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Fig. IO. SEM photograph of a specimen with Corr. d. = 5, with corroded (black) and uncorroded (white)
regions. x 100.

of corroded areas in the stress corrosion crack (Fig. IO). Such uncorroded areas only occur due to
transcrystalline slips at those locations where the hydrogen has reduced the cohesive force between
the atoms of the metal at the crack tip or, for example, by the pressure mechanism in which
molecular hydrogen occurs with the recombination of atomic hydrogen. As the molecules of
hydrogen occur, high pressures of up to IO6 bar are released, resulting in the occurrence of local
microcracks which join together into macrocracks thereby leading to failure of the wire. The final
result of all these mechanisms of cracking is the occurrence of the already mentioned brittle regions,
with the cleavage appearance of the fracture surfaces.

3. CONCLUSION

The fractographic analyses of the fracture surfaces of the corroded prestressing wires have shown
that the brittle regions with a cleavage appearance can only be the result of the operation of
hydrogen at the crack tip. The occurrence of hydrogen is made possible by the presence of the free
chlorides and the corrosion reaction, which provides the electrons needed for the transformation of
H + ions into atomic hydrogen, which then migrates into the crystal lattice of the metal.

REFERENCES

I. Burdekin, F. M. and Rothwell, G. P., Survey of corrosion and stress corrosion in prestressing components used in
concrete structures with particular reference to offshore applications, pp. 8-33. Cement and Concrete Association,
Wexham Springs, Slough, U.K., 1981.
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2. Brachet. M.. ReDOrt on Drestressine steel : 5. Stress corrosion cracking restistance test for Drestressine tendons. FIP-
Technical Repor;, 1980. '
3. FIP Commission on Prestressing Steels and Systems. Report on prestressing steel. FIP/5/3 1976.
4. Bergsma, F., Boon, J. W. and Etienne, C. F., Heron, 1977,22,4&70.
5. Vehovar, L., Corrosion of high-strength steel in prestressed concrete containing calcium sulphide. United Nations-Steel
Committee, Seminar Geneva, 1984.
6. ASM International, Metals Handbook, Vol. 13, Corrosion, pp. 108-1 13, ASM International, Metals Park, Ohio, U.S.A.,
1987.
7. Foroulis, Z. A,, Environment-sensitive fracture of engineering materials, Conference Proceedings, Metallurgical Society
of AIME, Chicago, 1977, pp. 379406.
8. Troiano, A. R., Trans. ASM, 1960, 52, 54 1960.
9. Beachem, C. D., Metall. Trans., 1972,3,437.