siandaii ch4 uniform corrosion

8/22/2019 SIandAII Ch4 Uniform Corrosion

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Surfaces, Interfaces, and their Applications II Uniform corrosion

Dr. Patrik Schmutz, Laboratory for Joining Technologies and Corrosion, EMPA Dbendorf, 2013 1

4 Uniform corrosion

4.1 Introduction relevant factors

Definition: Uniform corrosion is a corrosion attack with almost constant dissolution rates

other the whole surface (According to DIN 50900 Norm).

Reduced thickness: m

Mass loss: g/m2

Corrosion rate: g/(m2

. y) or m/ay

Figure 4.1: Schematic description of uniform corrosion with relevant parameters

The following facts can be mentioned about uniform corrosion based degradation process:

- Uniform corrosion is the most commonly found form of corrosion

- Rust formed on low alloyed steel is a special form of uniform corrosion

- Uniform corrosion occurs mainly on very active metals (low Reversible potential)

- Noble metals (gold, platinum) are immune against uniform corrosion

- Chromium, Titanium, Nickel, Stainless Steel are protected by a passive film and aretherefore not concerned by uniform corrosion

The most common uniform corrosion damages are found for example in water tubes,

buildings (rebars), bridges, a.s.o. Although they are very frequent, they are also the less

dangerous kind of corrosion processes because they are predictable. If failure still occurs, the

damages are the result of:

Wrong material choice

Wrong dimensioning of a structure

Insufficient protective coating

Usually, the failure can be easily prevented by increasing the component thickness or the use

of more corrosion resistant materials (passive materials, see chapter 3)


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In order to fully describe and understand uniform corrosion processes, it is important to

distinguish between two types of corrosion appearance:

1) Uniform corrosion with laterally homogeneous active dissolution rates. The wholesurface experiences a similar removal of material.

In this case, reaction kinetics are easy to obtain and can be fully predicted by the

electrochemical characterization methods presented in Chapter 2.3. By applying data analysis

procedures such as the polarisation resistance or the Tafel slope extrapolation methods, the

uniform corrosion rates can be directly obtained

Requirement for uniform corrosion:

Presence of a least a thin water layer (not visible in the case of atmospheric

corrosion) on the metallic surface

Homogeneous uniform corrosion:

Cathodic and anodic reaction occurs simultaneously at the same location of the

surface, an electron leaving the metallic surface is consumed by the cathodic

reduction at the same place.

Anodic dissolution:

Cathodic reductions:

In acids pH < 5

(H - Type)

In neutral and alkaline solution pH >5

(O - Type)

2) Uniform corrosion with laterally heterogeneous reactions on the surface.This type of attack results in the formation of craters with no sharp edges, Fig. 4.2.

Heterogeneous uniform corrosion is the result of local difference in the corrosion

condition due to materials heterogeneities or a difference in the solution aggressivity

and is the most common uniform corrosion process occurring in the practice.

Not a localized corrosion attack


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a) b)

c)

Figure 4.2: Laterally heterogeneous uniform corrosion for (a) cast ion tube, (b) mild iron and

(c) schematic representation of the surface roughening induced during heterogeneous

uniform corrosion

This type of attack is typically found for example in low alloyed steels that cannot passivateand that contain difference phases, Fig. 4.2-a,b. The heterogeneous dissolution results in a

roughening of the surface that should however not be referred as a localized attack (pitting).

Localized attacks follow completely different dissolution mechanisms and require stable

passivation of the surface next to the attack as a first requirement.

4.2 Aeration cells

The lateral reaction heterogeneity in the uniform dissolution process can also be related to

environmental parameters. A very important and practical relevant example is the aerationcell problem that is occurring as soon as lateral oxygen concentration gradients are

developing in a solution.

When a bulk solution with convection is considered, the oxygen concentration is everywhere

similar (homogeneous reaction) but in stagnant solutions, the oxygen concentration can vary

laterally. Figure 4.3 present the situation of a water droplet condensed on a pure iron surface.

Different O2 concentrations will be present at different location in the droplet (obviously

higher at the border).

The fast O2cathodic reduction reaction at the droplet border induces rapid pH increase. The

metal surface oxide is then thermodynamically stable and can then passivate (see chapter 3 of

Surfaces, Interfaces, and their Applications II) only at this specific location of the droplet.


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The result is the formation of a Macroelement (galvanic coupling element) with separated

anodic and cathodic reactions.

Figure 4.3: Schematic description of an aeration cell, an uniform corrosion processes with

anodic dissolution and cathodic reduction reactions laterally separated.

When the macro element is formed, the location of the different reactions is the following

(see also Fig. 4.3):

In the middle of the droplet:

The centre of the droplet will always continue to dissolve actively because the oxygen access

and reduction rate is lower and the pH stays in the neutral domain. Furthermore, the dissolved

iron cation can hydrolyse and this process keeps the pH low.

