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Progress in Organic Coatings 86 (2015) 147–163

Contents lists available at ScienceDirect

Progress in Organic Coatings

j o ur na l ho me pa ge: www.elsev ier .com/ locate /porgcoat

nvestigation of the anticorrosion efficiency of ferrites Mg1−xZnxFe2O4

ith different particle morphology and chemical composition inpoxy-ester resin-based coatings

ndrea Kalendová ∗, Petr Rysánek, Katerina Nechvílováaculty of Chemical Technology, University of Pardubice, 532 10 Pardubice, Czech Republic

r t i c l e i n f o

rticle history:eceived 8 November 2013eceived in revised form 24 February 2015ccepted 8 May 2015vailable online 2 June 2015

eywords:

a b s t r a c t

Mixed metal oxides with the ferrospinel structure ZnFe2O4, Mg0.2Zn0.8Fe2O4 and MgFe2O4 were syn-thesised by the high-temperature solid-phase process. A series of pigments containing the metalsMg–Zn–Fe was prepared from 4 different starting iron oxides: viz. goethite (FeOOH), hematite (Fe2O3),magnetite (FeO·Fe2O3) and specularite (Fe2O3). The properties of the ferrites as pigments were exam-ined in a solvent-based epoxy-ester resin-based coating material at a pigment volume concentrationPVCferrite = 10%. The anticorrosion efficiency of the paints with the ferrites was examined by exposing

erritesnticorrosion pigmentaintoating materialpoxy-ester resinon-isometric particle

panels coated with the paints to atmospheres with SO2, NaCl, or condensed moisture. Furthermore, thephysico-mechanical properties of paint films containing the pigments were evaluated by standardisedtests. Ferrites prepared from the needle-shaped FeOOH or lamellar Fe2O3 emerged as pigments withthe best anticorrosion properties. From the aspect of chemical composition, the paint films containingMg0.2Zn0.8Fe2O4, i.e. combinations of the cations Mg–Zn, were assessed as the best.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

A number of nontoxic anticorrosion pigment types are currentlyvailable for application in coating materials [1,2], with their anti-orrosion efficiency in the paints being mostly adequate. The abilityf paint to provide anticorrosion protection to metal substrates isoverned by the chemical composition of the anticorrosion pig-ent, its concentration in the coating material binder [3], and on

he amounts of other pigments present, such as fillers and spe-ial additives. The protective effect also depends on the method ofetal surface pre-treatment and a number of other factors [4,5].

he majority of available nontoxic pigments provide roughly theame level of protection; however, the efficiency of none of thesere close to that of the toxic chromate pigments [6]. The use of anti-orrosion pigments whose particles possess a non-isometric shapes an interesting approach. Such pigments in paints provide a higharrier protection, and adhesion of the paint to the substrate is also

mproved, meaning that the particles help to slow down corrosion

rocesses acting on the substrate. If the particles also exhibit activehemical and corrosion inhibiting effects, the pigments may showromise for use in anticorrosion protection paints [7].

∗ Corresponding author. Tel.: +420 728994274.E-mail address: andrea.kalendova@upce.cz (A. Kalendová).

ttp://dx.doi.org/10.1016/j.porgcoat.2015.05.009300-9440/© 2015 Elsevier B.V. All rights reserved.

The synthesis of ferrite-based pigments represents anotherpromising field in the development of anticorrosion pigments.Owing to their structure or chemical composition, ferrites have abeneficial effect of the paint’s anticorrosion properties and con-tribute to the stability of the coating system when exposed toelevated temperatures [8]. Even more efficient than ferrite-typespinel pigments [9,10], with combinations of 2 cations in the lat-tice (ZnFe2O4), are Generation II spinel pigments with 3 cations(Mg1–xZnxFe2O4 and Ca1–xZnxFe2O4). Their anticorrosion effectis based their reaction with a suitable binder which producesorganometallic soaps [11]. Among their assets is their alkalinity,owing to which they are able to neutralise acid binders, therebyshifting the pH to a range which is less favourable for corrosionreactions [12]. Their environmental friendliness is also an impor-tant aspect.

Spinel-type pigments are substances of crystalline nature whoseproperties depend on the properties of the host lattices. The crys-tal structure of spinel pigments derives from the mineral spinel(MgAl2O4). Three oxidation state combinations are conceivable forcharge compensation in oxides with the general formula AB2O4:A2+B2

3+O4, A4+B22+O4 and A6+B2

1+O4; the anion is largely O2−

[13,14]. The factors determining which atom combinations canform a spinel structure include the formal charge of the cation,the relative size of the cations [15]. A crystal lattice can be sta-ble only if the ion radii and their ratios (valency and valency ratio,

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ing in their colour and, in particular, particle shape were used inthe preparation of the ferrites from ferric oxide. The one iron oxidetype was red Fe2O3 with an isometric particle shape. The mate-rial was commercial Bayferrox 130 M (Lanxess-Bayer Leverkusen,

Table 1Amounts (weight and molar fractions with respect to the final product) of the start-ing substances taken to the synthesis.

Starting material: FeOOH

Pigment FeOOH ZnO MgCO3

Wt.% Mol.% Wt.% Mol.% Wt.% Mol.%

ZnFe2O4 68.59 66.6 31.41 33.3 – –MgFe2O4 67.82 66.,6 – – 32.18 33.3Mg0.2Zn0.8Fe2O4 68.43 66.6 25.08 26.6 6.49 6.6

Starting material: Fe3O4

Pigment Fe3O4 ZnO MgCO3

Wt.% Mol.% Wt.% Mol.% Wt.% Mol.%

ZnFe2O4 65.47 40 34.53 60 – –MgFe2O4 64.68 40 – – 35,32 60Mg0.2Zn0.8Fe2O4 65.31 40 27.56 48 7.13 12

Starting materials: �-Fe2O3, lam. �-Fe2O3

Pigment Fe2O3 ZnO MgCO3

48 A. Kalendová et al. / Progress in

oncentration proportions of the ions present) do not exceed cer-ain energy and space limits. Admissible radii assume the rangerom 0.4 × 10−10 m to 1.03 × 10−10 m. For instance, because of theiradius, the concentration of Ca2+ ions in a spinel lattice cannotxceed 30% [16]. Among the important properties of oxides pos-essing the spinel structure is their ability to mix together and formolid solutions almost without limitation. Spinels of the same typer different types (such as 2.3 and 4.2) can be combined in this man-er. As a result, simple spinels are able to form mixed spinels, whichre also referred to as substitution solid solutions. Combinations ofivalent and trivalent cations are largely present in ferrospinels, i.e.errites with a spinel structure. They can be usually described by theormula xMeaOb·yFe2O3, where Me = Mg or Zn. In the simplest case,here x = y = 1 and a = b = 1, the formula simplifies to MeO·Fe2O3,hich can be written in the simple form MeFe2O4.

.1. Goal of the study

Pigments based on mixed metal oxides, or ferrites–ferrospinelsf the Mg–Zn–Fe series (Mg1–xZnxFe2O4 where x = 0. 0.8, and 1)ere synthesised. The spinel lattice structure was chosen owing to

he outstanding properties of ferrites, notably their chemical andhermal stability, high covering power and insolubility [12,13]. Theuality and relevant properties of the pigments are affected by thetarting substances (oxide, carbonate, sulphate) and by the particleize, specifically surface area and morphology [17]. The synthesisf ferrites by decomposition of carbonates to oxides leads to theormation of oxides which are more reactive than those which aresed as the starting materials [18]. Alkaline earth (Ca) carbonatesre also more suitable starting materials for the preparation of pig-ents because the product cannot be contaminated by residues of

he unreacted starting substances, as is the case, e.g., with sulphatesnd chlorides.

The modelling and influencing of the properties of the finalroduct was based on the principle of isomorphous ion substi-ution in the initial spinel lattice MgAl2O4. Targeted selection ofations forming solid solutions can be employed to modify thenitial properties of the spinel lattice and change or create newroperties, especially those which affect the corrosion-inhibitingehaviour of the pigment in the coating material binder [19]. Zincnd magnesium ions were also selected owing to their favourableroperties affecting the structure and alkaline nature of the pig-ents. Four iron oxide types which differed in colour, structure,

nd primary particle shape, viz. goethite, magnetite, hematite andpecularite, were used for the preparation of the spinel-type pig-ents. The spinel structure was obtained by reacting the oxidesith zinc oxide (ZnO) as the Zn2+ ion source and with magnesium

arbonate (MgCO3) as the Mg2+ ion source. Calcination of the pig-ents was achieved by the high-temperature solid-phase process

iving rise to ZnFe2O4, MgFe2O4 and Mg0.2Zn0.8O4. The aim of thisrocedure was to obtain ferrites possessing a high anticorrosionfficiency, which is governed by a number of factors such as pig-ent particle morphology, acid-base properties, surface properties,ater soluble fraction, and the like.

