Views: 10 Author: Site Editor Publish Time: 2017-12-29 Origin: Site Inquire
Problems may be classified into two broad categories:
Natural or inherent problems based on the characteristics of the material and the conditions of the exposure
Vandalism and human-induced problems.
Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control.
Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur outinely.
Galvanized iron and steel's resistance to corrosion depends largely on the type and thickness of the protective zinc coating and the type of corrosive environment.
The zinc coating on galvanized iron and steel may be corroded by: Acids, strong alkalis, and is particularly vulnerable to corrosion by sulfur acids produced by hydrogen sulfide and sulfur dioxide pollution in urban atmospheres.
1.2 Natural Corrosion:
The zinc coating on galvanized iron and steel develops a natural carbonate on its surface by exposure to the atmosphere and by the action of rainwater. This coating, however, is usually not thick enough to protect the metal from further corrosion.
The carbonate can become brittle and crusty and eventually split, exposing fresh zinc for corrosion. Since the zinc coating on the iron or steel is very thin, it can corrode up to the base metal exposing the base to the atmosphere as well.
In industrial atmospheres, the zinc carbonate coating can be broken down by the same acids that attack zinc. These acids convert the carbonate to zinc sulfate, which is water soluble and washes away with rainwater, often staining the adjacent building elements.
1.3 Chemical Corrosion:
Galvanized iron and steel have good corrosion resistance to: Concrete, mortar, lead, tin, zinc and aluminum.
Galvanized iron and steel have poor corrosion resistance to: Plasters and cements (especially Portland cements) containing chlorides and sulfates, acidic rainwater run- off from roofs with wood shingles (redwood, cedar, oak, and sweet chestnut), moss, or lichen, condensation on the underside of zinc plates and ponded water on the exterior surfaces of the zinc features
Galvanic (Electrochemical) Corrosion: This type of corrosion is an electrolytic reaction between the zinc coating and dissimilar metals when in the presence of an electrolyte such as rain, dew, fog or condensation.
To prevent the corrosion of the zinc coating due to galvanic action, contact between galvanized items and copper or pure iron or steel should be avoided.
Galvanized iron and steel are corrosive to all metals except lead, tin, zinc and aluminum.
Applying a protective coating such as paint to galvanized iron and steel will alleviate the problems caused by corrosion of the protective zinc coating.
2. Vandalism or Human-induced Problems
2.1 Mechanical or Physical Deterioration:
Causes removal of the protective metal surface. The soft zinc coating on galvanized iron and steel make it vulnerable to abrasion damage, especially at roof valleys and gutters where the coating can be worn paper-thin by the scouring of rainwater.
Fatigue: A type of deterioration caused by cyclical expansion and contraction of sheet metal features, especially roofs, without adequate provisions for this movement.
Zinc is very vulnerable to fatigue failure because it has a relatively high coefficient of thermal expansion.
Fatigue failure may also occur when the metal sheets are too thin to resist buckling and sagging. It results in the bulging and tearing of the zinc coating and resembles a cut or a crack.
Creep: The permanent distortion of a soft metal which has been stretched due to its own weight. Thin areas of the metal are especially prone to failure. Creep may be prevented by the use of properly sized individual sheets and bays, properly designed joints, and an adequate number of fasteners.
Distortion: Permanent deformation or failure may occur when a metal is overloaded beyond its yield point because of increased live or dead loads, thermal stresses, or structural modifications altering a stress regime
2.2 Connection Failure:
Wind and thermal stress can damage a roof by pulling joints apart and loosening fasteners.