Molasses derusting of steel

This article describes a setup for derusting small steel components, mainly machine tool accessories, using a Molasses solution.

A 10% Molasses solution can be an effective way to derust steel. Feed grade Molasses costs about $2/kg at the local rural store.

The process is bacterial and activity depends on temperature. Experimentation suggests that optimal temperature is 30-35°, and derusting can be achieved in a few days at that temperature (subject to the degree of rust). At lower temperatures, the process may take many weeks. The nice thing compared to electrolytic derusting is that work is unlikely to be damaged by the process.

Above, the rust treatment system comprises:

  • Bain marie stainless 1/3 module 200mm deep with lid;
  • 1000W electric cooker;
  • 230VAC thermostat with thermistor probe immersed in the process liquid;
  • 230VAC dimmer to reduce the power of the cooker element.

Above is an internal view of the thermostat made from a Chinese 230VAC thermostat, a 3m extension cord and ABS plastic box.

The dimmer is a modified commercial dimmer off eBay, see WBT-4000W 230V AC dimmer / motor speed controller. Be aware that dimmers with significant hysteresis or ‘snap-on' effect may not start in the on state when set to low output power. This dimmer was modified for very low hysteresis.

Excessive heater power will result in temperature overshoot and may also burn the Molasses solution in contact with the heated pan. In this case, the dimmer is set for 70Vrms which corresponds to 100W from the 1000W cooker element.

Above is a capture of the temperature of the ‘soup' from cold using an independent logger tied to the thermostat's sensor. The tub in this test contains 2l of soup, and no work. There are some interesting departures from the expected smooth logarithmic curve cause by convection currents.

Once heated to operating temperature, the tub loses heat the to environment at a rate of about 30W at 20° ambient.

Should something go wrong, the soup is probably flammable if boiled dry and appropriate precautions should be taken.