Civil Engineering Reference
In-Depth Information
The procedure for painting and coating of the steel members should be car-
ried out as far as practicable in the shop, under strong supervision and with
appropriate conditions of humidity and protection from the extremes of
weather. The joint surfaces should, of course, be masked to permit welding.
It is essential to know that field coating of the joints and touch-up of shop
coats should be done only when the surfaces are dry and at the proper tempera-
ture. In some locations, portable tents or other protection will have to be pro-
vided and heaters or dehumidifiers, or both, may be required.
It is worth mentioning that coatings may delay initiation of corrosion by
10
20 years. It is extremely important that surface preparation be thorough
and in accord with the specified requirements. The offshore environment will
quickly degrade any coatings placed on damp steel or over mill scale or rust.
Morning dew can quickly degrade a well-prepared surface. DNV rules require
that the provisions for coating include the following, whcih can serve as a
checklist:
-
A description of general application conditions at the coating yard
●
Method and equipment for surface preparation
●
Ranges of temperature and relative humidity
●
Application methods
●
Time between surface preparation and first coat
●
Minimum and maximum dry-film thickness of a single coat
●
Number of coats
●
Minimum total dry-film thickness
●
Relevant drying characteristics
●
Procedure for repair of damaged coating
●
Methods of inspection (for example adhesion testing)
●
The relation between the coating breakdown as a percentage of area and the
structure lifetime, which is considered as a factor in designing the CP system, is
as shown in
Table 6.4
.
TABLE 6.4
Guide to Coating Breakdown Criteria
for Cathodic Protection Design
Coating Breakdown (% of Area)
Initial
Year
Mean
Final
10
2
7
10
20
2
15
30
30
2
25
60
40
2
40
90
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