Someone had asked what causes pitting in aluminum.
In simple terms, pitting is caused by deterioration / corrosion.
Here is a more detailed explaination of corrosion & pitting:
Corrosion
Chemical deterioration in metals is called corrosion. Physical damage to metal objects can leave them vulnerable to corrosion. For example, a scratch in tin plate leads to corrosion of the base metal.
Gold, silver and copper are the only metals that are stable enough to be found in their natural metallic state. All other metals are more stable when combined with other elements, to form oxides, sulphides, chlorides, carbonates or mixtures of these.
There is a natural tendency for most pure metals to return to their more stable, corroded states. Because of this, metal objects need to be protected from environmental conditions and pollutants which encourage corrosion.
In some cases—as the metal corrodes—the oxide film that forms acts as an insulating barrier, which slows the rate of corrosion to an acceptable level. Copper and aluminium (or aluminum if you prefer) are two metals in which oxide coatings form protective layers—called passivating layers.
Causes of corrosion
Moisture and oxygen are required for metals to corrode.
Chloride ions, which are found in common salt—sodium chloride—can speed up the corrosion rate, and are also capable of penetrating protective oxide layers.
Fats, oils and sweat can also enhance corrosion.
Acids formed when air pollutants combine with moisture will attack metals. Outdoor monuments and sculptures are particularly vulnerable to this type of attack.
Relatively un-reactive metals, such as copper and silver, can suffer significant corrosion if sulphide-containing materials are in the same environment as the metal.
These sulphide pollutants are usually associated with the breakdown of plant matter, and the decomposition of sulphur-containing proteins such as wool. A common pollutant is carbonyl sulphide.
Dust absorbs moisture readily. So areas with a build-up of dust can have quite high local humidity—higher than the surrounding environment. Dust will absorb pollutants and other reactive materials as well.
Corrosion also occurs if dissimilar alloys and metals come into contact with each other. This type of corrosion is called galvanic corrosion.
Under these circumstances, the more reactive metal or alloy corrodes while the less reactive metal is protected. For example, if iron and copper were in direct physical contact in the presence of moisture and oxygen, then the iron would selectively corrode while simultaneously protecting the copper.
The oxide film which forms on particular metals acts as an insulating barrier, slowing the rate of corrosion to an acceptable level. This type of passivation occurs with copper and aluminium.
Chloride ions and aluminium (aluminum) and its alloys
The presence of chlorides in aluminium alloys containing copper is a problem because chloride ions:
• penetrate the protective oxide coating;
• cause aluminium pitting corrosion; and
• promote localized copper corrosion from within the alloy.
As the copper corrosion products move to the surface, they interact with the aluminium corrosion products and form a blue-green, copper-stained aluminium hydroxide corrosion matrix.
The problem occurs when - as a result of electrochemical reduction by the parent metal - copper is re-deposited in metallic form on the surface of the alloy. The re-deposited copper acts as a cathode in a pitting corrosion cell.
The problem is to remove a relatively un-reactive metal deposit from the surface of a very reactive metal, while at the same time trying to remove the chloride ions.
One simple solution to the problem is to use a solution of ammonia and ammonium sulphate to wash away the chlorides and the metallic copper from the surface. This produces complex reactions, but is effective.
When a corroded sea plane float was treated this way, it took 12 months of steady soaking, scrubbing and cleaning to stabilize the corroded metal.
Someone had asked what causes pitting in aluminum.
In simple terms, pitting is caused by deterioration / corrosion.
Here is a more detailed explaination of corrosion & pitting:
Corrosion
Chemical deterioration in metals is called corrosion. Physical damage to metal objects can leave them vulnerable to corrosion. For example, a scratch in tin plate leads to corrosion of the base metal.
Gold, silver and copper are the only metals that are stable enough to be found in their natural metallic state. All other metals are more stable when combined with other elements, to form oxides, sulphides, chlorides, carbonates or mixtures of these.
There is a natural tendency for most pure metals to return to their more stable, corroded states. Because of this, metal objects need to be protected from environmental conditions and pollutants which encourage corrosion.
In some cases—as the metal corrodes—the oxide film that forms acts as an insulating barrier, which slows the rate of corrosion to an acceptable level. Copper and aluminium (or aluminum if you prefer) are two metals in which oxide coatings form protective layers—called passivating layers.
Causes of corrosion
Moisture and oxygen are required for metals to corrode.
Chloride ions, which are found in common salt—sodium chloride—can speed up the corrosion rate, and are also capable of penetrating protective oxide layers.
Fats, oils and sweat can also enhance corrosion.
Acids formed when air pollutants combine with moisture will attack metals. Outdoor monuments and sculptures are particularly vulnerable to this type of attack.
Relatively un-reactive metals, such as copper and silver, can suffer significant corrosion if sulphide-containing materials are in the same environment as the metal.
These sulphide pollutants are usually associated with the breakdown of plant matter, and the decomposition of sulphur-containing proteins such as wool. A common pollutant is carbonyl sulphide.
Dust absorbs moisture readily. So areas with a build-up of dust can have quite high local humidity—higher than the surrounding environment. Dust will absorb pollutants and other reactive materials as well.
Corrosion also occurs if dissimilar alloys and metals come into contact with each other. This type of corrosion is called galvanic corrosion.
Under these circumstances, the more reactive metal or alloy corrodes while the less reactive metal is protected. For example, if iron and copper were in direct physical contact in the presence of moisture and oxygen, then the iron would selectively corrode while simultaneously protecting the copper.
The oxide film which forms on particular metals acts as an insulating barrier, slowing the rate of corrosion to an acceptable level. This type of passivation occurs with copper and aluminium.
Chloride ions and aluminium (aluminum) and its alloys
The presence of chlorides in aluminium alloys containing copper is a problem because chloride ions:
• penetrate the protective oxide coating;
• cause aluminium pitting corrosion; and
• promote localized copper corrosion from within the alloy.
As the copper corrosion products move to the surface, they interact with the aluminium corrosion products and form a blue-green, copper-stained aluminium hydroxide corrosion matrix.
The problem occurs when - as a result of electrochemical reduction by the parent metal - copper is re-deposited in metallic form on the surface of the alloy. The re-deposited copper acts as a cathode in a pitting corrosion cell.
The problem is to remove a relatively un-reactive metal deposit from the surface of a very reactive metal, while at the same time trying to remove the chloride ions.
One simple solution to the problem is to use a solution of ammonia and ammonium sulphate to wash away the chlorides and the metallic copper from the surface. This produces complex reactions, but is effective.
When a corroded sea plane float was treated this way, it took 12 months of steady soaking, scrubbing and cleaning to stabilize the corroded metal.
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