mechanisms of non-toxic anticorrosive pigments in organic...

Progress in Organic Coatings 49 (2004) 358–371

Mechanisms of non-toxic anticorrosive pigmentsin organic waterborne coatings

Małgorzata Zubielewicza,∗, Witold Gnotba Paint and Plastics Department, Institute for Plastics Processing METALCHEM, Gliwice, Poland

b Chemical Department of Silesian Technical University, Institute of Chemistry,Inorganic Technology and Electrochemistry, Gliwice, Poland

Received 22 September 2003; received in revised form 22 September 2003; accepted 25 November 2003

Abstract

Investigations have been carried out concerning the mechanism of the behaviour of non-toxic anticorrosive pigments belonging to thegroup of phosphates, ferrites and ion exchange pigments in waterborne systems. The mechanism controlling the protective effectiveness oforganic coatings is complex and results from simultaneous activity of various agents, from among which the kind of the corrosion inhibitorand the structure of the coating are of fundamental importance. The effect of pigments on the protective properties of coatings was testedby means of electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET) as well as the salt spray andProhesion tests. For the investigation of the structure of coatings the porosymetric method and modulated-force thermomechanical analysis(mf TMA) were applied. The results of these investigations have shown that calcium zinc phosphate and zinc ferrite are the most effective.These pigments take part in the passivation of steel, which has been proved by the results of electrochemical investigations and by thepresence of the passive layers as has been found out by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDXA) and scanningelectron microscopy (SEM). Calcium zinc phosphate and zinc ferrite affect the structure of the coatings, increasing the glass transitiontemperature (Tg) of the coatings. Zinc phosphate and calcium-exchanged silica do not act in compliance with electrochemical mechanismneither do they improve the barrier properties of the binder.© 2003 Elsevier B.V. All rights reserved.

Keywords: Non-toxic anticorrosive pigments; Waterborne coatings; Mechanisms of action; Corrosion control

1. Introduction

Organic coatings can protect metal substrates from cor-rosion via the electrochemical mechanism, inhibiting cor-rosive reactions by sharing an anticorrosive pigment in thepassivation of the metal, forming stable and strongly adhe-sive layers on the substrate, or via the barrier mechanism,inhibiting the access of corrosive environmental agents tothe metal surface. Now these two mechanisms do not actseparately. The coatings protect the metal both by forminga barrier as well as electrochemically. Which of these willprevail depends on the chemical composition of the coating,mainly as to the pigmentation and to the chemical structureand the barrier properties of the binders.

Today, environmental protection requirements conditionthe formulation of protective coatings with waterbornebinders and non-toxic anticorrosive pigments. The best

∗ Corresponding author.E-mail address: [email protected] (M. Zubielewicz).

known and most frequently applied non-toxic anticorrosivepigments are phosphate pigments. Ferrites and ion-exchangepigments are of less importance.

Of the phosphate pigments, zinc phosphate is the mostimportant[1–10]. Thanks to its low solubility and reactivity,zinc phosphate can be used in many binders, particularly inthose where non-toxic basic pigments show limited stability,e.g. in alkyd binders with a high acid value, and in acidcatalysed systems or in waterborne binders.

The action mechanism of zinc phosphate is the phos-phatisation of the iron surface and the formation of com-pounds with carboxyl and hydroxyl groups of the binderagent, which react with the products of corrosion result-ing in a layer closely adhering to the metallic substrate[4–8,11–23]. The passive layer formed in the presence ofzinc phosphate consists of oxides (�-Fe2O3), hydroxyoxides(�-FeOOH,�-FeOOH) and iron phosphate[24–31]. The re-sult is a polarisation of the cathodic regions due to the for-mation of limited solubility basic salts, closely adhering tothe substrate. Other authors[32–38]are of the opinion that

0300-9440/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2003.11.001

M. Zubielewicz, W. Gnot / Progress in Organic Coatings 49 (2004) 358–371 359

the solubility of zinc phosphate is too low to produce a pas-sive layer. In a medium with pH 6.5–8 the solubility of zincphosphate is extremely weak, its effect is insignificant, sothat some authors[37] even think it acts only as an expen-sive extender. The mechanism of other phosphate pigments,such as aluminium triphosphate, calcium zinc phosphate,zinc polyphosphate and calcium phosphate, is interpretedsimilarly as that of zinc phosphate, although these pigmentshave not yet been investigated so extensively as zinc phos-phate[39–41].

