Finishers' Think Tank: Identifying Specific Oily Soils

Oily soils on parts can at times provide a real challenge. These suggested evaluation steps will help identify the real problem and the solution to your cleaning problem.


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� It’s not uncommon to be hit with a vexing cleaning problem. Oily soils on parts can at times provide a real challenge. Unfortunately these soils may be the result of a mixture of these materials, rather than one simple, uncomplexed, oily soil. Backtracking with the fabricator helps. They may submit, on request, technical data bulletins and MSDS that can identify or shed light on the unknown oils. Suppliers of cleaner blends may also have practical experience to tackle the specific cleaning problem. It may be the case that identifying the soil or soils may need to be undertaken, in order to develop a solution to the cleaning problem. Should this be the challenge, and then perhaps the following sequence may be helpful.
The oil or oils in doubt may encompass: drawing, sulfurized, quench, chlorinated paraffin, machining, types. The suggested evaluation steps that follow must be conducted using proper lab safety procedures and properly operating equipment, including technique and protective clothing. Handle chemical reagents properly. 
Determine the type of soil
  1. Add three or four drops of the soil to 15 milliliters of water in a test tube.
  1. If the soil dissolves in water, it would be synthetic water emulsion oil.
  1. Should the soil sink but not float to the surface, it would be chlorinated oil.
  1. If the soil floats, additional testing is required.
  1. Start over by adding one milliliter of the unknown soil to five milliliters of 50% hydrochloric acid solution. Heat this solution for five minutes.
  1. Carefully detect if an odor emanates from the heated sample. If it smells like “rotten eggs,” this confirms the oil is a sulfurized type or it contains this species.
Note: Sulfurized oils are commonly black or brown. This test in Step #5 is reliable, unless the sulfur is chemically-bonded in a complex way in the oil.
  1. If the heating step in #5 does not develop an odor, then the oil is either a mineral or fatty type.
  1. Should Step #7 be positive, start with a fresh sample as prepared in Step #1. Add five drops of 50% sodium hydroxide solution and mix well.
  1. No change in the oil layer confirms it is mineral type oil. If the sample soil dissolves with foaming, this indicates it is a fatty acid type (saponification).
Once the unknown soils have been identified, selecting an effective soak cleaner is easier. Suppliers provide a wide range of cleaners, blended specifically to remove many different types of soils. Surfactants and wetting agents have been developed with balanced alkalis, conditioning agents, solvents and dispersants, to address the most challenging cleaning issues. That is why there is not one universal cleaner. The metal forming and fabrication industries utilize more synthetic “green” oils in their processing operations, such as soy derivatives. These, in turn, may simplify cleaning demands.
Additional testing for confidence
When a candidate cleaner has been selected, it is of practical importance to aggressively prescreen it. Doing so improves confidence that cleaning will be a snap. I suggest adding the oil or mixture of oils to the cleaner prepared per recommended operating parameters (concentration, temperature). Gradually add the soil to the cleaner with good mixing.  Stop mixing and let the solution settle. Observe for any floating oils (lack of emulsification). Continue adding the soil by percentage increments until emulsification no longer occurs. Now, add the cleaner concentrate at 10 - 20% of the initial make-up. Mix well. Stop mixing and confirm that emulsification of the soil has been restored. Repeat this test as required. To this artificially-aged cleaner bath, immerse a previously cleaned steel panel for a few minutes, rinse in cold running water for 30 seconds, and observe for water breaks. If there are no water breaks, continue by immersing the panel in either 5% hydrochloric or sulfuric acid for 30 seconds. Rinse again in cold running water for 30 seconds. Again, observe for water breaks. If there are none, then the cleaner, artificially-aged with soils, appears to be a good selection for the cleaning challenge. Still not satisfied? Then why not coat a stamping grade steel panel with the oily soil or its mixture. Next bake the soils into the surface. Now, repeat the above-described cleaning test. This second test, if passed, should afford a high level of confidence that the selected cleaning will be effective.