Metalworking Fluid Spoilage: 5 Causes and Solutions

Category: Technical and educational guide | Author: Kimiya Sanat Savalan Research and Development (R&D) Unit

Water-based metalworking fluids (so-called soluble oils) are vital arteries in machining, turning and rolling processes. However, many workshops and industrial units face two corrosive challenges: Strong stench (like rotten eggs) and Phase Separation (oil separating from water).

The premature failure of the emulsion not only increases the cost of purchasing consumables, but also causes tool corrosion, damage to the color of the devices, rusting of production parts, and skin allergies for operators.

This technical article examines five common causes of metalworking-fluid degradation and explains practical measures to prevent them.

1. Water hardness: a hidden cause of emulsion failure

The water used for mixing with the oil contains various solutes including calcium ( $Ca^{2+}$ Ca2+) and magnesium ( $Mg^{2+}$ is Mg2+). The amount of these solutes is measured by the “water hardness” index (in ppm).

Degradation Mechanism: Calcium and magnesium ions can react with emulsifiers to form insoluble soap deposits. This disrupts the emulsion structure, allowing oil to separate and float on the sump surface.
solution:

As much as possible from purified water or water with controlled hardness (below $200\text{ ppm}$ 200 ppm).
If it is unavoidable to use hard water (such as well water in most industrial areas of Iran), from specialized grades resistant to hard water such as SVL FLUID E200 HW Use that their formulation is stable against calcium saltss.

2. Anaerobic bacteria growth (source of bad odor)

Due to the existence of the aqueous phase and the suitable operating temperature, the water-miscible metalworking fluid environment is a very suitable substrate for the growth of microorganisms (bacteria and fungi).

Degradation Mechanism: Sulfate-reducing bacteria (SRB) thrive in oxygen-depleted conditions, for example during extended machine shutdowns. Their activity can release hydrogen sulfide ( $H_2S$ H₂S), which causes the characteristic rotten-egg odour. Microbial activity lowers the fluid’s pH and can destabilise the emulsion.
solution:
Regular aeration of tanks on holidays (using simple aeration pumps).
Using formulations equipped with biological buffers and high-quality biocides that do not allow bacteria to grow.
Weekly measurement of pH and maintaining it in a safe range ( $8.8$ 8.8 to $9.5$ 9.5).

3. Entering tramp oils into the tank

Hydraulic oil, slideway oil and lubricating grease can leak into the coolant sump during operation. These contaminants are known as tramp oils.

Degradation Mechanism: These oils do not emulsify and form a floating layer on the coolant. This limits oxygen exchange and encourages anaerobic bacterial growth.
solution:
Using oil skimmer devices to continuously collect tramp oils from the surface of the tank.
Prevent hydraulic oil leaks into the coolant system.

4. Wrong method in initial mixing (addition order)

One of the most common operator errors in workshops is not following the correct order of mixing water and oil.

Degradation Mechanism: If you pour water on top of oil, the high concentration of oil in the initial phase prevents the emulsifier particles from being evenly distributed. The result of this is the creation of a thick and unstable inverse emulsion (water in oil) that becomes biphasic very quickly.
solution:
Always add the oil to the water slowly (O.W. Law: Oil to Water). While adding the oil, stir the mixture continuously so that the oil particles are evenly dispersed in the continuous water phase.

5. Failure to adjust the appropriate concentration (weakening of the dosage)

Some workshops drastically reduce the mixing percentage (e.g. less than 2%) to reduce costs.

Degradation Mechanism: Dilution reduces the concentration of corrosion-control and microbial-control additives throughout the sump. If the fluid is diluted below its specified range, lubrication and resistance to contamination can deteriorate rapidly.
solution:
Concentration of water-miscible metalworking fluid with the help of the device Refractometer Monitor daily or weekly.
Keep the concentration consistent with the manufacturer’s recommendation (usually between 3 and 7 percent depending on the type of machining operation).

Troubleshooting and corrective action summary table

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Signs of failure	Possible cause	Immediate corrective action
Smelling the smell of rotten eggs	Anaerobic bacteria activity due to liquid stagnation	Aeration of the tank, use of biocide or replacement of the entire fluid and flushing of the system
Separation of the oil layer on the surface	High water hardness or initial mixing error	Check water hardness, use grade resistant to hard water (`SVL-E200-HW`)
Rusting of parts and tools	Low pH (acidification) or too low a concentration	Increasing the concentration of the emulsion, checking and correcting the pH to high $8.8$ 8.8
Flaking and yellow deposit	Reaction of calcium salts with oil emulsifier	Washing the path of the nozzles, using soft water or oils with stabilizers

conclusion

Coolant service life depends strongly on maintenance. Managing water hardness, monitoring pH, removing tramp oil and choosing a suitable fluid can extend emulsion life and reduce purchasing and disposal costs. The improvement depends on operating conditions.

Specialists Kimiya Sanat Savalan They are ready to provide technical advice to analyze the water used and suggest the appropriate grade of lubricant according to your workshop conditions.

SAVALAN

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