. Experimental

.1. Preparation of the spinel type ferrites

.1.1. Preparation of the ferrites from FeOOHThe needle-shaped FeOOH served as the source of ferric ions,

nd zinc oxide (ZnO) and magnesium carbonate (MgCO3) were

he sources of the Zn2+ and Mg2+. The starting ferric oxide-ydroxide (FeOOH) was commercial Bayferrox 920 (Lanxess-Bayereverkusen, Germany), which possesses the goethite structure,ith a mean particle size 0.1–0.6 �m.

ic Coatings 86 (2015) 147–163

The starting materials were used to synthesise 3 spinel-typepigments: ZnFe2O4, MgFe2O4 and Mg0.2Zn0.8O4. In the reactionpathway, ferric oxide-hydroxide is first transformed to Fe2O3 at180 ◦C–200 ◦C (temperature T in reaction (1)), and this intermedi-ate reacts further with the Zn (ZnO) or Mg (MgO) cations (reactions(2)–(4)):

2FeOOHT−→Fe2O3 + H2O (1)

Fe2O3 + ZnO → ZnFe2O4 (2)

Fe2O3 + MgCO3 → MgFe2O4 + CO2 (3)

Fe2O3 + 0.2MgCO3 + 0.8ZnO → Mg0.2Zn0.8Fe2O4 + 0.2CO2 (4)

The above equations were used to calculate the amounts (pro-portions) of the starting substances with respect to the desiredamount of the resulting pigment (Table 1).

2.1.2. Preparation of the ferrites from FeO·Fe2O3Magnetite Fe3O4 with isometric particles was used as the start-

ing source of ferric ions in this case. Again, ZnO and MgCO3 servedas the sources of the other cations. The starting iron oxide wascommercial Bayferrox 316 (Lanxess-Bayer Leverkusen, Germany),chemically ferrous-ferric oxide FeO·Fe2O3, with a magnetite struc-ture. The proportions of the starting substances are given in Table 1.The reactions of the starting substances are described by the follow-ing equations:

2Fe3O4 + 3ZnO + 1/2O2 → 3ZnFe2O4 (5)

2Fe3O4 + 3MgCO3 + 1/2O2 → 3MgFe2O4 + 3CO2 (6)

2Fe3O4 + 0.6MgCO3 + 0.4ZnO + 1/2O2

→ 3Mg0.2Zn0.8Fe2O4 + 0.2CO2 (7)

2.1.3. Preparation of the ferrites from ˛-Fe2O3Two iron oxide types possessing identical structures and differ-

Wt.% Mol.% Wt.% Mol.% Wt.% Mol.%

ZnFe2O4 66.23 50 33.77 50 – –MgFe2O4 65.45 50 – – 34.55 50Mg0.2Zn0.8Fe2O4 67.37 50 26.95 40 5.75 10

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ermany), which is synthetic �-Fe2O3, with a hematite structure.he other oxide type was also �-Fe2O3 hematite, but its particlesad a lamellar shape (“lam. Fe2O3”). The material used was com-ercial Specularite MIOX–Sp44, which is natural �-Fe2O3 with a

amellar particle shape, containing very small amounts of the micalinochlore. The two iron oxide types served as the source of Fe3+

ons for the formation of the spinel structure. Again, ZnO and MgCO3ere the other reaction components. The reactions are represented

y Eqs. (2)–(4). The starting substance weight proportions are givenn Table 1. The morphology of the particles of the starting ironxides is illustrated by Fig. 1.

.2. Laboratory procedure for the preparation of the pigments

The calculated amounts of the starting materials were weighedith a precision of ±0.01 g, were mixed, and then subjected to dryomogenisation in an agate mortar for 30 min. The homogeneousixtures were transferred to refractory ceramic crucibles made

f sintered corundum and subjected to stepwise calcination in anlectric furnace. The synthesis of ZnFe2O4 included two calcinationteps. The first step was conducted at 600 ◦C, with a temperatureamp of 5 ◦C/min and the holdup time at the maximum temperaturef 2 h. The second calcination step was conducted at 1000 ◦C, with

temperature ramp of 10 ◦C/min and the holdup time at the maxi-um temperature of 3 h. In the synthesis of ferrites containing Mg,

.e. MgFe2O4 and Mg0.2Zn0.8Fe2O4, the calcination process encom-

assed three steps, which were conducted at higher temperaturesecause MgO is a more difficult starting point for the formationf spinels. The first step was conducted at 1050 ◦C, the temper-ture ramp was 5 ◦C/min and the holdup time at the maximum

ig. 1. Morphology of the particles of the starting Fe oxides: (a) goethite (�-FeOOH, ferretector; (c) hematite (�-Fe2O3, ferric oxide), SEI detector; (d) specularite (lam �-Fe2O3)

ic Coatings 86 (2015) 147–163 149

temperature was 2 h. In the second step, the calcination tempera-ture was increased to 1150 ◦C, the temperature ramp was kept at5 ◦C/min, and the holdup time was extended to 2.5 h. In the thirdstep, the calcination temperature was again increased, to 1180 ◦C,the temperature ramp was left at 5 ◦C/min, and the holdup timeat the maximum temperature was extended to 3 h. The calcinationtemperatures were selected based on previous X-ray analysis.

The pigments obtained by the calcination process were sub-jected to wet milling in ethanol. The pigment in ethanol wastransferred to a 500 ml zirconium oxide vessel containing millingballs that were 1 cm in diameter and were made of corundum. Thevessel was placed in a Pulversitte 6 planetary mill and milled at400 rpm for 1.5 h (ZnFe2O4) or 3.5 h (MgFe2O4, Mg0.2Zn0.8Fe2O4).The product was then filtered through a Büchner funnel equippedwith a KA 4 type (Czech product) filter paper for qualitative anal-ysis. The filtration cake was rinsed with 2 litres of distilled waterat 60 ◦C in order to remove water-soluble substances. Finally, thefiltration cake was rinsed with 100 ml of ethanol. The product wasdried at 80 ◦C for 24 h to remove ethanol and at 105 ◦C for another48 h to remove all moisture.

2.3. Elucidation of the pigment particle structure and morphology

X-ray diffraction spectra were measured on an X’Pert PRO MPD1880 X-ray diffractometer (PANanalytical, The Netherlands). The

diffraction data were evaluated by means of the X’Pert programs(X’Pert HighScore Plus Software version 2.1b and X’Pert Indus-try Software version 1.1g); the phases were identified using datafrom the ICCD PDF2 diffraction database. The pigment surface and

ic oxide-hydroxide), BEC detector; (b) magnetite (Fe3O4, ferrous-ferric oxide), SEI, SEI detector.

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article shape were examined on a JEOL-JSM 5600 LV scanninglectron microscope (JEOL, Japan) in the secondary electron mode.

.4. Examination of the physico-chemical properties of theigments

A helium AutoPycnometer 1320 (Micromeritics, USA) was usedo determine the pigments’ specific weight. Linseed oil consump-ion was measured by the pestle-mortar method [20]. The result ofhe measurement, the oil number (in g/100 g), is a necessary quan-ity for calculation of the CPVC and for the formulation of coating

aterials (paints). The pH of aqueous extracts of the pigments waseasured in accordance with ISO 789-9 by preparing 10 wt.% pig-ent suspensions in redistilled water (pH = 7), measuring their pH

eriodically over a period of 30 days, filtering the suspensions offfter that period (i.e. after the pH had reached a constant value), andeasuring the final pH of the filtrate (pH30). A WTW 320 pH meter

WTW, Germany) was used in conjunction with calibration bufferspH 4.01, 7.00. 10.01 and 12 at 25 ◦C) for the measurements. Specificlectric conductivity of aqueous extracts was measured conducto-etrically in 10% suspensions of the pigments in redistilled water

specific electric conductivity = 3 �S/cm) using a Handylab lF1 con-uctometer (SCHOTT, Germany) in conjunction with calibrationolutions (conductivity 37 and 1413 �S/cm at 25 ◦C). This proce-ure was based on the ISO 787-14 standard. Measurements wereaken periodically over a period of 30 days, after which the ulti-

ate (steady-state) specific conductivity (�30) was recorded. Theltrates of the 10% pigment suspensions obtained in this mannerere used to determine corrosion losses of steel panels submerged

n them [21]. The water-soluble fraction was determined gravi-etrically by extraction of the powdered pigment (weighed with

precision of ±0.01 g) in boiling distilled water (W100). This pro-edure was derived from the CSN EN ISO 787-3 standard [22]. AASTERSIZER 2000 particle size analyser was used for determina-

ion of the particle size distribution. This measurement is based onhe interaction of electromagnetic radiation with the particles ofhe dispersion system. Particle size is represented by the diameterf the equivalent sphere, i.e. sphere whose laser radiation disper-ion patterns are identical with those of the particle in question.

