Corrosion resistance of new epoxy–siloxane hybrid coatings A laboratory study

Progress in Organic Coatings 69 (2010) 278–286

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Progress in Organic

Coatings

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /p o r g c o a

t

Corrosion resistance of new epoxy–siloxane hybrid coatings.A laboratory study

I.Díaz,B.Chico,D.de la Fuente,J.Simancas,J.M.Vega,M.Morcillo ?

Department of Materials Engineering,Degradation and Durability National Center for Metallurgical Research (CENIM-CSIC),Gregorio del Amo 8,28040Madrid,Spain

a r t i c l e i n f o Article history:

Received 6April 2010

Received in revised form 9June 2010Accepted 28June 2010Keywords:

Epoxy–siloxane Zinc-rich primers

Accelerated corrosion tests Anticorrosive protection

a b s t r a c t

Traditional multilayer epoxy/polyurethane type anticorrosive paint systems are widely employed in the protection of steel structures due to their high ef?ciency against atmospheric corrosion.However,the use of isocyanate in the curing process,and the high volatile organic compound (VOC)content of such systems,makes it necessary to search for new isocyanate-free paints.

Hybrid organic–inorganic coatings,such as epoxy–siloxane coatings,represent a step forward in the ?eld of paint coatings for atmospheric corrosion protection.These new isocyanate-free hybrids present low VOC levels –due to the high solid content associated with their low viscosity –along with good heat and UV radiation stability and excellent chemical resistance.With the new polysiloxane inorganic resins it is hoped to improve the anticorrosive behaviour of traditional organic binders.

This work assesses the anticorrosive performance of one-layer epoxy–siloxane coatings compared to traditional two-layer epoxy/polyurethane coatings,in paint systems with and without an epoxy or silicate-type zinc-rich primer.

The anticorrosive properties of these coatings applied on steel substrates have been evaluated using a wide range of experimental techniques,namely measurement of water vapour and oxygen permeabil-ity in free ?lms,3-year outdoor testing,accelerated corrosion testing (condensing humidity,Kesternich,salt fog and Prohesion),and electrochemical impedance spectroscopy (EIS).Hybrid epoxy–siloxane ?lms show the lowest oxygen and water vapour permeability,capacitance values,and the highest ionic resis-tance values,along with excellent behaviour in humidity and Kesternich tests,outperforming traditional epoxy/polyurethane paints,while the behaviour of both types of coatings is similar in the salt fog,Pro-hesion and short-term outdoor tests.This type of paint requires the presence of zinc-rich primers to optimise its anticorrosive behaviour.

? 2010 Elsevier B.V. All rights reserved.

1.Introduction

In recent years great technological advances have been seen in the ?eld of anticorrosive paints.One of the basic objectives of the paint industry today is to develop products that are less aggressive to worker health and the environment.In this context,the synthe-sis of new polymers and pigments and the development of surface preparation and paint application equipment using advanced tech-nologies is a substantial contribution to the attainment of these goals.

One such product already available on the international market is a new type of polysiloxane resin based paints.One of the char-acteristics of these paints is that their high solid content,which means a low organic solvent concentration,allowing high paint layer thicknesses using conventional application methods,which is a fundamental factor for reducing application costs and execu-tion times.The technical literature presents hybrid epoxy–siloxane

?Corresponding author.Tel.:+34915538900;fax:+34915347425.E-mail address:morcillo@cenim.csic.es (M.Morcillo).coatings as excellent protective coatings due to their chemical structure,which combines the durability and hardness of epoxy coatings and notably improves on the gloss and colour retention of urethane type coatings.

Fig.1depicts the general chemical structure of organo-siloxane hybrids [1],where the R group may be hydrogen,methyl,ethyl,propyl,octyl,pentyl,acrylic or other organic substituents.The selection of these substituents allows the design of hybrid organo-siloxane polymeric networks with different reactivities and different physical,chemical and anticorrosive properties [2].Fig.2depicts the chemical structure of the epoxy–siloxane hybrid [3,4].

