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SVET and LEIS: following and quantifyingprotection process

For the fast evaluation of modern high quality coatings newefficient methods are needed which provide reliableinformation in a short time about the protective features ofthe coatings. Electrochemical techniques are the potentialcandidate for such an application, particularly the modernscanning methods like the local electrochemical impedancespectroscopy (LEIS) and the scanning vibrating electrodetechnique (SVET). Within this work both methods wereapplied for testing differently pigmented protective coatings.René Kleinegesse, Thadeus Schauer, Claus D. Eisenbach.By the development and optimisation of coatings a reliableand fast evaluation of their protective properties isnecessary for the screening and choice purposes. Theproven Florida or Arizona tests are extremely timeconsuming and for this kind of application useless. In orderto shorten the investigation time, different accelerated shorttime tests can be applied (e.g. Salt spray test,weather-o-meter, Kesternich test, prohesion test etc.). As analternative immersion tests in electrolytes, different cyclictests and accelerated atmospheric exposures (SCAB) arebeing used. The remaining problems with these methodsare their still too long duration, quantitative assessment ofcoatings and the correlation with the long time tests.

High sensitive methods monitored early corrosionsstagesModern scanning electrochemical measuring techniques likethe local electrochemical impedance spectroscopy (LEIS)and the scanning vibrating electrode technique (SVET) canbe of advantage by testing and evaluating the coatings.These methods operate on the basis of different workingprinciples and allow both the evaluation of the corrosionrelated changes in the phase of the electrolyte solution closeto the metal/coating phase (SVET) [1 to 3] or additionally thekinetic of the corrosion processes and the delamination ofcoatings (LEIS) [4 to 6]. For the efficient shortening of thetesting time, artificial micro damages are inflicted to coatingsand the corrosion and delamination related changes in theclose vicinity of these places can be scanned with thosemethods. Due to the high sensitivity of these methods, theearly stages of the corrosion and delamination progress atthe damage of the coating can be monitored and theobtained data can be used for the fast evaluation/screeningpurposes.

LEIS and SVETThe thermodynamic state of the metal surface (active orpassive), conversion or galvanic coatings, corrosionprotective pigments or different binders can however,meaningfully influence the performance pf LEIS and SVET.The purpose of this work was to evaluate the influence ofthese parameters by testing different pigmented protectivecoatings with LEIS and SVET.

Electrochemical activities visible with LEISTo illustrate the sensitivity of SVET and LEIS, selected linescans for the untreated steel covered with a referenceunpigmented coating are shown in Figure 1. The scans weretaken over the midpoint of the artificial damage of thecoating.In Figure 1 the different features of the SVET and LEISscans are readily visible. With SVET, the damage locationcan be better recognised and the further deterioration of the

coating at this place can be better followed. The LEIS dataprovide more resolved information about the electrochemicalactivities in the close vicinity to the damage. Analysis of theLEIS data for different distances from the midpoint of thedamage provide the additional opportunity to discriminatethe coatings.The data of SVET potentials over the midpoint of thedamage of the coating for the differently pigmented 2 packepoxy hydro-primer on untreated steel are represented inFigure 2.

Zinc dust provides efficient cathodic protectionWhile three tested systems show certain potential changes,no considerable changes are visible for the zinc dustpigmented system. This kind of behaviour of the zinc dust isdue to the efficient cathodic protection of the substrate and afast conversion of Zn2+ to the insoluble ZnO. The iron micapigmented and unpigmented systems take a middleposition. The relatively high SVET potentials for the zincphosphate pigmentation cannot be interpreted easily, sincethey can be either due to the electrochemical activity of thesubstrate or the soluble part of the pigment.Figure 3 represents the impedance data of the LEISmeasurements for the same systems as in Figure 2.

Pigments protect metal outside of the damageTwo measuring points in the distance of 1mm and 5mmfrom the midpoint of the damage are taken intoconsideration. The strongest impedance loss and clearly thelowest impedance values are measured for the unpigmentedsample. The data for the distance of 5mm from the midpointof the damage reflect predominantly the barriercharacteristics and the inhibition of the water and iondiffusion through the coating; these factors are influencedpositively by the tested pigments. The impedance values forthe short distance from the damage of the coating referrather to the corrosion activity within the damage and itsclose vicinity. They reflect the distance protection of thecoating on the one hand and the inhibition of the cathodicreaction in the vicinity of the defect on the other hand. It isevident that the zinc dust shows the best protection at adistance, in accordance with the SVET measurement. Theaction of iron mica should be however attributed to thesuppressed cathodic reaction in the area close to the defect.The results of the SVET measurements for the testedsystems on zinc-galvanised steel are plotted in Figure 4.