.5. Assessment of the anticorrosion efficiency of the pigments inhe paints

Model solvent-based epoxy-ester resin-based paints were for-ulated for investigation of the pigments’ anticorrosion properties.s model systems, the paints did not contain any other fillers ordditives that might appreciably affect the resulting efficiency ofhe paint. The pigment volume concentration (PVC) in the paintsas invariably 10%.

The binder for the paint formulation was epoxy-ester resin, a0% solution of a medium molecular weight epoxy resin esterifiedith a mixture of fatty acids of dehydrated ricin oil and soy oil

60% epoxide, 40% conjugated fatty acid), acidity 4.1 mg KOH, den-ity 1.07 g/cm3 viscosity 2.5–5.0 Pa s, flow time (DIN 53211-4 200)50 s, commercial name WorléeDur D 46. Co-octoate in a fractionf 0.3 wt.% was used as the siccative.

The paints were prepared by dispersing the powders in the liq-id binder in a Dispermat CV pearl mill (WMA GETZMANN GmbHerfahrenstechnik, Germany). Bentonite (0.5 g) was added to sup-ress unwanted sedimentation of the pigment particles.

.6. Preparation of samples for testing the anticorrosion

roperties

The test samples were prepared by coating the paints onto steelanels (deep-drawn cold-rolled steel manufactured by Q-panel,

ic Coatings 86 (2015) 147–163

UK) with a size of 150 mm × 100 mm × 0.9 mm by means of a box-type application ruler 200 �m slot width, as per ISO 1514. Afterthe layer had dried, a second layer was applied so that the totaldry film thickness (DFT) was 95 ± 10 �m. The dry film thicknesswas measured with a Minitest 110 magnetic thickness gauge fit-ted with a F16 type probe (Elektrophysik, Germany) in accordancewith ISO 2808. Ten panels were prepared for each paint. A test line8 cm long and penetrating through to the substrate was cut intothe paint film in the lower half of each test panel.

The paint films on the panels were allowed to dry in standardconditions (20 ◦C ± 2 ◦C, 50% RH) for 6 weeks in an air-conditionedlaboratory. Paint films were also prepared on polyethylene sheets,allowed to dry, peeled off, and cut into pieces 1 mm × 1 mm size.The squares of the unsupported films were used to prepare 10%aqueous suspensions of the paint films in redistilled water.

2.7. Laboratory corrosion tests

Cyclic three-phase corrosion test in atmospheres with conden-sation of water, with NaCl, and with SO2 was performed. In thistest, the test samples, i.e. steel panels coated with the paints, wereexposed to condensed water at 40 ◦C for 12 h (Phase 1 Stage 1) andallowed to dry at 23 ◦C for 12 h, as per CSN EN ISO 6270 (Phase 1Stage 2). Following exposure for 1152 h, the samples were removedfrom the condensation chamber and inserted for 240 h into a testchamber with a salt mist of 5% NaCl (ISO 7253). This was test Phase2, which was followed by Phase 3, for which the samples wereplaced into a test chamber with SO2 for 240 h, as per CSN EN ISO3231. The complete test took 1632 h.

Corrosion test with condensation of water in the presence of SO2was performed. This test was performed in 24-h 2-stage cycles, asper CSN EN ISO 3231. In the first stage, the samples were exposedto an environment with condensation of distilled water containingSO2 (0.2 mg/l) for 8 h. In the second stage, the samples were allowedto dry at 23 ◦C for 16 h. The first test panel batch was exposed for800 h, and the second, for 1600 h. Subsequently, the samples wereevaluated.

Corrosion test in an environment with sprayed solution of NaClwas performed. This test, which was based on ISO 7253, comprisedthree stages. First, the test panels were exposed to the mist of aneutral 5% NaCl solution at 35 ◦C for 10 h (Stage 1) and then to anenvironment with condensation of water at 40 ◦C for 1 h (Stage 1).Subsequently, the samples were allowed to dry at 23 ◦C (Stage 3)[19]. The first test panel batch was evaluated following exposure for800 h, while the exposure was extended to 1600 h for the secondtest panel batch.

2.8. Corrosion test evaluation methods

The corrosion tests were evaluated in accordance with the stan-dards ASTM D 714-87, CSN ISO 2409, ASTM D 610-85, and ASTMD 1654-92. The methods were based on a comparison of the paintfilm degradation effects between the sample and the standard. Thefollowing corrosion effects were assessed: formation (=size and fre-quency of occurrence) of blisters on the film surface, both acrossthe paint film area and near the test cut made in the film; extent ofsubstrate metal surface corrosion (corrosion-affected surface areafraction in %); and propagation (in mm) of corrosion in the vicinityof the test cut. Adhesion of the paint film to the substrate (glassplate) following the corrosion test and subsequent drying for 24 hwas also examined, as per CSN ISO 2409.

2.9. Determination of steel weight loss due to corrosion

In this test, steel panels of defined size and known weightwere submerged for 10 days (a) in filtrates of 10 wt.% aqueous

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uspensions of the powdered pigments and (b) in aqueous extracts

f 10 wt.% suspensions of pieces of the unsupported paint films con-aining the pigments tested [23]. In both cases, the suspensions hadeen allowed to stand for a period long enough (30 days in this case)

ic Coatings 86 (2015) 147–163 151

for steady-state pH and specific electric conductivity levels to be

established. The observed corrosion weight losses were convertedto relative data (%) with respect to the values in water. The relativedata obtained from the suspensions of the powdered pigments and

152 A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163

Fig. 2. (a) Results of X-ray diffraction analysis of ZnFe2O4 (starting substance: FeO(OH)). (b) Results of X-ray diffraction analysis of MgFe2O4 (starting substance: FeO(OH)). (c)Results of X-ray diffraction analysis of Mg0.2Zn0.8Fe2O4 (starting substance Starting substance FeO(OH)). (d) Results of X-ray diffraction analysis of ZnFe2O4 (starting substancemagnetite FeO·Fe2O3). (e) Results of X-ray diffraction analysis of MgFe2O4 (starting substance magnetite FeO·Fe2O3). (f) Results of X-ray diffraction analysis of Mg0.2Zn0.8Fe2O4

(starting substance Starting substance magnetite FeO·Fe2O3). (g) Results of X-ray diffraction analysis of ZnFe2O4 (starting substance hematite. �-Fe2O3)). (h) Fig Results ofX-ray diffraction analysis of MgFe2O4 (starting substance: hematite �-Fe2O3). (i) Results of X-ray diffraction analysis of Mg0.2Zn0.8Fe2O4 (starting substance Starting substancehematite. �-Fe2O3). (j) Results of X-ray diffraction analysis of ZnFe2O4 (starting substance specularite lam. �-Fe2O3). (k) Results of X-ray diffraction analysis of MgFe2O4

(starting substance: specularite lam. �-Fe2O3). (l) Results of X-ray diffraction analysis of Mg0.2Zn0.8Fe2O4 (starting substance Starting substance specularite lam. �-Fe2O3).

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f the unsupported paint films are denoted Xp and Xf, respectively24].

Tests were also performed to examine the paints’ adhesion-arrier properties. The factors measured included adhesion to theubstrate, hardness, and strength. Because unsupported paint filmsre difficult to prepare, the properties of the paints were measuredirectly on metal substrates in accordance with standardised tests.he tests imitate mechanical stresses in the external environment,uch as an object being dropped onto the surface (impact test) andeformations caused by bending and elongation (bending test andupping test).

Degree of adhesion of the paints was performed by the cross-cutest (ISO 2409). The lattice pattern was cut into the paint by meansf a special cutting instrument with cutting blades that were 2 mmpart. The degree of adhesion of the 1 mm × 1 mm squares to theubstrate was assessed.

Impact resistance (ISO 6272) measured the maximum height ofree drop of a weight (1000 g) at which the paint film still resistedamage. The test was performed by dropping the weight onto bothhe adverse and reverse sides of the test panel.

Resistance of the paint film against cupping was made in anrichsen cupping tester (ISO 1520). The objective of this test was todentify the resistance of the paint film against on-going deforma-ion of a coated steel panel caused by indentation by a 20 mm steelall. The cupping (in mm) giving rise to the first signs of disturbancef the paint film was measured.

Resistance of the coating during bending over a cylindrical man-rel (ISO 1519) provides the largest diameter of the mandrel (inm) causing disturbance of the paint film when the test panel is

ent over it.Pull-off adhesion test (CSN EN ISO 24624) was performed on

n Elcometer 106 pull-off adhesion tester (Elcometer, Germany).irst, the paint film was roughened and degreased, and then a testoller was glued to it. The adhesive was allowed to dry and thenhe roller was attached to the measuring instrument and the pres-ure acting on the paint film was slowly increased. The value (inPa) measured at the moment at which the film was pulled offas recorded.

