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.
Painting a vehicle is one of the most expensive operations in automotive industry. The painting process typically involves 30–50% of an automotive assembly plant’s costs [
Automotive paint has two main functions [
Despite the advances in surface coating technologies, priming remains the fundamental phase to prevent corrosion [
The purpose of priming in the automotive industry is [
Many researchers have focused on the chemical-mechanical priming performances. Kumar and Nigam [
The present work aims to characterize the mechanical, tribological, and corrosion behaviors of four commercial coating primers and a commercial topcoat applied on four different substrates (steel, hot-dip galvanized steel, electrogalvanized steel, and aluminium) in order to understand and thereby improve the durability of the final paint/metal system used in industry.
For the evaluation of coating systems performance, several tests have been used: 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 and electrochemical tests were carried out in 5% NaCl and 3% NaOH solutions.
Two sets of painted panels representing commercial primer coated substrates and commercial topcoat/primer coated substrates were examined. Each set contained a series of 16 panels that had been painted with or without a topcoat. Four commercial anticorrosive epoxy primers, namely, PPG primer (PP), AMERLOCK primer (AP), STANDOX primer (SP), and DEBEER primer (DP), were applied on four types of substrates: steel, hot-dip galvanized steel, electrogalvanized steel, and aluminium. Primer types, substrates, and their mechanical properties are summarized in Tables
Chemical composition of the used substrates.
Steel
Element | C | P | S | N | Fe |
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Weight (%) | ≤0.18 | ≤0.05 | ≤0.05 | ≤0.009 | Rest |
Steel for hot-dip galvanizing
Element | C | Mn | P | S | Fe |
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Weight (%) | ≤0.17 | ≤1.4 | ≤0.045 | ≤0.045 | Rest |
Steel for electrogalvanizing
Element | C | Si | Mn | P | S | Al | N2 | Fe |
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Weight (%) | ≤0.041 | ≤0.009 | ≤0.28 | ≤0.007 | ≤0.008 | ≤0.030 | ≤0.0034 | Rest |
Aluminum (Al5754H111)
Element | Si | Fe | Cu | Mn | Mg | Cr | Zn | Ti | Al |
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Weight (%) | <0.40 | <0.40 | <0.10 | <0.50 | 2.6–3.6 | <0.30 | <0.20 | <0.15 | Rest |
Mechanical properties of the used substrates.
Maximum stress | Yield stress | Elongation |
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Min | Max | Min | ||
Steel | 370 | 450 | 240 | 25 |
Steel for hot-dip galvanization | 360 | 510 | 235 | 24 |
Steel for electrogalvanization | 340 | 470 | 235 | 26 |
Aluminum | 190 | 240 | 80 | 18 |
Characteristics of different primers.
PP | SP | DP | AP | |
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Chemical base | Polyester-urethane chemistry | 1K Primer filler | Polyaminoamide | AMERLOCK 400AL |
Curing temperature | Ambient air | Ambient air | Ambient air | Ambient air |
Curing duration | 7 days | 7 days | 7 days | 7 days |
Color | Yellow | Gray | Gray | Gray |
Glossiness | Mate | Mate | Mate | Semigloss |
Primer coatings were applied using gravity pneumatic spray gun. Before application, viscosity and density of the different primers coatings were measured. The paint dry film thicknesses were maintained within the
The instrument consisted of a pendulum which was free to swing on two steel balls resting on a coated test panel. The pendulum hardness test was based on the principle that the amplitude of the pendulum’s oscillation would decrease more quickly when supported by a softer surface.
The stainless steel balls, 8 mm diameter, were of hardness HRC59. The total weight of the pendulum was 500 g. The period of oscillation was 1 sec and the time for damping from a 12-degree displacement to 4-degree displacement was taken. Three tests were performed to each coated sample and the final result was the average of the three experiments.
Impact tests have been performed over coated samples by making the impact from the substrate side (reverse impact) according to ISO 6272.
The impact tests consisted of checking the primer damage resistance to the collision of a ball (1000 g) dropped from a fixed height of 0.5 m directly onto the metal substrate side and evaluating if the coating on the other side had been damaged or not. The weight was dropped through a guiding tube whose height was incrementally marked.
