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Corrosion Safety in Sandwich Panels

Guidelines to avoid corrosion problems

A serious cause of deterioration of sandwich panels is galvanic or electrochemical corrosion. This occurs when the more noble of the two metals, in contact with an electrolyte (usually water), performs an electrochemical aggression on the less noble metal.

In fact, all metals have a characteristic electric potential. When metals of different potentials are in contact in the presence of an electrolyte (moisture, acid, etc.), a weak current of energy flows between them, causing corrosion of the metal with the higher potential (the less noble metal).

In the end, the less noble material will be so corroded that it will have to be replaced. In any case, with a careful choice of materials and protective coatings, galvanic action can be eliminated or greatly reduced.

The wider the gap between the metals in the frame, the more likely it is that corrosion will occur. For example, 2024 aluminum will corrode in the presence of 410 (active) stainless steel, but this corrosion will be even more severe in the presence of nickel. Metals of the same group do not normally give rise to galvanic corrosion as, for example, titanium and 304 stainless steel (passive). If the cathode metal (more noble) is larger than the anode metal (less noble), the corrosion will be much more severe, which would not be the case if the size were the opposite. For example, it is usually possible to use bronze seals in steel hydraulic pipes, which only causes slight corrosion of the steel. On the contrary, if steel gaskets were used in brass hydraulic pipes, these gaskets would corrode quickly.

Correct installation method of the panels

Significant corrosion problems can occur if sandwich panels are installed incorrectly on a steel support structure.

As an example, the illustrations shown in Fig. 5.2 show how to properly install a wall sandwich panel, leaving between the base of the panel and the brass underneath the necessary space to avoid electrochemical corrosion that would undoubtedly occur in case of rainwater.

Repair of deep scratches

The external surface of a sandwich panel is not unlike human skin. If damaged, it is exposed to attack by the elements and, if not properly protected, begins to become infected (except in the case of plastic or fiberglass panels where, at best, they can give a bad aesthetic impression).

Obviously, repairing a scratch requires time and money. In any case, if we allow rust to form on the outer surface of the metal, it begins to oxidize very quickly to the point of rotting. At this point, the repair can be very expensive.

After having taken all the safety measures established for handling chemicals, it is possible to follow the following process to repair the scratches:

consult the user’s manual or service manual for the exact code that uniquely identifies the paint. Take sandpaper, a polish and a soft cloth (Fig. 5.5);

remove the polythene film, if present, and thoroughly wash and polish the area to improve paint adhesion. Lightly sand the area with sandpaper, taking care that the transition between the sanded part and the painted layer is gradual (Fig. 5.6);

if the scratch extends deep into the metal base, apply a coat of primer and, once it has dried, spread with sandpaper (Fig. 5.7);

Carefully apply a coat of paint once the primer has dried completely. The final coating should be slightly higher than the undamaged paint (Fig. 5.8);

leave the paint to dry for a few days. Then rub the area with abrasive paste, taking care not to damage the paint that has just been applied. Once the procedure is completed, the finish should exactly reproduce the original (Fig. 5.9);

use wax to remove any possible imperfections and to add a protective coating

Repair of superficial scratches

Smaller, shallower scratches, as well as larger, deeper scratches, are areas where corrosion is more likely to occur. In any case, the procedure to follow to correct this type of defect is much simpler. In fact, in this case it is sufficient to cover the scratch using a suitable polyester-based correction paint, as shown in Fig. 5.11.

Corrosion problems due to remaining metal powder

A major cause of corrosion problems on the metal surfaces of sandwich panels is the remaining metal dust that remains on the metal surfaces as a result of normal operations performed during installation (cutting, drilling…).

The method to be followed consists of using a soft cloth wrapped around a piece of wood with rounded edges. Wood is a rigid material and, at the same time, softer than, for example, steel or aluminum. After removing this dust using a soft cloth, wash the surface thoroughly using a solution of hot water and liquid soap.

Synthesis of the characteristics of the different insulating materials.

Thermal conductivity

The heat flow through PUR/PIR foams is mainly due to heat conduction through the gas contained in the cells that constitute their structure and to the closed-cell structure itself. Thermal conductivity does not depend exclusively on density in the field of practical applications. Humidity can also play a role, since water has a better thermal conductivity than dry air.

When comparing foams with closed-cell structure, the heat flow in mineral wool is mainly influenced by convection and conduction of air through the fibrous structure, which accounts for about 75% of the entire heat flow.

In PUR foam, the value of the thermal conductivity is:

  • 0.020-0.024 W/m °C, immediately after production
  • 0.024-0.030 W/m °C, long-term value

The thermal conductivity of polystyrene varies depending on whether it is expanded or extruded:

  • 0.035 – 0.040 W/m °C, for expanded polystyrene (EPS)
  • 0.025 – 0.028 W/m °C, for extruded polystyrene (XPS).

The thermal conductivity measured in mineral wool is practically constant in the density range of 60-150 kg/m3, and is equivalent to 0.033-0.034 W/m °C.

From what we have just summarized, it seems clear that PUR and PIR foams perform better than polystyrene and mineral wool as insulating materials. In fact, as shown in Fig. 5.12, which shows the different thicknesses of insulating materials capable of ensuring equal values of thermal conductivity, approximately 15 cm of polystyrene or 18 cm of mineral wool are necessary to provide the same insulation guaranteed by 10 cm of PUR or PIR foam.