Reference experiments: for comparison, paints containing thetarting iron pigments and paints containing the non-pigmentedoating materials were also subjected to the same tests.

. Results and discussion

.1. Particle morphology and structure of the ferrites

The results of the X-ray diffraction analysis of the pigments syn-hesised are given in Fig. 2a–l). Morphology of the pigment particlesynthesised is shown in the SEM photographs in Fig. 3.

The pigments prepared were based on ferrites involving diva-ent cations: zinc ferrite (ZnFe2O4), (its solid solution with Mg,

g0.2Zn0.8Fe2O4), and magnesium ferrite (MgFe2O4). A total of 12errites were prepared from 4 different iron oxides differing in com-osition, i.e. cation combinations (Zn–Fe, Mg–Fe, and Mg–Zn–Fe),nd particle shape. The shape of the pigment particles followedhat of the starting iron pigment (Fig. 3). Particles of the ferritesrepared from FeOOH were needle-shaped; this shape was mostronounced in ZnFe2O4 (Fig. 3). The ferrites prepared from specu-

arite consisted of lamellar particles, which were smaller than thosef the starting substance owing to the efficient milling. The fer-ites prepared from magnetite and hematite had isometric nodulararticles. The cubic structure of ZnFe2O4 was confirmed in all sam-

les irrespective of the starting Fe oxide. The ferrites ZnFe2O4 wereree from the starting substances, except for the pigment preparedrom specularite, in which small amounts of the unreacted startingnO and Fe2O3 were present. Magnesium ferrite (MgFe2O4) was

ic Coatings 86 (2015) 147–163 153

cubic. Where magnesium ferrite had been prepared from specular-ite, magnetite or hematite, the product contained a small amountof Fe2O3 as a minority phase. The Mg0.2Zn0.8Fe2O4 solid solution

154 A. Kalendová et al. / Progress in Organ

Fig. 3. SEM photographs showing the morphology of particles of the pigments.(a1) ZnFe2O4 (FeOOH); (a2) ZnFe2O4 (FeOOH); (b1) MgFe2O4 (FeOOH); (b2)MgFe2O4(FeOOH); (c1) Mg0.2 Zn0.8 Fe2O4(FeOOH); (c2) Mg0.2 Zn0.8 Fe2O4 (FeOOH);(d1) ZnFe2O4 (FeO·Fe2O3); (d2) ZnFe2O4 (FeO·Fe2O3); (e1) MgFe2O4 (FeO·Fe2O3) (e2)MgFe2O4 (FeO·Fe2O3); (f1) Mg0.2 Zn0.8 Fe2O4 (FeO·Fe2O3); (f2) Mg0.2 Zn0.8 Fe2O4 (zFeO·Fe2O3); (g1) ZnFe2O4 (�-Fe2O3); (g2) ZnFe2O4 (�-Fe2O3); (h1) MgFe2O4 (�-Fe2O3); (h2) MgFe2O4 (�-Fe2O3); (i1) Mg0.2Zn0.8Fe2O4 (�-Fe2O3); (I2) Mg0.2 Zn0.8

Fe2 4 (�-Fe2O3,); (j1) ZnFe2O4 (lam. Fe2O3); (j2) ZnFe2O4 (lam. Fe2O3); (k1) MgFe2O4

(lam. Fe2O3); (k2) MgFe2O4 (lam. Fe2O3); (l1) Mg0.2 Zn0.8 Fe2O4 (lam. Fe2O3); (l2)Mg0.2 Zn0.8 Fe2O4 (lam. Fe2O3). SEI detector.

ic Coatings 86 (2015) 147–163

3.2. Physico-chemical properties of the pigments

The basic parameters of the pigments prepared from the vari-ous starting materials are summarised in Table 2. The parametersinclude density, oil number, CPVC, and particle size distribution.The particle size distribution values D(0.5), D(0.9), and D(0.1) showthat the size of 50%, 90% or 10% particles, respectively, in the vol-ume is smaller than the specified value. D(4.3) is the mean particlesize. The physico-chemical properties of the powdered pigments,pH, specific electric conductivities of pigment extracts and of theunsupported paint film extracts (in �S/cm), and fractions of sub-stances soluble in boiling water, W100 (in wt.%), are given in Table 3.The pH and conductivity data refer to Day 30, after the steady statesof the two parameters had established.

Milling was a successful way to reduce the ferrite particles to asize that is suitable for use as pigments in paints forming homo-geneous films free from aggregations. The mean particle size ofall ferrites lay within the range of 2.57–3.50 �m (Table 2). As fol-lows from the particle size distribution measurements, the meanparticle size of pigments synthesised from starting substances pos-sessing the needle shape or nodular shape lay within the range of1.20–2.49 �m. The mean particle size was smallest in the ferritesprepared from FeOOH, whereas ferrites prepared from speculariteconsisted of larger particles.

The pH and specific electric conductivity levels of the ferritepigments are given in Table 3. The highest pH values on day 30were observed for ferrites prepared by reacting Fe2O3 with MgCO3(represented by the formula MgFe2O4), followed by mixed fer-rites (Mg0.2Zn0.8 Fe2O4). The lowest pH values were found forpigments containing either zinc ions (ZnFe2O4) or the starting fer-ric oxides (FeOOH, Fe3O4, Fe2O3 and lamellar Fe2O3). The findingof the previous study [25], that ferrites whose particles possess anon-isometric shape give rise to basic solutions, was confirmed bythe present investigation. The pH of aqueous extracts of the paintfilms containing the ferrites tested exhibited an increasing trendduring the initial 30-day period, due to the slow release of ionswhich increased the alkalinity of the water. Magnesium ions werereleased most readily (from the ferrites MgFe2O4), and the pH oftheir extracts lay within the range of 6.1–8.11.

The specific electric conductivity levels increased with time, dueto the release of ions from the pigments or paint films. Ferriteswhich release ions give rise to a higher conductivity than the start-ing Fe oxides. An increase in conductivity during the 30-day periodwas also observed in aqueous extracts of the paint films contain-ing ferrites which had been prepared from FeOOH (Table 3). Onceagain, magnesium ions (MgFe2O4) were found to be released mostreadily. Generally, increasing conductivity accelerates the corro-sion process; in this specific case, however, the corrosion rate wasslowed down by the higher pH, at which corrosion was less severeowing to the properties of the steel.

The fraction of substances that are soluble in hot water is aparameter characterising the efficiency of the product rinsing stepand also an indicator of usability of the pigment in binder sys-tems that are sensitive to the water-soluble salt content. Sincethe water-soluble fraction never exceeded 1%, the hazard of theoccurrence of osmotic blisters was nearly negligible. The water-soluble fraction was lowest in the ferrite MgFe2O4 prepared fromlamellar Fe2O3 (MgFe2O4/lam. Fe2O3). It is noteworthy that thewater-soluble fraction of the ferrites prepared from FeOOH neverexceeded 0.04%, which is the level exhibited by the starting FeOOH(Table 3).

3.3. Corrosion-induced steel panel weight loss

The data of the steel panel corrosion losses in the aqueousextracts of the pigments (Xp) and of the paint films (Xf) are given

A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163 155

Table 2Physical properties (density, oil number, CPVC, and particle size distribution) of the powdered pigments.

Pigment Density* (g/cm3) Oil consumption* (g/100 g) CPVC (–) Particle size Per cent fraction below the specifiedsize in �m

D(0.1) D(0.5) D(0.9) D(4.3)

Goethite (FeOOH)ZnFe2O4 5.151 13.72 56.82 0.456 1.079 2.131 1.206MgFe2O4 4.411 14.76 58.82 0.891 2.188 4.449 2.446Mg0.2Zn0.8Fe2O4 5.049 11.63 61.3 1.457 3.206 1.701 1.701Magnetite (FeO·Fe2O3)ZnFe2O4 5.077 14.68 55.18 0.546 1.788 4.101 2.123MgFe2O4 4.347 13.08 62.06 0.871 1.939 3.763 2.157Mg0.2Zn0.8Fe2O4 4.824 11.79 62.05 0.489 1.597 4.057 2.009Hematite (�-Fe2O3)ZnFe2O4 5.077 19.58 48.33 0.343 1.018 3.666 2.166MgFe2O4 4.323 15.76 57.72 0.955 2.208 4.444 2.484Mg0.2Zn0.8Fe2O4 4.894 11.03 63.27 0.489 1.597 4.057 2.009Specularite (lam. �-Fe2O3)ZnFe2O4 10.73 5.167 62.65 0.552 2.569 7.656 3.496MgFe2O4 15.98 4.285 57.59 0.828 2.357 5.424 2.814Mg0.2Zn0.8Fe2O4 10.5 4.91 64.34 0.550 1.978 5.363 2.572Starting oxidesFeOOH 4.09 35.67 39.81 0.120 1.012 1.201 0.12Fe2O3 4.97 27.05 40.89 0.210 1.230 3.21 0.17FeO·Fe2O3 4.71 22.96 46.24 0.421 1.114 2.350 0.3lam. �-Fe2O3 4.88 13.5 58.54 1.230 2.334 7.612 5.3

* Parameters are given as arithmetic averages within 10 measured values.

