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CorrosionManagement | March/April 2012TECHNICALARTICLE2.1.1.3 Corrosion RateClassification for CarbonSteelWhen a designer is selecting a constructionmaterial, the corrosion rate figures for thematerial must be known so that the designercan determine how much sacrificial corrosionallowance is required when a preventivemeasure to corrosion is designed. Differentmaterials have varying corrosion ratesdepending on specific environments.Table 25 in BS6349 1 “Code of Practice forMaritime Structures” ( table 1 below) providescorrosion rates for unprotected structuralsteels in temperate climate in mm/side/year.Hong Kong Port Works manual 13 followsBS6349 1 with the exception that the corrosionin splash and below seabed zones are 0.5 and0.04 mm per year respectively.2.1.2 Influence of MarineEnvironment on CorrosionDavis et al states that Marine Environmentalconditions do influence the corrosion of marineFigure 2. Example of Modular Construction (Image courtesy of McConnell Dowell).Factorsstructures, and there is quite often a definiterelationship between the corrosion rate andthe protective current density required toprevent corrosion 14 . For example a cathodicprotection system designed for an offshoreplatform or drilling rig in the Gulf of Mexicomay not operate with the desired results inthe North Sea area. A cathodic protectionsystem designed for corrosion preventionon a structure in open sea water would notperform satisfactorily for the same structurein brackish water (i.e. mixtures of fresh andsalt waters found in tidal bays and estuaries)or polluted waters. Therefore in order tounderstand how the marine environment mayinfluence corrosion, it is important to havean understanding of the environment of thestructure and whether it will have an additionalaffect on the corrosivity of the structures asSeawater becomes more corrosive due to itshigher conductivity and chloride ion contentcompared to say Brackish waters 15 .There is also a degree of agreement that thecorrosion of carbon steel and its severity inthe marine environment depends on manyparameters including temperature, dissolvedoxygen content, salinity, water chemistry,pH, biofouling, fluid velocity and surfaceAverage(mm/side/year)Upper limitvalues (mm/side/year)Atmospheric zone – above splash zone andwhere direct wave or spray impingement is0.04 0.10infrequent (i.e. in the dry)Splash zone – above mean high water springlevel0.08 0.17Tidal zone – between mean high water and meanlow water spring level0.04 0.10Intertidal low water zone – between low waterspring and 0.5m below lowest astronomical tide0.08 0.17Continuous immersed zone – from 0.5m belowlowest astronomical tide to seabed level0.04 0.13Below seabed level - 0.015 max.Table 1: Corrosion rates of carbon steel in marine environment as per BS6349 table 25 116 & 17roughness, but the current velocity andtemperature variations particulary affect thecorrosion rate. For example some work hasshown that 18 the corrosion rate of carbonsteel is linear up to water velocity of 2 m/sand non linear between 2 and 6 m/s byusing an experimental method of holding acarbon steel pipe 150mm long by 12.5mmto 50mm diameter in a recirculating systemwith seawater at 23°C. There is a monotonicdecrease in the rate as water velocityincreases further while others 19 report thattheir field test data shows that the currentvelocity increases the corrosion rate of carbonsteel under tidal conditions non linearly up tomaximum current velocity of 1 m/s where1 m/s was the peak current velocity of thetest field location. However, the comparisonresults show that the overall corrosion loss inboth cases are similar so the overall designapproach will not be affected.As a general principle the corrosion rate alsoincreases with temperature but other variablessuch as oxygen concentration and biologicalactivity must also be considered 17 .2.1.3 Basis and Mechanismof Corrosion in CarbonSteel and how it relates topreventive measuresCorrosion manifests itself in many ways so anunderstanding of the basis and mechanismof corrosion and how and when it occurs willallow the designer to provide a design solutionto corrosion problems in marine environmentin order to achieve the intended design life ofthe structure.Unlike the Oil and Gas industry, a corrosionengineer is not usually part of the designteam of marine structures except for designof cathodic protection systems where acorrosion engineer is employed. Hencethe whole design process is designed andmanaged by a maritime engineer often withlittle knowledge of basis of corrosion exceptthat of code requirements. Zaki Ahmad statesthat 20 it is important the designer have anunderstanding and awareness of “corrosion”problems and more often, more attentionis paid to selection of “corrosion” resistantmaterials for specific environment, and aminimal consideration is given to design anddetailing.Therefore in order to understand therequirements for corrosion protection andto make an appropriate judgement on thecorrosion and corrosion prevention of marinestructures we need to have an appreciation of20
