Without its oxide, which grows naturally when
the surface of stainless steel is exposed to air,
stainless steels would not be corrosion resistant.
This applies to all the stainless steel grades that
are regularly used for the manufacture of springs,
whether they be of the austenitic type 302, 304,
316 or 17/7PH; martensitic type 420; or duplex
It might, therefore,
be presumed that a great
deal is known about this
oxide that confers such
a valuable service. It is
known that the corrosion-resistant properties
of the oxide appear when
there is >12% chromium
in the alloy. Indeed, this
was how stainless steels
were first invented in
Sheffield in 1913. Harry
Brearley was examining
the effects of alloy additions
to carbon steel
to improve the wear
resistance of gun barrel
steels, when discarded
steel with >12% Cr did not go rusty when left outdoors;
whereas all his other discarded trial steels
did go rusty.
The multi-layer oxide formed in air comprises
Cr2O3 that is about 3nm thick (0.120 × 10-6 in. or
10-15 atomic layers) with various layers over the
Cr2O3 inducing a top layer of Fe2O3.
This oxide can be made thicker by heating springs
in air or by acid passivation to ASTM A380 , but is
this thicker oxide better than the thin one, and why
is an acid passivation oxide better than that formed
when springs are stress relieved in air? Indeed, why
does 316 stainless steel
have better corrosion
resistance than 304
stainless when there
is no molybdenum
incorporated into this
protective oxide? Partial
answers to these
questions are available
from academics who
use X-ray photoelectron
but it would be very
interesting to have a
more complete answer
to the questions. Fundamentally
it would be
interesting and informative
to know if the
thickness or structure
of the Cr2O3 layer is affected by stress relief or
passivation, but IST doesn’t think results of this
study have ever been reported.
Notwithstanding, the point of this Cautionary
Tale is that the oxide that grows during stress relieving
is not detrimental. If stainless steel compression springs are stress relieved at 350°C (660°F), they go yellowy
brown in color – slight yellow at 350°C and darker at
480°C (900°F). This color arises due to refraction of
incoming light by the translucent multi-layer oxide.
The color is not real; it is merely a trick of light.
This yellow-brown oxide is corrosion resistant
and will not harm the function of the springs. Yet,
some customers still insist that they don’t want their
corrosion-resistant springs this color, but still want good functionality.
In these circumstances, IST recommends that the
oxide be cleaned off by pickling and passivating to
ASTM A380. Indeed, this passivation confers better
corrosion resistance (as measured by hours to onset
of red rust in an ASTM B117 salt spray test) to stainless
steel springs than the former brown oxide. This
is thought to be because the chromium oxide structure
is more tightly adherent (epitaxial).
As an aside, during the writing of this column, a
colleague was load testing two batches of stainless
steel springs that had large cracks on the inside
surface. He observed that the spring rate and load
values were only very slightly reduced when the
cracks were transverse and only 10-15% reduced
when the cracks were longitudinal. It is surprising
how little difference big cracks (to half way through
the wire section) make to the load /deflection characteristics
of compression springs. The point of this
cautionary observation is that load testing springs
as a means to sort the cracked from the non-cracked
ones requires a greater degree of precision than
might be expected. The cracking in both instances
had nothing to do with the state of the oxide on the
springs, but was associated with residual stresses.
Zubek, Luke. Technically Speaking, Austenitic
Stainless Steels in Springmaking. Springs, Oct. 2005.
Kerber, Susan J., and Tverberg, John. Stainless
Steel. Advanced Materials and Processes, Nov.
Mark Hayes is the Senior Metallurgist at the Institute of Spring Technology (IST) in Sheffield, England. Hayes manages IST’s spring failure analysis service, and all metallurgical aspects of advice given by the Institute. He also gives the majority of the spring training courses that IST offers globally.
Readers are encouraged to contact him with comments about this Cautionary Tale, and with suggested subjects for future installments, by phone at (011) 44 114 252 7984 (direct dial), fax at (011) 44 114 2527997 or e-mail at firstname.lastname@example.org.