Table 3Physico-chemical properties (pH, specific electric conductivity and water-soluble fraction) of the powdered pigments.

Pigment Particle shape pH Specific electricconductivity �30 (�S/cm)

Water-soluble fractionW100 102(%)

Pigment Paint Pigment Paint

Goethite (FeOOH)ZnFe2O4 Needles 7.05 6.28 37.6 21.2 3.48MgFe2O4 9.28 7.07 145.8 106.8 1.59Mg0.2Zn0.8Fe2O4 8.56 6.24 150.5 28 2.69Magnetite (FeO·Fe2O3)ZnFe2O4 Nodular 7.29 6.38 48.4 19.1 1.09MgFe2O4 8.85 6.7 131.5 45.8 6.54Mg0.2Zn0.8Fe2O4 8.65 5.68 103.1 17.1 1.67Hematite (�-Fe2O3)ZnFe2O4 Isometric 5.69 6.06 47.3 11.3 8.79MgFe2O4 8.76 6.1 131.5 42.4 7.79Mg0.2Zn0.8Fe2O4 8.42 5.44 75.1 21.3 1.97Specularite (lam. �-Fe2O3)ZnFe2O4 Lamellar 8.53 5.91 178.4 14.2 11.53MgFe2O4 11.13 8.11 300 339 0.60Mg0.2Zn0.8Fe2O4 8.98 6.65 224 29.2 7.95Reference Fe oxidesFeOOH Needles 4.18 5.48 349 18.8 3.82

5.686.176.37

ila

compifstmpvt

Fe2O3 Isometric 6.19

FeO·Fe2O3 Nodular 6.5

Lam-Fe2O3 Lamellar 8.07

n Table 4. For illustration, the pH and specific electric conductivityevels measured on Day 10 of the test (pH10 and �10, respectively)nd are also displayed.

Study of anticorrosive pigment properties always assumes aomplex view on the anticorrosive pigment/binder interactionsr those between the functional binder groups and surroundingedium. The greatest simplification from this point of view is

resented by following the corrosion losses and corrosion veloc-ties of anticorrosive pigment alone in the aqueous suspensionorm [26]. The corrosion losses were determined after 10 days ofteel panel submersion in aqueous extracts of the pigments or ofhe paint films. The pH and conductivity of the extracts play a

ajor role in the corrosion of the steel panels, as reflected by theanel weight loss data. The tabulated data are relative (per cent)alues with respect to the weight loss in demineralised water,aken as 100%. Corrosion inhibition by the pigments was most

36.9 24 2.47 406 34.6 16.54 191 36 3.23

pronounced for the ferrites prepared from FeOOH (Table 5). Thecorrosion loss in the paint film extracts was 13% higher if the paintinitially contained FeOOH than if the paint contained a ferrite pre-pared from it. Paints containing ferrites prepared from lamellarFe2O3 also exhibited outstanding corrosion-inhibiting properties.In this respect, all paints containing the ferrites synthesised weresuperior to the non-pigmented epoxy-ester coating material. Out-standing results were obtained with the paint films containingthe ferrite MgFe2O4 prepared from FeO·Fe2O3 or from lamel-lar Fe2O3 (MgFe2O4/FeO·Fe2O3, MgFe2O4/lam. Fe2O3) and withthe paint films containing the ferrite Mg0.2Zn0.8Fe2O4 preparedfrom lamellar Fe2O3 (Mg0.2Zn0.8Fe2O4/lam. Fe2O3). Outstanding

corrosion-inhibiting capacity was observed for paints containingferrite pigments prepared from lamellar Fe2O3.

The method of following the corrosion losses in the suspen-sions of organic coatings containing spinels gives evidence on the

156 A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163

Table 4Relative corrosion-induced weight losses of steel panels submerged in aqueous extracts of the pigments (Xp) and of the paint films (Xf), and pH and specific electric conductivity(�) levels of the extracts.

Pigment Aqueous extract of the pigments Aqueous extract of the paint film

pH (–) � (�S/cm) Xp (%) pH (–) � (�S/cm) Xf (%)

Goethite (FeOOH)ZnFe2O4 7.26 49.7 59.80 6.74 32.8 30.48MgFe2O4 7.95 208 75.81 7.0 101.2 29.74Mg0.2Zn0.8Fe2O4 7.78 196.5 48.09 6.89 51.1 28.69Magnetite (FeO·Fe2O3)ZnFe2O4 7.05 64.5 29.82 6.74 32.6 40.63MgFe2O4 8.06 207 52.51 6.91 74.1 24.43Mg0.2Zn0.8Fe2O4 7.77 159.1 49.05 6.69 44.8 40.66Hematite (�-Fe2O3)ZnFe2O4 6.78 52.5 38.92 6.45 26.8 44.71MgFe2O4 7.73 210 44.82 6.78 63 31.72Mg0.2Zn0.8Fe2O4 7.26 122.7 32.94 6.53 52.6 41.85Specularite (lam. �-Fe2O3)ZnFe2O4 7.44 227 49.77 6.61 30.3 35.30MgFe2O4 7.56 111.8 71.01 7.55 333 25.74Mg0.2Zn0.8Fe2O4 8.15 313 48.91 6.67 48.4 25.22Nonpigmented film, reference pigmentsEpoxy-ester film – – – 5.19 10.0 69.48FeOOH 6.64 357 56.79 6.68 43.8 43.01Lamellar Fe2O3 7.59 184.2 66.78 6.80 49.3 26.95Fe2O3 6.76 67.9 28.08 6.59 44.3 33.38FeO·Fe2O3 6.72 416 40.57 6.76 46.9 34.09H2O (blank) 6.75 9.3 100 – – –

Table 5Surface hardness data for the paint films on glass (DFT = 60 ± 10 �m).

Pigment Surface hardness (%)

Day 1 Day 2 Day 3 Day 7 Day 14 Day 21 Day 100

FeOOHZnFe2O4 9.51 13.70 14.32 24.35 35.83 40.09 46.64MgFe2O4 11.37 15.53 15.70 30.97 34.89 40.79 43.24Mg�0.2Zn0.8Fe2O4 9.98 13.70 20.32 27.90 37.24 41.96 43.76FeO·Fe2O3

ZnFe2O4 8.35 11.19 12.70 18.20 31.85 35.20 44.85MgFe2O4 10.67 13.93 14.78 28.37 34.66 39.16 42.27Mg0.2Zn0.8Fe2O4 9.05 12.33 13.86 25.30 34.89 39.16 44.08Fe2O3

ZnFe2O4 12.76 17.58 24.25 31.44 33.49 38.46 44.90MgFe2O4 9.28 12.33 13.63 22.46 32.32 38.46 40.82Mg0.2Zn0.8Fe2O4 10.21 13.24 15.24 29.08 37.70 41.96 44.55lam-Fe2O3

ZnFe2O4 11.14 15.75 18.01 33.10 40.52 45.45 48.49MgFe2O4 8.82 12.10 14.09 23.64 34.66 38.00 42.45Mg0.2Zn0.8Fe2O4 10.44 13.24 15.01 28.61 37.00 41.72 45.48Nonpigmented film. reference pigmentsEpoxy-ester film 9.05 12.79 14.32 23.64 38.88 40.09 41.10FeOOH 9.28 12.33 14.09 22.46 36.53 41.00 41.00FeO·Fe2O3 9.74 13.47 13.86 26.00 35.13 39.63 41.01

pufibhtataurtttp

Fe2O3 8.82 12.33 13.63

Lam. Fe2O3 9.74 13.01 15.01

ossible reactions inside the coating, namely that yet in the liq-id state, on the formation of film and partially also on ageing thelm already in the hardened state [27]. In an epoxy ester resininder, the acidic COOH binder groups are present, except theydroxyl groups. Epoxy esters are obtained by esterifying part ofhe hydroxyl groups and the addition of fatty acids to epoxy groupst higher temperatures (200–220 ◦C). To obtain optimum proper-ies of the binder is used only 40–60% of the esterification resinbility and the products contained a part of the hydroxyl groupsnesterified [28]. The carboxyl groups, contained in the epoxy esteresin binder, cause the acidity; the hydroxyl groups, contained in

he epoxy ester binder, cause the reduction of acidity. It was foundhat the film which does not contain any pigments and which wasransferred to the aqueous suspension, affects the water extractH value in such a way, that the resulting pH value is 5.2. When

18.44 31.85 36.83 41.3227.66 33.72 35.66 43.98

this slightly—acidic binder contains a spinel pigment, then the pHextract values are rising with most pigments used to a region of pHvalue = 6.5–7.5.

