The sun. [volume] (New York [N.Y.]) 1833-1916, 1907-07-05
THE PROTECTION OF IRON AND
STEEL.
Prefessor Gustave w. Thompson on the
Virtues or the Red Lead Coating.

To TIE EDITOR OF The SUN-Sir: In your
issues Of June is and June - there appeared
two interesting and extremely timely articles
on the corrosion OF iron and steel. As the
writers. Mr.. +. II. Cowles and MIR. Hutchinson
Scott. point out, the importance Of the pre-
ervation Of iron and steel IL growing every
day deserving Of greater consideration. "a
steel Ia entering to such a large extent into
every form Of structure. IL Ia pointed out
by these writers that the corrosion OF iron and
steel Is due to electrolytic action. This cleo
trolytic action. however. may come either
from without or within the system corroded.
Sometimes a stray electric current passes
through the system and seeks short cuts out
through the surface to other surfaces which
are more or less near. Two pieces OF metal
separated by moist substances form a system
through which a stray current will surely set
up electrolytic action. But. a. we have said.
corrosion does not always come through
electrical influences from without the system.
One part Of a single piece Of metal may have
a higher electrical potential than another
part Of the same piece Of metal. and from
this difference OF potential a current OF cleo
tricity tends 70 flow from the point OF high
such a current operates. when moisture Is
present. TO start and help c%orrosion.. Differ
ences OF electrical potential existing between
parts OF the surface of a piece of metal are
often due TO the presence of mill scale. IL il
customary for architects and engineers. TO
specify that during the process OF assembling
and before painting III not already done at
the mill all detachable mill scale shall he
era. kc. but as affecting the difference in po-
tenttal between pa rts Of the surface which
Is 30 much a ca use OF corrosion the undetach:
able mill scale y usually neglected and allowed
10 remain on the surface until oxidation and
weatberinR cause II 10 AXE on. The cleaner
the metal Is before painting the greater will
be the value of the protective coating, so that
we see the importance OF removing all mill
scale from steel surfaces that are subject to
Then again. some constituents In protec-
tive coatings act IA accelerators Of corrosion.
found that iron and steel. when treated with
certain req gents. are rendered mac
tive or passive: a chemical change has taken
we dc, not know for certain. but We do know
tnat me dinerepces ip potential on the sur
resistance to electrolytic corrosion has been
int
So with some pigments. and il is here that
would diner, with Mr.. Hutchinson Scott: for
have found by careful experiment that red
lead has the effect OF retarding the electrolytic
corrosion Of iron and steeI~red lead seems
10 have, an inhibitive effect. Furthermore.
RED LEAD T. a 9on1conductor of elect richly,
istlng coating. The opinion quoted by Mr..
hutchison Scott from Archuccturdl Art is not
borne out by the facts. Red lead does not
increase the rapidity OF the destruction OF
iron, by rust; on the ron trar ~, H. reta rds or
inhlblts oorrosion or rusting. The changing
by the, action Of carbonic acid from the air.
instead OF being a bad thing would be a good
thing. as carhonic acid In a free state Is an
accelerator OF corrosion and anything that
absorbs carbonic acid is a retarder OF corro-
sion.
Architects and engineers ave stuck IN red
lead as the material from which 10 make a
good protective coating because they have
iound mat red lead paint adheres so tena-
ciously TO iron and steel that even wire
brushes can detach IL only by prolong ed
amor They have also found that with a
Kind OF good paint can he applied for finish
ing coats. Many practices still exist in the
painting OF iron and steel which cannot lie
too strongly condemned. such, for insta nee,
~ the practice of applying n shop coat Of
nnseed oil. In order TO Obtain the protpctive
VHlne from q pigment IL must he close to the
steei. but a shop coat of linsepd oil prevents
TINS close contact which is 50 essential for
protection. a. w TUoupsnx"
BROOKLYN, july