The Development of Calcareous Films at a Defect in Coated Steel under

Intermittent Cathodic Protection Simulating Inter-Tidal and Splash Zones

W. Shi, S. B. Lyon

Corrosion and Protection Centre, School of Materials, University of Manchester,

Manchester, M13 9PL, UK

ABSTRACT

Cathodic protection (CP) should not work in the splash and intertidal zones because

there is no persistent electrolyte pathway for the protection current to flow.

Nonetheless, there is anecdotal evidence that CP can influence the corrosion processes

in these challenging regions of a structure and this is at least possible, since thin

electrolyte pathways can persist for some time on intermittently wetted surfaces even

after the bulk fluid has receded. Here we demonstrate that the Scanning Vibrating

Electrode Technique (SVET) may be used to measure local current at coating defects

in thin electrolyte layers, and can probe the influence of cathodic protection on the

local currents at the defect site. We confirm that cathodic protection is effective in thin

electrolyte layers and, significantly, that calcareous films form under these conditions

in seawater. Such films are found to be persistent in the absence of cathodic

polarisation and provide significant local inhibition of current flow at the defect site.

Key words: SVET; Cathodic Protection; Intertidal and Splash Zone; Calcareous

Films.

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HIGHLIGHTS

In-situ SVET was used to study corrosion at defects in epoxy-coated mild steel

in thin liquid layers of electrolyte under intermittent cathodic protection.

The formation of calcareous films was confirmed under intermittent cathodic

protection simulating inter-tidal and splash zone conditions.

Calcareous films were significantly protective under intermittent wetting

conditions and only failed when physically damaged.

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1. INTRODUCTION

The dominant form of corrosion protection for marine and estuarine environments

comprises the use of an appropriate organic coating system supplemented by cathodic

protection using either impressed current or sacrificial anodes. This approach works

well under fully immersed conditions whereby defects in the paint coating, whether

originating from initial application or arising during service, are protected by cathodic

polarisation. Thus, provided coatings are formulated to limit cathodic disbonding at

defects, then many years of service can be relied on. Corrosion in intermittently

wetted surfaces (i.e. in the splash and inter-tidal zones) is significantly more severe

than under fully atmospheric and fully immersed conditions [1, 2]. Consequently,

corrosion protection systems are commonly designed to provide additional protection

in this region of structures, for example: thermal sprayed coatings [3], metallic

sheathing [4, 5], petrolatum tapes [6] and high-build organic coatings [7]. Moreover,

cathodic protection is generally assumed to be largely ineffective under these

conditions because the return ionic path through the water is not continuously present.

However in the splash and inter-tidal zones, a thin wetted surface layer will generally

remain for a variable length of time and recent work has demonstrated that

electrochemical measurements can be performed in a simulated splash zone [8]. Also,

it is well-known that under cathodic protection conditions calcareous deposits formed

on the metal surfaces consist of aragonite [CaCO3] between -0.9 and -1.1 VSCE, of

brucite [Mg(OH)2] and aragonite [CaCO3] at -1.2 VSCE, and only of brucite [Mg(OH)2]

at potentials lower than -1.3 VSCE [9]. However, it remains to be answered whether

calcareous films can persist under intermittent wetting conditions so as to provide

significant protection.

Scanning reference electrode methods were introduced into corrosion studies in 1972

by Isaacs and Kissel [10]. A variant of this method, the Scanning Vibrating Electrode

Technique (SVET) has been used to investigate many materials and material systems

and finds application in studies of coating damage on coil-coated steel [11], corrosion

of epoxy-coated galvanised steel [12] and self-healing mechanisms in coatings over

metallic surfaces [13,14]. The overall aim of the work presented in this paper was to

measure the local current densities on epoxy-coated mild steel sample, in a thin layer

of electrolyte as a simulation of the marine inter-tidal zone. A further objective was to

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obtain an improved understanding of the effectiveness of intermittent cathodic

protection, and more specifically to study the development and persistence of

calcareous deposits under these conditions.

