03 Corrosion of Metals and Alloys Flashcards
Uniform Corrosion
Most common form of corrosion. Characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface.
Uniform Corrosion
Represents the greatest destruction of metal on a tonnage basis.
Crevice Corrosion
Metal deterioration that happens in tight spaces where a stagnant liquid gets trapped.
Galvanic Corrosion
Exists between two dissimilar metals with potential difference in a conductive solution.
Galvanic Corrosion
One metal (the cathode) is protected, whilst the other (the anode) is corroded. The rate of attack on the anode is accelerated, compared to the rate when the metal is uncoupled.
less
Galvanic corrosion is a serious threat because it accelerates the deterioration of the ____________ resistant metal.
Pitting Corrosion
A highly localized form of attack that eats away at the metal, creating small holes or cavities.
Pitting Corrosion
One of the most destructive and insidious forms of corrosion; as it can penetrate deep into the metal, leading to sudden and unexpected failures even when overall material loss seems minimal.
Titanium
Which material has the highest pitting resistance?
Intergranular Corrosion
Metals are made up of tiny crystals called grains. The boundaries where these grains meet are susceptible to __________________.
Intergranular Corrosion
Due to chemical composition difference at the grain boundaries from the bulk of the grain, this happens.
Selective Leaching
This type of corrosion targets alloys, which are mixtures of different metals.
Selective Leaching
During _______________, one specific component of the alloy is preferentially dissolved and removed from the surface by the corrosive environment.
Erosion-Corrosion
Occurs when a metal surface is repeatedly exposed to a fluid with abrasive particles. The repeated impact of the particles can cause loss of material that leads to pits, grooves, or cavities.
Stress Cracking Corrosion
Occurs when a metal is exposed to a corrosive environment under tensile stress. This combination can cause cracking or other forms of material failure.
Hydrogen Damage
Degradation that occurs when hydrogen penetrates the surface of a metal. Can lead to various forms of damage like embrittlement, blistering, and cracking.
Wet Corrosion
This occurs in the presence of a liquid containing ions (electrolyte).
High-Temperature Corrosion
This occurs at elevated temperatures above 400Β°C (750ΒΊF) from gases, solid or molten salts, or molten metals.
Wet Corrosion
Chemical process: ________________
High-Temperature Corrosion
Follows thermodynamic laws and electrochemistry kinetics: _____________________
Wet Corrosion
The kinetics are influence by various factors such as temperature, pH, and presence of impurities.
High-Temperature Corrosion
This type is not primarily electrochemical in nature, but involves chemical attacks from gases, solid or molten salts, or molten metals at elevated temperatures.
Tafel Equation
The _________________ is commonly used to describe corrosion kinetics, relating the current density to factors like activation energy and overpotential.
diffusion rates and electrolyte resistance
In High-Temperature Corrosion, the kinetics are influenced by factors such as _____________ and _____________.
Cast Iron
It is a generic term that applies to high carbon-iron alloys containing silicon.
Gray Cast Iron
Contain 2% to 4% carbon and 1% to 3% silicon. Considered as the least expensive of the engineering metals.
White Cast Iron
These have practically all of the carbon in the form of iron carbide. Extremely hard and brittle. Silicon content is low because this element promotes graphitization.
Malleable Cast Iron
These are produced by high-temperature heat treatment of white iron of proper composition. The graphite forms as rosettes or clusters instead of flakes and the material shows good ductility.
Ductile Cast Iron
The graphite is present as nodules or spheroids as a result of a special treatment of the molten metal. The mechanical properties of ductile irons can be altered by heat treatment similar to ordinary steel.
- graphite
For cast iron:
High Silicon Cast Iron
When the silicon content of gray cast iron is increase to 14%, it becomes extremely corrosion resistant to many environment. The excellent corrosion resistance of high silicon irons is due to the formation of a passive SiO2 surface later.
Duriron; Durichlor
_____________ contains about 14.5% silicon and 0.95% carbon. It becomes _____________ after the addition of 3% molybdenum.
Steels
Composed of up to 2.14% carbon.
Stainless Steels
Contained at least 11% chromium.
Martensitic Stainless Steels
Can be hardened by heat treatment similar to ordinary steel. Heat treatment increases tensile strength.
Austenitic Stainless Steels
Essentially nonmagnetic and cannot be hardened by heat treatment. Can only be hardened by cold working.
Ferritic Stainless Steels
Cannot be hardened by heat treatment. Can only be hardened by cold working, just like Austenitic stainless steel.
Precipitation-hardened Stainless Steels
Hardened and strengthened by solution-quenching, followed by heating for substantial times at temperature approximately in the range of 800 to 1000 F.
