Aluminum corrosion rates

Figure 9 shows the relationship between chip size and aluminum corrosion rate As chip size increases, rate increases rapidly for compound A which is not a low thermal stress molding compound. On the other hand, rate does not increase so much for compound B which is such a compound. As the chip surface is overcoated by passivation film such as SiOi and PSG (phosphorus silicate glass) to protect against contamination and damage such as scar and cracks, this passivation film sometimes cracks and its apriori defects are developed by the molding stress. Such problems are also found in the periphery of the chip surface. 

Fluorine can be handled using a variety of materials (100—103). Table 4 shows the corrosion rates of some of these as a function of temperature. System cleanliness and passivation ate critical to success. Materials such as nickel, Monel, aluminum, magnesium, copper, brass, stainless steel, and carbon steel ate commonly used. Mote information is available in the Hterature (20,104). 

The addition of stabilizers to tetrachloroethylene inhibits corrosion of aluminum, iron, and zinc which otherwise occurs in the presence of water (12). Where water in excess of the solubiUty limit is present, forming separate layers, hydrolysis and corrosion rates increase. System design and constmction materials should consider these effects. 

FIG. 28-2 Effect of pH on the corrosion rate, a) Iron, (h) Amphoteric metals (aluminum, zinc), (c) Noble metals. 

Aluminum corrodes at a fairly low rate between a pH of 5.5 and 8.5 at room temperature. At concentrations between 50% and 95%, sulfuric acid causes rapid attack below 10%, corrosion is much less. Hydrochloric acid is quite corrosive in all but dilute concentrations. The corrosion rate in hydrochloric acid increases 100-fold as temperature increases from 50°F (10°C) to 176°F (80°C) in a 10% hydrochloric acid solution. 

Aluminum is resistant to nitric acid at concentrations above 80%. At 50% nitric acid concentration at room temperature, corrosion rates are as high as 0.08 in. (0.20 cm) per year. 

Organic acids—except formic, oxalic, and some chlorine-containing acids—do not appreciably attack aluminum near room temperature. In most acids, the corrosion rate increases slightly with flow velocity. 

Figure 8.1 Effect of pH on corrosion of 1100-H14 alloy (aluminum) by various chemical solutions. Observe the minimal corrosion in the pH range of 4-9. The low corrosion rates in acetic acid, nitric acid, and ammonium hydroxide demonstrate that the nature of the individual ions in solution is more important than the degree of acidity or alkalinity. (Courtesy of Alcoa Laboratories from Aluminum Properties and Physical Metallurgy, ed. John E. Hatch, American Society for Metals, Metals Park, Ohio, 1984, Figure 19, page 295.)...

Zinc is attacked at high pH. However, in weakly alkaline solutions near room temperature, corrosion is actually very slight, being less than 1 mil/y (0.0254 mm/y) at a pH of 12. The corrosion rate increases rapidly at higher pH, approaching 70 mil/y (1.8 mm/y) at a pH near 14. Just as in aluminum corrosion, protection is due primarily to a stable oxide film that forms spontaneously on exposure to water. High alkalinity dissolves the oxide film, leading to rapid attack. 

Aluminum alloys are corroded at both high and low pH. Not all compounds that increase pH cause severe attack. Ammonium hydroxide only moderately increases corrosion rates. Wastage actually decreases above a pH of 12 in ammonium hydroxide solutions (see Fig. 8.1). A caustic solution causes corrosion rates to increase substantially as pH rises. The Al ion reacts vigorously with OH to produce A102. ... 

Fig. 2-11 Relation between corrosion rate of pure aluminum at 25°C 1 M Na SO after [39] A 1.5 g L NaCl o tapwater (about 0.002 mol L Na" ) ...

Aluminum and stainless steel are used almost interchangeably for any strength of nitric acid. Figure 3.7 compares the rate of attack of cold nitric acid on stainless steel and aluminum. Figure 3.7 shows that higher rates of aluminum corrosion occur up to about 80% nitric acid (HNO3), but aluminum is still to be preferred over stainless steel for any concentration above 80%. 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

Fruit and vegetable juices packed with 21-26 in. of vacuum and stored in uncoated aluminum cans caused severe corrosion as shown in Table III. The corrosion rate brought about by the juices depends more on the nature of the organic acid present and the buffering capacity of the juice than on the total titratable acidity (11). The use of coated aluminum containers considerably minimized corrosion problems. Product control under extended storage conditions may be achieved by using specific chemical additives. However, more work is needed in this area before final conclusions can be reached. 

Data of corrosion rate of carbon steel, copper, zinc and aluminum together with different TOW and contaminants were statistically processed (stepwise regression) and the following results were obtained ... 

In most corrosion processes passivity is desirable because the rate of electrode dissolution is significantly reduced. The rate of aluminum corrosion in fresh water is relatively low because of the adherent oxide film that forms on the metal surface. A thicker film can be formed on the surface by subjecting it to an anodic current in a process known as anodizing. In most electrochemical conversion processes passive films reduce the reaction rate and are, therefore, undesirable. 

Inhibiting the corrosion of aluminum alloys by adding 1-5% of transition metals is a dramatic case of corrosion protection because of the small amounts of additives that are successful in reducing the corrosion rate by 1-2 orders of magnitude. It turns out that the alloying materials shift the pzc toward the positive side on the potential scale. Thus, in many practical situations, the alloys of the transition metals are in a... 

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