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Technical Article

The development of anticorrosivepigments for the most diversecoating systems is extremely time

consuming and expensive, owing to thenecessary climatic testing, such as testingunder salt spray exposure.

To accelerate work on a new zinc-freepigment with improved protection,modern electrochemical investigationmethods have been used successfully andverified using traditional tests1.

Mode of action ofconventional phosphate-based anticorrosivepigmentsThe use of suitable anticorrosive pigmentshas a significant influence on theprotective properties of formulations formetal substrates. The mode of action ofan anticorrosive pigment is attributed tothe following factors2,3:l Increase in film strengthl Prevention of rust creepage and

underfilm corrosion at defect areasl General retardation of the corrosion

processl Cathodic and/or anodic passivation of

the metal surface

In the case of zinc phosphates, a very lowwater-solubility results in the release ofsecondary phosphate ions to the coating,which account for the formation of

inhibiting adhesive complexes on themetal surface and the associated anodicpassivation. Another theory on themechanism of action also describes theformation of tribasic ferric phosphatecomplexes, though only in weakly acidicmedia4,5. As an ampholyte, zinc, or afterhydrolysis, zinc hydroxide, alsodemonstrates solubility behaviour inacidic and alkaline media that isadvantageous for corrosion protection.The modified zinc orthophosphatepigments and zinc polyphosphatepigments that are established on themarket display a substantially increasedchemical and electrochemicaleffectiveness as compared withtraditional zinc phosphate, which enablesvery good protective properties to beachieved.

Besides economic considerations,ecological and regulatory factors play an

increasingly decisive role nowadays in theformulation of innovative coatingsystems, hence the call for zinc-freeanticorrosive pigments or those that donot require labelling has steadilyincreased in recent years.

Towards the design ofnew zinc-free pigmentsOnly a few metals qualify as suitablecountercations once heavy metals havebeen ruled out. Emphasis can be placedon the possible positive interactionsbetween calcium and magnesiumphosphate compounds.

The requirements of a new zinc-freepigment were defined as follows:l Zinc-free pigment technologyl Highly effective anodic corrosion

protection in solvent- and water-basedsystems

l Stability and universal applicationl Very good dispersion propertiesl Cost efficiency

Even at the early stage of theinvestigations, exceptional increases inperformance were observed when usingnewly-developed pigments with adifferent magnesium-calcium ratio, whichalso had a positive effect on the salt sprayresistance.

Towards new zinc-free anticorrosivepigments: identifying specific synergies using

electrochemical corrosion investigationsLars Kirmaier, Susanne Bender, Andreas Heyn, Heubach, Langelsheim, Germany

OutlineTwo modern electrochemical methods in the preliminary investigation of zinc-freeanticorrosive pigments are described, namely electrochemical rest potential analysis,and electrochemical noise analysis. The techniques are used to examine the usefulnessof aluminium and magnesium pigments in a styrene-acrylate copolymer emulsion. Theresults, which can be more rapidly obtained than those using traditional techniquessuch as the salt spray test, are then verified by comparison with the outcomes of saltspray testing. The most successful pigment is then tested successfully for applicabilityin solventborne alkyd and epoxy resin formulations, using the salt spray test for aduration of three weeks.

2PO43–+3Fe2+ Fe3(PO4)2

PO43–+Fe3+ FePO4

Steel

Figure 1: Mode of action of phosphate pigments binder

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Technical Article

Electrochemicalinvestigations directly incoating dispersionsAfter extensive preliminary investigations,test conditions could be determined fortwo electrochemical methods – firstly therest potential analysis and secondly theelectrochemical noise analysis – whichenable comparative statements to bemade on the protective effect ofanticorrosive pigments dispersed in anaqueous binder. For all electrochemicalinvestigations, a round bar of unalloyedsteel (C55, material no.: 1.1203) was usedas a sensor or working electrode. As theelectrolyte, an aqueous, organic coatingdispersion was used, which was producedby dispersing the binder and therespective anticorrosive pigment using adissolver. As the organic coatingdispersions were aqueous but highlyviscous, they were diluted in a 50:50 ratiowith de-ionised water.

Table 1 shows the electrolytes used,consisting of an aqueous binder and fourdifferent anticorrosive pigments orpigment combinations with a differentCa/Mg ratio. Variant B hereby displays ahigher magnesium content than variantA. It shows that pigment P2 (Heubach’sCMP pigment) displays the lowest pHvalue at the greatest solubility andconductivity. This makes it difficult topredict the actual effectiveness of thispigment combination in real coatingsystems. But it is a first indication ofdifferent electrochemical behaviour.

