Library Digitised Collections
Author/s:
Newman, S. H.
Title:
Paint and its relation to engineering (Paper & Discussion)
Date:
1941
Persistent Link:
http://hdl.handle.net/11343/24872
File Description:
Paint and its relation to engineering (Paper & Discussion)
68
VICTORIAN INSTITUTE OF ENGINEERS.
PAPER
PAINT AND ITS RELATION TO ENGINEERING.
By S. H. Newman.
In speaking on the subject of paint and its relation to Engineering, one must first define why metal surfaces are painted,
and in doing so, we consider that it is for either of the following
reasons : Firstly, to prevent corrosion, and secondly (and this
also combines with the first reason) to beautify the object in
question. In stating that the two are closely combined, it is clear
to everyone that there would be no beautification if rust stains
developed through the paint film. We can, therefore, define the
reason for painting metals as a prevention of corrosion.
My remarks to-night will refer chiefly to iron and steel, and
whilst the subject of this lecture covers other metals, we intend
first of all to discuss the corrosion of iron and steel and its prevention by painting. Large percentages of commercial iron and
steel are manufactured by hot rolling process, in which white
or red heated metal is exposed to the air. Whilst the metal is
at this high temperature, oxidation from the atmosphere is increased to a very considerable degree, and this is what forms
mill scale, which is well known to you all. No paint manufactured
will adhere satisfactorily to mill scale, due to the fact that the
mill scale flakes off the metal, and naturally will carry the paint
away at the same time. If mill scale would adhere to iron or steel
in a complete and continuous film, it would itself provide an
excellent protection against further oxidation.
Various methods are employed for removing this mill scale,
amongst which are pickling, sand-blasting, wire-brushing, chemical treatment and flame cleaning. Pickling is not practicable on
large steel structures; sand-blasting is quite efficient, but has the
disadvantage of making the exposed steel highly active, and will,
therefore, corrode very rapidly if not painted immediately, or if
the paint film is afterwards penetrated. At the present time,
wire-brushing appears to be the most satisfactory and economical method.
Some mill scale cannot be removed by this method, but any
scale which is so firmly adhered as not to be removed by wirebrushing, is not likely to flake off in service afterwards. Weathering is a method used frequently, but this is only on account of
its cheapness. A danger arises here in that the steel rusts where
the mill scale is not present, and excessive wire-brushing, or
other means, is necessary to remove it.
Chemical treatments, consisting of compounds containing
Phosphoric Acid, which convert the surface to iron phosphate,
are quite satisfactory for small steel structures, but impracticable for large structural work. The most recent development to
PAINT AND ITS RELATION TO ENGINEERING.
69
remove mill scale is what is termed flame cleaning. This is done
with an oxy-acetylene torch, moved over the surface, and the
heat loosens the mill scale sufficiently for it to fall off, or be
easily removed.
If the steel has no mill scale, it is still essential that surface
rusting be removed before painting, in order to obtain a satisfactory result. I have seen steel chimney stacks which were
erected and not immediately painted, on which rust developed,
and no matter how many coats of paint were applied afterwards,
wherever the rust had not been removed, it gradually showed
through the paint film.
The corrosion of iron is due largely to a form of electrolysis
which takes place in the presence of moisture and air. Impurities which are generally segregated throughout the metal, may
become electro-negative to iron, and in the presence of moisture
excite galvanic action which causes the more electro-positive
metals (the iron) to go into solution and form rust. This effect
may be prevented by placing on the surface of the metal substances which make the iron passive to electrolysis. Soluble
chromates and alkalies, or basis substances have this effect. If
chromate pigments or basic pigments are used in paint coatings,
their effect, when applied to metal, is to render the iron passive,
and thus prevent electrolysis and corrosion.
The extent of rusting is dependent upon the moisture content
of the atmosphere. In comparatively dry air, that is, with a low
humidity, very little rusting occurs. Atmospheres containing
chemicals, such as one finds around factory areas, seem to
increase the rusting rate. The rusting of steel is considerably
slowed down by the addition of chromium and nickel compounds,
making a low alloy steel along the lines of "rust proof" steel.
So far we have only discussed the rusting of iron and steel, but
the main theme of my talk is its prevention from a paint point
of view.
