Fusion-bonded epoxy composite coatings on chemically functionalized API steel surfaces for potential deep-water petroleum exploration
© Saliba et al. 2015
Received: 12 November 2015
Accepted: 9 December 2015
Published: 18 December 2015
Corrosion of oil and gas pipelines significantly reduces the service life of the pipelines, thus increasing costs, and more seriously, it can cause catastrophic environmental accidents. More recently, the exploitation of oil in ultra-deep seawater fields is facing new technological challenges in material selection owing to the extreme production conditions. Thus, the development of organic coatings as protective layers for steel pipelines is of crucial importance against highly corrosive environments. In this work, fusion bonded epoxy (FBE) coatings were deposited onto chemically functionalized carbon steel surfaces with organosilanes to investigate the potential applications in protection against corrosion and degradation in harsh marine environments. Carbon-steel API 5L X42 (American Petroleum Institute Standard grade) was chemically functionalized with two organosilanes, 3-APTES [(3-Aminopropyl)triethoxysilane], and 3-GPTMS [(3-Glycidyloxypropyl)trimethoxysilane], followed by the deposition of FBE composite coatings. The systems were extensively characterized with respect to each component as well as the steel-coating interface. The contact angle measurements and Fourier transform infrared spectroscopy (FTIR) results clearly indicated that the steel surface was effectively modified by the functional amine and glycidyl silane groups, leading to the formation of interfacial covalent bonds with increased hydrophobicity compared to bare steel surfaces. In addition, the morphological and chemical characterizations of FBE by scanning electron microscopy, atomic force microscopy, X-ray diffraction, and FTIR showed that it is composed of an epoxy-based organic matrix of bisphenol-A diglycidyl ether (DGEBA) reinforced with uniformly dispersed inorganic phases of calcium silicates and TiO2 particles. Moreover, the chemical functionalization of the steel surfaces with amino and glycidyl silanes significantly altered the interfacial forces with the FBE coatings, resulting in higher adhesion strength for 3-APTES-modified steel compared to 3-GPTMS-steel; however, both mostly showed cohesive rupture mode in the FBE component.
KeywordsCoatings Epoxy composite Steel protective coating Fusion-bonded epoxy Silane functionalization
The global oil and gas industry has been developing rapidly owing to the discovery of oil fields at ultra-deep seawater, and this has posed several new challenges for the exploitation and production of petroleum. Materials used in these conditions are subjected to extreme pressure, temperature, and corrosion and the associated technological issues must be addressed with consideration of engineering, economic, and environmental constraints [1–3]. Pipelines for oil transportation that are mainly made of steel have some of the following key advantages: (a) relatively low cost; (b) high efficiency; (c) short construction period; (d) safe and environmentally friendly application; (e) suitability for all global geographies. High-strength low-alloy (HSLA) steels have been widely used for crude oil pipeline transportation in the petroleum industry .
API-5L X-type grade steel is one of the most common pipeline materials in the oil industry. They are usually considered as more cost-effective and safer than any other types of steel. However, such pipelines used for the exploitation and transport of oil and gas are usually buried underground, undersea, or immersed in marine salty water, where they can get severely corroded [4–7]. Thus, single or multilayer protective coatings are generally used for external and internal corrosion protection of pipelines in gas/oil transmission and distribution. The key aspects for corrosion protection are adhesion of the coating to the steel surface, and resistance of the coating to permeation of water, oxygen, and/or ions. Hence, fusion-bonded epoxy (FBE) coatings or a combination of FBE with other materials forming multilayers are widely used for corrosion protection of gas/oil pipelines [8–10]. Epoxy-based polymers such as FBE are the widely used for gas/oil pipelines because they exhibit high chemical resistance, very low permeability to chloride ions, good mechanical flexibility, strong adhesion to steel, and suitable processing characteristics, leading to improved corrosion protection and operational lifetime of the pipelines. However, epoxy coatings can easily absorb moisture, and the diffusion of absorbed water into the epoxy-steel interface owing to the presence of hydrophilic hydroxyl groups in the cured network (epoxy/amine reaction) weakens the interfacial adhesion strength between the epoxy and steel in a corrosive environment, causing system failure [11, 12]. Thus, the improvement of the adhesion at the steel-epoxy interfaces by using organosilanes primers can be considered an important strategy for inhibiting the corrosion mechanisms. Surprisingly, although there have been a few studies on the use of steel that is chemically modified by silanes for improving the adhesion with epoxy coatings [13, 14], no report was found in the consulted literature in which a system composed of steel-primer-FBE was systematically and extensively characterized by considering each individual component and the interfaces between them. Hence, the main goal of this study was to prepare hybrid organic–inorganic FBE coatings using sol–gel chemistry for the functionalization of the steel surfaces to improve the interfacial properties for improved adhesion and protection against corrosion.
