Analysis of new methods for the determination of the cathodic disbonding behaviour of hull coatings

Marina DELUCCHI1,5, Alessandro BENEDETTI2, Andrea BERGO3, Muriel SALES4, Cedric HUBERT4,

Roberto STIFANESE5, Pierluigi TRAVERSO5

1DICCA, Department of Civil, Chemical and Environmental Engineering, Universtity of Genova, P.le Kennedy,1, 16129 Genova, Italy, marina.delucchi@unige.it

2 CNR-ICMATE – U.O.S. Genova, National Council of Researches, Via De Marini 6, 16149 Genova, Italy, alessandro.benedetti@cnr.it

3Centro Sviluppo Materiali S.p.A., Via di Castel Romano 100 - 00128 Roma, Italy, a.bergo@c-s-m.it 4 DGA Techniques aéronautiques, Division Matériaux et Technologies, 47 rue Saint Jean 31131 Balma, France,

muriel.sales@intradef.gouv.fr, cedric2.hubert@intradef.gouv.fr 5 CNR-ISMAR - U.O.S. Genova, National Council of Researches, Via De Marini, 6 16149 Genova, Italy,

roberto.stifanese@ge.ismar.cnr.it, pierluigi.traverso@ismar.cnr.it Abstract The work proposed here is devoted to the evaluation of cathodic protection effects on some commercial coatings prepared with an artificial holiday drilled in the centre of each testing panel. A new method for the determination of the cathodic disbonding behaviour of hull coatings was determined; it draws inspiration from the following standards: BS EN 10289, UNI EN and NACE TG-470. The main parameters are: synthetic sea water, replaced once a week, Pt anode, room temperature, applied potential = -1500 mV vs SCE, duration time = 13 weeks. In the cathodic side, brucite grew in areas of the thinner coatings far away from the site where the artificial defect was produced, witnessing intense coating damage. This phenomenon induces the increase of cathodic current request to maintain the fixed potential and the related anodic processes. In the anodic side, a pH drop, below 6, in times shorter than that planned for the solution renewal, was evidenced, inversely correlated to the current enhancement. The possibility of slowing down the acidification rate was examined, in presence and absence of anode isolation. Keywords: cathodic disbonding; brucite; anode isolation

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Introduction The work proposed here is devoted to the analysis of a new method for the determination of the cathodic disbonding behaviour of hull coatings. The new procedure draws inspiration from the following standards: • BS EN 10289:2002, Determination of requirements of epoxy/epoxy modified coatings for the

corrosion protection of steel tubes and pipeline fittings of on/offshore pipelines • EN ISO 15711:2004, Determination of the disbonding of epoxy coatings applied to metallic

substrates as ships or marine structures • NACE TG-470:2014, Cathodic disbondment test for coated steel structures under cathodic

protection The problems emerging by its application as testing procedure for coatings are here analysed and discussed as evidenced during 12 weeks of exposition. This experimentation can provide wider information about the phenomena occurring at the anodic and cathodic sides of the cell. Methods The new testing method The procedure by which the cathodic disbonding test was performed derives from the composition of three different standards. In Tab. 1 are resumed the main features of the reference standards as well as the adopted criteria for the new procedure (right column). Basically, the hybrid procedure investigated here was intended to verify the possibility to decrease the time of experimentation of standard EN ISO 15711 maintaining room temperature during the test, but decreasing the applied cathodic potential.

Tab. 1 Details on the reference standards and new procedure

BS 10289 EN ISO 15711 NACE TG-470 Our procedure Solution composition NaCl 3% Synthetic sea water NaCl 3 % Synthetic sea water Solution replacement - ≤7 days - 7 days

Electrolytic cell; electrolyte volume

Rigid plastic tube, diameter ≥ 50 mm;

≥150 ml

Tank, diameter ≥70 cm; 100 l

glass tube, inner diameter 75-

100 mm, 100 mm in length; -

Tank, diameter ≥70 cm; 100 l

Anode Pt Pt, graphite

Pt, Titanium coated with IrO2/Ta2O5

or other not corroding anode

Pt

Anode isolation - - Yes Yes / No Artificial defect,

cathode, diameter (mm)

10 6 6 5-6

Anode/Cathode ratio > 1 - Large < 1

pH 6 < pH < 9

(no indication about pH adjustment)

-

pH monitoring (no pH restrictions )

6<pH<9 NaOH 1M adjustment

Time duration (weeks) 0,3 26 4 13

Temperature (°C) 60 Room T>Room Room

Potential vs SCE (mV) - 1500 - 1050 - 1430 - 1500

The configuration of the tank is reported in Fig. 1.

