Question

Sketch and describe all the polarization curves obtained from different corrosion behaviours and give one example for each condition

Answer 1

There are some implications in the corrosion rate and its calculation. One of the most prominent one is the assumption that the surface corrodes homogeneously. We calculate the converted mass, from that the volume of that mass and that volume we spread evenly across the area of the sample to get the corrosion rate. This is often not true. Events like pitting corrosion appear, alloys corrode faster around phase boundaries, crevices corrode faster, etc. The corrosion rate is still a great parameter to compare the behavior of different materials in certain environments, but the values should be taken with a pinch of salt.

Polarization curves often give indications of what is happening at the surface.

t is not always easy to recognize and differentiate between a rather slow reaction and a diffusion limited reaction. Since the sample itself is usually the oxidizing partner, which is available in abundance, diffusion limitation is usually in the reductive part of the polarization curve.

Often the reduction is the reduction of oxygen, which is free diffusing in the solution and thus can be depleted. Unfortunately, the transition is in a real polarization curve from the linear part to the diffusion limited part quite hard to see.

Pitting corrosion can create misleading features as well. The onset is often characterized by a decrease of the Tafel slope, but if pitting corrosion appears already close to the E_corr it is difficult to recognize that pitting corrosion occurs.

The influence of other substances can sometimes be isolated by choosing a different measurement solution or choosing a different electrode. An iron electrode in alkaline sulfide solution will show sulfide oxidation next to the iron oxidation current. This can be identified with a platinum electrode, because platinum does not oxidize itself, so all measured current can be attributed to sulfide.

Polarization Curves of Thick and Thin Passivation Films

Quite interesting polarization curves are the ones of passive alloys or metals. There are two types of passive metals: thick and thin film. A thick film metal shows resistance to corrosion even when there is a driving force for corrosion (sufficient potential). The Evan’s diagram of such a system is shown in Figure 1.

Other metals or alloys show a decrease in the current density, if the potential is increased towards anodic potentials. These metals form a thin protective layer after reaching a critical potential. An Evan’s diagram can be seen in Figure

Important parameters are the primary passivation potential E_pp, the critical passivation current I_crit, the passive current I_pas, and the transmission potential E_t. At E_pp and I_crit the oxidation is strong enough to form a dense layer that protects the sample. The current drops to I_pas instead of the expected current predicted by a Tafel plot.

At the potential E_t oxidative processes can happen through the thin film und will quite likely lead to destruction of the protective layer. This behavior can cause quite some interesting effects on the polarization curves (see Figure 5.4 b). The processes below E_pp cause a polarization curve as we know it from the previous chapters, but the anodic part would not show a Tafel behavior for potentials above E_pp.

If the cathodic reaction has its intersection with the anodic reaction in the area of the passivation (above E_pp below E_t), the corrosion current I_corr is significantly lower than without passivation see Figure 5.5 a. If you want to estimate I_corr without the passivation for this figure, just extend the linear part of the oxidation curve below E_pp until you have the intersection with the reduction. That I_corr would be in the µA range.

2.

Figure 1 | Evan’s diagram of a thick film passive sample

Figure 2 | Evan’s diagram of a thin film passive sample

Important parameters are the primary passivation potential E_pp, the critical passivation current I_crit, the passive current I_pas, and the transmission potential E_t. At E_pp and I_crit the oxidation is strong enough to form a dense layer that protects the sample. The current drops to I_pas instead of the expected current predicted by a Tafel plot.

At the potential E_t oxidative processes can happen through the thin film und will quite likely lead to destruction of the protective layer. This behavior can cause quite some interesting effects on the polarization curves (see Figure 2). The processes below E_pp cause a polarization curve as we know it from the previous chapters, but the anodic part would not show a Tafel behavior for potentials above E_pp.

If the cathodic reaction has its intersection with the anodic reaction in the area of the passivation (above E_pp below E_t), the corrosion current I_corr is significantly lower than without passivation see Figure 3. If you want to estimate I_corr without the passivation for this figure, just extend the linear part of the oxidation curve below E_pp until you have the intersection with the reduction. That I_corr would be in the µA range.

