Aluminum corrosion examined

Fumeaux, R. C., G. E. Thompson and G. C. Wood. 1978. The application of ultramicrotomy to the electronoptical examination of surface films on aluminum. Corrosion Science 18 853-81. 

An irregular trough of metal loss is apparent along the circumference of the ring (Fig. 16.4). Metal loss is severe near the nozzle holes (Fig. 16.5). The corroded zone is covered with light and dark corrosion products and deposits. Analysis of these revealed substantial quantities of copper and zinc. Microscopic examinations revealed exfoliation of the aluminum ring in corroded regions. 

Panels of high strength aluminum alloy (7075-T6) were used in this study. The panels were approximately 10 x 3 x 0.032 inch (25 x 7.5 x 0.08 cm) in size. The test environments for coating evaluation were (1) a 5% NaCl spray (fog) chamber according to ASTM Standard Method of Salt Spray (Fog) Testing (B117-73), and (2) a modified 5% NaCl/S02 spray (fog) chamber with SO gas introduced periodically - ASTM Standard Practice for Modified Salt Spray (Fog) Testing (G85-84(A4)). In the latter case, a constant spray of 5% NaCl was maintained in the chamber and SO2 was introduced for one hour four tines a day (every 6 hours)( ). Coated test panels were examined for corrosion after one- and two- eek exposure periods. 

In a chemical plant nine aluminum alloys were tested for their resistance to corrosion. Four locations were chosen inside the chemical plant for this experiment and at each such place one plate of all nine alloys was placed. Exposure to chemical corrosion lasted one year. After the experiment four researchers examined the plates randomly and gave a mark from 1 to 10 to each plate depending on the observed resistance to corrosion. The experiment was aimed at asserting which of the offered aluminum alloys had the best resistance to corrosion at one or at all locations of the chemical plant. It was also interesting to see how much the researchers agreed or disagreed in their estimates of resistance to corrosion. Thus, the experiment had included three factors nine aluminum alloys, four locations and four researchers. Experimental results are shown in the following table ... 

Cold water at 2°C temperature is poured into the aluminum tube, taken away after three hours and the sample is then examined. Three samples are tested at once. In the case that none of them shows any sign of corrosion the testing is continued. 

The corrosion product plays an important role in the corrosion resistance of zinc, particularly in aqueous and certain atmospheric environments (cf. low-alloy or weathering steels). In such cases, the corrosion kinetics may be parabolic in nature, the rate decreasing with time (cf. aluminum), an aspect that has been extensively investigated. Taylor and Tolley [184] have examined the characteristics of sprayed zinc coatings and concluded that the decrease in permeability was... 

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

Inclusions of iron and lead may accelerate corrosion of zinc (HUbner et al., 1978) but the most damaging impurity is aluminum, especially in conjunction with certain other elements. Pelzel (1977) examined the relationships between the presence of aluminum and several other elements and the corrosion of zinc. The base material for these tests was zinc with 1.1% lead, to which various elements were added. Tensile specimens were exposed to both dry and moist air for 10 days at 95°C the resulting decline in tensile strength... 

Potentiodynamic polarization measurements are quite appropriate for determination of the pitting susceptibility of aluminum coatings, and/or the corrosion current density/ corrosion rate of coated steel products in general. ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, describes the calculation of corrosion rates and other information from electrochemical measurements. Another example of the use of DC electrochemical methods to examine the corrosion performance of coated sheet materials is a study by D. A. Jones et al. [48]. The study used polarization resistance measurements to examine the mechanism of steel and coated sheet degradation under conditions of alternate immersion. Jones compared the polarization resistance of samples of low-carbon steel, unpainted galvanized, aluminum-coated, and Zn-Ni alloy coated steel during continuous immersion and alternate immersion. Alternate immersion cyclic exposure produced a thick oxide that led to significant underfilm attack. Jones found that phosphate pretreatment tends to increase the resistance of these materials to underfilm attack. This study is an excellent example of the way electrochemical measurements can be used as a complement to other techniques to elucidate mechanistic information. 

SECM was also used to initiate pitting on steel and aluminum and to examine the pit growth and corrosion products (112a, 112b). In these studies, the UME tip was used to generate aggressive Cl ions in close proximity to stainless steel and aluminum surfaces. The tip and substrate current were then monitored to detect corrosion events. Fluctuations in the tip and substrate current were observed, which were indicative of the breakdown of the passive film and pit initiation on the metal. SECM and CV measuranents provided evidence that the large tip current fluctuations observed were caused by the reduction of Fe, which was released from the iron surface in the breakdown of the passive film of iron. 

In the early 1980s, new band pass filters were developed for use in mobile telecommunications systems. System performance degradation was observed when some of the filters exhibited inereased transmission loss. Examination of the internal surfaces of a cavity revealed extensive corrosion, especially near a tuning screw (Fig. 9-4). The cavities were constructed of aluminum,... 

Intergranular corrosion does not necessarily correlate with SCC in aluminum alloys. The 6000-series (Al-Mg-Si) alloys such as 6061 can suffer inteigranular corrosion but never fail by SCC. Examination of fiactuied specimens following intergranular corrosion shows crystallographic tuimeling attack on either side of... 

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