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The samples are placed at the edge of the tank at a distance of 5 cm from the bottom. In the centre are visible the reference and counter electrodes, connected to the potentiostat AMEL mod.2051 (see top left).

Fig. 1 Experimental setup

pH and Cl2 measurements Spatial/temporal pH and Cl2 measurements were performed at certain stages of the test in order to evaluate pH/Cl2 evolution with respect to the periodicity of solution renewal, fixed as once a week. pH and Cl2 measurements were performed collecting 20 ml amounts of solution at 0, 2, 5, 15, 25, 35 cm along the anode-cathode radial distance: 0 cm is referred to the anode (in the centre of the tank), 35 cm is referred to the cathode (on the wall of the tank). Collections were made every 2-3 hours after the solution replacement or pH correction. These measurements were performed both in presence and absence of anode isolation. Cl2 evaluation was performed with DPD colorimetric titration. pH adjustments pH adjustments were performed with NaOH 1M, aiming to control the acidification of the tank observed during the experiment. Each correction was made with up to 100 ml of NaOH 1M; this amount accounts for less than 1% of sodium cations present in 100 l of synthetic seawater featured by 3.5 g l-1 NaCl. At the end of the exposition period, it was necessary to adjust the pH 3-4 times every week; however chemical modifications induced by NaOH corrections resulted of negligible significance. Materials Samples were prepared by applying an abrasion resistant coating, in two different thicknesses, 150 and 250 µm, to steel panels. All the specimens were drilled in the centre to obtain a 6 mm diameter artificial defect. Three repetitions of each sample were prepared.

Tab. 2 Samples for cathodic disbonding tests

Coating Nature DFT, µm Total DFT, µm

1 Abrasion resistant epoxy 125

250 Abrasion resistant epoxy 125

2 Abrasion resistant epoxy 150 150

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In Fig. 2 it is presented the detail of the artificially produced defect. The diameter was comprised between 5 and 6 mm, hence, the exposed metal surface at the beginning of the immersion corresponded to the sum of 12 defects 2.35 mm2 each, i.e. 2.8 cm2 globally.

Fig. 2 Artificially produced defect

Results The aim of the present experimentation was to test the coatings behaviour under Mg-potential (-1500 mV vs SCE) applied for several weeks, highlighting possible criticisms of this procedure as well. A chronological criterion of results exposition is followed. Cathodic side In Fig. 3 it is presented the time evolution of the polarisation current. It is evident that the current enhances with time, reaching almost 300 mA after 12 weeks of testing. This evidence is correlated to the progressive damage of the coatings, witnessed by the growth of a white mineral deposit on their surface.

Fig. 3 Current intensity at -1500 mV vs SCE. Since the metal surface enhances with time due to coating progressive damage, current data cannot be reported as current density.

In Fig. 4 it is evident how the mineral growth occurs initially only in correspondence of the central drilled hole. The growth remains localised on the artificial defect for the thicker coatings (Fig. 4-a), while it propagates on the coated surface for the thinner coatings (Fig. 4-b). In some cases, even if the samples were stripe coated, the development of the mineral deposit occurred heavily on the edges (Fig. 4-c).

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a)

b)

c) Fig. 4 Visual appearance of the cathodes for a) the thicker coating, b) and c) two replicates of the

thinner coting 2. The three photographs, from the left to the right side, were collected after 3, 6 and 10 weeks of testing, respectively.

