Research Article Assessment of Automotive Coatings Used on Different Metallic Substrates
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Hindawi Publishing Corporation International Journal of Corrosion Volume 2014, Article ID 838054, 12 pages http://dx.doi.org/10.1155/2014/838054 Research Article Assessment of Automotive Coatings Used on Different Metallic Substrates W. Bensalah,1 N. Loukil,1 M. De-Petris Wery,2 andH.F.Ayedi1 1 LaboratoiredeGenie´ des Materiaux´ et Environnement (LGME), ENIS, Universite´ de Sfax, B.P.W. 1173-3038, Tunisia 2 IUT Mesures Physiques d’Orsay, UniversiteParisSud,PlateauduMoulon,91400Orsay,France´ Correspondence should be addressed to W. Bensalah; [email protected] Received 12 January 2014; Revised 20 March 2014; Accepted 26 March 2014; Published 29 April 2014 Academic Editor: Sebastian Feliu Copyright © 2014 W. Bensalah et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Four epoxy primers commonly used in the automotive industry were applied by gravity pneumatic spray gun over metallic substrates, specifically, steel, electrogalvanized steel, hot-dip galvanized steel, and aluminum. A two-component polyurethane resin was used as topcoat. To evaluate the performance of the different coating systems, the treated panels were submitted to mechanical testing using Persoz hardness, impact resistance, cupping, lattice method, and bending. Tribological properties of different coating systems were conducted using pin on disc machine. Immersion tests were carried out in 5% NaCl and immersion tests in 3% NaOH solutions. Results showed which of the coating systems is more suitable for each substrate in terms of mechanical, tribological, and anticorrosive performance. 1. Introduction The purpose of priming in the automotive industry is [1–4] (i) to inhibit corrosion of the metal substrate from Painting a vehicle is one of the most expensive operations in discontinuities in the paint system which may occur during automotive industry. The painting process typically involves application, (ii) to resist mechanical damage to the paint 30–50% of an automotive assembly plant’s costs [1]. This high system and to stop its propagation to the metal substrate, (iii) cost is attributed to the need to have high quality coatings toprovideadhesiontothesubstrateandtothesubsequent and the necessity of meeting strict environmental regulations. multilayerssystem,and(iv)tocontributetothecoating The coated surface must be finished in a way that provides an systems’ aesthetic aspect by reducing surface roughness. excellent surface quality [1, 2]. Manyresearchershavefocusedonthechemical- Automotive paint has two main functions [2, 3]: (i) to mechanical priming performances. Kumar and Nigam [5] make the vehicle look good and (ii) to protect the underlying have studied the effect of the corrosion of cold rolled mild metal from the harsh environment to which it is exposed. A steel beneath two types of epoxy primers after immersion for typical automotive paint system consists of many different six months in 3% NaCl. They have found that the chemical layers, each chosen for a precise function: (i) clearcoat, composition of the formed rust depended on the nature of expected to protect underlying layers and maintain a shiny the anticorrosive pigment present in the primer coating. appearance, (ii) basecoat, containing the visual properties of Lonyuk et al. [6] have investigated eight different coating colors or chromophoric layer, (iii) primer, acting as leveler systems containing different primers in terms of mechanical and as protector, and/or (iv) e-coat, to provide corrosion properties. They have observed failure by delamination protection. between the topcoat and primer for some coating systems Despite the advances in surface coating technologies, and adhesive failures between the primer and substrate priming remains the fundamental phase to prevent corrosion forothers.Otherauthors[7–10] have demonstrated the [3]. Protection against any form of corrosion and paint film great effect of the substrate type on the degradation of durability is and will be of major importance. the painted metal. Suay et al. [7] have shown that the best 2 International Journal of Corrosion performance properties of the primer were obtained when Primer coatings were applied using gravity pneumatic it was applied on phosphatized steel compared to the cold spray gun. Before application, viscosity and density of the rolled steel. Miller [8] showed that the reactivity of a metal different primers coatings were measured. The paint dry film substrate and the type of corrosion product formed had thicknesses were maintained within the 75 ± 5 m range and an influence upon the reliability of a protective coating. were controlled using