Alloy X-750 (UNS N07750) is a corrosion resistant Ni-Cr-Fe alloy. It is used in a wide variety of corrosive environments. Among these environments is seawater which is by far one of the most aggressive and complex environments. Performance of alloy X-750 in seawater has been investigated in terms of pitting and crevice corrosion tendency. For pitting corrosion resistance measurements accelerated electrochemical potentiodynamic technique has been implemented to determine the characteristic pitting parameters (primarily the pitting and repassivation potentials).Tests were done in normally aerated (no aeration/deaeration being applied) synthetic seawater at different temperature ranges (4 10 20 30 40 & 50°C) with a scan rate of 0.5mV/s. It has been found that at 203040 and 50°C the alloy X-750 suffered from pitting corrosion with pitting potentials of 493 508 444 and 444 mVSCE(25°C) respectively. This work has shown that as the temperature increases the pitting potential decreases to more active values which is consistent with other author's results. However in terms of pit dimensions and number interestingly it has been observed that as the temperature increases the number of pits decreases while their size becomes larger. On the contrary previous investigations by Z. Szklarska-Smialowska [15] and Rosenfeld [51] on iron and stainless steels observed large number of pits with small size at higher temperatures. The reason of discrepancy between these results is not so clear; however it might be due to different alloy being tested which might exhibit different response to temperature changes. The effect of temperature on pit size and number of alloy X-750 can be related to the conditions within the growing pits favor more growth of already formed pits to larger size and depth rather than forming new pits. Thus the formed pits will increase in size and provide protection to the surrounding to remain in the passive state. At low temperatures (10°C and less) the alloy underwent transpassive dissolution rather than pitting corrosion which was confirmed under optical microscope. However small pits were observed at the transpassive region. This has been reported by many authors [9]; and is more likely an indication that metastable pits occurred with subsequent repassivation and normal transpassive dissolution. Alternatively it can be due to the oxide film being destroyed and metal is attacked locally.CPT measurements revealed that the critical pitting temperature of alloy X-750 in seawater lies between 11 and 20°C. During potentiodynamic polarization measurements ennoblement in corrosion potential with time was found. The corrosion potential kept increasing in the positive direction with time and no steady state potential was attained even when the test coupons were kept in solution without polarization for more than 4 days. This ennoblement is thought to be due to passive film improvement with time by increasing in thickness and/or changing in composition. Crevice corrosion tests were done by long time exposure in artificial seawater for 30 days at two different temperatures (25 and 40°C). Interestingly the alloy did not suffer from crevice corrosion under the tested conditions. This may indicates that the alloy under crevice tests performs better than electrochemical accelerated pitting tests. The reason behind this can be due to the long time given for the alloy (in case of long term exposure crevice test) to develop the appropriate protective film that resists crevice corrosion attack. This can be supported by the relatively long time that was found to attain the steady state open circuit potential i.e. the passive film needs more time to improve and develop to the steady state thickness and composition. Correlation of PREN to the alloy resistance to pitting and crevice corrosion seems to be not applicable to alloy X-750 (with PREN of only 15.5) and thus it cannot be used as a ranking tool of alloy X-750 with respect to other stainless steels.