The objective of this test was to identify the resistance of the paint film against the ongoing deformation of a coated substrate panel with a pressed-in 20 mm steel ball. The result of the test gives the so-called “cupping” in mm during which the first disturbance of the coating occurred.
Determination was made by means of a special cutting blade with 6 cutting edges 2 mm apart and involved the degree of adhesion of the created 2 mm × 2 mm squares to a base substrate in accordance with standard ISO 2409.
This identified the resistance of the paint film against ongoing deformation of a coated substrate panel around a 2 cm diameter stainless steel mandrel, verifying the disruption of paint film cohesion during the bending of the painted substrate panel (ISO 1519).
Friction tests were carried out in dry conditions using a pin-on-disc tribometer. Coated samples with dimensions of 20 × 20 × 3 mm3 were brought into contact with 100Cr6 steel ball with a diameter of 6 mm. All tests were performed at the same sliding speed of 100 tr/min (0.052 m/s). The applied normal load was 1 N.
Friction tests were performed in ambient air (25–27°C) at 35–45% relative humidity (RH). During tests, the variation of the friction coefficient versus time was recorded.
The corrosion test applied to the coated substrates consisted only of samples after immersion into two solutions 3% NaOH and 5% NaCl at 25°C. A circular scratch surface (1 cm2) was made through the coating with a sharp instrument so as to expose the underlying metal to the aggressive environment. The panels were evaluated to assess failure at the scratch mark. Evaluation of specimens was performed after different exposure times (2, 7, and 15 days) and consisted in the measurement of scratched surface increase.
Potentiodynamic polarization tests of the coated samples were conducted with a conventional three-electrode cell and an Autolab PGSTAT302N controlled by NOVA software allowing data acquisition. Platinum sheet was used as a counter electrode and saturated calomel electrode (SCE) as a reference one. The tested coated samples were firstly allowed to reach a steady open circuit potential (OCP). Both OCP and polarization tests were carried out at room temperature (23°C) with a constant scanning rate of 5 mV/s under stirred conditions (300 rpm).
Surface profiles, using Surtronic 25 profiler from Taylor Hobson, were used to examine the surface topography of different paint coatings. The morphology of the corroded and delaminated paint coatings was studied using a LEICA optical microscope.
The Persoz hardness of the retained primers and primers/topcoat applied on the different substrates was measured. Knowing that standards require a Persoz hardness of the primers greater than 120 s, the surface hardness of both primers and primers/topcoat was around 120–140 s. In general, the hardness of a polymeric coating is an excellent probe of its chain stiffness [
The results of the impact resistance test of different primers and primers/topcoat are shown in Table
Results of the impact resistance test of different primers and primer/topcoat systems.
PP | SP | AP | DP | |
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Aluminum panel | D | D | D | D |
Steel panel | ND | ND | ND | ND |
Hot-dip galvanized steel | ND | ND | ND | ND |
Electrogalvanized steel | ND | ND | ND | ND |
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Primer/topcoat system | ||||
Aluminum | D | D | D | D |
Steel | ND | ND | ND | ND |
Hot-dip galvanized steel | ND | ND | ND | ND |
Electrogalvanized steel | ND | ND | ND | ND |
ND: not delaminated; D: delaminated.
During film formation and apart from the mechanism involved (evaporation of solvent, coalescence, chemical reaction, or their combination), in almost all cases the coating tends to contract. If this contraction is prevented by coating adhesion to its substrate and/or the mobility of macromolecular segments is hindered, a tensile stress will develop in the coating.
Standards require 4 mm as a minimum value of cupping for coating to be accepted. The cupping test is conducted with a relatively slow rate. The cupping action is stopped when cracking in the coating is visually detected.
As shown in Figure
Results of the cupping in an Erichsen cupping tester: (a) primer; (b) primer/topcoat system.
The presence of the topcoat seems to enhance remarkably this property for primers PP, AP, and DP/topcoat systems, whereas for a SP/topcoat system the cupping resistance is lower (Figure
This method is an adequate means for controlling the level of adhesion strength after the coating has been spread and cured on the substrate. Moreover, it allows the detection of any failure in the case of the dissolution of the bond between coating and substrate.
