DIN 51558-2 Testing of Mineral Oils - Determination of Neutralization Number - Part 2: Titration with Color Indicator, Insulating Oils

The standard “DIN 51558-2 Testing of mineral oils - Determination of neutralization number - Part 2: Titration with color indicator, insulating oils”, published by the German Institute for Standardization (DIN), describes a test method for determining the neutralization number of insulating oils, which are mineral oils primarily used as electrical insulation fluids in transformers, capacitors, cables, and similar equipment.

The neutralization number indicates the content of acidic components in the oil. It serves as an important quality and condition parameter.

New insulation oils typically have very low acidity levels (close to zero or slightly acidic).
During service, oxidation, thermal stress, or contamination can increase the number of neutralizations by causing the formation of acidic decomposition products.
This method helps assess aging, oxidation stability, and the need for maintenance or replacement by detecting relative changes in the oil's condition over time.

This parameter is critical in the electrical industry for monitoring the health of transformer oil, as excessive acidity can accelerate insulation degradation, corrosion, or sludge formation.

The test method is in the form of color indicator titration:

A sample of the insulating oil is dissolved in a suitable solvent mixture (typically toluene + ethanol or isopropanol).
A color indicator (usually p-naphtholbenzene or a similar pH-sensitive dye) is added to the solution, which gives a characteristic color in the presence of acids.
The solution is titrated with a standard alcoholic potassium hydroxide solution (e.g., 0,1 mol/L or 0,01 mol/L, depending on the desired acidity).
The endpoint is reached when the indicator changes color (from orange/red to green/blue, depending on the indicator used).
The neutralization number is calculated and expressed in mg KOH/g fat (the amount of potassium hydroxide required to neutralize acidic components in 1 gram of sample).

This method is a potentiometric or visual endpoint technique optimized for the typically very low acidity levels found in insulating oils.

While the DIN 51558-1 standard covers a general color-indicated titration method for other mineral oils (especially not insulating oils), the DIN 51558-2 standard is adapted to the specific requirements and low acidity range of transformer/insulating oils. This method is often used in conjunction with other tests such as water content (Karl Fischer), dielectric breakdown voltage, or oxidation stability.

This method remains a classic analytical procedure widely accepted in petroleum and electrical insulation testing laboratories worldwide.

In short, the DIN 51558-2 standard includes a test method for evaluating the corrosive effect of lubricants and related fluids on copper. This method is primarily used to determine whether mineral oils, gear oils, hydraulic fluids, or other petroleum-based products contain corrosive sulfur compounds or other aggressive substances that could damage copper or copper alloys.

This test is particularly important for lubricants used in systems containing the following components:

Bronze zodiac signs
Rice components
Copper deposits
Electrical coils and connectors

The main objectives of the DIN 51558-2 standard are as follows:

To evaluate the chemical compatibility between lubricants and copper.
Detecting the presence of active sulfur compounds.
To prevent premature corrosion of non-ferrous components.
To ensure the long-term reliability of mechanical systems.

Copper corrosion can lead to the following:

Surface pitting
Increased friction
Contamination of oil systems
Shortened lifespan of equipment

This test method is based on the principle of exposing a polished copper strip to lubricant under controlled temperature and time conditions. The test process essentially follows these steps:

A copper strip is polished to a standard surface.
The strip is dipped into the test oil.
The sample is heated to a specified temperature (usually around 100 degrees).
After a certain exposure time (e.g., 3 hours), the strip is removed.
The degree of discoloration or corrosion is visually compared using a standard grading scale.

After testing, the copper strip is evaluated using a classification system that typically falls within the following range:

Class 1: Slight tarnishing (acceptable)
Class 2-3: Moderate color change.
Class 4 Severe corrosion (unacceptable for many applications)

The darker and more irregular the discoloration on the surface, the more aggressive the lubricant.

The DIN 51558-2 standard is commonly used in the following areas:

Gear oils
Motor oils
Hydraulic fluids
Turbine oils
Transmission fluids

The sectors that rely on this test are:

Automotive production
Industrial machinery
Energy production
marine engineering

The DIN 51558-2 standard is technically comparable to the following:

ASTM D130 Corrosion of copper strip by petroleum products.
ISO 2160 Determination of corrosiveness to copper.

These standards follow similar principles, but specific test conditions (temperature, time, rating) may differ.

This test is critical in quality control because:

Even small amounts of active sulfur can cause damage.
Corrosion products can accelerate the deterioration of oil.
Manufacturers must verify the lubricant's compatibility with yellow metals.
Many original equipment manufacturer specifications require copper to pass corrosion tests.

In industries such as automotive and heavy industry, a lubricant that does not meet the DIN 51558-2 standard is generally rejected for use in systems containing copper alloys.

In conclusion, the DIN 51558-2 standard plays a crucial role in evaluating the chemical stability of lubricants and their compatibility with copper materials. By detecting abrasive tendencies early, manufacturers and users can:

It can prevent equipment damage.
You can extend the service intervals.
It can ensure compliance with technical specifications.
It can increase operational safety and reliability.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services in accordance with the DIN 51558-2 standard.