2. MATERIALS AND METHODS

2.1 Experimental approach

A commercial Uniscan M370 system was employed in SVET mode to measure the

localised corrosion activity. In order to obtain the local current density map, a voltage

to current calibration [15] was accomplished by placing the probe vertically above a

point-in-space (PIS) sample with a source of known current in an aqueous solution.

The PIS sample consisted of a gold wire, 0.2 mm in diameter in a 32 mm diameter

cylinder of resin.

SVET measurements were performed within a round shallow micro-cell 40 mm in

diameter and 5 mm in height. A small volume of electrolyte (~ 6 mL) was contained

within the cell so as to provide a thin layer of solution of 1-2 mm in depth. The probe

was located above the working electrode surface at an average height of ~100 μm

with amplitude of vibration of 30 μm. The SVET probe was a platinised platinum

microelectrode with 5 µm diameter probe tip. The electrolyte was nominally aerated

although during operation the cell was sealed to limit evaporation. Although many

replicated experiments were performed in order to confirm data reproducibility, the

results presented here comprise data taken from a single representative experiment.

2.2 Solution

SVET sensitivity is dependent on electrolyte conductivity, so as the concentration of

the solution is increased, the minimum current detectable increases while the

corrosivity also increases. Hence, the electrolyte conductivity was chosen in order to

optimise the SVET sensitivity at lower current densities while retaining sufficient

concentration for the corrosion processes to proceed. Trials to test the experimental

concept were carried out using 0.01 M sodium chloride giving a conductivity of about

0.12 S·m-1. The main experiments were carried out using artificial seawater diluted ten

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times giving a conductivity of about 0.50 S·m-1. This was prepared from the recipe of

Kester et al. [16]: NaCl (23.926 g), Na2SO4 (4.008 g), KCl (0.677 g), NaHCO3 (0.196

g), KBr (0.098 g), H3BO3 (0.026 g), NaF (0.003 g), MgCl2 (5.078 g), CaCl2 (1.147 g),

and SrCl2 (0.014 g) dissolved in deionised water to make 1 kg of artificial seawater.

2.3 Materials

Substrates were prepared using carbon steel Q-panels (102 mm × 152 mm × 0.8 mm)

that were coated with epoxy resin DER660-X80 from Dow Chemical Company

supplied as an 80% solution. A phenalkamine epoxy adduct (equivalent weight of

291) was utilised as curing agent. The solvent was a mixture of xylene and butanol

mixed at a 3:1 volume ratio. Trial experiments in 0.01 M NaCl used a 25% pigmented

epoxy coating at 142±18 µm, which contained fumed silica, tixogel (bentonite), china

clay and talc with total solids of 69.62 wt.% and a density of 1.236 g·cm-3. Defects in

this coating were created using a sharp scalpel blade. The main experiments used

substrates coated with clear epoxy at 60.92 wt.% solids content with a density of

1.007 g·cm-3 at a thickness of 130±6 µm. Defects of dimension 10 mm × 1 mm in the

latter coating were created via laser ablation using a KrF excimer laser Impex 848

(Lumonics Impact), with 248 nm wavelength and 15 ns pulse width using a projection

mask.

Cyclic exposures of 1 hour wet and 5 hours dry in dilute seawater were carried out to

simulate intermittent cathodic protection in the inter-tidal zone with impressed current

polarisation using a potentiostat and a saturated calomel reference electrode (SCE)

during the wet part of the cycle. X-ray diffraction (XRD) at the defect site was carried

out using a Philips X’ Pert Diffractometer with Cu Kα radiation (λ=0.154056 nm) in

order to identify any calcareous deposits.

