Aluminum Corrosion Protection

 

See. also Types of aluminum corrosion

Atmospheric corrosion of aluminum

Figure 1.1 – Atmospheric (general) corrosion of aluminum [3]

Factors atmospheric corrosion

Resistance aluminum and its alloys atmospheric corrosion depends on:

1) on climatic conditions, in which they are located:

• humidity;
• duration and intensity of rainfall;
• temperature;
• the number of sunny days per year;

2) on the degree of air pollution, that is, the concentrations:

• sulfur dioxide (SO₂);
• nitrogen oxides (NO_x);
• the amount and the chemical composition of the dust.

Climatic and degree of contamination of the atmosphere acting together. for instance, hung air contamination may reduce the critical value of the relative humidity, at which corrosion begins to develop. These factors can also have the opposite effect: rain increases the humidity in the air., but also washes away dust, and corrosion products, which may reduce the corrosion rate

relative humidity

The rate of atmospheric corrosion is dependent on relative humidity, and not just on the number or intensity of rainfall in the area. Rain is one, but not the only factor, which determines the relative humidity.

Relative humidity - the ratio between the actual water vapor pressure and a maximum pressure of water vapor at a given temperature. This ratio is expressed as a percentage.

At normal room temperature, air is considered:

• dry, if the relative humidity is not more than 30 %;
• normal, if the relative humidity is between 50 to 60 %;
• wet, if the relative humidity is higher 80 %;
• saturated with moisture, if the relative humidity of about 100 %.

In deserts and the arid zones of the relative humidity level seldom exceeds 10-20 %, whereas it is found mostly in temperate climates between 40 and 60 %. During the shower, it can reach 90-95 %, and during the tropical rains approach 100 % [2].

Dew point

The dew point is the temperature, at which moisture condensation will start. For a given level of humidity is the temperature, to which air must be cooled, to become saturated with moisture, and started her falling on nearby surfaces.

Duration moisture and sulphate electrolyte

Corrosion metals outdoors depends on the so-called wetting length and chemical composition of the electrolyte surface. Lasting moisturizing called period, during which sufficient moisture on the surface of the metal to corrosion. moistening duration typically determined, as the time, during which the relative humidity exceeds 80 % and, in the same time, on the metal surface temperature is higher 0 oC. Under these conditions at the metal surface can occur moisture condensation.

The critical relative humidity

Atmospheric corrosion of metals, including, aluminum, It occurs in thin films of moisture, which are arranged on the metal surface. There is a critical threshold for the relative humidity, below which aluminum and its alloys do not corrode. This is because, that lacks moisture in low humidity, to create a continuous electrolytic film on the metal surface. For a fresh surface-treated aluminum in the atmosphere is not contaminated, this threshold is about 66 % (drawing 1.2) [2].

Figure 1.2 - Influence of moisture on atmospheric corrosion of aluminum [2]:
curve 1 - a concentration of SO₂ near 1 %; curve 2 – without SO₂

No sulfates – no corrosion

In normal rural areas and atmospheres with a moderate degree of contamination of sulfates, Aluminum resistance to the environment to very high. Pitting (pitting) corrosion can occur on aluminum products in an atmosphere with a high sulphate content and high humidity.. Under such conditions may require aluminum corrosion protection.

chlorides

The presence of salts (especially chlorides) in the air reduces the durability of aluminum, but to a lesser extent, than for most other building materials. The maximum depth of corrosion pits is usually only a minor part of the thickness of the aluminum piece. In contrast to carbon steel strength properties of aluminum components, corroded, remain virtually unchanged.

Corrosion of aluminum in the soil

Corrosion behavior of aluminum in the soil - it is very important practical question. Electrical and telecommunication cables, water and gas distribution networks, as well as the base road signs, street lamps and various road constructions - all this is very often made of aluminum and aluminum alloys.

The acidity-alkalinity of the soil

Evaluation of corrosion resistance of metals, and including aluminum, contact pochvami, It is very difficult. Characterized Soil pH, which is closely related to the type and content of dissolved salts, the amount of carbon dioxide (CO₂), as well as possible contamination by industrial and domestic waste water. soil generally have an acidic pH in the range of 3,5 to 4,5. With depth from the surface of the earth, the pH value increases (figure 2).

Figure 2 - Dependence of the pH value in the soil on its depth [2]

 

The electrical resistance of the soil

Corrosiveness of the soil related to its electrical resistivity, which depends not only on the composition of the soil, but the water content and the concentration of inorganic salts . The figure 3 schematically shows the dependence of the corrosion rate of aluminum on the resistivity of various types of soils [2].

Figure 3 – The relationship between the specific electric resistance
soil and its corrosiveness [2]

corrosion of the aluminum shape in soils

Unprotected aluminum in soil can exhibit the following forms of corrosion [2]:

• pitting corrosion;
• galvanic corrosion (in contact with other metals);
• corrosion from stray currents.

Protection of aluminum in the soil

for aluminum, who works in the soil, most often used as a corrosion protection coating of the bituminous, and cathodic protection.

Aluminum in fresh water

Physical Chemistry of Water

Water is a strong solvent, which is able to dissolve:

• many inorganic and organic compounds,
• liquids, if they are polar and contain a hydroxyl group;
• gas.

Therefore, any water has a variable content:

• inorganic salts;
• dissolved gases;
• solids in suspension and
• organic substances.

