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CorrosionManagement | March/April 2012TECHNICALARTICLEto it. Detailing is important to ensure thatthe protective treatment can be applied toall surfaces. Narrow gaps, difficult to reachcorners, and hidden surfaces should beavoided whenever possible. Details that couldpotentially trap moisture and debris, whichwould accelerate corrosion, should also beavoided”.Clause 59 of BS 6349: Part 1 1 provides generalguidance on the use of structural steel andother metals in marine structures and theimportant points to note are as follows:Figure 6. Typical detail of berthing dolphin where reinforced concrete pile cap is supported by piles(Image courtesy of Mac Connell Dowell)n Fabrication details should be kept as simpleas possible and should be designed to avoidcorrosion and facilitate maintenance;n Tolerances for on-site connections shouldbe generous because of the difficultiesassociated with working in a marineenvironment;n As much prefabrication as possibleshould be undertaken, taking advantage ofmechanised welding and early paintingunder factory-controlled conditions; andn Maintenance strategy is developed toensure that periodic inspection is carriedout to enable corroded or otherdeteriorated members to be identified anddealt with at an early stage before itaffects the integrity of the structure.Penetration, retention or entrapment ofmoisture or water can be the result of manyscenarios. Examples are provided in figures5, 6 and 7 where water will be trapped andcorrosion will commence by forming acorrosion cell if good design detailing is notcarried out.Example 1: Figure 5 shows a typical gussetfree weld connection for a truss where thedesigner 28 has only shown the weld lengthsto resist the applied load but the joint asit is shown is prone to crevice corrosionas the connection has not been sealed bythe weld. The solution is to fillet weld theremaining part in order to seal the joint allaround.Figure 7. Bolt connection of top to base module on site (Image courtesy of Fortescue Metals Group(FMG) and Mac Connell Dowell)Example 2: Figure 6 shows a typical detail of aberthing dolphin where a reinforced concretepile cap is supported by piles. Possiblecrevice corrosion may occur at the pointof encapsulation where steel is capped byconcrete as there is no bond between themand the members are constantly exposed towetting and drying conditions. The solution iseither to coat the outer surface of steel pileright to the top and 50mm return inside thepile or adopt the reinforced concrete insitu filloption to the top 2 to 3m of pile and makethe section of the pile sacrificial.Possible crevice corrosion between steel pileand concrete cap.Example 3: Figure 7 shows a bolted connectionof a module to its base plate on site. Possiblecrevice corrosion may occur between the baseplates or bolts and base plates. The solution isto use a:n Membrane between the coated base platesso that the gap between the steel plates issealed; andn Non metallic and heat and UV resistantwashers in the bolts so that the gapsbetween the bolts and the base plates aresealed.By sealing the gaps the penetration,retention or trapping of moisture or watercan be prevented, hence the creation of acorrosion cell.2.1.3.4 Steel Membersconstantly exposedto wetting and dryingconditionsExposing the steel members to constantwetting and drying condition (i.e. splash zone)will result in formation of corrosion cells thatlead to aggressive and localised corrosionand has been the subject of intense study inrecent years 29,3,30 & 31 .Corrosion in the splash zone normallyencompasses several exposure zones ofdiffering aggressivity 3 & 30 and the corrosionperformance of marine structures in thesezones requires specific considerationwhen designing the marine structures. Theclassification of the zones is shown in figure8 and they are:Figure 5. Typical gusset free weld connection for truss (Image courtesy of Steel Designers’ Handbook, 6th Edition by B Gorenc, R. Tinyou and A. Syam, 1996 28 ).n Atmospheric Zone: The atmospheric zoneis located above the splash zones wherethe area is exposed to salt ladenatmosphere;22
TECHNICALARTICLEn Splash Zone: The splash zone is locatedbetween the tidal and marine atmosphericzones (i.e. above mean high-water);n Tidal and Low Water Zones: Tidal zone liesbetween the mean low water and meanhigh-water tidesn Submerged Zone: This is the zone that liesbetween the seabed level (includingscouring zone) and 600mm -700mm belowthe mean low water; andn Below the seabed level: Where structure isbelow the seabed-level.2.1.3.4.1Corrosion inAtmospheric ZoneThis is the area between the top of thestructure and the splash zone. The corrosionof an unprotected steel structures in this zoneis generally less than in the splash zone (Figure8) but it is still significant. The corrosionand degree of severity in the atmosphericzone depends on several variables such ashumidity, temperature, chloride content, windand sunlight 32 . For example saline particlesin the atmospheric zone accelerate metalliccorrosion process as chloride increases thesolubility of the corrosion products. It is alsoreported that marine chloride dissolved in thelayer of moisture also raises the conductivityof the electrolyte layer on the metal andtends to destroy the passive film existing onthe metallic surface 33 . Recent study has alsodemonstrated that bacteria are involved inthe longer term corrosion of mild steel in theatmospheric zone. 342.1.3.4.2 Corrosion inSplash Zone and BelowWaterlineA common localised and aggressive type ofcorrosion that occurs in splash zone and belowwaterline is defined as Accelerated Low WaterCorrosion (ALWA) and its existences wasn’twidely recognised until 1980s 30 . ALWA (Figure8) is aggressive and localised and occurs onthe surface of steel maritime structures in thetidal zones 30 .The detailed mechanism of ALWC is still notvery clear and continues to be a matter ofsome debate 3 & 30 but it is said it is to be oneform of Microbiologically Influenced Corrosion(MIC) 3 & 30 also known as microbial corrosionor biological corrosion. It is the deteriorationof metals as a result of the metabolic activityof microorganisms.Figure 8. Corrosion profile of an unprotected steel structure in seawaterIt has been reported that when metal is firstimmersed in seawater, a thin biological slimefouling layer tends to develop in a matterof hours, which itself may influence thesubsequent settlement of micro and macrofouling organisms 35 . Following the micro-ormacro fouling organisms attachment, largedifferences in oxygen concentration or pHchanges can be created on metal surfaceswhich results in localised corrosion 36 .In general the intensity of corrosion of anunprotected steel structure in seawater variesmarkedly with position relative to the meanhigh and low tide level 37 as shown in figure8. It states that 37 the splash zone is the mostseverely attacked region due to continuouscontact with highly aerated sea water andthe erosive effects of spray, waves and tidalactions.Corrosion in the idal zone (i.e. between MLWand MHW) is usually slow and uniform 3 . Thearea tends to accumulate dense barnaclegrowths with filamentous green seaweeds. Themarine growth does not corrode the materialbut makes the carbon steel more noble.Corrosion in the Low water zone is relativelysevere due to differential aeration at theuppermost point of continuous steelimmersion, where electrolyte is permanentand oxygen levels peak 3 , as well as that thelow water zone is acting anodic relative to thetidal zone.The area approximately 500mm below meanlow water, where severe corrosion rates areoften experienced is subject to AcceleratedLow Water Corrosion (ALWC) and PIANC 3summaries the corrosion mechanism of ALWCby stating that ALWC occurs in the presenceof sulphates in marine environments, whichare converted by sulphate-reducing bacteria(SRB) into hydrogen sulphide (H2S) thatcauses direct anaerobic corrosion of steelsurfaces. The H2S generated in this metabolicprocess also serves as a food source for23
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