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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