noncorrosive |
1a |
Light orange, almost the same as freshly polished strip |
1b |
Dark orange |
2a |
Claret red |
2b |
Lavender |
2c |
Multicolored with lavender blue or silver, or both, overlaid on claret red |
2d |
Silvery |
2e |
Brassy or gold |
3a |
Magenta overcast on brassy strip |
3b |
Multicolored with red and green showing (peacock), but no gray |
Corrosive |
4a |
Transparent black, dark gray, or dark brown with peacock green barely showing |
4b |
Graphite or lusterless black |
4c |
Glossy or jet black |
- It should be noted that the tarnish levels within the Noncorrosive classification are not to be considered progressively worse. For example, a tarnish level of 2e is not worse than a tarnish level of 1b.
- The common characteristic of the Noncorrosive classification is that there is no gray, black, or dark brown on the ending copper strip.
- The common characteristic of the Corrosive classification is that gray, black, or dark brown color is present on the ending copper strip.
The vast majority of Corrosive Sulfur tests we perform come out Noncorrosive. If, however, the result is Corrosive, then pending no contrary advice from the transformer manufacturer, oil supplier, or insurance company, our general recommendation follows:
If the risk of transformer failure from corrosive sulfur and/or the consequences of failure appear to be significant, then we recommend that the corrosive oil be replaced with oil that is deemed noncorrosive by ASTM D 1275 method B (48 hours at 150 C) and that contains a metal passivator. The metal passivator will not reverse damage already done, but will provide some protection for the coils and other metal parts in the transformer from reacting with any remaining corrosive sulfur after the retrofill. If it is not feasible to replace the corrosive oil, simply adding metal passivator to the oil will provide some protection to the transformer, but this will not be as effective as replacing the oil.
Processing corrosive oil has not been found to be effective at removing the sulfur compounds which are the precursors of the corrosive sulfur compounds, so the real problem will still exist.
Metal passivators are typically added to (or already in) transformer oils at a concentration of at least 100 ppm. Several transformer oil suppliers sell transformer oil already containing metal passivator at the proper concentration.
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