Anodic side A critical aspect of the test was related to the pH behaviour, with respect to which the 6<pH<9 criterion was adopted. In particular, the pH dropped below 6 since initial stages of polarization, even before the time planned to renew the solution, that was fixed at once a week. After 5 weeks (I≈120mA), one correction with 1 M NaOH was necessary before the weekly solution renewal, and the required amount of sodium hydroxide solution was ≈20 ml. Successively, after 2 months (I≈200 mA), three corrections were necessary, ≈50 mL of 1 M NaOH each, after 3 months (I≈270 mA), up to 150 ml of 1 M NaOH were necessary once a day.

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At initial stages of the experimentation the anode was intentionally left to work in contact with the solution (no isolation), in order to put in evidence the Cl2 production in the examined conditions, and evaluate Cl2/pH relationships. This investigation was performed as follows. When pH dropped to 6.5, two amounts of test solution (about 50 ml) were collected and stored in beakers, one closed with a cap, one left open in equilibrium with atmosphere. Successively, after the pH of the tank was adjusted to 8.8, other two 50 ml beakers were filled with the adjusted solution and were conserved with the criterion stated above. All the pH 6.5-8.8/open-closed samples initially contained 8 ppm of Cl2, i.e. the Cl2 amount produced until the sample collection. After the pH correction in the tank, the anode was isolated in order to interrupt the Cl2 diffusion in the tank. The Cl2 and pH time evolution was followed in the 50 ml samples and in the tank as well, where the polarization was not interrupted.

Fig. 5 Time evolution of Cl2 concentration in 50 ml samples collected from the tank, before

(pH=6.5) and after (pH=8.8) pH adjustments.

For all the solutions, the Cl2 concentration decreased (Fig. 5), and was accompanied to a pH decrease, too (Fig. 6). Nevertheless, in the tank, the pH decreased faster even below 6, while Cl2 concentration exhibited a slower diminution. Differently, in all the 50 ml samples, pH showed a slower decrease equilibrating to values ≥6, coupled to a faster Cl2 diminution. Considering the solutions of the beakers, the progressive disappearance of Cl2 is not accompanied by a significant pH variation, especially if initial pH is 6.5. On the contrary, in the tank there is a rapid acidification, which evidently did not depend on the Cl2 produced at the anode.

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Fig. 6 pH evolution of the samples presented in Fig. 5

After the anode isolation, the pH spatial/temporal evolution was monitored in the tank in successive stages of the cathodic disbonding test: after 5 and 9 weeks, respectively at I≈120 mA and I≈200 mA. It was verified that the Cl2 concentration in the bulk solution was actually < 0.1 ppm. It resulted that the pH=6 threshold was reached in correspondence of cathodes in nearly 2 days and 1 day, after 5 and 9 weeks of testing, respectively (data not showed). In both cases, the pH inside the anode isolating glass dropped within 1 hour to values in the range 1<pH<2. After 12 weeks of testing (I≈270 mA), the spatial/temporal evolution of pH and Cl2 was highlighted removing the anode isolation. Data are presented in Figs. 7-8-9.

Fig. 7 Temporal evolution of pH after 12 weeks of testing (I≈270 mA) along different positions

away from the anode. 0 cm is referred to the solution adjacent to the anode, 35 cm to the solution close to the cathodes

It is evident that after 12 weeks of testing (I≈270 mA), the time necessary for the solution close to the cathodes to reach pH=6 was reduced to about 8 hours. Nevertheless, after 8 hours, the Cl2 concentration close to the anode (Cl2 site production) and to the cathodes differed for more than one order of magnitude, with the lowest concentration of Cl2 in correspondence of the cathodes, 0.3 ppm (see Figs. 7 and 8).

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Fig. 8 Temporal evolution of Cl2 concentration after 12 weeks of testing (I≈270 mA)

Fig. 9 Spatial evolution of pH and Cl2 from data presented in Fig. 7 and Fig. 8. Two critical instants are compared: t=0 h, when pH was 8.8 and Cl2=0 ppm, and t=8 h, i.e. the time necessary to observe

pH=6 in correspondence of the cathodes.