At the border:

The highest cathodic reduction rate is obtained directly at the droplet border, because the

oxygen concentration is the highest but also the Nernst diffusion layer is thinner at this

location. The OH- anions generated are then increasing locally the pH value.


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In between (formation of hydroxide):

Between the clearly defined external passivated area and the actively dissolving centre of the

aeration cell, formation of corrosion product rings will be found. This correspond to the

transition pH region observed in the Iron Pourbaix Diagram (see chapter 11 of Surfaces,Interfaces, and their Applications I) where previously dissolved Fe cations will precipitate

when crossing the high pH stability boundary. These Iron hydroxide solid products will

subsequently convert in the formation of rust.

Further oxidation and formation of a ring of rust

Considering in a more general context the different corrosion equilibriums that can be

established in the case of an aeration cell, two situations can be distinguished, Fig.4.4.

- For a metal not passivating at high pH, Fig. 4.4a, the high cathodic reduction rate inthe aerated area will directly induce local increase of the anodic dissolution at the

same location. In the case of a droplet, the boundary would dissolve very fast resulting

in the formation of external corroded ring with a slower dissolving droplet centre.

- When the metal is passive at high pH, like for iron, there is an additional galvanic

coupling process occurring, Fig.4.4b. In this case, the high reduction current on thepassivated area will help maintaining a high equilibrium potential on the surface

(stable passive conditions) and the current will flow to the actively corroding area.

The dissolution rate in the less aerated area (centre of the droplet in the presented

example) will increase and will become equals to the sum of the cathodic reactions

iO2 (aerated) + iO2 (passive). Obviously the relative areas of the passivated andactively corroding areas have to be included in the calculation to find out the

acceleration rate. This acceleration factor and also the potentials difference that will

establish between the two areas will be explained in the next chapter about galvanic

corrosion mechanisms.

Note: Aeration cells can be found everywhere also where recesses are found or when two

metals are simply put in contact against each other (possible formation of crevices). Uniform

corrosion can be strongly accelerated in these hidden areas even if intuitively the slow

diffusion of oxygen to these locations would more let us think that uniform corrosion is

hindered.


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a)

b)

Figure 4.4: Equilibrium and corrosion rates obtained in aeration cells when: (a) the surface

can stay passive in the whole water drop, (b), the pH is increasing locally in a way for the

material to passivate in these areas


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4.3 Main parameters controlling corrosion rates

The uniform corrosion rates as function of pH evolution are directly related to the relevant

cathodic reaction occurring. Taking our example of iron, the pH dependence can be divided

in three distinct regions, Fig. 4.5:

1) For pH < 5, a strong pH dependence (E H2/H+ = -0.059 pH) is found.The corrosion process is following an H Type behaviour. As mentioned in the

chapter concerning reaction kinetics, all the reactions are under charge transfer

control (exponential reaction rates) and very high corrosion rates can be obtained.

Uniformly corroding materials should not be used (and are also not used in the

practice) in this pH domain. An alternative with a passivating material has to be

found for applications in acidic media.

2) Between 5 < pH < 10, the material removal rate is independent of the pH. Thereaction is diffusion controlled (usually O

2reduction) and accordingly follows an

O Type behaviour. The maximum corrosion rate is completely under control and

in this case, uniform corrosion can be assessed precisely and tolerated. This allows

the use of low alloyed cheap materials for industrial applications where price is an

issue.

Corrosion rates changes are then only observed within the variation boundaries of

the main parameters influencing the critical current density (diffusion layer

thickness, oxygen concentration, temperature). As neutral to alkaline environments,

are the main relevant ones where uniformly corroding materials are used, each single

relevant parameter will briefly be discussed below.

3) For pH > 10, iron is passive. Only very slow passive dissolution is occurring and inabsence of aggressive ions such as chlorides, the surface is efficiently protected from

corrosion.

a) b)

Figure 4.5: Uniform corrosion rate as a function of pH formulated in terms of (a) mass loss

and (b) corrosion current in the acidic pH domain


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In the neutral to alkaline pH domain, the controlling parameters are related to the diffusion

limiting current density.

Following influences are found from:

Temperature: increase of the diffusion constant

Oxygen concentration: function (T, solution)

Convection conditions: changes in the diffusion layer thickness

Considering first the temperature effect, Figure 4.6a shows the evolution of the materials

removal rate (corrosion rate) has a function of increasing temperature. The diffusion rate ofO2 is increasing inducing a linear growth of the corrosion rate in a closed system (curve 1). In

an open system (curve 2), water is evaporating at 100C so that the only oxidation will occur

at higher temperature and materials removal drops!

The detrimental effect of higher oxygen diffusion is partially compensated by the fact that the

O2 solubility limit is decreasing with temperature, Fig. 4.6b. Here again a little difference is

found between a closed system (curve 1), where an almost constant residual value is obtained

from 60C upwards and the open system (curve 2) where the oxygen concentration drops

quicker towards 0 at evaporation.

