Estimating Metal Pipelines Corrosion Using Ansys Fluent

In this application example, we will demonstrate how to set up an electrochemistry model to estimate corrosion in an iron pipe due to the presence of dissolved carbon dioxide in the fluid flowing through the pipe. In this case, we have a pipeline with two inlets and two outlets.

The idea is that as the flow mixes through the pipeline and heads towards the outlets, we'll be able to see the effect of the concentration of dissolved carbon dioxide on the corrosion rate. To set up such a model, we'll be using the pressure-based approach.

The same is true for the reaction-based approach. The reaction we have to set up is iron reacting with carbon dioxide in the presence of water, which will form iron-ions, CO3 2 negative. Hydrogen will be released as gas.

In this case, the cathodic reaction will be hydrogen, cation accepting two electrons, forming hydrogen gas. The acid dissociates into hydrogen gas and HCO3 negative. Iron dissociates into iron-ions and releases two electrons.

For the materials, we have: * Carbonic ion with a charge number of minus one * Carbonic acid with a charge number of zero * Iron metal with a charge number of zero * Hydrogen gas with a charge number of zero * Iron ion (Fe2+) with a charge number of two Now, using these materials, we'll create the mixture template.

We can look at how the mixture template looks like under the mixture species. Click on edit. These are the different available materials, and we can pull them to the selected species. The species which create the mixture template are carbonic ion, carbonic acid, Fe2+, H2, H+, and H2O.

The iron is carbonic. Then, we can see that the mixture template is selected as a solid species. With the mixture template set, we will set up the reactions. The first reaction is anodic, where iron dissociates into Fe2+.

The equilibrium potential is set to -0. 1. With the cathodic reaction, we have carbonic acid reacting to form hydrogen gas and carbon ion.

The second reaction is a solid species, where hydrogen changes to hydrogen cation, and the equilibrium potential is set to -0. 1. In the reaction mechanism, you have the option of selecting active reaction mechanisms. In this case, all three reactions are active.

Now, we move to setting up the inlet conditions. We have two inlets. One of the inlets will contain carbonic acid. The other inlet will only contain water. The idea is that we will be able to see where the corrosion rate is higher.

For inlet one: * The velocity is set to 0.1 meters per second * The temperature is set to 300 Kelvin * We have 0.05 mass fraction of carbonic acid in this inlet. The rest of the fluid is water Similarly, for inlet two: * The species are all set to zero.

That means only pure water is flowing through this inlet * The potential is set to zero * The temperature is 300 Kelvin * The momentum is 0.1 meters per second Both the outlets are set to atmosphere. So, the gauge pressure is set to zero. The temperature is set to zero.

The reverse flow condition is also set to water only. For solution methods, we're using the default coupled method. Controls are also default values. We initialize the problem for one of the inlets. In this case, we will use inlet 2 so that the boundary there is no carbonic acid in the system.

Add initialized condition and then run the calculation. The calculation converges monotonously and is completed in under 40 iterations. Now, we take a look at the corrosion rate. Corrosion rate is showcased in the unit of kg per meter square per second on the walls.

We can see the location from where carbonic acid was injected into the system (inlet 1) has the highest corrosion rate, and inlet 2, which had no carbonic acid (only pure water), has no corrosion or very, very minimal corrosion.

As the flow mixes and the concentration of carbonic acid reduces, the corrosion rate also reduces. Now, we can also convert this unit of kg meters per second into something which is more understandable for an engineer, say, for example, 2 millimeters of corrosion per year.

To perform this conversion, we can use the custom field function. Navigate to user-defined and custom field function. Create a custom field function.

In this case, the field function will take the iron corrosion rate, which is in terms of kg per meter square per second, divided by the iron density (H030) and then multiplied by 36524 and 3600, which converts the seconds to per year, and multiplied by 1000 to convert to per year.

The resultant units will be millimeters of corrosion per year. And we can plot it using a convert. So, we will use the custom field function millimeters per year, the name of the function. Now, we can see that the location where the maximum corrosion is is around 0.16 millimeters per year.

As the concentration decreases, it reduces down to 0.07 millimeters per year. Now, we want to see if we change the concentration of the inlet carbonic acid, whether that has an impact on the total corrosion rate or not. So, to do that, we will change the inlet condition.

In inlet one, we will change the species concentration, reduce the carbonic acid concentration to 0. 001. Apply, initialize the problem, run the calculation. As the calculation is complete, we take a look at the corrosion rate in terms of millimeters per year.

We can see the contour shows corrosion rate, but our scales are very, very large.

So, to change that, we double click on the contour, change the type to exponential, and now we can see the maximum corrosion rate is 3 minus 3 millimeters per year, which has reduced significantly from what it was previously reported when the concentration of the acid was higher.

This concludes today's presentation. Thank you very much for your time. Thank you.