13.07.2015 Views

In this issue: - the Institute of Corrosion

In this issue: - the Institute of Corrosion

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TECHNICALARTICLEFigure 10. Typical detail <strong>of</strong> providing good drainage to prevent commencement <strong>of</strong> corrosion cells (Image courtesy<strong>of</strong> Mac Connell Dowell)which are <strong>the</strong> catalyst for commencement<strong>of</strong> corrosion. For example if we can eliminate<strong>the</strong> formation <strong>of</strong> <strong>the</strong> electrolyte on <strong>the</strong>structures, <strong>the</strong>n <strong>the</strong> commencement <strong>of</strong>corrosion will be delayed. This means that<strong>the</strong> structural detailing should be in such away:n To maintain adequate space betweencomponents to facilitate corrosionprotection painting system to be applied;n To avoid formation <strong>of</strong> pools <strong>of</strong> water inparts <strong>of</strong> <strong>the</strong> structure at all time;n To ensure good drainage <strong>of</strong> water on steelmembers at all time;n To fully seal hollow sections;n To avoid placing steel members so <strong>the</strong>yare inaccessible for inspection; andn To insulate two metals from each o<strong>the</strong>r iftwo different metals are used in <strong>the</strong>design.Hence eliminating <strong>the</strong> formation <strong>of</strong> corrosioncells. The design should be based on <strong>the</strong>“eliminate <strong>the</strong> corrosion cells principle”.additional plate (see Figure 10) to providea sloped drainage system to eliminate <strong>the</strong>creation <strong>of</strong> corrosion cells in that area.Example 2Figure 11 shows detailing that providesgood drainage and adequate space betweencomponents to facilitate corrosion protectionpainting or galvanising;Example 3The detailing <strong>of</strong> <strong>the</strong> catwalk bottom nodesat an LNG/LPG jetty (i.e. bottom structuralchords intersect) was such that pondingwould have occurred. Hence “Megapoxy P1”was used (Figure 12) to eliminate <strong>the</strong> pondinghence elimination <strong>of</strong> corrosion cells.“Megapoxy P1” is a two component highstrength epoxy paste based on DGEBA epoxyresin and carbonate free filler. Easy to use,<strong>this</strong> product sets after mixing with excellentproperties for a wide range <strong>of</strong> applicationssuch as metal to metal bonding.Figure 11. Typical detail <strong>of</strong> providing good drainageto prevent commencement <strong>of</strong> corrosion cells (Imagecourtesy <strong>of</strong> Clough)3.0 ConclusionThis study has looked at a case study to showhow a design life <strong>of</strong> 50 years for steel marinestructures in a severe marine environmentin terms <strong>of</strong> corrosion can be achieved. Thestudy has shown that, corrosion affecting<strong>the</strong> durability <strong>of</strong> marine structures is ra<strong>the</strong>r acomplex phenomenon and <strong>the</strong>re is no singleapproach to <strong>the</strong> corrosion prevention <strong>of</strong>marine structures. An effective approach tocorrosion prevention is a combination <strong>of</strong> <strong>the</strong>following:1. Understanding <strong>of</strong> mechanism <strong>of</strong> corrosionand corrosion <strong>issue</strong>s;2. <strong>In</strong> <strong>the</strong> case <strong>of</strong> carbon steel marinestructures:n good detailing;n <strong>of</strong>fsite fabrication with good qualitycontrol; andn application <strong>of</strong> an appropriate coatingsystem that forms an impregnating typebarrier against <strong>the</strong> passage <strong>of</strong> seawater to<strong>the</strong> surface <strong>of</strong> <strong>the</strong> carbon steel structure.The common factor in all <strong>the</strong> above is <strong>the</strong>“prevent <strong>the</strong> formation <strong>of</strong> corrosion cellprinciple” which relies on <strong>the</strong> designer havingan understanding and awareness <strong>of</strong> <strong>the</strong>mechanism <strong>of</strong> corrosion and corrosion cellformation. The designer must consider <strong>the</strong>“prevent <strong>the</strong> formation <strong>of</strong> corrosion cellThe following are some examples <strong>of</strong> structuraldetails based on idea <strong>of</strong> “eliminate <strong>the</strong>corrosion cells principle”:Example 1The design was such that <strong>the</strong> top <strong>of</strong> <strong>the</strong> pile asshown in Figure 10 was sloped, hence <strong>the</strong> seawater would have been pooled and corrosioncell. The detail was modified by welding anFigure 12. Typical detail <strong>of</strong> prevention <strong>of</strong> ponding and corrosion cell (Image courtesy <strong>of</strong> Woodside)25

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