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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    Surfaces, Interfaces, and their Applications II Pitting Corrosion

    Dr. Patrik Schmutz, Laboratory for Joining Technologies and Corrosion, EMPA Dbendorf, 2013 2

    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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    Surfaces, Interfaces, and their Applications II Pitting Corrosion

    Dr. Patrik Schmutz, Laboratory for Joining Technologies and Corrosion, EMPA Dbendorf, 2013 3

    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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    Surfaces, Interfaces, and their Applications II Pitting Corrosion

    Dr. Patrik Schmutz, Laboratory for Joining Technologies and Corrosion, EMPA Dbendorf, 2013 4

    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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    Surfaces, Interfaces, and their Applications II Pitting Corrosion

    Dr. Patrik Schmutz, Laboratory for Joining Technologies and Corrosion, EMPA Dbendorf, 2013 5

    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