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PAINTING
METAL BRIDGES
;v-
BY
* WILLIAM B. MACKENZIE
MKM. CAN. SOC. CE., MEM. AM. SOa C.K., ASSISTANT ENGINEER
INTERCOLONIAL RAILWAY
MONCTON, CANADA.
^*»r
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Reprinted from The Canadian Engineer
1897.
V
PRICE, TWENTY-FIVE CENTS.
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PAINTING
METAL BRIDGES
BY
WILLIAM B. MACKENZIE
MKM. CAN. SOC. C.E., MEM. AM. SOC. C.E,, ASSISTANT ENGINEER
INTERCOLONIAL RAILWAY
MONCTON, CANADA.
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Reprinted .^rom The Canadian Engineer
1897.
PRICE, TWENTY-FIVE CENTS.
I
J
Painting Metal Bridges.
In the early part of 1895 I began to investigate the
subject of painting metal bridges. After reading what lit-
erature I could obtain, I determined to make a few experi-
ments for myself, the results of which are here presented.
I do not feel at liberty to give manufacturers' names in a
communication of this kind, particularly as my tests are
so few ; but I have no objection to giving, in a less public
way, such facts as I have, to persons interested in the
subject.
Twenty-four new wrought-iron plates, one foot square
and three-eighths of an inch thick each, were painted two
coats of different kinds of paint, under precisely similar con-
ditions, the boiled linseed oil being the same in all, excepting
those samples obtained ready-mixed from the manufac-
turers. These plates were provided with hooks on the
back, and hung on the lower chord eye-bars of a steel rail-
road bridge i ,900 feet long, across a strait or arm of the
sea, in latitude 45° 56' N., longitude 6o°59'W. They
stood vertically at a height of nine feet above the water
surface, and were exposed to the sun and salt air at all
times, and also to salt spray in rough weather. They
faced the prevailing wind from the north-east.
This strait, or arm of the sea, is situated about 35 miles
from the Atlantic Ocean, lies northeast and south-west,
and forms a narrow passage five miles long and 1,900 feet
wide, between two large inland basins. The strait is
bounded by high land on both sides, the bridge crossing it
in an east and west direction. The wind usually comes
from the north-east, and a light breeze causes spray to be
dashed against the floor. In storms, the spray is thrown
over the floor and across the bridge. The rise of tide in
4
the open sea, 35 miles distant, is five feet, but here the rise
is only six inches.
Metal corrodes here very rapidly. The bridge itself
was erected in 1890. One coat of iron oxide paint was
applied in the shop and another coat given after erection.
In 1H92, it was consi<lerably rusted, and two coats of iron
oxide paint were put on the lower chords and floor sys-
tem ; but without proper inspection or scraping. In 1894,
scales one-eighth of an inch thick were removed from the
end stiffeners of the floor-beams on the north side. Pieces
of this scale were trimmed to exact dimensions and the
cubic contents calculated. On. being weighed, the scale
was found to weigh slightly more than one-half that of
new steel.
A chemical analysis of iron-rust scale from the out-
side of the Conway Tubular Bridge, in England, is as
follows :
Sesquioxide of iron 92.9 percent.
Protoxide of iron 6.177 "
Carbonate of iron 0.617 "
Carbonate of lime 0.295
Silica 0.121
Ammonia trace.
100.000 "
In August, 1895, O"^ bridge was thoroughly scraped
and painted with two coats of iron oxide paint, the analy-
sis of which is as follows :
Color Indian red
Fineness 80 00
Body 85.00
Strength 700°
Iron oxide 48 iG percent.
Insoluble matter 5^-^4 "
Adulterated slightly with clay.
It is now in good condition ; but will require constant
attention lo rei:^ :dy defects in the painted surface as they
appear.
It will be seen by the record of the experimental
plates (see table) that the asphalt paints, the carbon
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paints, the coal-tar coatings, and some of the oxide of
iron paints are already out of the race, and that neither
the lead nor the graphite paints are holding out quite as
well as might have been expected.
