Materiali da costruzione avanzati - Parte 1

IMMUNITY

pH Passivation

Passive film naturally firmed, we have a very thin continuous passive film that is directly in

contact with the aluminum, over this we have a thicker porous oxide layer, the total thickness is

around 10 nanometers.

Generally the formation of this passive film is very fast if we compared to the formation of the

patina of weathering steel.

The resistance to corrosion is in a range of pH that is around 5 and 8.

Aggressive environments:

- industrial atmospheres with high concentrations of acid gases (SO2)

- marine

Rain washing is beneficial This graph represents the lose of performance.

Rural and urban atmosphere are poor of pollutants

and so there are no problems to corrosion.

The most dangerous corrosion is the localized

corrosion due to the chlorides.

It is enough a small amount of chloride to give the

localized corrosion of the aluminum

Localized corrosion

Promoted by:

- chlorides

- galvanic coupling (e.g. copper)

After pit initiation —> propagation stimulated by acidification within the pit Also ground can give problem

This is the Pirelli Building, during the of acidification, so also in rural

60’ and 70’ the atmosphere is full atmosphere we can have strong

of sulphur dioxide due to the burn of corrosion.

oil. Strong problem related to the

acidification of the rain and problems

of pitting corrosion.

Superficial change in color of the windows frame, there is leakage, this happens

because when we use the detergent to clean the glass, this detergent sometime

has a very strong alcaline media and this create problems related to corrosion.

Also salt used during the winter on the street cam

cause pitting corrosion in motorbike elements

When we have corrosion related to pit we have a very high propagation of corrosion due to the acidification in the pitting.

We have also generalized corrosion due to the increase of alkalinity in the area close to the pit. Externally we have the

pH S

> dissolution of the passive film

OH OH-

SpH and around the pit there is a

white area.

On of the way to increase the resistance to corrosion is related to the anodizing.

Anodizing s generally performed by producers to control the passive film to have a better resistance, with the increase of the thickness and the

reduction of the external porosity.

Anodizing (increase of corrosion resistance)

Aluminum —> Anode (corrosion —> consumption)

Formation of a more thick and compact protective oxide film

Anodizing force the metal to work as an anode, for this reason the producer corrode the aluminium element and produce the oxide on the surface.

The film is generally invisible, we can consider during procedure to put some colored particles to color the aluminium element, the advantage is that

we have a color that is related to electrochemical procedure, and this color is more resistant than paint.

Film is generally transparent, but can be coloured

Process (generally used for construction elements)

- Anodizing: put the aluminum element in an acid bath

- 5-10% H2SO4

- ia = 120-180 A/m2, E = 14-22 V

- T = 18-21°C

- Time = 10-60 min

- Thickness = 3-25 µm —> s(µm)=0.3 i(A/dm2)xt(min)

The corrosion resistance: thickness at least 15 µm

- Colouring (if necessary)

a) In the whole, put the pigment during the production of the film

b) With pigment (dipping), put the pigment in the porosity

c) Electrolytic Coloring

In this way we can obtain a lot of different colors.

The different techniques have different resistance to corrosion, generally the bast one is the coloring in the whole but in this way we can not obtain

va lot of colors.

After coloring it is important to seal the porosity

- Sealing —> generally in boiling water, after anodizing, this procedure create a partially seal of the corrosion

Effects:

- improve corrosion resistance

- improve colour stability

- prevent deterioration of anodized layer

Sealing is fundamental, Anodizing promote a good adhesion of paint or adhesive. Because we can obtain a surface that is perfect for the

mechanical adhesion.

The cost of anodizing is probably higher than painting. For the design we have to consider that there are a lot of

aluminum alloys, generally the pure aluminum is not used because

of its mechanical performances, we can use the alloys.

There are a lot of alloys and there are problems related to the

correct selection.

In the table are reported the different alloys with their characteristics, the commonly use in the construction is the series 6000 that contains

Magnesium and Silicon, this have a good corrosion resistance, it is easy to work, has good mechanical properties like good tensile strength and it

can be colored easily.

The serie 7000 is generally used for the aircraft and aerospace applications, because has high mechanical properties.

For the automotive is used the serie 5000.

The most used in the construction is the 6061T6.

Design and maintenance

- Avoid water and dust traps

- Avoid galvanic coupling

- Periodic cleaning with water

- Avoid contact with acid/alkaline solution (detergent, cementitious material ...)

- Do not expose in chloride bearing environments (marine or de-icing salts)

It is very important to consider that aluminum is a material with low resistance to corrosion if there is not the passive film, or without stability of the

passive film.

Corrosion of aluminium is very fast because the nobility of this is very low and its Equilibrium potential is Eeq = - 1,6 V. For example the Eeq of

steel is -0,4 V.

