Recent operations in the Persian Gulf have exposed military helicopter turbines to excessive amounts of ingested sand. Fine particles, less than 10 microns, are able to bypass the particle separators and enter the cooling and combustion systems. The initial sand chemistry varies by location, but is made up of a calcium aluminum silicate glass, SiO2 low quartz (Ca,Mg) CO3 dolomite, CaCO3 calcite, and occasionally CaCl rocksalt. The sand reacts in the hot combustion gases and deposits onto the turbine vanes as CaSO4, glass, and various crystalline silicates. Deposits up to 0.25 in. thick have been collected. Although cooling hole plugging is a considerable problem, excessive corrosion is not commonly observed due to the high melting point of GaSO4

Smialek, James L.

English

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NASA Technical Memorandum 105234
The Chemistry of Saudi Arabian Sand:
A Deposition Problem on Helicopter
Turbine Airfoils
James L. Smialek
Lewis Research Center
Cleveland, Ohio
Prepared for the
Gordon Conference on Corrosion
New London, New Hampshire, July 14, 1991
NASA
https://ntrs.nasa.gov/search.jsp?R=19920004961 2020-03-17T14:26:05+00:00Z
THE CHEMISTRY OF SAUDI ARABIAN SAND:
A DEPOSITION PROBLEM ON HELICOPTER TURBINE AIRFOILS
James L. Smialek
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
Recent operations in the Persian Gulf have exposed military helicopter turbines to excessive
amounts of ingested sand. Fine particles, less than 10 µm, are able to bypass the particle sep-
arators and enter the cooling and combustion systems. The initial sand chemistry varies by
location, but is made up of a calcium aluminum silicate glass, Si0 2 low quartz, (Ca,Mg)CO3
O,	 dolornite, CaCO3 calcite, and occasionally NaCl rocksalt. The sand reacts in the hot combustionLO 
Ln	 gases and deposits onto the turbine vanes as CaSO 4 , glass, and various crysalline silicates.
w Deposits up to 0.25 in. thick have been collected. Although cooling hole plugging is a consid-
erable problem, excessive corrosion is not commonly observed due to the high melting point of
CaSO4.
INTRODUCTION AND BACKGROUND
By December 1990, turbine performance losses were reported for Black Hawk personnel
transport helicopters operating in the Persian Gulf. Engine teardown and rehab tests verified
that sand erosion of compressor airfoils accounted for most of the loss. At this time, unusual
amorphous deposits were observed on the leading edges of the first stage turbine vanes. An
example of a clean vane (aluminized X-40) is shown in figure 1. While there was no indication
of hot corrosion and the deposits were easily removed in a cleaning procedure, they were also
quite thick (up to 0.25 in.) and covered a portion of the airfoil (up to 0.5 in. long). An example
of such a deposit is shown in figure 2. There was concern over the exact nature of these deposits
and the potential for future corrosion problems. The cooling passages in these engines also con-
tained a talc-like brown powder whose corrosive properties were also a concern. The connection
between ingested sand, vane deposits, and cooling passage powders was the focus of this analyt-
ical study.
ANALYSES AND DISCUSSION
Three sand samples were obtained for analysis from the Dhahran area, two from dunes and
one from a dried river bed. Deposits and cooling passage powders were obtained from four
turbine engines. They were analyzed by atomic emission and absorption spectroscopy, XRD,
SEM, DTA, and metallography.
Composition. - The chemical analyses shown in table I are the averages of similar sand,
powder, and deposit samples. The elemental compositions (including C and S) have been con-
verted to their corresponding oxides and normalized to 100 percent by weight. This facilitates
direct comparison to complex silicate, carbonate, and sulfate compositions in the literature.
However, the exact phases present can only be determined by the XRD results. The data is
displayed graphically for the major and minor elements in figures 3 and 4.
The dune and river bed samples were primarily SiO 2 , with minor amounts of Al20 33 CaO,
NiO, Fe2 O3 , MgO, and K 2 O. Carbon and sulfur were present as carbonates and sulfates. X-ray
diffraction identified low quartz as the primary phase, while some larger particles contained
CaCO3 calcite.