Figure 3 | Schematic Evan’s diagram with resulting polarization curve of a through passivation protected surface

Figure 4 | Schematic Evan’s diagram with resulting polarization curve of a cathodic loop caused by three intersections of the anodic and cathodic curve

A more complex polarization curve occurs, if the reduction Tafel plot and the oxidation Tafel plot have multiple intersections as shown in Figure 4. Despite a more anodic potential suddenly a reductive current appears, due to the fact that the surface is now passive. In such a case the E_corr of the sample is usually the highest or lowest of the three intersections.

Film formation can also be studied using cyclic polarization also known as cyclic voltammetry. The film formation can be observed during an anodic scan followed by a cathodic scan, which then shows the cathodic loop. Or it would be possible investigate the repassivation potential.

If during the anodic scan the film was destroyed due to very high anodic potentials, the film won’t be reestablished immediately. The potential needs to drop below a certain threshold, the repassivation potential, for the formation of a stable protective layer.

Pitting and Crevice corrosion

These are two quite complex topics, which are explained here briefly. Both are local processes, which can lead to a corrosion of the whole surface.

Pits are small spots where corrosion appears. Instead of spreading wide most of them penetrate the surface and then spread. This way a coating is undermined. During an anodic scan a sudden increase in the Tafel slope might indicate corrosion pitting. If this high Tafel slope is used for calculating the corrosion rate and the whole surface area of the sample is used, the corrosion rate is significantly underrated.

Passivated metals often show metastable pitting corrosion before the real pitting corrosion starts. The metastable pitting corrosion creates some current spikes in the polarization curve, which can be easily confused with noise. These spikes are the result of pit formation and these pits passivate again, so the pits have only a short lifetime.

Pits of a certain depth and crevices have the same problem. The diffusion inside the pits and crevices is quite limited. Mechanisms developed for steel corrosion are widely accepted as the foundation for the general process of local corrosion. First reduction of oxygen is happening everywhere at the surface, but the oxygen in the crevice is depleted after a while and diffusion is too slow to replenish significant amounts.

In the above discussion we have mainly (more or less implicitly) assumed that:

1) electrochemical corrosion is the only deterioration mechanism;

2) anodic and cathodic reactions take place all over the electrode surface, but not simultaneously at the same place, i.e. the anodic and cathodic reactions exchange places, constantly or frequently. Closely related to this dynamic behaviour it is assumed that:

3) there are no significant macroscopic concentration differences in the electrolyte along the metal surface, and the metal is fairly homogeneous.

These three assumptions lead to uniform (general) corrosion. But this is only one of several corrosion forms that occur under different conditions. The other forms of corrosion depend on the deviations from the mentioned assumptions. Such deviations may be due to:

a) the design (the macro–geometry of the metal surfaces)

b) the combination of metal and environment

c) the state of the surface (particularly cleanliness and roughness)

d) other deterioration mechanisms

These conditions will cause various deviations in the geometry and appearance of the attack compared with uniform corrosion, and it is convenient to classify corrosion just after the appearance of the corroded surface.The advantage of such a classification is that a corrosion failure can be identified as a certain corrosion form by visual inspection, either by the naked eye or possibly by a magnifying glass or microscope. Since each form of corrosion has its characteristic causes, important steps to a complete diagnosis of failure can often be taken after a simple visual inspection. On this basis, the following corrosion forms can be defined:

1. Uniform (general) corrosion

2. Galvanic (two–metal) corrosion

3. Thermogalvanic corrosion

4. Crevice corrosion (including deposit corrosion)

5. Pitting, pitting corrosion

6. Selective attack, selective leaching (de–alloying)

7. Intergranular corrosion (including exfoliation)

8. Erosion corrosion

9. Cavitation corrosion

10. Fretting corrosion

11. Stress corrosion cracking

12. Corrosion fatigue

A simple illustration of the various forms of corrosion is shown in Figure below:

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