Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.125
rimental to both the steel and the paint. Through electrochemical action, corrosion
sets in along the edges of the cracks in the mill scale and in time loosens the scale,
carrying away the paint.
Galvanic corrosion takes place when dissimilar metals are connected together.
Noble metals such as copper and nickel should not be connected to structural steel
with steel fasteners, since the galvanic action destroys the fasteners. On the other
hand, these metals may be used for the fasteners, because the galvanic action is
distributed over a large area and consequently little or no harm is done. When
dissimilar metals are to be in contact, the contacting surfaces should be insulated;
paint is usually satisfactory.
7.45 PAINTING STEEL STRUCTURES
Evidence obtained from dismantled old buildings and from frames exposed during
renovation indicates that corrosion does not occur when steel surfaces are protected
from the atmosphere. Where severe rusting was found and attributed to leakage of
water, presence or absence of shop paint had no significant influence. Consequently,
the AISC ‘‘Specifications for Structural Steel for Buildings’’ exempts from onecoat
shop paint, at one time mandatory, all steel framing that is concealed by interior
finishing materials—ceilings, fireproofing partitions, walls, and floors.
Structures may be grouped as follows: (1) those that need no paint, shop or field;
(2) those in which interior steelwork will be exposed, probably field painted; (3)
those fully exposed to the elements. Thus, shop paint is required only as a primer
coat before a required coat of field paint.
Group (1) could include such structures as apartment buildings, hotels, dormitories,
office buildings, stores, and schools, where the steelwork is enclosed by other
materials. The practice of omitting the shop and field paint for these structures,
however, may not be widely accepted because of tradition and the slowness of
building-code modernization. Furthermore, despite the economic benefit of paint
omission, clean, brightly painted steel during construction has some publicity value.
In group (2) are warehouses, industrial plants, parking decks, supermarkets, onestory
schools, inside swimming pools, rinks, and arenas, all structures shielded from
the elements but with steel exposed in the interior. Field paint may be required for
corrosion protection or appearance or both. The severity of the corrosion environment
depends on type of occupancy, exposure, and climatic conditions. The paint
system should be carefully selected for optimum effectiveness.
In group (3) are those structures exposed at all times to the weather: crane
runways, fire escapes, towers, exposed exterior columns, etc. When made of carbon
steel, the members will be painted after erection and therefore should be primed
with shop paint. The paint system selected should be the most durable one for the
atmospheric conditions at the site. For corrosion-resistant steels, such as those meeting
ASTM A242 and A588, field painting may be unnecessary. On exposure, these
steels acquire a relatively hard coat of oxide, which shields the surface from progressive
rusting. The color, russet brown, has architectural appeal.
7.46 PAINT SYSTEMS
The Steel Structures Painting Council has correlated surface preparations and primer,
intermediate, and finish coats of paints into systems, each designed for a