Epoxy–siloxane polymers are formed by skeletons of repetitive –Si–O–units,chemically bonded to lateral epoxy organic chains.The siloxane bond –Si–O–has a binding energy of 445Kj/mol,which is higher than the 358Kj/mol of the –C–C–bond that forms the repetitive unit of the organic polymer coatings [5–7].Consequently a higher activation energy is required to break the siloxane inor-ganic polymer skeleton,which means that these coatings must provide greater durability and better resistance to atmospheric degradation,heat,chemical attack,and UV radiation than organic polymers.Furthermore,the silicon in the polysiloxane polymer

0300-9440/$–see front matter ? 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2010.06.007

I.Díaz et al./Progress in Organic Coatings69 (2010) 278–286

279

Fig.1.Polymeric structure of organo-siloxane hybrids.

skeleton is bonded to two or three oxygen atoms,so the oxidative degradation that affects the C–C bonds present in organic polymers cannot occur in the oxidised polysiloxane chain,thus improving its resistance to attack by atmospheric oxygen and oxidising chemical products.

The siloxane bond has a partly ionic character of approximately 50%and hydrolyses easily,especially when catalysed by an acid or an alkali.The coating’s durability in the atmosphere is thought to be due to the reversible nature of the siloxane hydrolysis.The action of a high radiation energy photon in the presence of humidity causes the hydrolysis of the siloxane bonds,which are spontaneously regenerated by the curing reaction,not causing any permanent damage to the coating.

Furthermore,epoxy–siloxane hybrid paints do not use iso-cyanate in the curing process and present very low viscosity (1000–2000mPa s,at25?C DIN53019),which has allowed the development of very high solid coatings that meet all present and probable future requirements regarding volatile organic compound (VOC)contents.

All of these properties makes this type of hybrid paints an attractive alternative to traditional urethane type topcoat systems. However,the scienti?c literature contains little information on the anticorrosive resistance of this type of coatings,and so it is important to verify the performance in principle associated with organic–inorganic hybrid coatings.The fact that this is a new tech-nology makes it important to perform a general assessment(in laboratory and?eld tests)of their anticorrosive resistance and to compare them with traditional anticorrosive paints,especially aliphatic polyurethane paints,in order to ascertain their potential and the scope of their application.

2.Experimental

The study has used two epoxy–siloxane hybrid paint coatings(A and B)from different sources,and an epoxy/polyurethane coating system for comparative purposes.

Table1lists the9paint systems considered in the study and Table2details their formulation.The information has been obtained from the safety data sheets of the products.

The paints were applied by air-spraying on15×10cm panels prepared from5mm degreased cold rolled low carbon steel

plate.

Fig.2.Polymeric structure of epoxy–siloxane hybrids.

Table1

Design of paint systems.

Paint system Primer(DFT,?m)Intermediate(DFT,?m)Topcoat(DFT,?m)Total DFT(?m)

A1––Epoxy–siloxane(100?m)100±10

B1––Epoxy–siloxane(100?m)100±10

C1–Epoxy(100?m)Polyurethane(50?m)150±15

A2Epoxy zinc(50?m)–Epoxy–siloxane(100?m)150±15

B2Epoxy zinc(50?m)–Epoxy–siloxane(100?m)150±15

C2Epoxy zinc(50?m)Epoxy(100?m)Polyurethane(50?m)200±20

A3Zinc silicate(50?m)–Epoxy–siloxane(100?m)150±15

B3Zinc silicate(50?m)–Epoxy–siloxane(100?m)150±15

C3Zinc silicate(50?m)Epoxy(100?m)Polyurethane(50?m)200±20

Table2

Paint compositions.

Epoxy–siloxane A Mixture of hydrogenated epoxy resin and silicon resin that reacts with3-aminopropyltriethoxysilane and organosilane ester.

Epoxy–siloxane B Epoxy–siloxane hybrid combined with an aminosilane catalysed with tin dibutyldilarate(DBTL).

Polyurethane Polyurethane coating based on a combination of a hydroxylated acrylic resin with an aliphatic polyisocyanate.

