21.1 General
A durable steel structure is one that performs satisfactorily the desired function in the working environment under the anticipated exposure condition during its service life, without deterioration of the cross-sectional area and loss of strength due to corrosion.
The material used, the detailing, fabrication, erection and surface protection measures should all address the corrosion protection and durability requirements.
21.2 Requirements for Durability
21.2.1 Shape, Size, Orientation of Members, Connections and Details
The design, fabrication and erection details of exposed structures should be such that good drainage of water is ensured. Standing pool of water, moisture accumulation and rundown of water for extended duration shall be avoided.
The details of connections should ensure that,
a) all exposed surfaces are easily accessible for inspection and maintenance; and
b) all surfaces, not so easily accessible are completely sealed against ingress of moisture.
21.2.2 Exposure Condition
21.2.2.1 General environment
The general environment, to which a steel structure is exposed during its working life is classified into five levels of severity, as given in Table 28.
21.2.2.2 Abrasion
Specialist literature may be referred for durability of surfaces exposed to abrasive action as in machinery, conveyor belt support system, storage bins for grains or aggregates.
21.2.2.3 Exposure to sulphate attack
Appropriate coatings may be used when surfaces of structural steel are exposed to concentration of sulphates ( ) in soil, ground water, etc.
When exposed to very high sulphate concentrations of more than 2 percent in soil and 5 percent in water, some form of lining such as polyethylene, polychloroprene sheet or surface coating based on asphalt, chlorinated rubber, epoxy or polymethane material should be used to completely avoid access of the solution to the steel surface.
| Sl No.(1) | Environmental Classifications(2) | Exposure Conditions(3) |
|---|---|---|
| i) | Mild | Surfaces normally protected against exposure to weather or aggressive condition as in interior of buildings, except when located in coastal areas |
| ii) | Moderate | Structural steel surfaces:a) Exposed to condensation and rainb) Continuously under waterc) Exposed to non-aggressive soil/groundwaterd) Sheltered from saturated salt air in coastal areas |
| iii) | Severe | Structural steel surfaces:a) Exposed to severe frequent rainb) Exposed to alternate wetting and dryingc) Severe condensationd) Completely immersed in sea watere) Exposed to saturated salt air in coastal area |
| iv) | Very severe | Structural steel surface exposed to:a) Sea water sprayb) Corrosive fumesc) Aggressive sub-soil or ground water |
| v) | Extreme | Structural steel surfaces exposed to:a) Tidal zones and splash zones in the seab) Aggressive liquid or solid chemicals |
21.2.3 Corrosion Protection Methods
The methods of corrosion protection are governed by actual environmental conditions as specified in good practices [6-6(40)]. The main corrosion protection methods are given below:
a) Controlling the electrode potential,
b) Inhibitors, and
c) Inorganic/metal coatings or organic/paint systems.
21.2.4 Surface Protection
21.2.4.1 In the case of mild exposure, a coat of primer after removal of any loose mill scale may be adequate. As the exposure condition becomes more critical, more elaborate surface preparations and coatings become necessary. In case of extreme environmental classification, protection shall be as per specialist literature. Table 29 gives guidance to protection of steelwork for different desired lives.
21.2.4.2 Steel surfaces shall be provided with at least one coat of primer immediately after its surface preparation, such as by sand blasting to remove all mill scale and rust and to expose the steel.
21.2.4.3 Steel without protective coating shall not be stored for long duration in outdoor environment.
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
21.2.4.4 Surfaces to transfer forces by friction as in HSFG connections shall not be painted. However, it shall be ensured that moisture is not trapped on such surfaces after pre-tensioning of bolts by proper protective measures.
(Clause 21.2.4.1)
| Sl No. | Atmospheric Condition/ Environmental Classification | Coating System | |||||
| 1 | 2 | 3 | 4 | 5 | 6 | ||
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) |
| i) | Normal inland (rural and urban areas), mild | 12 years | 18 years | 20 years | About 20 years | About 20 years | Above 20 years |
| ii) | Polluted inland (high airborne sulphur dioxide), moderate | 10 years | 15 years | 12 years | About 18 years | 15 - 20 years | Above 20 years |
| iii) | Normal coastal (as normal inland plus high airborne salt levels), severe | 10 years | 12 years | 20 years | About 20 years | About 20 years | Above 20 years |
| iv) | Polluted coastal (as polluted inland plus high airborne salt levels), very severe or extreme | 8 years | 10 years | 10 years | About 15 years | 15 - 20 years | Above 20 years |
21.2.4.5 Members to be assembled by welding shall not be pre-painted at regions adjacent to the location of such welds. However, after welding, appropriate protective coatings shall be applied in the region, as required by the exposure conditions. If the contact surfaces cannot be properly protected against ingress of moisture by surface coating, they may be completely sealed by appropriate welds.
21.2.4.6 Pre-painted members shall be protected against abrasion of the coating during transportation, handling and erection.
21.2.5 Special Steels
Steels with special alloying elements and production process to obtain better corrosion resistance may be used as per specialist literature.