International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 12 Issue: 08 | Aug 2025 www.irjet.net p-ISSN: 2395-0072
Comparative Electrochemical Evaluation of Surface Treatments for Corrosion Protection of Marine Aluminum
Kaavya Koteshwar
1Student, IB Diploma Programme, Heritage International Xperiential School, Gurugram, Haryana, India
Abstract - This study comprehensively investigated the electrochemicalcorrosionprotectionperformanceofdifferent surfacetreatmentsappliedtomarine-gradealuminumunder simulated marine conditions. Four surface conditions were evaluated:untreatedcontrol,anodizedonly,paintedonly,and anodizedfollowedbypainting.Thespecimens,cutto5×5cm withathicknessof3mm,wereexposedtohalf-immersionina 3.5% NaCl solution for 21 days. Corrosion behavior was characterized using potentiodynamic polarization and electrochemicalimpedancespectroscopy(EIS).Thecombined anodizingandpaintingtreatment demonstratedthehighest corrosion resistance, with an 85% reduction in corrosion current density compared to untreated aluminum. It also exhibitedthelowestcorrosionrate(0.012mm/year)andthe highest polarization resistance (12,450 Ω·cm²). Anodizing aloneprovidedbetterprotectionthanpaintingalone,butboth weresignificantlylesseffectivethanthecombinedtreatment. The results highlighted the synergistic interaction between anodizing and painting, where the anodized layer improved paint adhesion and offered baseline electrochemical protection, while the paint provided an effective barrier against chloride penetration. This combination offered substantialdurabilitybenefitsformarineapplications
Keywords: electrochemical corrosion, marine conditions, anodization,anticorrosionpaint
1.INTRODUCTION
Marine aluminium alloys have been widely adopted in shipbuilding,offshoreplatforms,andmarineequipmentdue to their high strength-to-weight ratio, good machinability, andrelativelylowcost(Davis,1999).Despitethepresenceof anaturaloxidelayerthatprovidessomedegreeofcorrosion protection, these alloys are vulnerable to the aggressive chlorideionspresentinseawater.Inmarineenvironments, chloride-induced breakdown of the passive film leads to localized forms of corrosion such as pitting, crevice, and galvanic corrosion (Davis, 1999; Fan et al., 2023). These processes are accelerated by fluctuating temperature, oxygengradients,andvariablepHconditions.Thecombined effectof these factors reducesthestructural integrityand servicelifeofaluminiumcomponents(Davis,1999).
Tomitigatecorrosion,varioussurfacetreatmentshavebeen developed.Anodizingproducesathicker,denseroxidefilm through electrochemical oxidation, which can be further enhanced through sealing treatments (Davis, 1999; ISO,
2018b).Paintingsystems,particularlythoseusingmarinegradeepoxyprimersandpolyurethanetopcoats,providea physicalbarriertocorrosiveagentsandimproveaesthetics (ISO,2018a;StandardNorge,2022).Whilethesetreatments areeffectiveindividually,thesynergisticeffectsofcombining anodizingandpaintinghavenotbeenextensivelystudiedin marine-specific conditions using standardized electrochemical testing methods (ASTM International, 2023b;ISO,2016).
The aim of this work was to quantitatively compare the corrosionperformanceofuntreated,anodized,painted,and anodized-then-paintedaluminiumspecimensinacontrolled saline environment, to understand the potential synergy betweenanodizingandpainting,andtoprovideguidanceon the most effective surface treatment strategies for marine applications (ASTM International, 2021; ISO, 2018a; ISO, 2018b)
2. MATERIALS AND METHODS
Marine-gradealuminium sheetsweresectionedinto5×5 cm specimens of 3 mm thickness. Four surface treatment categorieswereprepared:untreatedcontrol(mechanically polished to 1200 grit), anodized only, painted only, and anodizedfollowedbypainting.Theanodizingprocessused 15%sulfuricacidat20°Cwithacurrentdensityof150A/m² andvoltageof15–18Vfor45minutes,followedbyhotwater sealing at 96°C for 30 minutes (ISO, 2018b). The painting processconsistedofa zinc-richepoxyprimerlayerwith a dryfilmthicknessof75μmandapolyurethanetopcoatwith a thickness of 50 μm (ISO, 2018a; Standard Norge, 2022). The coated specimens were cured for seven days at room temperaturebeforetesting.