3. RESULTS AND DISCUSSION

3.1 Proof of concept

Figure 1 shows maps of the current distributions around the scratched defect

pigmented epoxy-coated mild steel, before and after the application of impressed

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current cathodic protection in 0.01 M NaCl solution. In order to better study the

variation in current density in the defect region, a series of lines along the x-axis

direction at the central defect location are shown in Figure 2. After applying an

impressed galvanostatic current of −10 μA (−40 μA/cm2 of sample surface area) for

30 minutes, the defect location becomes less active. After 120 minutes of polarisation,

the current at the defect area moved from net anodic to net cathodic. This

demonstrates that it is possible to monitor current changes under cathodic protection

using SVET in thin electrolyte layers.

3.2 Intermittent cathodic protection in dilute seawater

In order to better visualise the development of calcareous films under intermittent

cathodic protection in dilute seawater, a clear, unpigmented, epoxy-coated sample

with a laser-ablated defect was used. In these experiments, the polarisation was

undertaken using a potentiostat at a constant potential of −1.0 VSCE. This was done as

a simulation of a sacrificial anode and to avoid the application of excessive potentials

during the drying part of the cycle that might occur under galvanostatic control.

Figure 3 shows the mapped changes in current density over the defect in the coating

as a function of time with the following exposure cycle under intermittent immersion

(1 hour wet and 5 hours dry): 48 hours under free corrosion; a further 96 hours under

cathodic polarisation of −1.0 VSCE; and finally 60 hours under free corrosion again.

Newly-formed calcareous deposits covering the exposed steel at the coating defect

were visually apparent after this experiment. Initially, under free corrosion, the defect

was a net anode, Figure 3a. After applying cathodic polarisation, the defect became a

net cathode, Figure 3b and 3c. However, after polarisation was stopped, the region

remained a net cathode for a time (60 hours) greatly in excess of the inter-tidal time

(10 hours), Figure 3d and 3e.

After the exposure cycle indicated above, XRD was then employed to analyse the

chemical composition of the newly-formed deposits. Figure 4, confirms that aragonite

and brucite were identified, which demonstrated that calcareous deposits are indeed

formed under intermittent cathodic protection and are stable after removing the

protection current.

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3.3 Effectiveness of calcareous films

SVET line scans across a coating defect under continuous measurements in a thin

electrolyte layer show the influence of the calcareous films in more detail, Figure 5.

Thus, as expected, application of an intermittent cathodic current causes the defect

site to become a net cathode from being a net anode and corrosion at the defect thus

ceases. However, more significantly when the cathodic current was discontinued 24

hours after polarisation was stopped, the current at the defect did not immediately

return to a high anodic value and the calcareous films were clearly providing

significant protection, Figure 6. Only when the deposits were physically damaged by

scratching with a scalpel, the defect region re-established itself as a significantly

corroding net anode.

4. CONCLUSIONS

1. It is shown that SVET measurements can be obtained in thin electrolyte layers and

provide detailed analysis of local current in the vicinity of coating defects under free

corrosion, cathodic protection and intermittent wetting conditions.

2. Calcareous films are shown to reliably form at coating defects under simulated

seawater tidal conditions (i.e. intermittent wetting) and having significant beneficial

influence on the cathodic protection process.

3. Such calcareous films are persistent and significantly protective for timescales

greatly in excess of the tidal time range and only cease to be protective when

physically damaged.

ACKNOWLEDGEMENTS

One of the authors (W. Shi) would like to express her appreciation to the late Prof.

David Scantlebury for his guidance and supervision during the early stage of this

research. Thanks are due to AkzoNobel for providing all coated samples.

REFERENCES

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[4] Y. Kawai, H. Iwami, H. Satoh, N. Baba, Welding Technology for Metallic Sheathing of Offshore Steel Structures Using Seawater-resistant Stainless Steel Sheet, Nippon Steel Technical Report, 95 (2007) 93-97.

