MS43220 Summary PDF

Preview

Summary

MS43220 – Corrosion Science – SummaryBasics and terminologyElectrode A conductor used to establish contact with a nonmetallic part of a circuit (e. an electrolyte)Electrolyte A nonmetallic part of a circuit, which provides medium for the passage of charge (ionic) carriersAnode The electrode where Ox...

Description

MS43220 – Corrosion Science – Summary Basics and terminology Electrode

A conductor used to establish contact with a nonmetallic part of a circuit (e.g. an electrolyte)

Electrolyte

A nonmetallic part of a circuit, which provides medium for the passage of charge (ionic) carriers

Anode

The electrode where Oxidation takes place (OA)

Cathode

The electrode where Reduction takes place (RC)

Metallic path

Provides passage of electrons between anode and cathode

Half-cell

A metal electrode in contact with a solution of its own ions

Remember:

Oxidation Is Loss (OIL) and Reduction Is Gain (RIG).

During the electrochemical corrosion of a metal the following reactions (can) occur: Anodic reaction 𝑀 → 𝑀 𝑧+ + 𝑧 𝑒 − Cathodic reactions 2𝐻2 𝑂 + 𝑂2 + 4 𝑒 − → 4𝑂𝐻 −

(dissolved oxygen)

2𝐻+ + 2 𝑒 − → 𝐻2

(acid solution, limited oxygen)

4𝐻+ + 𝑂2 + 4 𝑒 − → 2𝐻2 𝑂

(acid solution, oxygen)

𝑀 𝑧+ + 𝑛 𝑒 − → 𝑀(𝑧−𝑛)+

(metallic reaction)

Nernst potential The Nernst potential gives the dynamic equilibrium of metals in solution: 𝐸 = 𝐸0 +

[𝑜𝑥]𝑥 0.059 log [𝑟𝑒𝑑]𝑦 𝑛

Where 𝐸0 is the standard half-cell potential, n is the number of electrons transferred in the reaction and [𝑜𝑥]𝑥 and [𝑟𝑒𝑑]𝑦 are the concentration of the oxidizing and reducing metals respectively.

Pourbaix diagrams Pourbaix diagrams shows the susceptibility of metals to corrosion in an environment, this environment is specified using the pH value and the applied potential. Immunity is the lack of any noticeable attack on a metal when exposed to an operating environment. Passivity is caused by a buildup of a metal oxide layer on the surface of the metal. Prevents corrosion until the reactants diffuse through the oxide film, this takes a long time or may even never occur. The Pourbaix diagram does NOT provide information on the rate at which oxidation occurs. Butler-Volmer equation 𝐼 = 𝑛𝐹𝐴(𝑘𝑎 [𝑅]∗ − 𝑘𝑐 [𝑂]∗) Where 𝑛 is again the number of electrons transferred in the reaction, 𝐹 is Faraday’s constant (96485.3 𝐶 𝑚𝑜𝑙 −1 ), 𝐴 the surface area of the electrode and (𝑘𝑎 [𝑅 ]∗ − 𝑘𝑐 [𝑂]∗) the difference between the oxidation and reduction rates at the electrode surface. Anode size vs. Cathode size Consider the following example, where two plates are riveted together using a rivet of a different metal. Which assembly is preferred (a) or (b)? Because stainless steel is more noble than carbon steel situation (b) provides a larger anode surface which is preferred as this means that the corrosion is spread out over a larger area and thus the overall rate is slower. Situation (a) would lead to the rapid corrosion of the rivet and failure of the structure.

Standard Potentials (given during the exam)

Difficult to oxidize

Easy to oxidize

Corrosion rate The amount of metal thickness (𝑟) lost per unit time is given by: 𝑟=

𝑎𝑖 𝑛𝐹𝐷

Here 𝑎 is the atomic mass, 𝑖 is the current density, 𝑛 the number of electrons transferred, 𝐹 is Faraday’s constant (96485.3 𝐶 𝑚𝑜𝑙 −1 ) and 𝐷 the density of the metal. The value of 𝑖 is deduced from the polarization curve. Polarization curves (or Evans diagram) Polarization curves show the behavior of current density and potential. Also some other information can be obtained from these plots, such as active, passive and transpassive regions and anodic and cathodic branches.

ilimit It is important to realize that sometimes the axis (i.e. the potential and current density) are swapped, or that the direction of the positive and negative signs are flipped. In this case the cathodic branch always points towards the more negative potential and the anodic part towards the more positive potential. A rule of thumb which is sometimes employed for the determination of icorr is a potential difference of 50 mV between the anodic and cathodic branches. The points on the curve which correspond to this difference are to intersect with the tangential lines that are drawn to find icorr. Limiting species concentration If the transport of one of the species that is partaking in the reaction is limited by its diffusion rate towards the electrode surface, a limiting situation occurs which is displayed in a polarization curve according to the blue (–) line in the figure above.