Theoretical causes for the corrosion of iron and steel have
been discussed in detail by many authorities, but all agree that
complete protection would be afforded if a thoroughly waterproof coat of paint could be applied. However, no paint coat is
completely impervious to moisture, which is the basic cause of all
corrosion. Furthermore, the usual thickness of a paint film
being not more than 1/200th of an inch, it is likely to be broken
in service. For this reason, the initial coat of paint in contact
with the metal must contain chemically active pigments which
will retard or inhibit corrosion, even though the film permits
moisture to pass through and reach the metal surface.
To follow this matter farther, considerable, research work has
been done to find the best pigment that will retard corrosion. In
fact, more work has been done on the pigment question of metal
70
VICTORIAN INSTITUTE OF ENGINEERS.
paints than on the vehicle. I expect you are all familiar with
these paint terms, inasmuch as the pigment is the dry colour,
such as red lead, lead chromate, aluminium powder, white lead,
etc., and the vehicle is the oil or liquid part of the formula. The
only change made of recent years in the vehicle part of these
formulations is to use synthetic mediums, but whilst these have
shown results slightly better than raw and boiled linseed oil, it
has been proved that without the correct pigment they are for
from satisfactory. There is one important point in the selection
of a vehicle, and that is its rate of expansion and contraction
under heat and cold, which must closely resemble that of iron
and steel. Many metal paints have cracked owing to insufficient
expansion when the metal is exposed either to artificial heat
or summer conditions. Strangely enough, we have also seen
faults develop with an exterior metal paint which expanded too
rapidly, and the cracking occurred when cool conditions were
experienced, such as a sudden thunder storm, followed by heavy
rain, after a rather hot day. In engineering, you are quite
familiar with the expansion and contraction of metals, and can
fully realise the extent to which a paint film must be elasticised.
In the experience of most people, who have made a study of
painting metals, it has been proved, as stated before, that the
pigment used is the most important. Henry Gardiner, one of the
leading authorities on paint in America, conducted a series of
tests, in which he exposed several hundred steel and iron panels,
18 in. x 36 in. x 18 guage metal, and these were painted with
different pigments, using the same vehicle and same percentage
of vehicle in each case, this being 2/3 raw linseed oil and 1/3
boiled linseed oil. Altogether, thirty-five different pigments or
combinations of pigments were tested. Gardiner did these tests
to discover which pigments had the best rust-inhibiting properties, and after four years a committee of interested persons gave
the following ratings as their respective values. The highest rating was awarded to American vermilion (lead chromate), then
followed zinc and lead chromate mixed, zinc chromate straight,
willow charcoal, and whilst the list is very lengthy, and I do
not propose to give you the figures of every pigment, it may
interest you to know the positions on this list of many wellknown pigments used in metal paints. Red lead was No. 11 on
the list ; graphite, No. 12 ; white lead, No. 24 ; red oxide, No. 19 c
but magnetic black oxide of iron was No. 6 ; zinc oxide, No. 28
whilst ultramarine blue was a very bad last.
In one way, this information may not be wholly correct, because I know of other tests in which lead and zinc chromate,
which Gardiner rated as second best, together with red lead
(rated as eleventh), were combined to give a primer which was
rated as the most satisfactory.
PAINT AND ITS RELATION TO ENGINEERING.
71
Many people still prefer red lead as the best primer for metal,
but I am sure that although red lead will make a satisfactory
primer, as it has done for the last 50 years, later developments
can give us better primer mixtures than straight red lead.. It is
surprising how many people still demand red lead, and yet, if
you asked them to do any other part of their work in the same
manner as it was done fifty years ago, they would laugh you to
scorn._
With certain metal surfaces which are obscured from view,
and do not require painting from a decorative standpoint, it is
quite common practice to apply a low-priced bituminous coating,
representing both priming and finishing coats, which dual purpose in itself is quite a severe expectation. However, this will
give reasonable protection at a low cost, but has the further disadvantage of the fact that these coatings are only obtainable in
black, and cannot at any time be repainted in any other colour
if change of position necessitates their coming into prominence.
To sum up the priming coat on iron or steel, it is essential that
these being the foundation, even more care is needed in the selection of the paint for this coat than for the finishing coats.
Naturally, the finishing coat must be durable and weather resisting, but a breakdown of the finishing coat often means repainting only, whereas the breakdown of a primer means failure of
the finishing coat, and, if neglected, complete deterioration of the
metal.
Before leaving the subject of priming paints for iron and
steel, I would like to impress upon you the three important factors necessary to produce a satisfactory and durable finish.
Firstly, all rust and mill scale should be removed before application of the primer. Secondly, the metal surface should be free
from grease or oil, and naturally, any dirt. Thirdly, be certain
that a correctly formulated primer is selected.
