EUROCORR 2019: ‘New times,new materials,new corrosion challenges’: Part 2

Technical sessions on coatings

Technical sessions on coatings were convened throughout the week, sometimes with parallel sessions running on the same day. A total of 123 papers/posters were given, including 22 full papers on the complementary memory stick. Chairs included W. Fürbeth (DECHEMA-Forschungsinstitut, Germany), B. Normand (Institut National des Sciences Appliquées de Lyon, France), F. Montemor (Centro de Quimica Estrutural, IST, Portugal), F. Andreatta (University of Udine, Italy) and M. Olivier (University of Mons, Belgium), to name but a few. Herein, 12 full papers/posters are summarised under their specific topic headings.

Organic coatings

Michael Irmer (Fraunhofer Research Institution for Large Structures in Production Engineering, Rostock, Germany) spoke on the corrosion protection performance, after compression and impact, of coating systems for offshore wind structures. To simulate typical damage mechanisms during transport and assembly, compression, impact and corrosion resistance tests were performed in the laboratory. Statistical methods were applied to assess and evaluate the performance of 15 coating systems that differed in terms of the number of layers, resin–hardener mixtures, fillers/pigments, solids content and total dry film thickness (DFT). Representative conclusions included: increasing DFT had a significant positive effect on resistance to compression and impact; the use of glass flakes had no significant effect on resistance to the investigated mechanical loads; and coating systems with a polysiloxane topcoat showed the smallest damage areas compared to epoxy and polyurethane.

‘Calcium carbonate particles as natural oil carriers in polyurethane primers’ were investigated by S. Ranade (Indian Institute of Technology Madras, Chennai, India). Various templates (e.g. dextrose and carboxy methyl cellulose sodium salt) were used to produce calcium carbonate microparticles of different morphologies for loading with the inhibitors pongamia oil (from Millettia pinnata seeds) and benzotriazole. The oil-loaded microparticles were incorporated in an industrially used white primer and coated on mild steel plates. Electrochemical impedance spectroscopy (EIS) was used to measure the variations in the corrosion properties of the coatings exposed to 0.1 and 0.5 M sodium chloride solution for six days. A cyclic variation of impedance by one to two orders of magnitude was observed wherein a fall due to corrosion of the substrate was followed by a rise due to dissolution of the microparticles and release of inhibitor.

A poster by Jin Gaoa (University of Science and Technology Beijing, China) described ‘A correlation model between the surface aging of organic protective coatings and environmental factors’. Path analysis of the effects of various environmental factors upon the visible aging of acrylic PU coatings was performed using data from 13 atmospheric environmental test stations to obtain the coefficients of direct and indirect factors. While the direct effects of solar radiation and temperature were slightly greater than that of humidity (both reaching 0.492 when compared with 0.474), the indirect aggravation of other factors was greater for temperature than for solar radiation. Aging and environmental data from 10 test stations were subjected to k-means clustering analysis to obtain the classification and refinement criteria for typical environmental factors. A correlation model was then established via multivariate linear regression and verified using aging data from the three remaining test stations.

Inorganic coatings

The ‘Microstructural and corrosion properties of PEO-coated zinc-aluminised (ZA) steel’ were described by L. Pezzato (University of Padova, Italy). Plasma electrolytic oxidation (PEO) combines oxide film formation, dissolution and dielectric breakdown by using high voltages and current densities to generate persistent micro-discharges on the metal surface. Herein, PEO coatings were obtained using a solution containing sodium silicate and potassium hydroxide at high current densities and short treatment times in the DC mode. The typical PEO surface morphology was obtained, with numerous pores and micro-cracks. The coating thickness itself was strongly influenced by the process parameters. The PEO layer was mainly composed of aluminium and zinc oxides and silicates. The best samples in terms of corrosion resistance were (i) that treated for 3 min at 1.1 A cm⁻² and (ii) that treated for 2 min at 2.3 A cm⁻².