Passive layers A passive layer is typically an oxide layer which forms on a metal surface, it prevents the metal from oxidizing further by preventing the reactants (ions) from reaching the metal. The passive region of a metal is shown by a vertical line in the polarization curve, where the current density is constant over a range of different potentials. Typically the dissolution of a passive layer is very slow or even nonexistent, some layers also have self-healing properties where the passive layer is restored when it is damaged. It is generally assumed that the formation of oxide films is not due to deposition but instead it is formed in close connection with the crystal structure of the metal. Effects of corrosive environments on the polarization curve The corrosive environment, for instance the temperature or oxidizer concentrations influence the course of the polarization curve. The temperature generally increases the current density (left figure). In the right figure the lines 1 to 8 represent different oxidizer concentrations, the intersection with the polarization curve gives the corrosion rate (in the form of the current density) and the potential.

At some oxidizer concentrations (e.g. line 3) there is an intersection in both the passive and the active region of the polarization curve. Here it is important to realize that the dashed line is an artifact of the measurement technique and that point X is not a stable potential state. The microstructure (e.g. grain size) also has its effects on the polarization curve, a finer grain structure typically shifts polarization curve to smaller current densities. Five important points are mentioned in the lecture slides: (i) In the active state, corrosion rate is proportional to the anodic current density whether the alloy is of active-passive type or not. (ii) The current density rate of reduction must exceed the critical current density for passivation to ensure low corrosion rate in the passive state. (iii)Borderline passivity should be avoided in which either the active or passive state may be stable.

(iv) Breakdown of the passive film in oxidizing conditions due to transpassivity or initiation of localized corrosion should be avoided (v) The passive state in oxidizing conditions is essential for corrosion resistance, but reasonably small variations in the passive current density may not be significant. Break down of passivity Three mechanisms of passivity breakdown exist, depending on the situation more than one mechanism can be at play. The three mechanism are: penetration, film breaking and adsorption. The penetration mechanism involves the transfer of anions through the oxide film to the metal surface. The film breaking mechanism requires breaks in the oxide film that give the anions direct access to the metal surface. This is considered a fast mechanism. The adsorption mechanism starts with the adsorption of aggressive anions at the oxide surface. This adsorptions thins the oxide layer until it is completely removed. At which point intense localized dissolution can occur. The removal of the oxide film also introduces a possibility for pitting to occur. Pitting Pitting is a phenomenon where localized corrosion occurs in an unpredictable way, several pit shapes can be identified:

Pitting typically is associated with heterogeneities/discontinuities in the metal/environment system and the presence of certain aggressive ions. The figure on the right shows an overview of some pitting mechanisms. Inside the pit, iron ions get released and the electron moves through the metal to the surface of the electrode. Meanwhile an increased concentration of 𝐹𝑒 2+ ions accumulates inside the pit attracting more 𝐶𝑙− ions through the porous cap. The acidity of the solution in the pit increases and this further accelerates the corrosion even more.

During pitting a local anodic-cathodic couple in the alloy is present, typically in the form of a precipitate and a matrix metal. Depending on the potentials between the two materials two situations can occur: Anodic particle

Cathodic particle

Anodic particle

The matrix around the particle is cathodic and the particle anodic, hence the particle dissolves completely out of the matrix material.

Cathodic particle

In this case the matrix is anodic and thus dissolves, this slowly frees the particle from the matrix, the particle however does NOT dissolve but breaks loose from the matrix.

Prevention of pitting ▪

▪ ▪ ▪ ▪

Decrease aggressiveness of the environment o Decrease Chloride o Decrease Acidity o Decrease temperature Elimination of dead legs (pipes with a dead end) and other stagnant (non-moving fluid) areas Use of corrosion inhibitors Cathodic protection Increase alloy resistance by addition of Cr, Ni, Mo, N

Crevice corrosion Crevice corrosion is in essence very similar to pitting corrosion, inside the crevice an abundance of metal ions forms which attract chloride ions. Recombination takes place which increases the acidity inside the crevice accelerating the corrosion process even more. This process is autocatalytic as it accelerates itself as time progresses. Crevice corrosion is very geometry related, some causes of crevice corrosion are: badly aligned joints; stones or grit resting on a metal surface, poorly fitting gaskets and flange joints. This also means that by proper design crevice corrosion can for a great part be prevented.