Course +200 µm particles (stones) separated from the river bend sand contained substan-
tially less SiO2 and higher amounts of CaO, MgO, carbon, and sulfur. X-ray diffraction was
consistent with this result as CaCO 3 and (Ca,Mg)CO3 were identified as the major phases.
More recent sand samples show frequent occurrences of CaCO 3 and one case of NaCl from a salt
lake bed (King Fand International Airport region).
The cooling passage powders were high in silica but not as high as the as-received sand.
They were comparatively high in Al 2 O 3 and Fe 2 O3 , with increases in minor elements, such as
NiO, TiO 2 , and Cr 2 O3 . X-ray diffraction identified only quartz and possibly CaSO 4 . Erosion
of stainless steel and Ti-6A14V compressor hardware accounts for additional metals in the pow-
ders. The high Al 2O 3 content of these powders can be accounted for by the high Al 203 content
of the fines in the sand, 14 percent Al 2 O3 for -20 µm particles (not shown in table I) versus
2.5 percent Al 2 O3 overall. Since the powders were less than 10 µm, they more closely represent
the composition of the fine sand actually entering the turbine.
The resultant deposits were related to the chemical factors above. The levels of SiO2,
Al 2 O3 , Fe2 O3 , and minor compressor alloying elements were similar to those of the powders.
However, CaO and S increased, consistent with the CaSO 4 found on the inner surface of the
deposit by X-ray diffraction, while carbon decreased. CaO•MgO . 2SiO 2 diopside was the major
crystalline phase, while quartz was a very weak phase.
Deposit morphology. - The deposits formed at the base of the vane leading edges. In some
vanes the cooling air prevented hole-plugging (fig. 2), while others were completely covered over.
There was evidence of some viscous glassy flow (fig. 5). The metallographic cross-section shown
in figure 6 exhibits a featureless glassy matrix (grey), numerous large pores (black), and some
second phase precipitates (lighter grey). Some bright metallic particles were also present as
unreacted erosion products.
An SEM micrograph of the exterior surface (fig. 7) again reveals a smooth glassy surface
with porosity. EDS analysis showed (in descending order) high intensity peaks of Si, Ca, and
Al, intermediate levels for Fe (and S on the inner surface only), and low levels for Mg, Ti, Cr, K,
and Na. This is very consistent with all the chemical analysis results on the deposits. The inner
surface sulfur peak agrees with the CaSO 4 found on this surface by XRD.
Interpretation. - The chemistry and morphology of the deposits is explained as follows. The
calcium carbonate in the sand reacts with sulfur in the fuel to form a CaSO 4 deposit. This is
analogous to conventional deposition due to NaCl ingestion and Na 2 SO4 formation. CaSO4 is a
typical deposit in fluidized bed reactors where sulfur from the coal reacts with limestone and
dolomite. The high Si, Al, and Ca content of the deposits and cooling passage powders is a
direct result of the sand composition, especially for the fine sand, which contains more Al and
more carbonates. Impaction is therefore believed to be a major deposition mechanism with
respect to the silicates.
The reflective particles and Fe, Ti, Ni, and Cr in the deposits and cooling passage powders
appear to be a direct result of the upstream sand erosion of compressor hardware.
2
A
The melting points of the deposits and cooling passage powders was 1137 and 1136 °C,
respectively, as surmised experimentally from a broad endothermic DTA valley. This is con-
trasted to the high published melting points of some of the observed crystalline phases:
1713 °C for SiO 2 quartz, 1391 °C for CaO•MgO . 2SiO 2 diopside, and 1540 °C for CaSO4. A
CaO•Al 2 O3 . 2SiO 2-SiO 2-FeO phase diagram indicates a low quaternary eutectic at 1070 °C near
the deposit composition (this pseudoternary does not contain the exact deposit composition).
