CORROSION PREDICTION Heat-sensitive technique tracks early progress of corrosion

Offshore oil producers are constantly searching for materials which can resist the ageing process and the effects of harsh environments. The Thermal Activity Monitor, or microcalorimeter, could prove a valuable ally. The microcalorimeter is a device with an extreme sensitivity to temperature changes: it measures the heat from chemical reactions in millionths of a watt, or microwatts. Its ability to detect chemical reactions as they are under way can also be transferred to investigating

Calorimeter experiments shed new light on material performance in varied environments

Offshore oil producers are constantly searching for materials which can resist the ageing process and the effects of harsh environments. The Thermal Activity Monitor, or microcalorimeter, could prove a valuable ally.

The microcalorimeter is a device with an extreme sensitivity to temperature changes: it measures the heat from chemical reactions in millionths of a watt, or microwatts. Its ability to detect chemical reactions as they are under way can also be transferred to investigating corrosion.

"Corrosion is a chemical system: it generates heat, so you can measure it in the microcalorimeter," says Lars-Gunnar Svensson, who is responsible for R&D Microcalorimetric Methods - Shelf Life Technology at Celsius. "The advantage of the calorimeter is that with its sensitivity, we can detect the corrosion process at a very early stage."

The method can be used to compare corrosion rates of different materials and under different conditions, and also the effectiveness of different coatings.

Chemical reactions can also be studied under different environmental conditions. Factors such as temperature and humidity can be varied. The influence of oxygen can be studied in situations ranging from a totally inert atmosphere to a totally oxygen-filled one.

By continuously observing the heat flow over a period of perhaps several weeks, the rate of corrosion can be calculated, giving an indication of how long a material will last in a given environment. In one two-week long experiment, Celsius investigated the effect of humidity in causing corrosion in steel. A clean sample of steel was subjected to relative humidities of 40-85%, but no significant heat flow ensued.

The steel was then dipped in a weak solution of sodium chloride, dried and the test repeated. At 60% humidity a small heat flow was detected, which rose to high levels when the humidity was increased to 80-85%. The conclusion was that clean steel is highly corrosion resistant, but this resistance is undermined when exposed to salt.

Basic principle

Essentially the microcalorimeter is a simple device, developed by the Thermochemistry Laboratory, Chemical Center, University of Lund in Sweden. Designed for measurements in the temperature range 10-90C, it is based on the heat conduction principle. The heat flows between the sample and a large heat reservoir via special thermopile array sensors which convert the rate of heat transfer to an electric voltage.

The key to the successful functioning of the microcalorimeter, Svensson says, is its ability to maintain the temperature of the heat reservoir constant to within 0.0001C. Accurate heat flow measurements can only be made if this temperature is maintained unchanged. To eliminate baseline noise caused by minor variations in the temperature of the heat reservoir, the heat flow from an inert (non-reactive) reference sample is monitored.

Celsius has used the microcalorimeter extensively to investigate the stability of moisture-sensitive explosives and the ageing of propellants. For offshore users, its most interesting applications are likely to be corrosion investigations and compatibility testing.

Interaction analysis

In compatibility testing, the heat flow from each of the tested materials is first taken separately. "The materials are then tested together," says Svensson, "and if there is a net heat flow - that is, the amount of heat generated by the materials together differs from that generated by them separately - this indicates that a reaction is happening."

In a compatibility test of an epichlorhydrine rubber gasket material in a hydraulic fluid at 70C, the heat flow of the mixture was found to deviate from the non-interaction curve during the first 24 hours, an event which was attributed to swelling of the rubber in the fluid. Thereafter, no signs of chemical activity between the two components were found, suggesting that this form of rubber maintains its properties when exposed to this hydraulic fluid.

In another compatibility test of epoxy rubber and TNT, a high heat flow was generated when the two materials were mixed, showing that this was an unsuitable combination.

For more information contact Lars-Gunnar Svensson, Celsius Materialteknik: telephone +46 586 81605 or fax +46 586 58515.

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