The ‘Development of an anti-corrosive coating based on a ceramic nano-pigment’ was presented by G. Maria Fernanda (Institute of Ceramic Technology, Castellón, Spain). The Fe–Cr nano-pigment was synthesised by the sol–gel method, applied to SAE 1015 carbon steel by thermal diffusion, and characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), wavelength-dispersive X-ray fluorescence (WD-XRF) and oxygen elemental analysis via thermal decomposition and IR detection. When specimens were exposed for nine months to various real corrosive atmospheres, the coating showed excellent performance in all tested environments. Each environment generated different rust phases (Akaganeite (Fe₄Cl_0.25O_3.75(OH)_4.25(H₂O)_0.2), goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and/or magnetite (Fe₃O₄)) in the same steel, which could result in different corrosion behaviour. It is therefore important to test the materials in real-use conditions.

A poster describing the ‘Multi-criteria evaluation of self-protecting waterborne coating based on mesoporous silica nanoparticles in corrosion environment’ was given by I. Stojanović (University of Zagreb, Croatia). Two corrosion inhibitors, 8-Hydroxyquinoline (Oxine) and 1-hydroxybenzotriazole (HOBT), were encapsulated in mesoporous silica nanoparticles. The composites were embedded in a waterborne epoxy coating in two different concentrations (0.69 and 0.35 wt-%) of each inhibitor and applied to steel substrates. To evaluate the barrier properties and long-term corrosion resistance, electrochemical impedance spectra were collected every 24 h for 10 days. Salt spray chamber testing was conducted for 72 h under a simulated marine atmosphere (C5-M) in accordance with ISO 7253 and was followed by adhesion testing. The best protective properties were obtained with HOBT as an inhibitor.

J. S. Pinheiro (Federal University of Rio Grande do Sul) presented a poster describing a new method for improving the corrosion resistance of tartaric-sulphuric acid (TSA) anodised AA7075-T6 by immersion in fluorozirconic acid (H₂ZrF₆) at ambient temperature to precipitate nanometric Zr oxide. Various concentrations and solution pH values were evaluated and the resulting systems were characterised by EIS in 0.1 M NaCl after immersion periods of 1 h to 42 days. Treatment with 0.5% H₂ZrF₆ significantly increased the resistance and showed slightly improved durability at pH 3.5. For pH values of 3.0-3.5, the optimum H₂ZrF₆ concentration was 1%. The treated samples showed decreased pitting attack relative to non-sealed samples when exposed to 0.1 M NaCl. The least pitting was observed after treatment with 1% H₂ZrF₆ at pH 3.5.

Self-healing and smart coatings

‘Corrosion inhibition of AISI 1010 carbon steel coated with epoxy resin systems with 5% LDH–Zn–Al microcontainers’ was described by A. Seniski (Federal University of Parana, Curitiba, Brazil). The anti-corrosive effects of Zn–Al-layered double hydroxides intercalated with sodium dodecyl sulphate (SDS) and nitrite (NO₂) were evaluated after incorporation into an epoxy resin matrix (Bisphenol-A, 5% by mass) and application to AISI 1010 carbon steel. Epoxy coating systems were prepared with LDH Zn–Al–SDS, LDH Zn–Al–NO₂ and with a blank epoxy resin reference. The corrosion processes were characterised by cyclic salt spray testing for 100 h, EIS, open circuit potential (OCP) and linear polarisation resistance (LPR). The coating with LDH-NO₂ showed better polarisation resistance (Rp = 3.01 × 10⁴ Ω cm²) than the LDH-SDS (1.51 × 10³ Ω cm²) and the blank epoxy resin (1.56 × 10³ Ω cm²).

