SIandAII Ch6 Pitting Corrosion

8/22/2019 SIandAII Ch6 Pitting Corrosion

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Surfaces, Interfaces, and their Applications II Pitting Corrosion

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

6 Pitting corrosion

For passive metals and alloys, uniform corrosion is rare or only possible in a very aggressive

acidic (and alkaline for Aluminium) environments. In the presence of aggressive ions,

microscopic local corrosion attacks can take place under certain conditions. Localizedcorrosion can manifest itself as pitting, crevice or intercrystalline (intergranular) corrosion,

Fig. 6.1.

Figure 6.1: Manifestations of local corrosion: crevice corrosion (left), pitting (middle),

intercrystalline (intergranular) corrosion (right)

In the case of pitting corrosion, local corrosion attacks form at the passive surface. Crevice

corrosion is to a large extent a geometrical problem (more intense attack in the crevices

arising from stagnant media), and intercrystalline (intergranular) corrosion is a material

problem (sensitization or non-noble grain boundaries which are preferentially attacked).

6.1 Introduction relevant factors for pitting corrosion attack

Definition: In order for a passive metal to be susceptible to pitting corrosion, two conditions

have to be fulfilled:

1) Presence of aggressive anions (element: Cl, F, Br, I) inducing a local attack(dissolution) of the passive film

2) The equilibrium potential of the material must be higher than a material characteristic

potential called Pitting potential

The characteristic appearance (Fig. 6.2) of the pit in the early stage of localized attack is:

The attack has the form of geometrical features

Very well delimited holes (sharp boundary) are found

The material is absolutely intact next to the holes


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a) b)

Figure 6.2: Optical appearance of pitting corrosion attack: (a) Pitting corrosion of Cr-Ni

stainless steel in HCl solution, (b) cross-section through a typical semi-circular growing pit

of a few hundred micrometres.

The main reasons for localized corrosion failures are:

Insufficient passive film stability (for stainless steel, insufficient chromium content

cannot prevent chemical dissolution at the oxide-solution interface as discussed in

the chapter 3 about passivation)

Design errors: the presence of areas with stagnant solution will accelerate the

generation of local aggressive conditions (defined later in chapter 6.2)

Inadequate surface preparation: the presence of deep scratches or defects during

surface preparation can act as pit initiation sites.

In order to completely understand and assess the risk of a localized corrosion processes, it is

necessary to clearly distinguish three different phases, Fig. 6.3:

1) Pit (hole) initiationDuring this incubation phase, aggressive ions destabilize locally the nm-thick

passive oxide films or defects. It needs to be stated that the pit initiation stage

extends to micrometer large dissolved holes.

2) Pit growth or 3) repassivation

- Even if sufficiently large dissolution occurred to form micrometer size pits,corrosion can still stops if a new oxide film forms on the hole surface. This

situation is obtained when no local aggressive solution in the pit can be

generated.

- If the chemical environment is getting so aggressive (acidic) that repassivation isthermodynamically impossible, the corrosion will locally propagate in depth by

a dangerous autocatalytic process.


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Figure 6.3: Schematic description of the different stages of a localized corrosion processes

starting with an intact nm-thick oxide that is progressively dissolved. In the initial stage,

active dissolution can be suppressed if an oxide film reforms (repassivation) before

aggressive local chemistry is generated. Otherwise, a progressive in-depth pit growth occurs.

The pits are geometric features that can be investigated in-solution from their very early stage

of formation, Fig. 6.4. Missing atoms in crystalline passive films (example of Nickel oxidestructure, Fig. 6.4a) and adsorption layer can be detected by electrolytic Scanning Tunneling

Microscopy (STM) as well as nanoscale topographic changes related to pit formation,

Fig. 6.4b.

a) b)

Figure 6.4: Scanning Tunnneling Microscopy (STM) investigation of pit initiation process on

a Nickel single crystal in NaCl solution: (a) Atomically resolved oxide structure showingmissing atoms (white squares), (b) In-situ imaging of the growth of a nm-size pit


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a) b)

Figure 6.5: Optical microscopic images of pits formed on Fe immersed in phthalate buffer

solution (pH 5) with addition of: (a) 0.01KCl + 0.01 K2SO4, (b) only 0.01 KCl.

Although, most of the pits grow as semi-circular shaped holes (Fig. 6.5a) because of the local

diffusion processes, it is possible to generate crystallographic pits (for example hexagonal

holes) like for Fe immersed in KCl containing phthalate buffer, Fig. 6.5b. Here, acombination of crystalline plane dependant adsorption for specific anions and orientation

dependant dissolution rates is used to generate special pit geometries. Localized corrosion

processes are not always detrimental and can also be used to structure surfaces when the

corrosion mechanisms are well understood!