3.4. Mechanical tests of the paint films

The surface hardness data for the paint films on glass panels,determined with a Perzos pendulum, are displayed in Table 5. Theultimate values were read 100 days after the paints were coatedonto the glass substrate.

In the group of paints containing the pigment ZnFe2O4, the high-

est hardness was observed with the paint containing the ferriteprepared from lamellar Fe2O3 (ZnFe2O4/lam. Fe2O3). In the groupof paints containing the ferrite MgFe2O4, the highest hardnesswas observed with the paint containing FeOOH (MgFe2O4/FeOOH),

A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163 157

Table 6Results of mechanical tests of the paint films (DFT = 60 ± 10 �m).

Pigment Cupping [mm] Bending [mm] Impact Adhesion (deg.) Pull-off (MPa)

Reverse (cm) Averse (cm)

Starting substance: goethite (FeOOH)ZnFe2O4 9.79* <4 >100 >100 0 1.1MgFe2O4 9.84* <4 >100 >100 0 1.5Mg0.2Zn0.8Fe2O4 9.54* <4 >100 >100 0 1.9Starting substance: hematite (�-Fe2O3)ZnFe2O4 9.81* <4 >100 >100 0 1MgFe2O4 9.82* <4 >100 >100 0 1.6Mg0.2Zn0.8Fe2O4 9.49* <4 >100 >100 0 1Starting substance: magnetite (FeO·Fe2O3)ZnFe2O4 9.48* <4 >100 >100 0 1MgFe2O4 9.81* <4 >100 >100 0 2Mg0.2Zn0.8Fe2O4 9.73* <4 >100 >100 0 1.8Starting substance: specularite (lamellar �-Fe2O3)ZnFe2O4(S) >10* <4 >100 >100 0 1.8MgFe2O4 9.31* <4 >100 >100 0 1.8Mg0.2Zn0.8Fe2O4 >10* <4 >100 >100 0 1.3Nonpigmented film. reference pigmentsEpoxy-ester film >10* <4 >100 >100 1 1.6FeOOH 9.78* <4 >100 >100 2 1.9Fe2O3 9.8* <4 >100 >100 2 1.1FeO·Fe2O3 9.79* <4 >100 >100 0 2Lamellar Fe2O3 9.85* <4 >100 >100 2 1.2

* Panel break.

Table 7Corrosion effects on panels coated with the paints (DFT = 95 ± 10 �m), observed following 800 h of exposure to the atmosphere with NaCl mist.

Pigment Blisters on the filmsurface

Blisters in the cut(deg.)

Adhesion (deg.) Corrosion in thecut (mm)

Corroded metalsurface fraction (%)

Starting substance: goethite (FeOOH)ZnFe2O4 8F 4M 1 0.13 0.3MgFe2O4 8F 4M 0 0.22 0.1Mg0.2Zn0.8Fe2O4 – 6M 0 0.11 0.01Starting substance: hematite (�-Fe2O3)ZnFe2O4 8F 4D 0 0.09 1MgFe2O4 8F 6MD 0 0.4 0.03Mg0.2Zn0.8Fe2O4 – 6M 0 0.22 0.1Starting substance: magnetite (FeO·Fe2O3)ZnFe2O4 8F 2M 1 0.06 1MgFe2O4 8F 4MD 0 0.23 0.1Mg0.2Zn0.8Fe2O4 – 4M 0 0.1 0.03Starting substance: specularite (lamellar �-Fe2O3)ZnFe2O4 8F 2M 0 0.07 0.3MgFe2O4 8F 4F 0 0.42 0.3Mg0.2Zn0.8Fe2O4 – 8M 0 0 0.03Nonpigmented film, reference pigmentsEpoxy-ester film 8F 4F 2 0.3FeOOH 6F 6D 2 0.56 3

we(ufi

csttfeehlft

Fe2O3 8F 6F

FeO·Fe2O3 4M 2M

lam. Fe2O3 – 2M

hile in the group of paints containing Mg0.2Zn0.8Fe2O4, the high-st hardness was observed with the paint containing lamellar Fe2O3Mg0.2Zn0.8Fe2O4/lam. Fe2O3). Clearly, ferrites prepared from spec-larite contribute most to the surface hardness of the paintlms.

The spinel-type pigments are also capable of reacting with thearboxyl binder groups, which results in the formation of metaloaps [29]. The formation of soaps is however lower compared withhe alkyd resin. The formation of metal soaps becomes evident fromhe hardness values, which increase with the time. The lowest sur-ace hardness was found for the non-pigmented coating materialpoxy-ester resin (where all the samples were desiccated in pres-nce of constant amounts of siccatives). Due to the fact that the

ardness of coatings pigmented with starting iron oxides Fe2O3 is

ower than that of those pigmented with spinel-type pigments theormation of the zinc and magnesium soaps can be supposed toake place.

0 0.75 31 0.5 100 0.13 1

The results of mechanical tests of the paint films on metal sub-strates are given in Table 6. Films of the non-pigmented binderand of the paints containing the starting ferric pigments are alsoincluded for comparison. The data include the following tests: cup-ping (Erichsen) test, bending test, impact test (with weight droponto both the adverse and reverse sides of the panel), adhesiontest by the cross-cut method (blade spacing 1 mm), and pull-offtest. The mechanical tests (cupping, pending, impact, adhesion,and pull-off) of the various paints had a nearly identical course.The non-pigmented epoxy-ester resin film exhibited outstandingproperties. Non-pigmented coating materials possess high physicalproperties, determined by the nature of the binder. The mechanicalproperties of paint films are affected by the pigments contained in

the paints [30]. Many of the ferrites tested improved the adhesionof paint films; the results are given in Table 7. In the cupping test,the panels invariably broke before any paint film cracking could beobserved. The paint film never cracked in the test which involved

158 A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163

Table 8Corrosion effects on panels coated with the paints (DFT = 95 ± 10 �m), observed following 1600 h of exposure to the atmosphere with NaCl mist.

Pigment Blisters on the filmsurface

Blisters in the cut(deg.)

Adhesion (deg.) Corrosion in thecut (mm)

Corroded metalsurface fraction (%)

Starting substance: goethite (FeOOH)ZnFe2O4 4M 2MD 4 0.1 1MgFe2O4 6F 2MD 0 0.1 0.3Mg0.2Zn0.8Fe2O4 8F 2M 0 0.15 0.1Starting substance: hematite (�-Fe2O3)ZnFe2O4 6F 4MD 5 0.12 0.1MgFe2O4 4M 4MD 3 0.6 3Mg0.2Zn0.8Fe2O4 6F 4MD 1 0.18 0.1Starting substance: magnetite (FeO·Fe2O3)ZnFe2O4 4M 2D 5 0.11 1MgFe2O4 6M 4D 4 0.1 0.3Mg0.2Zn0.8Fe2O4 6F 4MD 1 0.21 0.1Starting substance: specularite (lamellar �-Fe2O3)ZnFe2O4 8M 4M 2 0.09 0.3MgFe2O4 6M 4F 4 0.2 1Mg0.2Zn0.8Fe2O4 4M 4F 4 0.08 0.1Nonpigmented film, reference pigmentsEpoxy-ester film 8F 4F 3 0.75 1FeOOH 4M 4D 5 0.7 3FeO·Fe2O3 4MD 2D 5 0.56 16Fe2O3 6M 2F 5 1 10lam. Fe2O3 4F 2MD 0 0.8 100

Table 9Corrosion effects on panels coated with the paints (DFT = 95 ± 10 �m), observed following 600 h of exposure to the atmosphere with SO2.

Pigment Blisters on the filmsurface

Blisters in the cut(deg.)