It is important to clean the surface to remove all the pollutants and the chloride that are present on the surface because the problem of aluminum is

related to localized corrosion. In ISO 9223 and ISO 9224 are reported the information related to corrosion, the generalized corrosion is not a

very big problem, the problem is the localized one because there are a very fast penetration in the metals.

With localized corrosion there is a strong reduction if the cross section and so a reduction of the properties.

For this reason are non used the aluminum elements in marine environment because there are a lot of problems to localized corrosion.

Also we have to pay attention to clean the aluminum element, if we put the aluminum element in dishwasher we destroy the materials, in this

machine there is an high alcalinity and this destroy aluminum.

Also during cleaning of windows frame, if we use a strong detergent there is the risk to remove the passive film and so there is the risk to

corrosion.

Also cementitious materials are alcaline, in this condition we can have a fast corrosion of aluminum.

In the soil we can have the ground that is acid or in atmosphere we can have sulphur dioxide that can corrode the aluminum elements.

COPPER

Copper: 20’417’159 ton (in 2016)

Main producers: Chile, Perù, China, United States, Congo, Australia, Zambia, Mexico Canada, Indonesia, Russia

Price: 6-6.7 €/kg (Carbon steel: 0.4-0.46 €/kg)

The price of copper is very high respect to the carbon steel or stainless steel. This because

it is present in a lot of areas.

The typical color that we can obtain after some years of exposure of the atmosphere, this

color is due to a patina that is product naturally. The patina give the resistance to

corrosion, the copper is a material with high conductivity, this material can transmit easily

the heat, inside the copper panel there are insulation materials.

The advantage is the service life.

Copper has a very high nobility, Eo = +0,34 V vs SCE —> low corrosion rate

The advantage is related to the low corrosion rate, the other advantage is that in environmental condition closed to

the neutrality, copper produce also the passive film.

The melting temperature is around 1000 Celsius degree, no problem to obtain copper and copper alloys.

Formation of a patina of Cu CO3 . Cu (OH)2 Thickness < 0.1 mm

near the sea coast Cu CI2 . 3Cu (OH)2 Time for formation > 1 year

We can obtain different colors in the time due to the formation of the patina. The advantage is the

strong resistance in marine environment. Copper do not give problem related the pitting corrosion.

The pre-oxidized copper are available to obtain patina before the use, with electrochemical

procedures.

Corrosion rate - ISO 9224 Are reported indications of corrosion rate in different environmental condition. We can observe that

the corrosion curves of copper are very low related to the curve of carbon steel.

If we put carbon steel in C2 (rural atmosphere) that is the best environmental condition, we can see a

strong corrosion related to the corrosion of copper in CX that is the worst environmental condition

that we can have. Copper has high resistance to atmospheric corrosion, the risk is related

to the carbon particles (products of combustion).

Carbon particles create cathodic size, because the carbon particles have

nobility that is higher than the nobility of copper.

Carbon particles create cathodic reaction and near this particles there is anodic reaction, this create a localized corrosion.

This is a problem of the copper pipe use in building because during construction there

are welding operation that can create particles that go inside of the pipes.

If the carbon particles are not removed, there are high localized corrosion.

Common example of the use of copper and copper alloy in the architecture, generally we use this element

for its service life and the service life depends on the thickness.

Dezincification of brasses Typical selective corrosion related to the use of brasses, brasses contain zinc, we can have some grain

that contain an high amount of zinc, and low amount of copper, grains that contain high amount of

cooper and low amount of zinc.

For this reason if we have the elements with low copper and high zinc there is the dissolution of the grain,

because the zinc has a nobility that is lower than the nobility of copper. This is a kind of selective

corrosion.

Galvanic coupling If we put copper in contact with other metal generally the other metals suffered by galvanic

coupling because copper is the cathode and the other metals have lower nobility.

In the picture there are steel elements put in contact with copper laminate, there is a strong

corrosion of the steel elements due to the high nobility of copper.

TITANIUM

Titanium: 6’877’550 ton (in 2016)

Main producers: South africa, China, Australia, Canada, Norway, Russia

Price: ~ 8-18 €/kg (Carbon steel: ~ 0.4 €/kg)

It is not so used because of the high cost, the production of titanium is very very low.

The cost is 20 times higher than the cost of carbon steel, the high cost is caused by the complexity of production.

4th metals in the Earth’s crust (1st iron, 2nd aluminum, 3rd magnesium) but few deposits with a significant amount of minerals rich in titanium

oxides, titanium is present anywhere but in low amount.

The majority of titanium is used as oxide (about 95%) in the paint industry as whitener or filler