A. Khan (Qatar University, Doha, Qatar) spoke on ‘Microcapsules reinforced polymeric smart coatings’. Urea-formaldehyde microcapsules encapsulated with linalyl acetate were synthesised by in situ emulsion polymerisation and a layer-by-layer technique was employed to synthesise alternatively charged polyelectrolyte (polyethylenimine and sulfonated polyether ether ketone) multi-layered microcapsules. The pH-responsive corrosion inhibitor dodecylamine was then entrapped in the multi-layers to make them doubly responsive. Three types of epoxy coatings, namely: (i) pure epoxy coating, (ii) single-layer smart coatings (SLSCs) and (iii) polyelectrolyte multilayer smart coatings (PMLSCs), were prepared by adding 5wt-% synthesised microcapsules into epoxy resin and applied to a polished steel substrate. Owing to the efficient release of the encapsulated self-healing agent and corrosion inhibitor, the coatings modified with multi-layered capsules demonstrated improved self-healing characteristics and higher corrosion resistance than the coating modified with urea-formaldehyde microcapsules.

Metallic coatings

A poster entitled ‘Performance evaluation of Al-based coatings replacing cadmium on high strength steels’ was given by Z. Sun (Beijing Institute of Aeronautical Materials, China). Seven potential alternatives for cadmium coating were examined, including ion vapour deposited aluminium coatings with and without non-chromium passivation (IVD Al + NCP and IVD Al+650E); electroplated aluminium (Ed Al); metallic ceramic sacrificial aluminium-based coatings (Al 962 + 570 and Al 962 + 1661); high-velocity oxygen fuel sprayed aluminium (Al HVOF); and arc sprayed aluminium (Al ARC). In the corrosion performance tests, the Al 962 + 570 and Al 962 + 1661 showed excellent corrosion resistance comparable to the cadmium coating. The Ed Al displayed white rust and a few pits after 495 h salt fog test, but no evidence of red rust after 1000 h. The IVD Al + NCP, IVD Al+650E, Al HVOF and Al ARC coatings displayed red rust after 408 h.

G. Roventi (Polytechnic University of Marche, Ancona, Italy) gave a poster on the ‘Corrosion resistance of Zn–Ni coatings containing ZrO₂ nanoparticles’. Zn–Ni alloy was electrodeposited onto mild steel from an industrial alkaline bath and nanocomposite coatings were obtained by the addition of 10 g L⁻¹ yttria-stabilised ZrO₂ nanoparticles (particle size < 100 nm, 10.14 wt-% Y₂O₃). Electrodeposition at 25 ± 1°C under galvanostatic control (current density: 30 mA cm⁻²) provided homogeneous and compact coatings with low grain size. The corrosion resistance was studied by polarisation resistance, potentiodynamic polarisation and EIS in 3.5% NaCl solution. The Zn–Ni coating displayed an E _corr of −0.767 V SCE and an I _corr of 1.16 × 10⁻⁵ A cm⁻², while the respective values for the Zn–Ni–ZrO₂ coating were −0.743 V SCE and 0.86 × 10⁻⁵ A cm⁻². The corrosion resistance of both coatings increased with exposure time due to enrichment in Ni and the formation of a uniform and compact layer of Zn₅(OH)₈Cl₂.

Pre-treatments

A. Guzanová (Technical University of Košice, Slovakia) spoke on the ‘Influence of abrasive blasting on the quality of coating systems with cathodic protection’. Various mechanically blasted surfaces were studied by roughness profilometry and 3D microscopy before application of a zinc-filled coating, followed by adhesion testing after curing and after exposure to corrosive environments. Surface blasting with steel grit showed good purity without secondary pollution. The subsequently applied coating displayed satisfactory adhesion. Among the evaluated non-metallic blasting abrasives, brown corundum created the highly irregular surfaces needed for optimal coating adhesion. All the high-humidity environments showed corrosion of the zinc in the coating, which led to a change in volume of the zinc particles and, hence, a change in the stress conditions in the coating. This culminated in a reduction in the proportion of adhesive failure compared to the as-sprayed state.

