This program has its primary objective: the development of hot corrosion life prediction methodology based on a combination of laboratory test data and evaluation of field service turbine components which show evidence of hot corrosion. The laboratory program comprises burner rig testing by TRW. A summary of results is given for two series of burner rig tests. The life prediction methodology parameters to be appraised in a final campaign of burner rig tests are outlined

Fryxell, R. E.

Leese, G. E.

English

NASA Technical Reports Server

Nss- 1 1s.o :
Effects of Surface Chemistry on Hot Corrosion Life*
R.E. Fryxell
General Electric Company
and
G.E. Leese
TRW Inc.
This program has as its primary objective the development of hot corrosion
life prediction methodology based on a combination of laboratory test data and
evaluation of field service turbine components which show evidence of hot
corrosion. The laboratory program comprises burner rig testing by TRW. The
discussion will summarize the results of two series of burner rig tests and
outline the life prediction methodology parameters to be appraised in a final
campaign of burner rig tests.
The two completed burner rig hot corrosion tests were performed under
identical conditions, as given in Table 1. Specimens included U700 and
Rene'80, uncoated and with the following coatings.
RT21 aluminide (Chromalloy American Corp.)
Codep aluminide (General Electric Co.)
Ni23CoI8CrI2AI0.3Y applied by vacuum (low pressure) plasma spraying
At approximately twenty cycle intervals, specimens were visually examined,
photographed, and coil inductance measurements made in a series mode at 10MHz
with a multifrequency LCR meter. These measurements, which are patterned
after studies at NASA-LeRC, afford a non-destructive means of following the
course of metal degradation. Ultimately the inductance data bank from this
program will be useful in the interpretation of the extent of oxidation/hot
corrosion phenomena, in particular subtle differences between materials.
Specimens were considered to have failed when visual evidence of hot
corrosion was noted in three successive inspections. Subsequent
metallographic evaluation has confirmed substantial Type I sulfidation for all
the uncoated specimens and, in most cases, coating penetration for the coated
specimens.
In the first series of hot corrosion tests, duplicate as-fabricated
specimens were exposed. In the second series of tests, specimens (coated
alloys only) were exposed which had been preaged at II00C under a variety of
conditions to determine the effect on hot corrosion behavior caused by surface
oxidation and/or interdiffusion between coating and substrate alloy. The
aging conditions were:
* Contract NAS3-23926
391
https://ntrs.nasa.gov/search.jsp?R=19880001798 2020-03-20T08:33:58+00:00Z
o Onehour cyclic, static air I00 hours
o One hour cyclic, burner rig I00, 300, 600 hours
o isothermal, static air 100, 300, 600 hours
o isothermal, vacuum 100 hours
Triplicate specimens were aged; one withheld for metallographic evaluation and
the remaining duplicates exposed in the hot corrosion tests.
In the baseline hot corrosion test, results in general were consistent
with previous experience.
Uncoated U700 corroded at about twice the rate of uncoated Rene'80,
paralleled by a similar difference in coil inductance changes as
reported at the 1984 Workshop.
o Coatings lives were similar on both substrates, i.e.
- 424 to 602 hours for Codep coated specimens.
- > i000 hours for NiCoCrAIY coated specimens.
On the other hand the RT21 coating which is compositionally similar to Codep
survived longer than Codep in this test: the coating was failed in two
specimens at 697 and at 1005 hours, but not completely penetrated in two other
specimens at 997 and at 1005 hours. A difference between these coatings also
was apparent in the coil inductance changes as was reported at the
1984 Workshop. Detailed electron microprobe analyses failed to reveal any
significant compositional differences between the two coatings and both have
the structural features of inward aluminide coatings. However the RT21
coating showed some evidence of entrapment of small oxide particles in the