Figure 4.6: Influence of Temperature on the uniform corrosion process: (a) materials

removal rate, (b) oxygen partial pressure (for 1 bar total pressure)

An increase in O2 concentration obviously increases the limiting current density and the

corrosion rate (icorr) directly, Fig. 4.7a. Additional uniform corrosion rate increase is obtained

by solution flow (convection), Fig. 4.7b. Changes in the diffusion layer thicknesses occur and

as a consequence also increase in corrosion rates.


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Figure 4.7: Influence of (a) oxygen concentration and (b) solution flow on the uniform

corrosion process

Finally, it is worth mentioning also the influence of salt concentration increases on the

corrosion rate, Fig. 4.8a. The rates are first increasing because of better electrolyte

conductivity up to 3-5% concentration, higher salt concentrations then decrease the O2

solubility. For uniform corrosion processes, concentrated salt solution are not a critical

parameter, this will be totally different in the case of localized corrosion processes. A last

parameter slowing down the uniform corrosion process is the slower oxygen diffusion

through rust layer, Fig. 4.8b. It has to be noticed that decrease in corrosion rate compared to

the initial values is usually not very large and that even in presence of corrosion product

layers, uniform corrosion will proceed at a constant rate.

Figure 4.8: Influence of (a) salt concentration and (b) adherent corrosion products on

uniform corrosion process

Note: Having discussed the main parameters influencing uniform corrosion in neutral and

alkaline environment, it is evident that all the factors affect the corrosion rate by a

factor of 2 to max 10. This justifies the statement made previously that this type of

corrosion is easily controllable and predictable.


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4.4 Electrochemical Impedance Spectroscopy characterization

After having discussed in general, the different parameters influencing the uniform corrosion

rates, a new electrochemical characterization technique is now introduced. Uniform corrosion

is a relatively fast process and involves a large spectrum of reactions from purely active

dissolution, to more or less protecting corrosion product formation associated with adsorptionphenomena. Linear polarization methods are usually not able to discriminate between all

these different phenomena, for this reason a frequency dependant measurement of the

electrochemical reactions, the Electrochemical Impedance Spectroscopy technique is

widely used in corrosion science. To illustrate the method and the different information that

can be obtained, the example of Magnesium alloys and their use as degradable implants is

presented. It is in fact a very complex system from different point of views:

- The alloys are structurally heterogeneous- Mg is very reactive (very low reversible potential) and hydrogen reduction is taking

place even in neutral to alkaline solutions

- Body fluids contains numerous species that can be integrated in corrosion products

4.4.1 Example of uniform corrosion degradable Magnesium metallic implants

Magnesium and its alloys are interesting candidates as biodegradable implants because

Magnesium is not only biocompatible... It is an important element in hundreds of metabolic

processes and the suggested daily ratio for adults is 350 mg/day.

Magnesium is furthermore very reactive and unstable in the pH domain of physiological

media, Fig. 4.9. This means that it should degrade in-vivo, but obviously a defined

degradation sequence during the first 6 months is aimed at and the alloys should not lose its

mechanical properties early during implantation. For this reason, temporary surfaceprotection need to be applied on the implant surface to avoid too fast initial dissolution, This

aspect will not be discussed at this stage of the lecture.

Figure 4.9: Pourbaix Diagram of Mg and pH range of physiological media


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Advantages of biodegradable implants in general are clear:

- No removal surgery- Avoidance of long-term adverse reactions (chronic inflammation)- Early load transfer

- Applications in paediatrics

In the case of magnesium, multiple additional advantages can be mentioned:

- Low corrosion resistance (see Pourbaix Diagram)- High biocompatibility- Favourable mechanical properties (better than degradable polymers for example)- Youngs modulus similar to bone, Fig. 4.10

a) b)

c) d)

Figure 4.10: (a) Schematic description of the bio-resorption process in Osteosynthesis

applications, (b) Magnesium mechanical properties compared to bone (Witte et al.

Biomaterials, 2005), (c) stents used for vascular application and (b) small pins used in

osteosynthesis

The currently most considered alloys for degradable medical implants are of the Magnesium-

Yttrium- Rare Earth system, Table 4.1. Some Mg-Al alloys are also investigated, but the

toxicity of Aluminum is still debated! The influence of Yttrium alloy content and microscale

distribution on the corrosion mechanisms and susceptibility in neutral electrolytes has been

recently investigated in detail [1]. The problem faced concerning implant lifetime predictionin the pH domain of physiological media is that Mg alloys do not degrade by direct active


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dissolution. The formation mechanisms of partially protecting Mg-hydroxide corrosion

products is quite complex [2].

