Atmospheric Corrosion

Atmospheric corrosion may occur when thin films of liquid water, in the range of up to hundreds of micrometers, forms on metal surfaces in contact with humidified air. The thickness of the film depends on the relative humidity of the surrounding air, but also on factors such as surface roughness and the presence of particles, especially salt crystals. The thin film of moisture acts as electrolyte, and may cause various corrosion phenomena, such as galvanic corrosion of a bimetallic element or crevice corrosion.

This tutorial model studies atmospheric galvanic corrosion as a function of the relative humidity of the surrounding air and salt load (NaCl) on a bimetallic aluminum alloy – steel surface. It is assumed that the electrolyte film solution is in equilibrium with solid salt particles, distributed uniformly over the surface at a given load.

The example uses parameter data from Ref. 1, Ref. 2, and Ref. 3.

Model Definition

The model set-up is shown in Figure 1.

Figure 1: Model set-up. Each metal surface is 12 mm wide.

The thickness of the electrolyte film depends on both the salt load density and the relative humidity, see Figure 2. The film grows significantly toward 100% relative humidity.

Figure 2: Thickness of the electrolyte for different relative humidities and salt load densities. The conductivity varies linearly with the salt load density. (Ref. 1)

The oxygen solubility, oxygen diffusivity, and the electrolyte conductivity also depend on the relative humidity, see Figure 3, Figure 4, and Figure 5.

Figure 3: Oxygen solubility vs relative humidity.(Ref. 1)

Figure 4: Oxygen diffusivity vs relative humidity.(Ref. 2)

Figure 5: Electrolyte conductivity vs relative humidity. (Ref. 1)

Electrochemical Reactions

The less nobler aluminum alloy is oxidized in the cell, with the electrode reaction kinetics described in terms of experimental polarization data from the corrosion material library.

On the steel surface, oxygen reduction occurs. The oxygen reduction reaction is limited by oxygen transport through the film. The limiting current density, ilim, O2 (SI unit: A/m2), depends on the film thickness, the oxygen solubility and the oxygen diffusivity according to:

where F (96485 C/mol) is Faraday’s constant, D (SI unit: m2/s) is the diffusivity of oxygen in the film, csol (SI unit: mol/m3) is the solubility of oxygen, and dfilm (SI unit: m) is the film thickness.

By assuming a first order dependency of the oxygen reduction kinetics on the local current density of the oxygen concentration, the following expression for the current density, ilim, O2 (SI unit: A/m2), can be derived:

where iexpr is the local current density of the electrode reaction in absence of mass transport limitations described in terms of experimental polarization data from the corrosion material library.

Results and Discussion

Figure 6 shows the local current density of the electrode reactions for a salt load of 0.5 g/m2 and various relative humidities. The cathodic currents reach a plateau close to x = 0 at a magnitude that is significantly affected by the relative humidity. This is due to a changing limiting current density for oxygen reduction. As the film thickness grows, the electrolyte transport length for oxygen increases, in combination with an increased oxygen solubility and diffusivity for higher relative humidities.

Figure 6: Local current densities along the metal surface at a salt loading of 0.5 g/m2 and relative humidities (RH) spanning from 80% to 98%.

Figure 7 shows the maximum anodic currents for various salt load densities and relative humidities. For all salt loads, a maximum current density is seen around a relative humidity of 90%.

Figure 7: Maximum metal oxidation anodic current density on the aluminum alloy metal surface for varied relative humidities and salt load densities (LD).

Looking at the maximum cathodic currents in Figure 8, it is seen that the maximum oxygen currents are about one order of magnitude smaller than the anodic currents, but that they follow the same trend with a maximum around a relative humidity of 90%. These currents are very close to the limiting current densities for oxygen reduction.

Figure 8: Maximum oxygen reduction cathodic current densities on the steel metal surface for varied relative humidities and salt load densities (LD).

Finally, Figure 9 shows the average anode current density, which gives us a measure of the total corrosion rate of the sample, for various relative humidities and salt load densities. The maximum is found for a salt load density of 3.5 g/m2 and a relative humidity of 95%.