6.2 Pitting corrosion mechanisms

The pitting corrosion susceptibility of a material can be assessed and discussed based on

electrochemical potentiodynamic polarization measurements, Fig. 6.6. A passive material will

usually show a very low current density (A/cm2

) up to the water dissociation potential or tothe chromium transpassive dissolution for steel in absence of aggressive anions. The

occurrence of a localized corrosion (pitting) process is evidenced on the electrochemical

measurement by a drastic reduction of the potential range for passivity and a rapid current

increase when the critical value for the pitting potential Epit is exceeded.

- For a given material, the higher the pitting potential is, the more corrosionresistant it will be. The polarization potential is giving the driving force for

anodic dissolution and will help stabilizing a localized corrosion process

(spontaneously with the cathodic reduction or externally with a potentiostat).

- When the potential is decreased below the repassivation potential Erep , thedriving force for dissolution is reduced and the attacked sites are deactivated by

formation of an oxide film in the pit.

On Figure 6.6, a schematic view of an anodic potentiodynamic polarization curve with the

different relevant parameters is given. Ideally, materials with Erep close to Epit will be more

corrosion resistant than if Erep is very low because when the two potentials are close together,

any potential drop in the pit will shut down its growth (current decrease). When a very low

repassivation potential is measured on a material, this means that every initiated localized

attack will growth.

Stable pitting condition: E > Epit

Existing pits have repassivated at: E < Erep


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Figure 6.6: Schematic description of electrochemical anodic potentiodynamic polarizationmeasurements (typical for stainless Steel) obtained with and without aggressive chloride

anions.

6.2.1 Pit initiation

The first stage of the pitting process called pit initiation is related to the breakdown of the

protecting passive oxide. Different mechanisms (Fig. 6.7) have been proposed for the various

passive materials depending on the thickness of their oxides and also on the chemical stability

at the oxide-solution interface (see chapter 3). A further aspect related to the passive oxide

has been discussed in the chapter 5 and is the very high potential drop (up to 107

V/cm)accommodated in the film that will favour ion migration. It is possible to distinguish between:

1) The penetration mechanism: the Cl- ions (or other halides anions) are integratedinstead of OH

-and migrate through the passive film inducing hole formation at the

metal-oxide interface. This type of initiation process is typical of corrosion resistant

material with very thin thermodynamically stable oxides like Chromium.

2) The island adsorption mechanism: the Cl- ions (or other halides anions) adsorbslocally on the surface replacing OH

-and decreases the stability of the oxide

surface. The passive layer dissolves faster until the metal surface is exposed. This

process is typical of less corrosion resistant materials such as stainless steels wherethe passive oxide is in an dynamic formation-dissolution state (see Quartz Crystal

Nanobalance results in the Passivation chapter)

3) The oxide cracking mechanism: in this case, the surface oxides (hydroxides) filmcracks open due to stresses (internal or applied). This initiation mode is rarer and

implies as prerequisite a poorly passivating surface (thicker oxide) or the presence

of a thick anodized layer for example. Magnesium in the alkaline domain would be

such an example of a material able to passivate but forming a thick oxide layer that

can crack in presence of chlorides.


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Figure 6.7: Schematic presentation of the three initiation mechanism often found

In industrially produced materials, there are other factors increasing the pit initiation

probability such as inclusions and defects, Fig. 6.8. Typically for stainless steel, the

manganese sulphide particles are dissolving very easily (they are not passivating, Fig.6.8a)

and generating very detrimental crevices where aggressive electrolyte can be trapped,

Fig. 6.8b. Sulphur addition is necessary to improve machinability of steels so that defect

engineering (size, shape, composition) is necessary to reduce the localized corrosion

susceptibility of these defects. For this reason, it has been necessary to develop dedicated

local electrochemical methods, suitable to perform corrosion resistance assessment on single

material defects. The electrochemical Microcell setup presented in detail in section 6.4 is

currently the most powerful one and has been widely used to elucidate the role of inclusion in

corrosion susceptibility of stainless steel.

a) b)

Figure 6.8: Influence of inclusions on pitting corrosion susceptibility: (a) weaker or absence

of passive film on MnS inclusions in stainless steel, (b) dissolution of the inclusion (imaged by

Focussed Ion Beam FIB) cut inducing the formation of microcrevices on a corrosion resistant

stainless steel

The pitting potential is defined by the threshold potential at which the measured current

rapidly increases because of the growth of a stable localized attack. Pit formation can

however already occur at potentials lower than the pitting potential.

