WÄRTSILÄ VASA 6L32, Instruction Manual

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Corrosion rate as a function of nitrite concentration
Nitrite based cooling water additives are so called anodic inhibitors
and require proper dosing and maintenance in order to serve as
intended. The nitrite of the additive is as such a salt and it will increase
the conductivity of the water. The conductivity is on the other hand
one of the main parameters affecting the corrosion rate once a
corrosion process gets started, the higher the conductivity the
higher the corrosion rate.
If the conditions (nitrite level, chlorides, pH, etc.) in the systems are
such that the nitrite based additive is no longer able to protect the
entire surface of the system there may occur a rapid, local corrosion
in the areas that are not protected. The corrosion rate at the
attacked areas will even be much greater than it would be with no
additive at all present in the system, see schematic graph of the
corrosion rate as a function of the nitrite dosage in Fig 02-4. Observe
that the position of the curve peak on the x-axis (= dangerous
condition for corrosion) is not stable, but will shift depending on
temperature, pH, chlorides and sulphates contents, etc. in the
cooling water.
The table below shows shows examples of the most common
cooling water additive types.
B
A
Corrosion r
ate
X ppm
Nitrite Concentration
To give full protection the Nitrite level
should be kept above X ppm. The
actual concentration is additive
supplier dependent.
A permanent lower level will lead to
an accelerated corrosion rate.
Fig 02-4
3202602002
200549-01 Fuel, Lubricating Oil, Cooling Water
W20/W32/W46, W46F, 32, 32LN 02 - 21