Alloy Mg Y RE Zr

WE43 Bal. 4.0 3.5 0.5

Table 4.1: Elemental concentration of the WE43 alloy

Electrochemical Impedance Spectroscopy characterization provides an interesting way to

identify the different reactions taking place on these surfaces

[1] M. Liu,P. Schmutz, P.J. Uggowitzer, G. Song, A. Atrens, The influence of yttrium on the

corrosion of Mg-Y binary alloys, Corrosion Science, 52, 3687-3701 (2010)

[2] N. Quach-Vu, P.J. Uggowitzer,P. Schmutz, Corrosion behavior of Mg-Y-RE alloys used

in Biomedical Applications studied by Electrochemical Techniques, Comptes Rendus de

Chimie, 11(9), 1043 - 1054 (2008)

4.4.2 EIS Principle

The Electrochemical Impedance Spectroscopy (EIS) measurements are usually performed at

the free corrosion potential. A very small voltage perturbation is applied by the potentiostat in

the three electrodes cell (see electrode kinetics, Chapter 2) and the induced current response is

analysed, Fig. 4.11. One advantage of the EIS measurement is the possibility to repeat themeasurements multiple times because the solid-liquid interface is not brought out of

equilibrium during the EIS measurement like in the potentiodynamic polarization

measurements. This allows following as a function of time the oxidation and corrosion

mechanisms occurring spontaneously on the surface or that can be induced by the presence of

an oxidant in the environment.

Figure 4.11: EIS measurement with small sinusoidal applied perturbation by the potentiostat

and recorded current response.


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A voltage perturbation (typically 10 mV) is applied and the current response as a function

of frequency is recorded.

If only resistances to charge transfer would be involved at the solid-liquid interface, then the

Ohms law would give a simple relation between the applied voltage und the current

response:

U = I * Rp

The polarization resistance Rp is then directly indicating the corrosion resistance, like in the

case of the linear polarization method. However, as mentioned in relation with reaction

kinetics, an electrochemical double layer is more complex and has a capacitive as well as a

resistive component in parallel, Fig 4.12b. Charging of the double layer (or surface oxide for

a passive system) induce a phase shift between the applied voltage signal and current

response meaning that this relationship is of complex nature.

Eac = Iac * Z Z (Z,Z): impedance

The 10mV perturbation is then applied over a wide frequency range, for example from 105

Hz

down to 100 mHz for the characterization of Magnesium in 0.01M Na2SO4, Fig. 4.12a. The

most common way of plotting the data is in the form of the Bode Plot (Fig. 4.12a) where the

absolute value of the Impedance modulus Z:

And the phase shift :

Are plotted versus the applied perturbation frequencies.

Very different information is then obtained at the different frequency giving a kind of

spectroscopic information on the surface processes. These measured data can be interpreted in

terms of equivalent electrical circuit, Fig. 4,12b.

At high frequencies, only the very mobile ions in solution are excited so that the solution

resistance can be assessed.

At lower intermediate frequencies, capacitive charging of the solid-liquid interface occurs.

The frequencies are still too high to induce electrochemical reactions (for example

oxidation/corrosion) and the information that is obtained concerns the dielectric properties of

the interface. This capacitance values C can provide very important information about oxide

properties when passivation or thicker oxides are formed on the surface. An alternative

measurement can also be to perform EIS measurements on thermal oxides previouslyproduced at higher temperature, or to perform solid-state impedance during service.


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Finally, at low frequency, the capacitive charging disappears because the charge transfer of

the electrochemical reaction can occur and this measured value of the resistance corresponds

directly to the oxidation/corrosion rate. For this reason, this low frequency impedance value is

referred to polarisation resistance Rp.

a) b)

Figure 4.12: EISmethod: (a) typical frequency dependant experimental measurement (Bode

Plot), (b) equivalent electrical circuit representation for the interpretation of data.

Bode plot in summary:

- Impedance is plotted as a function of the measured frequency.

- At high frequency the capacitance act as a short circuit and only the motion of charged

ions in solution can be characterized:RS

- At low frequency, the capacitance effect disappears so that the current passingthrough the solid-liquid interface is probed: RP

For Magnesium, an Rp value of 1000 Ohm/cm2 corresponds to a corrosion rate of

approximately 220 m/year. Now considering the spectra of Figure 4.12a, it can be stated that

the capacitive charging is very well defined, symmetrical and goes back to zero at low

frequency, indicating that only one charge-transfer electrochemical process is involved. The

only deviation from a perfect capacitive behaviour is the fact that the phase shift does not go

to -90 as would be expected in an electrical circuit. This phenomenon will be explained later.

When considering more complex alloys and media, frequency resolved electrochemical

impedance measurements allow characterizing in more detail multiple reaction mechanismsoccurring on a surface. The next example concerns a WE43 alloy immersed in a simulated

body fluid SBF 27 as base (concentration in mmol/l): 100.0 NaCl, 4.0 KCl, 27.0 NaHCO3,

