27 July 2008

Protective Coatings and Paints -02 Design Consideration

In the very first construction phase while the facilities still in conceptual design, we should keep in mind to considering corrosion affected by design. Although functional, safety, contructable and regulation requirement which the lead of design matter, we should put corrosion factor as well as previous consideration. Poor design of structures may accelerate deterioration or interfere with protection by coatings.

There are some examples of poor designs are listed below:
a) Water traps
Since water greatly accelerates deterioration, structures should be designed so that water is not collected. For example, angle irons and other configurations that can collect water should be oriented downward rather than upward. Weep holes of sufficient size should be placed where water collection cannot be otherwise avoided. Condensate water from air conditioners should not be allowed to run or drip on surfaces and steam or other vapors should not be allowed to impinge on surfaces.

b) Crevices
Crevices should be avoided in structures, because these oxygen-deficient areas accelerate metal corrosion. Thus, continuous welds should be used rather than skip welds. Back-to-back angles should also be avoided for this reason.

c) Rough and sharp surfaces
To obtain protection of a surface, a painter must be able to apply a uniformly thick, continuous film. Thus, irregular surfaces such as welds and other projections should be ground smooth to eliminate projections through the paint film. Weld-spatter, which is loosely-bonded to the steel, must be removed for two reasons. First, it creates crevices which lead to crevice corrosion and, second, as the mill scale becomes disbonded, the barrier film will be broken.
Sharp edges should be rounded (1/8 inch or more radius is ideal) because a uniformly thick coating cannot be applied over the edge. This is because wet coatings draw thin on them. Sharp interior corners should be avoided, since they may receive an excessive coating thickness.

d) Limited access
Areas to be protected should be readily accessible for inspection and maintenance. Difficultto-reach areas are not only difficult to prepare and coat, but reaching them may also constitute a safety hazard.

e) Incompatible environment
Materials must be compatible with the environment in which they are located. Thus, aluminum should not come into direct contact with concrete, because the alkalinity of the concrete will attack the aluminum.

f) Contact of dissimilar metals
Dissimilar metals probably present the biggest design problem. Because they have different corrosion potentials, they may corrode rapidly when in contact with each other. Examples of dissimilar metal (galvanic)
corrosion are:
* Steel pipe passing through an aluminum deck
* Steel nut on a copper valve
* Aluminum stanchion with a bronze lifelined) Aluminum plate with steel or different alloy aluminum rivets
* Mild steel attached to stainless steel Dissimilar surface conditions (e.g., threads, scratches, etc.) may also cause galvanic corrosion.

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