Corrosion tests were run with AISI 52100, AISI M-50 and AMS 5794 under conditions that simulate the crevice corrosion found in aircraft ball and roller bearings rejected at overhaul for corrosion. Test specimens were fabricated that simulated the contacts of balls or rollers and the raceways. Corrosion cells were assembled in the presence of a lubricant contaminated with water and chloride ions. The cell was then thermally cycled between 339 K (150 F) and 276 K (37 F). The corrosion observed after 14 cycles was that of crevice and pitting corrosion typically found in aircraft bearings. AMS 5749 showed a very slight amount of corrosion. No appreciable differences were noted between AISI 52100 and AISI M-50, but both showed much greater corrosion than AMS 5749. The corrosion pits observed in AISI M-50 appeared to be fewer in number but generally deeper and larger than in AISI 52100

Bamberger, E. N.

Nahm, A. H.

Parker, R. J.

English

NASA Technical Reports Server

https://ntrs.nasa.gov/search.jsp?R=19830011844 2020-03-21T04:06:22+00:00Z
IN THE PRESENCE OF CHU)RIDE+XMTMIWTUD LUBRICANT 
Richard J. Parker  
Nat ional  Aeronautics and Space Adminis t ra t ion 
Lewis Research Center  
Cleveland, Ohio 44135 
E r i c  N. Bamberger aad Alex 11. Nahr 
Ceaera l  E l e c t r i c  C a p a n y  
A i r c r a f t  Engine Bus iae rs  Group 
C i n c i n n a t i ,  Ohio 45215 
Cotros ion tests were run  w i t h  AISI 52100, AISI t4-50 and A S  5749 under 
c o n d i t i o n s  t h a t  s imula te  t h e  c r e v i c e  c o r r o s i o n  found i n  a i r c r a f t  b a l l  and 
r o l l e r  bear ings  r e j e c t e d  a t  overhaul  f o r  cor ros ion .  T e s t  s p e c i w a s  were fab- 
r i c a t e d  t h a t  s imulated t h e  c o n t a c t s  o f  b a l l s  or r o l l e r s  and t h e  raceways. 
Corrosion c e l l s  were assembled i n  t h e  presence o f  a l u b r i c a n t  contaminated 
0 with  water  and c h l o r i d e  ions.  The c e l l  was then  the rmal ly  cyc led  betveen 
h 
m 
339 K (150° F) and 276 K (37O F). The cor ros ion  observed a f t e r  1 4  c y c l e s  vas 
-. t h a t  o f  c r e v i c e  and p i t t i n g  c o r r o s i o n  t y p i c a l l y  found i n  a i r c r a f t  bear ings .  I 
w A I ( S 5 7 4 9 s h o w e d a v e r y s l i g h t m u n t o f c o r r o s i o n .  Noappreci~bledif ferences  
were noted between AISI 52100 and AISI PI-50, b u t  both showed much g r e a t e r  
cor ros ion  than AM5 5749. The c o r r o s i o n  p i t s  observed i n  AISI n-50 appeared t o  
be fewer i n  number bu t  g e n e r a l l y  deeper  and l a r g e r  t h a n  i n  AISI 52100. 
INTRODUCTION 
Corrosion of aerospace bear ings  has  become a s e r i o u s  and expensive prob- 
l e m .  The c o r r o s i o n  problem i s  most severe i n  systems wi th  long p e r i o d s  of 
nonuse. A suara ry  o f  causes  f o r  bear ing r e j e c t i o n  a t  a U.S. Navy f a c i l i t y  (1) 
shows t h a t  c o r r o s i o n  accounts  f o r  n e a r l y  one-third of t h e  b e a r i n g  r e j e c t i o n s  
from t h e i r  a i r c r a f t  s y s t e a a ,  inc lud ing  d r i v e l i n e s ,  wheels,  and accessor ies .  
A i r  Force exper ience ( 2 )  conf i rms t h a t  c o r r o s i o n  i s  a major c a u s e  o f  r e j e c t i o n  
a t  overhaul o f  a i r c r a f t  t u r b i n e  engine bearings.  
Cryogenic turbopump bear ings  i n  c u r r e n t  and f u t u r e  r o c k e t  eng ines ,  such 
as t h e  Space S h u t t l e  Main Engine, a l s o  r e q u i r e  t h e  use  o f  c o r r o s i o n  r e s i s t a n t  
bearings.  Cur ren t ly ,  AISI 440C is used f o r  b e a r i n g s  i n  cryogenic  app l ica -  
t ions .  I t s  hardness,  f a t i g u e  l i f e ,  and load c a p a c i t y  make AISI 440C a mar- 
g i n a l  m a t e r i a l  f o r  f u t u r e  cryogenic  turbopump bear ings  and inadequate  For 
turbine-engine main-shaft bear ings .  A s i g n i f i c a n t  mechanism of  c o r r o s i o n  i n  
a i r c r a f t  engine bear ings  was de f ined  i n  ( 3  and 4 )  as a r e s u l t  o f  obse rva t ions  
