Corrosion features alloys

Finally, pitting may be viewed as a special form of concentration cell corrosion. Most alloys that are susceptible to crevice corrosion also pit. However, many metals may pit but not show crevice attack. Further, although sharing many common features with concentration cell corrosion, pitting is sufficiently different to warrant a separate categorization. 

The oxychlorination reactor is a vertical cylindrical shell made of carbon steel with a support grid/air sparger system and internal cooling coils. Internal or external cyclones are used to minimize catalyst carryover. The reactor internal parts are made from corrosion-resistant alloy. The reactor has many design features depicted in Fig. 1. 

Undermining of Mg particles that may take place in a very severe corrosion process has been shown not to influence reaction (1.79) or (1.80) (Song et al, 1997b) either. Therefore, the hydrogen evolution phenomenon in an aqueous solution is one of the most fundamental and important corrosion features of Mg and its alloys. 

The rich literature on the corrosion of Ni-Cr-Mo alloys in GCR helium is briefly reviewed. Then, we report our preliminary results on the exposure tests of Haynes 230 in GCR representative conditions and we compare resistance of this material to the corrosion features of some reference Ni-Cr-Mo alloys. 

The objective of this section is to show by means of specific examples how the various crystalline defects and structural features described in Section 20.4 can affect the form, location and kinetics of the corrosion of metals and alloys. 

Previous considerations of pitting have been largely confined to metals and alloys that have a strong tendency to passivate, but since the pitting of copper has a number of unusual features it is appropriate to consider it in some detail. Reference to the potential-pH diagram for the Cu-H O (Section 4.2) system shows that in neutral solutions at the potentials encountered in oxygenated waters the stable form of copper is Cu O, and the corrosion resistance of copper thus depends upon whether or not the CU2O forms a protective film. 

The numerous metals and alloys used in practice show such a wide variation in response to various anions in acid and alkaline solutions that common features are difficult to discern and a basis for predicting corrosion behaviour is not very apparent. 

The distinguishing feature of the behaviour of the slow-rusting low-alloy steels is the formation of this protective rust layer. Corrosion in conditions where it cannot form is little different from that of unalloyed steel, although the particular alloying elements present will have some influence on the actual rate at which corrosion occurs. 

The effect of environmental variables upon the logarithm of velocity V5. K relationship has been examined for a few alloys in some conditions of heat treatment. While it cannot be certain that similar results would be obtained with all alloys, the results reported do show interesting features that may have points in common with all alloys. For an Al-Zn-Mg-Cu alloy (7075-T651) the stress-corrosion plateau velocity was a maximum in 5 m KI solution under potentiostatic conditions at -520 mV (v5. S.C.E.), reaching about 2 X 10 to 5 X 10 cm/s, whereas in 3% NaCl under open-circuit... 

Impure metals and alloys exhibit all the structural features and crystal defects of the pure meteils already discussed. In addition, however, impure metals and alloys exhibit many structures which are not observed in pure metals, and which, in many instances, have an extremely important effect on the properties, particularly the corrosion resistance. However, before dealing with the structure of impure metals and alloys, it is necessary to consider the concept of metallurgical components, phases, constituents and equilibrium phase diagrams. 

Consider first the corrosion of low alloy steel in HC1 per se, i.e. before the addition of organic inhibitors. As shown in Figures 1 and 2 for N80 steel in 15% and 28% HC1 at 65 C, Nyquist plots for steel in concentrated HC1 typically have only one distinct feature a single capacitance loop (a loop above the Z axis) with a hint of a second capacitance loop at lower frequencies. The low-frequency loop is more fully developed in 28% HC1 than in 15% HC1. Mass transport limitations are not evident except under extreme conditions, e.g. above 28% HC1 and 65 C. 

The general features described above for the metal dusting corrosion of Inconel 600 can also be extended to the other Ni based alloys except that the corrosion intensity decreases with increase of Cr content. The rate of corrosion is also a strong function of temperature. The maximum local metal dusting rate is plotted as a function of temperature in Fig.5. It is interesting to note that a maximum in... 

The most striking results were obtained with aehromatedZn-Fe alloy (0.3% Fe). It was confirmed that the particularly good corrosion resistance of this alloy is due to the properties of the chromating layer,which remained firmly attached to the metal surface until nearly all of the zinc alloy had been dissolved. This feature rules out chunk effects, so that a shift in the ratio of EC to CMT measurements from a little less than one to one half (over a long time and accompanied by an increase in corrosion potential by 120 mV) could best be explained as a result of a shift in... 

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