Active and barrier effects on zincgalvanised steelRemarkable for the data in Figure 4 is a relatively smallcorrosion activity of the substrate. The original activity forthe zinc phosphate pigmentation is probably connected withthe solubility of this pigment and the altogether small signalsrelate to the fast conversion of Zn2+ to ZnO.Similar as for the untreated steel, the LEIS data for thezinc-galvanised steel can be explained with both the activeand barrier action of the coatings (Figure 5).For the measurements conducted at a distance of 5mm fromthe midpoint of the damage a good barrier effect of zinc dustand iron mica is dominant. Regarding the results for thedistance of 1mm both the active protection of zinc dust andthe barrier effect of iron mica, strengthened by the easyoxidation of the zinc substrate within the cathodic area isevident.

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Figure 6 shows the SVET data for the differently coatedaluminium substrate. For this substrate no measurableelectrochemical activities could be determined with SVET.On the other hand however the results of the LEISmeasurements (Figure 7) for the same substrate show thatthe resolution of LEIS is sufficient for the discriminationbetween the tested coatings.

LEIS shows differences in pigment behaviour even forsand-blasted steelIt is well known that zinc has a negative electrochemicalpotential and consequently the passive status of thealuminium substrate (positive potential) can be disturbed.The effect of the galvanic coupling becomes apparent withthe LEIS measurements at 5mm distance from the defect. Inthe vicinity of the defect, the necessary oxygen transport forthe repassivation of the aluminium surface can take place,what is reflected by the higher impedance values for thedistance of 1mm from the damage. The lower impedancevalues for zinc phosphate could be assigned to the effect ofthe soluble part of the pigment.For the sand-blasted steel substrate, the SVET data do notreveal any considerable changes of the signal in the vicinityof the damage of the coating (Figure 8).It is evident that the good adhesive strength of the coatingdiminishes efficiently the cathodic reaction near the defect,so that the corrosion processes are no longer detectablewith SVET. Examining the same systems with LEIS (Figure9), it is still possible to discriminate between the differentpigmentations.

Correlation between short-time and conventionalcorrosion testOutside of the area of influence of the defect, higherimpedance values for the zinc dust than for the zincphosphate and iron mica were found. This behaviour of thezinc dust results from the electrochemical (cathodicprotection) and the physical barrier (oxidised zinc particles).Smaller impedance values for the zinc phosphate and ironmica can be connected either with the solubility or with a notoptimal orientation of the pigment particles. The sameinterpretation applies to the data for the area close to thedamage.The results of the salt spray test for the untreated steelsubstrate in Figure 10 point out at a good correlationbetween the short-time electrochemical SVET and LEISmeasurements and the conventional corrosion test.

Cathodic reactions are limited by good barrier andadhesion propertiesFor the purpose of a fast evaluation and screening ofanti-corrosive coatings, artificial defects in the coating areintroduced and the changes at these places as well as intheir vicinity are followed under immersion in an electrolyticsolution with the purpose to obtain preliminary informationabout the protective properties of the coating in anextremely short time. However, the electrochemicalprocesses on the metal surface of the damage place whichassist the coating damage are relatively complex anddepend on several factors.If a coating is applied on an electrochemically activesubstrate (e.g. carbon steel), corrosion reactions in thedefect are activated. Within a short time they lead to aspatial separation of the anodic reaction (Fe => Fe2+ + 2e-)and the cathodic reaction (O2 + 2H2O + 4e- => 4OH-) whichtake place in the damage and in its close vicinity,respectively. For the cathodic reaction, a diffusion supportedtransport of oxygen and water is crucial. Since the bothreactions are in equilibrium, the slowest step of the two