f r o m  Naval f l e e t  a i r c r a f t  bear ings .  It was concluded t h a t  t h e  c o r r o s i o n  
respons ib le  f o r  bear ing  r e j e c t i o n s  was caused by c y c l i c  temperatures  and 
lengthy s t a t i c  pe r iods  f r e q u e n t l y  experienced by a i r c r a f t  i n  t h e  f i e l d .  
Temperature of t h e  l u b r i c a n t  i n  t h e s e  engines  c y c l e s  from 508 K (455" F) 
during opera t ion  t o  273 K (32O F) during i n a c t i v e  pe r iods  i n  c o l d  c l i m a t e s .  
Moisture-laden air can get into the lubrication system in humid environrents, 
during rain, or during aircraft water wash. The moisture can condense on 
bearing components during the inactive periods. Additionally, it was found 
(3 and 4) that up to 3 ppm of chloride ion (most likely from sea water) was 
contained in the lubricant. Obserrations of rejected bearings showed that the 
corrosion was concentrated at the crevice f o d  at the coatacts of the balls 
or rollers and the raceways. The lubricant a d  cundensate coataiaing the 
chloride ion settled in this area, aad crevice corrosioa occurred. Tbe pres- 
ence of the chloride ions accelerates corro8ion (5). 
Although the observations in (3 and 4) were limited to bearings in Naval 
aircraft, it is likely that a similar mechaaism occurs in otber aircraft and 
aerospace bearings rejected for corrosioa. If the corrosion pits are of sig- 
nificant size, they can act as initiation sites for fatigue spalling aad 
shorten bearing life. 
The corrosioo problem is beiag attacked on three major fronts: corrosioo- 
resistant materials, corrosica-resistant coatings or surface d i f  icatioas, 
and corrosion-inhibited lubricants. Materials such as AISI U O C ,  &IS 5900, 
and M 5749 are called corrosion-resistant or stainless steels, but under 
some severe environmental conditions in aircraft bearings, corrosion will 
occur. They are, hwever, more corrosion-resistant than the c m a  aircraft 
bearing materials such as AISI M-50, AISI 52100, and the c m n l y  used case- 
carburized materials. Ceramics such as silicon nitride are t ~ l y  corrosioa- 
resistant in the aircraft environment. Although this material is not yet well 
developed for widespread aircraft application, it could be applied ia s a e  
specific cases. 
Chromium-ion inplantation has shown significant inprovacnt in the corro- 
sion resistance of AISl W-50 ( 6 ) .  ather surface modifications or coatings may 
also provide similar benefits but more work is needed in this area. It is 
probable that a cabination of approaches, such as the use of silicon nitride 
balls or rollers and coated raceways may be a viable solution for some appli- 
cations that require long periods of aonuse. 
Currently, the lubricant provides some measure of corrosion protection in 
aircraft bearings simply by keeping the components coated and moisture free. 
Corrosion inhibiting additives for the c m l y  used MIL-L-23699 lubricant 
have shovn promise in laboratory tests ( 3  and 4) and confirmation of the 
degree of improved corrosion resistance in actual aircraft systems is under 
way. 
Steels such as AllS 5749 and AHS 5900 combine the tempering, hot hardness, 
and hardness-retention characteristics of AISI M-50 steel with the corrosion 
and oxidation resistance of AISI 440C stainless steel. The typical chemical 
compositions of these materials are shown in Table 1. MIS 5749 and AHS 5900 
contain higher percentages of carbon and chromium than AISI M-50 for improved 
corrosion and wear resistance. Their hot hardness and hardness retention are 
better than AISI 440C and similar to AISI M-50 (7). 
AHS 5749 and AXS 5900 have shown excellent life in several accelerated 
rolling-element fatigue studies (7 to 10). These materials were double- 
vacuum-melted (VIM-VAR for vacuum induct ion melt, vacuum arc remelt) and 
showed lines equal to or greater than VIM-VAR AISI M-50 in similar tests. 
These life results are significant , since they show that a corrosion-resistant 
material that could potentially replace AISI M-50 in some aircraft bearing 
