No. 7 January 11, 2016

LUBRICANT’S CONTRIBUTION TO PREVENTION OF SHORTENED FATIGUE LIFE IN
HYDROGEN
The rolling four-ball tests as reported in our previous issue (KYTB 6) demonstrated a reduction in fatigue life in hydrogen environment, where repetitive contact between balls stimulated the formation of white structure accompanied by crack propagation leading
to flaking at an early stage. The same tests were carried out to examine the effect of lubricating oil additives on fatigue life in hydrogen and confirmed that certain types of rust inhibitors and anti-wear additives can prevent shortened fatigue life.
Fig.1 shows the rolling four-ball tester used in this study. Inthe cup filled with air or hydrogen, three bearing balls wererotated by another bearing ball placed on the top of them tosimulate fatigue flaking in rolling contact. In our previous test
(KYTB6), two PAOs of different kinematic viscosities wereused as a lubricating oil to provide two lubrication regimes, fullEHL (condition 1) and partial EHL (condition 2), where theratio of minimum oil film thickness to composite surfaceroughness was above 3.0 and 1.4 to 2.0, respectively. The
test results showed that a white structure formed only in hydrogen and the life of balls in hydrogen was shorter by one
order of magnitude than that in air. With these results in mind,
this study examined the effect of organometallic salts in lubricating oil on fatigue life in hydrogen.
Upper ball

Seal
OilLower balls

ExhaustH2 gas supply
Thermocouple

Fig.1

Table 1 shows the results of the tests under full EHL condition.
Under this relatively mild lubrication condition, some organometallic salts used as a rust inhibitor or an anti-wear additive,
especially organometallic salts A and B effectively extended
the fatigue life when added alone. With organometallic salts Aand B, no white structure formed likely because these rust
inhibitors formed a dense adsorption film on the ball surface
which prevented hydrogen from permeating into the steel.
Meanwhile, in the presence of solid contact under condition 2(Table 2), a white structure formed even when organometallic
salts A and B were added. Organometallic salt B had little
effect in extending the fatigue life. The adsorption film seemed
to have disappeared due to wear allowing hydrogen permeation. However, when used in combination with organometallic
salt antiwear additives D or E, these rust inhibitors prevented
white structure formation and significantly extended the flaking
life. It is considered that the antiwear action of organometallic
salts D and E helped the adsorption film remain and enabledextended life.

LoadMain part of rolling four-ball tester
Table 1Results of rolling four-ball tests in hydrogen (Condition 1)

AdditiveNone

L50 life, 106 rotations
L10 life, 106 rotations
White structure formation

6.4
4.3
Yes

Table 2

J. Imai and Y. Imai : Proceedings of the JAST Tribology Conference, Nagoya(2008-9) 351 (in Japanese).

Organometallic
salt A75<46No

Rust inhibitor
Organometallic
salt B
70<16No

Organometallic
salt C
22
16-

Anti-wear additive
Organometallic
Organometallic
salt D
salt E75<34
6.2
Yes
Yes

Results of rolling four-ball tests in hydrogen (Condition 2)

Additive

None

L50 life, 106 rotations
L10 life, 106 rotations
White structure formation

339.6
Yes

Rust inhibitorOrganometallic
Organometallic
salt Asalt B
112
326920Yes
Yes

Rust inhibitor + Anti-wear additive
Organometallic
Organometallic Organometallic
salt A + D
salt A + E
salt B + E
230<63230<24No
Yes
No

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