We assess the potential causes of damage to mould box ends and find that both HOT corrosion, due to burning of emitted volatiles, and COLD corrosion, due to the presence of high humidity (and possibly high chloride ion concentration), are likely to contribute to the damage. We suggest strategies to minimise the damage, including venting of the volatiles, ventilation of the pallet grooves during cooling and minimisation of brushing effects during cleaning. Some calculations are described in an attempt to quantify the various effects discussed, but we are unable to accurately estimate their significance

Jenkins, D.R.

McElwain, S.

English

Mathematics in Industry Information Service Eprints Archive

123
CORROSION AND WEAR IN MOULDING BOXES
D.R. .lenkins! and D.L.S. McElwain2
We assess the potential causes of damage to mould box ends and find
that both HOT corrosion, due to burning of emitted volatiles, and
COLD corrosion, due to the presence of high humidity (and possibly
high chloride ion concentration), are likely to contribute to the dam-
age. We suggest strategies to minimise the damage, including venting
of the volatiles, ventilation of the pallet grooves during cooling and
minimisation of brushing effects during cleaning. Some calculations
are described in an attempt to quantify the various effects discussed,
but we are unable to accurately estimate their significance.
1. Introduction
Toowoomba Foundry makes a range of cast iron metal products including
brake drums, wheel spiders and pump housings. These are cast in large sand-
filled moulds. The mould boxes are made of metal and consist of separate top
(cope) and bottom (drag) parts, which sit on a pallet car that travels around the
mould line (see Figure 1). The top surface of the pallet car is ribbed, to allow
gaps between the bottom of the mould box and the car. The gaps provide vents
for steam volatiles and water to escape during the casting process. A mould is
filled with a mixture of damp sand, coal dust and binder, which is impacted into
the mould by compressed air. Blanks are placed in the mould boxes to create
the casting shape.
When liquid metal is poured into the mould, steam and volatiles are pro-
duced in the vicinity of the casting, and they vent through both the top and
bottom of the mould. The volatiles burn in the bottom vents and occasionally
at the mould top. The mould and pallet traverse a cooling tunnel for around 90
minutes, during which time the casting continues to cool and water, steam and
gases may continue to escape from the mould.
The mould boxes edges are observed to suffer damage, known to be caused
by corrosion, but only in the regions where the gases vent from the pallet car.
lCSIRO Mathematics and Information Sciences, Locked Bag 17, North Ryde NSW, Aus-
tralia 2113. Email David.Jenkins@cmis.csiro.au.
2School of Mathematics, Queensland University of Technology, GPO Box 2434, Brisbane
QLD, Australia 4001. Email s.mcelwain@qut.edu.au.
124 Toowoomba Foundry
Six vents for
steam and gas
from the mould
Casting
MouldingBox Cope
MouldingBox Drag
Cast Iron0 Pallet Car
Figure 1: Schematic of the mould box and pallet car.
The edges of the mould have corrosion-resistant metal welded to them, but the
corrosion pattern is still apparent.
The aim of this project is to investigate possible mechanisms that cause the
corrosion damage and to develop strategies that decrease the corrosion effect(s)
or even eliminate them.
2. Possible mechanisms
Corrosion is, in general, a complicated process, and it is not possible in a
study of this nature to consider the corrosion mechanisms in depth. There are
several useful textbooks on corrosion (e.g. Evans, 1976; West, 1980) that are
helpful, however, in describing general aspects of corrosion, and giving infor-
mation about rates of corrosion in particular circumstances. Our approach to
identifying the mechanisms of corrosion was to attempt to understand as many
aspects of the casting process as possible, and assess whether any of them con-
tribute to the corrosion.
The first observation we made was that, in normal operation, the mould
boxes cycle around the casting line with a period of about 2 hours. During that
time they are filled with sand, the metal is cast, cooled, the box is emptied
and then cleaned ready for the next cycle. We also observed that, for around
10 minutes after the metal is poured into the mould, flames emanate from the
grooves in the pallet car. The flames are caused by burning of the volatiles, and
the combustion products form a "cocktail" of gases that can promote corrosion.
While it is difficult to obtain a good estimate of the rate of corrosion, a "typical"