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[6] W. Qu, Y. Huang, X. Yu, M. Zheng, D. Lu, Effect of Petrolatum Tape Cover on the Hydrogen Permeation of AISI4135 Steel under Marine Splash Zone Conditions, Int. J. Electrochem. Sci., 10 (2015) 5892-5904.

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[10] H.S. Isaacs, G. Kissel, Surface Preparation and Pit Propagation in Stainless Steels, Journal of The Electrochemical Society, 119 (1972) 1628-1632.

[11] A.G. Marques, A.M. Simões, EIS and SVET Assessment of Corrosion Resistance of Thin Zn-55% Al-rich Primers: Effect of Immersion and of Controlled Deformation, Electrochimica Acta, 148 (2014) 153-163.

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Signal Detection of Point Source Activity, Electrochimica acta, 49 (2004) 2871-2879.

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LIST OF FIGURES

Figure 1: Changes in the current distribution over a scratched defect in pigmented

epoxy-coated mild steel immersed in 0.01 M NaCl: (a) freely corroding, (b) 30

minutes after application of cathodic protection, and (c) 120 minutes after application

of cathodic protection.

Figure 2: Selected line plots showing the current variation with time over a scratched

defect in pigmented epoxy-coated mild steel immersed in 0.01 M NaCl before and

after applying cathodic polarisation.

Figure 3: Current distribution at a laser-ablated defect in clear epoxy-coated steel in

dilute seawater under intermittent wetting (1 hour wet and 5 hours dry): (a) under free

corrosion for 48 hours; (b) as (a) after 48 hours of intermittent cathodic polarisation at

−1.0 VSCE; (c) as (b) after another 48 hours of intermittent cathodic protection; (d) as

(c) after cessation of cathodic polarisation for 12 hours; (e) as (d) after 48 hours.

Figure 4: X-ray diffraction pattern of deposits formed at the defect in clear epoxy-

coated mild steel after 9 days’ exposure in a simulated tidal environment (1 hour wet

and 5 hours dry) in dilute seawater.

Figure 5: SVET line scans of local current density across the centre of the coating

defect region in a simulated sea tidal environment with and without cathodic

protection.

Figure 6: Changes of current response as a function of time on selected point at X-

position of 7.2 mm in Figure 5 showing the output values above a defect region of the

epoxy-coated mild steel sample, when immersed in a simulated sea tidal environment

and under cathodic protection.

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Figure 1: Changes in the current distribution over a scratched defect in pigmented epoxy-coated mild steel immersed in 0.01 M NaCl: (a) freely corroding, (b) 30 minutes after application of cathodic protection, and (c) 120 minutes after application of cathodic protection.

Figure 2: Selected line plots showing the current variation with time over a scratched defect in pigmented epoxy-coated mild steel immersed in 0.01 M NaCl before and after applying cathodic polarisation.

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(a) (b) (c)

(a) (b) (c)

(d) (e)

Figure 3: Current distribution at a laser-ablated defect in clear epoxy-coated steel in dilute seawater under intermittent wetting (1 hour wet and 5 hours dry): (a) under free corrosion for 48 hours; (b) as (a) after 48 hours of intermittent cathodic polarisation at −1.0 VSCE; (c) as (b) after another 48 hours of intermittent cathodic protection; (d) as (c) after cessation of cathodic polarisation for 12 hours; (e) as (d) after 48 hours.

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White

Brucite,Mg(OH)2Aragonite, CaCO3

Figure 4: X-ray diffraction pattern of deposits formed at the defect in clear epoxy-coated mild steel after 9 days’ exposure in a simulated tidal environment (1 hour wet and 5 hours dry) in dilute seawater.

Figure 5: SVET line scans of local current density across the centre of the coating defect region in a simulated sea tidal environment with and without cathodic protection.

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Figure 6: Changes of current response as a function of time on selected point at X-position of 7.2 mm in Figure 5 showing the output values above a defect region of the epoxy-coated mild steel sample, when immersed in a simulated sea tidal environment and under cathodic protection.

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