Pitting, aluminum coatings

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. 

Aluminum coated steel is subject to pitting in seawater. The presence of copper in small amounts causes severe microgalvanic corrosion. 

Corrosion has been encountered infrequently to date and has been a surface type, as opposed to pitting corrosion that can result in perforations. Entrapped air in the beverage or in the cans headspace increases the corrosive action of the product according to Koehler et at (21). As with beer and other canned foods, aluminum ends provide electrochemical protection when combined with tinplate or tin-free-steel can bodies. The level of iron pickup is reduced while the amount of aluminum dissolved in soft drinks increases without detrimental effect. Aluminum containers with vinyl epoxy and vinyl organosol coatings are compatible with carbonated soft drinks. 

Uniform and pitting-type corrosion of various materials (carbon steels, stainless steels, aluminum, etc.) could be characterized in terms of noise properties of the systems fluctuation amplitudes in the time domain and spectral power (frequency dependence of power) of the fluctuations. Under-film corrosion of metals having protective nonmetallic coatings could also be characterized. Thus, corrosion research was enriched by a new and sufficiently correct method of looking at various aspects of the action of corrosive media on metals. 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

It has been recently shown that as-synthesized template-containing high-silica MFI coatings on aluminum alloys have superior corrosion resistance to chromate conversion coatings in strong acids, bases, as well as pitting aggressive and the in situ... 

Initial attempts to use aluminum for automotive trim were unsuccessful due to the corrosion behavior of the metal. It is therefore anodized for automotive trim applications to provide a protective oxide surface which acts as a barrier coating for corrosion pro tec tion. > 2 Aluminum and its alloys are susceptible to pitting and crevice corrosion in chloride containing environments. The corrosion resistance of anodized aluminum is therefore highly dependent on the quality of the anodized surface and the absence of scratches and other damage sites. 

Pitting of aluminum occurs at defects in the anodized coating due to poor process control or mechanical forces in service such as stone pecking.3,4 characterized by a central pit which is surrounded with a white halo of corrosion product. Pitting is more prevelant in high chloride containing environments. 

A second coating of nickel is then electrodeposited onto the mother disc. This nickel layer, the stamper master, is peeled off and used to stamp the data impression onto melted polycarbonate plastic discs. The polycarbonate disc now has all of the pits found on the original glass master disc etched by the laser. Because polycarbonate is clear, it is vacuum coated with a thin aluminum film to produce the reflective layer required by the laser. This delicate aluminum layer is covered with a protective layer of polycarbonate to prevent aluminum oxidation and marring of the data surface. The back of the CD can now be covered with information in the form of art and lettering. 

Chemical and Corrosion Resistance The corrosion resistance of CCCs depends on thickness and coating age. Corrosion resistance has been observed to scale with total chromium content [153]. Some studies have found that corrosion resistance does scale with Cr(VI) content [154], while others have found no such correlation [155]. Corrosion resistance is evaluated by continuous or cyclic accelerated exposure testing and electrochemical methods. On aluminum alloys, heavy CCCs will resist pitting for as long as 400 to 1000 h [156]. CCC-coated surfaces will exhibit total impedances of 1 to 2 Mf2 cm after exposure to aerated 0.5 M NaCl solution for 24 h. Such coatings can be expected to withstand 168 h of salt spray exposure without serious pitting [157]. CCCs usually perform well in mild neutral environments, but do not fare as well under... 

Polycarbonate is used for making compact audio discs, which are based on digital recording and playback technology and can store millions of bits of information in the form of minute pits in an area only which is read by the laser. Each track comprising a spiral of these its is laid in polycarbonate which is backed with reflective aluminum and coated with a protective acrylic layer. 

The structures that have evolved for ablative-mode optical discs make use of interference effects to minimize the reflectance (R) of the disc in the absence of a hole. A typical ablative-mode optical disc has the structure shown in Figure 5.51. The substrate is an optically transparent material such as polycarbonate, poly(methyl methacrylate), poly(ethylene terephthalate), or poly(vinyl chloride), topped by a subbing layer to provide an optically smooth (to within a fi-action of a nanometer) surface for the recording layer. A metal reflector (typically aluminum) is then incorporated next to a transparent dielectric medium such as spin-coated poly(a-methyl styrene) or plasma-polymerized fluoropolymers. This dielectric spacing layer serves both to satisfy the quarter-wave (2/4) antireflection conditions and to insulate thermally the A1 reflector from the top absorbing layer where the information pits are created. 

Voevodin N, Jeffcoate C, Simon L, Khobaib M, Donley M (2001) Characterization of pitting corrosion in bare and sol-gel coated aluminum 2024-T3 alloy. Surf Coating Technol 140 (l) 29-34... 

The welded decks of 10 barges, heavily pitted and totally perforated in places, were repaired in 1963-1964, sprayed with 250 pm zinc, and sealed with one coat of aluminum-pigmented unmodified phenolic resin sealer. At 3-4 year intervals, one coat of ordinary red oxide paint is applied for decorative purposes only. 

In prerecorded CD-ROM disks, the information is stamped as low reflectivity pits on a high reflectivity background. The disk has a reflectivity of 70% (achieved byusing an aluminum layer), whereas the pits have a reflectivity of 30% (these reflectivity specifications have been defined in the Red Book). The CD-ROM drive uses this difference in reflectivity to sense the information stamped on the disk. To be compatible with CD-ROM readers, a CD-R disk must also use this reflectivity difference when recording data. To accomplish this, a CD-R disk is coated with an organic polymer that can change its local reflectivity permanently upon sufficient heating by a laser spot. The structure of a CD-R disk is shown in Fig. 14.19. 

ASTM B 117 is the standard test used for evaluating the corrosion resistance of unpainted aluminum alloy surfaces when treated by anodizing, conversion coating, and for plating processes. Aluminum surfaces, which are anodized and then sealed for corrosion protection, must resist the salt spray environment for at least 336 h without pitting. Conversion... 

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