rate of corrosion in such an environment would be about 50 mm/yr (West, 1980)
Corrosion and wear in moulding boxes 125
if it occurred continuously. If we consider that this type of corrosion, which we
call HOT Corrosion occurs for only 10 minutes of a 2 hour cycle, and taking
into account the number of hours per year the foundry operates, we find that
a continuous corrosion rate of about 100 mm/yr would be required to account
for the amount of corrosion that is occurring (known to be about 2 mm/yr).
Thus we infer that HOT Corrosion is responsible for some of the damage that
is occurring.
A third observation is that there is a considerable amount of moisture lying
in the pallet car grooves while it traverses the cooling tunnel. This, presumably,
leads to a high humidity environment in the vicinity of the mould box edges,
which can promote corrosion. The presence of chloride ions in the water enhances
the corrosion rate further. Toowoomba Foundry uses town water in the process,
and this water has a relatively high chloride content. This type of corrosion,
which we call COLD corrosion, can occur at rates of around 4-5 mm/yr (Evans,
1976) if the moisture has a moderate chloride concentration, but this rate is
strongly dependent on the chloride concentration. Again, taking into account
the time available for this type of corrosion to occur during a foundry cycle, we
find that a rate of about 11 mm/yr would be required to account for the amount
of corrosion that is occurring. So, we infer that COLD corrosion is responsible
for some of the damage.
Another observation from the industrial representatives is that some sand
can escape through the damaged sections during the process of filling the mould
box with sand. The high pressure air forces the sand out. This phenomenon can
occur only once the damage is sufficient to allow the passage of sand through
the gaps between the cope and drag boxes. It was postulated that this type of
sand "blasting" may wear the box edges and thus contribute to the problem.
However, the fact that no such wear is seen on the cope boxes implies that wear
is not an important mechanism. An important feature of corrosion, though, is
that scale (the corrosion products) build up on the surface of the metal and can
act as a protective layer. The sand-blasting effect may act to remove the scale,
and thus providing a clean surface for corrosion to continue.
Similarly, the effect of cleaning the boxes once they are emptied may also
be removing scale, and thus contributing to corrosion. It has been observed
that the damaged areas show striations consisting of about 20 parallel, roughly
equally spaced grooves along the damaged portions of the mould box edges.
There appear to be too many such grooves across the width of the edges for
them to be associated with the process of welding the edges. (All box edges
have been welded with special hard metal material. This metal is applied as a
wire run around the box edge. About 5 wire thicknesses are required to cover the
edge, and it was speculated that variation in metal composition could account
for the striations, but this was discounted.)
126 Toowoomba Foundry
The cleaning process includes passing the mould box edges over a spinning
wire brush, made of hard steel. The brush wires move in the same direction
as the striations in the damaged sections, and we speculate that they cause the
striations. A straightforward stability argument shows that once a groove has
formed the wire strands will tend to slide into the deepest part of the grooves
which deepens the grooves even further. In any case, the action of the brush
would be to remove some scale and thus promote corrosion.
A final observation is that the damage pattern on the mould-box edge has
another interesting characteristic - it is deepest in the centre (in the direction
going from outside the box to inside). We believe this to be due to the effect of
the HOT corrosion, for the following reason.
We assume that HOT corrosion occurs with a rate dependence of the form
r = kCe-E/RT (1)
where k is a rate constant, C is the concentration of a constituent of the burning
volatiles that promotes corrosion, E is an activation energy, R is the universal
gas constant and T is the temperature of the surface of the mould box, at which
the reaction is occurring.
Now, we expect that the temperature of the mould box will decrease across
the mould box edge, towards the outside of the box, and that the presence of the
corrosion promoting contaminant may increase in the same direction. In that
case, it is easy to show that r will, in general, have a maximum value across the