Thespecimenswerehalf-immersedin3.5%NaCl solution prepared with distilled water, maintained at 25°C and pH 7.0, for a duration of 21 days (ASTM International, 2021). Electrochemical testing was conducted using a threeelectrodecell,withthespecimenastheworkingelectrode,a saturated calomel electrode (SCE) as the reference, and a platinum mesh as the counter electrode. Potentiodynamic polarizationscanswereperformedfrom−250to+250mV vs.OCPata scanrateof0.125mV/s (ASTMInternational, 2023b; Stern & Geary, 1957). Electrochemical impedance spectroscopymeasurementscoveredfrequenciesfrom100 kHzto10mHzwitha10mVRMSamplitude(ISO,2016).
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 12 Issue: 08 | Aug 2025 www.irjet.net p-ISSN: 2395-0072
Fig -1:Photographsoftestspecimensbeforeimmersion testing:(a)untreatedcontrol(b)anodizedonly(c)painted onlyand(d)anodizedthenpaintedspecimens.
Fig -2:Experimentalsetupshowinghalf-immersedtest specimensinsaline:(a)untreatedcontrol(b)anodized only(c)paintedonlyand(d)anodizedthenpainted specimens.
3. RESULTS AND DISCUSSION
-3:Potentiodynamicpolarizationlines
Fig -4:NyquistPlots
The open circuit potential (OCP) measurements revealed that the control specimens shifted from −820 mV to −780 mVovertheimmersionperiod,indicatingactivecorrosion (Davis, 1999). Anodized specimens maintained a stable potential of approximately −750 mV, while painted specimens exhibited moderate stability at −680 mV. The anodized-then-paintedspecimensdisplayedthemoststable performance,maintaininganOCPof−650mVwithminimal fluctuations(Davis,1999).(Note:OCPindicatesinterfacial conditionbutisnotadirectmeasureofcorrosionrate;rate metrics are derived from i_corr and/or R_p per standard practice.)(ASTMInternational,2023a;Stern&Geary,1957).
Potentiodynamic polarization results indicated that the untreatedcontrolspecimenshadacorrosioncurrentdensity of 8.2 μA/cm², corresponding to a corrosion rate of 0.095 mm/year. Anodized specimens showed significant improvementwithani_corrof2.1μA/cm²(0.024mm/year), while painted specimens exhibited 3.8 μA/cm² (0.044 mm/year). The anodized-then-painted treatment demonstrated the best performance with an i_corr of 1.2
Fig
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 12 Issue: 08 | Aug 2025 www.irjet.net p-ISSN: 2395-0072
μA/cm² (0.012 mm/year), reflecting an 85% reduction in corrosion current compared to the control. These interpretationsfollowstandardTafel/Stern–Gearyanalysis and the accepted conversion of i_corr to corrosion rate (ASTMInternational,2023a;Stern&Geary,1957).
Electrochemicalimpedancespectroscopyresultssupported thepolarizationfindings.Thepolarizationresistancevalues were1,850Ω·cm²forthecontrol,8,750Ω·cm²foranodized, 4,200 Ω·cm² for painted, and 12,450 Ω·cm² for anodizedthen-painted specimens. The coating resistance of the combinedtreatmentwasapproximately3timeshigherthan that of painted-only specimens, highlighting the effectiveness of the anodized underlayer in preventing chlorideingress(ISO,2016;GamryInstruments,n.d.).The Nyquist/Bode behavior expected for high-barrier organic coatings larger semicircle diameter and higher lowfrequency|Z|isconsistentwiththeseR_ptrends(ISO,2016; GamryInstruments,n.d.).
Thecalculatedsynergisticparameter(S=1.34)confirmeda positive interaction between anodizing and painting. This was attributed to the anodized layer providing enhanced surfaceroughnessandchemicalbondingsitesforimproved paintadhesion,combinedwiththebarrierpropertiesofthe paintlayer(Davis,1999;ISO,2018b;StandardNorge,2022).
3. CONCLUSIONS
Thecombinedanodizingandpaintingtreatmentofferedthe most effective corrosion protection for marine-grade aluminum in saline environments. It achieved the lowest corrosion rate, the highest polarization resistance, and demonstratedclearsynergisticbenefitscomparedtoeither treatment alone (ASTM International, 2023a; ISO, 2016). Anodizing alone provided superior performance over paintingalone,indicatingtheimportanceofelectrochemical protection in addition to physical barriers (Davis, 1999). These findings suggested that for applications where aluminum components are exposed to aggressive marine environments,asequentialtreatmentofanodizingfollowed bytheapplicationofmarine-gradepaintprovidedthemost durable and cost-effective solution (ISO, 2018a; Standard Norge,2022).
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