Discussion The discussion is turned to some evidences and criticisms regarding both the cathodic and anodic sides occurring during the test, as a consequence of the appliance of the new testing procedure. Cathodic side The growth of a mineral deposit is an important evidence; it was initially formed on the drilled hole and successively on other parts of the panels (see Fig. 4). The growth of mineral deposits, composed mainly by CaCO3 and Mg(OH)2, deriving from the alkalinity developed at the cathode, is a well documented phenomenon in the cathodic protection literature since 80’ [1]. The interest for this topic is due to the lower current requirement necessary to sustain the polarisation potential in presence of deposits, which means current savings and economical advantages in the cathodic protection process. The best protective performances are related to deposits featured by large aragonitic fractions, growing at -900>E>-1050 mV SCE. At potentials more cathodic than -1200 mV SCE only brucite deposition occurs: brucite has scarce protecting properties [2]. At the potential applied in our test, the mineral deposit is only brucite, with weak or null protecting properties [3], even worsened by the hydrogen evolution; the produced pressures cause mechanical

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disbonding [4], as witnessed by large (mm to cm size) brucite scales frequently forming on the surface and detaching from it. As evidenced in Fig. 1, the current necessary to maintain the polarisation potential enhances with time, due to gradually emerging defects exposing new metallic surface. These new bare metal surfaces are mainly due to the rapid degradation of the thinner films, that are easily permeated by the electrolyte in several locations, probably due to defects in the application of the one layer coating. The larger is the current requirement to sustain the imposed potential, the more intense are the correlated anodic processes, whose consequences are discussed below. Anodic side The acidification of the tank solution is an important phenomenon that was detected. It was induced by anodic processes occurring as faster as current intensity enhanced. This increase of the rate of acidification implied to achieve pH values equal to 6 within one week. The possibility of slowing down the acidification rate is discussed below, evaluating the presence/absence of anode isolation, considering that in the present experimentation a Pt anode was used. Anode isolation Firstly, it is briefly reminded that at the imposed potential of -1500 mV vs SCE, the main anodic reactions occurring at the electrodes are oxygen and chlorine gaseous evolution:

1) 2𝐶𝑙− → 𝐶𝑙2 + 2𝑒− 2) 2𝐻2𝑂 → 𝑂2 + 4𝐻+ + 2𝑒− (acidic solution) 3) 2𝑂𝐻− → 𝑂2 + 2𝐻+ + 2𝑒− (alkaline solution)

In general, despite the equilibrium potential for O2 evolution is less noble than equilibrium potential for Cl2 evolution, the second reaction exhibits faster kinetics. Hence, in order to make the acidification rate slower, in presence of anode isolation, anodes featured by large O2 overpotentials and small Cl2 overpotential can be looked for, as alternative to Pt anode. Indeed, larger Cl2 evolution would not represent a problem, since gaseous Cl2 is deliverable to the atmosphere by anode isolation. Hence, Cl2 selectivity (i.e. a larger Cl2/O2 ratio) would allow a lower acidification rate. As suggested by applications like electro-oxidation of organic matter [5] or chlor-alkaly industry [6, 7], where oxygen evolution is as a side reaction and anodes with large O2 overpotential are desirable, TiO2/RuO2 anodes, known as DSA (Dimensionally Stable Anodes), deserve to be mentioned. DSA are regarded to be selective versus Cl2 evolution until RuO2<20% [8], hence, unless an existing practical advantage of DSA vs. Pt anode in retarding O2 vs. Cl2 evolution, their stability has to be verified in the operating conditions encountered here (1<pH<2, 10-2<i<10-1 A cm-2). Some aspects drive to think about a positive behaviour in this sense, considering that DSA stability is well documented since decades [9] in the operative conditions of chlor-alkaly industry (pH≈4, 10-1<i<1 A cm-2), where their life-service is evaluated in years. In addition, even if the loss of activity is related to anodic processes as the formation of non conductive Ru oxides [10][11] and oxygen evolution [11][12], the low pH values (1<pH<2) encountered in the isolating anode glass would minimize RuO2 corrosion rate [13][14]. In addition, the threat of O2 evolution would be reduced due to lower O2 anodic evolution in the low acidic pH region. Anyway, referring to conditions highlighted here, the practical advantage in reducing the acidification rate using DSA instead of Pt anodes with isolating glass has to be investigated.

