Fuel, Oil and Coolant Specifications
more commonly used corrosion inhibitors are chromates, borates, nitrates, nitrites and soluble oil. Depletion of all types
of inhibitors occurs through normal operation. Therefore, strength levels must be maintained by the addition of inhibitors
at prescribed intervals. Always follow the supplier's recommendations on inhibitor usage and handling.
Sodium chromate and potassium dichromate are two of the best and most commonly used water system corrosion
inhibitors. However, the restrictive use 'of these materials, due to ecology considerations, has de-emphasized their use
in favor of non-chromates. Care should be exercised in handling these materials due to their toxic nature.
Chromate inhibitors should not be used in permanent type antifreeze solutions. Chromium hydroxide, commonly called
"green slime", can result from the use of chromate inhibitors with permanent type antifreeze. This material deposits on
the cooling system passages, reducing the heat transfer rate (Fig. 2) and results in engine overheating. Engines which
have operated with a chromate-inhibited water must be chemically cleaned before the addition of permanent antifreeze.
A commercial heavy-duty de-scaler should be used in accordance with the manufacturer's recommendation for this
Soluble oil has been used as a corrosion inhibitor for many years. It has, however, required very close attention relative
to the concentration level due to adverse effects on heat transfer if the concentration exceeds 1% by volume. For
example: 1-1/4% of soluble oil in the cooling system increases fire deck temperature 6% and a 2-1/2% concentration
raises fire deck temperature up to 15%. Soluble oil is not recommended as a corrosion inhibitor.
Non-chromate inhibitors (borates, nitrates, nitrites, etc.) provide corrosion protection in the cooling system with the basic
advantage that they can be used with either water or a water and permanent antifreeze solution.
An inhibitor system (Fig. 3) is a combination of
chemical compounds which provide corrosion
protection, pH control and water softening ability.
Corrosion protection is discussed under the heading
Corrosion Inhibitors. The pH control is used to
maintain an acid-free solution. The water softening
ability deters formation -of mineral deposits. Inhibitor
systems are available in various forms such as coolant
filter elements, liquid and dry bulk inhibitor additives,
and as an integral part of permanent antifreeze.
Coolant Filter Elements
Replaceable elements are available with various
chemical inhibitor systems.
Compatibility of the
element with other ingredients of the coolant solution
cannot always be taken for granted. Problems have
developed from the use of the magnesium lower
support plate used by some manufacturers in their
coolant filters. The magnesium plate will be
attacked by solutions which will not be detrimental
to other metals in the cooling system. The dissolved
Fig. 2 - Heat Transfer Capacity
magnesium will be deposited in the hottest zones of the
engine where heat transfer is most critical. The use of an aluminum or zinc support plate in preference to magnesium is
recommended to eliminate the potential of this type of deposit. High chloride coolants will have a detrimental effect on
the water softening capabilities of systems using ion-exchange resins. Accumulations of calcium and magnesium ions
removed from the coolant and held captive by the zeolite resin can be released into the coolant by a regenerative
process caused by high chloride content solutions.