applications would be expected to have at least equivalent lives. 
' J .  
-. 
The e x t e n t  o f  c o r r o s i o n  resistance improvement o f  t h e s e  naterials over 
t h e  nore c o r o n  a i r c r a f t  b e a r i n g  u t e r i a l s  such  as AISI H-50 and AISI 52100 
h a s  n o t  been  well documented or pub l i shed .  It was t h e  o b j e c t i v e  o f  t h e  work 
r e p o r t e d  h e r e i n  t o  d e t e r m i n e  t h e  c o r r o s i o n  r e s i s t a n c e  o f  AI(S 5749 r e l a t i v e  to  
t h a t  o f  AISI N-50 and AISI 52100. 
The o b j e c t i v e  was accaplishcd by pe r fo rming  a series o f  c o r r o s i o n  tests 
w i t h  spec imens  f a b r i c a t e d  to s i m u l a t e  t h e  crevice : o r r o s i o n  i n  t h e  p r e s e n c e  
o f  a s p e c i a l l y  p repa red  l u b r i c a n t - c h l o r i d e  s o l u t i c n  and exposed to  them1 
c y c l i n g ,  
TEST HATERIALS 
The c h e n i c a l  c a p o s i t i o n  o f  t h e  materials used  i n  t h e s e  tests is g i v e n  i n  
Tab le  1. The test spec imens  were made from r o l l i n g - c o n t a c t  (RC) r i g  test b a r s  
(11).  Rod m a t e r i a l s  o f  app rox ima te ly  1 3  r (0.5 i n )  i n  d i a m e t e r  were h e a t  
t r e a t e d  to  o b t a i n  a nominal Rockwell C h a r d n e s s  o f  62. The h e a t  t r e a t i n g  
c y c l e s  are d e s c r i b e d  i n  T a b l e  2. C y l i n d r i c a l  b a r s  76.2 m(3.0 i n )  l ong  and 
9.5 rr (0.375 i n )  i n  d i a m e t e r  were ground from t h e  h e a t  t r e a t e d  rods .  Half  
o f  t h e  test b a r s  were ground w i t h  s a d d l e s  as shown i n  Fig.  1 v i t h  a  r a d i u s  
s l i g h t l y  g r e a t e r  t h a n  t h e  bar r a d i u s .  T h i s  s a d d l e  p r o v i d e s  a  c o n t a c t  and 
c r e v i c e  s i t u a t i o n  to  s i m u l a t e  t h e  c o n t a c t  o f  a h a l l  or r o l l e r  and a b e a r i n g  
racevay as shown i n  Fig.  1. Both t h e  c y l i n d r i c a l  and t h e  s a d d l e  s u r f a c e s  were 
ground t o  a f i n i s h  o f  0.15 to  0.20 ua ( 6  to 8 p i n ) .  
The l u b r i c a n t  used on t h e  b a r s  d u r i n g  t h e  c o r r o s i o n  tests was a s p e c i a l l y  
p repa red  l u b r i c a n t - c h l o r i d e  s o l u t i o n .  I t  c o n s i s t e d  o f  MIL-L-23699 t y p e  l u b r i -  
c a n t  t o  which was added t h r e e  p a r t s  p e r  m i l l i o n  (ppm) by wcight  o f  c h l o r i d e s  
( s u p p l i e d  as ASRl D665 s y n t h e t i c  s eawa te r ) .  The w a t e r  c o n t e n t  o f  t h e  l u b r i -  
c a n t  s o l u t i o n  was then  a d j u s t e d  to  600 ppm by v e i g h t  by t h e  a d d i t i o n  o f  d i s -  
t i  1 led water .  
CORROS ION TEST PROCEDURE 
The procedure  used  f o r  t h e s e  c o r r o s i o n  tests was similar t o  t h a t  used i n  
3 .  The test b a r s  of  e a c h  n a t e r i a l  were c l e a n e d  by s u c c e s s i v e l y  washing i n  
s e p a r a t e  b a t h s  o f  t o l u e n e ,  e t h a n o l ,  hexane,  and ace tone .  T h e r e a f t e r ,  t h e  b a r s  
were handled  o n l y  by rubber-gloved hands  and were  a i r - d r i e d  a t  room tempera- 
t u r e .  The b a r s  were t h e n  inmrersed f o r  1 hour  i n  t h e  l u b r i c a n t - c h l o r i d e  s o l u -  
t i o n .  During t h i s  exposu re ,  t h e y  were p e r i o d i c a l l y  r o t a t e d .  The b a r s  were  
removed and d r a i n e d  f o r  30 minu te s  a t  room tempera tu re .  
The b a r s  were then  p l aced  i n  a  s p e c i a l  f i x t u r e ,  shown i n  Fig .  2 ,  w i t h  t h e  
lower two b a r s  having  t h e  s a d d l e ,  and t h e  mat ing  two b a r s  p l aced  a c r o s s  t h e  
s a d d l e s .  T h i s  arrangement produces  a  c o n t a c t  and c r e v i c e  s i t u a t i o n  s i m i l a r  to  
t h a t  i n  a b a l l  o r  r o l l e r  b e a r i n g  and p r o v i d e s  t h e  o p p o r t u n i t y  f o r  t h e  c r e v i c e  
c o r r o s i o n  mechanism. The f i x t u r e  was made o f  p o l y e t h y l e n e  m a t e r i a l .  The f i x -  