outer 5_m while the Codep coating did not. Whether this difference is the
cause of differences in hot corrosion lives cannot be concluded with
certainty; yet it may be noted that, as will be shown, no consistent
difference in life exists for specimens preaged at II00C.
Before discussing the results of the second series of hot corrosion tests
some comments are required about preaging. In furnace exposures even up to
600 hours there was frequently localized pitting but no examples of coating
failure. However in burner rig preaging localized pitting was significantly
more severe and approached penetration in a few instances even at i00 hours.
For the longer exposures coating penetration was often observed. Since
coatings on the specimens preaged in the burner rig for 300 hours or more had
essentially failed during that time, their hot corrosion lives are of minimal
value and will not be discussed.
Limiting the results in this manner it became clear that preaging caused
substantial decreases in coatings lives relative to the baseline results cited
above, particularly for the aluminides (RT21 and Codep). What is most
striking (see Figure I) is that for all three coatings life degradation is far
more severe for coated U700 than for coated Rene'80. This cannot be
392
attributed to metal recession during the preaging which was minimal in i00
hours in air and zero in vacuum. It is interesting to note that on average
the consequences of preaging in vacuum and in air appear to be similar. This
suggests (as will be discussed below) that compositional changes resulting
from coating/substrate interdiffusion predominate over surface oxidation in
affecting subsequent hot corrosion performance.
As shown in Figure 2 coatings lives for some coating/substrate systems
degrade progressively from i00 to 600 hours preoxidation; while for other
systems maximum degradation has occurred in I00 hours preoxidation.
In an attempt to explain the large and consistent differences in the
coated U700 and Rene'80 specimens, quantitative electron microprobe analyses
of coatings were obtained on selected specimens in the as-preaged condition
(prior to hot corrosion testing). Within measurement error, Codep and RT21
compositions were indistinguishable for a given preage time as were those at
i00 hours oxidation whether isothermal, furnace cyclic or burner rig cyclic.
Data thus consolidated are given in Table II.
For both aluminide and NiCoCrAIY coatings there are no apparent major
differences between the U700 and Rene'80 systems.
For both aluminide and NiCoCrAIY coatings there are no significant
differences between vacuum and air preaging, reinforcing the above
comment that interdiffusion effects are far more important than
surface oxidation in determining subsequent hot corrosion coating
life.
The composition changes for the aluminide coatings are much greater
(especially AI content) than for the NiCoCrAIY coating. This
parallels the differences in hot corrosion life shown in Figures I
and 2. Note also coil inductance changes (Figure 3) for cyclic
oxidation preaging which parallel the lesser composition changes for
the NiCoCrAIY coated specimens. Here again there is no hint as to
reasons for differences in the coated U700 and Rene'80 systems.
The data in Table II represent average compositions of the coating
additive layer. Another aspect to consider is the makeup of the diffusion
zone (in particular for aluminide coatings) since prior experience suggests
that while beta NiAl corrodes rather rapidly, the rate slows down when the
diffusion zone is reached. Conceivably the makeup of the diffusion zone is
equivalent in a corrosion sense for aluminide/U700 and aluminide/Rene'80
systems in the as-fabricated condition but changes in different manners (or
rates) in the IIOOC preaging. There are, in fact, visual differences after
aging which did not exist before aging. Before aging, the diffusion zone in
both systems contains a closely spaced needle like phase (sigma) in a beta
matrix, oriented normal to the surface. As aging at II00C progresses this
393
ORIGINAL PAGE LB
OF POOR QUALIT_/
phase gradually disappears and a blocky phase (carbides) forms. The rate of
this transformation is considerably faster for the U700 system and is nearly
