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Preparing steel
Precautions against corrosion
It is the duty of the designer to determine the specification most suitable for the project in hand.
High humidity or a corrosive atmosphere can exist or be created internally and these should be recognised as special situations.
Class A - Seaboard and Industrial Areas:
Where a building or structure is to be erected within 8km of the sea, a salt water inlet, or an industrial plant likely to emit pollutants, such as gas works, chemical plants, steel mills and oil refineries.
Class B - Areas other than those covered by Class A.
Class A conditions
Preparation
All structural and /or decorative steelwork, including cleats, bolts, fixings and other parts should be cleaned as follows:
- Grease and oil should be thoroughly removed by washing with a suitable solvent;
- Mill scale, rust, dirt and /or other deleterious substances should be removed by grit or shot blasting to a Class "3" standard in accordance with the appropriate standard;
The profile of blasting should not exceed 0.04mm or be less than 0.025mm.
Surfaces for both inorganic and organic zinc rich primers should be prepared as indicated in (1) and (2) above.
All surfaces should be thoroughly dry and free from dirt, dust and other deleterious matter before the coating is applied.
Coating
Immediately after preparation and within a period of 20 minutes, apply a zinc rich primer to an average thickness of 0.07mm (minimum 0.06mm and maximum 0.075mm).
The primer should be organic or inorganic containing the appropriate quantity of zinc to give an acathodically protective system and have one of the following bases:
- Epoxy resin - one or two part
- Self curing ethyl silicate
The primer should be spray applied and air cured except where the designer approves the substitution of brushing for spraying.
A coat of tar epoxy to a dry film thickness of not less than 0.8mm may be used in lieu of zinc primer on steelwork that is concealed from view.
After erection all damaged portions should be cleaned back to near white metal and made good to match the original coating in thickness, texture and colour, using a primer compatible with the original coating.
For decorative treatment of exposed steelwork refer to the Painting section of the Specification.
Class B Conditions
Materials
Red oxide zinc chromate (ROZC) should comprise a polyurethane oil or vinyl copolymer vehicle with a minimum of 20% zinc chromate and not less than 6% red iron oxide.
Preparation and application
Grease and paint should be thoroughly removed by washing with a suitable solvent.
ROZC should be applied by spray over a grit blast Class 2.5 ASCL 9.4 or acid de-scale process, to a minimum thickness of 0.05mm. Wire brushing by any means is unacceptable as a preparation.
When acid de-scaling is used all faying or abutting surfaces must be fully sealed by welding so as to exclude the possibility of acid lodging in cavities and interstices.
The specification for Class B conditions should not, under any circumstance, to be applied to Class A conditions.