Intergranular corrosion If a cathodic intermetallic particle is present in the grain boundaries of a metal this can lead to intergranular corrosion. The matrix around the particle, being anodic, dissolves slowly releasing the particle from the grain boundary, this process continues if another particle is available further along the grain boundary. Depletion of alloying components may occur near the grain boundary due to the formation of intergranular carbides (sensitization). This depletion makes the matrix more susceptible to corrosion and this results in faster intergranular corrosion. Stress Corrosion Cracking (SCC) Stress corrosion cracking is the initiation and slow growth of cracks under the simultaneous influence of tensile stresses and aggressive environment. Some characteristics of SCC include: macroscopic material breakdown without deformation perpendicular to the stress direction; no measurable material removal; no visible corrosion products and difficult identification of corrosion initiation. For SCC to occur three requirements have to be met: Tensile stress, Environmental composition and metal composition and microstructure (MS), this is illustrated to the right. The origins of these requirements are as follows:

Tensile stress

Residual stresses from cold working, welding or surface treatments Applied stresses from service, such as hydrostatic pressure or bending loads.

The greater the stress on the material the quicker it will crack, this can already occur at very low stresses up to as low as 5% of the yield stress. This aspect is also dependent on the environmental conditions in which the material is loaded. Environmental composition

Combinations of alloys and environments in which the alloy is susceptible to SCC are documented extensively in literature.

Metal composition and MS

Pure metals are more resistant to SCC, but for alloys the composition, microstructural features and heat-treatment are important. Grain boundary precipitates may induce intergranular SCC, sensitization can also have an effect here.

The morphology of the attack of SCC on the alloy always follows a path perpendicular (normal) to the stress direction. The path may both be intergranular or transgranular depending on the combination between environment and alloy.

Mechanisms of SCC For stress two events are of importance, the initiation of a crack and the propagation of this crack into the metal. Crack initiation Crack initiation is in one part mechanical and thus surface defects, grooves, pores welds and other discontinuities are possible crack initiation sites. For SCC however also the environment inside these defects needs to develop such that a stress corrosion crack can initiate. Crack propagation Crack propagation can be sub-divided into two different mechanisms, dissolution based mechanisms and cleavage based mechanisms. Dissolution based mechanism function based on the dissolution of atoms near the crack tip, in a direction normal to the applied stress. This is promoted by a chemically (pre-existing) active path, for instance segregates along grain boundaries (left figure) or by an active path generated by stress/strain concentration at the crack tip (right figure). For strain generated paths (right) distinction can be made between disruptive and slip-dissolution (red circle) type mechanisms. In both cases the continuity of the oxide film is disrupted. Allowing for further dissolution inside the crack.

The dissolution based mechanism is limited by the rate of corrosion at the crack tip (the dissolution rate) and the re-passivation rate (the rate at which the oxide layer restores). If the restoration rate is sufficiently high, the stress corrosion cracking might halt. Cleavage based mechanisms are considered when the corrosion causes the crack tip to embrittle, no material is actually removed by chemical process with this mechanism. This embrittlement effect allows further growth of the crack under mechanical loading. The discussed mechanism which causes this embrittlement is adsorption induced. Species adsorb to the crack flanks near the tip and weaken the metallic bonds causing the material to embrittle. This allows the crack to propagate further under the applied mechanical loading. For this the following factors should be considered: Stress state, corrosion products, time of failure and the environment.

Challenges in SCC ▪

▪ ▪ ▪

Finding Stress Corrosion Cracks o Cracks my initially be shallow o May not be visible to unaided eye Evaluation of the crack when it is found Mitigation (stopping the crack propagation) and Management Service Disruptions

Preventing SSC ▪ ▪ ▪

Reducing stresses by either annealing (residual stresses) or changing the design (applied stresses). Changing the environment-alloy combination, depending on which of the two can be changed. Applying cathodic protection or inhibitors if feasible also other surface treatments such as shot peening can be useful in some cases.

