Polarization resistance, aluminum alloys

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. 

Pure aluminum easily undergoes passivation, and the formed oxide layer has a significant resistance and strongly retards the polarization current flow. Only aluminum alloys are used for sacrificial protection. It is known that manganese and copper alloy additives cause a shift of the potential in the positive direction, while mercury, tin, and indium cause a shift in the negative direction. 

Anodic polarization of steels, aluminum alloys and copper alloys, decrease their resistance to fatigue crack initiation, whereas cathodic polarization increases the resistance. 

Yu et introduced a process for the formation of double layer REM conversion coatings on aluminum alloy A1 2024. The results of polarization measurements showed that the corrosion resistance provided by the double layer REM conversion coatings was superior to that of chromate conversion coatings (CCC). Heller et examined the corrosion performance, morphology and... 

The beneficial effect of the addition of different concentrations of CeClj on the resistance to corrosion of the aluminum alloy 8090 is shown in Fig. 3.4 (Davo, 2004). The polarization curves for AA8090 alloy in a 3.56wt.% NaCl solution with different concentrations of CeCl3 are shown. Addition of CeClj leads to a shift of the cathodic branch toward more negative potentials and lower current densities, revealing a reduction in the oxygen reaction rate and in the corrosion... 

The most reported application of eiectrodeposited silane sol-gel films has been the corrosion protection of metals. The corrosion resistance of the films is usually evaluated by polarization curves and electrochemical impedance spectroscopy (EIS) measured in corrosive media. Mandler and coworkers [73] reported the electrodeposition of TEOS, MTMS, and PhTMS films on aluminum alloys. They found that all the three eiectrodeposited silane films protected the aluminum alloys, and the PhTMS film provided the highest corrosion resistance due to its hydrophobic aromatic ring. Figure 12.20 shows that the eiectrodeposited PhTMS film performs five orders of magnitude higher in impedance modulus... 

Conversion coatings or surface passivations have been employed on the surfaces of metals and alloys for more than a century. Conversion coatings involve (but are not limited to) molybdenum, zirconium, chromium, aluminum, vanadium, phosphate, potassium, cerium, nickel and zinc-rich layers to increase the polarization resistance of the surface and hence decrease current and potential... 

Gehring (Ref 1) studied attack on aluminum by high-velocity seawater, using the polarization resistance technique to estimate metal removal as well as weight loss. Typical data for 5456 alloy show that following a plateau of calculated, constant metal-rmioval... 

The development of stainless aluminum has also been claimed for cerium-treated pure aluminum and aluminum alloy 6061-T6 Qess satisfactory behavior was obtained for aluminum alloy 2024-T3) (Ref 22). The treatment involves a 2 h exposure to three sqtarate solutions boiling 10 m M Ce(N03)3, boiling 5 m M CeQ3, and anodic polarization in the passive region in deaerated 0.1 Af Na2Mo04. Excellent corrosion resistance was found upon immersion of treated samples in 0.5 Af NaCl. Scratched surfaces also showed excellent resistance. No mention of salt spray testing of the cerium-base treatments was made, however. 

Mo are single phase, supersaturated solid solutions having an fee structure very similar to that of pure Al. Broad reflection indicative of an amorphous phase appears in deposits containing more than 6.5 atom% Mo. As the Mo content of the deposits is increased, the amount of fee phase in the alloy decreases whereas that of the amorphous phase increases. When the Mo content is more than 10 atom%, the deposits are completely amorphous. As the Mo atom has a smaller lattice volume than Al, the lattice parameter for the deposits decreases with increasing Mo content. Potentiodynamic anodic polarization experiments in deaerated aqueous NaCl revealed that increasing the Mo content for the Al-Mo alloy increases the pitting potential. It appears that the Al-Mo deposits show better corrosion resistance than most other aluminum-transition metal alloys prepared from chloroaluminate ionic liquids. 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

The corrosion resistance of stainless steels and nickel-based alloys in aqueous solutions can often be increased by addition of chromium or aluminum. " Chromium protects the base metal from corrosion by forming an oxide layer at the surface. Chromium is also considered to be an important alloying metal for steels in MCFC applications. Chromium containing stainless steel, however, leads to the induced loss of electrolyte. Previous studies done to characterize the corrosion behavior of chromium in MCFC conditions have shown the formation of several lithium chromium oxides by reaction with the electrolyte. This corrosion process also results in increased ohmic loss because of the formation of scales on the steel. Aluminum additions similarly have a positive effect on corrosion resistance. " However, corrosion scales formed in aluminum containing alloys show low conductivity leading to a significant ohmic polarization loss. 

A passive metal is one that is active in the Emf Series, but that corrodes nevertheless at a very low rate. Passivity is the property underlying the useful natural corrosion resistance of many structural metals, including aluminum, nickel, and the stainless steels. Some metals and alloys can be made passive by exposure to passivating environments (e.g., iron in chromate or nitrite solutions) or by anodic polarization at sufficiently high current densities (e.g., iron in H2SO4). 

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