The general mechanism of the activity of ferrite pigmentsassumes their hydrolysis in the coating. During this, hydrox-ides are formed and counteract the formation of corrosiondue to the higher pH, the formation of soaps of alkalineearth metals in some binders (alkyds, epoxy esters) and dueto passivation brought about by the presence of OH− ions[8,42–45].

The anticorrosion effect of ion-exchange pigments can bedescribed considering two mechanisms: the exchange of cal-cium ions for hydrogen ions penetrating the coating from theenvironment, neutralising the acid compounds and formingon the substrate a protective layer consisting of calcium andiron silicate[39,46–50].

The aim of this research was to investigate the mecha-nism of the behaviour of anticorrosive pigments belongingto the group of phosphates, ferrites and ion-exchangers inwaterborne systems. The protective mechanism of organiccoatings derives from the simultaneous effect of various fac-tors, among which the type of the applied corrosion inhibitorand the structure of the coating are the most important. Boththese factors have been taken into consideration in this pa-per.

Table 1Characteristics of anticorrosive pigments

No. Kind of the pigment/accepteddetermination

Content of the maincomponent, % (m/m)

Resistivity(� m)

pH 10% (m/m)suspension

Density(g/cm3)

Oil absorptionnumber, %(m/m)

Median diameterof the particlesa,d50 (�m)

1 Zinc phosphate/‘1’ 95.5b, Zn3(PO4)2 70.89 6.23 3.4 21.3 4.542 Calcium zinc phosphate/‘2’ 99.2b, (Zn,Ca)PO4 28.25 7.64 3.1 35.0 2.513 Zinc ferrite/‘3’ 99.5, (Fe,Zn)O2 90.5 6.89 5.0 20.0 0.624 Calcium-exchanged silica Ca/Si/‘4’ 5.0 Ca, the rest

mainly SiO2

33.3 8.05 1.8 48.5 1.92

a Sedigraph 5100, Micromeritics.b After ignition at 600◦C.

Table 2Characteristics of waterborne binders

No. Kind of the binder/accepteddetermination

Content of solidsubstances, % (m/m)

Density(g/cm3)

pH Viscosity,d (Pa s)

Glass transitiontemperature,Tga (◦C)

Median diameter ofpartclesb, d50 (�m)

1 Fatty acid modified dispersion/‘A’ 48± 2 1.00 8.0 25 17.8 0.382 Styrene acrylic dispersion/‘B’ 50± 1 1.00 8± 0.5 50–80 27.7 0.28

a DMTA MkII, Polymer Laboratories.b Mastersize S, Malvern Instruments.

2. Experimental

2.1. Samples

Four anticorrosive pigments were applied in our inves-tigations, differing in their chemical structure and physicalproperties, viz. zinc phosphate, calcium zinc phosphate, zincferrite and calcium-exchanged silica. Two dispersive binderswere also examined, with differing chemical compositionsand particle sizes, viz. fatty acid modified urethane disper-sion and styrene acrylic dispersion. We show the fundamen-tal physico-chemical properties of the pigments investigatedin Table 1, while Table 2 presents the physico-chemicalproperties of the applied binders.

In paints, moreover, red iron oxide, barium sulphateand microtalc were used. The content of red oxide wasconstant in all formulations. The content of anticorrosivepigments and extenders in paints was based on the de-termination of regions with a composition warranting thehighest value of the critical pigment volume concentration(CPVC) of the mixture (pigment+ two extenders)[51].The concentration of the anticorrosive pigment in theseregions also ensures optimal protection effectiveness of thecoating. The concentration was determined by Ruf’s testas well as gravimetrically, taking into account the entirerange of concentrations (0–100%, v/v) of each compo-nent.

