Supplementary Section

Hot Corrosion Studies on Ni-base Superalloy at 650 °C under Marine-like Environment

Conditions using Three Salt Mixture (Na2SO4 + NaCl + NaVO3)

Venkateswararao Mannava1, A. Sambasiva Rao2, Neeta Paulose3, M. Kamaraj1, and Ravi

Sankar Kottada1*

1Department of Metallurgical and Materials Engineering, Indian Institute of Technology

Madras, Chennai – 600 036, India2Structural Failure Analysis Group, Defense Metallurgical Research Laboratory, Hyderabad

500058, India3Materials Group, Gas Turbine Research Establishment, Bangalore 560093, India

Total number of pages: 7

Number of Figures: 3

Number of Tables: 2

* Corresponding author. E-mail: ravi.sankar@iitm.ac.in; raviskottada@gmail.com Ph: +91 44 2257 4779, Fax: +91 44 2257 0545

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S1. Detailed literature survey on hot corrosion of superalloys

Considerable work on hot corrosion of Ni-base superalloys was carried out in the past

in order to understand the hot corrosion mechanisms at high temperatures above 750 C. For a

long time, it was believed that primarily Na2SO4 is the salt that degrades the microstructure of

the engine components above 884 ºC in both aircraft and marine environments [R1]. Thus,

very early work on this field started with deposition of only Na2SO4 at different gas mixture

ratios of SO2/O2. Seybolt [R1] opined that hot corrosion is a cyclic process (sulphidation

followed by oxidation) while detailed studies of Bornstein et al. [R2, R3] showed that hot

corrosion follows the fluxing model (basic or acidic fluxing) where reaction between molten

salts and oxides occur on the surface of the alloys. In basic fluxing, oxides react with O2-/or

Na2O when SO3 partial pressure is less. However, acidic fluxing leads to the dissociation of

oxides and forms cations and O2- ions. Goeble et al. [R4, R5] studied the hot corrosion of

B1900 Superalloy, Ni-Al and Ni-Cr alloys with Na2SO4 deposition and supported the fluxing

mechanism. Further, Gupta et al. [R6], Zhang [R7], Zhang et al. [R8] and Hwang et al. [R9]

extended the fluxing mechanism for individual oxides by studying the relationship between

Na2O ion activity and solubility of oxides. The following discussion provides a detailed

literature survey on hot corrosion behavior of different Ni-base superalloys.

For a long time, LTHC (<750 C) was not recognized as a deleterious hot corrosion

mechanism in superalloys. However, failure of gas turbine engine components in operation at

lower temperatures (<750 C) in marine-like environment has lead to the studies on hot

corrosion at lower temperatures (600 – 750 ºC) [R10]. Hocking et al. [R11], and Luthra [R12,

R13] worked on Ni and Co-base alloys and reported that Na2SO4-NiSO4 and Na2SO4-CoSO4

eutectics form at 671 ºC and 555 ºC, respectively in SO2 and O2 atmospheres. Thus, these

investigators concluded that presence of SO3 gas is necessary for hot corrosion to sustain at

low temperatures.

Table S1 presents the hot corrosion behavior of different superalloys in marine-like

environment in the temperature range of 630 – 900 °C. It was understood (Table S1) that

addition of NaCl (5wt.% or 20wt.%) to Na2SO4 enhances the oxidation rapidly without any

incubation period, and scale disruption sites act as pits for further oxidation. Moreover, SO3 is

the principal oxidant to occur during hot corrosion. Subsequently, Otero et al. [R21] and

Sidhu et al. [R22, R23] had shown that besides Na2SO4, V2O5 was also influential salt in

aircraft environment by showing the hot corrosion phenomenon of Ni-base superalloys at

temperatures ranging from 700 to 900 ºC. It was also concluded that formation of NaVO3

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accelerates hot corrosion. Later, Sidky et al. [R24] worked on Ni-base alloys in Na2SO4-

NaVO3 salt mixture environment and found that Cr-depletion is more in the presence of VO3-

melt as compared to SO42- melt at 630 – 780 ºC. Subsequently, the work by Deb et al. [R25]

on near composition of Mar-M246 with 60 wt.% Na2SO4 + 10 wt.% NaCl + 30 wt.% NaVO3

shows that Ni3V2O8 compound prevents the hot corrosion at 900 – 975 ºC.