reactions determines the kinetic of the whole system.If the coating exhibits good barrier properties or/and a goodadhesion, both factors will limit the cathodic reaction and atthe same time the corrosion within the defect. If the coatingpossesses active corrosion protection pigments which areable to impose a certain distance protection, this can lead tothe inhibition of the anodic reaction in the defect. Whentesting coatings on passive substrates as aluminium alloys,no considerable corrosion reactions are detectable in theearly stages of the exposition. Using SVET and LEIS, theprocesses discussed above can be followed and quantified.Since SVET is sensitive only for changes of the electrolytephase due to the electrochemical processes like the anodicand cathodic reactions of an active metal, this method canbe used for the investigation of uncoated and coated activemetals. Concerning passive or easily passivated metals andalloys (stainless steels or aluminium alloys), the LEISmethod offers more advantages, since with this method notonly the changes in the electrolytic phase can be evaluated,but also changes on the metal surface and within thecoating can be examined.

Screening of corrosion protective pigments by SVETand LEISSVET and LEIS represent an option for the fast investigationof different aspects of the corrosion protection of metals withcoatings. Regarding different types of pigmentation ofcoatings, SVET and LEIS measurements can be used forthe evaluation and screening of corrosion protectivepigments. In the case of coatings on an electrochemicallyactive substrate (e.g. carbon steel), both the active corrosionprotection and the barrier effects can be determinedquantitatively. For an efficiently working zinc dust pigment acomplete cathodic corrosion protection is expected, whichbecomes apparent by small and constant SVET potentialswithin the area of an artificial defect of the coating. Theapplication of SVET for the optimisation, formulation andquality control of coatings pigmented with zinc dust appearsas advantageous.

LEIS enquiry suitable for passive substratesBy testing coatings pigmented with iron mica predominantlybarrier effects can be expected, with a limited diffusion ofwater and ions throughout the coating. This inhibition effecton the cathodic reaction can be however partly or to highextent masked by the corrosion processes within thedamage due to the lacking distance protection of thispigment. Since with LEIS both the area within the damageand in its vicinity can be resolved, it is well suitable in thiscase. With pigments that exhibit a certain solubility in water,certain difficulties can be faced by the analysis andinterpretation of both SVET and LEIS data. In such casesmeasurements with complementary methods should becarried out.Because the undercutting and delamination of coatings canbe evaluated with SVET and LEIS, the testing of theadhesion of coatings can also be included into theinvestigation. The conducted SVET and LEISmeasurements show that the surface state of the metal(untreated or sand blasted steel) can have a great effect onthe corrosion protection. If the metal substrate is in apassive state (e.g. stainless steel or aluminium alloys), thenLEIS suits usually better for the evaluation of coatings andprotective pigments.

References[1] I. Sekine, T. Suzuki, M. Yuasa, K. Handa, K. Takaoka, L.Silao, Prog. Org. Coat. 31 (1997) p 185[2] E. Bayet, F. Huet, M. Keddam, K. Ogle, H. Takenouti, J.

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Electrochem. Soc. 144 (1997) L87[3] F. Zou, D. Thierry, H. Isaacs, J. Electrochem. Soc. 144(1997) p 1957[4] E. Bayet, F. Huet, M. Keddam, K. Ogle, H. Takenouti,Mater. Sci. Forum 289-292 (1998) 57[5] R. Lillard, P. Moran, H. Isaacs, J. Electrochem. Soc. 139(1992) p 1007[6] M. Wittmann, S. Taylor, Electrochem. Soc. MeetingAbstr. 94-1 (1994) 74[7] H. Isaacs, Corrosion 43 (1987) p 594[8] M. Wittmann, S. Taylor, "Advances in CorrosionProtection by Organic Coatings II", Electrochem. Soc. Proc.95-13 (1995) p 158 [9] F. Maile, Dissertation, University of Stuttgart, Stuttgart(2000)

AcknowledgmentThe authors acknowledge the financial support of theresearch project (AiF-No. 12210 N) from the GermanFederal Ministry of Trade and Commerce which providedfunds through "Arbeitsgemeinschaft IndustriellerForschungsvereinigungen Otto von Guericke e.V. (AiF)".