In this potential domain (E < Epit), these local events are often very numerous (Fig. 6.9a), butsolution aggressivity for stable growth is not achieved. We are in presence of metastable


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pitting events. The hole formation process for the metastable pitting process is measurable as

a dissolution current transient, Fig. 6.9b. When local dissolution occurs and the current starts

to increase, the polarization in the hole in formation will be automatically decreased because

of the ohmic drop U=RI effect. The concept that solution resistance is slowing down the

reaction rate has already been discussed in relation with galvanic coupling. In the case of a

localized attack, it means that as soon as the current locally increased, the driving force(potential) for dissolution is decreased at the pit bottom and passivation can be re-established.

But during, the dissolution phase, local solution aggressivity is also increasing and once a

given threshold passed charge is reached, one of the metastable pit will transfer in a stable

growing pit.

Figure 6.9: Electrochemical characterization of metastable pits: (a) complete polarization

curves with current transients in presence of chlorides, (b) Enlargement of a single current

transient corresponding to one initiation event

a) b)

Figure 6.10: Scanning Electron Microscopy (SEM) characterization of metastable pitting

corrosion event on stainless Steel in NaCl: (a) before removing the passive film remnant,(b) after removal of the cover passive film to show the real pit size


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Note: A metastable pit, even before being in a stable growth condition is already a large

microscopic attack as can be seen on Fig. 6.10. It is however important, for a correct

assessment of the localized corrosion susceptibility of a material/structure, to determine if

you are in presence ofmetastable events that will not result in safety relevant damages or if

dangerous stable pits are growing. Sometimes, the effective size is not seen because of the

presence of the passive film, but once the cover is removed, metastable pit size of a fewmicrometer are found, Fig. 6.10b.

6.2.2 Pit growth

Once a metastable pit reached stabilized growth, in-depth propagation will occur. The

question is now why a localized attack can be so detrimental. The answer becomes clear when

considering the electrochemical reactions involved.

Following reaction steps are found during pit growth processes, Fig.

1) Dissolved metallic ions are hydrolyzed (see equation below). As a result, largeamount of protons (H3O

+) are formed and the pH will drop quickly in the pit.

2) This hydrolysis process induces a local excess of positive charge (H3O+). Theelectroneutrality in the local pit solution volume has to be achieved at any time so

that fast Cl-

migration in the pit will occur producing locally more aggressive HCl

solutions.

3) The diffusion of O2 is more difficult in the occluded pit area resulting in local O2depletion. Obtaining stable passivation equilibrium condition is difficult with small O2

diffusion current as shown in Figure 6.12 so that constant activation of the pit is

guaranteed. When an active-passive transition is present for the passive material, a

decrease of the pH will result in an increase of the critical current density (i crit),

Fig. 6.13. The repassivation process is therefore even more hindered.

The combination of all these phenomena induces an autocatalytic process for the localized

attack where the dissolution rate is accelerating during a first phase. If the pH drops below a

value of 4, it is also necessary to take into account an additional cathodic reaction taking

place only in the pit, the hydrogen reduction (H-type corrosion process). This cathodic

reaction is charge transfer controlled so that it can induce very high dissolution rate in the pits

and acidification will accelerate the pit growth rate.

2H+ + 2e- = H2


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For a passive metal, an active passive element (microgalvanic coupling) is furthermore

created and maintained during the pit growth phase:

Active anode (pit)

Passive cathode (intact passive layer around the pit)

Concerning the galvanic coupling conditions, the area ratio is catastrophic with a small anode

surrounded by a very large cathode. If the electrolyte conductivity is good and the passive

film supports oxygen reduction, then the pits will grow very fast !

Figure 6.11: The different processes occurring during the pit growth process generating very

acidic local environment and autocatalytic acceleration of the dissolution

a) b)

Figure 6.12: Equilibrium situation that can be spontaneously obtained for the corrosion

reaction: (a) stable passivation outside of the pit in aerated neutral solution, (b) active

corrosion in acid pit media with highericrit and lower O2 diffusion current


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Figure 6.13: Increase oficrit with decreasing pH stabilizing active corrosion in presence of O2

reduction reaction

6.3 Pitting susceptibility of MaterialsDue to the fact that oxygen is almost always present in the environment, and that equilibrium

conditions up to its reversible potential can be established:

E rev, O2 = 1.23 SHE - 0.059 pH

most of the industrially relevant materials will spontaneously suffer from localized

corrosion attacks if their pitting potential does not surpass the Erev, O2. Fig. 6.14 schematically

presents this situation with the dangerous pitting potential domain extending up to the blue

point.

Figure 6.14: Potentiodynamic polarization curve of metal that can be passivated in the

absence and activated in the presence of pitting inducing anions.