A chemical analysis of the water beneath the bridge
was arrived at as follows :
The average density of ocean water is 1.026, and the
composition is as follows :
Water 96.5 per cent.
Salts 3.5
The composition of the salts is as follows :--
Chloride of sodium 77-758
C^ilcride of magnesium 10.878
Sulphate of magnesium 4 737
Sulphate of lime 3.600
Sulphate of potash 2 465
Bromide of magnesium 0.217
Carbonate of lime 0.345
Total salts 100.000
The density of the water under the bridge was deter-
mined roughly by weighing a certain volume and compar-
ing it with the weight of a like volume of fresh water.
This gave a density of i. 0081 3, which would make the
composition as follows : —
Water 98.215 per cent.
Salts , 1.785
100.000 per cent.
The composition of the salts would be the same as
that given above for ocean water.
The highest summer temperature of the air in 1895
was 82° F., and the lowest temperature in winter of 1895-6
was — 2° F. The highest summer temperatureof theair in
1896 was 72° F., and the lowesi temperature in winter of
1896-7 was — 7°. The summer temperature of the sea-water
under the bridge is 60° F. to 63° F., and the winter tem-
perature 30° F. to 35° F. Total pre^^ipitation in 1895 was
probably about 70 inches. Total precipitation in 1896
was 69.86 inches. Snow lies on the ground, more or less.
i
j
7
in depths from three inches to three feet, between the
middle of December and the middle of April. The great-
est velocity of the wind during north-east autumn and
winter storms is about 60 miles per hour.
CORROSION.
Corrosion is most active in autumn and winter, be-
cause there is then more moisture deposited upon the
metal, and the water contains a larger proportion of oxy-
gen and carbonic acid.
At 32" F. water will absorb 4.9% of its own bu)k of oxygen.
At 5o°F. " 3.8% ••
At 68*'F. '• 3.1% •• «•
Snow water contains more oxygen v'lan rain or river
water and will rust metal quicker. Cole' water dissolves
more carbon dioxide than warmer water. At -32° F. water
will dissolve i.3 volumes of carbon dioxide, and at 60*^ F.
only one-half as much. Pure rain-water contains '2^
volumes of air in 100 volumes of water. If water is freed
from oxygen by boiling, iron will not rust in it, nor will
it rust in perfectly dry air. Rust consists of iron, oxygen
and water and it requires a simultaneous action of oxygen
and water to produce it. Damp oxygen and damp car-
bon dioxide in combination produce rust quickly. Neither
will do so when dry either together or separately, and
only to a very slight extent when damp, separately. It
requires the combination of both, damp, to rust quickly.
Steel, when unprotected and ex-
posed to the weather and sea-
water, corrodes at the rate of
^2 of an inch per year or i inch in 82 years.
Wrought-iron, under same condi-
tions, corrodes at the rate of ^hv
of an inch per year or 1 inch in 190 years.
Steel, unprotected and exposed to
the weather and fresh water, cor-
rodes at the rate of y|^ of an
inch per year or i inch in 170 years.
8
Wrought-iron, under same condi-
tions, corrodes at the rate of
^Ijf of an inch per year or i inch in 430 years.
Steel, unprotected and submerged
in sea-water, corrodes at the
rate of y^,y of an inch per yjar, or i inch in 130 years.
Wrought-iron, under same condi-
tions, corrodes at the rate of ^}^
of an inch per year or i inch in 310 years.
Steel, unprotected and submerged
in fresh water, corrodes at the
rate of ^^ of an inch per yea/, or i inch in 600 years.
Wrought-iron, under same condi-
tions, corrodes at the rate of ^^^
of an inch per year or i inch in 700 years.
Wrought-iron in an overhead bridge, subjected to
coal-smoke from locomotives, corroded in 25 years from
39.5 per cent, or 1.8 per cent, per year to 100 per cent.,
or 4 per cent, per year, some of the members being en-
tirely eaten away.