After a lattice pattern is cut into the coating, the examination of the created grid area was conducted using an illuminated magnifier. This method is used for a quick pass/fail test. In the rather qualitative standardized tape-test the scales used to classify the specimens are from 0 to 5; that is, 0 corresponds to a very poor and 5 to a very good adhesion. Table
Results of the lattice method.
PP | SP | AP | DP | |
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Aluminum | 0 | 0 | 0 | 0 |
Steel | 0 | 1 | 0 | 0 |
Hot-dip galvanized steel | 0 | 0 | 0 | 0 |
Electrogalvanized steel | 0 | 1 | 0 | 0 |
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Primer/topcoat system | ||||
Aluminum | 0 | 0 | 0 | 0 |
Steel | 0 | 1 | 0 | 0 |
Hot-dip galvanized steel | 0 | 1 | 0 | 0 |
Electrogalvanized steel | 0 | 0 | 0 | 0 |
Automotive coating must have high flexibility and formability to overcome the harsh conditions such as cutting, pressing, and stamping process.
This test is often used for evaluating the flexibility of the coating. Although it is difficult to control the strain rate in this manually operated test, it can provide very useful flexibility ratings.
In Table
Resistance of the coating during bending on a cylindrical mandrel.
PP | SP | AP | DP | |
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Aluminum | NC | NC | C | C |
Steel | NC | NC | NC | NC |
Hot-dip galvanized steel | NC | NC | NC | C |
Electrogalvanized steel | NC | NC | NC | C |
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Primer/topcoat system | ||||
Aluminum | NC | NC | C | C |
Steel | NC | NC | NC | NC |
Hot-dip galvanized steel | NC | NC | NC | NC |
Electrogalvanized steel | NC | NC | NC | NC |
C: cracked, NC: not cracked.
Abrasion resistance is a basic factor in the durability of the polymeric coating. Abrasion is caused by mechanical actions such as rubbing, scraping, or erosion from wind and water. It can take two general forms: marring or wearing. Abrasion resistance is related to physical characteristics such as hardness. The general agreement is that the major source of mar damage is a carwash, a normal periodic activity for many automobile owners. During a car wash, dust embedded in the brush, and in some cases the brush itself causes numerous microscale scratches on the surface.
Prior to friction tests the roughness measurements, in terms of arithmetic roughness Ra, of the retained primers/topcoat systems were conducted (Figure
Surface profiles of the different primers/topcoat: (a) SP, (b) DP, (c) PP, and (d) AP.
Friction coefficient of the studied primers/topcoat applied on the aluminum substrate as a function of sliding time.
Before studying coatings, friction behaviour of the aluminium substrate was investigated. The evolution of the friction coefficient reveals three regions: friction first increases abruptly, then gradually increases, and finally achieves a steady state value for the rest of the sliding distance.
Turning to primer/topcoat systems, their friction coefficients show globally similar tendencies. The friction coefficients are stable at values between 0.3 and 0.4, and then they increase to values similar to that of the aluminium substrate indicating so the rupture of the coating system. For all coatings, there is adhesion at the beginning of the test which suggests the formation of a junction over the area of real contact and the presence of shearing during sliding [
One possible measure for mar and wear resistance is based on the appearance of the transition point. Based on this criterion the relative ranking of different coating from best to worst would be
During the course of environmental exposure, water, oxygen, and salts diffuse through the paint system [
The results obtained for circular scratch specimens after 15 days of exposure in 5% NaCl are shown in Figure
Delamination of primers applied on the different substrates after 15 days of exposure in 5% NaCl solution.
Initially the area that is exposed to the NaCl solution, where the circular scratch was made on the steel panel, suffers corrosion. As the oxide film grows, this scratch behaves as a cathode to neighboring zones in contact with the NaCl solution and function as anodic regions. The Fe2+ ions in contact with the solution (O2 and HO− produced in the cathodic zone) give rise to corrosion products Fe3O4, Fe(OH)3, or Fe2O3 [
Figure
Zinc is chemically more active than steel; thus the corrosive elements tend to feed upon the zinc, preventing rusting of the underlying steel parts.
