Transactions

494 TRANSACTIONS OF T H E A.S.M.E. AUGUST, 1941
inhibition, and what molecular-weight acids might give rise to
rusting. I t was found, as might be expected, that the acids of
lower molecular weight, such as formic, acetic, and even propionic,
were rust producers. In fact, more rust was produced by such
acids than by a neutral untreated white oil. Ascending the scale
of acids, complete protection was found through the use of 0.25
per cent caproic acid, having a molecular weight of 116. All acids
of higher molecular weight also give protection. Later experiments
indicated that 0.1 per cent of caproic acid was equally
effective.
Fig. 7 shows a series of ten steel-corrosion specimens, indicating
the relative rusting and rust-inhibiting qualities of 0.25 per cent
of various organic acids in white medicinal oil. All tests were
carried out at 140 F, the oils with 1 per cent water mixture having
been stirred for a total of 16 hr, and left in a quiescent state for a
total of 13 hr. The results are given in Table 2.
T A B L E 2
R ESU L T S OF C O R R O S IO N TESTS O N ST E EL
S P E C IM E N S S H O W N IN F IG . 7
Carbon Molecular
Percentage
atoms in weight of of surface
acid acid rusted
No acid............................................................................................................5
Formic a c id ................................ 1 46 100
Acetic acid .................................. 2 60 100
Propionic a c id ............................ 3 74 50
Butyric a c id ................................ 4 88 20
Valeric a c id ................................ 5 88 2
Caproic a c id ............................... 6 116 0
Caprylic a c id .............................. 8 144 0
Palmitic a c id .............................. 16 256 0
Stearic a c id ................................. 18 284 0
■Fig. 8, which gives the structure of one of the forms of caproic
acid, illustrates how caproic acid or any organic acid of higher
F i g . 8 A c t i o n o p C a p r o i c A c i d a s a C o r r o s i o n I n h i b i t o r
molecular weight tends to inhibit iron corrosion in contact with
water suspended in oil.
The water-soluble or hydrophilic part of the molecule of organic
acids is called the carboxyl group. This is the same group which
attaches itself to iron surfaces. After a layer of such molecules
has become attached to the iron, the oil-soluble or hydrophobic
parts of the molecules are presented to the oil and prevent water
globules, suspended in motion in that oil, from coming in contact
with the iron, thereby, inhibiting rusting of the iron.
O t h e r S o u r c e s o p R u s t I n h i b i t i o n
Organic acids are not the only oxidation products of oil which
inhibit rusting of turbine oils since, while the organic acids are
being formed, there are probably compounds with hydroxyl
groups and ketone groups which are likewise hydrophilic and
capable of wetting the iron. It is therefore possible to treat a new
oil with the nonacid oxidation products of mineral oil and, thereby,
effect iron-corrosion inhibition without raising the neutralization
number.
Because the traces of organic acids and associated oxidation
products in turbine oils tend to inhibit rusting under turbineoperating
conditions, it must not be assumed that a used turbine
oil, for example, can be used as a rust-inhibiting coating for iron or
steel under all conditions. Such an oil-treated specimen if placed
out in the rain or in salt water would certainlv rust.
F i g . 1 0 A p p a r a t u s D e v e l o p e d t o D e t e r m i n e R e l a t i v e O x i d a ­
t i o n R a t e s o f V a r i o u s O i l s