Pitting and Crevice Corrosion Resistance of Alloy UNS N07750 in Seawater

Abstract

Ni-based UNS N07750 alloy is a corrosion resistant Ni-Cr-Fe alloy, 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 UNS N07750 in seawater has been investigated in terms of pitting and crevice corrosion tendency.

For pitting corrosion resistance measurements, accelerated electrochemical potentiodynamic polarization technique was conducted to determine characteristic pitting parameters. Tests were done in normally aerated (no aeration/de-aeration being applied) synthetic seawater, at different temperature ranges (4, 10, 20, 30, 40 & 50⁰C), with a scan rate of 0.5mV/s. It has been found that at 20, 30, 40 and 50⁰C, alloy UNS N07750 suffered from pitting corrosion with pitting potentials of 493, 508, 444 and 444 mV_SCE(25⁰C), respectively.

This work has also shown that as temperature increases, pitting potential decreases to more active values. However, in terms of dimensions and number of pits, it has been observed that as temperature increases, number of pits decreases while their size becomes larger. On the contrary, previous investigations by Z.Szklarska-Smialowska,¹ and Rosenfeld, ² on iron and stainless steels, observed large number of pits with small size at higher temperatures. At lower temperatures (10⁰C and less), the alloy underwent transpassive dissolution rather than pitting corrosion. However, small pits were observed in the transpassive region, which has been also reported by many authors. ³

Critical Pitting Temperature (CPT) measurements revealed that CPT of alloy UNS N07750 in seawater lies between 11 and 20⁰C.

During potentiodynamic polarization measurements, ennoblement in corrosion potential with time was noticed, and no steady state corrosion potential was attained even when the test coupons kept in solution without polarization for more than 3 days.

Crevice corrosion tests were done by long-term exposure in stagnant artificial seawater for 30 days at two different temperatures (25 and 40⁰C). The alloy did not suffer from crevice corrosion under the tested conditions. This may indicate that the alloy under crevice tests performs better, compared to accelerated electrochemical pitting tests.