The pitting potential values of common materials measured versus the Standard Hydrogen

Electrode (SHE) in 0.1M NaCl solution is presented in Tab. 6.1. It is obvious that materialslike Aluminium are very susceptible to pitting initiation (very low pitting potential). The


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different Cr-Ni stainless steels and Nickel show better pitting corrosion resistance but can all

spontaneously fail in aerated media. Chromium and Titanium with pitting potential higher

than 1 Volt are immune against pitting corrosion in this chloride containing media. For

chromium, it must not be forgotten that the material surface will experience transpassive

dissolution at higher potential, but the detected current increase is not related to a localized

corrosion attack.

Table 6.1: Pitting potentials for different metals and alloys

Metal Epit ( versus SHE, 0.1 N NaCl)

Aluminium -0.37

Nickel 0.28

18/8 CrNi steel 0.26

12% Cr steel 0.20

30% Cr steel 0.62

Chromium > 1.0

Titanium > 1.0 (1 N NaCl)

For high-alloyed steel, the resistance against pitting increases with higher chromium and

molybdenum content, Fig. 6.15. It is worth noticing that in order to obtain a significant

increase in pitting corrosion resistance, very large amount of chromium are necessary because

chromium is needed to stabilize efficiently the passive film on stainless steel that is first in a

very dynamic state of constant formation-dissolution from 12% Cr up to approximately 30%

Cr. This aspect needs to be checked for any new alloy development because the alloying

element concentration necessary for passivation and for pitting corrosion resistance of thepassive film can be very different. For Molybdenum, the situation is different. If sufficient

amount of Chromium is present in order to guarantee passivation, addition of small amount of

Molybdenum (a few percent) is sufficient to obtain similar pitting corrosion resistance,

Fig.6.15b.

a) b)

Figure 6.15: Influence of alloying element of stainless Steel on the pitting potential increase

in NaCl solutions


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Molybdenum acts in a different way on the localized corrosion process. Addition of Mo will

decrease the critical current density for the active-passive transition in very acidic media,

Fig. 6.16. The influence of small Mo addition in obtaining order of magnitude current

decrease is shown in Fig. 6.16b. One first beneficial effect is that pits can repassivated

because even with low oxygen contents in the pits, the only possible equilibrium is in the

passive domain, Fig. 6.16a. Molybdenum has a second very beneficial effect that is related tothe dissolved molybdate ions that can act as a buffering agent in the pit. The pH drop is

smaller and obviously the hydrogen reduction rate can also be kept low.

a) b)

Figure 6.16: Potentiodynamic polarization measurement and critical current density as

function of Molybdenum content

Figure 6.17 shows an example of the pitting potential change with the alloy composition.

Three steel types with different compositions (Tab. 6.2) were potentiodynamically polarized

in neutral NaCl of LiCl solutions. Fig. 6.17 shows that steels with the highest Ni and Mo

content possesses the highest pitting potential.

a) b)

Figure 6.17: Potentiodynamic polarization measurement of different steels (see Tab 6.2 for

composition) in: (a) 0.1M NaCl and (b) 6M LiCl


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Table 6.2: Some steel and nickel qualities sorted in groups with increasing pitting and

crevice corrosion resistance; depending on the qualities, alloys indicated in brackets can be

present in different groups. The second part of the table indicate where the different group of

alloys can be used

group materials

I 1.4301, 1.4303, 1.4306, 1.4541, 1.4543

II 1.4401, 1.4404, 1.4406, 1.4429, 1.4435, 1.4436, 1.4438, 1.4571

III 1.4439, 1.4462, 1.4503, (1.4539)

IV (1.4539), 1.4563

V 1.4529, 1.4565, Avesta 254 SMO, (2.4856)

VI 2.4602, 2.4610, 2.4819, (2.4856)

The localized corrosion resistance of stainless steels against pitting (and crevice corrosion) ispositively influenced by the already mentioned elements chromium (Cr), molybdenum (Mo),

and nitrogen (N), and negatively influenced by the elements sulphur (S), manganese (Mn,

together with sulphur), and carbon (C). The elements that have a positive effect are

summarized in the PRE (Pitting Resistance Equivalent). To calculate the PRE, the following

equation is used:

PRE = a %Cr + b %Mo + c %N.

For the constants, a = 1, b = 3.3 und c = 0 30 are used. The PRE has no strict scientific

meaning. Nevertheless, it is a suitable guideline to a comparative evaluation of the resistanceof solution annealed / quenched stainless steel (and nickel alloys). Impurities, inclusions,


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degree of deformation, previous heat treatments and the surface condition of the steel are not

considered in the PRE.

