Stress-corrosion cracking mechanisms anodic current density

Numerous observations show that cracks often advance by a succession of rapid jerks, which is contrary to what one would expect if crack propagation is due to anodic dissolution at the tip. In addition, the transient propagation rates can be very fast, difficult to reconcile with anodic current densities at the crack tip even if these can be quite high as discussed above. Other mechanisms therefore contribute to crack propagation in these cases, a most likely one is crack propagation by successive brittle fractures of the metal. A case in point is hydrogen induced stress corrosion cracking (HISCC). 

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

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