How nickel based alloys perform against localized corrosion
Nickel based alloys containing chromium, molybdenum, tungsten and other elements are commonly used for equipment production in the petrochemical, chemical process and power plants due to their supreme corrosion resistance in the oxidizing and reducing conditions. The availability of sufficient chromium content enhances the development of passive barrier oxide layer of Cr2O3 on the surface and molybdenum and tungsten seem to prevent the propagation of localized corrosion if the passive layer is breached. So the evaluation of the concentration of chromium, molybdenum and tungsten is the key aspect in developing NiCrMo and NiCrMoW alloys that prevent uniform and localized corrosion and stress corrosion cracking.
The passive layer characteristics of various NiCrMo alloys were evaluated at different potentials and temperatures in acidic media by using different electrochemical and surface characterization methods. Potentiostatic and potentiodynamic tests described that Hastelloy C22 has lower passive current density than Hastelloy C276. It was features to higher chromium concentration in Hastelloy C22 that results in the growth of a more secured Cr/Ni oxide barrier layer at the alloy oxide interface. The influence of chromium, molybdenum and tungsten on passive film characteristics and to some level, on localized corrosion were evaluated for the Hastelloy C24, C276, Inconel 625, Hastelloy C2 and C2000.
Alloys containing high chromium content showed superior passive film characteristics as compare to low chromium containing alloys. While enhanced passivity was normally featured to the alloy Cr concentration, Mo and W were noticed to segregate to the external layers of the layer and at high potentials when oxidation of Cr3 to C4 begins, were noticed to play a significant role in suppressing the onset of transpassivity. As a result, Mo and W have a major contribution in the control/ inhibition of localized corrosion in vigorous conditions. Alloys containing high chromium and molybdenum content such as Hastelloy C22 and C2000 offer higher localized corrosion resistance than grades C276 and C4.
Electrochemical and surface evaluation methods used to find the relationship between passive film impedance properties and composition in vigorous environments. The oxide layer properties of Hastelloy C22 bar were evaluated at different potentials in concentrated chloride solution. A direct relationship between film resistance and chromium oxide content of the barrier layer oxide film was noticed.
To understand the influence of chromium and molybdenum on the prevention of localized corrosion behaviour, crevice corrosion studies on Inconel 625, Hastelloy C276 and Hastelloy G3 were conducted. An increase in molybdenum concentration resulted in a lower passive current density. A crevice corrosion resistance ranking:
Hastelloy C276 > Inconel 625 > Hastelloy G3, depending on an increase in molybdenum concentration. The crevice corrosion sensitivity of Inconel 625 and Hastelloy C22 in an acidic solution is observed. The depth of penetration caused by crevice corrosion was higher for Inconel 625 as compare to Hastelloy C22. These alloys have a similar chromium concentration, so, the variation in the crevice corrosion behaviour was featured to their molybdenum concentration, higher molybdenum concentration in Hastelloy C22 results in higher crevice corrosion resistance.