All of the reagents and precursors, (3-Glycidyloxypropyl)trimethoxysilane (3-GPTMS, Aldrich, USA, ≥98 %, C9H20O5Si), (3-Aminopropyl)triethoxysilane (3-APTES, Aldrich, USA, 99 %, C9H23O3SiN), methanol (Synth, Brazil, 99.5 %, CH4O), ethanol (Synth, Brazil, 99.5 %, C2H6O), acetic acid (Synth, Brazil, 99.7 %, C2H4O2), acetone (Synth, Brazil, 99.5 %, C3H6O), hydrochloric acid (Synth, Brazil, 37 %, HCl), sodium hydroxide (Synth, Brazil, 99 %, NaOH), and hydrogen peroxide (Synth, Brazil, 29 %, H2O2) were used as received. FBE powder (Scotchkote™ 226 N, 3 M™, River Place Blvd., Austin, USA) was used as the coating material. Deionized (DI) water (Millipore Simplicity™) with resistivity of 18 MΩ cm was used in the preparation of all solutions. All of the preparations and syntheses were performed at room temperature (25 ± 2 °C) unless specified otherwise. Potassium bromide (Sigma-Aldrich, USA, ≥99 %, KBr), suitable for spectroscopy, was used to prepare the FTIR samples.
Chemical and mechanical requirements for API 5L X42 PSL2 steel 
Chemical composition (%wt)
By agreement between the purchaser and the manufacturer, elements other those listed above may be used
Fe content, balance
Typically >97 %
Carbon equivalent (CE, Pcm), maximume
Carbon equivalent (CE, IIW), maximumf
Mechanical properties (MPa)
Yield Strength, minimum
Yield Strength, maximum
Ultimate tensile strength, minimum
Ultimate tensile strength, maximum
Methods and protocols
Characterization of steel substrate
The chemical composition and mechanical properties of the steel plate were supplied by the manufacturer. Microstructural characterizations were carried out using optical microscopy (OM) and scanning electron microscopy (SEM) coupled with energy dispersion X-ray spectroscopy (EDX, EDAX GENESIS). The specimens for the OM and SEM observations were prepared using Nital (2 %) etching solution on the specimen’s surface. Before the chemical attack, the specimens were ground using sandpaper of 100–1000 mesh. Then, a polishing process was performed using diamond paste (1 μm) as the abrasive material. Finally, the grinding and polishing residues were removed using water, ethanol, and drying.
Atomic force microscopy (AFM) was also used for steel plate characterization. Analyses were conducted using the Park Systems/XE70 microscope in non-contact mode and a cantilever with a force constant equal to 42 N/m.
Surface cleaning and activation of API-5L X42-PSL2
Prior to the deposition of FBE coatings, the steel samples were subjected to a specific surface treatment process. First, the specimens were cleaned by immersion in acetone for 10 min, followed by immersion in ethanol for 15 min, at room temperature, for mild degreasing and organic removal. Next, the chemical etching method with HCl solution (0.58 % v/v) was used for removing any oxidized species on the steel surface. The pieces were immersed in an acid bath for 20 min at 57 ± 3 °C under stirring. Finally, the steel surface was stabilized by immersion in hydrogen peroxide for 30 min, and dried in oven at 57 ± 3 °C for 4 h.
Functionalization of steel surface by organosilanes
The 3-GPTMS was added in an aqueous/alcohol solution (80 % distilled water: 20 % methanol, pH = 5.0 ± 0.2 with acetic acid) to yield a 2 % (v/v) final concentration. The solution was allowed to hydrolyze for 48 h prior to use for the hydrolysis of alkoxide groups and silanol formation. Next, the specimens were immersed in this solution for 20 s and then cured at 150 ± 5 ◦C for 1 h for covalent bonds formation (Additional file 1: Figure S1).