edge thickness, leading to the concave corrosion pattern seen. This argument
may be somewhat tenuous, because it is very difficult to know what are the
constituents of the burning volatiles, but we expect that the oxygen content will
increase from inside to outside the box, and that this will likely increase the
corrosion rate, so that C may well represent oxygen concentration.
3. Prevention strategies
In the remainder of this report we present some strategies for minimising
or preventing the corrosion effects identified above and attempt some simple
modelling approaches that may assist in evaluating their effectiveness.
3.1 Venting
The volatile gas that is generated when the molten metal is poured escapes
from both the top and bottom of the mould boxes. It is only at the bottom of the
Corrosion and wear in moulding boxes 127
boxes that these gases cause a problem with corrosion, so we wish to investigate
whether it is possible to divert a significant amount of the volatile gases from
escaping to the lips of the bottom of the mould (where the corrosion is occurring)
by the strategic placement of vents in the sand mould. The moulds themselves
are comprised of compacted moist sand containing coal dust and vents could
be stamped into the compacted sand at the same time as the mould pattern.
When the molten metal is poured into the mould the gas, comprised of air, water
vapour and volatiles, at the molten metal/sand interface will undergo a rapid
increase in pressure. The equations which describe the mechanisms of general
heat and mass transfer through a multiphase system are reasonably well known
and could in principle be solved by a number of techniques, probably involving
numerical methods. Here we avoid any complex modelling of the multiphase
system and examine the effectiveness of the vents by the following simplified
problem.
Consider a porous medium, n, in two dimensions, containing a single gas
phase under pressure at a boundary, I'j, On the boundaries r2 zero flux con-
ditions are imposed and at the porous medium/atmosphere boundaries, r3, the
gas pressure is set to atmospheric. We assume that the gas phase is governed
by Darcy's Law:
K
q= --\lp,
J..L
(2)
where q is the specific discharge, K is the intrinsic permeability of the porous
medium, J..L is the viscosity of the gas phase and p is the pressure relative to the
atmosphere. The domain and boundaries, showing a rough placement of the air
vent, are shown in Figure 2. Assuming also incompressible flow, i.e. \l.q = 0,
the problem can be posed in the nondimensional form
p = 1, x E r1; n.\lp= 0, x E r2; p = 0, x E r3
(3)
(4)
where n is the unit outward normal vector on r2 and we have ignored the grav-
itational term. This is a standard problem and can be solved either analytically
(e.g. using conformal mapping techniques) or numerically. We use the finite
element package Fastfto to obtain the pressure solution and then calculate the
streamlines shown in Figures 3 and 4. From the streamlines one can get an
immediate indication of the tendency of flow of the gas. Figure 3 shows the
flow trajectories for a vent with a relatively moderate depth of penetration into
the mould. Only a relatively small amount of gas is diverted from escaping the
lower boundary. Figure 4 shows an air vent placed closer to the source of gas
(i.e. the edge of the casting) with a greater penetration. Most of the streamlines
128
r3 r3
r2 r3 r3
- r;r1 r2
-
r2
r3
Toowoomba Foundry
Figure 2: Definition of the solution domain, which represents half of a mould box.
The casting, which causes the release of gas, is at the portion of the boundary
marked r1.
emanating from the boundary r1 are intersected by the vent in this case, so
most of the gas will be diverted away from the lower boundary.
Of course, a real casting has a complicated shape in three dimensions, so we
expect that several such vents would be required to achieve sufficient diversion
of gas away from the bottom of the mould. However,the principle of placing the
vents close to the casting would still apply, but this would need to be carefully
considered, in order that the integrity of the casting itself was not compromised
by weakening the sand structure.
There are other possible means of re-directing the gas flow. During the
course of the MISG, the Toowoomba Foundry representatives began to investi-
gate means of diverting the gases away from the pallet grooves and into gaps in
the sides of the boxes, where any effects of corrosionwould be of no consequence
to the process.
3.2 Groove ventilation
It is clear that the high humidity, due to the presence of moisture in the
