Carla Sofia Jorge dos Reis

Transcription

1 OPTIMISATION OF AN ANTICORROSION COATING PROCESS USING THE CATHODIC ELECTRODEPOSITION METHOD IN THE AUTOMOTIVE INDUSTRY Carla Sofia Jorge dos Reis INSTITUTO SUPERIOR TÉCNICO UNIVERSIDADE TÉCNICA DE LISBOA ABSTRACT The electrodeposition method is regarded as an excellent technology and is widely used by the automotive industry to apply anti-corrosive primers. One of the most recent advances in this field is based upon the replacement of low thickness anticorrosive coating paints with high thickness anticorrosive coating paints. This new technology has many advantages, such as providing a superior performance against corrosion. Samples of the two paint types were analysed, employing electrochemical impedance spectroscopy as the experimental method. The anticorrosion performance benefits associated with the high thickness paint were confirmed. The implementation of a new paint, as with other process changes, implies re-characterizing the critical parameters. As part of the work presented those critical parameters were identified, as well as their influence on the formation of the anticorrosion coatings, which must be uniform and defect free. Keywords: Electrodeposition, Cataphoretic Paint, Critical parameters. 1. INTRODUCTION The quality of the vehicles available for sale can be compromised by the corrosion of the material they are made of. High resistance steel and aluminium are examples of materials characterized by their low resistance to corrosion. The application of high quality coatings is, therefore, essential for the protection of such materials present in a vehicle s structure, especially in the highly competitive automotive industry. One process of applying anticorrosion coatings, highly used and considered the most efficient, is the cathodic electrodeposition (CED) method. Related to its improvement is the replacement of the low thickness 1

2 anticorrosive coating paints used with high thickness anticorrosive coating paints. The implementation of such a change implies re-characterizing the critical parameters of the CED process, in order to optimize it. vehicle s structure are all uniformly coated with paint. This results in an excellent anticorrosion protection. Figure 1 shows a schematic diagram of the several elements of the CED process. 2. CATHODIC ELECTRODEPOSITION There are several reasons the cathodic electrodeposition (CED) process is regarded as an excellent technology and is widely used by the automotive industry to apply anti-corrosive primers. This process yields thin and uniform paint layers, characterized by their excellent performance against corrosion, even in difficult access areas of the vehicle structure. In addition, this process is environmentally friendly, due to the reduced production of volatile organic compounds (VOC s) and the total absence of heavy metals. It is highly automated and has minimal product losses, which makes it an efficient process, both from a technical and an economical point of view. The CED is a complex painting technology, consisting on the immersion of the material that is to be coated. It is based on the movement of the electric charged particles (paint) in a preestablished electrical field. These particles are attracted to the opposite signal pole, the metallic material that is to be coated. Through this process the surface, edges and cavities of the Figure 1. Electrodeposition paint. In the CED method the material or substrate to be coated, the component in Figure 1, works as the cathode, which is immersed in a bath of paint, the solution. The ink particles that are positively charged are attracted to the cathode, and local interactions are established between its surface and the active molecule groups [1]. The substrate is connected to one of the direct current electrodes to which a voltage is applied. This is related to the current by the Ohm law. As the tension is constant, the current reaches a value, I 0, which is a function of the liquid paint resistance between the electrodes (R) and the voltage (V), as in equation 1. The resistance is directly proportional to the bath resistance (ρ) multiplied by the distance that separates the electrodes (I). V I 0 = (1) R 2

3 3. CATAPHORETIC PAINT Usually the painting of a vehicle structure involves three different layers of paint: low thickness anticorrosive primary, an intermediate primary and a finishing paint. The intermediate primary is applied because coating composed solely by the cataphoretic paint and the finishing paint is unable to provide the required characteristics: thickness, anticorrosive resistance, colour (little ability to cover), roughness and brightness. However, the recent development already mentioned provides a high thickness anticorrosion coating, which enables the removal of the intermediate primary from the process and therefore reduces the emission of VOC's [2]. This new technology, called the 'two coats', allows the application of only two layers of paint to the vehicle structure, the high thickness anticorrosive primary and the finishing paint (top coat), without compromising the quality of the coating. This new generation of paint, based on paint products with high solids content, offers a superior performance against corrosion. These products provide an effective material protection that can exceed 2000 hours of exposure to saline environments [2]. An electrochemical impedance spectroscopy test (EIS) was conducted in order to analyse and compare the performance of the low and high thickness anticorrosive paints. 3.1 ELECTROCHEMICHAL IMPEDANCE SPECTROSCOPY TEST Impedance is a measure of the opposition of an element in a circuit to the passage of electric current. The application of this technique to the electrochemical systems is quite complex. Nevertheless, it allows the understanding of the development of corrosion with time [3]. In the specific case of the polymeric coatings the electrochemical impedance spectroscopy (EIS) generates quantitative data that are related to the coating quality in a material. Such data indicates the changes that can occur with time in a coating long before any deterioration is visible. The experience s main objective was to compare the anticorrosive protection capacity of the two cataphoretic paint types, low thickness (A) and high thickness (B). Samples of the painted material where placed in contact with a sodium chloride electrolyte conductor. By applying a sinusoidal potential a current through the polymeric paint is measured. This allows the impedance value to be calculated using equation 2. V ( t) Z = (2) I( t) If the paint deteriorates as a consequence of the electrolyte, allowing the passage of current, then the paint is named non capacitive. This paint is 3