The CPVC was calculated using Bierwagen’s method[52], as to the following formula:

CPVC= φ∑pi=1xi(1 + uia)

360 M. Zubielewicz, W. Gnot / Progress in Organic Coatings 49 (2004) 358–371

whereφ is the densest packing factor,p the number of pig-ments,uia the adsorbed volume of oil on pigmenti, andxithe volume fraction of pigmenti.

In order to determine the effect of excess binder on an-ticorrosion properties of the coatings, three versions of thepaints were prepared for three values of the ratio of pigmentvolume concentration to the critical volume concentration(Λ = CPV/CPVC), the so-called reduced pigment volumeconcentration, e.g.Λ = 0.45 (‘I’), Λ = 0.60 (‘II’), andΛ = 0.80 (‘III’).

The applied marking of the paint samples consists ofthree variable elements: the binder, the pigment and the re-duced pigment volume concentration, e.g. A/1/I denotes:urethane dispersion/zinc phosphate/Λ = 0.45. The pigmentmarkings are shown inTable 1, those of the binders inTable 2.

In porosity and visco-elastic tests free-standing filmswere used. The thickness of the coatings amounted to about70�m. The anticorrosion properties of the coatings weretested in corrosion chambers and by electrochemical meth-ods. For these purposes attached films were used, depositedon St3S steel. Plates with dimension of 7.5 cm × 15 cmwere degreased using a mixture of xylene and acetone andpainted by air-spraying. These coatings were approximately50�m thick. Previous to the investigations the coatings hadbeen conditioned for 2 months at room temperature.

All the investigations concerning the anticorrosion prop-erties of the coatings were carried out on five samplesobtained for each formulation of the paint. The results arepresented as arithmetic means.

2.2. Porosity measurements

We determined the porosity of coatings using a mer-cury penetrometer of the type Autopore II 9220 V2.03 No.16-006, Micrometrics, with the following parameters:

• penetrometer constant: 21.63�l/pF,• maximum head pressure: 3.277× 104 N/m2,• contact angle: 130◦.

The size of the pores was determined within two ranges:350–0.002 and 60–0.002�m. In order to interpret the resultsobtained, the below 60�m was chosen, assuming that largerpores are the result of random errors. The porosity of thecoatings was characterised by quoting the total volume ofthe poresVt in cm3/g of coating and the volume of poresbelow 10�m (V10).

2.3. Analysis of the coatings by mf TMA

Both the pigments and the extenders affect the structure ofthe coating, and thus also their visco-elastic properties andglass transition temperatureTg. In most casesTg is higherin pigmented than in non-pigmented coatings, due to theadsorption of the polymer on the pigment particles and thereduced mobility of the polymer segments. The determina-

tion of Tg and the mechanical properties of the coatings bymeans of thermomechanometric methods lets us investigatethe interaction of the pigments with the binders, indicatingthe packing in the coatings and hence also their cohesionand tightness, which properties considerably influence theeffectiveness of corrosion protection[53]. Modulated-forcethermomechanical analysis (mf TMA) allows us to deter-mine the mechanical properties of polymers in relation totemperature and to frequency of deformation[54–56]. Thistechnique measures the storage modulusE′ and the loss mod-ulus E′′ of the materials. The ratio of these moduli, tanδ,varies with temperature and reaches its maximum value atthe glass transition temperatureTg, permitting measurementof this parameter.

These investigations were carried out by means of theapparatus DMTA MkII, Polymer Laboratories, applying thetension method at a frequency of 1 Hz and an amplitude ofdeformations 16�m within the temperature range of−10 to110◦C, the heating rate being 5◦C/min. The test results werepresented in the form of diagrams of the loss tangent tanδand the storage modulusE′ as a function of temperature,which diagrams were also used to determineTg.

2.4. Testing the corrosion protection

The following methods were applied to test the corrosionprotection of coatings:

• Salt spray test according to ISO 7253.• Prohesion test: 1 h spraying 0.5 g NaCl+ 3.5 g (NH4)2

SO4/dm3 at 25◦C, 1 h dry conditions at 35◦C.• Electrochemical impedance spectroscopy (EIS).• Scanning vibrating electrode technique (SVET).

