Corrosion Metallurgy

Corrosion Metallurgy

Advanced Metallurgy

Presented byOmar Mohamed YousriAhmed Hassan Awad

Ain Shams University Design & Prod. Eng. DepFaculty of Engineering MDP 610

IntroductionCorrosion is a spontaneous electrochemical process and is defined as degradation of material due to its reaction with the environment.

To one degree or another, most materials experience some type of interaction with the environment, a deterioration in mechanical properties of the material occur, other physical properties or appearance also.

Forms of corrosion

• The different forms of corrosion• represent corrosion phenomena categorized according

to their appearance.• The three categories used are:– Group 1. Readily identifiable by ordinary visual examination

“Uniform – Pitting – Crevice – Galvanic” Corrosion – Group 2. May require supplementary means of examination.

“Erosion – Intergranual” Corrosion– Group 3. Verification is usually required by microscopy

(optical, electron microscopy, etc.). “Selective leaching – Stress corrosion cracking”

Uniform (or general) corrosion

• Uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surface area or a large fraction of the total area. General thinning takes place until failure.

• The breakdown of protective coating systems on structures often leads to this form of corrosion.

• Surface corrosion can indicate a breakdown in the protective coating system, however, and should be examined closely for more advanced attack.

• If surface corrosion is permitted to continue, the surface may become rough, and surface corrosion can lead to more serious types of corrosion.

Pitting corrosion• Pitting corrosion is a localized form of corrosion by which cavities,

or “holes,” are produced in the material.• Pitting is considered to be more dangerous than uniform corrosion

damage because it is more difficult to detect or predict it. • Corrosion products often cover the pits. A small, narrow pit with

minimal overall metal loss can lead to the failure of an entire engineering system.

• Pitting corrosion occurs when discrete areas of a material undergo rapid attack while most of the adjacent surface remains virtually unaffected.

• Apart from the localized loss of thickness, corrosion pits can also be harmful by acting as stress risers.

Crevice corrosion

• Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the microenvironmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation materials. Because oxygen diffusion into the crevice is restricted.

• A differential aeration cell tends to be set up between crevice (microenvironment) and the external surface (bulk environment).

• Creation of highly corrosive microenvironmental conditions in the crevice, conducive to further metal dissolution.

Galvanic Corrosion• Galvanic corrosion occurs when dissimilar metallic materials are brought into

contact in the presence of an electrolyte.• An electrochemical corrosion cell is set up due to differences in the corrosion

potentials of the dissimilar materials.• The material with the more noble corrosion potential then becomes the

cathode of the corrosion cell, whereas the less noble material is consumed by anodic dissolution.

• Effect of surface area ratio:

If aluminum rivets were used on steel plates, the rivets would corrodeextremely rapidly

• Corrosion rate increase with the increase in potential difference.

• Corrosion current density strongly increased with the decrease in anode surface area.

Selective leaching

• Selective leaching refers to the selective removal of one element from an alloy by corrosion processes.

• A common example is the dezincification of unstabilized brass, whereby a weakened, porous copper structure is produced.

• The selective removal of zinc can proceed in a uniform manner or on a localized (plug-type) scale.

• brass dissolves with Zn remaining in solution and Cu replating out of the solution.

• Graphitization of gray cast iron, whereby a brittle graphite skeleton remains following preferential iron dissolution, is a further example of selective leaching.

Uniform layer dezincification in brass heat-exchanger tube

0.110 mm(0.0042”)

Brass (Cu-30% zinc) exposed to room temperature salt solution for 80 days

Reprinted with permission of ASM International®. All rights reserved.

Outer tube wall

Dezincified metal

Erosion Corrosion• Erosion corrosion is the cumulative damage induced by

electrochemical corrosion reactions and mechanical effects from relative motion between the electrolyte and the corroding surface.

• Erosion corrosion is defined as accelerated degradation in the presence of this relative motion.

• The motion is usually one of high velocity, with mechanical wear and abrasion effects.

• Grooves, rounded edges, and waves on the surface usually indicating directionality characterize this form of damage.

• Erosion corrosion is found in systems such as piping, valves, pumps, nozzles, heat exchangers, turbine blades.