Molten glassy phases were thus likely only during the impaction process and would ac-
count for some viscous flow and the glassy appearance of the deposit. Since the melting point
exceeds normal vane temperatures by more than 200 °C, the glass and the potentially corrosive
CaSO4 phase are not expected to be totally molten for any period of time. This is consistent
with the absence of severe corrosion effects.
The porosity in the deposit can be rationalized by CaCO 3 decomposition in the deposit, if it
is assumed that it first survives the hot gas path. CO 2 gas bubbles would result with an en-
richment in the CaO content of the glass. This trend is consistent with the decreased C and
increased Ca contents of the deposits compared to the powders (table I).
Exploratory crucible tests were performed on bulk NiAI samples to assess the corrosive
behavior of the relatively pure "dune" sand, the cooling passage powder, CaCO 3 , and CaSO4 at
1000 and 1200 °C for 100 lir. The first two reagents were relatively innocuous at both temper-
atures. CaCO3 formed a thick 12CaO . 7AI 203 scale at 1200 °C. CaSO caused accelerated
oxidation via NiAl 2 O4 and NiO formation, also at 1200 °C (95 mg/cXeight change). This
preliminary test suggests that some form of accelerated corrosion may be possible due to Ca
compounds under severe conditions.
CLOSING REMARKS
Compositional and phase analyses have aided in the interpretation of sand-induced depos-
its on helicopter turbine vanes from the Persian Gulf. Fine silica-based sand containing Ca(Mg)
carbonates is able to deposit as Ca-Al-Fe-silicates plus CaSO 4 . Compressor erosion products are
also found in cooling passage powders and the deposits. Based on the analysis of deposits to
date, a serious hot corrosion problem is not expected. The situation would be more severe for
aircraft operating in high NaCl regions. At this time the major concern is blocking of cooling
holes and loss of aerodynamic surface. The adoption of external filter bags has been shown to
reduce sand injection and is expected to also reduce deposition problems.
ACKNOWLEDGMENTS
This study would not have been possible without the concerted and timely effort of the
Analytical Services Branch, especially Fran Archer (atomic emission spectroscopy) and Ralph
Garlick (X-ray diffraction).
3
TABLE I. - CIIEMICAL ANALYSIS OF RAW SAUDI SAND,
INGESTED FINE POWDERS, AND VANE DEPOSITS
[Emission spectroscopy, normalized to 100 percent oxide, averages of ( ) samples.
Chemical SAND(3) STONE(1) POWDER(4) DEPOSIT(4)
Si0 2 90.79 12.26 50.20 43.44
CaO 1.46 25.89 4.11 17.73
Al 203 2.51 5.19 16.44 11.65
Fe203 .84 .85 15.06 11.43
Mg0 .54 15.64 2.01 5.77
NiO .99 .15 6.12 4.67
Ti02 .19 .20 1.53 2.18
Na20 .25 .16 .62 1.41
K 20 .51 .13 1.26 -------
Cr203 .05 .07 2.19 .98
M003 ------- ------- .65 .55
Nb 2 0 5 ------- ------- .45 .25
Coo ------- ------- .81 .04
Cu 2 0 ------- ------- .33 .32
CO 2 5.57 39.37 3.70 .27
S0 2 1.27 8.54 1.01 4.56
Si0 2 quartz sand contained Al 2 03 and CaCO 3, especially in fines. River bed
stones were primarily CaCO 3
 and (Ca,Mg) CO3 . Ingested powders collected
from upstream compressor erosion. Vane deposits, resulting from powder im-
paction and reaction with sulfur, contained glassy silicates, Ca0•Mg0.2SiO2
diopside, and CaSO4 . See also figures 3 and 4.
4
C-s
I
J
Figure 1.—First stage vane pair from T-700 turbine engine used in Black Hawk helicopters. (Codep aluminide
coated X-40 cobalt alloy).