t u r e  was clamped t o g e t h e r  and suspended i n  a l a r g e  beake r  which was t h e n  
l o o s e l y  cove red  w i t h  aluminum f o i l .  T h i s  assembly ,  c a l l e d  t h e  c o r r o s i o n  c e l l ,  
was t h e n  c y c l e d  a l t e r n a t e l y  between a 339 K (150' F) oven and a 276 K (37' F) 
r e f  r i g e r a t o r .  T o t a l  exposu re  was 14 c y c l e s ,  e a c h  c y c l e  c o n s i s t i n g  o f  8 h o u r s  
i n  t h e  oven and 16  hour s  i n  t h e  r e f r i g e r a t o r .  Cor ros ion  ce l l s  were assembled 
w i t h  e a c h  o f  t h e  t h r e e  m a t e r i a l s .  
Upon comple t ion  o f  t h e  exposu re ,  t h e  b a r s  were removed, c l e a n e d  w i t h  a n  
a l k a l i n e  s o l u t i o n  i n  an  u l t r a s o n i c  c l e a n e r ,  and photographed.  T y p i c a l  c o r r o -  
s ion s ites were then examined metal lographical l y to determine corms ion 
characteristicr. 
RESULTS AND DISCUSS ION 
Corrosion test8 were performed with tart bar8 of AllS 5749, AISI SZlOO, 
and AISI H-50 assembled to rimlate crevice corrosion in the preamce of a 
lubricant-chloride solution. After the 14 thermal cycler were completed, the 
test bars were cleaned and examined. Visual inspection revealed that AM1 
M-50 and AISI 52100 had extensive corrosion damage. A slight amount of corro- 
sion was present on the AWS 5749 bars. As shown in Figs. 3 m d  4, the corm- 
sion was not limited to crevice corrosion, but pitting corrosion also occurred 
on exposed surfaces of both materials. The slight mount of corroaion on the 
AllS 5749 bars appeared to be limited to the raddle area where crevice corm- 
sion occurred, as shown in Fig. 5. 
For a detailed examination of the corrosion damage, scanning electron 
microscopy was employed. Typical corrosion areas are s h w n  in Figs. 6 to 8 .  
In general, there was no significant difference between the corroaion in the 
crevice areas and that on the expored surfaces. Differences were noted 
between the appearance of the AISI N-50 and the AISI 52100. The corrorion on 
AISI 52100 appeared to cover more surface area, but the corrosion pits on the 
AISI M-50 bars appeared to be deeper. This observation is not readily apparent 
in Figs. 6 and 7, but test bars of both materials were sectioned through typi- 
cal corrosion pits for further observations. 
Optical photorricrographs of the section6 through the pits are shown in 
Figs. 9 and 10. The greater depths of the AISI N-50 corrosion pits are ap- 
parent by corrparing these photos. This observation agrees with those of (12) 
which illustrate that effect in bearings reroved from service. 
The corrosion pitting on the U S  5749 was very limited in area and only 
in the crevice area near the edge of the saddle as shown in Figure 8. These 
few pits are very shallow, as shown in Fig. 11. 
The pits shown in Figs. 9 to 11 were selected as typical of the deepest 
pits observed in each material. These deepest pits had depths of approxi- 
mately 120, 70, and 20 urn (0.005, 0.003, and 0.0008 in) in the AISI N-50, AISI 
52100, and M S  5749, respectively. The deepest pits in a bearing rutface are 
the most detrimental, since they are moat likely to be initiation sites for 
rolling-element fatigue cracks and subsequent spalling. Thus, the corrosion 
observed would tend to reduce rollingzlorcnt fatigue life in AISI W-50 more 
than in either AISI 52100 or AHS 5749. 
SUMMARY OF RESULTS 
Corrosion tests were performed with test bars of AWS 5749, AISI 52100, 
and AISI M-50 assembled to simulate crevice corrosion in the presence of a 
lubricant-chloride solution. The assembled corrosion cells were thermally 
cycled between 339 K (150' F) in an oven for 8 hours and 276 K (37. F) in a 
refrigerator for 16 hours. Total exposure time was 14 cycles. Following this 
exposure, the corrosion cells were disaraeebled, cleaned, observed, m d  
metallurgically examined. 
The following results were obtained: 
1. The corrosion resistance of AMS 5749 was much better than that of AISI 
M-50 or 52100. 

