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The stress corrosion cracking (SCC) behaviour of aluminium alloys has been studied for the past five decades and is still a research area of high interest due to the demand for higher strength aluminium alloys for fuel saving. This chapter brings out the general understanding of the SCC mechanism(s) and the critical metallurgical issues affecting the SCC behaviour of aluminium alloys. The developments made so far with regard to alloying and heat treatment of aluminium alloys for high SCC resistance are discussed. An overview of the available literature on the SCC of aluminium alloy weldments and aluminium alloy metal matrix composites are also presented.

In this work coatings were developed on the surface of AM50 magnesium alloy using four different electrolytes containing 10 wt.% each of K3PO4 and Na3PO4 in combination with either potassium or sodium hydroxides. Electrolyte conductivity and breakdown voltage were measured in order to correlate the property of the coating to the nature of electrolyte. Further, the coatings were examined using scanning electron microscopy for surface morphology and cross sectional investigation, X-ray diffraction for phase determination, and electrochemical impedance spectroscopy for corrosion resistance evaluation. The effect of employing different ions in the electrolytes results in different surface morphologies, chemical phases and, consequently, the corrosion resistance of the coatings. The EIS results indicate the presence of porous and compact layers in the structure of the PEO coatings, whilst the overall coating resistance mainly results from the compact layer, the role of the porous layer as a barrier against corrosion is negligible. Finally, a correlation between the passive current density of the bare alloy and the corrosion resistance of the PEO coating is proposed. (c) 2007 Elsevier B.V .

Publisher Summary Austenitic stainless steels are known to be prone to stress corrosion cracking (SCC), mainly due to their low stacking fault energy. This increases the probability of stacking faults on the surfaces, and also enables coplanar movement of dislocations during straining. This chapter evaluates the influence of low-temperature sensitization (LTS) on the degree of sensitization (DOS) in two grades of 304LN stainless steels with 0.12 and 0.15 wt.% N. A heat treatment at 500°C for 11 days is used to simulate LTS at 300°C for 100 years. The DOS of the heat-treated specimens is evaluated in double-loop electrochemical potentiokinetic reactivation tests. The constant strain U-bend specimens under annealed, sensitized, and LTS-treated conditions are used for stress corrosion cracking (SCC) tests. In the first SCC test, specimens are exposed to a boiling solution of acidified NaCl. In the second test, specimens are exposed to a re-circulating loop with oxygenated water at 280°C and 8.96 MPa for 1000 h. A similar test is conducted in high-purity water. The 304LN with 0.15 wt.% N is more susceptible to sensitization and LTS. For the comparison, the sensitized 304 stainless steel is also tested in high-purity water. While the sensitized 304 steel shows intergranular SCC, the DOS developed in the annealed stainless steels after the LTS treatment is not sufficient to make them susceptible to intergranular SCC.

Corrosion behavior of deformed galvanneal steel sheets has been evaluated as a function of their degree of deformation under four different strain paths namely, biaxial, uniaxial (drawing), plane strain and tensile mode. A correlation between the degree of deformation and the loss in the extent of corrosion protection offered by these coatings in 3.5 wt.% NaCl solution has been established by carrying out electrochemical studies such as potentiodynamic polarization and electrochemical impedance spectroscopy. An almost linear relation has been found to exist between the effective strain applied to the specimen and damage caused to the coating. While uniaxial, plane strain and tensile modes of deformation have been found to affect the coating corrosion resistance almost to the same extent, the biaxial deformation has been found to cause a greater damage to the coating., © Elsevier

The purpose of this work is to show that during slow strain rate testing (SSRT), the laboratory air might not be a non-corrosive atmosphere for evaluating the Iscc (stress corrosion cracking susceptibility index) of high strength Al alloys. Instead, it is proposed that glycerin could be effectively used as the non-corrosive atmosphere to find out the correct Iscc. This is demonstrated while examining 7010 Al alloy in the under aged condition., © Elsevier

The oxide films formed during early stage of oxidation at 800°C on dendritic and interdendritic regions of the cast Fe–16Al–1C (wt.%) alloy were studied using scanning Auger electron spectroscopy. Microhardness measurement and elemental depth profiles by Auger spectroscopy reveal that the carbide, Fe3AlC0.69, is the major constituent of the interdendritic region, while dendrites are predominantly Fe3Al phase. Between the two, the interdendritic region is found to be more prone to oxidation than the dendritic region, which was attributed to presence of carbides with low-Al content. In spite of the difference in oxide film thickness exhibited by both the phases, they consist of an inner aluminium oxide layer and an outer iron oxide layer., © Elsevier

Iron aluminides produced by the electroslag refining technique, having the compositions: (1) Fe–16Al–1C, (2) Fe–10Al–1C, and (3) Fe–8Al–1C were used to investigate the effect of Al on the oxidation behaviour of the Fe–1C–Al system at 700 to 1000 °C. Prior to oxidation studies, phase and microstructure of alloys were analysed. The carbide phase, Fe3AlC0.69, was found to be distributed in the Fe3Al matrix in alloy 1 and α (Fe–Al) matrix in alloys 2 and 3. The low Al content alloys displayed inversion in the oxidation kinetics below 800 °C, while, high Al content alloy displayed inversion phenomena at 1000 °C. The mechanism involving inversion in oxidation kinetics was found to be different in the two cases. In the former, it was attributed to the preferential oxidation of Al, while in the latter, to the phase transformation within the Al2O3. Carbides in the alloy having low Al content showed instability during oxidation., © Elsevier

Electrochemical corrosion behaviour of iron aluminides, produced by electroslag remelting technique, having the compositions (1) Fe–15.6Al–0.05C, (2) Fe–15.6Al–0.14C, (3) Fe–15.6Al–0.5C and (4) Fe–15.6Al–1C were investigated in 0.5 N H2SO4 media. Corrosion rates of these alloys were found to increase with carbon content. This was attributed to the preferential attack of the carbide phases. These alloys exhibited typical active–passive–transpassive behaviour. In addition, they displayed a secondary anodic current maxima during polarization. The resistance of aluminides to breakdown of passivity was assessed by varying addition of chloride ions in the same media., © Elsevier