Copper
This is different from most other metals in that it combines corrosion resistance with high electrical and head conductivity, formability, machinability, and strength when alloyed.
Copper - zinc alloys
______________ alloys have better physical properties than copper alone, and they are also more resistant to impingement attack; hence, brasses are used in preference to copper for condenser tubes.
Dezincfication or Dealloying
Factors due to high temperatures, stagnant solutions, and porous inorganic scale formation.
Stress-Corrosion Cracking (Season Cracking)
This happens due to presence of ammonia.
Aluminum
Lightweight metal having good corrosion resistance to the atmosphere and most aqueous media. Excellent electrical and thermal conductivity
8; 4
Aluminum corrosion can occur at Extreme pH greater than pH of ______, or less than pH of ______).
High
____________ levels of pH can break down the aluminium oxide coating. In this case, the coat is broken down faster than it can repair itself.
Magnesium
Has the lowest density of all structural metals (1.7g/cm3 vs steel with 7.85g/cm3). Good electrical and thermal conductivity
magnesium oxide
Pure magnesium readily forms a layer of ____________.
strength-to-weight ratio
Magnesium alloys are sought after for its high ________________.
atmosphere, distilled water, and alkalis
Magnesium and magnesium alloys are resistant to ______________, ______________, and ______________.
nickel
Pure ____________ is ductile and tough mainly because of its face-centered cubic structure. This can be alloyed while maintaining its structure, inheriting its property to the alloy.
Cupronickels
Consists of 10-30% copper and 70-90% nickel. In seawater, this forms a stable layer of Cu2O that provides excellent resistance to biofouling. Has good thermal and electrical conductivity
Nickel-Chromium Alloys
Also known as Nichrome.
Metal Composition
Impurities within a metal can act as tiny battery cells, accelerating corrosion. Conversely, purer metals generally exhibit better corrosion resistance.
crystal lattice
The arrangement of atoms in the metalβs _______________ can influence its susceptibility to attack by corrosive agents.
pH level
Highly acidic or alkaline environments (low or high pH, respectively) can significantly increase corrosion rates for most metals.
higher
Generally, _____________ temperatures accelerate corrosion reactions, though specific mechanisms may vary.
Oxygen
________________ often plays a crucial role in many corrosion processes, acting as an oxidizing agent. However, in some cases, its absence can also lead to specific types of corrosion.
moisture
The presence of _______________ is essential for most corrosion processes to occur, as it provides the necessary conductive medium for ionic movement.
Mechanical stresses
_________________ on the metal can make it more susceptible to corrosion, especially localized forms like stress corrosion cracking.
Microbial activity
In some environments, microorganisms like bacteria can accelerate corrosion through various mechanisms, a phenomenon known as microbiologically influenced corrosion (MIC).
Oxygen
______________ is a primary cause of uniform corrosion in steels, alloys, and other metals in natural environments.
thickness loss per unit time
The rate of uniform corrosion is typically given in terms of _____________ per _____________.
Intergranular Corrosion
This is a specialized type of attack that takes place at the grain boundaries of a metal.
Intergranular Corrosion
This form of corrosion is often caused by impurities in the metal, leading to chemical or electrochemical attack on the grain boundaries.
Galvanic Corrosion
Is also known as dissimilar metal corrosion or bimetallic corrosion
Crevice Corrosion
Is a localized type of corrosion occurring within or adjacent to narrow gaps.
Pitting Corrosion
It is characterized by a highly localized loss of metal. The initiation of a pit is associated with the breakdown of the protective film on the metal surface.
Erosion Corrosion
It is usually caused by an aqueous or gaseous corrodent flowing over the metal surface or impinging upon it.
Erosion Corrosion
It is affected by velocity, turbulence, impingement, the presence of suspended solids, temperature, and prevailing cavitation conditions.
Stress Corrosion Cracking (SCC)
It is a phenomenon where cracks form and propagate in a material due to the combined effects of sustained tensile stress and exposure to a corrosive environment.
Biological Corrosion / Biologically Influenced Corrosion
It refers to the degradation of metals caused by the activity of living organisms. This is also referred to as microbial-induced corrosion (MIC).
Dezincification
It is the term to describe the leaching of zinc from brass.
plug-type; layer-type
Low zinc alloys favor ___________ while ___________ attack is more prevalent in high-zinc alloys
Plug-Type Attack
Is found in neutral or alkaline water, high salt content, and above room temperatures.
Graphite Corrosion
It is the term used to describe the selective leaching of iron from gray cast iron.
hydrogen
The _____________ may be initially present in the metal or it may accumulate through absorption. In most cases, the damage is associated with residual or applied stresses.