Impedance measurements provide much informationon the processes at the interface metal/coating/electrolyte[57–75]. By analysing the obtained impedance spectra wecan detect the adsorption of corrosion inhibitors, draw con-clusions concerning the protective properties of variouskinds of coatings and their changes as a function of time,determine the diffusive control of the corrosion process etc.An advantage of this method is the possibility of investigat-ing the adhesion of the coatings and their structure, as wellas the degree and mechanism of degradation.

Tests were carried out using the 9121 Atlas FrequencyResponse Analyser and the Atlas 9181 Electrochemical In-terface. Each sample was immersed in 3% NaCl solution. A10 mV sinusoidal perturbation was applied within the fre-quency range of 10−2 to 105 Hz. The reference electrodewas a saturated calomel electrode (SCE) and the counterelectrode was a platinum grid.

One of the more recent methods for determination of thebehaviour mechanisms of protective coatings is the scan-ning vibrating electrode technique (SVET). The essenceof its operation consists in the detection of the sites ofelectrochemical activity (corrosion) on a metal surfacecovered with an insulating layer, e.g. oxides, conversion


coating or organic coating. SVET is, therefore, adequatefor investigations of local corrosion and for the detectionand scanning of the developing delamination of coatings atpoints of local failures[76–80].

The SVET measurements were made with the apparatusSVET SVP100, Uniscan, Buxton. The measuring electrodewas placed at 250�m from the surface of the coating, andmeasurements were taken during oscillation at a frequencyof 80 Hz with amplitude 50�m, the immersion time being24 h. To allow the measurements, a 0.5 mm hole was drilledin the coatings on steel specimens to initiate the delamina-tion while being immersed in an electrolyte of 10−3 mol/dm3NaCl with 10−3 mol/dm3 EDTA as sequestrant of the cor-rosion products.

The properties of the coatings were characterised by theimpedance modulusZ and phase angleϕ (EIS), the SVETpotential and the global destruction of the coatings in cor-rosion chambers in the conventional scale 0–15 (0, no de-struction; 15, maximum destruction).

2.5. Analysis of passive layers

The formation of passive layers was determined by meansof X-ray diffraction (XRD), energy dispersive X-ray analysis(EDXA) and scanning electron microscopy (SEM).

Layers which had been formed on:

(a) steel immersed for 24 h in an aerated water suspensionof the pigments,

(b) steel under a coating immersed for 72 h in aerated wa-ter were analysed with reference to steel immersed inaerated water for 24 h.

The X-ray phase analysis was carried out making use of adiffractometer of the type XRD 7, Seifert-FPM. Character-istic Co K� radiation was applied as well as a Fe filter. Thediffractograms were carried out within the range of angles2θ from 10.00◦ to 90.00◦, which corresponds to the range ofinterplanar distancesdh k l from 0.1027 to 0.12659 nm. Thecontents of the crystalline phases in the investigated phaseswere analysed based on catalogues:

Fig. 1. Changes ofZmax vs. immersion time depending on the kind of the binder; calcium zinc phosphate;Λ = 0.60.

• Powder Diffraction File Search Manual (HanawaltMethod), Inorganic, JPCDS, 1979.

• Powder Diffraction File, Sets 1-32, JCPDS, 1974.The results of these investigations are shown in the form

of diffractograms, in which the intensity of reflections appearas the maximum height on the diffractogram, with the heightof the highest reflection taken as 100. The SEM micrographsand EDXA of the layer formed on the substrate were carriedout on a scanning microscope LEO 1525 with an X-raymicroanalyser RENTEC.

3. Results and discussion

3.1. Protective properties of coatings

EIS results have been interpreted based on changes ofthe impedance modulusZmax at a frequency 10−1 Hz. Thisquantity is the sum of the resistance of the coatings, theelectrolyte and the charge transfer and may be attributedto electrochemical processes occurring under the coatings[34,52,81]. Some authors consider this to be the most usefulmeasure of the protective properties of a coating[82–84].

Fig. 1illustrates the change ofZmax during the immersionof coatings pigmented with calcium zinc phosphate, depend-ing on the applied binder. The large difference in behaviourof coatings, based on the binders A and B, may be causedby the film forming process and their cross-linking.

