PATINA Network-Performance of coil coating in natural atmospheres of Ibero-America

The research work performed on 12 coil coating materials in the frame of the PATINA Network (Anticorrosive Protection in the Atmosphere) sponsored by CYYTED is discussed. It includs the task accomplished by Science and Technology institutions of Ibero-America on formulations supplied by different prcoduction Sector companies from the participating countries, between 1996 and 2000. Coil coating schemes were exposed outdoors, according to ISO 2810 standards, in 9 ambient conditions of the MICAT (IberoAmerican Test Stations Network), CYTED. The protective characteristics on the steel base metal was determined as a function of time, following ISO 4628 and ASTM D 3274 standards. Electrochemical impedance spectroscopy (EIS) was applied on various materials after 1, 2, 3 years and 42 months exposure, to evaluate the weathering effect in different atmospheric conditions.


INTRODUCTION
Coil coating or pre-painting steel, is a novel and one of the most advanced steel anticorrosive protection technologies, developed to promote steel utilisation in the most varied environmental conditions.It consists of a metallic and then an organic coating, both of low thickness, applied in continuous on thin steel coil.Steel metallic substratum is generaly coated with zinc or a Zn alloy, as galvalume (with 55 % Al and 1.5 % Si), which provides extra protection and great performance improvement against atmospheric corrosion.
Electrochemical impedance spectroscopy (EIS) was applied after various exposure times to determine mechanistic parameters affecting the protectiveness of the distinct schémas, such as the barrier effect of the organic films, the influence of the 2 different Zn base coatings, base and finishing paints, the underlying corrosion of sustrata, etc.
A good description of the experimental impedance diagram can be obtained from the analysis of the transference function using nonlinear fitting routines, like those developed by Boukamp^^\ This research work was carried out by Group 6: "Coil Coating" of PATINA Network, in the XV Sub-program "Environmental Impact on Materials", CYTED, for the 1996-2000 period.

EXPERIMENTAL
Results after the final 42 month's testing, of 12 formulations described in table I, in 9 of the 72 test sites of the MICAT Latin American Network, characterised in table II, are discussed.
Atmospheric exposure evaluation of defects in the organic coatings, with and without incisions, was performed every 6 months, according to ISO 4628/1 to 6 t^l and ASTM D3274 ^^^ standards.
EIS was yearly applied on Al to A6 weathered panels.The electrochemical experiments were carried out at 20±2 °C in a Faraday cage, after 1-hour immersion in NaCl solution 3.5 % w/v, pH 8.2, as electrolyte.A Solartron 1255 frequency    response analyser coupled to a Solartron electrochemical interphase 1286 and a PC were used.Impedance data was collected starting at 1 MHz up to 1 mHz and interpreted on the basis of equivalent electrical circuits, using a suitable fitting procedure.

RESULTS
-Outdoor exposure.Rain, dew, high relative humidity and important temperature decrease cause the liquid water contact with the test pannels, designed as "time of wetness" (TOW).The sun radiation synergises the TOW's effects, enhancing deterioration of components pigments and polymers of the coating.When water and the environmental contaminants reach the underlying metal, the corrosion phenomenon is added to the coating damage, accelerating the irreversible damage.
Once steel corrosion starts on samples in which the organic coating was applied, rapidly growing blisters appear and high deterioration rate occurs at the incision and borders.Conversely, materials in which steel was coated with Zn or galvalume, only evidence slow white corrosion, even in the most aggressive environments.When the Zn coating locally dissappears its cathodic protection stops, and orange corrosion points appear due to the base steel attack at the incision and borders.Away from the borders, corrosion and blistering are slower than at the incision because they occur through paint films.EIS.When there is no electrolyte solution within the metal/organic coating interface, no electrochemical double layer formation nor faradaic reactions occur, being the only information obtained from impedance data that related with the dielectric properties of the organic coating.A first interpretation of the complex plane plot may be described by a transfer function corresponding to an equivalent circuit built up from a series combination of the electrolyte resistance (Rg) with the paint film dielectric capacitance (Cj).As the exposure time goes on, the amount of permeating water, oxygen and ions increase the coating conductivity, becoming measurable the resistance (R^) associated to the dielectric capacitance C^.In such conditions, the impedance diagram and equivalent circuit, together with information about the membrane ionic resistance (Rj, which describes paths of lower resistance short-circuiting the organic coating), and the dielectric capacitance (C^, whose value is associated with the water uptake) can be obtained.Once the permeating species reach the metallic substrate corrosion processes may initiate, causing the emergence of the electrochemical double layer (C2) and the charge transfer resistance (R2) proper of a faradaic process, related as I/R2 with the corrosion rate.In this case, the impedance response and the corresponding equivalent circuit modelling the coated metal / 3.5 % NaCl solution are those exhibited in figure r .Ionic resistance (Ri) values ranged from 10 Qcm up to greater than 10 Ocm .These resistance values are inversely related to the average cross-section of the conductive pathways between the metal and the electrolyte.Therefore, they suggest that penetration of the paint by the electrolyte solution after 12 or 24 months exposure to different atmospheric media was highly dependent on both the painting system formulation and the aggressiveness conditions.Accordingly, the lower values obtained coincided with large rust spots on the specimens, confirming their poor barrier and anticorrosive properties under the experimental conditions.
On the other hand, the high anticorrosive protection found in samples where Ri>10 Qcm^ was attributed to the great influence of the mechanism described by the barrier and insulating effect model (i.e., the paint capacity for delaying the diffusion of aggressive species up to the metal paint interface), added to an effective anticorrosive action of the pigments incorporated to the primers.In agreement with this mechanism, it was assumed that the metal surface was effectively kept isolated from the 3.5 % NaCl solution during EIS measurements.Results related with the charge transfer resistance (R2) of the different samples show that the values of this parameter vary between lO'^'lO^ Ocml The lower denote a progress of the active (corroding) metal surface beneath the paint film due to an increase of both the pore density and the metal electrolyte contact area, while the higher indicates that there is an effective insulating effect provided by the painting system.Correspondingly, the double layer capacitance (C2) can provide information about the extent of the underfilm corroding area as a consequence of the protective coating degradation^ \ Blistering of organic coatings applied on steel is a very common phenomenon under service conditions.According to Kreese^ % osmosis plays an important role in blistering due to the fact that unidirectional transport allows the accumulation of water between the coating and the substrate.