Adhesion (deg.) Corrosion in thecut (mm)

Corroded metalsurface fraction (%)

Starting substance: goethite (FeOOH)ZnFe2O4 – 6F 0 0.95 1MgFe2O4 – 4M 0 0.88 0.3Mg0.2Zn0.8Fe2O4 – 8M 0 0.94 0.03Starting substance: hematite (�-Fe2O3)ZnFe2O4 – 8MD 0 0.73 0.3MgFe2O4 – 8M 0 0.25 0.3Mg0.2Zn0.8Fe2O4 – 6F 0 0.46 0.03Starting substance: magnetite (FeO·Fe2O3)ZnFe2O4 – 6MD 0 1.13 0.1MgFe2O4 – 8MD 0 0.68 0.1Mg0.2Zn0.8Fe2O4 – 8F 0 0.15 0.1Starting substance: specularite (lamellar �-Fe2O3)ZnFe2O4 – 6F 0 0.72 0.1MgFe2O4 – 8M 0 0.78 1Mg0.2Zn0.8Fe2O4 – 8F 0 0.43 0.3Nonpigmented film, reference pigmentsEpoxy-ester film – 8F 4 0.4 0.1FeOOH 8F 8F 0 0.69 1FeO·Fe2O3 8F 6M 0 0.88 1

bnwswfitTtp

3

moT

Fe2O3 6F 2F

lam. Fe2O3 8F 6M

ending the panel over the 4 mm diameter mandrel. The paint filmever cracked during the impact test, either when dropping theeight from a height of 100 cm onto the reverse side or the adverse

ide of the panel. The adhesion of all paints with the pigments testedas degree 0, while that of the non-pigmented epoxy-ester resinlm was degree 1. The adhesion degree was 2 for the paints withhe starting oxides FeOOH and Fe2O3 and with the lamellar Fe2O3.he pull-off strength was between 1 and 2 MPa. The best result inhe pull-off test (pull-off strength 2 MPa) was obtained with theaint containing MgFe2O4 (FeO·Fe2O3).

.5. Corrosion tests of the paint films on steel panels

The results of the corrosion tests of the paints containing the pig-ents examined are included in Tables 7–10. Corrosion changes

n the paint films and on the substrate metal were assessed.he effects included the formation of blisters on the paint film,

1 0.18 10 1 3

corrosion of the metal surface, and corrosion of the metal at thecut made in the film. The evaluation was based on the followingstandards: ASTM D 714-87 (blister size and frequency of occur-rence), ASTM D 610-85 (corroded fraction of the metal surfacearea), and ASTM D 1654-92 (corrosion propagation near the testcut). Scores which are acceptable according to the standards areshown in parentheses. The degree of paint film adhesion measuredby the cross-cut test (1 mm × 1 mm squares) was assessed as perthe CSN ISO 2409 standard 24 h after removing the panels from thecorrosion environment.

The data obtained from the tests consisting of 800 h and 1600 hof exposure to the atmosphere with the NaCl mist are given inTables 7 and 8, respectively. The data obtained from the tests con-

sisting of 600 h and 1200 h of exposure to the atmosphere withSO2 are given in Tables 9 and 10, respectively. The data obtainedfrom the 1632-hours’ cyclic test comprising exposure to the variouscorrosion environments are given in Table 11.

A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163 159

Table 10Corrosion effects on panels coated with the paints (DFT = 95 ± 10 �m), observed following 1200 h of exposure to the atmosphere with SO2.

Pigment Blisters on the filmsurface

Blisters in the cut(deg.)

Adhesion (deg.) Corrosion in thecut (mm)

Corroded metalsurface fraction (%)

Starting substance: goethite (FeOOH)ZnFe2O4 8F 6MD 0 2.06 1MgFe2O4 8F 6MD 0 2.11 0.1Mg0.2Zn0.8Fe2O4 – 6M 0 1.98 0.03Starting substance: hematite (�-Fe2O3)ZnFe2O4 8F 6MD 1 2.58 1MgFe2O4 8F 6MD 0 2.08 1Mg0.2Zn0.8Fe2O4 8F 6M 0 2.07 0.1Starting substance: magnetite (FeO·Fe2O3)ZnFe2O4 8F 4MD 0 2.45 0.3MgFe2O4 8F 6MD 0 2.44 0.3Mg0.2Zn0.8Fe2O4 8F 6MD 0 2 1Starting substance: specularite (lamellar �-Fe2O3)ZnFe2O4 8F 4MD 0 2.82 0.3MgFe2O4 8F 6M 0 1.79 1Mg0.2Zn0.8Fe2O4 8F 6M 0 2.08 0.3Nonpigmented film, reference pigmentsEpoxy-ester film – 4MD 5 1 1FeOOH 8F 4D 0 1.94 3FeO·Fe2O3 8F 4D 0 2.02 3Fe2O3 8F 6D 0 2.19 3lam. Fe2O3 8F 6MD 1 2.43 3

Table 11Corrosion effects on panels coated with the paints (DFT = 95 ± 10 �m), observed following 1632 h of cyclic exposure to the various corrosive atmospheres.

Pigment Blisters on the filmsurface

Blisters in the cut(deg.)

Adhesion (deg.) Corrosion in thecut (mm)

Corroded metalsurface fraction (%)

Starting substance: goethite (FeOOH)ZnFe2O4 – 4M 1 0.35 3MgFe2O4 – 8F 1 0.2 0.1Mg0.2Zn0.8Fe2O4 – 4F 0 0.4 0.03Starting substance: hematite (�-Fe2O3)ZnFe2O4 6F 4F 1 0.3 1MgFe2O4 6F 4F 2 0.4 0.3Mg0.2Zn0.8Fe2O4 6M 8F 3 1.2 0.3Starting substance: magnetite (FeO·Fe2O3)ZnFe2O4 8F 4MD 0 0.5 0.3MgFe2O4 – 4F 5 0.6 0.1Mg0.2Zn0.8Fe2O4 8F 4M 0 0.3 3Starting substance: specularite (lamellar �-Fe2O3)ZnFe2O4 6F 4F 0 0.1 0.1MgFe2O4 – 6F 1 0.25 0.1Mg0.2Zn0.8Fe2O4 6F 8F 0 1.25 3Nonpigmented film, reference pigmentsEpoxy-ester film – 4F 5 0.8 0.1FeOOH 6M 6MD 4 0.4 0.3FeO·Fe2O3 6M 4MD 3 0.6 1

3m

o(f(r

tnfrwa

w

Fe2O3 6M 4M

lam. Fe2O3 – 6M

.5.1. Cyclic corrosion test in the combined test chamber with saltist

This test encompassed total exposure for 800 h and 1600 h. Thebserved data are included in Tables 7 and 8. The best resultsin nearly all parameters) for 800 h of exposure were obtainedor the paint with ferrite ZnFe2O4 prepared from lamellar Fe2O3ZnFe2O4/lam. Fe2O3), both in comparison with the other zinc fer-ite types synthesised and with the starting ferric pigments.

The group of paints with MgFe2O4 gave better overall resultshan the paint with ZnFe2O4; the only parameter where this wasot true was corrosion in the cut. The best results were obtained

or paints containing Mg0.2Zn0.8Fe2O4. The weakest parameter wasesistance to the formation of blisters on the film near the cut,

hich was affected by the disturbance of the protective coating

nd penetration of chloride ions beneath the film.The corrosion effects observed following 1600 h of exposure

ere very similar to those observed after 800 h of exposure with

2 0.55 163 1.3 10

regard to the influence of the various groups of pigments. Theresults are given in Table 8. Examples of corrosion effects on thepanels after the exposure and after removing the paint film areshown in Fig. 4. Once again, the best results were obtained withthe paints containing the ferrite Mg0.2Zn0.8Fe2O4.

Conclusions from the exposure of the panels to the environmentwith NaCl mist:

• Paints containing ferrites prepared from starting oxides of lamel-lar shape (lam. Fe2O3) or needle shape (FeOOH) are superior topaints with ferrites possessing an isometric particle shape.

• All of the paints containing the ferrites are superior to those con-taining the starting ferric oxides.

3.5.2. Cyclic corrosion test in the condensation chamber with SO2Evaluation of the corrosion effects following 600 h of expo-

sure to the environment with SO2 is presented in Table 9.

160 A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163

F paint

a int film

Tcssffdwpepcmli

ig. 4. Examples of corrosion effects of the NaCl mist on a steel panel coated with and 1600 h (B) of exposure; (C and D) Steel substrate surface after removing the pa

he presence of any pigment in the epoxy-ester resin system appre-iably improved the degree of adhesion of the paint film to theubstrate following exposure to the corrosion environment. Out-tanding results were obtained with the paints containing theerrites prepared; corrosion blisters never developed. The corrodedraction of the substrate metal surface is very small and largelyecreases from paints containing the Zn ferrite (ZnFe2O4) to thoseith the Mg ferrite (MgFe2O4), and was lowest when using theaint with the mixed ferrite Mg–Zn (Mg0.2Zn0.8Fe2O4). Due to theasier diffusion of the mobile SO2, corrosion in the cut was moreronounced on this exposure than on that exposed to salt mist. The

orrosion effects observed after 1200 h of exposure to the environ-ent with SO2 are listed in Table 10. From the corrosion effects

isted, corrosion in the cut was the most marked. The best resultsn this respect were observed with the paint containing MgFe2O4

containing MgFe2O4 prepared from Fe2O3. (A and B) Paint films following 800 h (A), following 800 h (C) and 1600 h (D) of exposure.

(MgFe2O4/lam. Fe2O3). As regards protection against corrosion ofthe substrate metal surface, paints with any of the ferrites weresuperior to those with the starting ferric oxides. The formation ofblisters was nearly negligible. A comparison of changes on panelsexposed to the corrosion environment for 600 h and for 1200 h ispresented in Fig. 5.