Clear coatings display considerable differences in porosity(Table 3). The total volume of poresVt per gram of coating isnearly twice that found with binder B. These differences inthe continuity of the coatings should be accounted for by the

Table 3Porosity of clear coatings

Kind of the binder Vt (cm3/g) V10 (cm3/g) d50 (�m)

A 0.0834 0.0220 0.38B 0.1367 0.0247 0.28


Fig. 2. Temperature dependence of tanδ and Tg for clear coatings.

differing physical properties of these two binders, althoughthey are both dispersions with similar average diameters ofthe particlesd50. It seems that the homogeneity of coatingA is due mainly to better packing of the particles than incoating B, which better packing is connected to a differingparticle size distribution.

The curves presented inFig. 2show distinctly differentTgas well as a different cross-linking in the two binders. Theheight and width of the peaks demonstrate the uniformityand cross-linking density of the coatings[85]. An increasein cross-linking frequently leads to a decreased value oftanδ and a broader peak, as can be seen the case of binderA. Coating A is characterised by greater uniformity and ahigherTg than coating B. The shape of the curve indicatesthat a coating with binder A retains its mechanical propertiesover a wider range of temperatures.

Results of salt spray andProhesion tests have confirmedthe better properties of coatings based on binder A, withrespect to coatings based on binder B.

Fig. 3. Changes ofZmax vs. immersion time depending on the kind of the pigment; binder A;Λ = 0.60.

Fig. 3 illustrates the change ofZmax in time, dependingon the type of pigment applied. The highest values ofZmaxare displayed by coatings with binder A and pigmented withzinc ferrite and calcium zinc phosphate. During the entireperiod of investigation, the capacity character of the spec-trum could be detected, as also the high value of the resis-tance (about 1E8� cm2), which shows the inhibiting effectof these pigments, due to the existing conditions that allowformation of a passive layer.

The analysis of changes in the frequency relation of thephase angle has proved the stable barrier character of thecoatings, stabilised by good wet adhesion and probably alsoby successful inhibition of corrosion. This becomes particu-larly evident in the case of zinc ferrite as an increase of thephase angle in the low-frequency range and its insignificantchanges during the whole period of immersion of coatingsin the electrolyte (Figs. 4 and 5). During the whole assessedtime of immersion no development of corrosion could bedetected in the pores of the coating, and the constant low


Fig. 4. Bode plots for pigmented coatings after 24 h immersion in 3% NaCl; binder A;Λ = 0.60.

value of frequency break point (about 5 Hz) indicates thatthe surface of direct contact of the electrolyte with the sub-strate does not develop, which indicates good adhesion ofthe coatings.

Coatings pigmented with zinc phosphate and calcium-exchanged silica behaved similarly during the whole time ofimmersion. These coatings displayed weaker barrier prop-erties than coatings pigmented with calcium zinc phosphateand zinc ferrite, so resistive paths were formed and corro-sion could develop, probably due to incomplete inhibitionof electrode reactions.

The inhibition of electrode reactions (electrochemicalmechanism) can be detected only in the cases of zinc ferriteand, to some extent, calcium zinc phosphate. No electro-chemical action of zinc phosphate and calcium-exchangedsilica has been recorded. Neither do they seriously affectthe increase in barrier properties of the coatings.

Figs. 6 and 7illustrate the SVET results of the coatingsin relation to the kind of the pigments whenΛ = 0.60.The negative sign of the measured potential corresponds toan anodic process at the surface of the electrode, whereasthe positive sign refers to a cathodic process. The absolutevalue of the negative potential correlates with the intensityof the anodic process. The absolute value of the positive

potential concerns not only the intensity of the cathodicprocess at the point of failure of the coatings, but also itsintensity under the coating and the effect OH− ions formedthere have on the passivation of metal at the point offailure.

Positive SVET potentials and small changes of potentialin time have been found for coatings pigmented with cal-cium zinc phosphate and zinc ferrite. In the case of calciumzinc phosphate, however, the absolute values of the poten-tials are higher. The exposed surface of steel, where the coat-ing has been damaged and for both these pigments, playsthe role of a cathode and is effectively protected againstcorrosion.

