EFFECT OF MULTI-FUNCTIONAL INHIBITORS ON THE ELECTROCHEMISTRY WITHIN A CORROSION CRACK

The electrochemical and mass transport mechanisms in stress corrosion cracking, which depend on the rate of metal dissolution and production of hydrogen, have been used to establish analytically the electrode potential distribution within the crack. When crack growth occurs by enhanced anodic dissolution of the plastically strained tip, the electrode potential at the crack tip always is more active than at the crack mouth because of the electric potential gradient which exists in the electrolyte within the crack. This also gives rise to additional or alternate electrochemical reactions such as hydrogen evolution and anodic dissolution at the crack tip. Furthermore, because of the potential difference from the crack mouth, the electrochemical driving force becomes more favorable for the development of corrosion inside the crack.; The analysis predicts the distribution of electrode potential within a crack, and theoretical results have been compared with experimental measurements recorded from a model electrode system. Under free corrosion, a small potential difference may cause a concentration change of Cl('-) ion and increase the chloride attack. In order to reduce the chloride and hydrogen attack, multifunctional inhibitors, such as borax-nitrite with small amounts of surfactant such as MBT or amino-methyl-propanol, are excellent inhibitors. The surfactant interferes in the dissolution reaction and blocks active chloride ion and hydrogen ion by interacting synergistically with the passive film produced by the borax-nitrite, which results in development of a stronger and thicker protective film.