The type of final coat on iron or steel is dependent largely
upon the particular requirements of the individual case. All
paints may be classified under two headings, viz., external and
internal. In using the word "external," we refer to surfaces
which are exposed to outside conditions, whilst by "internal,"
we mean such surfaces as are not subjected to the elements, and
are, generally speaking, inside buildings. Let us, therefore, consider finishing paints under these two headings.
Finishing metal paints for external use have to be of such a
nature as to withstand the destructive influence of moisture and
sunlight. In the case of finishing coats for external work, its
anality is mainly derived from the vehicle part of the formula.
The pigments do not play such an important part as they do in
the case of primers, although naturally some pigments assist the
vehicle in its weather-resisting properties better than others.
fl
72
VICTORIAN INSTITUTE OF ENGINEERS.
From a decorative point of view, they are required in colours,
probably to tone in with the surroundings, and are often required in high gloss finishes. Providing the paint is so formulated to give best outside durability, and is to be applied over a
good priming coat, the range of satisfactory finishing metal
paints is extensive.
In selecting the colour, it may be necessary to consider the
heat reflective properties, or in other words, the changes in temperature of any building or large container painted in colours.
To show you the effect the selection of colour will have on temperature, I shall quote from a bulletin issued by the Educational
Bureau, Scientific Section of the Paint Manufacturers' Association of United States, on the heat reflecting properties of colours
applied to oil and petrol storage tanks.
"In considering the effect of the different types of rays of
which light is composed, we find that the calorific or heat-producing rays are conducted by painted or finished objects in
widely varying degree. This fact should be studied by the constructive painter who is called upon to paint the enormous areas
presented by metal oil tanks. Such tanks may contain light distillates which, upon becoming warm, produce highly expansible
vapors. When black or dark-coloured paints have been used,
rapid absorption of heat takes place, and considerable losses by
evaporation are apt to occur. White or light-coloured paint
should, therefore, be used for the finishing coats of oil storage
tanks. Paints presenting a high gloss are, moreover, less absorptive of thermal rays than those presenting a matte surface. The
following figures give the rise in temperature of benzine contained in small tanks painted in varnish colours (gloss finish),
when subjected to rays of carbon arc for periods of fifteen
minutes
Rise in Degrees
Colour
Fahrenheit.
Tinplate .. .. .. .. .. .. ..
19.8
Aluminium Paint
.. .. .. .. ..
20.5
White Paint .. .. .. .. .. ..
.. .. .. ..
22.5
Light Cream Paint .. .. ..
23.0
Light Pink Paint .. .. .. .
23.7
Light Blue Paint .. .. .. ..
24.3
Light Grey Paint .. .. .. ..
26.3
Light Green Paint .. .. .. .. .
26.6
Red Iron Oxide Paint .. .. .
29.7
Dark Prussian Blue Paint ..
36.7
Dark Chrome Green Paint ..
39.9
Black Paint .: .. .. .. .. ..
54.0
i
PAINT AND ITS RELATION TO ENGINEERING.
73
You will note from these figures that tinplate gives the lowest
rise in temperature. It would, of course, be impossible to manufacture a building or large container solely of tinplate, and,
therefore, the painted surface which came a close second, viz.,
aluminium paint, is the one mostly selected for this work. That
is the reason why most of the petrol companies paint their tanks
with an aluminium finishing paint, but this pigment, due to the
war position, is now practically unobtainable.
Another test conducted in America on some extremely large
tanks showed that the one painted black had an evaporation of
over 8000 gallons, in one year, more than a similar tank painted
with aluminium. Therefore, the paint industry undoubtedly
plays a big part in assisting the supply of petrol, which is certainly a problem in these rationing days.
My remarks this evening have been more or less restricted to
structural iron and steel work, but before leaving the question
of external metal paints, a few remarks covering the motor . car
industry would be appropriate. The subject of the painting of
motor vehicles and the like can be extended indefinitely, but
to-night I propose to touch only briefly upon this aspect. Remarks which have been made previously on the priming coat
still apply in this industry, excepting that in most cases the
primer is stoved or baked on to the metal. This is done only to
save time and space. For example, a long oil primer will give
equally good results, but this would not air dry under fortyeight hours at ordinary temperatures. The same material could
be stoved onto a car body in one hour without affecting the life
of the film.
Finishing coats in the motor car industry at the present time
are nitro cellulose lacquer and synthetic stoving enamels. Nitro
cellulose lacquers have been used by the motor car industry
fairly extensively for the last fifteen years, but have been displaced in America by synthetic stoving enamels, and the same
trend is gradually developing in this country.
