MMET Exam 3

Electrochemical reaction

Chemical reactions between electrons and ions.
-Producing and consuming electrons

Oxidation

-Produces an electron
-Loses an electron
-Happens at anodes

Reduction

-Consumes an electron
-Gains an electron
-Happens at cathodes

Oxidation

Fe -->Fe^+2 + 2e-

Reduction

2^H+ + 2e- --> H2

Cathode/anode/electrolyte

For an electrochemical reaction, _______ and an _____ and an ___________ is needed.

EMF Series

Direction of reaction and potential determined by thermodynamics
-More anodic (bottom) will corrode more than those that are cathodic (top)

Galvanic Series

Ranks metals and alloys based on their nobility, or how cathodic or anodic they are.
-Electrolyte used is seawater
-Top is more anodic while bottom is more cathodic

Factors affecting corrosion

1. Materials properties
2. Environment

Material properties

-Redox potential
-Passivity
-Metallurgical factors

Redox potential

Is corrosion thermodynamically favorable?

Passivity

Formation of a protective film

Metallurgical factors

When an anodic area is formed
-Chemical segregation
-Presence of multiple phases
-Inclusions
-Cold work
-Nonuniform stresses

Environment

-Chemical nature
-Operating conditions
-Polarization

Operating conditions

-Intended service life
-Temperature
-Velocity of corrodent
-Concentration of solution
-Impurities in solution
-Aeration

Polarization

The change in corrosion potential with the change in corrosion current
-An issue in batteries

Types of corrosion

-Uniform
-Pitting
-Crevice
-Galvanic
-Stress corrosion cracking
-Intergranular attack
-Dealloying
-Erosion

Uniform

All surface area is exposed to the corrodent (rust)
-Prevented by material choice and removal of electrolyte

Pitting

Local corrosion damage
-Prevented by se available data on corrosion

Crevice

Local attack in a crevice
-Prevented by good gasketing

Galvanic

When 2 dissimilar metals are connected and one is corroded and consumed to save the other one
-Prevent by using insulating bush in between, don't let them touch

Stress corrosion cracking

Spontaneous corrosion induced cracking of a material under static stress (environmentally assisted cracking)

Intergranular attack

Preferential corrosion at the grain boundaries.

Dealloying

One element in the alloy is preferentially removed

Erosion

• Liquid impact
• Liquid erosion
• Slurry erosion
• Cavitation erosion

Liquid impact

Protective film removal
Can be combined with mechanical material removal

Liquid Erosion

Like liquid impact, but here fluid is parallel to the surface
-Minimized by lowering fluid speed

Slurry Erosion

Like liquid impact, but abrasive particles enhance it
-Ceramic or elastomer coatings can be helpful.

Cavitation corrosion

Formed when the operational pressure is dropped below the vapor pressure of the fluid
-Causes the formation of gas bubbles that collapse at an increased velocity on the surface of the material
-Induces initial cavitation.

Corrosion data

-Corrodent chemistry
-Corrodent concentration
-Temperature
-Aeration
-State of stress

Corrosion characteristics

1.Corrosion Data
2.Standardized lab experiments

Uniform and Pitting ASTM

Use immersion corrosion

Dissimilar metals ASTM

Galvanic attack

Crevice ASTM

Rubber band around sample put in liquid

Stress corrosion cracking ASTM

Introduce stress, observe the arc portion

Liquid Corrosion

See effect of velocity on corrosion rate of metals (determined by weight loss)

Corrosion control

1.Material Selection
2.Environmental control
3.Design

Material Selection

-Coatings
-Cladding: Cold working one material onto another material
-Heat treatment
-Diffusion Treatment: Introduce chromium to steel
-Corrosion data
-Surface finish:

True

The rougher the surface the faster the corrosion rate

Environmental Control

• Temperature
• Velocity
• pH: dissolved gasses
• Cleaning: can prevent buildups that lead to crevice or concentration corrosion
• Inhibitors: alter the environment

Heat Treatment

Stainless steel after welding becomes sensitized, ____ __________ and subsequent quenching addresses it.

Inhibitors

-Remove oxygen (makes metal less corrodible)
-Absorpitive inhibitors: slows down anodic and cathodic reactions through a passive film

Design

Corrosion happens (reduce/delay it w/ design)
-Design cleanability to avoid residue corrodent
-Provide proper drainage

Cathodic Protection

Use electrochemical reaction in our favor
-Use sacrificial anodes like Mg or Zn for steel applications (protects multiple metals)

Anodic Protection

Senses load and applied reverse to balance it out, thus no corrosion
-More complex than cathodic protection

Chromium

If you want to protect steel, add ________

Stainless steels

-Resist corrosion even at high temperatures
-Have at least 11% Cr
-Forms a passive layer in oxidizing env.