Negative SVET potentials of coatings pigmented withzinc phosphate indicate an active corrosion of steel at thecoating defect site, although the low absolute values of thepotentials and the small change of potential in time showthat the rate of these processes is rather slow, particularly inthe case of coating based on binder A.

In the case of calcium-exchanged silica, we found that thevalues of negative potential are the highest and in time thereare large changes of potential. This pigment cannot inhibitthe anodic process at the point of failure of the coatingsinvestigated.


Fig. 5. Bode plots for pigmented coatings after 240 h immersion in 3% NaCl; binder A;Λ = 0.60.

Taking into account both the sign and the absolute valueof the measured SVET potentials, it can be seen that calciumzinc phosphate and zinc ferrite clearly display the best pro-tection effectiveness. Both these pigments have a tendencyto passivate the steel substrate and to impose the cathodicfunction at the site of failure. The other two pigments do

-0,5

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0 200 400 600 800 1000 1200 1400 1600

Immersion time (minutes)

SV

ET

pot

entia

l (m

V)

A/1/II A/2/II A/3/II A/4/II

Fig. 6. SVET potential vs. immersion time depending on the kind of the pigment; binder A;Λ = 0.60.

not display any electrochemical activity in the investigatedcoatings.

The results of salt spray andProhesion tests (Figs. 8and 9) indicate different protective properties of the coatings,dependent on the kind of the pigment. The best propertieshave been found in coatings pigmented with calcium zinc


-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0 200 400 600 800 1000 1200 1400 1600

Immersion time (minutes)

SV

ET

pot

entia

l (m

V)

B/1/II B/2/II B/3/II B/4/II

Fig. 7. SVET potential vs. immersion time depending on the kind of the pigment; binder B;Λ = 0.60.

Fig. 8. The degree of destruction of coatings depending on the kind ofthe pigment; salt spray test; exposition time 360 h; binder A;Λ = 0.60.

phosphate and zinc ferrite. The results of tests in corrosionchambers correlate well with the results of electrochemicaltests.

The behaviour of coatings under corrosive conditionsmay be due to both the effect of the pigments and to thestructure of the coating. The pigmentation of waterbornesystems favourably affects the reduction of pores in the

Fig. 9. The degree of destruction of coatings depending on the kind ofthe pigment;Prohesion test; exposition time 1000 h; binder A;Λ = 0.60.

coating. The total volume of pores per gram of the coatingdecreases two- to four-fold (Table 4), perhaps due to thegood wetting of the pigments with the binder as well tothe content of the pigment in the packing while the film isbeing formed. Calcium zinc phosphate and similarly alsocalcium-exchanged silica reduce the porosity of coatingsto a greater extent in the pore-size range below 10�mthan other investigated pigments. This becomes particu-larly evident in the case of binder A modified with fattyacids, so that calcium soaps may be formed sealing thecoating.

The kind of applied pigment affects essentially the struc-ture of the coating (uniformity, cross-linking) as may beseen from the shape of the curves and value ofTg presentedin Fig. 10. The flattest peaks and highest values ofTg oc-cur in coatings with zinc ferrite and calcium-exchanged sil-ica, which proves the interaction pigment–binder and alsopigment–pigment, as well as the favourable influence ofsmall pigment particles on the structure of the coating (cf.Table 1). The increase ofTg due to pigmentation can be ex-plained by the adsorption of the binder on the surface of thepigment particles, the increased density of the cross-linkingand consequently also by the weaker mobility of the poly-mer segments[53].

Tests results concerning coatings with a different levelof pigmentation, expressed by value ofΛ, indicate betterprotective properties of coatings withΛ = 0.45 andΛ =0.60 than coatings withΛ = 0.80 (Fig. 11).

Table 4Porosity of coatings dependent on the kind of the pigment

Kind of the coating Vt (cm3/g) V10 (cm3/g) d50 (�m)

A/1/II 0.0294 0.0211 7.12A/2/II 0.0307 0.0130 2.51A/3/II 0.0277 0.0156 0.62A/4/II 0.0224 0.0108 1.35


Fig. 10. Temperature dependence of tanδ and Tg depending on the kind of the pigment; binder A;Λ = 0.60.

