Corrosion and wear in moulding boxes

Toowoomba Foundry

Corrosion and wear in moulding boxes can be reduced by incorporating vents in the sand mixture in the boxes. This allows the gases generated during casting to rise to the top of the sand, rather than passing underneath the edges of the drag boxes where corrosion is most severe. Dehumidifying the air in the cooling tunnels, and treating the water to reduce the level of chlorine, also helps.

Established in 1871, Toowoomba Foundry first served Australia's farming communities making parts for windmills and irrigation pumps. It has a reputation for high-quality metalwork. Now a subsidiary of National Consolidated Limited, it has installed an air impact mould line to manufacture cast iron metal products such as brake drums, wheel spiders, pump housings and impellers.

Drag box
Moulded parts in the lower half of a sand mould. The boxes containing the sand become badly corroded. This corrosion can be reduced by putting vents in the sand mixture, by dehumidifying the air in the cooling tunnels, and by reducing the level of chlorine in the water used during processing.

Moulds are fashioned in compacted sand carried in two boxes made from mild steel sitting one atop the other. These boxes‹known as the cope (top) and drag (bottom)‹are transported through the moulding cycle on flat-top pallet cars. But, in less than three years, the ends of the drag boxes become badly corroded.

The boxes themselves are expensive to make, and the foundry has developed a repair process which has limited the corrosion mainly to the lips protecting the upper and lower surfaces of the ends of the drag boxes. The corrosion also takes a specific concave form, being deeper in the middle of the lip.

The company asked the Mathematics-in-Industry Study Group to investigate possible mechanisms for the damage, and ways to decrease or eliminate the wear. The corrosion displays a highly characteristic pattern both in where it occurs and the form it takes. The Study Group generated a model of interplay between temperature and oxygen which could account for the shape of the corroded area, and for its magnitude under the hot conditions of moulding and as the boxes cool. The group also found several factors, such as the chemical nature of the water and the way in which the drag boxes were cleaned, that served to accelerate the damage.

I was impressed with what mathematics could achieve in helping to sort out a process problem, and very pleased with the suggestions and outcomes. Some of the recommendations will be implemented shortly and others will be followed-up later.

Mithiran Kirupairajah
Toowoomba Foundry

Moulding boxes
Assembled moulding boxes, showing the top (cope) and bottom (drag) sections.

The casting process forces moisture, gas and even some sand out of the mixture in the boxes. In fact, the pallet cars are ribbed to allow the gases and water to flow out underneath the edge or lip of the drag box.

The Study Group explored several possible mechanisms for the corrosion. Initially the group developed a simple model for the corrosion (rusting) of the steel boxes by oxygen under hot conditions.

Given that it is hotter inside the drag box than outside, it was assumed that the temperature falls steadily through the width of the lip. But as the temperature rises, the solubility of oxygen in water decreases. So the best conditions for corrosion should occur in the middle of the lip, which is exactly the pattern observed.

Besides oxygen, other elements in the system likely to cause corrosion include sulphur and chlorine.

A natural source of sulphur is coal, some of which is included in the sand mixture. But the coal used by the foundry had only low levels of sulphur and only minimal amounts of sulphides were found in the scale produced by corrosion. By contrast, the level of chlorides in the scale is very high.

So the group developed equations for the addition and loss of chlorine during the moulding process. The water used in the plant was found to be naturally high in chlorides, which tend to be retained during the cycle. In fact, chloride levels were concentrated by a factor of four during the moulding process.

The sand blown out through the pallet vent was not likely to cause the corrosion, but it could well accelerate wear after a patch had begun to corrode.

In fact, the sand could well act like a sand blaster, and clean off the scale exposing new metal underneath. Scale tends to seal off fresh metal from oxygen and further corrosion.

Brushing during cleaning of the drag boxes also cleans off the scale, exposing new metal to corrosion. The impact of the brushing appears as a pattern of about 20 parallel grooves across the corroded patches on the lip of the drag box. One member of the Study Group developed a theory that narrow grooves caused by brushing were initiators of a process known as crevice corrosion. Analysis showed that the conditions of the moulding cycle supported crevice corrosion, which occurs in the absence of oxygen and in the presence of chlorine.

MI*Net consultant: Sean McElwain
Queensland University of Technology