How Is the Corrosive Sulfur Test Performed?

This is Part 3 of a three-part series on corrosive sulfur testing. Part 2 addressed the practical question of when corrosive sulfur testing is warranted — and which transformer characteristics should drive that decision. Here we will cover the mechanics of ASTM D1275 — how the test is conducted, how results are classified, and what corrective action is recommended when oil tests Corrosive.

Test Procedure Overview

The test method is ASTM D1275, Standard Test Method for Corrosive Sulfur in Electrical Insulating Liquids. The procedure for copper corrosion follows a defined sequence:

• Approximately 250 mL of oil sample is placed in a glass flask. A copper strip of specified purity is cut to size, polished to remove existing tarnish, and bent into a V shape.
• The prepared copper strip is placed on its edge on the bottom of the oil-filled flask.
• Nitrogen gas is bubbled through the oil for five minutes to purge dissolved oxygen.
• The flask is sealed and placed in an oven at 150°C for 48 hours.
• After 48 hours, the flask is removed from the oven, the copper strip is extracted and cleaned of oil, and its color is observed.
• The observed tarnish color is compared against the ASTM D1275 classification table to determine result and tarnish level.

Interpreting Test Results

Noncorrosive Classification

Oil is classified as Noncorrosive when the ending copper strip shows no gray, black, or dark brown coloration. Multiple tarnish levels fall within the Noncorrosive classification, but these levels are not ordinally ranked — a result of 2e is not considered worse than a result of 1b. Tarnish level within this classification carries no progressive significance.

Corrosive Classification

Oil is classified as Corrosive when gray, black, or dark brown coloration is present on the ending copper strip. The presence of any of these colors is the defining indicator regardless of the extent or distribution of discoloration.

The vast majority of corrosive sulfur tests return a Noncorrosive result. When a Corrosive result is returned, further action is warranted.

Recommended Response to a Corrosive Result

Oil Replacement with Metal Passivator

When the risk of failure or the consequences of failure are significant, the preferred course of action — absent contrary guidance from the transformer manufacturer, oil supplier, or insurer — is to replace the corrosive oil with oil that tests Noncorrosive per ASTM D1275 and contains a metal passivator.

Metal passivators function by reacting with metallic surfaces to form a protective layer that inhibits further corrosive attack. It is important to note that passivators do not reverse damage already done — their role is protective, not restorative. Following a retrofill, the passivator provides ongoing protection against residual corrosive sulfur that may remain in the system.

When Oil Replacement Is Not Feasible

If a full oil replacement is not operationally or logistically feasible, adding metal passivator directly to the existing corrosive oil will provide a degree of protection. This approach is less effective than a complete retrofill but represents a meaningful interim measure. Metal passivators are typically dosed at approximately 100 ppm in transformer oil.

Why Oil Processing Is Not a Sufficient Remedy

Processing corrosive oil — through filtration, degassing, or similar methods — has not been shown to effectively remove the sulfur precursor compounds responsible for corrosive sulfur formation. Processing may address other oil quality parameters but does not resolve the underlying corrosion risk. Oil replacement remains the recommended solution when a Corrosive result is confirmed and risk is assessed as significant.

Summary:

• ASTM D1275 uses a copper strip immersed in oil at 150°C for 48 hours; post-test tarnish color determines whether the oil is classified as Corrosive or Noncorrosive.
• The classification threshold is straightforward: gray, black, or dark brown on the copper strip indicates Corrosive; absence of these colors indicates Noncorrosive.
• Tarnish levels within the Noncorrosive classification are not ordinally ranked — a higher level designation does not indicate a worse result.
• The majority of corrosive sulfur tests return Noncorrosive results; a Corrosive result is the exception but requires a defined response.
• When a Corrosive result is returned and failure risk is significant, the recommended action is oil replacement with Noncorrosive oil containing a metal passivator.
• Metal passivators protect against further corrosive attack but do not reverse existing damage — their role is preventive following a retrofill.
• If full oil replacement is not feasible, adding passivator to existing oil offers partial protection at approximately 100 ppm dosing.
• Oil processing has not been shown to remove sulfur precursor compounds and is not a substitute for oil replacement.

In This Series:

• Part 1: What Is the Corrosive Sulfur Test for Transformer Oil?
• Part 2: When Should the Corrosive Sulfur Test Be Performed?
• Part 3: How Is the Corrosive Sulfur Test Performed?