Epoxy Epoxy coating formulated with laminar pigments(micaceous iron oxide)and magnesium silicate.Based on a mixture of two epoxy resins –one a solid in a low molecular weight solution and the other a liquid–which react with a polyamidoamine with

dimethylaminomethylphenol.

Epoxy zinc Primer based on a low molecular weight epoxy resin(epichloridrine+bisphenol A)combined with a high molecular weight polyamide.

Contains metallic zinc in powder at a concentration in accordance with SSPC Paint20level3.

Zinc silicate Inorganic ethyl silicate based zinc primer.Contains metallic zinc in powder,allowing it to provide cathodic protection.

280I.Díaz et al./Progress in Organic Coatings69 (2010) 278–286 Prior to the application of the paint coatings the specimens were

shot-blasted with shot steel abrasive to Sa3grade according to ISO

8501-1[8],reaching an average roughness of6.3?m.

The panels were air dried for7days prior to testing.A scribe of

0.3mm in width and6cm in length was cut in the lower zone of the

panels to evaluate the inhibitive properties of the paint coatings in

that zone.

2.1.Permeability measurements

For the determination of water vapour and oxygen per-

meation rates through the paint coating[9,10],free?lms

with an area of11.34cm2of the coating material were used.

The thicknesses obtained for free?lms of the epoxy–siloxane

coatings were125±10?m,while those corresponding to the

epoxy/polyurethane coating were notably higher,210±10?m.

Water vapour permeability was ascertained by means of perme-

ability cups using a50%relative humidity gradient(ASTM D1653

[11]).The temperature was kept constant at25?C.

Oxygen permeability measurements were performed following

the electrochemical technique,using a selective oxygen electrode

[12,13].The free?lm,with an area of12.56cm2,was located in a

permeation cell between two compartments of0.027L each,one

with distilled water kept constantly saturated with oxygen and the

other with deaerated distilled water in which an Orion selective

oxygen electrode(model97-08)was placed.The water in both

compartments was maintained at25±1?C.Readings of the oxy-

gen transferred through the paint?lm were taken with an Orion

analyser-processor,model SA720.Permeabilities were calculated

according to the following expressions:

water permeability=

( m1×t)

s

,

oxygen permeability=

( m2×t×v)

s

where m1is the daily mass loss,in units(mg/day); m2 the amount of oxygen permeated per unit of time,in units (mg O2/min L);t the free?lm coating thickness;s the coating area; v is the volume of water in which the oxygen is dissolved.

2.2.Adhesion tests

The method used to evaluate the adhesion of all the paint sys-tems was the adhesion test,according to standard ASTM D4541-02 [14].The test was performed with an AT-1000portable adhesion meter,which measures the pulling force in units of kgf and applies a progressive pulling rate of not more than1MPa/https://www.360docs.net/doc/be7586541.html,e was made of a20mm diameter dolly?xed to the coating using a two-component epoxy glue,considering a tested area of3.14cm2.

Prior to the performance of the test the contact surface between the dolly and the different coatings was polished with a400-grain?ne abrasive.This serves to improve the contact between the two surfaces,preventing premature failure of the paint–glue joint. After cleaning the surfaces with ethanol,the glue was applied and allowed to dry for24h at room temperature.The coating was scored through to the substrate around the loading?xture with a circular hole cutter before running the test.

Two pieces of information are obtained from a pull-off test.The ?rst is the pull-off strength of the coating and the second is where the split occurred in the paint system.The split could be an adhesive break,a cohesive break,a combination of both,or a failure of the glue[15]

.

Fig.3.Equipment used in impedance measurements.

2.3.Corrosion tests

The paints were subjected to the following accelerated corrosion tests:humidity(ASTM D4585-07[16]);Kesternich(ASTM G87-02, 2L SO2[17]);salt spray(ASTM B117-09[18]);and Prohesion(ASTM G85-09[19]).