For interior metal coating, the problem becomes one of the
individual conditions required. The metal may have to face the
fumes of acids or alkalies under factory conditions. It may have
to be heat-resisting for certain other work. Then again, machine
tools on which are used emulsified cutting oils present another
problem, as well as some interior metal parts requiring hardness
of finish due to the particularly severe usage to which they are
subjected. This by no means covers all types of finishes called
for. For example, dozens of different finishes are being manufactured for use on interior metal surfaces, each one being made
with a definite requirement in view. Take, for instance, white
enamel on a metal refrigerator, the appearance of which is well
74
VICTORIAN INSTITUTE OF ENGINEERS.
known to you all. This would have to withstand the action of
butter, milk, grease, etc., and tests conducted have proven how
injurious is the action of butter on the ordinary paint film. It
must, therefore, be tough and hard to withstand such foods, as
well as standing wear and tear without being brittle. It must not
discolour under heat, sunlight, humidity and darkness. It must
not absorb the odour of food, or give out any odour of its own.
With such qualifications necessary, production of a suitable line
was not an easy matter, and represented a considerable amount
of research work.
In the case of alkali and acid fumes being present, special
formulations of paint are manufactured to withstand these conditions. They are mainly based upon chlorinated rubber.
Chlorinated rubber was first prepared in the middle of the nineteenth century, but it was not prepared commercially until 1917.
At that time the method employed was to chlorinate a 4% solution of rubber in carbon tetrachloride, obtaining a product with
65% chlorine. Since then, numerous improvements have been
made in the manufacture of chlorinated rubber, and the most
recent is a product with greatly improved flexibility, which it
retains on aging. This material is sometimes used on its own, or
in conjunction with other gums.
Formulations of paints for application to metal to withstand
heat have changed rapidly during recent years, and the latest
material is a special synthetic medium, to which is added zinc
dust before application to the metal. Metals coated with a paint
of this nature can be heated to redness, and the paint film
remains unaffected, and is, in fact, improved, for it has the
effect of baking the finish hard onto the surface. This has even
surpassed results obtained by the use of aluminium paints.
In engineering workshops, the selection of a satisfactory paint
for finishing machine tools presents a problem. This is chiefly
due to the emulsified cutting oils which are solvents for most oil
paints, and in addition, the paint applied must be sufficiently
hard to withstand severe contact by other tools, etc. Synthetic
enamels based on phthalate resins do stand up to such conditions.
At one time a machine was regarded purely from the engineering work which it turned out. Nowadays, these machine tools are
painted, often in bright colours, and it has been proved that a
clean looking machine encourages the workman to keep his work
and the workshop clean.
Before leaving iron and steel, we should cover the painting of
galvanised iron. This type of iron causes more trouble from a
painting angle than most metals; yet if the correct procedure is
followed, the difficulty is easily overcome. You all know that the
exterior coating on galvanised iron is zinc, and even the old
PAINT AND ITS RELATION TO ENGINEERING.
75
masters knew that paint would not stick to zinc. Years before
this metal attained commercial importance, zinc palettes were
used on which to mix colours, because even after drying the
paints could be easily scraped from the zinc surface with a
palette knife. This paint-shedding characteristic of zinc was later
found to be equally true of cadmium. While it was a decided
advantage to the old masters, nevertheless this peculiar characteristic is a source of annoyance to the moderns, who, due to this
property, are forced to repaint galvanised structures frequently
in order to prevent the corrosion of the zinc coating.
The problem of making paint to hold to galvanised iron is as
old as galvanised iron itself. Even the first to observe the peeling of the paint felt that something had to be done about it. The
number of medicines proposed has been enormous. They have
ranged from salts of silver to raspberry vinegar, and have included lemon juice, ammonia, blue vitriol, ordinary vinegar,
muriatic acid, nitric acid, sulphuric acid, acetic acid, the
chlorides of copper and antimony, the acetates of copper and
ammonia, zinc dust, and many similar chemicals and
conjectures.
The reason why paint peels off galvanised iron that has not
been treated is a problem that has been solved only by careful
study.
