Corrosion testing of nickel base superalloys and coatings with molten sulphates at 900°C

The work refers to the application of electrochemical testing procedures on the corrosion properties of different commercial nickel base superalloys. This included the following alloys: IN 738 LC, IN 939, Nimonic 81, Nimonic 90, IN 713 LC, IN 792 5 A, IN 100, IN 853, IN 617, Nimonic 105, B 1925, N 33 Hastelloy X, Mar M 246, Mar M 421, Mar M 432, IN 671, Nicrofer 7520, IN 800. Beside a short galvanostatic test with subsequent metallographic measurement of the resulting corrosion damage, potential/current density curves and potential/time curves were taken from selected alloys. In all cases the corrodent was a melt of (Na_0.9, K_0.1)₂SO₄ at 900°C. Thermogravimetric tests were run for comparison of three alloys (IN 738 LC, IN 939, IN 713 LC). The existence of ranges of passivation and induction periods could be established. The galvanostatic corrosion test was also applied to the testing of two different coatings (S 57 and LDC 2) on IN 738 LC and Mar M 246 or IN 100. In all cases scale compounds and intermetallics in coatings were analyzed by XRD. The electrochemical test for nickel base superalloys is a useful tool for a quick ranking of alloys. For the assessment of coatings more work will be necessary.