Function of Nickel based super alloys in powerful acids media

However wide corrosion data is available for prolong functional stainless steels, although data still lacks for the high performance alloys. It includes Nickel based super alloys with relatively high molybdenum concentration of about 30% for the chemical industry and super alloys for the gas turbine plant.

The corrosion rates in different acids are noticed. As anticipated, the highest corrosion rate occurred in boiling 10% HCl acid, the most powerful reducing acid. Highest corrosion resistance by Hastelloy wiregrade B2 is seen in HCl acid. Unlikely, it showed minimum resistance in boiling 10% nitric acid. As already seen in other study, the concentration of molybdenum is a major factor in deciding the corrosion function in hydrochloric acid, whilst the concentration of chromium is decisive in nitric acid.

The extent of corrosion resistance of Hastelloy B2 is not severely affected by cold deformation and welding. For cold forming of about 50%, cold forming with after welding and cold forming subsequent to welding, the stable corrosion rate was below 0.25 mm/a.

Behavior of alloys with regards to types of local corrosion called pitting, crevice and stress corrosion cracking is investigated. The concentration of chromium and molybdenum concentration is crucial for the resistance to pitting and crevice corrosion. These types of corrosion occurred by chloride ions and the availability of oxidizing agents increases the effect. 10% FeCl3 solution is normally used to evaluate the pitting and crevice corrosion of stainless and chemically resistant steels. 

Although this test solution is not sufficiently extreme for nickel based alloys that usually contain large magnitudes of molybdenum. To evaluate the pitting corrosion of nickel based alloys, solution comprising of 7% sulfuric acid + 3% HCl + 1% FeCl3 + 1% CuCl2 was used to stimulate the conditions in scrubbers. In these test solutions and at the higher temperatures of 323K and 375K, Hastelloy C276 did not receive pitting or crevice corrosion due to high pitting index of about 70. It should be considered that at this stage in presence of crevices, crevice corrosion occurs before pitting. The materials active in crevice and pitting corrosion on the surface is prevented by cathodic protection by the anodic crevice region.

Incoloy 825 was also evaluated. The main objective to correctly determine the critical pitting corrosion potential using electrochemical methods because this potential allows a reliable comparison of the pitting corrosion behaviour of different material in a medium. The problem related with the early appearance of crevice corrosion on surface should be understood as it prevents the correct determination of the critical pitting corrosion potential. Besides, it is observed that the potentiodynamic quick test with a ramping rate of 1000mV/min that is usually used, does nt permit differentiation of the pitting corrosion specifically of the nickel based alloy.

The study aimed on potentiostatic holding analyses with the holding periods of 24 hours for every potential step of 100mV, measured against a saturated calomel electrode. The sample was evaluated for local corrosion after each holding period. For practical applications, there is a damage limit, irrespective of different types of corrosion. Hastelloy G is evaluated for 24 hours tests in five different solutions at 343K or 70oC. Nickel based alloys Inconel 625, Hastelloy C22 and Hastelloy C276 showed suitable performance.