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Free anode (not isolation) Another possibility is to eliminate isolation, and leave the anode in direct contact with the solution. This arrangement implies a delivery of anodic oxidation products into the tank, avoiding the anode to face a 1<pH<2 solution (as inside the anode isolating glass). As a consequence, in opposition to what suggested in the case of anode isolation, a Cl2/O2 relatively lower ratio would be desirable, in order to avoid a possible threat of hypochlorite deriving from Cl2 dissociation. This aspect is of importance especially in presence of coatings with components sensitive to hypochlorite oxidation power, e.g. organic materials [15][16]. The impact of this variable in the procedure examined here is evaluated considering that i) the used anode was Pt, ii) standard BS 10289 recommends to keep pH in the range 6-9, iii) standard NACE TG40 recommends anode isolation, iv) Cl2 spatial/temporal distribution exhibited the behaviour presented in Fig. 7,8,9. Basically, the spatial/temporal Cl2 distribution is a balance between: a) the production rate of chlorine in function of current intensity, b) the diffusion of Cl2 from the production site (anode) towards the target site (cathodes), c) the Cl2 dissociation in function of pH spatial/temporal variations. In particular, it is known that Cl2 dissociations to hypochlorous acid and hypochlorite anion by the reactions

4) Cl2 + H2O → H+ + Cl- + HClO 5) HClO → H+ + ClO-

are pH-dependent processes: the highest the pH, the more efficient the dissociations. At pH=5 the Cl2 to HClO dissociation is 100%, while the HClO to ClO- is 0%. In the pH range regarded by standard BS 10289, i.e. pH 6, 7, 8, 9, the ratio between ClO- and HClO is 0.05, 0.2, 0.7, 0.9 respectively. Besides 6<pH<9 recommendation, 6<pH<7 corrections are suggested, in order to limit the hypochlorite conversion by almost 1 order of magnitude vs. corrections at pH=9, whatever the Cl2 concentration. Practical evaluations are referred to chlorine data after 12 weeks (I=270 mA, see Figs. 7,8,9). At this stage of polarisation it can be seen that, when pH near the cathodes is 6, Cl2 concentration is 0.3 ppm, which means ClO-≈10-2 ppm for 6<pH<7. Even considering Cl2=2 ppm as spatial averaged concentration at the same time, ClO- would be ≈10-1 ppm, i.e. negligible values of hypochlorite. This result depends on the geometrical configuration adopted here. The 35 cm radial distance between anode and cathodes obliges Cl2 molecules to equilibrate from the anode to the cathodes with a certain delay (see Fig. 9), in manner that when cathodes reach pH=6, Cl2≈10-1 ppm is met in opposition to a Cl2 anode concentration larger than one order of magnitude. Of course, this is not the case of the geometrical set-up in the standard NACE TG40. Here, the dimensions of the cell (inner diameter not exceeding 100 mm) make possible the HClO → H+ + ClO- conversion driven by the cathodic alkalinity due to the exiguous anode-cathode distance. Finally, the set-up here adopted suggests that a free working anode (not insulated) can make the hypochlorite threat not so significant as in a small cell indicated in standards BS 10289 and NACE TG40, especially if pH is controlled between 6 and 7. Rather, pH corrections would be necessary with daily frequency when I≥250 mA. Geometrical arrangement Finally, the possibility of keeping the pH>6 by means of modification of the geometrical parameters of the cell seem not reliable. In principle, both the reduction of the cathodic surface and/or the enhancement of the volume of the solution would result in slowing down of the acidification rate, and/or pH enhancement. Nevertheless, due to the log dependence of the proton concentration with volume, reduction of cathodic surfaces and/or enhancement of solution volume might be performed not linearly, but exponentially, which is not actually practicable.

