Aluminum long-time corrosion

The anodic oxidation of sheet aluminum has been used for a long time to protect aluminum against corrosion by a well-adhering oxide layer. Porous oxide layers are formed if acid electrolytes are used that can redissolve the aluminum oxide (mostly sulfuric or phosphoric acid). A compact oxide layer is formed at the beginning of the electrolysis (Fig. 20.3). Simultaneously, the current decreases, due to the electric resistance of the oxide. Subsequently follows a process in which the oxide is redissolved by the acid, and the current increases until it reaches a steady state. The electrochemical oxidation continues to take place with formation of pores. At the end of a pore, where it has the largest curvature, the electric field has its largest gradient and the process of redisolution is fastest. 

Gonzalez et al. [43] and Lopez et al. [44] investigated the atmospheric corrosion of aluminum with different layers of anodic films in eleven different environments with estimated levels of salinity. Electrochemical corrosion techniques and electrochemical impedance spectroscopy were used to estimate the corrosion rates. The results indicated that at critical levels of anodic film thicknesses, the aluminum could be protected for a long time even when exposed to high levels of salinity. 

Anodizing of aluminum provides long-term corrosion resistance and decorative appearance. Corrosion of the anodized film is induced by SO gas and depositions of grime, sulfates, and chlorides. These depositions promote corrosion since they tend to absorb aggressive gases and moisture, thereby increasing the time of wetness and decreasing the pH of the electrolyte at the interface between the depositions and the surface. 

PMS liquids are corrosion-inert substances. Under normal conditions and heated to 100-150 °C they do not cause corrosion and for a long period of time do not change in airflow when in contact with aluminum and magnesium alloys, bronzes, carbon and doped steels, as well as titanium alloys. PMS liquids do not change their properties under 100 °C in air for 200 hours in contact with the above-listed alloys as well as with beryllium, bismuth, cadmium, Invar alloy, brass, copper, mel-chior, solder, lead, silver. The stability of the properties of PMS liquids in these conditions is usually accompanied by the absence of metal and alloy corrosion, although the colour of the metal surface may slightly change. 

However, the corrosion resistance of aluminum as well as the durability of joints made with epoxy adhesives is very dependent on the type of aluminum alloy used. Bonds made with relatively corrosion-resistant 6061-T6 aluminum alloy will last about 4 times as long as equivalent joints made with 2024-T3 alloy when exposed to marine environments. 

Chromium(VI) oxide nitrate is a dark red liquid which boils at 63 to 65° at 0.7 mm. Hg pressure. It is soluble in carbon tetrachloride. In water, it reacts immediately to form chromic and nitric acids. It is a more powerful oxidizing agent than vanadium (V) oxide nitrate, and care must be taken to avoid contact with hydrocarbons. It is corrosive to most metallic surfaces, except aluminum, and reacts in the same manner as vanadium (V) oxide nitrate does toward paper, wood, and rubber. It cannot be stored for so long a time as vanadium (V) oxide nitrate but is relatively stable in a sealed ampul in the absence of light and moisture. It can be purified by distillation in vacuum over lead(IV) oxide. 

With galvanoaluminum coatings like zinc-nickel alloys, long duration times were reached. Regarding thermal and mechanical stress, the corrosion-protective effect of the zinc-nickel alloy layers is clearly inferior to that of galvanoaluminum. The short life span of IVD-aluminum deposits in comparison to galvanoaluminum is notable. This difference may result from the different microstructures of these two types of aluminum coatings [97, 156]. 

Work at Sandia National Laboratory has looked into the fundamentals of corrosion. Macroscopic results of experiments into the pitting of aluminum wire when exposed to sodium chloride solution indicate that the pitting potential is not a thermodynamic value but rather the potential associated with the kinetics of oxide breakdown. As a result, as a device becomes increasingly small, the probability of oxide breakdown will likewise decrease. At nanoscale a device made of this material would be highly unlikely to undergo oxide breakdown, and such a device would be expected to exhibit stability for long periods of time. 

CO2 is a slightly toxic, colorless gas with a pungent, acid smell. It will not bum or support combustion. The gas is 1.4 times heavier than air and sublimes at atmospheric pressure at minus 78°C. CO2 is not corrosive to steel, as long as it is free of water. With water it reacts to H2CO3, which can cause rapid corrosion to steel. Chromesteel or aluminum should be used if contact with water is unavoidable. CO2 will react violently with strong bases, ammonia and amines. 

The second method involves measurement of the temperature, time of wetness, amount of sulfur in the atmosphere, md the amoimt of chloride in the atmosphere. Temperature and hiunidity information can be used to estimate the time of wetness. This estimation is based on the percentage of time that the temperature is above freezing, O C, and the relative humidity is at the same time above 80 %. Once the time of wetness is known, it is then possible to determine a time of wetness class (Table 2). Sulfur dioxide content of the atmosphere can be estimated either by measurement of the concentration in the atmosphere over some period of time or by means of the sulfation plate or candle. This information is then used to develop a sulfur dioxide class or P class (Table 3). The chloride dry plate or wet candle method is used to obtain the chloride deposition rate of the atmosphere that is then converted to a chloride class or S class (Table 3). The corrosion class or C class can be obtained for the time of wetness, chloride, and sulfur dioxide classes (Table 4). Once the corrosion class is known, it is possible to estimate the corrosion damage that will occur in either short-term or long-term exposures for the five metals, steel, weathering steel, aluminum, copper, and zinc (Table 5). The detailed information on this method is discussed in Tables 2-5. 

---