1 mm
Figure 2.—Macrograph of leading edge vane deposit showing openings at cooling
hole locations.
normalized weight %
100
0Saudi sand	 . ' ; I stones	 cooling powders	 vane depositsEl
80
60
40
20
0
Si02	 CaO	 Al203	 Fe203	 Mg0	 NiO	 CO2	 S02
major component oxide species
Figure 3.—Analyzed composition of Saudi Arabian sand and helicopter engine deposits. (Presented as oxide species normalized to
100% total. Averages of 3 sand and one river bed stone sample; averages of cooling passage powders and vane deposits from 4
engines). Major components.
6
normalized weight %
3
Saudi sand	 stones	 cooling powders	 vane deposits
a
2
1
0
TiO2	 Na2O	 K2O	 Cr2O3	 MoO3	 Nb2O5	 Coo	 Cu2O
minor component oxide species
Figure 4.—Analyzed composition of Saudi Arabian sand and helicopter engine deposits. (Same considerations as figure 3).
Minor components.
7
1 mm
Figure 5.—Macrograph of four leading edge vane deposits showing viscous flow
patterns around sides.
a
	 r	 °	 `•
. r	
•^
!t
a'
^^ ti	
tli^^r'r ^ +G
	 '1 y^^! s ^ 4t ^ ^^ .k}4
3	 G	
w 
f	 °_ : M	 +^o
:•.^	 ^4
• ,	
d	 ^ +^
r	 ^tq
•^^ ^• "^	 B
I	 I	 P 	 sJ M 	 f °	 M'f
100 m	 –
Figure 6.—Optical micrograph of vane deposit. Polished cross-section showing glassy
Ca-AI-Fe silicate matrix with CaO-MgO-2SiO2 diopside precipitates from impacted
sand, Fe-Ti metallic inclusions from compressor erosion, and gas bubbles from
CaCO3 decomposition. (G = glass, P = diopside precipitates, M = metallic
inclusions, B = gas bubbles).
8
- i
AdW	 loogm
Figure 7.—SEM micrographs of glassy external deposit surface with gas bubbles.
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
Technical Memorandum
4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
The Chemistry of Saudi Arabian Sand: A Deposition Problem on
Helicopter Turbine Airfoils
WU-505-62-OK6. AUTHOR(S)
James L. Smialek
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
REPORT NUMBER
National Aeronautics and Space Administration
Lewis Research Center E - 6559
Cleveland, Ohio 44135 - 3191
9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
National Aeronautics and Space Administration
Washington, D.C. 20546- 000 1 NASA TM- 105234
11. SUPPLEMENTARY NOTES
Prepared for the Gordon Conference on Corrosion, New London, New Hampshire, July 14, 1991. Responsible person,
James L. Smialek, (216) 433 -5500.
12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE
Unclassified - Unlimited
Subject Categories 26 and 27
13. ABSTRACT (Maximurn 200 words)
Recent operations in the Persian Gulf have exposed military helicopter turbines to excessive amounts of ingested sand.
Fine particles, less than 10 µm, are able to bypass the particle separators and enter the cooling and combustion systems.
The initial sand chemistry varies by location, but is made up of a calcium aluminum silicate glass, Si0 2 low quartz,
(Ca,Mg) CO 3 dolomite, CaCO3 calcite, and occasionally NaCl rocksalt. The sand reacts in the hot combustion gases
and deposits onto the turbine vanes as CaSO 4, glass, and various crystalline silicates. Deposits up to 0.25 in. thick have
been collected. Although cooling hole plugging is a considerable problem, excessive corrosion is not commonly
observed due to the high melting point of CaSO4.
14. SUBJECT TERMS 15. NUMBER OF PAGES
Sand; Persian Gulf; Black Hawk helicopters; Si0 2 ; CaCO 3 ; CaC04; Deposits I O
16. PRICE CODE
A03
17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT
OF REPORT OF THIS PAGE OF ABSTRACT
Unclassified Unclassified Unclassified
Standard Form 298 (Rev. 2-89)
Prescribed by ANSI Std. Z39-18
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The chemistry of Saudi Arabian sand: A deposition problem on helicopter turbine airfoils

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