Regarding their resistance to pitting and crevice corrosion, stainless steels can be roughly

divided into groups (Tab. 6.2). This division into groups of alloys provides only a general rule

for corrosion resistance but should avoid that alloys with low pitting corrosion resistance are

used in completely wrong environment. This can represent a major safety issue. In specificcases, the material impurities (Mn and S content, see Fig. 6.17) and especially the surface

quality (ground, brushed, polished, honed) can play an important role. Lower classified steel

with better surface quality behaves in a more resistant way than higher classified steel with

bad surface (rough, grooves, scratches).

Figure 6.17: Microstructural observation of the MnS inclusions distribution in a Stainless

steel as function of the nominal Sulphur content in the alloy.

Higher chloride concentrations (more aggressive conditions) shift the pitting potential to more

negative values (Fig. 6.18). This effect is however relatively marginal compared to the

corrosion resistance changes that can be obtained by modifying and alloy or changing the

materials.

Figure 6.18: Pitting potentials of different metals depending on the Cl- concentrations


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Finally, it needs to be mentioned that pitting corrosion is only an issue as long as the material

is passive and the surface is protected. Reducing the chloride concentration in neutral

environments will result in slow passive film dissolution process (left-bottom conditions of

Fig. 6.19). In presence of higher chloride concentration in neutral to slightly acidic HCl media

will induce severe localized attack (middle part of the graph) until passivation cannot be

obtained typical at pH around 0 (right part of Fig. 6.19).

Figure 6.19: Pitting versus uniform corrosion of stainless steel as function of chloride

concentration and pH

6.4 Local electrochemistry - the Microcell technique

6.4.1 Introduction

The durability of technical passive alloys (e.g., stainless steels or aluminium alloys) is often

limited by chemical or structural heterogeneities. Inclusions play a key role in being potential

initiation sites of pitting and crevice corrosion, Fig 6.17. Many electrochemical methods to

study localized corrosion are based on large-scale experiments with exposed areas in the

square millimetre to square centimetre range. To understand the mechanisms of pit initiation

and propagation, it is useful to also study the corrosion processes at the micro- and nanometreranges. Such investigations often provide a better insight into the initiation mechanism of

localized corrosion. The electrochemical techniques for localized corrosion tests in

microscopic and nanoscopic dimensions can be divided into two major groups:

1. Scanning techniques: Immersed samples (immersed area is in the square millimetre to

square centimetre range) are scanned using microelectrodes or ultramicroelectrodes. These

methods allow obtaining information about the local distribution of one or several parameters

during corrosion experiments. The experiments might be performed under open-circuit

conditions or under potential or current control. Depending on the particular technique, a

lateral resolution down to a few nanometres is possible. However, many scanning methods do

not allow measuring of local corrosion currents. The potentiostat controls the current flow ofthe whole immersed surface area.


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2. Small-area techniques: By decreasing the size of the exposed area, it is possible to

localize the electrochemical process. This can be achieved by thin embedded wires,

photoresist techniques, a droplet cell, or small glass capillaries touching only small areas of

the specimen surface, Fig. 6.20. These techniques allow, in contrast to the scanning methods,

polarization of the microscopic surface areas. All common electrochemical techniques, for

example, potentiostatic or potentiodynamic measurements, galvanostatic measurements, andcyclic voltammetry, can be applied. Local corrosion currents can be evaluated directly. With a

high-resolution potentiostat, corrosion processes on a molecular level can be recorded, even if

the small-area techniques do not show such a high lateral resolution as the scanning methods.

Figure 6.20: Different examples of methods allowing local determination of

electrochemical reaction rates

6.4.2 Electrochemical Microcell principle

The setup for microelectrochemical investigations based on the microcapillary is shown

schematically in Figure 6.21a. The entire setup is mounted on a microscope, allowing for

precise positioning of the capillary. The fully assembled microcell is shown in Fig. 6.21b. The

microcell basically consists of a pulled microcapillary filled with electrolyte. The tip diameter

of the capillary can be varied from about 1 to 1000 m depending on the experimentalrequirements. Between the front end of the microelectrode and the surface of interest, a layer

of silicone rubber is applied as sealant.

The microcell is fixed at the revolving nosepiece replacing an objective, and the specimen is

mounted on the microscope stage. This arrangement enables the search for a site with

different magnifications before switching to the microcapillary measurement. In this way,

simple, precise, and fast positioning of the microcell is possible. A reference and counter

electrode are connected to the capillary allowing electrochemical control of the investigated

surface.


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Figure 6.21: (a) Schematic description of the Microcell system and (b) photographic view of

the mounted setup

a) b)

Figure 6.22: (a) Glass microcapillary with silicone sealant used and (b) measurement

principle with positioning of the capillary precisely on different locations of a heterogeneousstructure.