Electrochemical restpotential analysisThe rest potential analysis (RPA) is basedon a rest potential measurement using atwo-electrode arrangement, whereby aAg/AgCl electrode was used as areference electrode and the unalloyedsteel C55 as a working electrode. Theaqueous coating dispersion (see Table 1)served as the electrolyte, which wasagitated throughout measurement with amagnetic stirrer to prevent the pigmentssettling. At defined time intervals, a 1MNaCl solution as a corrosion stimulatorwas added to the electrolyte by means of

computerised pump arrangement. As akey measurement parameter, thepotential curve was observed andevaluated by determining the criticalquantity of chloride at which a significantpotential drop occurs. For allmeasurements the potential curve wasfirst recorded over 60 minutes without theaddition of chloride.

As a characteristic example of the restpotential analysis, in Figure 2 a restpotential curve is shown for eachanticorrosive pigment, P1 to P4. Using thepotential curves, behavioural differencesof the pigments are observed. For P1 toP3 a marked potential drop can be seen asa result of adding a defined chloridequantity. For P4 it is noted that the restpotential arises right at the start of theinvestigation at -400 mV. At this potentialthere is strong metal dissolution which isfurther accelerated after 60 min as a resultof the chloride being added. The slightrise in potential to about -250 mV isattributed to the corrosion or secondaryproducts on the metal surface and is not aresult of inhibiting mechanisms.

Pigments P2 (CMP) and P3 are pigmentcombinations of pigments P1 and P4 indifferent ratios. The pigmentcombinations first have the characteristicinitial potential drop at about -400 mV

and then a potential increase dependingon the quantity of pigment P4 present.The greater the quantity of pigment P4(100% magnesium component) in thepigment combination, the longer themetal needs to develop a passive-likesurface state and the longer therefore ittakes for the potential rise to occur.

Figure 3 illustrates the critical chloridequantities identified at the characteristicpotential drop for the anticorrosivepigments P1 to P4 dispersed in the binder.The mean value is shown in each case,calculated using a minimum of threeindividual measurements. Pigment P2(CMP) displays the highest criticalconcentration value, pigment P1 andparticularly P4 the lowest.

Electrochemical noiseanalysisElectrochemical noise analysis (ECN) is avery sensitive method for recording localcorrosion processes and material changes,and has been successfully used for manyyears for various applications6, includingin the field of coatings testing7.Measurements for investigating thedissolution behaviour of the unalloyedsteel in the aqueous coating dispersionswere performed without external current,using a three-electrode arrangement. Forthis purpose, two macroscopicallyidentical working electrodes made fromC55 were short-circuited via a zeroresistance ammeter and connected to ahigh-ohm potentiometer and a Ag/AgClreference electrode. The measured noisesignals are filtered by means of a bandpass. This enables them to be separatedfrom the steady-state components

Table 1: Electrolytes used, consisting of binder and anticorrosive pigment

Sample Binder Pigment composition pH Water-soluble Conductivityvalue fraction [%] [μS/cm]

P1 based on a 100% calcium component 6.8 0.10 79P2 styrene-acrylate Ca/Mg: Variant A, (CMP) 6.4 1.7 944P3 copolymer Ca/Mg: Variant B 6.7 1.2 735P4 emulsion 100% magnesium component 7.2 1.2 726

P2

P1

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Dosage

–0.1

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Figure 2: Rest potential curves for the unalloyed steel, measured in anaqueous binder with the anticorrosive pigments P1 to P4

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(current and potential) and they can beamplified separately. After a test period of20 minutes without chloride addition,0.04 ml of a 1M NaCl solution was addedto the electrolyte at five-minute intervals.The potential and potential noise, as wellas the current noise between the steelelectrodes, were measured and evaluated.Using the calculation of noise chargequantities and noise resistances, whichwere established in extensive preliminaryinvestigations as characteristic values forthe protective effect of the pigments, aneven finer differentiation of the resultscan be achieved as the charge quantitiesshow a direct relationship to pigmenteffect.

Figure 4 shows the current noise-timecurves over the entire test period of 180minutes for the unalloyed steel inaqueous binder with the anticorrosivepigments P1 to P4.

To illustrate the relationships andprocesses taking place at the metalsurface, Figure 5 presents the cumulativecharge quantities determined from thecurrent noise-time curves.

Clear differences are shown between thevarious pigments. While P1 at the start ofthe measurement has a low noise activityand thereby a low metal dissolution, astrong initial activity can be identified inthe first test interval for pigments P2, P3and P4 due to the magnesiumcomponent. In the test period from 120 to180 minutes, the noise activities of thepigment combinations P2 and P3decrease greatly while for P1 there is anincreased dissolution. In fact, pigment P4shows a sharp drop in the noise currentand cumulative charge quantity, yetactive metal dissolution continues, whichcould be evidenced by the detected noiseresistances.