The 3-APTES was added in an aqueous/alcohol solution (90 % distilled water: 10 % methanol, natural pH) to yield a 2 % (v/v) final concentration. The solution was allowed to hydrolyze for 30 min prior to use for the hydrolysis of alkoxide groups and silanol formation. Next, the specimens were immersed in this solution for 30 s and then cured at of 70 ± 5 °C for 1.5 h for condensation reactions (Additional file 2: Figure S2).
Chemical functionalization process parameters using 3-GPTMS e 3-APTES on steel API 5L X42 substrate
3-GPTMS/(deionized water/methanol solution)
3-APTES/(deionized water/ethanol solution)
Hydrolysis Time (h)
5–5.5 (acetic acid)
Molar ratio water/alkoxide
Immersion time (s)
Cure temperature (°C)
150 ± 5
70 ± 5
Cure Time (h)
The influence of surface functionalization on the hydrophilic/hydrophobic behavior of the steel surface was estimated via contact angle measurements carried out by depositing DI water droplets (50 µL) on the steel surface. The apparatus used for measurements was a digital camera Canon Rebel T3 and image analysis software (ImageJ, version 1.44, public domain, National Institutes of Health).
Fourier transform infrared spectroscopy (FTIR) was used to characterize the presence of chemical groups at the surface of steel, in order to demonstrate the effectiveness of the developed procedure for functionalization. The attenuated total reflectance (ATR) mode was used (Nicolet 6700, Thermo Electron Corp.) within the range between 4000 and 675 cm−1. Each spectrum was recorded with a resolution of 4 cm−1 with a total of 32 scans. Further, to obtain image mapping of the chemical groups, Nicolet iN10 Infrared microscope (Thermo Scientific) with OMNIC Picta microscopy software (Thermo Scientific) was used to collect images from the aminosilane-modified steel surface. The motorized stage and the Slide-On MicroTip Ge ATR crystal accessory obtained spectra over an area of 400 μm × 400 μm with 100 μm steps (X and Y) allowing area map collection (4000–675 cm−1, 16 scans, 8 cm−1 resolution).
Characterization of FBE powder and coating
The moisture content was determined gravimetrically after drying the sample powder at 110 ± 5 °C for 12 h. The material obtained (solids content) was calcined in an oven at 610 ± 5 °C for 2 h and the residue of calcination was associated with the filler present in the material. The resin content was obtained by calculating the difference between the solid content (100 %) and the filler content. The results were expressed as the percentage (% mass) of material obtained in each step of the assay.
Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were performed using SDT Q-600 simultaneous TGA/DSC instrument (TA Instruments, New Castle, DE, USA). Samples were used for the experiments at temperatures of up to 600 °C with a heating rate of 10 °C min−1. The samples were loaded into an open platinum crucible. The TGA and DSC curves were recorded simultaneously with 0.1 μg sensitivity. The analysis was performed under continuous flow of dry nitrogen gas (50 mL min−1).
The morphology of the FBE powder and FBE coating was evaluated using scanning electron microscopy (SEM, FEI-INSPECTTM S50) coupled with energy dispersion X-ray spectroscopy (EDX, EDAX GENESIS). Before examination, the samples were coated with a thin carbon film by sputtering under a low deposition rate, substrate cooling, and maximum distance between the target and sample in order to avoid sample damage. Images of secondary electrons (SE) were obtained with an accelerating voltage of 15 kV. The FBE particle size and size distribution data were obtained based on the SEM images by measuring at least 200 randomly selected nanoparticles using an image processing program (ImageJ, version 1.44, public domain, National Institutes of Health).
The crystalline phases present in the FBE powder were characterized by X-ray diffraction (XRD) patterns recorded using a PANalytical Empyream diffractometer (Cu-Kα radiation with λ = 1.5406 Å). Measurements were performed in the 2θ range of 15°–75° with steps of 0.06°.
The FBE powder was also analyzed by the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method (Nicolet 6700, Thermo Fischer) over the range of 4000–400 cm−1 using 64 scans and a 2 cm−1 resolution with subtraction of the KBr background. The powder samples were mixed in a ratio of 1 % (wt%) to KBr powder dried at 110 ± 5 °C for 2 h.
The FBE coatings were evaluated in the range of medium and near infrared (NIR) wavelengths using attenuated total reflectance (ATR, 4000–675 cm−1, 32 scans, and a 4 cm−1 resolution) and NIR integrating sphere (7400–4000 cm−1, 16 scans and a 8 cm−1 resolution), respectively, with background subtraction.