An atmospheric exposure test was also conducted on a corro-sion rack located on the roof of the CENIM laboratory in an urban atmosphere of corrosivity category C2–C3,for low carbon steel, according to ISO9223[20].After3years of exposure none of the tested coating systems(see Table1)had suffered deterioration due to blistering or rusting of the metallic substrate.

2.4.Electrochemical impedance spectroscopy(EIS)

The anticorrosive performance of paint systems A1,B1and C1was also monitored using electrochemical impedance spec-troscopy(EIS)in a classic three-electrode cell with a working area of9.62cm2.EIS measurements were performed using a potentio-stat/galvanostat(AutoLab EcoChemie PGSTAT30)equipped with a FRA2frequency response analyser module(Fig.3).Frequency scans were carried out by applying a±5mV amplitude sinusoidal wave perturbation,close to the corrosion potential.Five impedance-sampling points were recorded per frequency decade.The analysed frequency range was from100kHz to1mHz and the electrolyte used was a0.1M sodium sulphate solution.The impedance data was analysed using the electrochemical impedance software ZView?(Version2.9c,Scribner Associates,Inc.,USA).

3.Results and discussion

3.1.Permeability measurements

3.1.1.Water vapour permeability

Table3summarises the different water vapour permeability grades obtained with the traditional two-layer epoxy/polyurethane topcoat system(C1)and the two one-layer epoxy–siloxane hybrid coatings(A1and B1).It must be pointed out the higher water vapour

Table3

Water vapour permeability results.

Free coatings Thickness

(?m)

Slope

(mg/day)

Permeability

(mg mm/cm2day) Epoxy–siloxane(A1)125±107.99660.0957

Epoxy–siloxane(B1)125±1010.55300.1107

Epoxy/polyurethane(C1)210±109.08500.1734

I.Díaz et al./Progress in Organic Coatings69 (2010) 278–286281

Fig.4.Adhesive failure in system B2,between the intermediate epoxy coating and the epoxy–siloxane topcoat.Left:adhesion test.Right:test panel scribe zone in test panel.

Table4

Oxygen permeability results.

Free coatings Thickness

(?m)Slope

(mg O2/L min)

Permeability

(mg mm/cm2day)

Epoxy–siloxane(A1)125±100.01900.0085

Epoxy–siloxane(B1)125±100.01950.0083

Epoxy/polyurethane(C1)210±100.02020.0133 permeability of epoxy–siloxane B1in relation to epoxy–siloxane A1.

According to this data,the entry of electrolyte necessary for the corrosive process to the metallic substrate is delayed by the pres-ence of the protective one-layer epoxy–siloxane hybrid coatings (A1,B1)due to their considerably lower water vapour permeabil-ity,64%and55%respectively,than the conventional two-layer epoxy/polyurethane system(C1).

3.1.2.Oxygen permeability

Table4displays the permeability values obtained for the tested free?lms and the thickness of the latter.

The one-layer epoxy–siloxane free?lms(A1,B1)possess con-siderably lower oxygen permeability,64%and62%,respectively, than the two-layer epoxy/polyurethane?lm(C1).This hinders the corrosive process,polarising the cathodic reaction of atmospheric corrosion by limiting the access of oxygen to the cathodic areas of the metal.

3.2.Adhesion tests

Table5shows the pulling force of all the tested paint systems and the type of rupture with the coatings forming the paint systems.

Both the traditional epoxy/polyurethane coating systems and the epoxy–siloxane hybrid coatings present similar pulling forces. Adhesion values of epoxy–siloxane A are higher than the corre-sponding to epoxy–siloxane B.Most failures occur at the paint–glue interface during the pull-off test,thus indicating the great adhe-sion between the different coatings of the paint systems.A higher percentage of cohesive failure is observed in the interme-diate layer(epoxy)of the traditional coatings compared to the epoxy–siloxane hybrids.The only exception is the case of the epoxy zinc/epoxy–siloxane coating(B2),where the failure is completely adhesive between the topcoat and the primer,showing a lower pulling force than the rest of the paint systems[21–23](see Fig.4).