3.5.3. Accelerated corrosion test of consecutive exposure tovarious corrosion environments

This test served to assess how the coating responds to a chang-ing corrosion environment. The coated panels were exposed to

an environment with condensed water for 1152 h, then to a saltmist environment for 240 h and finally to an SO2 environment foranother 240 h. The results are listed in Table 11. Photographs oftwo paint films and the steel substrates are presented in Fig. 6.

A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163 161

F ared

fi the p

Tsstirc

MZsTand

ig. 5. Corrosion effects on a panel coated with the paint containing MgFe2O4 preplm after 600 h (A) and 1200 h (B) of exposure; (C and D) steel panel after removing

he corroded fraction of the substrate metal surface was con-iderably higher when using paints with ferrites based on thetarting oxides Fe2O3 and lamellar Fe2O3 than when using any ofhe remaining paints. The non-pigmented epoxy-ester film exhib-ted very good properties, except for adhesion, which undoubtedlyanks amongst the most important parameters of protectiveoatings [30].

The best anticorrosion properties were found for paints withgFe2O4 (MgFe2O4/FeOOH and MgFe2O4/lam. Fe2O3) and with

nFe2O4 (ZnFe2O4/lam. Fe2O3). The corroded fraction of the sub-trate metal surface was 0.1% and the adhesion degree was 1.

he adhesion degree of the paint with ZnFe2O4 was 0. The resultsttained with the paints containing isometric shape ferrites wereot as good as those attained with the non-isometric ferrites. Thisifference is illustrated by Fig. 6.

from Fe2O3, observed after exposure to the environment with SO2. (A and B) Paintaint film, following 600 h (C) and 1200 h (D) of exposure.

3.6. Conclusions from the corrosion tests and mechanicalresistance tests of the paint films

(1) The pigment particle shape was found to substantially affectsurface hardness of the paint films. Surface hardness washighest for the paint film containing ZnFe2O4 prepared fromlamellar Fe2O3 (ZnFe2O4/lam. Fe2O3). The same hardnessdegree was found for the paint films containing ZnFe2O4or MgFe2O4, both prepared from FeOOH (ZnFe2O4/FeOOH,MgFe2O4/FeOOH). Paints with any of the pigments preparedshowed a better degree of adhesion than those with the starting

ferric oxides.

(2) Corrosion-induced weight losses of steel panels submergedin aqueous extracts of the powdered pigments and in aque-ous extracts of the unsupported paint films serve to assess

162 A. Kalendová et al. / Progress in Organic Coatings 86 (2015) 147–163

F g0.2ZF ) Subs

ig. 6. Corrosion effects on panels coated with two chemical identically pigments, MeOOH, (B and D) pigment prepared from FeO·Fe2O3. (A and B) Paint films, (B and D

the effect of the cation which is slowly released from thepigment and possesses basic properties. The corrosion lossesin the powdered pigment extracts were higher than in thepaint film extracts, due to the higher electric conductivity,which affects corrosion appreciably. The best results in thistest were obtained with paint films containing MgFe2O4 orMg0.2Zn0.8Fe2O4 prepared from lamellar Fe2O3 (MgFe2O4/lam.Fe2O3 and Mg0.2Zn0.8Fe2O4/lam. Fe2O3). Low corrosion losseswere also measured for the paint with MgFe2O4 prepared fromFeO·Fe2O3 (MgFe2O4/FeO·Fe2O3). Results of such a measure-ment give us information on the alkalinity of the pigment

considered and the solubility thereof in water medium. Thisresults can be summarized as follows: the synthetised spinelpigments exhibit alkaline nature and differ by solubilities inaqueous phases; they shift the pH water extract value of the

n0.8Fe2O4, differing in their particle morphology: (A and C) pigment prepared fromtrate metal after removing the paint film.

coating films to the alkaline region also on using a binder show-ing acid reaction; the binder containing carboxy groups/spinelpigment interactions lead to the appearance of metal soaps.

(3) The orders of corrosion effects on the test panels subjected to1600-hours’ exposure to the atmosphere with the mist of a5% NaCl solution were as given below, starting with the pig-ment for which the corrosion effects were least pronounced.The respective starting ferric oxides are given in parentheses:

(a) Blister formation on the paint film: Mg0.2Zn0.8Fe2O4(FeOOH) < MgFe2O4 (FeOOH), Mg0.2Zn0.8Fe2O4 (FeO·Fe2O3,Fe2O3), ZnFe2O4 (Fe2O3)

(b) Propagation of corrosion in the cut: ZnFe2O4 (lam.Fe2O3) < ZnFe2O4 (FeOOH), MgFe2O4 (FeOOH, FeO·Fe2O3)

(c) Corrosion of the substrate metal surface: Mg0.2Zn0.8Fe2O4(FeOOH, FeO·Fe2O3, Fe2O3, lam. Fe2O3)

Organ

(

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A. Kalendová et al. / Progress in

4) The orders of corrosion effects on the test panels subjected to1200-hours’ exposure to the atmosphere with SO2, again start-ing with the pigment for which the corrosion effects were leastpronounced, are listed below:

(a) Blister formation on the paint film: Mg0.2Zn0.8Fe2O4(FeOOH) < all the remaining pigments

b) Propagation of corrosion in the cut: MgFe2O4 (lam.Fe2O3) < Mg0.2Zn0.8Fe2O4 (lam. Fe2O3) < ZnFe2O4 (FeOOH)

(c) Corrosion of the substrate metal surface: Mg0.2Zn0.8Fe2O4(FeOOH) < MgFe2O4 (FeOOH), Mg0.2Zn0.8Fe2O4 (Fe2O3)

5) The orders of corrosion effects on the test panels subjected toconsecutive exposure to the 3 corrosion environments (con-densed H2O, NaCl mist, SO2) were as outlined below, startingwith the pigment for which the corrosion effects were leastpronounced:

(a) Blister formation on the paint film: ZnFe2O4 (FeOOH), MgFe2O4(FeOOH, FeO·Fe2O3, lam. Fe2O3), Mg0.2Zn0.8Fe2O4 (FeOOH)

b) Propagation of corrosion in the cut: ZnFe2O4 (lam.Fe2O3) < MgFe2O4 (FeOOH) < MgFe2O4 (lam. Fe2O3)

(c) Corrosion of the substrate metal surface: Mg0.2Zn0.8Fe2O4 (lam.Fe2O3) < ZnFe2O4 (FeOOH), MgFe2O4 (FeOOH, FeO·Fe2O3, lam.Fe2O3)

6) The above results show that the pigment particle shape has amajor effect on the pigment’s corrosion resistance. Irrespec-tive of the corrosive environment, pigments prepared from theneedle-shaped FeOOH or from the lamellar Fe2O3 were foundto be the most resistant. The cations in the pigments also playa role in the corrosion protection properties. Paints contain-ing the mixed pigment Mg0.2Zn0.8Fe2O4. were among the best.Good protection against corrosion in the cut caused by the NaClmist was also found for ZnFe2O4.

. Conclusion

The anticorrosion efficiency of some ferrites as pigments inpoxy-ester resin-based paints was examined depending on theirtructure and chemical composition. The ferrites synthesisedncluded ZnFe2O4, MgFe2O4 and Mg0.2Zn0.8Fe2O4 prepared fromerric oxides possessing different structures and primary particlehapes, viz. �-FeOOH, FeO·Fe2O3, FeO·Fe2O3, and lam. �-Fe2O3.he paints tested contained the pigment s at a pigment volumeoncentration (PVC) = 10%. Two types of tests were performed:hysico-mechanical tests of the paint films and accelerated cor-osion tests of steel panels coated with the paints exposed tonvironments with salt mist, with SO2, and with condensation of

ater. The best anticorrosion efficiency was found for ferrites pre-ared from ferric oxides consisting of non-isometric particles.

The accelerated corrosion tests provided evidence that all paintsontaining the pigment Mg0.2Zn0.8Fe2O4 prepared from FeOOH,

[

[

ic Coatings 86 (2015) 147–163 163

FeO·Fe2O3, Fe2O3 or lam. Fe2O3 are well suited to use in corrosionenvironments of category C5-I (according to CSN EN ISO 12994).Those paints have medium-to-long lifetimes. Hence, the pigmentsare very high quality and can be recommended for application inindustrial high humidity/moisture environments with aggressiveatmospheres. The pigments synthesised can be used in coatingmaterials that are designed to protect metal substrates from corro-sion in harsh corrosion conditions.

Nontoxic pigments can be obtained based on a suitable choice ofthe cations in the oxide lattice. Solubility and toxicological effectsare important aspects from the environmental protection point ofview. It is concluded that the starting substances used are toxico-logically insignificant and virtually insoluble.

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