When designing a building, it is important to carefully evaluate the insulating material to be used and the optimum thickness of the panel, in order to obtain the desired degree of thermal insulation.

In addition, the possibility of combining different insulating materials should be considered, according to the various requirements of the project.

Mechanical properties

The most significant mechanical properties for defining the resistance to applied loads of an insulating material are its tensile, compressive and shear strength. With respect to mechanical properties, it can be observed that PUR and PIR foams generally perform better than mineral wool when mechanically loaded. In fact:

the shear strength of PUR and PIR foams is undoubtedly higher than that of mineral wool;

the tensile strength of PUR and PIR foams is generally higher than that of mineral wool, although high quality mineral wools can provide tensile strength values comparable to those of PUR and PIR foams;

The compressive strength of PUR and PIR foams is generally comparable to that of mineral wool.

All these considerations justify the difference between the value of the ripple resistance of a panel with PUR and PIR foam insulation (~130 N/mm2) and a panel with mineral wool insulation (~100 N/mm2).

Since the curl stress of a sandwich panel identifies the value of the applied load at which the panel collapses (if subjected to a bending test), it is obvious that a panel insulated with foam and, therefore, characterized by a higher curl stress, is also able to ensure higher values of total support spacing for equal values of insulation layer thickness.

Fire behavior

As a consequence of their organic base, all foams are combustible. Fire performance can be improved by choosing suitable raw materials or special foaming processes, by using retarding agents or by injecting inorganic filler material.

In any case, the additives have, above all, the function of delaying the combustion process, but do not significantly influence the temperatures at which the foams begin to decompose chemically and ignite.

Polystyrene, both expanded and extruded, has the negative tendency to melt at temperatures just above 100 °C, resulting in its melting even before it reaches combustion. It also tends to form incandescent droplets.

It begins to decompose at approximately 300 °C, and ignites at immediately higher temperatures. When it burns, it releases smoke and carbon particles.

The main products emitted are carbon dioxide CO2 and styrene;

  • polyurethane and polyisocyanurate foams do not melt when exposed to fire (as is the case with expanded or extruded polystyrene), but form a carbon layer.
  • PUR B3 foam begins to decompose at 150-200 °C and becomes flammable at about 300 °C and releases dense smoke when burning.
  • Mineral wools with a low organic binder content can be considered practically non-combustible. The fibers do not burn; rather, they melt:
  • the glass wool fibers melt at 650 °C
  • rock wool fibers melt at 1,000 °C

Metallic paints

Key features

Metallic paints are composite paints whose most significant particularity is given by their shine: under direct illumination, the surfaces coated with this paint appear to have a kind of glitter enriched by a slight sparkle effect, with a color generally different from that of the background.

Because metallic paint is essentially composed of a substrate, a binder (resin) and flakes dispersed in the binder, the light it reflects is composed of three components:

  • light reflected by the substrate
  • light reflected by the binder surface
  • light reflected by the flakes

Of course, in addition to these fundamental components of light, it is possible to obtain various combinations with light: for example, a light reflected by the substrate can be reflected backwards by a flake and thus be reflected back to the substrate towards an observer.

In any case, any reflection substantially attenuates the light, while directly observed flakes are more intense and are perceived as sparks to the human eye.

The sparks come from the light reflected by the flakes directly toward the observer.

However, the light can suffer many reflections, which attenuates its intensity and makes the sparkle effect of the surface invisible.

Because the flakes do not cover the entire surface, the light they reflect seems to fluctuate, providing a certain texture to the painting. In other words, these paintings have an irregular weave, with uneven fluctuations in brightness and, in some cases, color.

Under direct illumination, the flakes act like a mirror on a sunny day; that is, you can see the reflections only with the correct orientation. If this orientation is slightly deviated, the observer’s eye may not perceive these reflections. These sparks, due to the angle, do not happen very often (e.g., every 100 flakes). This is why they are separated by a sufficient distance for the human eye to distinguish them.

When illuminated by diffuse light, the sparks are much less pronounced. In fact, it is possible to see almost all the flakes, but their brightness is low; that is, the sparks are weak and dense and, in this case, the eye cannot distinguish the flakes separately.

Therefore, we can conclude that the weave of the painting, while clearly visible under direct sunlight, almost disappears when illuminated by diffuse light.

Problems to avoid when metallic paints are used for building envelopes

  • During the production of sandwich panels, the external surfaces of the panels are obtained by profiling metal coils with the desired final characteristics for the cladding.
  • In any case, metal coils, even if painted with the same color, do not have exactly the same final appearance, since the appearance of a coil can vary significantly from one production campaign to the next. This variation is quantified by the delta E or ?E.
  • If the variation is low, the color difference will be barely perceptible to the human eye. For higher color variations, this difference may become more evident. This circumstance becomes more critical with metallic paints, since their high capacity to reflect direct light can accentuate the color differences of the coating.
  • For these reasons, the characteristics of metallic paint, which are favorable from an aesthetic point of view, can in some cases cause problems in the coating of building walls, since great care must be taken to prevent the walls of buildings from showing areas characterized by different shades of color.