Corrosion control checklist - National Physical …

1 Corrosioncontrolchecklist The National Physical Laboratory is operated on behalf of the DTI by NPL Management Limited, a wholly owned subsidiary of Serco Group plcCORROSIONSTOP IT NOW!4590/DOC 9/1/03 4:54 am Page 31 Corrosion control begins during designThe cost Failure to control Corrosion can lead to: increased costs reduced safety negative environmental impactCorrosion costs the United Kingdom a significant percentage of its GNP. Engineering employs similarmaterials world-wide, therefore comparable proportionsof Corrosion costs are found in other industrialised countries. It has been estimated that a quarter of all Corrosion problems could be prevented easily byusing well established techniques. The economic,social or ecological consequences of major corrosionfailures can be ruinous. Corrosion increases runningcosts and reduces plant efficiency, availability and product quality.

2 The causeCorrosion results from the chemical interaction of a metal with its environment and may beuniform in nature or localised. Physical and chemicalconditions of the environment influence both the rateand type of attack, the nature of Corrosion products, the way in which metallic properties themselves are affected, and the Corrosion control measures needed. Advances in materials technology The continueddevelopment of material technology and monitoring and control methods mean the potential for saving Corrosion costs is higher today, and that for favourable applications very much greater savings are now achievable. In some types of project, uncontrolled Corrosion could amount to 30% of the total commitmentof capital expenditure. Design awareness and the life-cycle Good control of Corrosion requires the awareness and co-operation of the entire design team, including engineers anddesignersnot only in each specialised discipline but in project management and cost control .

3 Adequatemeans for collecting, reporting and recording Corrosion information from operational situations must also 9/1/03 4:54 am Page 1 Multi disciplinary activityThe design of most functional components and systems is a multi-disciplinaryactivity. Large engineering design contractors integratethe materials, welding, painting and Corrosion controlfunctions into the design team on a project basis. Thisallows decisions on materials and Corrosion control to be implemented from the earliest stages of design andconstruction, and ensures all materials and corrosioncontrol requirements are adequately documentated. Insmaller organisations this is commonly achieved throughindependent materialsand Corrosion the life-cycle of a metallic fabrication. Due attention to design, choice of materials and manufacturing processes, transportation,storage, start-up, operation, inspection, maintenanceand shut-down, will reduce the cost of Corrosion and risk of failure.

4 Effective and timely consideration ofdesign, Corrosion monitoring, Corrosion control and theconsequences of Corrosion failure not only reduce theextent and cost of Corrosion but enhance the safety,functionality, useful lifetime and appearance of the fabrication. This checklistis not for dealing with Corrosion after it has happened, but for preventing Corrosion at the designstage. 24590/DOC 9/1/03 4:54 am Page 2 Design considerationsFactors that can influence Corrosion :EnvironmentChemicalNaturalStor age, transitStressResidual stress from fabricationStatic, variable and alternating operating stressesShapeJoints and flanges Crevices and depositsTrapped and contained liquid CompatibilityMetals with metalsMetals with other materialsQuality control of materialsMovementFlowing fluidsParts moving in fluidsTwo- and three-phase flowEntrained solidsVibration and pulsingTemperatureOxidation, scales and tarnishesHeat-transfer effectsMolten depositsCondensation and dewpointsControlSurface cleaning and preparationCoatingsCathodic protectionInhibitorsData loggingInspectionPlanned maintenance34590/DOC 9/1/03 4.

5 54 am Page 34 Rule of thumb For many aqueous Corrosion reactions a 30 C temperature changecould result in a 10-fold change in Corrosion rate, while a 20 C temperature change will approximately double the Corrosion rate. Chemical industrialSea waterimmersedRuralLowertemperatureHigher temperatureCorrosion rate increasingMarine splash zone Urban/Automotive Splash zone High tideLow tideMud-lineDepth of immersionCorrosion rate mm/yearMetalIndustrial AirMarine AirRural Air(microns/year)(microns/year) different types of atmosphere affect Corrosion rateswith two years exposure (Note: 10 microns = mm) Corrosion rates and influence of aggresiveenvironmentsCorrosion rates at different positions for steel partly immersed in sea water4590/DOC 9/1/03 4:54 am Page 45 EnvironmentThe type of environment to which a material is exposedis a major factor in determining the type and severity ofcorrosion undergone.

