POWER August 2016 - 18

cess by forming new radicals that are more
stable.
Secondary antioxidants react with peroxides,
which are often present as the
lubricating oil reacts with oxygen. These
antioxidants are responsible for breaking
the cycle and preventing branching and
further propagation. Most often, grease
formulators will use a combination of
primary and secondary antioxidants to
maximize the protection of the oil against
oxidative degradation.
Extreme Pressure and Antiwear
An expert at the Massachusetts Institute
of Technology claims that high friction
and wear results in the wastage of
resources totaling more than 6% of the
gross national product. Therefore, taking
action to minimize the effects of friction
and wear can save a lot of money.
There are several different types of
mechanical wear mechanisms that can
occur in a lubricating system, including
adhesive wear, abrasive wear, pitting, and
spalling. Wear preventative additives are
a necessary component in lubricants intended
for applications that are prone to
such damage.
Wear preventative additives fall into
two categories: extreme pressure agents
and antiwear agents. Extreme pressure
and antiwear additives both function by
depositing a protective barrier via a reaction
on the metal surface.
Antiwear additives are generally used
under mild conditions of low loads and
high speeds to reduce the rate of continuous
and moderate wear. The additive
helps coat the application surface
to protect the metal from wear over
time. Common examples of antiwear additives
are triaryl phosphates and zinc
dialkyldithiophosphate.
Extreme pressure additives are usually
used under heavier loads, at high
temperatures and low speeds to prevent
catastrophic failure or seizing of the application.
Common examples of extreme
pressure additives are molybdenum disulfide,
graphite, sulfurized olefins, and dialkyldithiocarbamate
complexes.
Anticorrosion and Antirust
Corrosion is defined as the destructive alteration
of metal by a chemical or electrochemical
reaction between the metal
and its environment, resulting in change
and weakening of the metal's properties.
Corrosion is a process, not a property.
All metals except the noble metals are
unstable under atmospheric conditions,
which allows them to be converted into
their oxidized form. There are two primary
types of corrosion: electrochemical corrosion
and chemical corrosion.
Electrochemical corrosion involves the
reaction of ferrous (iron-based) metal or
its alloys in a two-step process in the
presence of an electrolyte, most typically
water or moisture. The result of this reaction
is commonly known as rust.
Rust inhibitors can be included in a
formulation to slow corrosion of iron alloys.
Rust inhibitors function by physically
adsorbing onto the metal surface, thus
blocking the surface of the metal from the
effects of water, acids, and air.
Chemical corrosion involves the attack
of aggressive chemical species like acids,
bases, and sulfur. This is often the result of
the oxidation of hydrocarbons and sulfurcontaining
additives, or additive by-products,
on a metal surface or metal oxide
layer, resulting in the formation of ionic
metallic or oxidized metallic compounds.
Unlike electrochemical corrosion, chemical
corrosion does not require an electroCIRCLE
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POWER August 2016

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