Corrosion and protection of metals in the rural atmosphere of ' Tl Pardo ' , ' Spain ( PATINA / CYTED project )

Atmospheric corrosion tests of metallic and organic coatings on steel, zinc and aluminium have been conducted in "El Pardo" (Spain) as part of the PATINA/CYTED project "Anticorrosive Protection of Metals in the Atmosphere". This is a rural atmosphere with the following ISO corrosivity categories: C2 (Fe), C2 (Zn), C3 (Cu) and CI (Al). Its average temperature and relative humidity is 13 °C and 62.8 %, respectively, and it has low SO2 and C r contents. Results of 42 months exposure are discussed. Atmospheric exposure tests were carried out for the following types of coatings: conventional paint coatings for steel and hot-dip galvanized steel (group 1), new painting technologies for steel and galvanized steel (group 2), zinc-base metallic coatings (group 3), aluminium-base metallic coatings (group 4), coatings on aluminium (group 5) and coil-coatings on steel, hot-dip galvanized steel and 55 % Al-Zn coated steel (group 6).


INTRODUCTION
Different means have been developed, including the use of protective coatings, to protect materials from the degradation they suffer when exposed to the atmosphere for certain lengths of time.Due to the large number of industrial applications and situations that can arise in practice, the simulation of natural exposure tests^ ' % though slow and costly, can be of great value.Constant changes in coatings technology, as a result of the appearance of new coatings with improved characteristics and properties, justifies the performance of studies such as that presented here.
The present work forms part of an ambitious project entitled "Anticorrosive Protection of Metals in the Atmosphere" (PATINA/Thematic Network XV.D -CYTED).The experimentation phase was carried out in a network of atmospheric corrosion stations in the Ibero-American area, considering a broad range of environments and corrosivities^ \ The station at "El Pardo" (Madrid, Spain) forms part of this network and is classified as a rural atmosphere, according to ISO 9223^ % with the following corrosivity categories: C2 (Fe), C2 (Zn), C3 (Cu) and CI (Al).The average temperature is 13 °C, the average relative humidity 62.8 % and the SO2 and CI contents recorded are low.Specimens belonging to the six working groups comprising the project (see Table I) were first exposed in the early months of 1996, arranged in two special racks.

EXPERIMENTAL PROCEDURE
The test specimens were in some cases supplied by the manufacturer and in other cases, such as the materials with organic coatings, prepared in the laboratory following the corresponding specifications.
On a significant number of the specimens an incision was made in the coatings, penetrating to the base metal, in order to determine the behaviour and delamination of the system in unprotected zones.The coatings were evaluated at periodic intervals during the exposure time (42 months) following the ISO standards selected for that purpose^^ '''' '^ ^l In view of the amount of specimens and the variety of coatings exposed in the atmospheric testing station at "El Pardo", a summary is presented below for each group.More detailed information about the characteristics of the coatings has been reported elsewhere^ \

RESULTS AND DISCUSSION
The results presented below indicate the defects found after 42 months of experimentation in "El Pardo" testing station.The periodic evaluations of the state and deterioration of the paint coatings were carried out by visual inspection and comparison with ISO standards^ ^^^ .
-GROUP 1: Table II   group, noting slight losses of: colour, shine, chalking and also blistering in the incision.Fungus also appear on specimens A and CH, It is well known that degradation is suffered by all materials as a consequence of the action of the atmosphere, as a function of the macroclimate (oxygen, humidity, contamination, solar radiation) and microclimate (time of wetness of the surface, temperature of the material), surrounding the exposed specimen.Both the macroclimate and the microclimate play a decisive role in the durability of materials exposed to the action of the atmosphere.
This work evaluates the deterioration of metallic and organic coatings from the point of view of the protective function rather than the decorative function of the coatings.
The defects observed, considering the experimental results obtained after 42 months of exposure, are related with rusting in the incision on steel base specimens (in the other cases the zinc

CONCLUSION
In general, after 42 months of exposure in the rural atmosphere a minimum degree of degradation (in many cases almost nil) of both the organic and metallic coatings is observed."El Pardo" testing station may be considered a reference station in view of its low aggressiveness and the slight deterioration experienced by all the specimens exposed, even at the lowest thicknesses.

Table VII.
Defects encountered on Group 6 specimens after 42 months of exposure in "El Pardo" testing station Tabla VII.Defectos encontrados en las probetas del Grupo 6 después de 42 meses de exposición en la estación de ''El Pardo" protection); slight losses of colour, shine and chalking on organic coatings.On the other hand the most generalized defect in the metallic coatings has been corrosion in the incision and at the bare edges.The field study has been conditioned by the effect of the low corrosivity of the atmosphere.

Table 1 .
Groups of specimens exposed in ''El Pardo" station These average thicknesses do not include the thickness of the galvanizing (60 mm).All the specimens have edge protection and include an incision.

Table II .
Defects encountered on Group 1 specimens after 42 months of exposure in "El Pardo" testing station DefectsColourChalking Incision (blist.)