3.2. Protective layers

No passive layers could be detected on the surface ofplates immersed in zinc phosphate and calcium-exchangedsilica suspensions, nor under coatings with these pigments.Diffractograms merely displayed peaks which are character-istic for �-Fe, as well as for standard samples.

In the case of calcium zinc phosphate and zinc ferrite,both in the suspension and under the coating, the formationof layers was detected (Figs. 12 and 13). In the calciumzinc phosphate suspension a well-adhering layer of Fe3O4is formed, as may be seen in the diffractograms inFig. 12.Under the coating in both binders, besides Fe3O4, phosphatewas also found with the composition Fe3(PO4)2(OH)3 (cf.the standard inFig. 14). In the case of zinc ferrite on thesubstrate, both in the suspension and under the coating, alayer of Fe3O4 forms, as can be seen in the diffractogramsin Fig. 13(cf. the standard inFig. 14).

Figs. 15 and 16present SEM micrographs of the surfaceand composition of the surface layer on steel plates previ-ous to and after 24 h immersion in calcium zinc phosphate

Fig. 11. Changes ofZmax vs. immersion time depending onΛ; binder A; calcium zinc phosphate.

suspension. On steel immersed in the pigment suspension(Fig. 16), there is distinctly visible the compact layer identi-fied by means of XRD. The EDXA results confirm the for-mation of an oxide layer on the steel surface in the presenceof calcium zinc phosphate.

4. Mechanisms of anticorrosion pigments

The EIS results indicate that the investigated pigments donot increase the barrier protection provided by the binders.Pigments reduce the porosity of the coatings; the smallerthe pigment particles, the lower the porosity and the greaterthe stiffness of the polymer due to the geometry of the pig-ment or the specific interaction pigment–binder, resultingfrom the adsorption or formation of chemical bonds withfunctional groups. No share of the investigated pigmentsin the barrier mechanism of anticorrosion effects has beenfound.

The active role of anticorrosion pigments consists in theelimination or inhibition of corrosive reactions on the anode


Fig. 12. Diffractograms of a layer formed in the presence of calcium zinc phosphate.

and/or cathode. If the pigment is to meet these requirements,the inhibitor in the corrosive medium must be released. Thepigment can pass over to the electrolyte thanks to its havingbeen dissolved, by hydrolysis or ion exchange. A rapid re-lease of the inhibitor leads to destruction of the coating byosmotic blistering, whereas slow release leads to too low aconcentration to initiate corrosion inhibiting reactions.

The determination of the inhibiting properties of the in-vestigated pigments in an aerated aqueous suspension andunder the coating has proved the lack of any passivation ef-fect of zinc phosphate and calcium-exchanged silica. Theabsence of a passive layer proves that the inhibitor contentin the corrosive medium is very small and the corrosion isuncontrolled. The lack of activity of these pigments has beenconfirmed by means of the XRD, EDXA and SEM as wellas in corrosive investigations by means of electrochemicalmethods and in corrosion chambers. The obtained resultscontradict the mechanism of the protective effect of thesepigments quoted in reference literature.

The solubility of zinc phosphate under neutral conditionsis too weak to take part in reactions inhibiting corrosion.More soluble phosphates, e.g. zinc polyphosphate and cal-

cium phosphate, form protective layers which contain phos-phates[40,41].

Similarly also as in the case of calcium-exchanged silica,the concentration of the inhibitor in the corrosive medium isvery small. The assumptive mechanism of the action of thispigment assumes only a low solubility of SiO2 in alkalineconditions[48,49]. The absence of a passivating effect ofthis pigment proves that it does not take part in the protectionof steel when the pH conditions are nearly neutral.

Calcium zinc phosphate and zinc ferrite passivate the steelsubstrate, as can be proved by the results of X-ray and elec-trochemical investigations. XRD and EDXA results haveproved in the case of both these pigments the presence of anoxide layer on the substrate with a stoichiometry approxi-mately the same as in the case of Fe3O4. In the presence ofcalcium zinc phosphate, iron phosphate Fe3(PO4)2(OH)3 isalso formed.