In aerated and near neutral pH NaCl solution, the passive layer from zinc does not form [ The cathodic reaction corresponds to the reduction of oxygen and leads to a pH increase:
The anodic reaction involves the dissolution of zinc and leads to weight loss:
Accordingly, the corrosion layer will occur on the area where the circular scratch was made. This layer is porous and much hydrated, the passivation of zinc does not occur, and the corrosion of zinc will proceed especially under the polymeric coating [
Figure
The corrosion resistance of aluminum arises from its ability to form a natural oxide film on the surface in a wide variety of environments. This oxide film can readily undergo corrosion reactions as has been reported in many investigations into the electrochemical behavior and corrosion resistance of aluminum in different environments and especially the ones containing chloride ion [
In NaCl solution (aerated and near neutral pH) we assist the formation of aluminum chlorides, AlCl3, or chloride complexes,
Polymeric coatings on aluminum are generally subjected to filiform corrosion which usually starts at cut edges [
Visual inspection of immersed AP, SP, and DP coatings showed no blistering, gloss change, or delamination from the aluminum substrate after 15 days of exposure in 5% NaCl solution (Figure
In order to more investigate the corrosion behavior of all the retained substrates and at the interface coating/substrate in NaCl solution, morphology of the corroded surfaces was examined using optical microscopy (Figure
Optical micrographs of corroded surface after 15 days of exposure in 5% NaCl solution: (a) primed steel, (b) hot-dip galvanized steel, (c) steel, and (d) aluminium.
From these optical observations different behaviors can be seen: (i) the delamination interface of primed steel and the initiation of corrosion products at the proximity (Figure
After topcoat application, coating systems do not exhibit a significant enhancement in anticorrosive behavior after exposure in 5% NaCl solution during 15 days. Primer/topcoat system seems to be subjected to delamination after any defect has occurred. For illustration, Figure
Delamination of primer/topcoat system applied on hot-dip galvanized steel after 15 days of exposure in 5% NaCl solution.
In this section, the coated hot-dip galvanized steel was retained to run electrochemical tests.
The free potential evolution of the steel, hot-dip galvanized steel, and all the retained primers applied on the latter in 5% NaCl solution is presented in Figure
(a) OCP evolution of bare steel, hot-dip galvanized steel, and different primers applied on hot-dip galvanized steel immersed in a 5 wt% NaCl solution; (b) potentiodynamic polarization curves of bare steel, hot-dip galvanized steel, and different primers applied on hot-dip galvanized steel immersed in a 5 wt% NaCl solution.
Figure
On the other hand, SP and PP primers presented lower
Literature includes many reports on the way that polymeric materials are easily hydrolyzed in the presence of alkalinity [
After 7 days of immersion in 3% NaOH solution, all primers were delaminated from their substrates. Figure
The use of topcoat on different primers improves notably the anticorrosive behavior of the coatings. In fact, Figure
Delamination of primers and primer/topcoat systems applied on hot-dip galvanized steel after exposure in 3% NaOH solution.
Four commercial automotive primers and a topcoat were applied on different metallic substrates, namely, aluminium, steel, and electrogalvanized and hot-dip galvanized steel. The mechanical, tribological, and anticorrosive performances of the studied coating systems were evaluated. The mechanical characterization shows that (i) all coating systems have a good Persoz hardness, (ii) coating delamination occurs with only aluminum substrate with impact resistance testing, (iii) among the primers, DP has worse cupping behavior, (iv) PP and SP have better flexibility than other coatings, (v) and all coating systems have good adhesion strength.
For tribological test, results revealed, globally, good wear resistance for the whole coating systems.
Immersion tests in 5% NaCl solution show that (i) DP/topcoat is the best system for steel; (ii) hot-dip galvanization is more adequate than electrogalvanization as a pretreatment for steel; (iii) and PP primer is to be avoided on aluminum substrate.
Electrochemical tests in 5% NaCl solution show that PP and SP primers applied on hot-dip galvanized steel present more protective action than DP and AP primers.
Immersion tests in 3% NaOH showed the benefic effect of topcoating in alkaline resistance of different coating systems.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by “Les Industries de Carrossage Automobiles (ICAR-Tunisia)” and Ministry of High Education and Scientific Research, Tunisia. The authors thank Mlle. E. BEN HROUZ and Mlle M. ESSID, engineers, for help and contribution to this research.