Active vs. Inactive Sulfur Additives and Copper Corrosion in Lubricants & MWF.

Active vs. Inactive Sulfur Additives and Copper Corrosion in Lubricants & MWF.

Active and Inactive Sulfur Additives widely work as extreme pressure additives in lubricants and metal working fluids( MWF).

 

What is an active sulfur additive? And what is an inactive sulfur?

Generally, the active sulfur is defined as the sulfide that reacts with metal (especially referring to copper) under certain temperatures.
If the sulfide does not react with copper, it is inactive sulfur.

The fundamental difference between the two lies in whether they react directly with copper and cause copper corrosion at a certain temperature (usually room temperature).

e,g.
Synthetic Calcium Sulfonate TBN 400, even though it has a sulfur content approximate 1.5%, and are almost inactive sulfer, which has almost no corrosion to copper, and the content sulfur has no direcet anti-wear and EP performance in the lube formulas.

 

How to measure the active sulfur content in the lubricant additive?

Active sulfur content is defined as the amount of sulfur that can react with metals (copper) to form metal sulfides at a specific temperature. Standard ASTM D1662-92 shows its testing method, which measures the sulfur consumed by reacting with copper powder at 149°C for 1 hour.

 

ASTM D1662 － 92 Standard Test Method for Active Sulfur in Cutting Oils

This test method measures the quantity of sulfur available to react with metallic surfaces to form solid lubricating aids at the temperature of the test. Rates of reaction are metal type, temperature, and time dependent.

 

What is the relationship between active sulfur and temperature?

Active sulfur content is closely related to temperature. At different temperatures, the same sulfide additive will exhibit different activity, meaning the amount of components that react with metals to form sulfides will vary significantly. Generally speaking, active sulfur content increases significantly with temperature increasing. Therefore, the performance of sulfene is directly related to its operating conditions, particularly operating temperature. At lower temperatures, the extreme pressure and corrosion resistance of active sulfur ep additives will be significantly weakened.

 

What are the main sources of sulfur in lubricating oil?

Sulfur in lubricants primarily comes from sulfur-contain additives, including antioxidant and Anti-corrosion additives, e.g., ZDDP; detergents such as sulfurized alkylphenols and alkyl sulfonates; and sulfur-based extreme additives & antiwear additives, friction modifiers such as sulfized olefin, MoDTP. And some mineral base oils also contain very small amounts of sulfides.

 

Does the sulfur content of the base oil significantly affect the copper corrosion performance of a lubricant product?

Normally, the sulfur content of the base oil is relatively very low, which is far lower than the sulfur content in the oil additives.
e.g.:
A Zinc Butyl Octyl Primary Alkyl Dithiophosphate (ZDDP) has a typical sulfur content of 16%.
And an extreme pressure oil additives Sulfurized Isobutylene T321 has a typical sulfur content of 44%.

According to the API (American Petroleum Institute) base oil classification standard, base oils with sulfur content exceeding 0.03% are categorized into Group 1. In contrast, both Group 2 and Group 3 base oils are required to have a sulfur content strictly below 0.03%.

So, the sulfur from the base oils has a very slight impact on the copper corrosion performance.

What are the main applications of active sulfur additives in lubricants and metalworking fluids?

In lubricants, the primary and most typical application of active sulfur is in automotive gear oils and industrial gear oils, which have excellent extreme pressure and high-temperature friction protection.
When discussing gear oil, one cannot fail to talk about its most important component: the active sulfur extreme pressure agent, Sulfurized Isobutylene (SIB).
SIB is a typical active sulfur-containing extreme pressure oil additive with a high sulfur content, typically between 40% and 46%. Due to its excellent thermal stability and extreme pressure activity, SIB effectively prevents metal surface damage under high-speed impact loads. Therefore, it is widely used in automotive gear oils and industrial EP gear oil formulations. It is currently the most widely used sulfur gear oil additive in gear oils.

 

Applications of Active Sulfur Additives in Metalworking Fluids

The extreme pressure properties of metal working fluids primarily come from the introduction of EP/AW additives such as sulfur, phosphorus, and chlorine. Active sulfur EP MWFs are particularly effective for machining difficult-to-machine metals like stainless steel and alloy steel, as well as for difficult machining processes such as tapping, broaching, deep-hole drilling, stamping, and deep drawing.

For example, when machining difficult-to-machine materials like alloy steel, using inactive sulfur cutting oils has poor anti-sticking properties, which can easily lead to a built-up edge on the tool tip, impacting cutting performance. However, switching to EP cutting oils containing active sulfur can overcome this drawback.

The application of inactive sulfur additives in metalworking fluids. Why is it used?

The inactive sulfur additive does not react with copper or copper alloys, causing corrosion, because of its chemical stability at room temperature.

For example, in metal cutting fluids, inactive sulfur is primarily used for machining copper-containing workpieces, where its extreme pressure and anti-wear properties will be activated at the high-temperature and high-pressure cutting process.

In metal cutting fluid formulations, inactive sulfur is often combined with P & Cl extreme pressure and anti-wear additives.
Generally, Chlorine and phosphorus additives provide anti-wear protection at low and medium temperatures (180-600°C), while sulfur is most effective in the highest temperature range (600-1000°C). These additives provide a wide temperature protection range, creating a synergistic effect.

 

 

Chorus^® offers a wide range of EP/AW Additives for engine oils, gear oils, hydraulic oils, compressor oils, metalworking fluids, greases, and other lubricants at competitive prices. Get in Touch for more technical data about these oil additives.