Verification usingtraditional corrosiontestingTo verify the evidence of theelectrochemical investigation methodsusing conventional corrosion testing,cold-rolled steel sheets were coated witha formulation that is used in practice, alsobased on an aqueous styrene-acrylatebinder with the anticorrosive pigments P1to P4. The results after 408 hrs of ageing inthe salt spray test (DIN EN ISO 9227) arepresented in Figure 6. The combination ofthe calcium and magnesium component(P2) in the anticorrosive pigment results ina significant increase in corrosionprotection.

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Technical Article

45

05

10 15

25 30 35 40

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P1 P2 P3

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Figure 4: Current noise-time curves for the electrochemical noise analysis inthe aqueous coating dispersions

Figure 5: Cumulative charge quantities for two test intervals, determinedfrom the noise current-time curves in the electrochemical noise analysis inthe aqueous coating dispersion

Figure 3: Critical chloride quantity in the investigation of anticorrosivepigments P1 to P4, dispersed in aqueous binder

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To investigate the performanceproperties of pigment combination P2(CMP) in other binder systems, salt spraytests were performed with formulationsbased on a solventborne short-oil alkydresin (see Figure 7) and a solvent-borneepoxy resin (see Figure 8). Forcomparison, the pigment combinationwas tested against a zero sample withoutanticorrosive pigment, magnesiumphosphate, calcium phosphate and finallya reference sample containing zinc (zincbenchmark). The dry coating thickness ineach case was 70 µm. To evaluate thedegree of rusting and the rust creepage atthe cross-section, the lower half of thecoating was removed after the salt sprayexposure test.

The result of the salt spray exposuredemonstrates the good performanceproperties when using CMP comparedwith magnesium phosphate. Even theprotective effect of the reference samplethat contained zinc was outperformedusing CMP. Furthermore, magnesiumphosphate exhibits serious problemsregarding adhesion, rusting andblistering.

A significant improvement in adhesionand rust creepage at the cross-sectioncould also be achieved in this system byusing CMP.

SummaryThe investigations using rest potentialanalysis and electrochemical noise

analysis show differences in terms of thepigment composition and its effect on thesubstrate dissolution. The proportion ofmagnesium in the tested pigmentvariants P1-P4 has a decisive influence onthe anticorrosion performance of theoverall system. Rest potential analysisshows that an increasing quantity of themagnesium component in theanticorrosive pigment causes the criticalconcentration value, which gives anindication of the anti-corrosion effect, todrop. A concentration of 100%magnesium component in theanticorrosive pigment results in activemetal dissolution and no anticorrosiveproperties. On the basis of thisinvestigation, P2 (CMP) could bedetermined as an optimal pigment for thissystem with a defined Ca/Mg ratio.

With regard to electrochemical noise,firstly a trend is identified whereby theinitial noise activity in the time intervalfrom 0 to 60 minutes increases withincreasing concentration of themagnesium component. This behaviourpoints towards increased substratedissolution in the first 60 minutes of themeasurement at increasing magnesiumconcentration. After an extended testperiod and the addition of chloride, thepigment combination P2 displayed thebest anticorrosive properties. Pigment P4(100% magnesium component) doesshow a reduction in the noise activity, butgiven the noise resistances determined itcould be demonstrated that increasedmetal dissolution still occurs.

References1 Bender S, M Babutzka, L Kirmaier,

Moderne elektrochemischeKorrosionsuntersuchungen gezielteingesetzt, Farbe und Lack 2014, inprint

2 Kirmaier L, Farbe und Lack, 120-123,115, 2009

3 Vogelsang J, Basics about AnticorrosivePigments and Corrosion Inhibitors andPossibilities for their Usage, EuropeanCoatings Conference, Berlin, 2000

4 Ruf J, Organischer Metallschutz,Vincentz Verlag, Hannover, 260, 1993

5 Yongsheng H, F Liu, E-H Han, S Anjum,G Xu, Corr Sci, 69, 77-86, 2013

6 Heyn A, J Göllner, Analysis andMonitoring of Corrosion usingElectrochemical Noise - Part 5, Materialsand Corrosion, 64, 663, 2013

7 Plagemann P, O Yezerska, ABrinkmann, L Kirmaier, Farbe und Lack,115, 94-7, 2009

Figure 7: Test result after 408 hrs exposure in the salt spray test, basis:solventborne short-oil alkyd resin

Control Phosphate CMP Zinc benchmark

Figure 6: Test result after 408 hrs exposure in the salt spray test, base:aqueous styrene acrylate

P1: 100% P2: Ca/Mg: P3: Ca/Mg P4: 100%Ca component Variant A (CMP) Variant B Mg component

Figure 8: Test result after 504 hrs exposure in the salt spray test, basis:solventborne epoxy resin

Zero sample Calcium phosphate CMP Zinc benchmark

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