Polymers in organic coatings and advanced materials

A joint session on polymers in organic coatings and advanced materials was convened on the Tuesday afternoon, chaired by J. Heinemann (DIN CERTCO, Berlin, Germany) and W. Fürbeth (DECHEMA-Forschungsinstitut, Germany). A further general session on polymers and advanced materials took place on the Wednesday morning, with J. Heinemann as chair. In total, 17 papers/posters on these topics were given, with 5 full papers appearing on the complimentary memory stick. One full paper and one abstract are summarised herein.

D. Mills (University of Northampton, UK; Figure 1) delivered a full paper on ‘Investigating the long-term effects of immersion in salt solution on the protective properties of bio-content polyurethane coatings’. Resins were synthesised using the pre-polymer and single-step methods and castor oil was added to the systems to decrease the amount of artificial chemical compounds in the final products. The samples were subjected to immersion in 3% NaCl solution for different periods of time. Coatings with greater amounts of castor oil exhibited enhanced DC resistance. As a result of immersion, some pits and cracks in the surface were revealed by SEM imaging. Dynamic mechanical analysis showed that an increased castor oil content leads to higher cross-link density and higher glass transition temperature. Hence, it appears that the castor oil not only confers environmental friendliness but also improves the properties. OPEN IN VIEWER

L. Villareal (CIRIMAT/CNRS, France; Figure 2) delivered a paper (abstract only) on ‘Barrier properties of solvent-based and water-based epoxy coatings for the corrosion protection of carbon steel in aggressive environments’. An electrochemical impedance study on a high-solids solvent-based paint and a water-based paint, both applied to carbon steel, was described. The evolution of the impedance response in 0.5 M NaCl solution was followed for up to five months of immersion time. For both systems, the low-frequency impedance modulus was high, from 10⁸ to 10¹⁰ Ω cm² depending on the immersion time, indicating an obvious barrier effect. For both coatings, the modulus decreased during the initial 24 h due to water uptake but subsequently increased. Gravimetric measurements on the free solvent-based films indicated a low water uptake (∼1.5%) and mass loss associated with plasticiser leaching. The impedance increase for the water-based system was associated with accelerated film formation during exposure to the electrolyte, although the water absorption was limited (1-2%). OPEN IN VIEWER

CO₂ corrosion in industrial applications

There were a total of five papers/posters on this topic; three of the four full papers are summarised herein. Sessions were held on Tuesday and chaired by Ralph Bassler (Federal Institute for Materials Research and Testing (BAM), Berlin, Germany).

The ‘Effect of elemental sulphur on the corrosion behaviour of X80 steel under a supercritical CO₂ environment’ was outlined by Q. Gong (China University of Petroleum, Beijing, China). Molten sulphur was poured over X80 steel and autoclaved at 40°C and 8MPa with 200 ppmv NO₂ and 1000 ppmv O₂ in water-saturated supercritical (SC-CO₂) and SC-CO₂-saturated water. The corroded specimens were analysed by SEM/energy-dispersive X-ray spectroscopy (EDS), XRD and X-ray photoelectron spectroscopy (XPS). Uniform corrosion rates were determined by weight-loss. Both uniform and localised corrosion were found. Corrosion products comprised FeCO₃, α-Fe₂O₃, FeSO₄ and hydroxyl oxide iron (FeOOH; α-FeOOH or γ-FeOOH). Sulphur decreased the uniform corrosion rate, but increased localised corrosion. The mechanism of localised corrosion was discussed.

