Factors governing protection of steelwork
To expand uponlecture General corrosion, giving the practical means of protecting steelwork at a level suitable for young architects and engineers.
This lecture covers the assessment of the required life design for the successful use of protective systems and surface preparation. The coatings commonly used to protect steel are described, and the use of stainless and weathering steels are briefly discussed. Finally, a general discussion of maintenance is given.
1. LIFE EXPECTANCY
Table 1 classifies the principal types of environment that have a significant influence on the life expectancy of steel.
In dry, heated buildings, e.g. offices, hospitals, and warehouses, the corrosion rates of carbon steel are usually very low. Steel can even sometimes be used without protection in such environments when it is hidden. Elsewhere, it is coated for aesthetic or hygienic reasons.
Many interiors are not dry, however, and steelwork requires protection in these situations, as well as in exterior environments.
Structures and buildings usually have a 'design life.' If, after execution of the structure, access is impossible, the initial protective system needs to have the same life as the steel. Economic pressures often increase the functional life of building significantly beyond the design life. Changes in expectation usually occur after the initial protective system is in place. It is sensible, therefore, to consider this possibility at the start of every new project.
1.1 Likely time to first maintenance
Table 2 gives (in column a) typical lives in the general environment quoted to prevent deterioration of the steel using various coating systems. Column b gives the likely time to first refurbishment where good appearance and the maintenance of a readily cleaned surface are important. Neither set of figures can allow for the influence of local conditions, e.g. heavy overnight condensation due to the unplanned shutting down of ventilating systems to save money.
Protective systems require regular inspection allowing unexpected local failures to be repaired. Ideally, the base steel should never be exposed. If the first coat of the system is zinc galvanising or metal spray, then it should be considered part of the structure with the paint coats being refurbished at intervals, which ensure it remains unexposed.
1.2 Life between maintenances
When there is data on the performance of a protective system on similar structures or plant, prediction of the intervals to maintain the top coat(s) is fairly easy. Since the initial failure of a protective system may be sooner than anticipated, the estimation of the interval for some breakdown to bare steel can be complicated.
1.3 Assessment of life requirement
It may be necessary to assess each part of a structure separately. For each assessment the following points should be taken into account:
a. Required life of structure/plant
b. Decorative and hygienic requirements. The decorative life of a coating (and its ability to be readily cleaned) is rarely as long as the protective life of the system (see Table 2).
c. Irreversible deterioration if scheduled maintenance is delayed
d. Difficulty of access for maintenance
e. Technical and engineering problems in maintenance
f. Minimum acceptable period between maintenance
g. Total maintenance costs, including partial or total shut-down, closure of roads, access, etc.Read more