Behaviour of painted aluminium in Ibero-American atmospheres

Aluminium generally presents good corrosion resistance to the atmosphere. However, unprotected aluminium and aluminium alloys weather outdoors to an ugly grey colour, which deepens to black in industrial atmospheres, and undergo superficial pitting in marine atmospheres, etc. Finishing technologies are applied for their protection and decoration in a wide range of applications. These technologies basically consist of two protection processes: anodizing and painting; the latter going from conventional solvent base paints to modem water-borne, high solids and powder coatings. This paper considers the weathering performance of three paint systems: alkyd, polyurethane and polyester, after more than three years of exposure in a wide spectra of Ibero-American atmospheric conditions. The information reported includes resistance to undercutting corrosion at the scribe, filiform corrosion, fungal attack and change in the physical-chemical properties of the paint surface (loss of gloss, colour changes, chalking, etc.).


INTRODUCTION
This research forms part of the Ibero-American 'PATINA' project (Anti-Corrosion Protection of Metals in the Atmosphere), which involves the participation of fourteen countries of the Ibero-American region^^l One of the main tasks of this network is to evaluate the corrosion protection behaviour of a wide range of conventional and advanced coatings in typical atmospheres of the IberO'American region.
The different coating technologies were distributed between six working groups.Group 5 included protective coatings on aluminium: anodizing^^^ and painting^^ ^' ^^ "^l Aluminium generally presents good corrosion resistance to the atmosphere.However, unprotected aluminium and aluminium alloys weather outdoors to an ugly grey colour, which deepens to black in industrial atmospheres, and undergo superficial pitting in marine atmospheres^ \ This work reports the main results obtained in the project with reference to paint coatings applied on aluminium, after 36-42 months of exposure in different atmospheres of Ibero-America.

EXPERIMENTAL
Specimens of 10x15 cm were cut from aluminium plate of 2 mm thickness, L-3350 alloy (UNE 38335), for paints A and B, and L-3140 alloy (UNE 38314) for paint C.
After surface conditioning by means of degreasing, washing and drying, the specimens received the following protection systems: -Systems A and B. Yellow chromating by means of immersion for 2 minutes in a dissolution of Na2Cr203 0. Once the paint coatings had cured in the laboratory atmosphere, a scribe of 0.3 mm thickness was made in the lower zone of the specimens, with an angle of attack of 60 ± 15 °, penetrating the paint coating and reaching base aluminium.Another series of specimens was unscribed.
Series of specimens were exposed in a variety of atmospheres of the Ibero-American region representing a broad spectrum of atmospheric corrosivities (Table I). Figure 1 shows a grid corresponding to ISO standard 9223^^, on classification of atmospheric aggressivity according

RESULTS AND DISCUSSION
In general terms the anticorrosive behaviour of the paint coatings in the different atmospheres may be considered to be good after three years of atmospheric exposure, with some exceptions that are mentioned below.The deterioration observed on some test specimens and in certain atmospheres has mainly affected the decorative characteristics of the specimens more than their anticorrosive properties.Table II indicates the deterioration recorded for the three paint systems in the different atmospheres.
With regard to the anticorrosive behaviour of the three paint systems after three years of atmospheric exposure, all have provided good protection of the aluminium in the different atmospheres.Attention is drawn only to the occurrence of slight undercutting and/or blistering of the paint films in the zones adjacent to the scribe in the case of paints B and C, the latter in all the atmospheres where it was tested (Table II).Paint B only showed this type of damage in Lima and La Voz atmospheres.In La Voz station, the blistering of some parts of the edge of the specimens seemed to show a similar morphology to filiform corrosion, which nevertheless does not develop in a clear way.In Lima atmosphere, however, filiform corrosion is patently observed on the coating adjacent to the scribe (Fig. 2), and the filaments progress notably with exposure time, after an incubation period of approximately 18 months of exposure.
With regard to the decorative properties of the different coatings, the main changes observed refer to the following aspects: changes in colour, loss of gloss, chalking, presence of fungi on the paint surface and existence of firmly adhered soiling on the paint surface.
With regard to adhered soiling, this damage only occurred in the industrial atmospheres participating in the study (Esmeraldas and to the SO2 and Cl" (salinity) content of the atmosphere, that situates the different testing stations participating in the study.As can be seen in figure 1, the network of testing stations goes from the rural atmosphere of El Pardo, with very low SO2 contamination, to more industrial atmospheres (Esmeraldas and Cubatao) with a higher SO2 content.Marine atmospheres are also considered, from those practically without SO2 contamination (Jubany in the Antarctic, Punta del Este and Tablazo) to others which are more contaminated by SO2 and with increasing levels of salinity: Panama, Lumiar, Lima, Viriato and La Voz.
Visual inspections and photographic records of the state of the different coatings were carried out every 6 months and the study ended after approximately three years of exposure (36-42 months depending of the different test sites).The specimens corresponding to Esmeraldas station were disappeared after two years of exposure.The inspections in the testing stations referred mainly to the following aspects: -Presence of dust, adhered soiling, fungi, etc. on the paint surface.-Qualitative changes in colour and gloss and chalking^^ of the paint films.-State of the scribe and adjacent coating (undercutting, blistering, filiform corrosion, etc.).
Once the specimens had been withdrawn from the testing stations and taken to the laboratory, an Fungi, preferentially on the downward facing side. Fungi.
Slight blistering at the scribe and edge area, similar to filiform corrosion.