The ‘Effect of CO₂ gas on carbon steel corrosion in an acidic-saline based geothermal fluid’ was presented by G. Aristia (Freie Universität Berlin, Germany). The corrosion behaviour of carbon steel exposed to synthetic acidic-saline geothermal brine of pH 4 and 1500 mg L⁻¹ chloride (replicating geothermal well conditions at Sibayak, Indonesia) was assessed at 70 and 150°C. At 70°C, exposure tests and electrochemical tests were performed at ambient pressure. Surface morphology and cross-sections were analysed using a scanning electron microscope (SEM) and energy-dispersive X-ray (EDX). OCP and EIS measurements were performed on carbon steel in CO₂-containing solutions at 70 and 150°C. Localised corrosion was observed to a greater extent at 70°C due to lower protectiveness of the corrosion product layer cf. that at 150°C, whereas FeCO₃ had a high corrosion resistance at both temperatures. Longer exposure (28 days) revealed deeper pitting compared to the seven-day exposure.

Le Q. Hoa (BAM – Federal Institute for Materials Research and Testing, Berlin, Germany) presented ‘On the corrosion behaviour of CO₂ injection pipe steels: role of cement’. Representative low-cost materials including carbon steel 1.8977 (grade X70: 0.04-0.16% C) and low alloyed steel 1.7225 (AISI 4140: Fe–Cr–Mo alloy) were investigated in simulated pore water at 333 K and under worst-case scenario CO₂ saturation conditions. Simulated pore waters were made from cement powders (Dyckerhoff Variodur^® and Wollastonite). In electrochemical tests, including OCP measurement for seven days followed by cyclic voltammetry, both steels showed negative OCPs in the −650 to −600 mV region, indicating active corrosion. Variodur^® performed better than Wollastonite in terms of both pit depth and cement powder hardness. In all cases, 1.4562/alloy 31 (UNS N08031: Fe–Ni–Cr–Mo alloy with 0.15-0.25% N addition) is highly corrosion resistant and therefore recommended for long-term use.

Corrosion control in aerospace

There were two full papers out of a total of 25 papers/posters on this topic, which was held on Tuesday. Chairs were shared by Theo Hack (EADS Innovation Works, Munich, Germany) and Mikhail Zheludkevich (Helmholtz-Zentrum Gmbh, Geesthacht, Germany). Two full papers and two abstracts are summarised herein.

S. Garcia-Vergara (Universidad Industrial de Santander, Colombia) presented a full paper on ‘Characterization of anodic oxide film formed on Ti6Al4V by alkaline NaTESi (a mixture of sodium tartrate and silicate) anodization’. Although anodising has proved most successful in activating titanium surfaces to create strong, durable adhesive bonds, a highly toxic and carcinogenic mix of chromic acid and hydrofluoric acid (CAA) was hitherto used. Herein, anodising was carried out in alkaline solutions with various additives such as sodium tartrate and sodium silicate. SEM, contact angle measurements with the sessile drop method and Raman spectroscopy were used to study the morphology, wettability and crystallography of the anodic films. The surface wettability was improved with respect to Ti6Al4V without anodising; the change in contact angles being from ∼60° to <12°. Additionally, the anodic films showed better corrosion resistance than the pristine surfaces along with greater durability of the adhesive bond, making these anodising solutions a suitable replacement for CAA.

Z-W. Zhan (Beijing Institute of Aeronautical Materials, Beijing, China) presented a full paper on ‘Corrosion behaviour of two aluminium coatings in marine environment’. Ion vapour deposition (IVD Al) and ionic-liquid-based electroplating (ILEp Al) were studied. The morphology of the coatings was characterised by optical microscopy (OM) and SEM. Atomic force microscopy (AFM) was used to measure the surface roughness and EIS was used to examine the coatings at different times during the 1000 h alternating immersion test in artificial sea water. The scanning Kelvin probe (SKP) was used to map the electrical potential of the scratch area of the coatings. Although both aluminium coatings experienced a certain amount of corrosion after a two-year exposure, both coatings conferred good protection for the steel substrate.

