White Metal Blast Cleaning removes all traces of rust, mill scale,
and all other detrimental matter and defects from the surface. This standard is
the most expensive and is justified only for the most corrosive atmosphere or when
special precautions are required to ensure positive paint adhesion.
Completed structural components and weldments blast cleaned to
commercial, near-white, or white metal standards, should be primed in the shop.
Those that require field welding must have their welded areas blast cleaned during
the erection. In either case, these blast cleaned areas must be primed before any
rusting can occur. The normal practice is to blast clean only as much surface as
can be primed the same day.
18.104.22.168 Paints and Application. The common paint system for crane structures are
one or two coats of primer and two or three coats of topcoat. Normal dry film
thickness of each coat is 2 to 5 mils. The dry film thickness of a primer coat
intended to protect a freshly blast cleaned surface against rusting is usually
less than 2 mils. Primers on faying surfaces of structural bolted connections
must meet the requirements of the Manual of Steel Construction (Specification for
Structural Joints Using ASTM A325 or A490 Bolts). Such paint protection is
appropriate for all service environments, but are intended mainly for use in
conditions of high humidity or marine atmosphere.
Primers with inorganic zinc offer excellent resistance to weathering and
abrasion; primers with organic zinc are tolerant of variations in surface
preparation quality, have better compatibility with topcoats, and are more
flexible than the inorganic types. To avoid any doubts about the quality of
bonding between them, the primers and topcoats should be the products of the same
manufacturer. Furthermore, the manufacturer's application (and curing)
instructions should be followed to obtain the expected performance and service
life of the paint system. (In some paint systems tie coats may be required
between zinc-rich primers and certain special topcoats.) Many zinc-rich primer
formulations exhibit high reactivity initially, but develop a layer of zinc
corrosion products that retards further zinc sacrifice until damage to the coating
exposes the surface. However, the greater the galvanic demand on the zinc, the
faster the primer coating breaks down, unless protected by the zinc corrosion
products or by the topcoats. Water ballast tanks of floating crane barges require
special paints formulated for that purpose, which may be inorganic zinc, urethane,
epoxy, or coat tar epoxy. The acceptability of these paints for the intended
service must be demonstrated by their manufacturers' laboratory testing.
Primers and topcoats are best applied by spraying, but brushing-on may be
preferred for smaller areas, touch-ups, and to force them into surface
22.214.171.124 Cathodic Protection. The immersed portions of floating crane barges
require cathodic protection to retard corrosion. A cathodic protection system,
with an automatic impressed current, is used for this purpose. It consists of
platinum-tantalum anodes and silver/silver chloride reference cells, permanently
installed on the barge exterior at least 5-feet below the waterline (lightship
condition), and a saturable reactor power supply with an automatic controller.
The system must have sufficient capacity to supply a minimum current density of 2
milliamperes per square foot to the immersed barge area. The minimum distance
between any anode and reference cell should be 40 feet.