Table S1. Hot corrosion behaviour of different Ni-base superalloys studied in marine-like environment using various salt mixtures at temperature >700 C

Author/year Alloy Salts in (wt.%) andtemperature ( °C) Environment Comments

McKeeet al.1978 [R14]

IN738IN713IN100

90%Na2SO4+10%NaCl at 75080%Na2SO4+20%NaCl at 85075%Na2SO4 +25%NaCl at 950

76% O2- 0.1% SO2-N2

50% O2 -1% SO2-N2

50% O2-1% SO2-N2

At 750 °C, addition of small concentration (1-10 wt. %) of NaCl to Na2SO4 enhanced corrosion rate. If SO2 or NaCl is absent, the corrosion rates are much lower.

Johnsonet al.1978 [R15]

Nimonic 90Nimonic

105, IN587, IN597

IN713C

NaCl vapour at 650-950 Air

NaCl is more aggressive at high temperature. Destroys the oxide scales rapidly, and enhances oxidation. Does not allow the incubation period to form oxides.

Santoro1979 [R16] IN617

0, 0.5, 2, 5, 10 % synthetic sea salt and 4 ppm Na2SO4 by weight in air.

Type A-1 fuel

Corrosion occurs by small amounts of salt depositing on gas turbine materials during heat-up following cooling cycle.

Misra1986 [R17] IN738 Na2SO4+50 ml%Li2SO4 at 750

Na2SO4-CoSO4 at 750 O2 - 0.12% SO2-SO3

SO3 is the principal oxidant, and CrS acts as a pit.

Kameswari 1986 [R18] Nimonic90 Na2SO4,NaCl,

Na2SO4+1%NaCl at 700-900 Air Addition of NaCl to Na2SO4 enhance the oxidation.

Nichollset al.1992 [R19]

IN738 NaCl at 700NaCl at 700

Air-0.2% (SO2 + SO3),AirAir-0.2% (SO2 + SO3),

Local scale disruption sites act as initiation for Type II hot corrosion.

Gurrappa1999 [R20] CM247LC

95% Na2SO4+5% NaCl90% Na2SO4+10% NaCl85% Na2SO4+15% NaCl80% Na2SO4+20% NaCl at 900

AirCorrosion rate is high with 5 or 20 wt.% of NaCl, than with 10 wt.%.

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Figure S1. The BSE micrographs of hot corroded samples of GTM718-3SM at (a) 60 h, and (b) 80 h, as seen on the top surface

Figure S2. SEM/ X-ray mapping of hot corroded samples of GTM718-3SM on their top surfaces (a) 40 h, and (b) 100 h

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Figure S3. BSE micrographs of the cross section view of hot corroded samples of GTM718-3SM after (a) 60 h, (b) 80 h, showing an increase in depth of the Cr-depletion layer with increasing duration of exposure

Table S2. Contaminant levels in various types of fuels and predicted composition of deposits [R26]

Types of fuels in gas turbine engine

Impurities (ppm) Deposits (mol.%)

S Na K V Pb Na2SO4 K

2SO4

PbSO4 V2O5 NaVO3

Contaminate distillated 1 2 - 1 2 75.2 - 24.8 - -

Residual oil 1 20 - 50 5 92 - 5.1 3.1 0.3Contaminate distillated without V

1 2 - 2 75 - - -

Coal derived oil0.11 4.8 1.5 0.4 84 15 1 - -

Coal derived low melting deposit - - - - - 53 12 - - -H-Coal boilergrade

1.5 5 16 - - 18.4 34.5 0.8 - -

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