3.3.Accelerated corrosion tests

3.3.1.Humidity test

After5304h of permanent humidity exposure all the coat-ing systems,both the conventional epoxy/polyurethane and the epoxy–siloxane hybrid types,show very low degrees of rusting.The systems including epoxy–siloxane hybrid coatings,which as noted above have lower water vapour permeability rates,present consid-erably less blistering than the traditional systems[23],although the difference is smaller when the primer used in the hybrid systems

is epoxy zinc.Hybrid system B2is the only one that experiences slightly greater blistering than the traditional systems(see Table6),Fig.5.Evaluation of blistering degree for the different paint systems exposed in permanent humidity condensation conditions.

282I.Díaz et al./Progress in Organic Coatings69 (2010) 278–286

Table5

Results of adhesion test.

Paint system(Table1)Adhesion pull-off(MPa)Type of rupture

%Glue%Adhesive%Cohesive

Intermediate/topcoat Intermediate Topcoat

A19.8100–––

B17.7100–––

C18.172–28–

A27.2100–––

B2 2.3–100––

C29.428–72–

A37.57129––

B37.485–15–

C3 6.8100–––

Table6

Results obtained in permanent humidity condensation cabinet after5304h of exposure.

Paint system Rusting degree(ASTM D-610[24])Blistering degree Observation at scribe

ASTM D-714[25]Numerical scale[26]a

A19–10–10b Rusting

B110–10b Rusting

C19–10D31Rusting

A29–10D75Rusting

B210D20Rusting adhesion fail at intermediate/topcoat C29–10D42Very slight rusting

A310–10b Very slight rusting

B310–10b Very slight rusting

C39–10D53Absence of rusting

a The values of size and frequency of blisters have been converted,according to ASTM D-714,into numeric values using the Keane conversion table.

b6720h.

Table7

Kesternich resistance test after20exposure cycles.

Paint system Rusting degree(ASTM D-610[24])Blistering degree Delamination from scribe(mm)

ASTM D-714[25]Numerical scale[26]a

A110F89 1.5

B110F892

C110M663

A210F890.5

B210F890.75

C210M880.75

A310D860.5

B310F890.5

C310M440.5

a The values of size and frequency of blisters have been converted,according to ASTM D-714,into numeric values using the Keane conversion table.

as a consequence of the poor adhesion between the primer paint and the topcoat(see Table5and Fig.4).

With regard to the behaviour of the paint coatings in the scribe zone,the systems that include a silicate-type zinc-rich primer show a certain preponderance in terms of cathodic protection of the bare base steel.It is well known that inorganic ethyl silicate-type zinc-rich primers afford better and longer lasting cathodic protection than epoxy type zinc-rich primers,provided that a high metallic zinc content in the coating is assured[27].This is because wetting properties of the vehicle have a decisive in?uence on the elec-tronic conductive of the paint coating;the non-polar nature of ethyl silicate-type binder makes it dif?cult to wet zinc particles(“pseudo-

Table8

Resistance to salt fog after3456h of exposure.

Paint system Rusting degree(ASTM D-610[24])Blistering degree Delamination from scribe(mm)

ASTM D-714[25]Numerical scale[26]a

A17–1016

B12–1016.5

C18MD8715

A29–102

B29–103

C28–102

A38–10 1.25

B38–10 1.5

C39F89 1.5

a The values of size and frequency of blisters have been converted,according to ASTM D-714,into numeric values using the Keane conversion table.

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283

Fig.6.Evolution of blistering degree for the different paint systems exposed in conditions of 2L SO 2in the Kesternich test.

wetting”vehicle),whereas epoxy type binder,being of polar nature,wet them relatively easily (“effective-wetting”vehicles)[28].

Fig.5displays the evolution of blistering for the different paint systems with exposure time,showing the excellent performance of the hybrid coatings compared to the traditional paint systems.Hybrid system B2is an exception to this behaviour.The traditional epoxy/polyurethane type systems start to deteriorate after 350h of

exposure.

Fig.7.Kesternich test.Aspect of the scribe zone.