TECHNICALARTICLEhow the corrosion cells are created which leadto corrosion. 21A basic corrosion cell consists of:n Anode site, where corrosion occurs andfrom which current flows;n Cathode site, where no corrosion occursand to which current flows;n Electrolyte, a medium capable ofconducting electric current by ioniccurrent flow (i.e. water, or saturatedconcrete);n Metallic path, connection between theanode and cathode, which allows currentreturn and completes the circuit.and they are all required for corrosion tooccur. The driving force behind corrosion isa corrosion cell that is electrochemical innature; that is, involves chemical reactionsand the transport of electrons. The drivingforce behind the corrosion cell is a potentialor voltage difference between the anode andcathode, which leads to corrosion to occur.The corrosion cell (i.e. potential difference)can be created if:n Unprotected steel surface is in contactwith electrolyte which could be either afilm of moisture or seawater;n Dissimilar metals are in contact or joinedtogether;n A detail exists that allows penetration,retention or trapping of moisture or water;andn Steel member is constantly exposed towetting and drying conditionsThe above are some examples of the corrosioncell creators in their simplest form and if theformation of corrosion cell is prevented, thencorrosion can be prevented and the intendeddesign life of the marine structure can beachieved. We name this approach “prevent theformation of corrosion cell principle”. In orderto accomplish this, the designer must havean understanding and awareness of corrosioncell formation during the design and detailingprocess of the structure. We will explore thisin some detail to understand the mechanismand formation of the corrosion cell and how itcan be prevented.2.1.3.1 Unprotected steelsurface in contact withelectrolyteAtmospheric zone adjacent to jetty is ladenwith wind-swept fine particles of sea salt(i.e. sodium chloride) that is highly corrosiveand deposits on nuts and bolting and surfaceof the structure surfaces 22 & 23 . The sodiumchloride could be dispersed either as liquidaerosol or dry particles 24 , and it will providecatalyst for corrosion which is known to occurat relative humidity as low as 35% 25 .When the unprotected steel surface is contactwith an electrolyte such as seawater or a filmof moisture containing dissolved salts, anelectrochemical process occurs which leadsto corrosion 26 . All metals in contact with anelectrolyte have an electrochemical potentialthat is specific for the metal/electrolytecombination. Different metals have differentpotentials in a specific electrolyte. At thesurface of one metal in an electrolyte, thereare anodic and cathodic areas, which havesmall differences in potential. They form activeelectrochemical cells in which current flowsfrom the anodic areas into the electrolyte, andfrom the electrolyte into the cathodic areas.The prime example of this scenario is theprimary and secondary steel members of ajetty structure and if it is left unprotected, orbadly protected, cell corrosion will be createdon its surface which will lead to corrosion.Hence to prevent the creation of the corrosioncells, all the primary and secondary membersas shown in Figure 2 are coated with an anticorrosionsystem to prevent the creation ofthe corrosion cells.2.1.3.2 Dissimilar metalsare in contact or joinedtogetherAlthough dissimilar metals such as stainlesssteel anchor bolts aren’t encouraged for usein marine environment, there are places thatstainless steel anchor bolts are used due totheir higher yield strength. Where there arestainless steel anchor bolts in concrete, there ispossibility of corrosion of the bolts, base plateor embedded carbon steel reinforcement andit is a concern for the designers as cathodicprotection. Therefore proper detailing ofthe holding down bolts by understandingthe corrosion mechanism is the key for thepreventing corrosion in the structure.Due to the nature of this Jetty Structureand its design all these details are in theatmospheric zone of the structure and figures3 and 4 provide two examples of stainlesssteel holding bolts.2.1.3.3 A detail that allowspenetration, retention ortrapping of moisture orwaterGood design detailing is the first and mostimportant step in corrosion control andimproper detailing will create a corrosioncell in maritime structures that will leadto corrosion. The codes for steel structuresprovide general guidance on use of structuralsteel and good detailing but do not providean in depth knowledge of the subject. BS ENISO 12944-3 guidance for the prevention ofcorrosion by good design 27 detailing statesthat “the design of a structure can affect thedurability of any protective coating appliedFigure 3. Stainless steel holding down bolts of the concrete deckguard rails (Image courtesy of Mac Connell Dowell)are not isolated.are not isolated.are not isolated.Figure 4. Detail of holding down bolts for a stanchion (Image courtesy of Mac Connell Dowell)if the stainless steel boltsif the stainless steel boltsif the stainless steel bolts21
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