This study indicated that paint, irrespective of its quality or
the care exercised in applying it, is pervious to the atmosphere
which among other things contains moisture, oxygen and carbon
dioxide. It is these materials which slowly filter through the
paint and react with the zinc (or cadmium) to form oxides and
carbonates between the paint and the metal. These oxides and
carbonates are sufficiently alkaline to attack the constituents of
the paint, forming metallic soaps that break the bond between
the paint and the metal. The bond being broken, and the paint
being suspended in mid-air, so to speak, the slightest force
exerted on the paint, as, for instance, the friction furnished by
the wind or the relative expansion caused by the sun, caused the
paint to leave the metal and to expose the soap. This film of soap
offers protection for the zinc beneath it, but not sufficient to make
it weather-proof. Sooner or later, depending on the severity of
the climate, the zinc is converted into non-adherent oxides and
carbonates which are easily removed by the elements, thus leaving the steel free to rust.
It is considered by many that the most satisfactory method of
painting galvanised iron is to allow it to age in the weather for
some six months, prior to painting. This removes all of the destructive elements. If the galvanised iron is to be painted
immediately, the most satisfactory method that has been proven
76
VICTORIAN INSTITUTE OF ENGINEERS.
in a commercial way, is a wash over with copper sulphate solution, in the proportion of 2 lbs. of copper sulphate to the gallon
of water. After standing for approximately fifteen minutes, the
galvanised iron should be hosed with water to remove surplus
copper sulphate. Galvanised iron under this process will turn
black in colour, and when thoroughly free from moisture, can
be painted • in a similar manner to any iron surface.
You will be aware, probably more fully than I am, of the hundred and one forms in which iron and steel can exist, and whilst
I have not covered by any means, from a painting angle, every
type of iron and steel, what I have said covers the general question of paint problems on iron and steel.
Chief amongst other metals requiring paint is aluminium and
its alloys. This metal originally gave considerable trouble, and
adhesion of a paint film was perfected only after extensive
research work had been carried out. The most satisfactory method
for preparing aluminium and aluminium alloy surfaces for
painting is the anodising process which should be used wherever
possible.
Anodising is an electro chemical process which removes all
surface impurities and deposits a microscopic film of aluminium
oxide in such a condition as to provide excellent "tooth" for
subsequent painting coats. Whether the aluminium has been
anodised or not, the most successful priming coat is one in which
the main pigment is zinc chromate. Very thin coats should be
applied, and this brings to light a very important point, namely,
that paint films applied to metals which have a natural tendency
to resist the adhesion of such a film, should only be of the very
minimum film thickness.
Other metals which can be covered briefly are copper and its
alloys, including brass, etc. All of these metals have what is
termed in the painting trade a very greasy surface, and are very
resistant to adhesion of a paint film This does not mean grease,
but is a name describing the nature of these metals. Most satisfactory results are obtained by buffing, sand-blasting, or any
method of producing an etched surface on the metal. In priming
these metals, a thin film of a primer, the pigment of which is zinc
chromate, gives the best results.
Another metal surface which often gives trouble is cadmium
plating. Some little while back I was connected with an investigation regarding metal signs. The manufacturers of these
signs, in order to deliver an excellent piece of workmanship,
decided to cadmium plate them, with the idea that if the paint
were chipped off, after completion and erection, no corrosion of
the metal would take place. In this regard, the idea was excellent, but difficulty was immediately experienced in obtaining
DISCUSSION.
77
satisfactory adhesion by the paint which was to go on these
signs. Many experiments were conducted, and here again the
primer, in which zinc chromate was the pigment content, gave
the only satisfactory results. Red lead and other well-known
priming paints tried in this case were hopless failures.
From my remarks you may think that the painting of metals
is a difficult matter. I do not want to leave you with this impression, for you will notice that I have covered in this paper more
of the troubles which can and do exist, and their remedies, rather
than the better known path of painting. In doing this, I have
considered the instructive value, as we all have to face the difficult tasks at some time or other, and I hope the foregoing will
be of some assistance when you strike any of these problems.
DISCUSSION
In reply to questions from various members the author said :
Bonderising had proved very successful in reducing corrosion ;
a penetration of one two-thousandth of an inch had been
secured. Regarding the use of rust inhibitors, costs were a factor retarding progress in their use. Pigments played a greater
part than the vehicle in heat absorption. Many persons believed
that aluminium paint was a good rust preventive, but he considered chrome pigments were much superior. Driers were added
to paint because the public demanded quick drying; but they
had a detrimental effect. In reply to Mr. Ed. Watson, the author
said that the use of infra-red rays for rapid drying had proved
very successful. Paint, which under ordinary conditions would
take many hours to dry, had required only ten minutes, or even
five minutes, under infra-red rays. Moreover, the paint was, if
anything, improved by this treatment.