© 2006 by Taylor & Francis Group, LLC

68 Corrosion Control Through Organic Coatings
4.1 INTRODUCTION TO BLAST CLEANING
By far, the most common pretreatment for steel constructions prior to painting is
blast cleaning, in which the work surface is bombarded repeatedly with small solid
particles. If the individual abrasive particle transfers sufficient kinetic energy to the
surface of the steel, it can remove mill scale, rust, clean steel, or old paint. The
kinetic energy (E) of the abrasive particle before impact is defined by its mass (M)
and velocity (V), as given in the familiar equation:
E = (MV 2 )/2
Upon impact, this kinetic energy can be used to shatter or deform the abrasive
particle, crack or deform old paint, or chip away rust. The behavior of the abrasive,
as that of the old coating, depends in part on whether it favors plastic or elastic
deformation.
In general, the amount of kinetic energy transferred, and whether it will suffice
to remove rust, old paint, and so forth, depends on a combination of:
• Velocity and mass of the propelled abrasive particle
• Impact area
• Strength and hardness of the substrate being cleaned
• Strength and hardness of the abrasive particle
In the most-commonly used blasting technique — dry abrasive blasting —
velocity of the blasting particles is controlled by the pressure of compressed air. It
is more or less a constant for any given dry blasting equipment; the mass of the
abrasive particle therefore determines its impact on the steel surface.
In wet abrasive blasting, in which water replaces compressed air as the propellant
of the solid blasting media, velocity of the particles is governed by water pressure.
In hydrojet blasting, the water itself is both the propellant and the abrasive (no solid
abrasive is used). Both forms of wet blasting offer the possibility to vary the velocity
by changing water pressure. It should be noted however that wet abrasive blasting
is necessarily performed at much lower pressures and, therefore, velocities, than
hydrojet blasting.
4.2 DRY ABRASIVE BLASTING
Only heavy abrasives can be used in preparing steel surfaces for painting. Lighter
abrasive media, such as apricot kernels, plastic particles, glass beads or particles,
and walnut shells, are unsuitable for heavy steel constructions. Because of their low
densities, they cannot provide the amounts of kinetic energy that must be expended
upon the steel’s surface to perform useful work. In order to be commercially feasible,
an abrasive should be:
• Heavy, so that it can bring significant amounts of kinetic energy to the
substrate
• Hard, so that it doesn’t shatter into dust or deform plastically (thus wasting
the kinetic energy) upon impact
© 2006 by Taylor & Francis Group, LLC