3.3.2.Kesternich

After 20exposure cycles in the SO 2atmosphere (2L),no rusting is observed with any of the studied paint systems.How-ever,all the paint systems that include hybrid coatings,without exception,show notably less blistering than the conventional epoxy/polyurethane type systems (see Table 7),once again con-?rming the results obtained in the measurements of water vapour permeability (see Table 3).

Fig.6shows the evolution of blistering experienced by all the studied paint systems exposed to cycles of 2L SO 2.The paint coat-ings start to deteriorate due to blistering in SO 2atmospheres after approximately 10testing cycles.The absence of a zinc-rich primer leads to greater delamination of the coatings in the scribe zone (Fig.7).

3.3.3.Salt spray

The epoxy–siloxane coatings (without zinc-rich primer)show a considerable rusting degree,especially coating B1,which is greater than that of the conventional epoxy/polyurethane system (C1)(Table 8).However,the conventional system is the only one of all the studied coating systems to experience slight blistering after 3456h of exposure to the saline atmosphere.The one-layer hybrid systems (A1,B1)and the traditional two-layer system (C1)present notable rusting of the steel at the scribe and delamination as a con-sequence of the absence of zinc-rich primer.Here again it is possible to see the preponderance of the silicate-type zinc-rich primer over the epoxy type [27].

3.3.

4.Prohesion test

After 4896h of testing,all the paint systems without zinc-rich primer –both hybrids (A1,B1)and conventional (C1)–have a larger corroded area than the rest of the paint systems.This is most clearly seen in the case of hybrid system (B1),with a corroded area of 33%.On the other hand,except for the slight blistering experienced by the traditional paint system (C1),the coating systems do not present blistering (Table 9).

With regard to delamination at the scribe (Fig.8),the best results correspond to the paint system containing an epoxy zinc-rich

284I.Díaz et al./Progress in Organic Coatings69 (2010) 278–286

Table9

Resistance to Prohesion test after4896h of exposure.

Paint system Rusting degree(ASTM D-610[24])Blistering degree Delamination from scribe(mm)

ASTM D-714[25]Numerical scale[26]a

A18–1010

B12–1011

C18M88 5.5

A29–100.5

B29–10 4.5

C29–100.5

A39–1010Voluminous rust formations

B39–1026Voluminous rust formations

C39–109.5Voluminous rust formations

a The values of size and frequency of blisters have been converted,according to ASTM D-714,into numeric values using the Keane conversion table.

Table10

Prohesion test.Time(h)to formation of rust along entire length of scribe.

Paint system Time(h)for scribe fully rusted

A15

B15

C110

A2360

B2440

C2380

A31060

B31060

C31100

primer,in contrast to what happened in the condensing humidity and salt fog tests,where coating delamination in the scribe area was lower with the coatings applied on zinc silicate primer.This appar-ently anomalous result may be due to the strong attack of the ethyl silicate zinc-rich primer,and of the base steel once the cathodic protection capacity of this primer has been exhausted(Table10),Table11

Electrochemical parameters(C p,R p,/Z/at1mHz)of the tested coatings.

Paint systems C p(F)R p( )/Z/( )

Epoxy–siloxane(A1) 5.06×10?10–1011 Epoxy–siloxane(B1) 6.06×10?10 6.58×108109 Epoxy/polyurethane(C1)8.8×10?10 2.4×109109.5

leading to considerable lifting of the topcoat paint at the scribe. Observation of cross-sections of these paint systems in the scribe area(Fig.9)reveals strong attack of the zinc-rich primer and the presence of abundant voluminous rust at the scribe area in

the paint

system A3containing the zinc silicate primer(Fig.9top),in com-

parison with the relatively low attack at the scribe area in the paint

system A2containing the epoxy zinc primer(Fig.9down).EDS

spectra of the corrosion products reveals the presence of zinc from

the dissolution of zinc particles of the primer.

This strong attack may be due to the special microstructure of

the silicate-type zinc-rich primer’s with its pseudo-wetting vehicle, Fig.8.State of the different paint systems after4896h of exposure to Prohesion cyclic test.