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Conclusions Cathodic disbonding tests on coatings of different thickness were performed with a new hybrid procedure adopting recommendations presented in standards EN 15711, BS 10289 and NACE TG40. It consisted in applying -1500 mV vs SCE to the coated panels, prepared with an artificial defect, immersed in synthetic sea water at room temperature for 12 weeks. Conclusions resumed here are related to observations and criticism emerged during the experimentation:

- Cathodic side. Brucite growth occurred in areas of the thinner coated panels different from the area of bare metal produced with the artificial defect on the coating, witnessing progressive coating damage. This phenomenon allows verifying the possible degradation of the coating. In addition, the application of the coating must be perfectly continuous, also on the edges of the panels, in order to avoid preferential occurrence of defects thereby, which induce the increase of cathodic current request to maintain the fixed potential and related increase of anodic processes.

- Anodic side. A very important evidence was related to the pH drop below the value of 6 in times shorter than that planned for the solution renewal, inversely correlated to the current enhancement. The possibility of slowing down the acidification rate was so examined. In presence of anode isolation, acidification rate has to be aimed looking for a material with good stability at 1<pH<2 values (as in the isolating glass) featured by small O2 and large Cl2 overpotentials, since Cl2 can be delivered to the atmosphere. DSA anodes deserve verification. In absence of isolation, anode is prevented to face 1<pH<2 values, but Cl2/O2 production ratio might not be too large, unless possible hypochlorite poisoning from Cl2 dissociation. In case of Pt (used here), it was found that near the half-life of the experiment, when pH dropped to 6, Cl2 was delivered to the cathodes to an extent figuring out 10-2<ClO-<10-1 ppm.

Acknowledgements The financial support of the project “Corrosion Control for Navy Ships” is acknowledged. The authors wish to thank Giulia Spadotto for the help in the assessment of the experimental set-up and for the execution of a lot of measurements. References [1] W.H. Hartt, C.H. Culberson, S.W. Smith, Corrosion, 40 (1984) [2] C. Barchiche, C. Deslouis, D. Festy, O. Gil, P. Refait, S. Touzain, B. Tribollet, Electrochmimica Acta, 48 (2003) 1645 [3] C. Deslouis, D. Festy, O. Gil, V. Maillot, S. Touzain, B. Tribollet, Electrochmimica Acta, 45 (2000) 1837 [4] G. Salvago, S. Maffi, A. Benedetti, L. Magagnin, Electrochimica Acta 50 (2004) 169 [5] H. Särkkä, A. Bhatnagar, M. Sillanpää, Journal of Electroanalytical Chemistry, 754 (2015) 46 [6] S. Trasatti, Electrochimica Acta, 45 (2000) 2377 [7] V. Trieu, B. Schley, H. Natter, J. Kintrup, A. Bulan, R. Hempelmann, Electrochimica Acta, 78 (2012) 188 [8] R. K.B. Karlsson, H. A. Hansen, T. Bligaard, A. Cornell, L. G.M. Pettersson, Electrochimica Acta 146 (2014) 733 [9] L.D. Burke, O.J. Murphy, J. Electroanal Chem., 112 (1980) 39 [10] T. Loucka, Journal of Applied Electrochemistry, 7 (1977) 211 [11] T. Kishi, Y. Sugimoto, T. Nagai, Surface Technology, 26 (1985) 245 [12] Lj.M. Gajic-Krstajic, T.Lj. Trisovic, N.V. Krstajic, Corrosion Science, 46 (2004) 65 [13] A.A. Uzbekov and VS. Klement'eva, Soviet Electrochem., 21 (1985) 698

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[14] M.M. Pecherskii, V.V. Gorodotskii, N.Ya. Bun'e, and V.V. Losev, Soviet Electrochem. 22 (1986) 615 [15] Cathodic Disbondment Test for Coated Steel Structures under Cathodic Protection, NACE, TG40, 2014 [16] E. Broesder, Stopaq B.V., Stadskanaal Coatings and Cathodic Disbondment - The True Story, the Netherlands K.C. Lax, Asset Integrity Services Ltd., Aldermaston, United Kingdom

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