Deposition and cure of FBE on API 5L X42 steel surface
FBE powder was deposited on the steel substrate and spread with a mold to guarantee homogeneous thickness. In the sequence, the system was heated at 248 ± 5 °C for 5 min and cooled down to 25 °C at room temperature, as schematically represented in Additional file 3: Figure S3.
The coating was applied on the clean steel surface (bare steel) and on the chemically modified steel surface using 3-GPTMS or 3-APTES.
Adhesion tests according to ASTM D3359-09  and ASTM D4541-09  were carried out to measure the adherence in the FBE/steel interface after cleaning and after chemical modification with silanes. The ASTM D3359-09 test used 3 M® adhesive tape Scotch® 8809, an adhesive-based resin and rubber. For the ASTM D4541-09 test, the portable PosiTest brand (AT-M Manual Adhesion Tester) was used, with detection limit of 23.36 ± 0.01 MPa.
Results and discussion
Characterization of steel substrate
Mechanical properties (MPa)
Ultimate Tensile Strength
Characterization of steel substrate after cleaning (bare steel)
Because metallic materials do not absorb infrared radiation, the FTIR spectrum of cleaned steel (not shown) can only reveal the presence of the metal hydroxides and oxy-hydroxides that are expected to be formed after the treatment with hydrogen peroxide.
Characterization of functionalized steel substrate
Moreover, characteristic peaks from organofunctional groups of silanes were also detected. In the amino-modified steel substrate (Fig. 4a), a broad band from 3500 to 3200 cm−1 is attributed to νs absorptions from N–H overlapped with Si–OH groups, and the bands at 1573 and 1318 cm−1 are associated with the in-plane scissoring bending of primary amines (–NH2) and the stretching of C–N bonds, respectively . The FTIR analysis of glycidoxy-modified support (Fig. 4b) also presented peaks from CH2OCH– bonds at 1260 and 854 cm−1 characteristic of the organofunctional group . The detection of these organofunctional groups at the surface indicate that they are available for creation of covalent bonds with the epoxy coating, favoring the adhesion behavior .
Characterization of FBE
Thermal treatment of the as-supplied FBE powder at 110 ± 5 °C indicated a moisture content of 0.5 ± 0.1 %. After calcination at 610 ± 5 °C, the weight percentage of resin and inorganic content were calculated as 66.7 ± 3.0 and 32.8 ± 3.0 wt%, respectively. These results are in agreement with the manufacturer specifications that indicate moisture content lower than 6 wt%, inorganic content in the range of 21–43 wt%, and resin content from 52 to 76 wt%.
ν(C–H) of the oxirane ring
ν(C–H) of CH2 and CH aromatic and aliphatic
ν(C=C) of aromatic rings
ν(C–C) of aromatic
ν(C–O–C) of ethers
ν(C–O) of oxirane group
ν(C–O–C) of oxirane group
δ (N–H) of primary amines
δ(Si–O–Si) and δ(O–Si–O)
The method proposed for organosilane modification of steel solid support was efficient and resulted in homogeneous organically functionalized steel pipelines, as measured by contact angle and FTIR assays. 3-GPTMS and 3-APTES surface modifiers have improved adherence between the steel and fusion bonded epoxy, as detected by the change in the mode of rupture from interfacial to a combined interfacial–cohesive mode for the cured FBE coating. Such adhesion enhancement was attributed the development of new covalent bonds among the cured epoxy chains and organofunctional groups at the interfaces.
HSM designed the experimental procedure of the study, performed the analysis, and drafted the manuscript. PAS and AAPM performed the preparation, characterization, and analysis of the systems, and drafted the manuscript. DBS performed the SEM–EDX characterization and analysis of steel substrates and epoxy-based composite. All authors read and approved the final manuscript.
The authors acknowledge financial support from the following Brazilian research agencies: CAPES (PROEX-433/2010;PNPD), FAPEMIG (PPM-00202-13;BCN-TEC 30030/12), CNPq (PQ1B–306306/2014-0; UNIVERSAL-457537/2014-0), and FINEP (CTINFRA-PROINFRA 2008/2010). The authors express their gratitude to P. Trigueiro for the SEM and AFM images.
The authors declare that they have no competing interests.
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