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Experiencing Rusting
on Machine Surfaces and Tools.
One role of metal working fluids is to resist
and control rust. However, several factors can prevent the
metalworking fluid from doing so. If rust is prevalent on
machine surfaces and/or tools, it could be caused by one or
more of the following factors; improper dilution, water condition,
bacterial contamination, contaminating fluids, and/or metal
being machined. In order to control rust, these factors need
to be controlled or eliminated.
Improper dilution
Weak concentrations will negate the effects of a rust inhibitor.
To prevent weak dilutions, concentration levels should be
checked at least once a week with a refractometer. The refractive
index can be obtained by the supplier of the metalworking
fluid. Concentrate should be added when needed based on refractive
index.
Water conditions
Water hardness is a big factor in the performance of the metalworking
fluid. Water that is too hard will deplete the rust inhibitor.
When hard water is used, minerals are left behind as the water
evaporates. The evaporation increases the concentration of
the coolant. To make up for evaporation loss, more water is
added to the coolant with additional minerals. This makes
the mineral level in the coolant much higher than the original
mixture. Hard water above 10 grain hardness (170 PPM) is very
destructive to a metalworking fluid. Water purity of less
than 1 grain hardness (15 PPM) is ideal. A sample of water
supply should be tested for hardness before mixing with a
metalworking fluid. If water is hard it is suggested to use
distilled or deionized water. Deionization, using ion-exchange
resins, is one of the most effective and widely used ways
to remove minerals from the water before it is mixed with
the coolant concentrate. Deionizing units are generally connected
to a shop's water supply between the source of the water and
the point at which the water is used. Deionization removes
all dissolved solids, leaving the treated water almost as
mineral-free as distilled water.
Contaminating fluids
Contaminating fluids are tramp oils that enter into the metalworking
fluid. Tramp oils are hydraulic oils, spindle oils, gear lube,
way lube, rail lube, oil coated on raw material, cleaner,
or any other contaminant that the metalworking fluid can collect
during use. Tramp oils can be easily noticed because they
coat the surface of the metalworking fluid. When tramp oils
exist, they start to react with the water to form an acid.
This acid starts to deplete the metalworking fluid and any
additives in it, including the rust inhibitor. To prevent
tramp oils, leaks should be detected and fixed immediately
and installation of a continuous tramp oil separating system
is recommended. Regular cleaning of the sump should be done
to prevent bacterial growth due to tramp oils.
Bacterial contamination
A sump full of dirty oil, tramp oils, and chips help bacteria
to grow. The bacteria produces corrosive acids and salts,
which could damage machinery and workpieces, besides depleting
the metalworking fluid itself. Bacteria can be eliminated
if sumps are kept clean on a regular basis, chips are removed
from the coolant with a good filtration system, and tramp
oils are eliminated. Bactericides may be added to the coolant
to retard the growth of bacteria. However, if bacteria is
prevalent, the coolant is already weak and adding a bactericide
will only weaken the coolant more.
Metal being machined
Most metalworking fluids have a negative charge. When some
metals are in a machining state they create a positive charge.
The reaction between the two will cause a breakdown in additives
in a metalworking fluid such as; emulsion stability, wetting
ability, and rust preventives. Problematic metals include
cast or ductile iron or magnesium. These metals create a positive
charge and may react with the metalworking fluid. Before machining
these types of metals, verify that the metalworking fluid
being used is compatible with these metals.
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