Hydrogen Embrittlement (HE) Internal Hydrogen Embrittlement (IHE)

Concentrations of hydrogen pre-exist in a material in regions of high hydrostatic stress, resulting in cracking under sustained stress well below the yield stress.

Hydrogen Environment Embrittlement (HEE) This particular type of HE occurs when a structure is operated in a hydrogen rich environment which enables the diffusion of hydrogen into the lattice. Hydrogen Assisted Cold Cracking (HACC)

Typically occurs during welding, when hydrogen gets trapped in the vicinity of the weld and upon cooling the pressure of the hydrogen causes the material to crack.

Sources of hydrogen (general cases) ▪ ▪ ▪ ▪ ▪ ▪

In the production of material In the fabrication process During welding In storage of hydrogen gas As a result of a corrosion process (e.g. cathodic reactions) From addition of hydrogen to reactor coolant in order to remove oxygen

Case specific sources IHE

Chemical solutions used for: applying coatings, electroplating, pickling, paintstripping and cathodic cleaning. Alternatively also from the reduction of water or acid at the surface.

HEE

High pressure gaseous (and liquid) hydrogen used as fuel.

Trapping sites Trapping of hydrogen in a lattice can occur at: a) b) c) d) e) f)

Lattice matrix Surface sites Sub-surface sites Grain boundaries Edge dislocations Vacancies

Examples of susceptibility of metals ▪ ▪ ▪

High strength martensitic steels are extremely susceptible to IHE. Ferritic steels need high hydrogen concentrations. Nickel, aluminium metastable austenitic steels and copper alloys exhibit little (if any) susceptibility to IHE.

Diffusion of Hydrogen Hydrogen diffuses through the lattice in response to gradients in: (i) Hydrogen concentration (ii) Temperature (iii)Hydrostatic stress fields ▪ ▪ ▪ ▪

Hydrogen tends to region of high triaxial tensile stress Hydrogen assists in the fracture of metal by developing intense local plastic deformation. Hydrogen is more likely to diffuse in BCC than in FCC because of the close packing of FCC Increasing temperature can remove/bake the hydrogen out of the metal (it diffuses out).

If a crack tip is present hydrogen will tend towards that location (high stress state) in the case of HEE the hydrogen gas disassociates and subsequently adsorbs near the crack tip. In the case of IHE, solute hydrogen will diffuse to and adsorb at the internal crack tip surfaces. This hydrogen will weaken interatomic bonds and facilitate decohesion leading to crack propagation. Factors that influence HE ▪ ▪ ▪ ▪ ▪ ▪ ▪

Source of hydrogen (external/internal) Exposure time Temperature and pressure Material properties (Alloy composition, microstructure, etc.) Surface treatment Heat treatment Stress levels (applied and residual)

Atmospheric corrosion Three categories exist for atmospheric corrosion: ▪ ▪ ▪

Rural Industrial Marine

(dry climatic conditions) (Sulphur) (Chloride)

Additionally other more specific conditions can also be considered: ▪ ▪ ▪

Urban Arctic Tropical (wet or dry)

Atmospheric conditions should be defined in terms of temperature, humidity, and contaminants, as well as their corrosivity to specific materials of construction being considered. Atmospheric corrosion is an electrochemical process meaning electrolyte is required. This is supplied by the surrounding in the form of rain, fog, dew, melting snow or high humidity. Since this is not always the case atmospheric is a discontinuous process. An important parameter here is the time of wetness, which is the sum of all the durations of individual wet periods. Some other factors influencing the atmospheric corrosion are: ▪ ▪ ▪ ▪

Temperature Sheltering Wind velocity Nature of corrosion products (additional contaminants)

Testing Corrosion Immersion testing

During immersion testing a specimen is placed in a solution and left over time to corrode, specimens can remain immersed until signs of rust occurs or for a fixed time to monitor the corrosion as a function of time. Gradation (A-D) are used to indicate how much the specimen has corroded, A meaning the specimen is stain and rust free, B up to 1% of surface area covered, C meaning 1-25% is covered and D more than 25%. Immersion testing gives no mechanism information.

Salt spray

Salt spray test are typically employed in industry, their use is well standardized. The material is subjected to a cycle where it is first sprayed by a 5% NaCl solution, then left to dry, followed by a condensation humidity phase and a controlled humidity/humidity phase.

Electrochemical testing

Two options, potentiodynamic measurements and Electrochemical Impedance Spectroscopy (EIS). Potentiodynamic measures potential and current whereas EIS uses small period signals to perturb an el...