Corrosive Sulfur in Mineral Oil - A Killer App for Electronics

Updated: Sep 17, 2018

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

In this post, I want to highlight the corrosive sulfur problem and the impact it can have on your servers and electrical equipment. Let’s start by clarifying that this is not a new problem; in my thirty year career as a developer of dielectric fluids for use in the power 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 corrosive sulfur problems and the concentration of sulfur in mineral oils has been increasing over the past decade. This is due to changes in the petroleum industry’s source of raw 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”).

When mineral oils are heated, the sulfur can change form, into aggressive, corrosive forms of sulfur. The most aggressive is dibenzo-disulfide, or DBDS. These corrosive sulfur species then attack electronics in one of two ways:

1) Corrosive Direct Attack

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.

Figure 1: Direct Attack by Corrosive Sulfur on Copper

Figure 2 - Shows corrosion of a bearing race, where silver plating has been attacked and removed, causing failure of the bearings.

Attack on silver race for bearings

2. Copper Salts Accumulations

As sulfur erodes the copper, zinc, and other metals on circuit boards the sulfur ions combine in solution with copper ions forming increasing amounts of copper salts such as Cu2S (Copper Sulfide), CuSO3 (Copper Sulfite) and CuSO4 (Copper Sulfate) in the oil. These metallic salts are not soluble in mineral oil and immediately begin to precipitate, or in laymen’s terms – to form crystals - onto “substrates”, as they’re called, which in this case means on circuit boards, insulation, or spacers - any structure where the salts can begin to crystallize and build up their lattice structures. Copper-sulfur salt crystals are highly conductive and build rapidly so they easily bridge connections the large windings in power transformers with explosive results. This phenomenon is only seen when mineral oil-based dielectric heat transfer fluids are used. Short circuiting by copper salts accumulations causes 10’s mlns of dollars in damage every year in the power transformer industry and it happens even easier on circuit boards due to their highly dense traces.