Mineral Oil and Corrosive Sulfur: What is eating away at your immersed electronics?

Updated: Jul 30, 2018

Mineral and white oils are often considered for use as heat transfer fluids for immersion cooling of bit miners and electronics due to its low cost and dielectric properties. Unfortunately, most potential and current users of mineral oil are unaware of the corrosive effects of the many naturally occurring sulfur compounds that exist in mineral and white oils.

In this post, I want to highlight the corrosive sulfur problem and the impacts it can have on your servers and electrical equipment. Let’s start by clarifying that this is not a new problem; in my thirty career as a developer of dielectric fluids for use in the power and transformer industry I’ve wrestled with this in transformer cooling applications for decades (See Notes 1,2, 3 below). However, what I have noticed is that the frequency of sulfur corrosion and the sulfur levels in many mineral oils has been increasing over the past decade due to changes in the petroleum industries source of crude oils and in their bid to reduce the costs of processing.

Sulfur is a naturally occurring element found in almost all crude oils extracted from the ground. In the past, refiners would avoid using high sulfur crude as it caused corrosive damage in the refineries and required additional processing to meet product specifications. However due to changes in the market availability of “sweet crude” we are seeing a far greater variety of different sulfur compounds make it through the refining process unchanged with the result being an increasing sulfur content in most mineral and white oils (going forward I’ll refer to both of these as “mineral oil”).

Not all sulfur is inherently aggressive - but several types can later turn into one of the corrosive forms of the compound, the most corrosive being dibenzo-disulfides (DBDS). Yes, you read that right - over time, noncorrosive types of sulfur can change in use into a corrosive type. These corrosive sulfur species then attack electronics in one of two ways:

Attack Mechanism One: Direct Attack on Copper and Other Metals

Corrosive sulfur, as its name implies, directly attacks the copper and zinc materials in electronics. The corrosive sulfur erodes these metals from circuit boards and electronic components typically attacking areas where current is flowing, and the charged sulfur particles are attracted. This is often seen when mineral-oil based heat transfer fluids and lubricants are used with copper or silver bearings, or with exposed copper and zinc on circuit boards. See Figure 1, which illustrates a relatively early, but widespread sulfur-induced corrosion of a circuit board that has been immersed in mineral oil.

corrosive sulfur in mineral oil
Figure 1, Direct Corrosive Attack

Figure 2 shows corrosion of a bearing race, where silver plating has been attacked and

removed, causing failure of the bearings.

Bearing Raceway Corrosion by Sulfur