pallet grooves, increases the rate of COLD corrosion. Therefore, reducing the
Corrosion and wear in moulding boxes
i i : .
. / ./ / i .:'!
.'.' /
-----_._--
'",-
-,
\,
129
Figure 3: Calculated streamlines for a mould containing a vent with a moderate
penetration.
Figure 4: Calculated streamlines for a mould containing a vent that is both close
to the source and has significant penetration into the mould.
130 Toowoomba Foundry
humidity in the grooves of the pallet cars may significantly reduce the rate of
COLD corrosion. One possible means of doing this would be to blow air along
the grooves as the pallet cars traverse the cooling tunnel, where most of the
COLD corrosion is likely to occur. Toowoomba Foundry representatives were
considering the feasibility of devising a means of providing such ventilation.
3.3 Chloride reduction
The presence of chloride in the water supply may be a significant factor in
the corrosion. In fact, the concentration of chlorides in the damp sand may be
considerably higher than that in the water. To appreciate this, we again need
to consider the process. After a casting is removed from a mould box, some of
the sand used is lost, due to it being attached to the casting. Sand is therefore
added to make up that which is lost. In addition, water is added to the mould
box during a water cooling stage. If we assume that all of the chloride in the
water attaches to the sand, then chloride in the sand continually increases, as
shown by the difference formula
(5)
where Ci is the chloride concentration of the sand after being used i times in a
mould, Vb is the volume of sand in the box, Vi is the volume of sand lost (and
consequently the volume added back in), Ps is the bulk density of the sand, Pw
is the density of water, fm is the mass fraction of moisture in the added sand,
Vw is the volume of water added, per box, during cooling and Cw is the chloride
content of the town water supply.
The solution of this difference equation has a limiting value of
(6)
and using estimates of the quantities in the above equation we find that
and, as noted before, the chloride concentration in the incoming town water
supply is already high.
While this looks slightly alarming, it must be noted that we have assumed
the chloride ions remain attached to the sand, but if this is the case then they will
not be in solution, where they can contribute to corrosion. As a result, it is not
clear that the build-up of chloride ions in the sand is necessarily deleterious. The
Corrosion and wear in moulding boxes 131
only solution to reducing the chloride ions in the system is to use water with
a low chloride content. This would require the demineralisation of all water
used in the mould line, which we were assured by the industry representatives
is economically not practical.
3.4 Brush modification
Should our observation relating to striations in the damaged areas be correct,
then it would be desirable to consider modifying the method of cleaning the
mould box ends, particularly in the region of the corrosion damage. This may
prevent removal of scale on the damaged portions, and may also eliminate the
striations.
4. Discussion
Although we have identified plausible mechanisms for the corrosion occur-
ring on the mould box ends, and have suggested strategies aimed at reducing
the corrosion, we have been unable to clearly quantify the effects. We have
shown that both HOT and COLD corrosion can contribute to the damage, but
it proved impossible to determine whether one type dominated the other. There
is a tradeoff between the high corrosion rate of HOT corrosion with its short
duration, and the lower corrosion rate of COLD corrosion with a considerably
longer duration. Nevertheless, we would expect that with a combination of the
strategies suggested above, the amount of corrosion should at least be reduced.
Acknowledgements
The moderators, Sean McElwain and David Jenkins, were greatly assisted
by Doug Harland, Mithiran Kirupairajah, Malcolm Johnson and Vance Wolff of
Toowoomba Foundry. A number of people contributed to the work presented
here, but special mention goes to Garry Pantelis for producing the venting so-
lutions and Nina Notomi for preparing an excellent report on the causes of
corrosion. Other contributors included Leigh Brookshaw, John Cogill, Julie
Clutterbuck, Otto Konstandatos, David Mander, Graeme Pettet, Kashif Rasul,
Michael Simpson and Madjid Waladan.
References
U.R. Evans, The Corrosion and Oxidation of Metals (Edward Arnold, 1976).
J.M. West, Basic Corrosion and Oxidation (Ellis Horwood, 1980).


Corrosion and wear in moulding boxes

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