Ionic resistance vs. exposure time for AI to A6 materials
This EIS results showed a clear decrease of the organic coating resistance with time for samples Al to A6, after the first yearly period.On the contrary, coating resistance increased after the second year in the most aggressive of the testing atmospheres (La Voz) for samples Al, A3 and A6 when corrosion of the metallic sustratum produced Zn corrosion products, causing a blocking effect on paints pores.For other schémas and also for the same samples but in milder marine test sites, decresing values were determined for longer periods.
In the 3 marine test stations, with very different yearly mean T, sun radiation and hours, SO2 and [CI'], etc. the most sensitive failure for all the formulations in the exposed atmospheric conditios was blistering at the incision.The changes with time in the magnitude of this failure for Al, A2 and A3 showed that the most significant environmental parameter was the TOW.In spite of the great difference among those marine atmospheres, other data than sun radiation and [CI'] did not show a significant correlation with the magnitude of the damage variation.Comparing the changes with the time of the paint schema resistance for the 3 materials it could be concluded that the best one was A2, the following was A3 and finally Al.This result was also observed in all the continental marine atmospheres.In the antarctic marine atmosphere of Jubany Base practically no variations were detected with time in paint resistance for all materials, up to the final test period.In good agreement with the absence of color change and the lowest gloss decrease, as compared with the rest of the atmospheres, it could be attributed to the low accumulated sun radiation and sun hours during each test period.
Materials A4 and A5 showed very little change in the organic coating resistance after the first year exposure in La Voz, appearing at the end of the second year a sudden change in A5, that destroyed completely the paint film in most atmospheres since that time.
In marine atmospheres best behaviour was observed for A4 schema and worst for A5.In urban ambients best behaviour was found for material A2 and the worst for A6.

CONCLUSIONS
-The best performances of the tested coil coating materials were observed after exposures in Test Stations of lowest aggressiveness, this being mainly determined by the TOW and by their pollutant content.-Industrial solid pollutants at the Cubatao Station had a strong negative aesthetic effect on all formulations, determined by the hardness of particles emitted by chimneys of the steel mill works, where the Test Station is located.This effect was more evident in samples which organic film surface hardness was lowest.This solid pollutant effect did not allow to appreciate the expected result of the high SOj concentration on the steel protectiveness.
-Best behaviour before corrosion after 42 month's test was observed when metallic coating was deposited on steel.Better results were obtained for galvalume than for Zn.-The Ri, Ci, R2 and C2 values obtained from EIS kept good correlation with the characteristics observed during visual inspection.-Among the metal coated samples designed A the best performance was shown by formulation A4 and the worst was A5, -Among the metal coated samples designed E the best performance was shown by formulation A3, followed by E2 and the worst behaviour was observed for A4.-The protective schema thickness greatly affects the behavior.The great dispersion determined with different samples introduced uncertainty in results.-Among the overall set of formulations the main causes of the improved resitance to most aggressive atmospherestwere the metallization with galvalume vs.to Zn and the total paint coating thickness.Better protection could be attributed to the PVDF as compared to polyester finishing coatings.
Norway) (**) Elements detected through EDX, in decreasing order of relative abundance.

Table I .
CharacterisHcs of the tested materials