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SAISC TECHNICAL
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Photo 2 requires, e. g. failure of entire coating system to a specific RI condition? Many times the client says he has a guarantee but does not know what is guaranteed?( Also get a guarantee for the colour not breaking down due to UV exposure)
Preparation for the system, preparation and more preparation Those were very wise words my first boss after graduation taught me about corrosion protection. The success of the system will always be totally dependent upon how well the preparation of the steel surface has been done. By preparation we could include the following separated into those falling under the planning stage and those during the fabrication process.
1. Plan the steelwork at design and detailing stage to suit the chosen process. a. Before a draughtsman puts a finger to“ the proverbial key board” to commence the production of structural steel detail drawings he needs to understand: i. Is this an architectural finish project where‘ spit and polish’ and high quality finishes are required? ii. What corrosion protection or decorative system has been chosen for the project? iii. Will the steelwork be accessible for future maintenance work if necessary? If not, then a more comprehensive specification may be necessary. iv. Other specified requirements. b. Is continuous welding( all be it seal welds in some cases) a corrosion protection requirement, or conversely should welding be kept to an absolute minimum to prevent possible distortion which could impact on the finished product? Always remember the 6 metre rule: if a part of structure is 6 metres or more away from any seeing eye then that eye will not pick up imperfections in the finish, so it is not necessary to go crazy with the‘ spit and polish routine’.
c. If a paint system is chosen for corrosion protection, one would try to avoid welded double angle construction with a small gap between the heels of the angles because it is impossible to paint properly between these angles. This could be solved by using bolted construction and doing the full paint system before doubling up the angles or by using a‘ Tee’ bar profile to emulate the double angles. The latter is obviously preferable when future coating maintenance is required.
d. This form of construction with hot dip galvanizing can present a similar problem. Providing there is at least a 2mm gap between the two surfaces, cleaning and subsequent galvanizing will not present any problems, however, with smaller than 2mm gaps( not recommended), sealing with molten zinc may not necessarily occur and the crevice may lead to weeping of acid salts( from the previous cleaning processes) from the crevices following water quenching and cooling. Weeping of acid salts can be cleaned and if necessary after cleaning, sealed by some sealant or product such as Galvpatch or Zincfix recommended by HDGASA.
LEFT TO RIGHT: Photos 1 – 3.
e. Specifically for architectural finish hot dip galvanized steelwork requires some additional up front planning. i. It is important to ensure that the steel ordered for this purpose falls into an ideal range of so called silicon killed steel. Certain alloying elements found in steel, in particular silicon and phosphorus, depending on their percentage presence can lead to very thick coatings when hot dip galvanized. In theory from a corrosion point of view, thick coatings will have a proportionally longer life, but in practise these thick coatings can damage easily( which for technical reasons is not detrimental to the life of the system) but more importantly from a decorative point of view these thick coatings also do not oxidise their surfaces into that delightful patina( spangle finish) that architects so love. The Hot Dip Galvanizers Association of SA( HDGASA) have good advice available in this regard. ii. In the galvanizing process, steel components get dipped into various solutions as well as molten zinc. It is important not to have any enclosures in the steel component where air can be trapped preventing the liquids from doing their intended work or to trap molten zinc on withdrawal. To prevent this it becomes necessary to add suitable drain / breather holes into corners where such air entrapment can occur. This should be done by showing holes on steel detailed drawings. Once again HDGASA has good information on the subject. iii. If in doubt ask you friendly galvanizer or HDGASA to come around and advise you!
Steel Construction Vol. 40 No. 1 2016 21