1.0 MgSO4 * 7H2O, 2.5 CaCl2 * 2H2O, 1.0 KH2PO4, TRIS Buffer (pH 7.4).

The solution was usually buffered with tris(hydroxymethyl)aminomethane TRIS to maintain

physiological pHs around 7.4 during the measurement. In the next example, measurements

have been performed with the same ionic concentrations but once with the TRIS buffer and

once without, Fig. 4.13. Keeping the pH at 7.4 (triangles), a single uniform corrosion

process is occurring identified by a low impedance modulus (102.5

ohm*cm2) and a uniform

attack of the surface (grey picture on Fig. 4.14). When the buffer is removed, the pH at the

surface is increasing and the overall impedance modulus is increasing of more than one order

of magnitude (104

ohm*cm2). However, what is striking on the plot is that the phase shift


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shows two maxima. A new one aroused at higher frequency (around 105

Hertz). The higher

the frequency of a process, the faster it can proceed, this means that the fact that most of the

surface is protected is generating localized breakdown of the surface protecting corrosion

product layer. This fact is also evidenced by the fact that in this solution, the surface stays

shiny (coloration is an indication of oxidation of the surface) with clearly visible black spots

corresponding to localized attack (mechanisms will be discussed in relation with pittingprocesses).

Figure 4.13: EIS measurement on WE43 immersed in SBF 27: squares indicate the

measurement without buffer, triangles with buffer.

Figure 4.14: Schematic description of the corrosion process with uniform corrosion in

buffered solution and protection of the surface with very fast localized corrosion attack in

unbuffered solution


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Another way of plotting the EIS data is the Nyquist plot where the imaginary part Z of the

complex impedance is plotted versus the real part Z, Fig. 4.15b and c. Each measurement

point (Fig. 4.15b) is identified by its modulus and phase shift. The information obtained is in

principle similar than for the Bode representation. On Fig. 4.15, the Rs and Rp values can be

obtained at the intersection of the semi-circle with the real axis. If only one semi-circle is

obtained, this means that a well-defined single electrochemical reaction is occurring.

a) b)

c)

Figure 4.15: Complex plane representation of the impedance values (a) phase shift and

sinusoidal amplitude response, (b) corresponding value in the Nyquist plot, (c) EIS

measurement for a single electrochemical process (1 semi-circle)

The Bode plot is more used in corrosion science because it gives qualitative information

about polarization resistances. The Nyquist plot is then common in electrochemistry and its

main advantage is that the lower frequency measurement domain is better visible in this

representation. For example, diffusion and adsorption processes will be monitored at low

frequency because they are slow process compared to charge transfer. First information that is

better visible on the Nyquist plot is the deviation of a measurement from an ideal capacitive

behaviour when the surface is rough. On Figure 4.16, two examples of measurement with

different roughness are shown. On a perfectly flat surface, the surface capacitance should be

homogenous everywhere on the surface. The result is a perfect semi-circle and the phase shifton the Bode plot would be 90. When the surface is rough, the capacitance is different in the


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scratch valleys then on flat areas. This induces a capacitance distribution around an average

value and can be modelled by a so-called constant phase element CPE (Q). The exponent n issimply a measure of the capacitance distribution and if n=1 than the single capacitance

situation is found again. When the capacitance values are scattering, then the semi-circle is

depressed and the phase shift on the Bode plot would not reach -90. Not only roughness but

also other heterogeneities such as material composition gradients or non-uniform corrosionproducts/passive oxides can also result in CPE behaviour.

Flat polished surface Rough scratched surface

Figure 4.16: Capacitive behaviour measured by electrochemical impedance spectroscopy as

function of the surface roughness

Pure capacitance C

Constant phase element Q

C

jZ

C

=

( )Q n

0

1Z

Y j=


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The last examples presented allow discussing the different mechanistic information about

electrochemical reaction that can be obtained by EIS technique. WE43 sample was immersed

in 100 mM NaCl with an initial pH of 7.4 and analyzed after different immersion times up to

24 hours, Fig. 4.17. A single measurement typically lasts 45 minutes for this active system.

On the Nyquist plot, two semi-circles are now visible for every measurements, this indicates

that two electrochemical processes with very different reaction rates are generated by thevoltage perturbation. The impedance values are constantly increasing as function of

immersion time and are the largest for 24 hours. This indicates that the surface is stabilizing

during immersion in this solution. For the interpretation of the measurement, it should also

be kept in mind that this WE43 alloys contains numerous intermetallics in an almost pure Mg

matrix, Fig. 4.18.

Figure 4.17: Time dependant EIS response of WE43 immersed in 100 mM NaCl, (pH 7.4)

Figure 4.18: SEM image of a polished and Ar Ion sputter etched WE43 surface revealing the

intermetallic phase in the microstructure


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From the experimental data, it can be stated that there is a first process that is fast and

corresponds to a direct charge transfer at the metal-solution interface (process 2), Fig. 4.19

The reason why the second process is so nicely separated from the first one is that it occurs at

a much slower rate. The most plausible interpretation is that part of the surface is covered

with porous corrosion products. The ions generated at the metal-corrosion product interface

will slowly diffuse through this porous corrosion products layer (process 1). In summary,when oxidation from the Mg occurs, the Mg

2+cations either directly go in solution (only

through fast process 2) or they have to diffuse slowly through the corrosion products (process

1 + process 2).