4 characterized by low impedance values and leaves the metallic material susceptible to corrosion [5]. On the other hand, if the paint resists the passing of current it means that the paint is intact and that the solution was unable to penetrate the paint. In this second situation the attack to the material by the solution is ineffective and the paint is considered capacitive, showing high impedance values [5]. Therefore, these paints provide a good anticorrosive protection even when damaged by overheating. The test was conducted on two regular painted steel samples from an industrial installation. After being painted, these samples were placed in the oven for polymerization (curing). Since vehicles are in permanent contact with aggressive environments the samples were submitted to a second curing, in order to simulate such aggressions. Tests were conducted after the samples were immersed for one hour and repeated after one month. A sinusoidal disturbance, with 10 mv rms amplitude and frequencies from 50 khz to 10 mhz, was applied. 3.2 EXPERIMENTAL RESULTS Figure 2 shows the impedance results and the phase angle obtained for samples A and B after one hour and a month of electrolyte exposure. Both samples show high impedance and phase angle values during the tests. One can conclude that the paints are intact, not allowing the passage of current. Figure 2. Impedance and phase angle for the A ( ) and B ( ) cataphoretic samples after one hour of immersion (above) and one month of immersion (down). Nevertheless, results for the two paint types indicate that cataphoretic paint B shows higher resistance than cataphoretic paint A. The impedance values of the cataphoretic paint B are much higher, so it has a better performance after overheating. It was concluded that the high thickness paint is, in fact, more effective in protecting the anticorrosive material. 4

5 4. CHANGE IMPLEMENTATION TO THE PROCESS AND OPTIMISATION When any part of a production line is to be changed the elaboration of previous studies is advisable. Once a change is considered feasible its implementation implies the optimisation of the new process, which should follow a prepared test plan. Such plan must take into account the critical parameters of the process, so that they can be adjusted, individually or together, to the process, so as to fulfil the manufacturer s product requirements. A study of the influence of the most important parameters in a CED process for paint coating is shown below. The knowledge of such parameters is essential to define in advance the plan to conduct tests during the optimisation process, as well as for the interpretation of the results. 4.1 ANODE-CATHODE AREA The anode/cathode area must be kept in a ratio of 1:4 [6] in order to maintain the good functioning of the electrodeposition and thus contribute to the performance of the film of deposited paint. When the cathodic area increases, unbalancing the mentioned proportion, the acid removal is no longer efficient and the density of current decreases. This increase of cathodic area causes ph fluctuations and a reduction in the electrodeposited film quality. In addition, it may cause the deterioration of the equipment, namely of the anodic cells. 4.2 BATH CONDUCTIVITY Bath conductivity is a parameter that must be strictly controlled, since it is related with the electrical properties of the bath. A low conductivity bath can result in a low penetration power and in a film of reduced thickness and high roughness [6]. However, when conductivity exceeds the normal values the coating can become too thick, with roughness or even present ruptures [6]. 4.3 ANODE-CATHODE DISTANCE The anode-cathode distances must be well defined before the construction of the electrodeposition tank. These distances are important because they can influence the penetration power of the electrodeposited paint [7] and, consequently, lead to the deficient coating of difficult access areas in the vehicle structure. The optimisation of this parameter may imply high costs since it can involve changes in the layout of the anodic cells or of the parts to be coated (cathode) in the transporter. It s mandatory that these distances allow a good performance of the mixing system. This ensures the homogenization of the paint and compensates its deposition. 5