Unstrained members corrode more quickly than
strained members. Shaded parts will corrode more slowly
than parts exposed directly to the sun's rays.
Real iron-rust does not promote further rusting be-
cause of any chemical influence on the iron, but being a
spongy mass, it retains in its pores 24 per cent, of water
deposited as rain or dew. It does not, therefore, prevent,
but rather encourages rusting, and in this way has a phy-
sically injurious effect upon iron. Corrosion accelerates
with time, the second year's being 50 per cent, greater
than the first.
{
r
LINSEED OIL.
In paint, oil is king ; any finely ground pigment, inert
toward the metal and oil, will last until the oil decays and
wastes away, and against this decay and waste there is no
remedy. The raw oil is obtained by both cold and hot
preoi,ure from linseed, or the seed of the flax plant, Linum
I
r
usitatissimum. When cold-drawn, the color of the oil
is golden-yellow, and when hot-pressed, brown-yellow
The specific gravity is :
At ^o" F. = 0.9385
' 53^' F. = 0.9364
. " 55° F. = 0.9350
" 68° F. = 0.9325
" 77° F. = 09300
" 266° F. = Boiling-point
" — i6^° F., the oil congeals to a solid yellow mass.
It is sold under different forms : Raw, refined, boiled
and artist's oil. The seed should be ripe, and from two
to six months old. The quality of the oil is affected by
the quality of the seed, which is in turn ruled by the soil
and climate in which it is grown. Boiled oil is heated to
a temperature of from 266° to 600° F., and agitated me-
chanically for five or six hours. Water evaporates, and
the scum and froth is removed from the surface with
ladles; this scum is afterwards used in making putty.
Equal quantities of litharge and red lead are added by
slow degrees as dryers, to the extent of three per cent, of
the oil, a small proportion of umber being also thrown
in. The heat is continued for two or three hours, when
the fire is suddenly withdrawn and t^e oil left covered
over for ten hours longer. It is now known as "boiled
oil," and is stored in settling-tanks for a few weeks, during
which time the uncombined driers settle to the bottom
as " foots." The heating process darkens the color and
causes the oil to dry quickly, producing a hard firm surface.
Pure, unadulterated linseed oil is not a common article.
Driers are not infrequently added through the bunghole
without boihng. Cotton-seed oil, Niger oil, hemp-seed oil,
pippyseed oil, colza oil, rape seed oil, Lucca oil, resin
oil, turpentine oil, benzine, fish oil, aniaial oil and water
are often mixed with it, all or any of which shortens the
life of the paint. There are about 17 vegetable drying oils
which may be used in paint, and over 30 vegetable non-
drying oils, which may be used as adulterants. The
greater number of these oils are cheaper tlian linseed oil.
lO
In addition, there are the fish and animal oils, so that the
rarity of pure linseed oil is not to be wondered at.
The purity of linseed oil may be roughly tested by
shaking it well ; if iridescent bubbles appear on the sur-
face, it is adultorated with benzine or mineral oil ; if sul.
phuric acid is present, the paint when shaken and then
allowed to stand will thicken into a brown paste. Other
rough tests are : Brush it upon brown paper, and
expose to the sun's rays ; the water, benzine, etc., Avill
evaporate and leave the oil. Dip a sheet of well-sized
paper into the oil, and hang it up to dry ; when dry the
whole of the sheet should show a well -varnished coating ;
if only the bottom of the paper is varnished, the oil is in-
sufficiently boiled. Brush the oil on a smooth wood sur-
face ; if it turns white, " blooms " in drying, it is adulterated
with resin.
Oil, when spread out thinly, dries by absorbing oxygen
from the air ; the water and vapor passing out create mul-
titudes of very minute holes in the oil cover, where water
may enter ; these holes are partly filled up by the second
coat of paint. While the oil is absorbing oxygen, it adds
13 to 14 per cent, to its weight.
PIGMENTS.
After reading what the different manufacturers say
about the price, covering capacity, and durability of their
own particular paints, and the folly of using anything else,
a person is inclined to believe fully in David's hasty asser-
tion that " all men are liars."