an Elcometer 355 Top Thickness Gauge Kittelberger and Elm [9] concluded, in their work, that the equipped with eddy current probe. The commercial topcoat type of the substrate was responsible for paint blistering was a two-component polyurethane resin with a thickness of susceptibility. Walter [10] focused on paint coatings on steel, 75 ± 5 m. galvanized steel, Zn/55% Al coated steel, and aluminium ∘ immersed in 5% NaCl solution at 50 C. He concluded that 2.2. Testing Methods and Equipment the corrosion performance of these coatings, as measured by their delaminating time, was directly related to the corrosion 2.2.1. Persoz Pendulum Hardness Test (ASTM D 4366-95). The resistance of the unpainted substrate. instrument consisted of a pendulum which was free to swing The present work aims to characterize the mechanical, ontwosteelballsrestingonacoatedtestpanel.Thependulum tribological, and corrosion behaviors of four commercial hardness test was based on the principle that the amplitude coating primers and a commercial topcoat applied on four of the pendulum’s oscillation would decrease more quickly different substrates (steel, hot-dip galvanized steel, electro- when supported by a softer surface. galvanized steel, and aluminium) in order to understand and The stainless steel balls, 8 mm diameter, were of hardness thereby improve the durability of the final paint/metal system HRC59. The total weight of the pendulum was 500 g.The used in industry. period of oscillation was 1 sec and the time for damping For the evaluation of coating systems performance, sev- from a 12-degree displacement to 4-degree displacement was eral tests have been used: mechanical testing using Persoz taken. Three tests were performed to each coated sample and hardness, impact resistance, cupping, lattice method, and the final result was the average of the three experiments. bending. Tribological properties of different coating systems were conducted using pin on disc machine. Immersion and electrochemical tests were carried out in 5% NaCl and 3% 2.2.2. Impact Resistance Test. Impact tests have been per- NaOH solutions. formed over coated samples by making the impact from the substrateside(reverseimpact)accordingtoISO6272. The impact tests consisted of checking the primer damage resistance to the collision of a ball (1000 g) dropped from a 2. Experimental fixed height of 0.5 m directly onto the metal substrate side and 2.1. Materials. Two sets of painted panels representing evaluating if the coating on the other side had been damaged commercial primer coated substrates and commercial top- or not. The weight was dropped through a guiding tube whose coat/primer coated substrates were examined. Each set con- height was incrementally marked. tained a series of 16 panels that had been painted with or without a topcoat. Four commercial anticorrosive epoxy 2.2.3. Resistance of the Coating against Cupping in an Erichsen primers, namely, PPG primer (PP), AMERLOCK primer Cupping Test (ISO 1520). The objective of this test was to (AP), STANDOX primer (SP), and DEBEER primer (DP), identify the resistance of the paint film against the ongoing were applied on four types of substrates: steel, hot-dip deformation of a coated substrate panel with a pressed-in galvanized steel, electrogalvanized steel, and aluminium. 20 mm steel ball. The result of the test gives the so-called Primer types, substrates, and their mechanical properties are “cupping”inmmduringwhichthefirstdisturbanceofthe summarized in Tables 1, 2,and3.Theelectrogalvanizingand coating occurred. hot-dip galvanizing of steel panels were carried out under industrial subcontracting. The hot-dip galvanizing process 2.2.4.DegreeofCoatingAdhesionbyaLatticeMethod. involves three steps prior to the immersion of the panels ∘ Determination was made by means of a special cutting blade into the molten zinc bath (460 C): (i) caustic cleaning in with 6 cutting edges 2 mm apart and involved the degree a hot alkali solution to eliminate organic contaminants, of adhesion of the created 2 mm × 2mmsquarestoa base (ii) pickling in a heated sulphuric acid solution, and (iii) substrate in accordance with standard ISO 2409. fluxing of the steel panels by immersion into an aqueous solution of zinc ammonium chloride and then drying before zinc deposition. The thickness of the zinc coating is about 2.2.5. Coating Resistance during Bending on a Cylindrical 50 m. For the electrogalvanizing process, steel panels were Mandrel. This identified the resistance of the paint film mechanically polished, chemically decreased in commercial against ongoing deformation of a coated substrate panel alkaline solution, and then etched in 20% HCl solution at around a 2 cm diameter stainless steel mandrel, verifying the room