6 4.4DRY EXTRACT The dry extract, defined as the non volatile substance of the bath, influences the thickness of the film of paint. The coating s thickness increases proportionally with the increase of the dry extract. If the content of solids in the paint bath is low then its penetration power decreases and its conductivity will be lower than expected. This can result in a low thickness film [6]. The opposite situation leads to a high thickness film, as expected, and can eventually make the mixing difficult [7]. 4.5 PIGMENT/BINDER (P/B) and promotes the formation of coatings with rude aspect and low resistance to corrosion [1,6]. 4.6 BATH PH The ph of the bath is another parameter that should be well defined and permanently controlled, in order to obtain a slick and uniform film. If the ph value is too high it can cause coagulation of the binder, which renders the bath dirty and increases the number of defects [1,6]. On the other hand, low ph values can lead to the dissolution of the deposited paint or to the appearance of pinholing in the coating [1,6]. RELATION Cataphoretic paints are essentially composed by pigments and resins (or a binder). The pigments are introduced into an ink in order to give it colour and opacity. It contributes to the stability of the bath and is very important for anticorrosive performance [1,8]. Nevertheless, the binder has the function of maintaining the particles of pigment linked to the film of dry ink. Binders are important in obtaining coatings with good properties of adhesion, durability and chemical resistance [8]. The deficient control of the pigment/binder relation can lead to defects in the film of paint. High binder content promotes crater formations and diminishes the paint penetration power. An exaggeratedly low binder content increases the paint penetration power 4.7 BATH TEMPERATURE The temperature of the paint bath in the CED tank is a very important variable in the electrodeposition processes and its values should be between 28 and 35 ºC [1]. Bath cooling must be efficient so that the limit temperature is not exceeded. Should it exceed the maximum temperature solvents would rapidly evaporate and the rest of the paint would deposit in the tank, leading to irreparable damages in the installation. Higher temperatures result in a high thickness film with a larger number of craters and poor visual appearance [7]. This will also cause a reduction in the paint penetration power [7]. On the other hand, lower temperatures have the 6

7 advantage of promoting the penetration power and the brightness of the coating, but lead to low thickness values [7]. 4.8 IMMERSION TIME The immersion time of metallic material in a CED bath is important because it influences the thickness of the electrodeposited film. If the immersion time increases, the coating thickness will equally increase until the current density starts to diminish and stops completely. After this, positive particles will no longer move to the cathode, meaning that thickness won t increase even if the material continues immersed in the conductor bath. The material immersion time may affect the process s efficiency in terms of productivity, which diminishes with the increase of the electrodeposition time. There are plants with large production numbers, operating 24h a day, and with big time limitations. Increasing 1 minute of the immersion time could lead to a significant loss of units produced annually and a significant loss of income. 4.9 TENSION Tension is one of the most important parameters in a CED process. It significantly influences the coating thickness and the resulting number of defects. When the applied tension is too high the penetration power [1] increases, which can result in a paint film of excessive thickness. In addition, it can lead to defects such as coating rupture and small imperfections that are a consequence of excess gas release through the paint film [1]. However, when the applied tension is too low a defect free film is obtained but with a low paint thickness RECTIFIER RIPPLE The rectifiers used in electrodeposition are used to convert alternate current into direct current, so as to minimize voltage variations during the paint film formation. There are no ideal rectifiers, all of them present voltage fluctuations known as rectifier ripple. Hence, they must be maintained under permanent control, so variations don t exceed more than 5% the designated set-point defined for the tension [6]. If these variations are too big the paint film can present many defects and non-uniform thickness in all exterior points of the car structure SOLVENTS CONTENT Organic solvents are added to the immersion bath to facilitate the resin solubility and provide a proper consistency for the paint application. The excessive addition of these solvents could lead to a high thickness coating and low penetration power [6,7]. None the less, if the solvent content is too low, the electrodeposited film will have a lower 7

8 thickness and brightness than expected, as well as a higher roughness [6,7]. conditions, which will lead to a uniform coating free of defects. 5. CONCLUSIONS The electrochemical impedance spectroscopy test allowed a deeper analysis of one of the most recent technological advances, which involves the replacement of low thickness anticorrosive coating paints by high thickness ones. The high thickness coating paint proved to be more resistant, even after being exposed to excessive heating caused by two successive healings. This is a consequence of its high thickness and new paint properties that slow its deterioration and the consequent metal corrosion. The electrodeposition process is an excellent solution when the goal is to paint metallic surfaces with anticorrosive paint. It is a sensitive process that responds to any disturbance of its critical parameters. Its optimisation should be based on a plan of experimental tests prepared in advance by the manufacturer who, in this phase, will work in partnership with the suppliers. The process s sensitivity requires establishing well-defined operating 6. REFERENCES [1] Almeida, M. Elisabete; Guia sobre protecção anticorrosiva na indústria automóvel, Protap, Maia, 2000 [2] 6.html ( ) [3] Fernandes, João Salvador; Curso de EIS fundamentos, Instituto Superior Técnico, Lisbon, 2007 [4] /Evaluation-of-organiccoatings-with.html ( ) [5] Simões, Alda; Curso EIS EIS em sistemas pintados, Instituto Superior Técnico, Lisbon, 2007 [6] 2.html ( ) [7] /paint.html ( ) [8] Manual de protecção do ferro e aço por pintura, Instituto Nacional de Investigação Industrial, Lisbon,