Mix corrosion

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In geomorphology and related sciences, mixed corrosion is understood to be the corrosion of limestone or dolomite , which is caused by the mixing of two lime-saturated solutions of carbonated water.

(Meteoric) water from precipitation, which penetrated deep into the rock through crevices, was already saturated with calcium carbonate (lime, CaCO 3 ; in dolomite analogous to calcium-magnesium-carbonate CaMg (CO 3 ) 2 ), which means it can no longer dissolve lime. Nevertheless, in the depths, at places where rivulets meet in hairline cracks or crevices, large cavities form over long periods of time without any noteworthy inflows and outflows. Pools also arise in water-filled cavities at those points where water is pressed in through a crack.

As a rule, the concentration of carbon dioxide (CO 2 , in the form of carbonic acid H 2 CO 3 ) - depending on the respective temperature - in the lime-saturated waters of the various channels of a cave system is different. Therefore, a solution with a new concentration forms at their confluence. Because the amount of dissolved CaCO 3 does not increase linearly with the CO 2 concentration in the water, further lime can be dissolved by this mixed solution.

Diagram of mixed corrosion according to Bögli 1963
Mixture corrosion according to Bögli (1963): The mixed solution with the CO 2 and CaCO 3 concentration T, which was created from two CaCO 3 -saturated solutions with the concentrations P1 and P2, is in the aggressive range.

The starting point for the above figure is the reaction equation for the solution of calcium carbonate by carbonic acid (see carbonic acid weathering ):

.

Through empirical investigations it was determined at which substance mass of dissolved CO 2 which substance mass CaCO 3 is present in an aqueous solution in a state of equilibrium ( saturation ). The black equilibrium curve (saturation curve) in the above figure was thus obtained. The relationship between CO 2 concentration and CaCO 3 saturation is apparently not linear.

By mixing two equal volumes of Karst water of the composition P1 (mg in Example 1 above / l CO 2 and 74 mg / l CaCO 3 ) and P2 (mg in the above example 47 / l CO 2 and 273 mg / l CaCO 3 ), in this way a mixed solution is obtained whose composition T corresponds to the mean values ​​of the concentrations P1 and P2 (in the above example 24 mg / L CO 2 and 173.5 mg / l CaCO 3 ). T is the midpoint of the line P1-P2. Accordingly, T does not lie on the equilibrium curve because it is not a straight line. It follows that the mixture with the composition T, although it was created from two lime-saturated waters, is now undersaturated with lime and can therefore dissolve further lime. In phreatic (water-filled) karst cavities, no additional CO 2 can be absorbed by the water . This means that only as much lime can be dissolved as the amount of CO 2 that has become “free” through the mixing process . The mass of the CO 2 newly available for the lime solution as a result of the mixing process corresponds to the length of the distance CT.

If you want to find out how much lime can also be dissolved by the mixing process, you have to take into account that 1 mmol CO 2 is required to dissolve a substance amount of 1  mmol CaCO 3 (see reaction equation above), where 1 mmol CaCO 3 has a substance mass of 100 mg and 1 mmol CO 2 correspond to a substance mass of 44 mg. There is a proportionality between the two quantities, and the associated constant of proportionality corresponds to the slope of the straight line that ran through points (0; 100) and (44; 0) in the graphic above. If you move this straight line parallel to the y-axis so that it goes through point T, it intersects the equilibrium curve at point A. The distance AB between point A and segment CT corresponds to the amount of lime that would also be loosened by the mixture corrosion.

Individual evidence

  1. ^ Alfred Bögli: A contribution to the formation of karst caves . In: Die Höhle - magazine for karst and caving . tape 14 , no. 3 , 1963, p. 63-68 ( PDF ).

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