Figure 4.19: Model used for the interpretation of the EIS data with two dissolution processes

In the next examples, Calcium and Phosphate ions are added to the previously presented

solution, Fig. 4.20. The EIS spectra in the high and middle frequency domain show relatively

similar behaviour. However, an additional inductive loop is found at the very low frequencies.

In figure 4.18, all the measured values have positive imaginary impedance values, whereas in

figure 4.20, the signal first goes in the positive value and then turns back. The different

components of the measured signal are schematically depicted in Fig. 4.21. They can be

separated in correlated processes, this means charge transfer or diffusion where the current

response is directly following the applied voltage perturbation. The two processes will have

semi-circle in the upper right quadrant of the complex plane representation. On the other side,

uncorrelated processes are independent of the applied voltage. They typically correspond to

adsorption processes where molecules or ions deposit on the surface. These processes areinducing inductance components (release of energy stored in the solution). They are identified

by semi-circles in the lower right quadrant of the complex plane.

Correlated processes:

1 Charge transfer reaction (direct)2 Diffusion processes in corrosion products (delayed)

Spontaneous processes (uncorrelated)

3 Adsorption processes generating inductive components


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a)

b)

Figure 4.20: Time dependant EIS response of WE43 immersed in

(a) 100 mM NaCl + 25mM CaCl2, (pH 7.4), (b) 100 mM NaCl + 2.5mM CaCl2 + 1mM

K2HPO4 (pH7.4)

In the case of calcium and phosphates, integration of these species in the corrosion product

layer is probably the reason for the non-correlated component of the impedance. The presence

of these species in the surface oxy-hydroxide has been confirmed by Scanning Electron

Microscopy (SEM) with Energy Dispersive Spectroscopy EDX measurements. Integration of

phosphates has a stabilizing effect showed by the impedance increase compared to pure NaCl

solution. For Calcium, its integration is detrimental, a slight impedance increase is observed

after two hours but then the corrosion processes accelerate and his the fastest after 24 hours

(smallest impedance measured).


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Figure 4.21: Schematic description of the different components measured in presence of

Calcium in solution; (1) charge transfer, (2) diffusion, (3) adsorption processes

Summary

Macroscopic Electrochemical Impedance Spectroscopy (EIS) with the frequency dependant

specific information obtained is a very versatile technique

1) The method can be considered as a spectroscopic one and can be used to investigatesimultaneously dielectric properties (through capacitive response) and different

electrochemical reactions (charge transfer or diffusion controlled) taking place with

very different speeds

2) The technique allows to identify multiple reactions on a surface by means of theirspecific time constant determined from experimental datas even if the method is

probing macroscopic areas

3) Adsorption phenomena can also be identified and followed even if they are not

triggered by the voltage perturbation (uncorrelated processes)

The large amount of information to be gained in one single EIS experiment can however

make data interpretation very difficult. EIS measurement can also be performed locally.

Interpretation is then even more difficult because the localization of the information in

conducting electrolyte is poorly defined.


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4.5 Atmospheric corrosion

When considering uniform corrosion processes, it is necessary to also mention the

atmospheric corrosion of metals. Atmospheric corrosion is known to be an electrochemical

process governed by climatic factors like temperature, relative humidity and the chemical

composition of the atmosphere. The main difference between aqueous corrosion where thebulk solution insures defined corrosion processes and the atmospheric corrosion is that the

thin water layer present in this case is much less defined, Fig. 4.22. In atmospheric conditions,

the diffusion layer is completely determined by the electrolyte thickness inducing:

- Higher diffusion rates for cathodic reactions and corrosion products removal- Faster changes in electrolyte parameters (pH, precipitation, )

Most (estimation of around 80%) of the uniform corrosion processes and problems are of

atmospheric nature, but the phenomena are still poorly investigated and documented. The

main reason is that precise electrochemical characterization of these corrosion mechanisms is

almost impossible. The available characterization methods are then mostly gravimetric mass

loss analysis after field exposure and analytical chemistry determination of the released ions,

Fig. 22.

Another reason for the difficulty in predicting atmospheric corrosion rates is the numerous

environmental parameters involved and influencing the corrosion mechanisms, Tab. 4.2.

Relative humidity and wet/dry cycles (Time of Wetness) can have a crucial influence on

corrosion product formation and stabilization. These products are influencing the corrosion

rates of materials like, for example, copper or zinc that would corrode actively in neutral

aqueous solutions. Other parameters like air pollutants will influence corrosion products

stabilization or dissolution as well as contribute to the acidification of the water film. Their

influence is however not completely understood and the definition ofWetness is also debatedbecause it can depend on the material considered. The 80% RH humidity threshold is an

arbitrary value based on practical experience and observations.

Figure 4.22: Schematic description of the difference between aqueous and atmospheric

corrosion phenomena and of used characterization methods for each case


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a) Time of wetness = when temperature is higher than 0C and the relative humidity ishigher than >80%.