The following comparative statement of the cost of
painting a loo-foot span steel rridge, with a number of
kinds of good paint, is taken from a statement published
by C. E. Fowler, C.E., in the Engineering News of Feb.
6th, 1896. The cist of painting spans from 20 feet to 300
feet was accurately determined, and it will be seen that,
after all, there is very little difference in the first cost
between good qualities of the usual kinds of paint used
for general railroad work ; there being only $9.25 differ-
ence between oxide of iron and red lead for a span of 100
feet.
II
Cost of painting a loo-fooi railway bridge (clean new
iron) with different kinds of pigments and linseed oil :
Oxide of iron — 14 gals, ist coat, at 50c; 10 gals. 2nd coat, at 50c.
Latxjr, $68.00 = $80.00
Red lead— 10 gals, ist coat, a* $1.25 ; 7 gals. 2nd coat, at $1.25.
Labor, $68.00 — $89.25
White Lead— 14 gals, ist coat, at 85c. ; 10 gals. 2nd coat, at 85c.
Labor, $68.00 = $88.40
Graphite— 14 gals, ist coat, at 70c. ; 10 gals. 2nd coat, at 70c.
Labor, $68.00 = $84.80
Asphalt— 23 gals, ist coat, at 40c. ; 14 gals. 2nd coat, at 40c.
Labor, $68.00 = $82.80
Carbonizing coating— 7 gals, ist coat, $1.50 ; 5 gals. 2nd coat, $i 50.
Labor, $68.00 = $86.00
SCRAPING.
The labor of scraping the metal and applying the
paint is from four to eight times the cost of the pigment
and oil ; so that there can be no economy in using cheap
paint. To remove scale, loose rust, oil, dirt and cinders,
use benzine, chisels and hammers, wire brushes and
scrapers, as may be necessary. Unless the metal is per-
fectly clean and dry, no paint can be successful. Scraping
should not proceed far ahead of the painting. Should salt
spray touch the scraped metal, it should be scraped again
before painting.
KIND OF PAINT AND ITS APPLICATION.
It is absolutely necessary that the first coat should
contain a large quantity of pigment, and should dry
quickly with a dense, firm surface, to receive the second
coat in from 48 to 72 hours. If not dry and firm, the
paint will blister, because of the separation of the first
coat from the metal. To secure this, a pigment of high
specific gravity must be used in boiled linseed oil, wl h a
considerable quantity of turpentine, and we cannot do
better than use a heavy, finely ground oxide of iron pig-
ment, inert toward both metal and oil, and which has
already been tested for durability, such as No. 1 of my
experimental plates. Iron oxides often contain acids, lime,
sulphur, clay, etc., so that there is much room for choice.
Red-lead has been largely used for first-coat work, because
12
it Jries quickly, with a hard surface ; but if the air, from
local causes, contains obnoxious gases, such as sulphuretted
hydrogen gas, produced by the passage of trains, the red-
lead will be quickly destroyed. J. Newman, author of
"Corrosion and FouHng," says by letter of 21st January,
1897 : "Probably the worst paint you can use for either
iron or steel is ordinary lead and oil paint." Oil alone
should not be used for priming in the shop ; it collects dirt
and cinders ; besides, the pu'-e oil dries, but never hardens.
As it contains no pigment, it is quite porous and pervious
to water; the surface will consequently expand and pres-
ent a shrivelled appearance and blisters will eventually
appear.
For second-coat work, an elastic but firm surface is
required to follow the expansion and contraction of the
rnetal and resist the mechanical impact of strong dust or
cinder-laden winds and rain, spray, hail and snow. In
this coat more boiled oil, a less weighty pigment, and a
less quantity of turpentine is required, so that it will dry
more slowly and for a longer time resist the sun's influ-
ence, which is ever tending to harden and crack the surface
and allow the entrance of water to the metal. A pigment,
then, of low spec^' -c gravity must be used. Crude graphite
ore powder has a specific gravity of about 0.7, and as
graphite cannot be affected by chemical influences, it would
seem to offer a suitable materia' for second-coat work. I
have tested it for a period of one-and-a-half years, and so
far it has done fairly well. Objections have been urged
against it as follows :
ist. It is expensive to grind to a high degree of fine-
ness, because of its oily and flaky character.