Thin microcapillaries are obtained by heating borosilicate glass tubes and pulling them when

they reach the glass temperature. For sealing, the glass capillaries are dipped in a one-

component silicone lacquer. A stream of ethanol is pressed through the micropipette in order

to flush out the silicone inside the capillary without destroying the fine tip. Very thin layers of

silicone are applied to the capillary tip by repeating this process many times. The hardening

process proceeds at a slow rate, which allows the silicone surface to energetically minimize

under the effect of surface tension, resulting in smooth and dense coatings, Fig. 6.22a. This

process allows production of capillaries with tip diameters below 1m.The quality of the

silicone sealant is a most critical factor, as it determines to a high degree the quality of anelectrochemical experiment.


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Figure 6.23: Different examples of application directly of final products (implants, machined

watch components, electrical circuit)

Measurement of the local electrochemical response and corrosion susceptibility of metallic

materials can be performed on curved substrates or complex shaped components because of

the sealing at the end of the capillary, Fig. 6.23.

6.4.3 Pitting corrosion mechanisms information obtained

The first information that is of interest for a heterogeneous material is the distribution of

pitting corrosion susceptible areas on a surface. For this purpose, the influence of the size of

the exposed area on pit initiation can be investigated by means of microcapillaries of

decreasing sizes. Figure 6.24 shows how the pitting potential might change if the exposed

area is decreased. Local polarization curves were measured at random sites on the low

sulphur 1.4301 (Fig, 6.17; also called 304 SAE Grade) plate in 1M NaCl. The diameter of the

microcapillaries varied from 50 to 1000 m. Common large-scale measurements show a

pitting potential of about +300mV (SCE). Measurements performed with 1000 m

microcapillaries show similar pitting potentials. Diminishing the exposed surface to an area of

50 m in diameter led to an increase of the pitting potential to about +1200mV (SCE).Microelectrochemical experiments revealed that the pitting potential is an area dependent

value. Additional measurements showed that the increase of the pitting potential is caused by

a decrease of the number of weak points when the exposed area is diminished.


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Figure 6.24: Pitting potentials of a 1.4301 plate (0.003% S) as a function of the diameter ofthe microcell. The polarization curves were measured at random sites in 1M NaCl

Figure 6.25 shows a polarization curve measured on the medium-sulphur 1.4301 rod. A

capillary with a tip diameter of about 100 m was used to select a single MnS inclusion. The

electrolyte was 1M NaCl. The curve reveals onset of pitting at +300mV (SCE). Large-scale

measurements showed stable pitting at a similar potential, so that it can be stated that this type

of inclusion represent the most susceptible defect in the alloy. Microelectrochemical

measurements performed on areas without MnS inclusion did not indicate pitting at potentials

below the onset of oxygen evolution. Figure 6.25b shows the SEM picture of the inclusionafter the corrosion experiment. It indicates that the inclusion/bulk interface is mainly attacked.

a) b)

Figure 6.25: Pitting behaviour of a single MnS inclusion on an 1.4301 rod (0.017% S): (a)

Polarization curve measured in 1M NaCl and (b) SEM picture of the attacked MnS inclusion

taken after the corrosion experiment


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Getting even more local with smaller capillaries, Figure 6.26 shows polarization curves

measured on the high-sulphur 1.4301 plate. A capillary with a tip diameter of about 2.5 m

was chosen to study the corrosion behaviour of different zones of a single MnS inclusion in

1M NaCl. The investigated spots are indicated with white circles on the SEM picture taken

before the corrosion. The electrochemical experiments were discontinued at a current limit of

10 nA (corresponding to a current density of about 200 A/cm2). After a measurement, thecapillary was positioned on the next spot. The polarization curves of the 3 spots provide

information on the pitting behaviour of different zones of the inclusion. The measurements

performed on the adjacent bulk matrix indicate no pitting at potentials below +1000mV

(SCE). The current fluctuations in the passive range were probably caused by activation and

repassivation processes. The measurements performed on the inclusion/bulk interface show

that pit initiation occurred in the potential range between +100 and +300mV (SCE). Curves

measured on the interface showed numerous current fluctuations caused by pit nucleation

processes. The pitting potentials of the polarization curves measured on the MnS inclusion are

shifted to values about 200mV more positive compared to the interface. The fact that the

centre part of a MnS inclusion shows active behaviour only at higher potentials supports the

hypothesis that MnS inclusions are covered by an oxide film. The oxide film shows most

defects at the interface inclusion/bulk due to the change of the crystallographic structure. This

might explain why different polarizations are necessary to activate the interface and the centre

part of an inclusion.