complete in i00 hours. Further investigation of these differences is planned
and will be factored into life prediction modeling in the remainder of this
program.
The rationale for life prediction modeling and confirmatory hot corrosion
testing i_ as follows. Since in the mission of an aircraft engine, a small
percentage of time is spent at turbine temperatures considerably higher than
cruise (takeoff and thrust reverse, analogous to the iI00C aging in this
program), coating/substrate interdiffusion with consequent accumulative
decrease in remaining coating life can take place. A series of age treatments
followed 5y hot corrosion tests will provide input for one element in mission
analysis/life prediction. For an aircraft flying short missions, high
temperature operation will accumulate faster in terms of total flying hours
and hot corrosion coating life would be expected to be less based on the
results $iscussed above. Such a correlation was qualitatively apparent in the
analysis of field service turbine components performed earlier in this
program_ as reported at the 1984 Workshop.
It is clear that a life prediction model must include not only mission
analys_s and coating identification but substrate identification as well.
To hav_' _,Laximum opportunity to vary aging times (and test conditions)
_;ithin the _cope of planned tests, the remaining burner rig tests will be
limited to Co_ep coate& U700 and Codep coated Rene'80. Aging will be limited
to isotherma_ oxidation. In the first of these tests, now underway under the
eondltion_ given in TaDle I, preaging t_mes of 15 and of 25 hours are
included,, Evaluation will include detailed electron microprobe analyses of
as-aged specimens and fl_rtber consideration of possible differences in the
diffusion zone between the two substrate systems. Further tests will include
add_t_ona) eging times and/or d_fferent salt levels.
T_hle 5urner Rig Operating Conditions.
Specimen Temperature - 900 _ C ÷ 9" C
Test Cy_:le "- _ tLo_r at temperature fo]low_d by 6 minu_es of forced
ai_ r_olin_
_@i,m* C6_cent_ _t_oz - 0.5 pp_ _cdium _i_I0%} in the combustion gases
_ntroduced as aqueous NaC1
_owbus£_ Aiz F_,:_af: Tem_eratar_ -" 2_Z" C * iO" C
:pecimeD_ - _gh? positioned equally on a 4.2 _m (1.64 _nch) diameter
circle of e _o]der rotating at 600 rpm
Buzn_r W_,zzl_ _o_t tim me<or - 2.54 cm (I.0 inch)
_urner Pressure - _ 0 Dsi_
_ozzlt _xc_ _ _eerc_[ Spec_me_ - _._5 cm (1.75 inch)
394
TABLE II Average Compositions of Coating Additive Layer
as Function cf Aging Time at I!00C (Weight Percent)
Alum!nide
Rene'80
U700
NiCoCrAIY
Rene'80
U700
AI Cr Mo Ti Cc NiARe , Hours
0 o 31 5 0.6 0 7 6 _o
I00 Vacuum 2 18 7 0.4 3.8 9 62
i00 Oxidation 2 18 6 0.2 3.4 9 63
0 2 31 5 0.6 0.8 !0 52
i00 Vacuum 1 17 7 0.6 3.1 12 61
i00 Oxidation 3 18 7 0.6 2.9 13 61
0 1 13.0 20 0.2 19 43
i00 Vacuum 1 ii.0 14 0.7 1.7 19 53
i00 Oxidation 2 11.3 16 0.6 1.8 17 54
600 Oxidation 1 7.8 19 1.3 2.5 16 54
0 i 12.0 20 0.4 21 44
!00 Vacuum 0 Not Analyzed
i00 Oxidation 2 12.8 15 0.3 1.6 19 54
300 Oxidation i 7.4 17 1.9 2.6 19 55
600 Oxidation ! 7.5 !7 1.9 2.2 19 54
_Entries in this column are numbers of specimens analyzed.
%
C)
_t
g
c_.
C.)
QO
O
(-9
tOO0
8OO
6oo
4O0
2OO
_] VA CUUt.!
_ CYCLIC IN AIR150_HERHA[ IN AIR
BUR:_ER RIG
U,700 U/00
CODEF' RT21
A A
7
!Ill
U700 RENE 80 REN[ O(J RENE 80
NICOCRALY CODEP RT2I NICOCRALY
Figure 1 Hot corrosion coating life at 900C after i00 hours
preaging at ii00 C. (Each bar is an average for two
specimens except for NiCoCrA_IY coated U700, isothermal
and burner rig, for which individual results are
shown).
395
1000
CD
:z:
,-- BOO
CD 600
Q:
CO
c_
F= _oo
C-9
200
0
i___ __ A--i UTOOICODEP
-"_. l--IllU7OOIRT21
"-. i--l_ uTOOINICOCRALY
"--.\ \ _--_ REN[80 ICODEP
t \ \ "-... I--ORENEOO/RI21
\\\ \ "-.. T- -IIRENE BO INIC0 CR ALIf
-il
I I f I I
!00 200 3D0 400 500 600
Hours
Figure 2 Hot eorrosion life at 900C versus isothermal preaging
time at II00C.
r4z
(J
c
,ej
_a
,-4
,!
:7
c_
i /_ Codep/Rene' 80UlO0
RT2] /U700
o ,k/ .
I_ -2:- _ 1 ._. I I i
20 "CJ 6q 80 I00
flOURS
Figure 3 Changes _l..C.,II Inductance it: Static Oxidation at IIOOC, Cyclic Exposure.
396


Effects of surface chemistry on hot corrosion life

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