TABLE 1. - CHWICAL COMPOSITIONS OF TEST MTBRIALS 
TABLE 2. - W T  TREATMENT OF TEST MATERIALS 
P 
0-00s 
-007 
0003 
- 
S 
0.003 
a002 
-003 
CEVn 
AISI 52100 
V IM-VAR 
AISI M50 
V IM-VAR 
AMS 5749 
Heat 
treatment 
Preheat 
Austenitize 
Quench 
Temper 
Deep freeze 
Temper 
Material 
C 
1.04 
-82 
1.11 
Si 
0-30 
-24 
030 
Iln 
0.34 
-28 
-17 
W 5749 
816' C (1500' F) 
in salt 
1121' C (2050' F) 
In salt 
In 593' C (1100' F) 
in salt, air cool 
to room temperature 
538' C (1000' F) 
for 2 hours 
-73' C (-100' F) 
for 2 hours 
538' C (1000' F) 
for 2 hours 
' 
AISI 52100 
None 
843' C (1550' F) 
in salt 
In 54' C (130' F) 
oil, air cool to 
room temperature 
177' C (350' F) 
for 2 hours 
-73' C (-100' F) 
for 2 hours 
177' C (350' F) 
for 2 hours 
AISI U-50 
816' C (1500' F) 
in salt 
1107' C (2025' F) 
in salt 
In 566' C (1050' F) 
in salt, air cool 
to room temperature 
524" C (975' F) 
for 2 hours 
-73' C (-100' F) 
for 2 hours 
524' C (975' F) 
for 2 hours 
Cr 
1.46 
4.20 
13.92 
no 
0.03 
4 1 6  
3.87 
V 
--- 
1-01 
1-15 
Ni 
0.04 
008 
-01 













Corrosion of simulated bearing components of three bearing steels in the presence of chloride-contaminated lubricant

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server