Table 4.2: Most important climatic and air pollutant parameters influencing atmospheric

corrosion

In recent years, the discussion also started about metal run-off caused by atmospheric

corrosion and becoming an environmental problem. As run-off, we understand the total

amount of corrosion products washed off by rainwater, and reported as amount of metal

cations released. Run-off rates and corrosion rates depend on the climatic and pollution

situation as well as on the applied metallic material and exposure time. Figure 4.23 presents

the standard environmental exposure tests used to determine the corrosion rates (a) andadditionally collect the washed metal ions to be analysed by analytical methods (b). The

metallic panels are hanging at 45 inclination facing the south. These normed parameters

insure a consistency of the results obtained at different sites worldwide. Databases with

experimental measurement of all the relevant atmospheric parameters and the related

corrosion and run-off rates are shared between countries. They serve the formulation of

statistical mathematical models for the estimation of the critical parameters controlling

atmospheric corrosion

a) b)

Figure 4.23: Standard corrosion test setup used to determine: (a) uniform corrosion rates,(b) run-off washing away of soluble corrosion products and particles


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The prediction of atmospheric corrosion damage is usually performed by a Dose Response

function analysis with the environmental parameter mentioned above as input parameter and

a correlation with the time-dependant measurement of corrosion rates at different relevant

sites, Fig. 4.24. In order to have a full correlation, different environments such as industrial

(near dense Highways: Hrkingen), urban (Dbendorf), agricultural (Payerne) and mountains

(Davos) have to be chosen and will be defined as reference sites. A mathematical treatmentperformed by varying each considered parameters and assessing their influence on variation

of the corrosion rate will allow identifying and extracting the relevant ones. Figure 4.25,

shows the corrosion rate extrapolation obtained for copper and where sulphur dioxide in

conjunction with humidity and ozone concentrations play a crucial in increasing the

corrosion rates. These pollutants are typical from industrial areas and there, the highest

uniform corrosion rates are expected for copper.

Figure 4.24: Dose-Response assessment of key parameters influencing uniform corrosionrates based on mathematical statistical analysis

Figure 4.25: Relation between the measured uniform corrosion rate and influence of the keyparameters for copper


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Once the critical parameters have been identified for a given materials and a relation of these

parameters with corrosion rates has been validated on the tests sites, then overall prediction

maps can be calculated for countries. In Figure 4.26, the risk of copper corrosion is presented

for Switzerland and it clearly indicates that using large copper structure does not represent a

major problem in the mountains (white areas), but that the valleys of the Ticino is a

problematic area. The main identified problem there, is the pollution coming from theindustrial activities in Northern Italy. These maps serve the government to define

environmental strategies and regulations. In the case of copper, it is interesting to note that the

sulphur content in the air is currently decreasing but Ozone is increasing so that the corrosion

problem is not disappearing.

Figure 4.26: Mapping of the corrosion rates as function environmental parameters

Atmospheric corrosion of metals can of course in general not just be modelled by a relation

between some main environmental parameters giving directly the corrosion rate. These

environmental species will interact with the corrosion products and can more or less stabilize

them depending on their nature. Figure 4.27 shows for copper how the corrosion products will

evolve as a function of exposure time. At the beginning, cuprite (copper oxide) will beformed and will give the surface a brown appearance. Afterwards, the surface products will

transfer in hydrated sulphate containing (with addition of chloride near seas) compounds.

They usually turn in more or less intense green colour as function of the chloride content and

are then referred as Patina. This Patina is currently generated by purpose for architectural

effect and this is a positive effect of a corrosion process!


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Figure 4.27: Schematic description of the very different corrosion product structure that can

be obtained depending on the air exposure process.

Run-off

The formation of stable corrosion products slowing down uniform corrosion brings us to the

question of the Run-offrates from these surfaces. This process is then environmental relevant

because of the leached ions that will be found in water and soils.

Figure 4.28: Description of the run-off products formation and their characterization.


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Determination of run-off rates is relatively simple but time consuming. The rainwater, which

flowed over each test roof (simulated by panels, Fig. 4.23b), was sampled in 50-liter plastic

containers. The sampling periods were 1 to 6 months depending on the frequency of rain

events. The sampled rain amount was measured, and a specimen of 1000 mL was taken after

having intensively stirred the sampled rainwater container. To this specimen 10 mL of

concentrated nitric acid was added to dissolve solid corrosion products (get total run-offamount) and to stabilize the cations.

Different materials have been assessed for their Run-off susceptibility, Fig.4.29. The

following elements were analysed by Inductively Coupled Plasma Optical Emission

Spectroscopy (ICP-OES): aluminium (Al), cadmium (Cd), chromium (Cr), copper (Cu), iron

(Fe), lead (Pb), nickel (Ni), tin (Sn), titanium (Ti) and zinc (Zn). For measuring tin, the water

sample was acidified with hydrochloric acid instead of nitric acid. Another portion of the

sampled rain was filtrated through a 0.45 m-membrane filter to get a separation between the

dissolved matter and the particulate matter (so-called insoluble matter). The filtrate were

treated and analysed in the same way as mentioned above. The particulate matter in the

sampled run-off is the difference between the total run-off and the filtrated run-off

measurement.