‘Flash-PEO coatings for corrosion protection of Mg alloys’ was outlined by E. Matykina (Universidad Complutense de Madrid, Spain). Cr (VI)-free corrosion protective alternatives such as Cr (III)-, phosphate- and fluorozirconate-based conversion coatings have recently been offered for Mg in order to comply with international environmental regulations. However, these plasma electrolytic oxidation (flash-PEO) coatings underperform compared with chromium conversion coatings (CCC). Hence, short-term anodic treatments, based on plasma electrolytic oxidation (flash-PEO) as alternatives to CCC, have been investigated. The coatings were developed on AZ31B magnesium alloy based on: (i) environmentally friendly electrolytes; (ii) low thickness (∼5 μm); (iii) short treatment times; (iv) excellent paint adhesion and (v) similar or better neutral salt spray test (NSST) performance compared with CCC. In situ and ex situ incorporation of several inhibitors was also studied. EIS, paint adhesion test and NSST revealed that the coatings outperformed CCC (no corrosion after 1000 and 168 h NSST).

L. Ma (CNRS – Chimie ParisTech, France) discussed ‘Corrosion behaviour and surface modifications of the AA2050-T8 (an Al–Cu–Li aluminium aerospace alloy) in mild and aggressive electrolytes’. The alloy, containing Li to reduce density, was studied in 0.001 M NaCl + 0.01 M Na₂SO₄ and in 0.1 M NaCl. OCP measurements showed much lower values in 0.1 M NaCl than in milder electrolytes, indicating different corrosion mechanisms. Optical microscopy showed significant pitting in the 0.1 M NaCl. No pits were seen in the milder electrolyte. Chemical mapping of pits formed at OCP in 0.1 M NaCl by time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed enrichment in iron, zinc, manganese oxides and copper. In the mild electrolyte, Li accumulated in the inner part of the oxide layer; in the 0.1 M NaCl, the lack of Li in the vicinity of pits indicated high Li activity and fast dissolution of the Li-rich layer during the first stages of corrosion.

Corrosion in concrete and cathodic protection

There were three full papers (two reviewed here) out of a total of five papers/posters presented on the Tuesday. Chairs were shared between Prof. Michael Raupach (RWTH, Aachen, Germany) and Jerome Crouzillac (BAC Corrosion Control, Voisins-le-Bretonneux, France).

‘Grout acidification of ribbon anode in impressed current cathodic protection systems in concrete structures’ was described by M. Cheytani (University of New South Wales, Sydney, Australia). Acidification, associated with chlorine gas generated at the anode, probably due to insufficient anode embedment and inadequate quality control/workmanship during ICCP construction, permitted water ingress to the ribbon anode. Experiments were conducted using cylindrical concrete samples installed with ribbon anodes. Four mild steel bars were placed in the centre of each cylinder, acting as cathodes. To simulate marine environments, samples were placed in 35 g L⁻¹ sodium chloride solution and an equivalent of circa 15 years of current applied at 20 mA m⁻² of the steel surface. The results confirmed that application of a 2.5 mm cementitious waterproofing coating eliminates water ingress, minimising grout acidification problems in new and existing ICCP systems.

C. Stone (Concrete Preservation Technologies, Nottingham, UK) addressed ‘Anode reactive behaviour in galvanic protection of reinforced concrete’. When there is little risk of corrosion of the structure being protected, sacrificial anodes will not corrode. Anode dormancy, therefore, leads to limited current needing to be supplied during dry periods. By only corroding and protecting the steel during times of corrosion risk, galvanic anodes can have extended design lives, providing efficient and effective protection. Over 10 years of data collected from real installations and laboratory studies were reviewed. The prototype system was installed at Whiteadder Bridge, Scotland. It has been found that the low maintenance currents during this period allowed galvanic systems to be on track to successfully surpass their design lives by over a decade.

Corrosion of steel in concrete

There were nine full papers on this topic, out of a total of 34 papers/posters, four of which have been chosen for précis here. Lectures took place on the Wednesday and were chaired by Prof. Michael Raupach (RWTH, Aachen, Germany).

