Silicon Doesn't Always Mean Dirt

One of the most fundamental truths that many oil analysis users learn is that elevated silicon levels from elemental spectroscopic data equate to dirt ingression. The logic is simple: common dirt from road dust and other sources contain high levels of silica.

Therefore, elevated silicon from spectrometric analysis means dirt ingression. But is this true in every situation? What other possible sources of silicon exist in a lubricating oil environment that may cause silicon levels to rise?

Depending on the environment in which the equipment is operating, silicon may appear in the lubricant for various reasons.

Possible Ingressed Contaminants

• Fly ash in a coal-fired power plant

• Airborne dust from a cement plant

Possible Internal Contaminants

• Silicone sealant used as a sealing agent

• Silica from mechanical seals

• Fibers from a decomposing fiberglass-type filter element

• Silica gel from desiccating breathers

• Silicon-based grease

• Silicon dry-film lubricants/antiseize compounds

• Components that have been alloyed with silicon carbide to increase their hardness or thermal expansion characteristics.

In addition, silicon, in the form of polymeric methyl silicone, is a common antifoam additive used in many types of oils. How can the educated oil analysis user differentiate between silicon in all these different forms, without expensive and elaborate chemical analysis? The key to identifying the silicon’s form and source is lock-step trending.

Lock-step trending refers to the process of observing multiple parameters rise and fall simultaneously. This type of trend pattern often indicates a correlation between the two parameters, pointing toward the real root cause. An example of lock-step trending can be found in the following case.

Landfill Gas Cogeneration

A local municipality installed a cogeneration facility designed to predominantly operate on methane gas, which is produced from the city landfill, to generate electricity. After installation, oil analysis revealed rapidly rising levels of silicon in the oil, suggesting that the engine was ingesting dirt.

Perhaps more disconcerting was the resultant increase in antimony, a common wear metal used in some sleeve-type bearings. The most obvious explanation was dirt ingression causing abrasive wear, but how could dirt be ingested if the air filter was in good shape and the fuel gas was prefiltered?

The clue to this mystery was the observation that the rise in silicon was accompanied by a rise in acid number as shown in Figure 1.

Figure 1. Sample Report Indicating Silicon and Acid Number on the Rise

Acid number is usually measured to indicate oil degradation due to oxidation. While prolonged dirt ingression can lead to oil degradation as a result of increased friction and the associated increase in temperature, it seemed unlikely that was the cause for rapid oxidation in this case.

After a meeting in which individuals were invited to share their views, they decided that some silicon-containing contaminant, other than dirt, was entering the engine and causing acids to build up in the oil. What was the contaminant?

The team decided to analyze a fuel gas sample to help identify the type of silicon. The fuel gas showed high levels of siloxanes, an acidic gas, which is commonly found in North American landfills. The question about the source of the silicon-containing contaminant was revealed; the siloxane gas in the fuel was accumulating in the oil, resulting in an increase in silicon levels, and the resulting acid was corroding the bearing.

The solution was to scrub the fuel gas with activated charcoal to remove the siloxanes, which resulted in more normal acid numbers and a decreased antimony levels, suggesting that bearing wear rates were also back to normal.

Although this case study illustrates a rather unique situation (unless of course you’re maintaining a waste gas cogeneration facility); it does illustrate the idea that lock- step trending, together with lateral thinking can lead to problem solving and root cause analysis.

Elevated silicon levels should always raise concern. However, just like every other oil analysis alarm, the root cause of the problem should be investigated in order to formulate an appropriate maintenance response to address the root cause of the problem, without jumping to the obvious, but sometimes wrong assumption. Get into the habit of using all the data from an oil analysis report.

Tracing the Source of Silicon

The following examples illustrate some of the likely causes of increasing silicon in a diesel engine, and how through simple lock-step analysis, the root cause can be determined. For the following examples, read the parts per million (ppm) for each element. See if you can identify the cause of the distribution.

Example 1: Severe Dirty
In this example, elevated silicon is observed along with high wear readings. In addition, there is a rise in aluminum, which although partly due to piston wear, also indicates dirt ingression. Common dirt is comprised predominantly of silica and alumina.

The typical ratio of these two, depending on the geology of the region in question is 3.4:1 (Si:Al). The rise of both elements simultaneously is usually an indication of simple dirt ingression, resulting in abrasive wear to rings, liners and pistons.

Example 2: Coolant Leak
In this case, the rise in silicon is not accompanied by a rise in aluminum, ruling out dirt ingression. However, the elevated sodium level is the key to this case. Sodium is a commonly used corrosion inhibitor in engine coolants. Its rise, together with the rise in copper, indicates a coolant leak likely from the cooling core. But why silicon? Silicon is often used in engine coolants as an antifoam agent (just like in engine oils), or as a corrosion inhibitor, most commonly when aluminum-cooling cores are in use.

Example 3: Silicone Sealant Leaching
A sudden rise in silicon with no other indicator usually means a silicon containing contaminant, just like the cogeneration example above. In engines and many other components, this is often a sign of leaching of a silicone-based gasket sealant. If the component is new or rebuilt, a similar pattern may also be observed from casting sand used to fabricate certain components. It is not uncommon to see high levels of silicon (but no aluminum) during the break-in period of a new engine for this reason.

Example 4: Piston Torching - Fuel System Fault
This example looks like another dirt ingression problem. However, the silicon to aluminum ratio of 1:1 indicates there is more to this example than initially meets the eye. This is an example of piston torching. If a fuel injector is faulty, it can allow excess fuel to lie on top of the piston and burn. The resulting high temperatures can cause the piston to melt with a resultant increase in aluminum (piston), iron (liner) and chrome (ring) wear.

The rise in silicon is the result of silicon carbide being alloyed with the piston material in order to reduce the coefficient of expansion of the aluminum. In this case, silicon is a wear element identified as such based on the ratio of aluminum to silicon.