6 Minor constituents of the surroundings can be critical, aggravating or suppressingcorrosive attack. Geographical location is also important,as is the influence of weather and season at the time ofconstruction, repair, environmentsAirRural (farming, fertilisers, ammonia)Coastal, marine (chloride)Industrial, urban (sulphur dioxide, soot and acid rain)WatersRivers, lakes, wells, water pipelines, dams; sea water(tidal, "splash zone"), estuaries, harbours (in most ofthese, chlorides are significant contaminants, as well as a variety of corrosive biological agents).SoilsClays, chalk, loam; drainage; acidity; access of air; bacterial action (sulphate reducing bacteria).Chemical environmentsUrban and industrial atmospheres Environmental pollutants creating aggressive mistsInterior of reaction and storage vessels in chemical environmentsEmbedded metals may react with non-metallic materialssuch as wood, plastics, and concrete.

7 The porosity ofsome of these media allow contact with stagnant temperature usually causes increased ratesof Corrosion . A notable exception is when temperatureincrease reduces relative humidity or dries out the radiationSunlight, ultraviolet light and thermal radiation, can activate short-lived reactive chemical species from contaminants in the environment, aggravating 9/1/03 4:54 am Page 56 Cracks from residualstresses in poorfabrication (weld).CracksStressCorrosiveenvironmentC racksStress corrosionSodiumchloridesolutionWaterAir fatigue limitNumber of cycles350300250200105106107 Maximum stress (N/mm2)AirEffect of environment on fatigue behaviour of mild steelCommon locations for stress Corrosion cracking4590/DOC 9/1/03 4:54 am Page 6 StressGeneral or uniformcorrosion causes progressive loss ofcross-section and increased stress levels, while unevenand localised Corrosion causes high stress should control extremes of stress and environmentarising at any time in the life-cycle of the about the following.

8 Residual and thermal stresses arising from fabrication Transportation shocks and vibration Installation Performance and pressure testing, leading to over-stressing, test fluids Corrosion Operating and fault conditions Maintenance, including effects of modifications and repairsStatic stressesThe environment both inside and outside a structure can affect the strengths of the materials specified. Foreach alloy, stress Corrosion Cracking(SCC) requiresspecific environmental corrosive conditions, with thealloy subjected to a critical tensile stress (residual orapplied). SCCis very dangerous because of the progressive nature of the cracking, with ever rising risks of rupture, leakage, or loss of function. Fluctuating stresses and Corrosion fatigueIn a corrosive environment the stress level at which itcould be assumed a material has infinite life is lowered or removed completely.

9 Corrosion fatigue and frettingare both in this class. Much lower failure stresses andmuch shorter failure times can occur in a corrosive environment compared to a situation where the alternating stress is in a non-corrosive effects and stressAtomic hydrogen released by cathodic reactions duringcorrosion can enter metals, causing several significanttypes of damage. Hydrogen has a strong tendency totravel along stress gradients and accumulate wherethere are high tensional stresses. By several mechanisms,this can cause cracking in may also beintroduced during welding or due to cathodic 9/1/03 4:54 am Page 7 Design tips Good inspection will often minimise premature Corrosion traps and provide access for cleaning. 8 CrevicesDepositsMetalCorrosionDebris, dirt, particulatesDrainageAccessibilityCrevice WeldAccessPoorLiquid trapsImprovedCorrosionCreviceCreviceCrev iceCreviceCrevice4590/DOC 9/1/03 4:54 am Page 8 Shape"Trap" geometry is poor geometry.

10 Corrosion may befostered by retention of liquids or contact of the metalsurface with a local environment differing from thatexpected by the designer. Features shown oppositeencourage poor drainage, possibilities for solution concentration, differential aeration and poor access for inspection, repairs or about the effect of the following ..AccessInspection and maintenance requires adequate accessto all parts of a structure. Design must allow for easyreplacement of material where the risk of Corrosion andthe consequences of failure are , flangesCrevices let stagnant liquid accumulate, causing locallyaccelerated Corrosion from differential aeration. Therecan also be contact between dissimilar metals. Slowerdrying at joints can increase concentration of aggressivesalts. Consider the use of appropriate and safe weld profile, lack of fusion and stitch welding canall lead to crevices and stress raisers.