Figure 4.29: Runoff rates measured for different materials during normed atmospheric

exposure

As can be seen in Fig. 4.29, which shows the run-off rates of all investigated materials

averaged over 5 years exposure excluding the first year, the passive layer forming metals like

aluminium, stainless steel and titanium have run-off rates less than 0.01 g m-2

a-1

(the release

of metal was smaller than the detection limit (


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electrodeposited tin layer giving the atmosphere and the rainwater contact to the substrate.

From the tinned copper (1 m) and tinned stainless steel ,a total tin run-off rate of 0.02 to

0.05 g m-2

a-1

could be found in the beginning of the exposure. After one year of exposure the

total tin release was smaller than 0.01 g m-2

a-1

.

The differently treated zinc specimens show runoff rates from 2 to 3 g m-2 a-1 . The runoff ratefrom the as rolled titanium-zinc and the hot-dip galvanized steel are very similar (2.6 respect.

2.4 g m-2

a-1

). The black phosphated titanium-zinc has a smaller runoff rate than the untreated

titanium-zinc (1.9 g m-2

a-1

). The black sheet changes its visual appearance in the course of

time from black to grey. This is a sign for changes, which took place in the structure and

composition of the corrosion products. At longer times, the runoff rate of the phosphated

material could therefore increase.

The grey pre-patinated titanium-zinc shows the highest runoff rate of all investigated zinc

materials (3.2 g m-2

a-1

). This could be explained by the formation of corrosion products with

different crystallinity and specific surface area during the industrial pre-patination process.

The runoff rate is identical

The same way as for the corrosion rates, run-off maps can then be established for specific

materials using the Dose Response function statistical analysis, Figs 4.30 and 4.31. Globally,

for copper, Run-off relates to uniform corrosion processes. The rate controlling parameters

for the amount of run-off cations (MR) is again sulphur dioxide and ozone, but the relation

with humidity is different. Rain is an uncorrelated parameter in this case. As a result, Run-off

rates are high in the central part of Switzerland (Lucerne area) although the corrosion rates

were not very high. The Ticino is then also very much exposed to the run-off problem, but

there the corrosion rate was already high. On Figures 4.30 and 4.31, the 8 measurement sites

in Switzerland are also shown.

Figure 4.30: Run-offcopper release map for Switzerland based on 8 measuring sites and

dose response analysis


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For copper, a relatively simple DoseResponse function relation is found with only few

parameters influencing the run-off rates. In the case of zinc, the other widely used uniformly

corroding materials, the corrosion product stabilization is much more complex. As a

consequence, other environmental parameters are influencing the run-off rates in synergy

with sulphur dioxide and ozone. Exposure time and Temperature (Temp) will influence the

formation of the corrosion products and the run-off (MR) together with the pollutants.Fig.4.31. As a consequence, a different local distribution of high run-off rate areas can be

found for zinc compared to copper. The Ticino case with high run-off rate is clear because the

high amount of pollutants present. In the central part of Switzerland, a finer discrimination of

the local run-off rates to valley and to the surrounding of cities is found. Both for copper and

zinc, the mountains area are obviously immune towards run-off problems. Agricultural area

around Payerne for example also shows low run-off rates. These maps are useful for

governmental authorities in order to evaluate the environmental impact of materials and

formulate regulations. Also companies can use them to design their materials and coating as

function of the environment. Corrosion and run-off rates can be used to tailor sacrificial

coatings thickness for materials like zinc currently widely used in outdoor applications.

Figure 4.31: Run-offzinc release map for Switzerland based on 8 measuring sites and doseresponse analysis

Summary

Understanding and predicting uniform corrosion mechanisms has two benefits:

1) Knowledge of the corrosion rates of metals is an economical advantage

Components and structure design can be optimized and risks can be evaluated


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2) Knowledge about Run-off amounts

is and ecological advantage / requirement

Water, soils and foods contaminated through corrosion products can be toxic for humans but

more important is that the ecosystem can be deteriorated by toxic corrosion products alreadyin presence of trace amount.

Avoiding uniform corrosion

There are simple measures to avoid/decrease the uniform corrosion rates and this is valid as

well for aqueous as for atmospheric corrosion processes:

1) A pH increase above 9.5 is in general beneficial (especially for steel)

2) Reduction of the oxygen content of the solution

3) Addition of corrosion inhibitors in the solution (only possible in closed systems)4) Optimized the design of the component, see Fig. 4.32. Avoiding the presence of

areas with stagnant water will already prevent a lot of uniform corrosion related

failures

5) If the design cannot be changed, a passive material has to be chosen for the

structure

Figure 4.32: Simple construction measures avoiding the presence of stagnant water in

structure and reducing uniform corrosion damages

siandaii ch4 uniform corrosion

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