2nd. Because of its lightness, no great body of it can
be got into the oil.
3rd. It settles out of the oil.
Some of these objections may be more imaginary than
real.
A prominent manufacturer says that the composition
of the best graphite now mined, ground and used for
Dajnt, is 9S follows :
^3
ANALYSIS OF THE CRUDE ORE.
Moisture 0.15 per cent.
Graphite 33.^8 "
S''|ca 37.54
Oxide of iron 14.25 "
Pyrites of iron 1.27 "
Oxide of alumina .... 12.35 "
Lime 0.54 "
Magnesia 0.48
100.00
On referring to the statement of cost, it is seen that
for a 100-foot span bridge, the cost of graphite is $4.80
greater than iron oxide.
There exists in Canada, in large quantities, a cheap
natural product, the specific gravity of which, compared
with No. I iron oxide powder, is as 0.31 to i. This
material, after an inexpensive treatment, can be mixed
with the iron ore powder to reduce the weight and in-
crease the bulk, and on adding oil, a paint will be pro-
duced affording an elastic silicious surface which will re-
sist cracking, peeling and blistering for a much longer
time than the iron oxide alone.
On a clean, dry surface, paint applied at a tempera-
ture of 70° F., will last longest. Painting should not be
done in damp, wet or freezing weather, and on summer
mornings time must be given for the dew to evaporate
before work begins, as oil paint will not adhere to wet
metal. Painting should never be done by contract, but
by day work, under a competent foreman. In painting it
is best to begin at the top and work down. Buy paint in
powder form when possible ; next best in paste form.
Light colors quickly become covered with a layer of dust,
which absorbs as much heat as dark surfaces. The greater
the number of pigments mixed together, the shjrter the
life of the paint.
LIFE OF METAL BRIDGES.
Heretofore, owing to increases in train loads, flimsy
construction and neglect, the average life of iron bridges
has not been over 25 years, and it is now generally believed
among well-informed engineers that the average existence
14
of such properly designed metal bridges as have lately
been constructed throughout the country will not greatly
exceed fifty years. Where corrosion is particularly active,
as at the bridge of which I have spoken, the Hfe of the
structure must be much shorter, probably not over 35
years.
An English oxide of iron paint much used in this
country contains 48 per cent, of iron oxide and 52 per
cent, of insoluble material, and some American oxide of
iron paints contain as little as 33 per cent, of iron oxide
and 60 per cent, of silica. We have in Canada unlimited
quantities of iron ore, which requires no preparation ex-
cept grinding. Why should we then continue to import
that with which nature has so liberally endowed us, and why
should we continue to use unsuitable paints on locomo-
tives, stationary engines, cars, roofs, bridges, freight and
store-houses, when good iron oxide paints are cheaper and
more lasting ?
The company which will produce iron oxide paint
from Canadian ore, as good as the best, push its sale, and
show the people its advantages for works of this kind, will
benefit both themselves and those to whom they sell.
Hundreds of iron ore deposits exist in Canada, some afiford-
ing 72.4 per cent, metallic iron, and several extensive
graphite deposits, some of which produce 50 per cent, of
pure black lead, while the natural product of low specific
gravity for mixing with the iron ore powder can be sup-
plied pure in any desired quantity.
For part of the information contained in the foregoing
article, I am indebted to Prof. J. Spennrath's prize essay on
" Protective Coverings for Iron," 1896 ; J. Newman's
" Corrosion and Fouling," 1896, and to several articles in
engineering periodicals by such high authorities as W. P.
Wood, E. Gerber, W. G. Berg, A. H. Sabin, Samuel
Wallis, J. H. Stanwood, and J. E. Greiner.