Stress corrosion cracking• SCC is the cracking induced from the combined influence of

tensile stress and a corrosive medium.• The required tensile stresses may be in the form of directly

applied stresses or residual stresses.• Cold deformation and forming, welding, heat treatment,

machining, and grinding can introduce residual stresses.• SCC usually occurs in certain specific alloy-environment-stress

combinations.• Usually, most of the surface remains unattacked, but with fine

cracks penetrating into the material. In the microstructure, these cracks can have an intergranular or a transgranular morphology.

• SCC is classified as a catastrophic form of corrosion because the detection of such fine cracks can be very difficult and the damage not easily predicted.

SCC in aircraft component

Intergranular corrosion

• Intergranular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected.

• This form of corrosion is usually associated with chemical segregation effects or specific phases precipitated on the grain boundaries.

• Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.

• A classic example is the sensitization of stainless steels. Chromium-rich grain boundary precipitates lead to a local depletion of chromium immediately adjacent to these precipitates, leaving these areas to corrosive attack.

Metallurgical Factors affecting corrosion

• Material properties• Effect of Alloy composition• Microstructure• Degree & type of deformation• Heat treatment• Passive layer formation

1. Material properties

• Electrode potential is the major metallurgical factor affecting corrosion rate.

• Electrode potential depends mainly on the electrode chemical composition (alloy composition), the presence of impurities (non metallic inclusions), electronic structure, crystal structure.

22

Electromotive Series(emf series)

in reduction form

Metal crystal Structure

• BCC having an open structure while FCC & HCP having close packed structure.

• FCC & HCP are more resistance to corrosion than BCC.• As the structure become close packed, the corrosion

affinity decrease.

2. Effect of alloy composition

Effect of Ni and Cr in carbon and alloy steel:

• As Cr % and Ni % in steel increase the exchange current density decrease.

• The exchange current density represent corrosion rate.

• So as Cr and Ni % increase corrosion rate decrease.

3. Microstructure

• The microstructure phases and micro constitutional structure such as grain boundary, second phase precipitations, phase morphology greatly affect corrosion rate.

• It may form micro galvanic corrosion cells as a result of the difference in the stability or free energy. Where less stable (high energy) area will be anode relative to more stable area (lower energy) which act as cathode.

Grains are more stable than Grain boundary due to the arrangement of atoms, so Grain boundaries are anode

and Grains are cathode. (anode corroded)

• Microstructure (micro galvanic cell between different phases for example between ferrite and cementite in carbon steel)

• Cementite is intermetallic compound which is more stable than Ferrite solid solution

• Cementite is cathode while ferrite is anode.

Dark area is cementite while bright area is ferrite

4. Degree & type of deformation

• Type of deformation:a. Cold deformation: cold deformed structure is

less resistant to corrosion due to the presence of large amounts residual internal stresses and large amounts of structural defects.

b. Hot deformation: hot deformed structure is relatively more stable and more corrosion resistant due to the presence of recrystallized structure which is free from internal residual stresses and structural defects.

Degree of deformation:• Structural defects and internal energy increased

with increasing degree of deformation. • Corrosion resistant decreased with increasing

the degree of deformation.A mechanically deformed metal or alloy can experience galvanic corrosion due to differences in atomic plane distortion and a high dislocation density.

5. Heat treatment

• Types of heat treatment:1. Annealing (Having high phase stability with very low thermal

and internal stresses; very slow cooling rate) “very low corrosion rate”

2. Normalizing (Relatively stable phase with low thermal and internal stresses; slow cooling rate in air) “low corrosion rate”

3. Hardening or quenching (very unstable phase with high thermal & internal stresses; high cooling rate) “ very High corrosion”

4. Aging or precipitation hardening (unstable phase with relatively high thermal & internal stresses; high cooling rate followed by tempering) “High corrosion rate”

6. Passive layer formation

During corrosion of metals, metal oxide layer formed at the surface of the metal.• If metal oxide is stable; further corrosion of

metal is prevented by the formed metal oxide (passive layer)

• If metal oxide is unstable; corrosion process will continue (porous metal oxide – volatile metal oxide)

Characteristics of self protective layer (passive layer)

• Adherence • Compacted • Thermal & electrical

conductivity• Good Mechanical

properties• Formability