Corrosion exfohation

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Another approach involved the addition of a corrosion inhibitor. Sinyavsky et al. [4], using an addition of 10 g/L potassium dichromate to the standard EXCO solution, reported that the exfohation trends were the same as in the standard solution for all of the alloys examined (AA2124-T3, T8 AA2618-T6 and AA7175-T6, T76, T73), and because of less general corrosion the exfohation was easier to identify and to assess in the inhibited solution. 

Historically, various investigators in Europe have used hydrochloric acid-px>tassium dichromate solutions for testing the exfoliation susceptibility of all types of aluminum alloys. An example is the Russian Stanchtid GOST 9.904-82, Aluminium Alloys Methods of Accelerated Exfohation Corrosion Testing [22]. It should be noted, however, that hexavalent chromium (Cr ) solutions are considered undesirable for health and environmental considerations and have been prohibited in some countries. 

Electrochemical and Conductivity Tests—These are useful techniques that indicate expected exfohation corrosion performance but do not actually produce corrosion. Both the electrochemical corrosion potentials and the electrical conductivity are secondary characteristics that relate to the metallurgical condition of the metal and must be correlated to actual exfohation corrosion tests. 

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