I.Díaz et al./Progress in Organic Coatings69 (2010) 278–286

285

Fig.9.Cross-section optical micrographs(100×).Top:zinc silicate/epoxy–siloxane paint system(A3).Far from the scribe area(a),and at the scribe(b).EDS spectra(c)of the corrosion products.Down:epoxy zinc/epoxy–siloxane paint system(A2).At the scribe(d).

which facilitates the consumption of the zinc particles in its interior [27,28],and to the high corrosivity of the Harrison’s solution used in the Prohesion cyclic test[29],whose saline concentration in the scribe area also increases as a consequence of the successive drying stages in the different cycles.

3.4.Electrochemical impedance spectroscopy(EIS)

Only coatings A1,B1and C1were tested,i.e.the paint sys-tems without zinc-rich primer.Table11presents the resistance and capacitance values obtained by?tting of the respective impedance diagrams to equivalent circuits,and the impedance modulus values (/Z/)at the lowest measured frequency,1mHz,obtained from the Bode diagrams(Fig.10).

The models used are represented in Fig.10,where R e is the electrolyte resistance;R p is the electric resistance of the protective coating,and C p is the capacitance of the protective coating.These parameters may be taken as indicators of the state of the protective coating subjected to a medium that is aggressive to the metal.

The characteristic low frequency arc associated with the corro-sion process on the metallic surface,which can take place in pores or possible defects in the coating if the coating permeability allows diffusion of the electrolyte[30–32],is not observed in any of the three studied coatings(A1,B1or C1).

The epoxy–siloxane coating(A1)presents excellent barrier properties,acting as an almost perfect capacitor with a capacita-tive behaviour and an impedance modulus(/Z/)of the order of 1011 at a frequency of1mHz[33].Due to the absence of cor-rosive processes and diffusion processes on the metallic substrate, the impedance modulus for this coating at the lowest measured fre-quency,1mHz,may be taken as the ohmic or ionic resistance of the coating.

With regard to the epoxy–siloxane and epoxy/polyurethane coatings(B1and C1),the drop in R p may be associated to the pene-tration of electrolyte in the coating,slightly raising the capacitance compared to the epoxy–siloxane coating(A1)due to the increase in the dielectric constant of the medium.

This can be seen in the Nyquist diagram,which exhibits the characteristics of a semicircle in the Randles circuit(see Fig.10

). Fig.10.Nyquist and Bode diagrams,and associated equivalent circuits,of the tested coatings after504h of exposure to a0.1M Na2SO4solution.

286I.Díaz et al./Progress in Organic Coatings69 (2010) 278–286

However,all the studied coatings present an ionic resistance of more than107 ,a value above which coatings are considered to be in a good state and without deterioration[34].

Attention is drawn to the fact that a new time constant( )starts to appear at low frequencies for the epoxy–siloxane coating(B1), probably due to the charge transfer resistance of the metallic sub-strate or incipient diffusion controlled mass transport processes.

4.Conclusions

?Great differences in behaviour are seen between the two tested epoxy–siloxane formulations.While hybrid coating A equals or surpasses the protective capacity of hybrid coating B and the epoxy/polyurethane paint system in all the tests,hybrid coat-ing B frequently shows poorer anticorrosive properties than the conventional epoxy/polyurethane paint system.Differences in performance of the two epoxy–siloxane coatings could be attributed to the differences in water vapour permeabilities and adhesion values on steel base.

?The presence of zinc-rich primers generally optimises the good anticorrosive behaviour of epoxy–siloxane hybrid paints.?Epoxy–siloxane hybrid coating A presents excellent blistering behaviour,both with and without a zinc-rich primer(organic or inorganic),in the permanent humidity,Kesternich and salt fog tests,generally surpassing the performance of the conventional epoxy/polyurethane systems and proving to be highly suitable for environments with high humidity.

Acknowledgements

One of the authors,I.Díaz,would like to thank the State Agency Consejo Superior de Investigaciones Cientí?cas(CSIC)for the I3P programme contract granted,thanks to which it has been possible to undertake the work presented in this paper.

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