ExperimentalThe working principles of SVET and LEIS are described indetail elsewhere [7 to 9]. The SVET and LEISmeasurements were conducted by the same conditions withModel SVP100 and LEIS 270 from Uniscan Instruments,Buxton, UK. The distance between a probe tip and asubstrate was 200µm by a vibration amplitude of 50µm(SVET) and a frequency of 1kHz (LEIS). The scanned areaaround the artificial defect with a diameter of 500µm was10mm x 7,5mm. The electrolyte solution was 1mM NaCl and1mM EDTA with a conductivity of 300µS/cm. Thesalt-spray-test was carried out according to DIN 53 167.Four different substrates were chosen for the investigation,i.e. untreated carbon steel ST 14/05, zinc-galvanized steeltype ST 14/05 and aluminium alloy AlMg 1, all fromChemetall GmbH, Frankfurt/Germany, and sand-blastedcarbon steel ST 14/05 from Krüppel, Krefeld/Germany.As a coating two pack epoxy hydro-primer on the basis of"Beckopox 613" and "Beckopox 385", both from VianovaResins GmbH & Co KG, Wiesbaden/Germany, were tested.The coatings were pigmented with such pigments as zincaluminum polyphosphate hydrate ("Heucophos ZAPP"), ironmica ("Heucorox FL"), both from Heubach GmbH,Langelsheim/Germany, and zinc dust ("Zincoli Superfine620") from Grillo Zincoli Germany, Stolberg/Germany. Thepigments were used in coatings by Q = 20,55. As areference the unpigmented coating was used. All coatingshad a thickness of 21 ± 2µm.

Results at a glance- Important information can be gained about the efficiencyand the working principle of anti-corrosive coatings in anextremely short time by the investigation of artificial microdefects in coatings under immersion in an electrolytesolution.- Electrochemical scanning methods such as SVET andLEIS are particularly well suitable for the investigation ofartificial micro defects in coating. For the electrochemicallyactive metal substrates SVET suits well. In the case ofpassive substrates and efficient barrier coatings, the use ofLEIS appears as advantageous.- SVET and LEIS are generally suitable for the preliminaryevaluation of the active corrosion protection, barrier effectsas well as adhesion and delamination of coatings withinextremely short time periods.

LIFELINE-> René Kleinegesse studied macromolecular chemistry atthe University of Stuttgart. On his thesis which he received2003 he has worked both at the Research Institute forPigmenzs and Coatings and at the University of Stuttgart. InFebruary 2003 he joined the Oemeta Chemische WerkeGmbH in Uetersen.-> Thadeus Schauer studied chemistry at the TechnicalUniversity of Gdansk and received his PhD from the Instituteof Physical Chemistry at the Polish Academy of Science. Atpresent he is working at the Research Institute for Pigmentsand Coatings in Stuttgart.-> Prof. Dr. Claus D. Eisenbach gained his doctorate at theUniversity of Mainz in 1974. After a postdoc residency in theUSA he moved to the Institute for MacromolecularChemistry at the University of Freiburg where he qualified asa lecturer in 1980. In 1984 he accepted a post at thePolymer Institute of Karlsruhe University. Between 1986 and1995 he held the professorial chair in MacromolecularChemistry II at the University of Bayreuth. Since the autumnof 1995 he has been a full professor of MacromolecularChemistry at the University of Stuttgart where he is head ofboth the Institute for Applied Macromolecular Chemistry andthe Research Institute for Pigments and Coatings inStuttgart.

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Figure 1: SVET and LEIS line scans for the epoxy hydro-primer on untreated steel.

Figure 2: SVET data for the epoxy hydro-primer on untreated steel.

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Figure 3: LEIS data for the epoxy hydro-primer on untreated steel.

Figure 4: SVET data for the epoxy hydro-primer on zinc-galvanized steel.

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Figure 5: LEIS data for the epoxy hydro-primer on zinc-galvanized steel.

Figure 6: SVET data for the epoxy hydro-primer on aluminum.

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Figure 7: LEIS data for the epoxy hydro-primer on aluminum.

Figure 8: SVET data for the epoxy hydro-primer on sand blasted steel.

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Figure 9: LEIS data for the epoxy hydro-primer on sand blasted steel.

Figure 10: Results of the salt-spray test on untreated steel according to DIN 53209 andDIN 53210 with the scale ranging from 0 to 5 for less corrosion and bubbles at 0 and

high corrosion and numerous bubbles at 5.

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