Pitting or pitting
Pitting corrosion is the most common kind aluminum corrosion and aluminum alloys. It is often referred to as pitting. Ulcers are formed in the oxide film of aluminum, exposed to the atmosphere, fresh or salt water and other neutral electrolytes. Since in solution with high acidity or high alkalinity, an oxide film is usually not stable, the pitting corrosion occurs only at pH values ranging from 4,5 to 9,0.
Ulcers can be the smallest and concentrated or widely scattered and different in size depending on the chemical composition of the aluminum alloy, quality oxide film and corrosive properties of the medium. pitting phenomenon may be enhanced by the presence of cracks and aluminum contacts with other metals.
Morphology pitting corrosion of aluminum
It is known, that chloride ions are able to "break through" the oxide film. Aluminum chloride (AlCl3) are usually present in the solution inside the ulcer and its concentration increases with the development of corrosion or with a decrease in the humidity of the ambient air. A saturated solution of aluminum chloride having a pH value of about 3,5, so the bottom of the pitting corrosion continues until, while there still may migrate oxygen and corrosive electrolyte. The shape of corrosion pits can vary from a small saucer to hemispherical and beyond - to the deep circular cylindrical holes. This distinguishes pitting of intergranular corrosion, at which subsurface formed "tunnel" along the grain boundaries, usually visible only under a microscope.
Figure – Galvanic chemistry pitting
Rating resistance to pitting
Pitting corrosion resistance of aluminum alloys depends essentially on the degree of their purity. Pure aluminum has maximum resistance to pitting, and alloys of various series in terms of pitting corrosion resistance are arranged in the following order (in European and international designations): 1xxx - 5xxx - 3xxx - 6xxx - 7xxx - 2xxx.
Pure aluminum (99.00 % and cleaner) is more resistant to pitting corrosion, than any of the aluminum alloys. Rapid dissolution of the oxide film occurs only in highly acidic or highly alkaline solutions. At pH values ranging from 4 to 9 an oxide film can arise only light spots and local pitting. Ultrapure aluminum (99.999 % and cleaner) resists pitting corrosion much more successfully, than industrial purity aluminum.
Of all the industrial aluminum alloys 5xxx alloys (Al-Mg) with magnesium content not more than 3 % They have the best resistance to pitting corrosion and lowest propagation velocity ulcers, especially in sea water and aqueous solutions with a content of chlorides. Increasing copper content only to 0,2 % drastically reduces the resistance of these alloys to pitting.
Following on the resistance to pitting corrosion are 3xxx aluminum alloys (Al-Mn), such as 3003 (AMc) i 3004 (D12). Aluminum-manganese alloys in these particles have an electrochemical potential similar to the potential of aluminum. Therefore, with a low copper content (up to 0,05 %) alloys 3003 and 3004 pitting corrosion resistance are almost the same as that of pure aluminum. Increasing the content of copper increases the tendency for pitting corrosion in chloride solutions and copper content of about 0,15 % this effect becomes apparent.
Copper and iron for pitting corrosion of aluminum
Copper and iron have the greatest influence on the tendency for pitting corrosion of aluminum alloys. Although 6xxx alloys (Al-Mg-Si), for example, alloy AD31, They are generally prone to moderate pitting, their resistance to it can greatly decreased with increasing content of copper and iron, for example, raft AD33 (alloy 6061). Aluminum Alloys 7xxx (Al-Zn-Mg) и alloys 2xxx (Al-Cu-Mg and Al-Cu-Mn), especially those, that contain a lot of copper, have the least resistance to corrosion pittinigovoy. Therefore, the surface sheets from these alloys usually clad – Technical coated with a thin layer of aluminum, for example, aluminum brand AD1pl (99.30 %) by GOST 4784-97 or a special aluminum alloy containing about 1 % zinc (АЦпл) according to the same standard. 7xxx copper-free alloys (with or without manganese) show the best pitting corrosion resistance of all high strength alloys.
Preventing pitting corrosion of aluminum
Typical approaches to reduce or minimize pitting corrosion are based on the following principles:
- To reduce the aggressiveness of the environment, for example, the content of chloride ions, lowering the temperature level, acidity of active oxidising agents and.
- Carefully selected materials of aluminum constructions. for instance, possible to use aluminum alloys and the 5xxx 3xxx. High-strength aluminum alloys used only CLAD.
- Carefully design structures: avoid crevices, apply the circulation and mixing of fluids to prevent stagnation, ensure effective drainage of liquids designs.
Источник: Corrosion of Aluminum and Aluminum Alloys. Edited by J.R. Davis. – ASM International, 1999.