© 2006 by Taylor & Francis Group, LLC

74 Corrosion Control Through Organic Coatings
Sharp [8] lists nitrites, amines, and phosphates as common materials used to
make inhibitors. He notes problems with each class:
• If run-off water has a low pH (5.5 or less), nitrite-based inhibitors can
cause the residue to form a weak but toxic nitrous oxide, which is a safety
concern for workers.
• Amine-based inhibitors can lose some of their inhibitive qualities in lowpH
environments.
• When using ultra-high pressure, high temperatures at the nozzle (greater
than 140°F [60°C]) can cause some phosphate-based inhibitors to revert
to phosphoric acid, resulting in a contaminant build-up.
In the 1966 edition of the manual Good Painting Practice, the Steel Structures
Painting Council recommended an inhibitor made of diammonium phosphate and
sodium nitrite [9]. Other possibilities include chromic acid, sodium chromate,
sodium dichromate, and calcium dichromate. The 1982 edition of this manual does
not make detailed recommendations of specific inhibitor systems [1].
Van Oeteren [10] lists the following possible inhibitors:
• Sodium nitrite combined with sodium carbonate or sodium phosphate
• Sodium benzoate
• Phosphate, alkali (sodium phosphate or hexametasodium phosphate)
• Phosphoric acid combinations
• Water glass
He also makes the important point that hygroscopic salts under a coating lead to
blistering and that, therefore, only inhibitors that do not form hygroscopic salts
should be used for wet blasting.
McKelvie [11] does not recommend inhibitors for two reasons. First, flash rusting
is useful in that it is an indication that salts are still present on the steel surface; and
second, he also points out that inhibitor residue on the steel surface can cause blistering.
The entire debate over inhibitor use may be unnecessary. Igetoft [12] points out
that the amount of flash rusting of a steel surface depends not only on the presence
of water but also very much on the amount of salt present. The implications of his
point seem to be this: if wet blasting does a sufficiently good job of removing
contaminants from the surface, the fact that the steel is wet afterward does not
necessarily mean that it will rust.
4.3.3 ADVANTAGES AND DISADVANTAGES OF WET BLASTING
Wet blasting has both advantages and disadvantages. Some of the advantages are:
• More salt is removed with wet blasting (see 4.3.4).
• Little or no dust forms. This is advantageous both for protection of
personnel and nearby equipment, and because the blasted surface will not
be contaminated by dust.
© 2006 by Taylor & Francis Group, LLC