Aluminum Corrosion resistance
Generally, aluminum and aluminum alloys have good corrosion resistance in the following environments: atmosphere, fresh and sea water, most soils, Most food and many chemicals. The expression "good corrosion resistance" means, that in most cases the product or parts made of aluminum alloys can be used without special surface protection in a long service life.
The degree of corrosion effects on aluminum product depends on the aggressiveness of the environment, in which it operates, as well as the functions and features of its life expectancy. If necessary, taking measures to prevent or limit exposure to the corrosion of aluminum and aluminum alloy, including, application of surface coatings, such as, for example, powder coating and anodizing.
The influence of alloying elements
Alloying elements aluminum alloys have an effect on their corrosion properties. It is therefore necessary to choose the most appropriate alloy for each of the corrosive environment.
The main alloying elements, which are used in aluminum alloys, They are copper, magnesium,, manganese, silicon and zinc, and unavoidable impurities industrial alloys - iron and silicon. The effect of, that these elements exert on the corrosion of aluminum and aluminum alloy, is as follows:
- copper reduces corrosion resistance greater, than any other alloying element;
- iron reduces corrosion resistance and its content must be kept low, if maximum corrosion resistance is required;
- magnesium has a positive effect on corrosion resistance;
- manganese has a slight positive effect on corrosion resistance;
- silicon has a slight negative effect on corrosion resistance;
- zinc in most environments little effect on corrosion resistance.
Corrosion deformable aluminum alloys
Aluminum (more 99,9 % Al)
Ultrapure aluminum (more 99,9 % Al) shows the highest corrosion resistance. It dramatically decreases with increasing impurity content, especially copper and iron.
Aluminum alloys of Al-Mn
Al-Mn alloys have very good corrosion resistance and are used outdoors without any protection.
Aluminum alloys Al-Mg
- Floats Al–Mg They have the best corrosion resistance among all wrought aluminum alloys.
- When the content of magnesium over 4 % Corrosion of articles made of these alloys are greatly influenced by manufacturing techniques.
- After prolonged heating at temperatures above 60 ° C alloys with a large content of magnesium are susceptible to stress corrosion and corrosion subsurface.
Aluminum alloys Al-Cu
- Alloys with a copper content of more than 0,25 % – all duralumin (duralumin, duralumin) have low corrosion resistance and are not used in aggressive environments of sea air or industrial atmosphere without protective coatings.
- Previously, one of the most common error was the use of duralumin in corrosive environments without adequate protective coating.
Aluminum alloys Al–Zn–Mg–Cu
- Alloys, containing zinc, magnesium and copper, for corrosion behavior similar to Al-Cu alloys and also require protection to corrosive environments.
Aluminum alloys are Al-Zn-Mg
- For this family of alloys of critical importance for the corrosion resistance of play technological methods, especially heat treatment, as well as the chemical composition of the alloy.
- Aluminum alloys are Al-Zn-Mg may be susceptible to stress corrosion cracking and subsurface corrosion.
Corrosion of cast aluminum alloys
For aluminum castings corrosion is typically much less of a problem, than, let us say, for sheet products, since the cross section castings typically much thicker and longer can prevent surface corrosion.
- Casting aluminum alloys Al–Mg typically have good corrosion resistance and are used in marine environments.
- Casting aluminum alloys Al–Si commonly referred to as alloys, that have good corrosion resistance in various media air and water.
- Casting aluminum alloys Al–Cu и Al–Si–Cu require a protective coating when the corrosive environment.
Rating alloys for resistance to corrosion
- The corrosion behavior of aluminum alloys depends on the type of medium, they must resist.
- Fig. 2 gives a relative rating of the corrosion resistance of various aluminum alloys in acidic and alkaline environments. The lower the number - the higher the rating.
Fig. 2 – Behaviour of Different Alloys in Acids and Alkalines