a) b)

Figure 6.26: Corrosion behaviour of different zones of a single MnS inclusion on a 1.4301

plate (0.026% S) in 1M NaCl: (a) Polarization curves measured on different spots of the MnS

inclusion using a microcapillary with a tip diameter of about 2.5m, (b) SEM picture of a

MnS inclusion taken before the corrosion measurements


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Modification of the microcells allows the evaluation of various parameters during a corrosion

experiment or performing of corrosion measurements under different stress parameters

(temperature, mechanical loading and flow)

pH measurements: A 25-m tungsten wire insulated with 5mm polyimide can be used to

measure the pH at variable positions over the sample surface during corrosion experiments(26). The wire, with a freshly cut tip, is inserted into the microcapillary and positioned with a

fine threaded micrometer screw with a resolution of +2m. Prior to an experiment the

tungsten wire is calibrated in different buffer solutions to check the reproducibility. Only the

results of measurements with reproducibility better than 0.5pH units are usually accepted. The

experimentally determined response time of the tungsten wire is in the order of 1 sec, which

is about 10 times longer than the time of the diffusion of the hydrogen ions to the probe.

Measurements with temperature control: A Peltier element is used to control the

temperature of the sample during a corrosion experiment. The Peltier element allows varying

of the temperature between 0 C and 100 C. A special temperature control unit with a linear

control characteristic was developed in order to avoid interference with low currentmeasurements.

Measurements with applied mechanical stress: A special sample holder is used for

experiments with applied stress. For constant deflection tests, thin samples are used as

specimens. Stress is applied to the specimen by holding it down at the border and bending it

over a centred screw. The amplitude of the applied stress is calculated. The calculations are

verified with the help of the minimum deformation which led to indications of plastic

deformation after relaxation of the applied stress.

Measurements with friction: A rotating aluminium oxide tube inside the microcapillary

allows corrosion measurements with friction. This modification allows testing of the

behaviour of a metal probe in an electrolyte under rubbing conditions on the micrometrescale. Rotational speed, load, and applied electrochemical potential or current can be varied

so that it is possible to analyse the corrosion rate during rubbing at various controlled

conditions. In order to quantify the tribocorrosion, profilometer measurements of the treated

zone were performed. An accuracy of 10% was found. With this microtribometer the testing

object could be directly placed on the sample holder without any particular pretreatment.

Measurements with flow: An additional microcapillary inside the microcell enables testing

of the vertical fluid flow. The bottleneck shape of the microcapillary allows application of

very high flow rates. The inner capillary is positioned with a fine threaded micrometer screw

with a resolution of +2 m.

Measurements with sensors

- pHA coated 25 m tungsten wire was used to determine the pH before and after pit initiation at a

MnS inclusion in 1M NaCl, Fig. 6.28. A capillary with a tip diameter of about 100 m wasused to select a single MnS inclusion on the high-sulphur 1.4301. Before each polarization

experiment, a cathodic potential of -500mV (SCE) was applied for 2 min. The pretreatment

led to a pH of about 9.5 because of the cathodic reduction reaction. During the polarization

experiment the pH decreased slowly back to the value of the bulk electrolyte. A value of pH 5was measured during the dissolution of the MnS inclusion and a pH value around 2 after the


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onset of stable pitting. Diffusion calculations were performed to estimate the pH inside the pit

nucleation site. The results showed that a measured pH of 2 correlated with a pit pH of 1.5

and a measured pH of 5 correlated with a pit pH of 4.5.

Figure 6.27: Schematic representation of the different sensors that can be implemented in the

microcapillary.

a) b)

Figure 6.28: (a) Polarization curve and pH values measured on a single MnS inclusion of a

1.4301 plate (0.026% S) in 1M NaCl, (b) Polarization curves measured on single MnS

inclusions of a 1.4301 rod (0.017% S) in 1M Na2SO

4at different temperatures


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- TemperatureFigure 6.28b shows polarization curves measured in 1M Na2SO4 on the medium sulphur

1.4301 plate at different temperatures. A capillary with a tip diameter of about 70 m was

chosen to study the dissolution behaviour of single MnS inclusions in 1M Na2SO4 at elevated

temperature. In sodium sulphate, MnS inclusions are active, but the 1.4301 alloy does not pit.Dissolution of the Mns inclusion starts at lower potentials when the temperature is increased

- Mechanical StressMicroelectrochemical corrosion measurements were performed at single large MnS

inclusions in order to evaluate the influence of applied mechanical stress on the pitting

behaviour of stainless steels at room temperature. The rolling process of the high-sulphur