‘Investigations of corrosion due to calcium leaching’ were described by Y. Schiegg (TFB AG, Switzerland). If cement-based materials are in contact with liquids of lower ion concentration, the chemical potential gradient induces ionic diffusion, e.g. Ca²⁺, from the pore solution towards the solvent (typically water). This increases the porosity and enhances the ingress of corrosion promoting substances. Leaching investigations and electrochemical measurements give similar initiation times for ordinary Portland cement (OPC) and for OPC blended with fly ash (FA) and limestone (LS). For OPC blended with high amounts of blast furnace slag, de-passivation takes about twice as long compared with that found by electrochemical measurements. Finally, it was shown that the time for corrosion initiation increases with CaO content.

M-I. Ana (Universitat de València Vera, Spain) outlined ‘Voltammetric sensors for oxygen availability detection’. Oxygen diffusion through the concrete matrix is heavily dependent on concrete porosity, tortuosity of the pore network and moisture content. Two types of concrete were studied by cyclic voltammetry and impedance spectroscopy: w/c = 0.6 and w/c = 0.5. Oxygen concentrations were: (1) atmospheric pressure; T = 22°C; RH = 80% and (2) vacuum; T = 2 2°C; RH = 80%. The resistor element in a simplified Randles equivalent circuit (Rs) obtained by means of impedance spectroscopy allowed quantitative models of humidity availability inside the concrete matrix to be developed. Information provided by oxygen concentrations and humidity allowed the presence of cracks in the concrete to be detected as a result of step changes in these quantities close to the cracks.

‘Formalization of the conditions under which iron passive films break down in high pH environments’ was discussed by H. Nagate (Tokyo University of Science, Japan). A theoretical model of passive film breakdown was developed using complex chemistry such that, at a given time, the thickness of a passive film will increase with increasing iron potential, hydroxide ion concentration and dissolved oxygen concentration. The derived model of cross-link density within a passive film implies that assuming a constant iron potential, the critical halide ion concentration for passive film breakdown can be expressed as an exponential function in terms of solution pH. Moreover, assuming a constant concentration of hydroxide ions in solution, a passive film is predicted to break down once the concentration ratio of halogen-replaced to unreplaced Fe complexes reaches a certain critical value, given by [a _FeOOHx(H2O)3 + a _FeOx(H2O)4]/a _FeOOH(H2O)4 > ξ, where ξ is the critical threshold for passive film breakdown and a represents activity.

S. K. Hellner (IFT/UiB, Bergen, Norway) explained ‘Non-contact corrosion monitoring of steel reinforcement’. Norway has over 17 000 road bridges, many of which are concrete-based. Currently, the condition of the reinforcement is evaluated by measuring the potential of the steel versus a reference electrode (typically Cu/CuSO₄). The possibility of monitoring the corrosion of concrete reinforcement (rebar) with field Kelvin probe (FKP) technology has been evaluated. Whereas the SKP vibrates a fine tip perpendicular to the sample, which has to be at a known fixed height above the rebar, the FKP rotates a probe around an axis parallel to the sample. Unlike the traditional half-cell potential mapping, which requires direct contact with the test sample and surface preparation, the corrosion potential of the rebar is readily obtained by FKP without direct contact. More testing and refinements of the probe will be necessary to increase the measurement sensitivity and measurement speed.

Marine corrosion

There were seven full papers out of a total of 28 papers/posters on this topic, which was held on the Wednesday. Five papers have been selected for précis here. The Chair was Prof. Philippe Refait (University of La Rochelle, France).

P. Refait (University of La Rochelle, France) discussed ‘Corrosion of Fe–Cr (0.3-1 at.-% Cr) coated carbon steel in marine environments’. The resistance to marine corrosion was investigated for thin films prepared on the laboratory scale by physical vapour deposition (PVD) and subsequent heat diffusion treatment (1000°C). Carbon steel coupons were covered by 40 μm thick films of Fe–Cr alloys containing 0.3, 0.5 or 1 at.-% chromium, then immersed for three to seven months in artificial sea water (ASTM D1141) or in natural marine environment (Les Minimes harbour, La Rochelle). EIS, OCP and LPR measurements were used. Corrosion product layers were characterised by μ-Raman spectroscopy and XRD. In both environments, the Fe–Cr-coated carbon steel coupons showed an improved resistance to corrosion. Moreover, the corrosion resistance, from polarisation resistance Rp and EIS data, increased with time for the coated coupons, while remaining constant for carbon steel.