The formation of a passive layer consisting of oxides andphosphates in the presence of phosphate ions may be ac-counted for by the formation of iron phosphates during thefirst stage, followed by the oxidation of Fe2+ ions to do Fe3+ions and the formation of Fe3O4 and/or Fe2O3 [45,86]. Due


Fig. 13. Diffractograms of a layer formed in the presence of zinc ferrite.

Fig. 14. Standard diffractograms of Fe3O4 and Fe3(PO4)2(OH)3.


Fig. 15. SEM micrograph and surface analysis of a steel plate previous to immersion in pigment suspension.

to the presence of phosphates in the passive layer, this ismore compact[24,41,86].

The formation of a passive layer on the steel surface inthe presence of zinc ferrite may be the result of pH controlof the medium due to the hydrolysis of ferrite. This pigmentbuffers the medium, and the corrosive reactions on the cath-ode stimulate its hydrolysis.

Most models of the passive layer on steel assume itsdouble-layer structure: Fe3O4 on the side of the metal and�- or �-Fe2O3 on the side of the electrolyte[24,29,86,87].The Fe3+ oxide in the external layer can be hydrated and ex-ists as Fe2O3 + H2O or oxyhydroxide FeOOH. More recentinvestigations have shown that the passive layer is not con-structed of compounds with strictly determined stoichiome-try, but contains compounds with variable stoichiometry andis amorphous. The main role of anticorrosive pigments may

be a reduction of the activation energy aiming at the forma-tion of Fe3−xO4 with higher non-stoichiometry[29]. Pig-ments which catalyse the formation of Fe3−xO4 with highernon-stoichiometry warrant a better protection of steel.

The identification of substances formed on steel substratein the presence of calcium zinc phosphate and zinc ferritemerely by means of X-ray method did not allow exact recog-nition of the structure of the passive layers and the kineticsof their development, but together with the results of elec-trochemical investigations it proves the active mechanism ofthe effect of these pigments.

The test results indicate that in spite of the fact that thecoating structure can be improved by all the tested pigmentsapplied up to some given volume concentration, effectiveprotection of a steel substrate is guaranteed only by those be-having in compliance with the electrochemical mechanism.


Fig. 16. SEM micrograph and surface analysis of a layer formed on a steel plate after 24 h immersion in calcium zinc phosphate suspension.

5. Conclusions

• The protective properties of waterborne coatings dependessentially on the chemical structure of the binders andon their film forming mechanism, as expressed, amongothers, by their cross-linking, their porosity and the glasstransition temperatureTg. These parameters determinethe barrier properties of the coatings.

• The pigmentation of coatings up to a given volumeconcentration, expressed by the valueΛ, improves thestructure of the coatings, although an effective corrosionprotection of steel substrate can be guaranteed only bythose pigments which behave with the electrochemicalmechanism.

• Among the non-toxic anticorrosive pigments investigated,zinc ferrite and calcium zinc phosphate behave in com-

pliance with the electrochemical mechanism. None ofthe investigated pigments increases the barrier propertiesof the binders.

• The joint application of investigations in corrosion cham-bers and electrochemical and mechanical investigationsallows us to assess the protection effectiveness of coat-ings and to offers an insight into the mechanism ofanticorrosion pigments.

Acknowledgements

The authors wish to express their gratitude to Dr. ElżbietaKamińska-Tarnawska for her participation in the realisationof this research project. This research work was sponsoredby the Polish Committee for Scientific Research under theproject number T09B 023 22.


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Mechanisms of non-toxic anticorrosive pigments in organic waterborne coatingsIntroductionExperimentalSamplesPorosity measurementsAnalysis of the coatings by mf TMATesting the corrosion protectionAnalysis of passive layers

Results and discussionProtective properties of coatingsProtective layers

Mechanisms of anticorrosion pigmentsConclusionsAcknowledgementsReferences

mechanisms of non-toxic anticorrosive pigments in organic...

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