EFFECTS OF OXIDATION AND HOT CORROSION IN A NICKEL DISC ALLOY

1. 5th July, 2015 – JOURNAL CLUB EFFECTS OF OXIDATION AND HOT CORROSION IN A NICKEL DISC ALLOY A. Encinas-Oropesa1, G.L. Drew2, M.C. Hardy2, A.J. Leggett2, J.R. Nicholls1 and N.J. Simms1 1Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK 2Rolls-Royce plc., PO Box 31, Derby, DE24 8BJ, UK. Rudolph J. Kashinga 1

2. Mechanics of Advanced Materials Research Group AIM: To characterise oxidation and hot corrosion of Nickel base alloy, RR1000; using laboratory experiments

3. Mechanics of Advanced Materials Research Group Outline Introduction • Background • Experimental setup • Results Oxidation HotCorrosion • Experimental setup • Results • Oxidation • Hot Corrosion Conclusions

4. Mechanics of Advanced Materials Research Group Introduction Current high acquisition and life cycle costs in modern gas turbine engines demand Increasingly high levels of fuel economy Reduced NOx emissions and noise, and Reduced module weight. Therefore, high overall pressure ratios, and higher compressor discharge and turbine entry temperatures are inevitable. Hence, materials for the high pressure & temperature sections of the compressor and turbine are to be strong enough to stand the higher temperatures and stresses.

5. Mechanics of Advanced Materials Research Group Introduction Cont. ... Prolonged durations at high temperature in HP turbine discs result in surface oxidation and hot corrosion. Oxidation is known to reduce fatigue life in nickel disc alloys at temperatures exceeding 500ºC due to: Surface cracking of oxide scales Increased crack growth by higher propensity for transgranular and intergranular cracking. Hot corrosion is more severe in aircraft engines travelling near coastal, deserts and volcanic regions. High chloride salts and fine sulphate sands are ingested in the engines, combines SO2, to Na2SO4 which react with Ni alloys and result in erosion.

6. Mechanics of Advanced Materials Research Group Oxidation Experiments RR1000 is a γ’ precipitation hardened Ni base alloy developed by Rolls-Royce for disc rotor applications Nominal composition of 18.5Co-15Cr-5Mo-3.6Ti-3Al-2Ta-0.5Hf-0.03C-0.02B-0.06Zr wt.%, balance Ni. Isothermal oxidation at temp. of 700, 725, 750, 775 and 800ºC in a Setaram ‘Setsys Evaluation’ TGA system for durations of 200h. Cyclic oxidation testing was at 700 and 750ºC, via a resistance furnace and microbalance for durations of 100, 200, 500 and two at 1000hrs.

7. Mechanics of Advanced Materials Research Group Results - Oxidation Parabolic oxide scale growth was observed. 2= .t = Q ≈ 270 kJ/mol(256kJ/mol)

8. Mechanics of Advanced Materials Research Group Oxidised Surfaces/substrate

9. Mechanics of Advanced Materials Research Group Hot Corrosion Experiment ‘Deposit recoat’ method Samples coated with controlled quantities of Na2SO4/NaCl deposits were exposed to 700ºC For durations of 200, 300 and 500 hours in a gas stream of air-300 vpm SO2. These simulations give an acceleration of 5-10 times reality within the 500 hour test period

10. Mechanics of Advanced Materials Research Group Results – Hot Corrosion Damage was quantified by dimensional metrology of samples before and after exposure. Shallow/broad pitting damage, with the inner corrosion product/deposit layer rich in S and Cr, and the outer layer with higher levels of Ni and Co were observed. The cumulative probability, P(x(i)), of the ith largest maximum pit size is calculated from: Variation in metal loss data with salt composition and rate of deposition for RR1000. RR1000 shows little sample-to-sample variation. Given the steep curves, the data also indicate that there is a small variation in pit size within each sample.

11. Mechanics of Advanced Materials Research Group Conclusions Oxidation Damage Hot Corrosion Damage It has been found that hot corrosion in nickel disc alloys takes the form of broad, shallow pitting damage, which is typical of type II sulphidation. Levels of corrosion have been found to increase with increasing rate of salt deposition, Decreased levels with increasing Cl content in the salt deposit, which was varied between 95 and 98 % Na2SO4, were observed. Thin oxides were generated (parabolic wrt) in the majority of these exposures. Slowly growing Cr and Ti rich oxide scales formed on the surface of sample during oxidation in air at temp. btwn700 and 800°C. From FIB microscopy, oxide scale thicknesses were consistent with average values calculated from mass change data.

12. Mechanics of Advanced Materials Research Group End! Thank you for listening!