‘Crevice corrosion testing of C276 material in chlorinated sea water at high temperature for a heat exchanger system’ was presented by M. E. Mitzithra (TWI, Cambridge, UK). A laboratory rig was constructed from C-276 to simulate the in-service environment of a shell-and-tube cooler. Artificial sea water (ASTM D1141) with 0.8-1.0 ppm free chlorine and 7.0 ppm dissolved oxygen (DO) at 80°C was the corrosive medium. A torque of 16 N m was applied. Specimens taken from the weld overlay suffered from corrosion. The autogenous weld and parent tube specimens did not exhibit signs of corrosion. Tube to tube-sheet welds were of finer microstructure than that of weld overlays. This difference in dendritic structure could have also contributed to the presence of the localised corrosion in the weld overlay. It was concluded that C-276 should not be used for chlorinated sea water at 80°C.

M. Abbas (Cranfield University, UK) described ‘Evaluation of the effects of highly saline and warm sea waters on corrosivity of marine assets’. Short-term corrosion of cupronickel (Cu–Ni) 90/10 alloy and mild steel in highly saline and warm sea waters were assessed. Field experiments were conducted in pollutant-rich sea waters (site 1) and natural sea waters (site 2) in the North Indian Ocean, coupons being examined after 15, 30, 45 and 60 days. Corrosion rates were evaluated using standard mass loss methods. Sea water temperatures at both sites were assumed to be the same. However, due to pollutants and high nutrient content at site 1, the chemical environment was different. Comparatively higher average corrosion rates were observed for both alloys in natural sea water at site 2 which may be attributed to high temperatures and salinity levels.

‘Corrosion damage effects on the structural integrity assessment of offshore structures’ was outlined by S. Aghasibayli (University of Strathclyde, Glasgow, UK). Offshore wind turbine structures are subjected to the aggressive sea water environment, temperature cycles, tidal fluctuations and variable cyclic loads due to wave and wind impact. The choice of corrosion protection method for a particular part of the structure depends upon its location (atmospheric, splash, tidal, submerged, buried zone). Standard DNVGL-RP-0416 calls for the minimum corrosion rate for submersible parts to be 0.10 mm/year for internal and 0.30 mm/year for external surfaces (North Sea environment). Current corrosion mitigation methods include corrosion allowance, cathodic protection (CP), coating/painting and inspection/monitoring. The latest monitoring trends include acoustic emission. Since pitting is difficult to detect and relies on destructive testing which is inapplicable to functioning structures, a better understanding of pit formation in welds is needed.

M. A. Knoch (Aachen University, Germany) addressed the ‘Influence of mechanical loads on the corrosion behaviour of thermally sprayed ZnAl15’. Although pure Zn coatings can offer reliable cathodic corrosion protection to steel in most atmospheres, ZnAl may confer superior protection in marine environments. Samples were exposed to mechanical loading (body impact/scratching etc.) according to ISO 17872 and subsequently exposed to a neutral salt spray test for 554 h. After testing, EDS analysis revealed that the corrosion products comprised a mixture of ZnAl-oxides (Zn _x Al _y O _z ). Whereas platelike products (assumed to be Simonkolleite crystals) were identified on the thin films, the thick-film corrosion products consisted of a porous layer (possibly zinc hydroxide or hydrozincite) on top of Zn _x Al _y O_z. The coatings exhibited ductile deformation behaviour, which is preferable to the brittle failures often observed in organic coating systems.