The corrosion resistance and electrochemical behavior of two commercial nickel alloys, one a Nickel-Molybdenum alloy (N10675) and the other a Nickel-Chromium- Molybdenum (N06200) alloy were investigated in pure hydrochloric acid and HC1 solutions containing different levels of ferric ions. Oxidizing ferric ions are commonly encountered in various chemical processes, and they markedly affect the corrosion behavior of the alloys, even at ppm levels. It was of interest to use electrochemical techniques to compare the anodic behavior of the alloys since the classic immersion tests do not yield sufficient knowledge regarding the effect of ferric ions on their corrosion behavior. Standard polarization resistance and potentiodynamic polarization test methods were used. The results clearly showed that the electrochemical behavior of the two alloys was drastically affected by the addition of ferric ions in the acidic solutions. The Ni-Mo and Ni-Cr-Mo alloys were resistant to general corrosion in pure HC1 at ambient temperature; however in HC1 solutions containing ferric ions, the Ni-Mo alloy showed poor corrosion resistance, while the Ni-Cr-Mo alloy exhibited excellent corrosion resistance when the level of ferric ions was higher than 250 ppm, because of passivation.
Hydrochloric acid is one of the most important mineral acids with many uses, including pickling of steel, treatment of oil wells and chemical cleaning such as in the pharmaceutical sector. This acid is extremely corrosive and its aggressiveness can change drastically depending on its concentration, temperature and contamination by oxidizing impurities. Such oxidizing species are found in process streams and cleaning applications where a strong reducing acid is required, such as hydrochloric acid. In general, steels, stainless steels and copper alloys can not tolerate HC1, hence the need to use nickel alloys. 14 The outstanding resistance to hydrochloric acid of Ni-Mo (B-type) alloys such as HASTELLOY ® B-3 ® and Ni-Cr-Mo alloys (C-type) alloys as HASTELLOY C-2000 ® has been one of the main reasons for their commercial success in the chemical process industry. Yet, the difference between B and C- types of alloys is that the Ni-Mo alloys can be used at all concentrations and temperatures of the pure acid, even at the boiling points, whereas the performance of Ni-Cr-Mo alloys is strongly temperature-dependent, at medium and high concentrations. 6-8
On the other hand, B-type alloys were not designed to be used in acids with the presence of oxidizing species such ferric, cupric, dissolved oxygen and other multivalent metal (oxidizing conditions), because the presence of these oxidizing contaminants in solution tends to greatly accelerate the rate of corrosion attack, and C-type alloys are recommended. 9-13. This well known claim concerning the performance of (N10675) alloy and (N06200) alloy, in oxidizing conditions, was revealed mainly from studies using classic standard immersion tests. References dealing with the detrimental effects of oxidizing species on corrosion behavior of the nickel alloys, using electrochemical techniques, are almost non-existent. Recent work was conducted by Rebak 14' 15 concerning the influence of ferric ions on the corrosion behavior of nickel alloys in several reducing acids using both techniques. It has been established that the influence of ferric ions was stronger in hydrochloric solutions than sulfuric acid, for similar acid concentrations. In addition, as predicted by the chemical composition of the alloys, the (N 10675) alloy (Ni-28.5Mo) had the lowest corrosion rate in reducing conditions and the (N06200) alloy (Ni-23Cr-16Mo-l.6Cu) had the lowest rate in oxidizing conditions.