Hydrogen Damage
The damage may be in the form of:
liquid metal embrittlement (LME)
The most drastic form of liquid metal attack is the instantaneous fracture of the solid metal in the presence of stress, a condition described as β__________________.β
Exfoliation
When intergranular corrosion takes place in a metal with a highly directional grain structure, it propagates internally, parallel to the surface of the metal.
Exfoliation
This requires elongated (parallel-shaped) grains, a susceptible grain boundary condition, and a relatively severe environment.
Corrosion Fatigue
It is the cracking of a metal or alloy under the combined action of a corrosive environment and repeated or fluctuating stress.
Filiform Corrosion
Metals with semipermeable coatings or films may undergo a type of corrosion resulting in numerous meandering threadlike filaments of corrosion beneath the coatings or films.
Filiform Corrosion
Under this corrosion, the growth formation is explained by the formation of a differential aeration cell.
Dissimilar electrode cells
__________________ may be formed when two dissimilar metals are in contact or due to the heterogeneity of the samemetal surface.
Concentration cells
__________________ are formed when the electrodes are identical but are in contact with solutions of differing composition
Differential Temperature Cell
Formed when electrodes of the same metal, each of which is at a different temperature, are immersed in an electrolyte of the same initial composition.
Metal Ion Concentration Cells
This situation arises when a significant difference in metal ion concentration exists over a metal surface.
Oxygen Concentration Cells
This develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points.
Pourbaix Diagram
Potential-pH Diagrams are also known as:
Potential-pH Diagrams / Pourbaix Diagram
This is constructed from calculations based on the Nernst equation and the solubility data for various metal compounds.
Sloping lines
In the Pourbaix Diagram shown:
Passive zone
In the Pourbaix Diagram shown:
Polarization Curves
The intersection of the two polarization lines (curves) closely approximates the corrosion current and the combined cell potentials from the freely corroding situation.
Cathodic Polarization
This reduces the corrosion rate due to increased current resistance. It is also the most common type of polarization.
Activation Polarization (ππ΄)
It is a manifestation of the relative changes in the activation energies for dissolution and deposition, when equilibrium is disturbed. It is always a component of the total polarization, whether or not there are also significant contributions from concentration and resistance effects. The polarization is positive, i.e., anodic, or negative, i.e., cathodic, according to whether the net current is a dissolution or deposition current.
Concentration Polarization (ππΆ)
Polarization of an electrolytic cell resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface.
Resistance Polarization (ππ )
The change in voltage associated with effects of the environment and the circuit between the anode and cathode sites. It includes the effects of the resistivity of the media, surface films, corrosion products, etc.
Ohmic Polarization or IR Drop
Resistance Polarization (ππ ) is also known as ______________ or ______________.
Atmospheric Corrosion
It is a complicated electrochemical process taking place in corrosion cells consisting of base metal, metallic corrosion products, surface electrolytes, and the atmosphere.
Rusting
______________ is negligible in climates where the air temperature is below the freezing point of water because ice is a poor conductor and does not function effectively as an electrolyte.
Time of wetness
This refers to the length of time during which the metal surface is covered by a film of water.
Adsorption Layers
The amount of water adsorbed on a metal surface depends on the relative humidity of the atmosphere and on the chemical and physical properties of the corrosion products.
critical relative humidity
For such adsorption to take place, the relative humidity must be above a certain relative value known as the __________________.
dew
Occurs when the temperature of the metal surface is below the _______ point of the atmosphere.
fog
High acidity and high concentrations of sulfates and nitrate can be found in ___________ droplets in areas of high air pollution
Rain
This creates even thicker layers of electrolyte on the surface than dew. It affects corrosion by adding a phase layer of moisture and corrosion stimulators.
Dust
This can promote corrosion because of their hygroscopic nature.
- Stationary Period
- The metal is covered with a spontaneously formed oxide and aqueous layer.
dry deposition velocity
Wind speed and type of wind flow have pronounced effects on the atmospheric corrosion rate. This is illustrated by the _______________________, which is defined as the ratio of deposition rate of any gaseous compound and the concentration of that compound in the atmosphere.
dissolution β coordination β reprecipitation
Atmospheric corrosion rate is influenced by the formation and protective ability of the corrosion products formed. The eventual thickening of the film of corrosion products can be described in a sequence of consecutive steps:
Initiation
_________________ of corrosion is the result of the presence of solid particles on the surface. Greater corrosive effects result when particles of hygroscopic salts settle on the surface and form a corrosive electrolyte.
3 to 5 yrs
When small amounts of copper, chromium, nickel, etc. are added to carbon steel, low-alloy carbon steel results. These steels are known as weathering steels. The corrosion rate becomes stabilized within _____ to _____ yrs. A dark brown to violet patina develops over this period.