1.4301 plate leads to stretched inclusions. Depending on their orientation, different corrosion

behaviour is observed. Experiments on unstressed specimens with single shallow MnS

inclusions showed metastable pitting in 1M NaCl, whereas stable pitting was initiated at deep

inclusions. Figure 6.29a shows the polarization curves for sites with single large and shallowMnS inclusions measured in 1M NaCl without stress and with an applied stress equal to 80%

of the yield strength. The curve for an inclusion without applied stress indicates MnS

dissolution between +300 and +550mV (SCE). Large current peaks were observed 100mV

after the onset of MnS dissolution. The curve indicates metastable corrosion, but not stable

pitting. SEM pictures (Figure 6.29b) showed several small corrosion sites at the

inclusion/bulk interface. The MnS inclusion did not dissolve completely in 1M NaCl; it

dissolved just partly.

a) b) c)

Figure 6.29: Schematic Dissolution and pitting behaviour of single shallow MnS inclusion in

1M NaCl without and with applied stress: (a) Polarization curves measured on single shallow

MnS inclusions of a 1.4301 plate (0.026% S) in 1M NaCl, (b) SEM image of the shallow MnS

inclusion after the electrochemical experiment without applied stress, (c) SEM image of the

shallow MnS inclusion after the electrochemical experiment with applied stress.


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Weld joints are often a major target of corrosion. The welding process often results in

decreased material properties in the heat affected zone and the weld itself. The microcell

technique has been successfully used to test welds of stainless steels. Local measurements of

different areas of welds have indicated that different zones suffer corrosion attack depending

on the type of weld. The measuring technique proved easy to handle and apply. Unlikeimmersion tests, which may take several days, it yields a reliable judgment of the quality of a

weld in only a few minutes. In many applications, it is required that the corrosion resistance

of the entire construction is tested. The microcell technique allows also a non-destructive

testing of welds. The corrosion resistance of laser welds was evaluated to optimize a new

laser welding process to join 1.4301 sheets. Figure 6.30a shows SEM pictures of a laser weld.

The production pieces with the laser weld were tested as received in 1M NaCl solution. In

Figure 6.30b, an example of polarization curves measured on different spots of the laser

welding is shown. It is clear that the corrosion resistance of the matrix and the zone adjacent

to the weld trench is significantly higher than that of the weld trench itself. The bottom of the

trench showed a low corrosion resistance. It was found that the holes at the bottom of the

trench (Figure 6.30a) significantly decreased the corrosion resistance. Too high laser intensitycaused these holes. This meant that the process parameters had to be optimized.

a) b)

Figure 6.30: Microelectrochemical testing of a laser weld on a 1.4301: (a) SEM pictures of

the laser weld and (b) polarization curves and pitting potentials of different zones of the laser

weld measured in 1M NaCl


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Summary

- GeneralIn order to evaluate correctly the failure risks related to localized corrosion processes, it is

important to consider that the bulk solution chemistry is often irrelevant and that propagationrate is completely determined by the very aggressive local chemistry that can establish in a

pit, Fig. 6.31. Local microcell measurements have shown that pHs down to a value lower

than 2 can be found. At this pH, most of the metals cannot passivate properly.

Geometry of defects also plays a crucial role: deep microcrevices generated by defect

dissolution (Fig 6.31 right: dissolved MnS) or inadequate surface preparation can stabilize a

growing pit on material that would otherwise not suffer from pitting corrosion.

Figure 6.31: Schematic description of the critical parameters in pitting corrosion

- Microcell techniqueThe microelectrochemical technique using microcapillaries as microcells is an elegant method

to study local phenomena on metal surfaces. Due to the enhanced current resolution down to

picoamperes and femtoamperes, local processes in the nanometer range can easily be

measured. Incorporation of microsensors into the microcapillary cell generates additional

chemical parameters such as pH values simultaneously with electrochemical measurements.

This provides important analytical information especially, with respect to nonstationary

processes.

The results of localized corrosion experiments on stainless steels in 1M NaCl clearly showed

that in moderately concentrated solutions, the presence of MnS inclusions is of primary

importance for pit initiation, whereas the size and geometry as well as the amount of chlorides

and the temperature are critical for the transition from metastable to stable pit growth. Pit

initiation studies at MnS inclusions on 1.4301 stainless steels were performed at different

scales. Electrochemical experiments using a capillary with a tip diameter in the range of

100 m allowed evaluation of the corrosion behaviour of areas with one single inclusion.

Electrochemical measurements using a capillary with a tip diameter in the range of 1m

allowed investigation of different spots of a single inclusion. Hence, the corrosion behaviour

of the weakest zone could be determined.

Date post:	08-Aug-2018
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SIandAII Ch6 Pitting Corrosion

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