La Voz
Fungi, preferentially on the downward facing side. Fungi.
Slight undercutting at scribe edges.Fungi.
Very slight undercutting at scribe edges.
Slight undercutting of the paint adjacent to the scribe, which progresses with time. Fungi.
Fungi, preferentially on the downward facing side.Slight blistering at edges and zones adjacent to the scribe.
Slight blisterint at the scribe.

-Not tested
Chalking was observed on all three systems and in all the testing stations, to a greater or lesser degree.Figuro 2. Corrosión filiforme en la pintura B en la atmósfera de Lima.Préstese atención al significativo progreso de los filamentos con el aumento del tiempo de exposición.
Cubatao), and on the two paints (types A and B) tested in these atmospheres.The presence of fungi, which could be seen in some inspections and testing stations, did not constitute very significant damage to the paint films.The slight staining that was produced (Fig. 3) disappeared in most of the cases with simple washing of the paint surface with water.The fungi identified on the specimens exposed in Viriato were of the types Aspergillus Niger and Curvularia Harveyi.
Chalking was observed to a greater or lesser degree on the three paint systems in all the testing stations.Qualitative evaluations of the degree of chalking of the three systems were made during the inspections, leading to the following conclusions: -Paint C is that which presents the highest degree of deterioration due to this phenomenon, and paint B is the most resistant in terms of this property.
-The degree of chalking increases in general with exposure time.-As was foreseeable, the degree of chalking is a function of the intensity of solar radiation and hours of sunshine per year at the exposure site.Thus, the highest degrees of chalking were presented in the testing stations of La Voz, Panama and Lumiar, and the lowest in Lima.
The changes of colour have been very slight on the three paints In relation with the loss of gloss experienced by the three paint systems in the different atmospheres.Figure 4 displays the results obtained in the laboratory on specimens aged in the atmospheres for three years of exposure.Data is not presented for Cubatao, since, as has already been mentioned, the specimens suffered heavy soiling in this testing station.From observation of this figure the following conclusions can be drawn: -Paint C (alkyd) is that which experienced the greatest loss of gloss and paint B the least.-A certain relationship is observed between the loss of gloss and the hours of sunshine per year at the exposure site.Thus, Viriato (2723 h of sunshine per year) always registers great losses of gloss, while in Lima (1376 h of sunshine per year) the losses of gloss are very low.(industrial atmospheres of Esmeraldas and Cubatao), slight staining by fungi in many of the atmospheres, which disappears after simple washing, as well as chalking and loss of gloss in all the atmospheres.These last two types of deterioration, most pronounced on paint C and least on paint B, present a higher intensity the greater the number of hours of sunshine at the exposure site.

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/m'.day Behaviour of painted aluminium in Ibero-American atmospheres M. MORCILLO, ].SIMANCAS, F. CORVO, B. ROSALES, F. FRAGATA, J. PEÑA, M. SANCHEZ, S. FLORES, E. ALMEIDA, S. RÎVERO AND O.T. DE RINCÓN in-depth inspection was made of the state of the different coatings.Of particular importance was the quantification of the variation in colour and loss of gloss experienced by the coatings in the different atmospheres.These measurements were made in the Paints and Coatings Laboratory of INETI (Lisbon, Portugal) with the assistance of a 948 spectrocolorimeter (X-Rite) and a statistical glossmeter (Novo-Gloss), in accordance with the standards ISO 7724 ^^^ and ISO 2813 ^^^ respectively.

Figure 1 .Figura 1 .
Figure 1.Characterization of atmospheres corresponding to the test stations with regard to SO2 contamination and salinity, according to ISO 9223^^1Figura1.Caracterización de las atmósferas correspondientes a las estaciones de ensayo en función de la contaminación por SO2 y la salinidad, de acuerdo con ¡SO 9223^^1

Figure 2 .
Figure 2. Filiform corrosion of paint B in Lima atmosphere.Note the significant progress of filaments with increasing exposure time.

Figure 3 .
Figure 3. Presence of fungi on the surface of paint C exposed in Lumiar atmosphere.

Table II .
Damage observed on the paint systems exposed in the different testing stations Tabla II.Daños observados en los sistemas de pintura expuestos en las diferentes estaciones de ensayo