However, not all of these elements affect the dissolved aluminum corrosion. The main effect on the corrosion of aluminum is exerted by those dissolved in water [2]:

• chlorides;
• heavy metal ions.

The effect of chloride concentration

Overall recognized, that among all anions, chloride ions have the highest ability to penetrate into the natural oxide film on the aluminum surface [2]. This is because, These ions are very small and very mobile. It is known, that chlorides, and fluorides, bromide and iodide anions are, which activate corrosion of aluminum in water (figure 4), whereas sulfates, nitrates and phosphates less activate such corrosion (figure 5.1) or do not activate it at all. Chlorides feature is, they may substitute the oxygen atoms in the oxide film of aluminum. This leads to a weakening of the resistance of the oxide film to corrosion [2].

Figure 4 – The effect of chloride concentration on the resistance to corrosion
alloy 3003 in fresh water [2]

Figure 5.1 – Effect of sulfate concentration on the resistance to corrosion
alloy 3003 in fresh water [2]

pit corrosion

In natural fresh water and tap water, aluminum can undergo pitting (pitting) corrosion (Figure 5.2). However, With regular cleaning and drying of the risk of serious corrosion is very small. aluminum pans, pots and pans, as well as the soldiers' aluminum bowls, spoons and cups served faith – the truth for decades without any signs of corrosion.

increases the likelihood of corrosion, if the water is stagnant, and aluminum is wet for extended periods.

Figure 5.2 – Pitting corrosion of aluminum [3]

Effect of copper

The presence of aluminum in copper alloys greatly reduces their resistance to corrosion. Such alloys are used only subject to reliable corrosion protection.

Aluminum in seawater

Chlorides in seawater

Usually seawater contains about 35 g / l of dissolved inorganic salts, of which chlorides are about 19 %. A related improved corrosion seawater Activity.

The quantity of sea water pH

The quantity of sea water pH near the surface of the seas and oceans is very stable and is about 8,2. This pH value is within the range of the natural oxide film stability. This explains the good resistance to corrosion of aluminum in seawater.

Aluminum alloys for seawater

In seawater, aluminum-magnesium alloys (AlMg) with a magnesium content of no more than 2,5 %. Of these alloys are manufactured hulls and other supporting structures. For deck superstructures, the corrosion resistance of aluminum alloys of the 6xxx series (AlMgSi alloys) is quite enough.

Aluminum in contact with concrete

The use of aluminum in the construction industry makes him come into contact with most materials, which are used in construction: concrete, gypsum, polymers, etc.. P.

Exposure to concrete

Aluminum is highly resistant to the effects of concrete and cement mortar, despite their high alkalinity with a pH value of about 12. When the concrete starts to set, always occur slight aluminum etching depth of not more 30 m. This impact, However, slows down after a few days of contact (figure 6). This leads to a very localized decrease in pH value to 8 units and the formation on the aluminum surface protective film of calcium aluminate.

Similarly concrete acts on aluminum alloy items. This enhances the adhesion between these materials. After that, how concrete will harden (dry), Corrosion is not usually occurs. However, there, where moisture accumulates and persists, can develop corrosion. The increased volume of corrosion products can cause cracks in the concrete.

aluminum protection from the concrete impact

Therefore, spray wet alkaline building materials, for example, mortar and concrete, leave the surface, but clearly visible stains on aluminum surfaces. Since these stains are difficult to remove, the visible aluminum surfaces must be protected, for example, on construction sites.

This type of corrosion can be effectively prevented by coloring an aluminum or bitumen paints, resistant in alkaline media. Because the oxide coating is destroyed in a strongly alkaline medium, anodizing the aluminum in this case is not suitable as a corrosion protection.

It is believed, that in dry atmosphere inside the building in contact with the completely hardened concrete aluminum requires no corrosion protection.

influence of chlorides

The concrete is usually added to the 3 % calcium chloride, to accelerate concrete setting, as well as prevent it from freezing in winter. This leads to a substantial reduction of the specific electrical resistance of concrete, as well as its ability to retain moisture. All this contributes to concrete corrosion activity.

Aluminum and chemicals

Due to the protective properties of the natural oxide layer, Aluminum exhibits good resistance to many chemicals. However, in an environment with low or high pH (less 4 and more 9) the oxide layer dissolves and, Consequently, Accelerated corrosion of aluminum. Therefore, inorganic acids and concentrated alkali solutions are highly corrosive to aluminum.

Exceptions are the above said concentrated nitric acid and ammonia solutions. They do not cause corrosion of aluminum.

In mild alkaline aqueous solutions of corrosion can be slowed by the use of silicates as inhibitors. Such inhibitors include cleaning solutions.

Most inorganic salts are not particularly corrosive to aluminum. Salts of heavy metals make an exception here. As a result, the reduction reaction of heavy metals on an aluminum surface, it can be subjected to severe galvanic corrosion. An example of such a heavy metal is copper.

Aluminum has good resistance to many organic compounds. Aluminum equipment used in the production and storage of many organic chemicals.

Aluminum and dirt

Mud, which accumulates the aluminum surface, may to some extent reduce its durability. This primarily relates to the aluminum surface, which undergoes hydration for extended periods. Inhibit the accumulation of dirt and drying of the surface creates conditions for corrosion. Therefore, depending on the degree of contamination aluminum constructions wash once or twice a year.

Sources:

1. Materials Corporation SAPA.
2. CORROSION OF ALUMINIUM /Christian Vargel, 2004
3. TALAT 1252