False

SS is well suited for reducing environments (e.g. sulfuric acid).

Manufacture stainless steel

-Adding ferrochromium to low carbon steel scrap
-Use electric furnaces
-Ladle treatments
-Argon-oxygen decarburization (AOD)

Oxygen/carbon

Manufacturing SS is challenging because Cr reacts with ______ and ______, so special processes are used.

Ni/C/N

Different alloying elements change the range
of stability of phases.
__, _ and _ extend the austenite region.

Microstructures of SS

1. Ferrite (BCC)
2. Martensitic (BCT)
3. Austenitic (FCC)
4. Precipitated hardness
5. Duplex

Ferritic Stainless steel

BCC Structure
Low carbon (

Martensitic Stainless steel

BCT Structure
Chromium carbide is present in the structure
12-18% Cr (Alloys Fe/C/Cr)
Up to 1.2% C
Quench hardening (hardenable)
Poor weldability and notch sensitivity
Magnetic
Risk of stress corrosion
Low impact strength

Austenitic Stainless steel

FCC Structure
-Fe, Cr (16-26%), C(lowest), Ni (8-24%)
Work hardenable
Non-magnetic
Risk of stress corrosion
Highest impact strength
High weldability
Ni is austenizer

True

Austenitic stainless steel is a metastable phase

False

Quench is not needed to maintain the FCC structure of austenite

SS Identification System

AISI: 3- digit system
1st letter shows composition
-200 = Cr, Ni, Mg
-300 = Cr, Ni
-400 = Cr

Carbon/weldable

Lowering ______ makes them more ________

True

Molybdenum prevents pitting

What are ferritic SS used for

Non-structural and high temperature applications

What are martensitic SS used for

Structural applications and cutting tools

What are austenitic SS used for

Chemical and creep resistance, tanks, piping

Physical properties of SS

• Density: similar to other iron-based alloys
• Structure: affects mechanical properties and magnetism
• Conductivity: low electrical (one sixth of carbon steels) and thermal conductivity (less than half of carbon steels)
• Expansion: austenitic alloys can have 50% larger thermal expansion. Can be problematic in bimetal strips. Other structures are similar to carbon steels.
• Modulus of elasticity: slightly lower than carbon and alloy steels --> for the same section size SS has more elastic deformation

Forming

-Austenitic SS has high ductility --> no fracture in huge deformations
-Ferritic SS as a group are not as formable as carbon steels

Machining

-If not modified (adding sulfur and phosphorous), much lower machinability compared to B1112
-Ferritics are gummy
-Austenitics tend to cold work

True

Corrosion resistance from chromium oxide layer

Pickling and Passivation

-To achieve the maximum corrosion resistance a uniform passive film is needed
-Pickling removes oxides (made from welding, heat treatment)
-For passivation nitric acid, phosphoric acid or citric acid is used

Sensitization

Chromium carbides in austenitic SS can form at grain boundaries, making them susceptible to corrosion.

How to prevent sensitization

-Reduce exposure time to high temperatures
-Reducing carbon shrinks region of _____________, making them more weldable

Heat treatment

-Ferritic: only annealing useful
-Austenitic: after annealing quenching is needed to prevent sensitization

Sensitization

Cb (Nb), Ta and Ti prevent ___________

Sulfur/selenium

______ and ________ lower corrosion resistance (but easier machining)

Limitations of SS

• Prone to pitting
• Best in oxidizing environments
• Susceptible to crevice corrosion
• Prone to attack in chloride and reducing acids (bleach solution, sea water, other Cl water)
• Some prone to stress corrosion cracking
• Susceptible to intergranular corrosion when sensitized
• Susceptible to galvanic corrosion between grains

Ferritic/Martensitic/Austenitic

-430(_______)
-416, 420, 400C (___________)
-302, 304, 316, 304L, 316L (__________)

Cupola

Cast iron is often melted in a ______

Cast Iron Types

1. Gray- General Purpose
2. Malleable- Heat treated for ductility
3. White- Hard and wear resistant
4. Ductile- Some ductility as cast

Fluidity/cast

Silicon in cast iron increases ________, making it easier to ____

True

Cast iron has b/w 2% and 4% carbon

Graphite flakes in pearlite or ferrite matrix

Graphite nodules in pearlite or ferrite matrix

Cementite and pearlite

Graphite rosettes in pearlite or ferrite matrix

Vibrational dampening

Ductile

Hard and brittle

Strong and ductile

Fe,C,Si

20/60/increase

• As increase from __ to __, the following ________:
-All strengths
-Quality of finish for machined surfaces
-Resistance to wear
-Modulus of elasticity

20/60/decrease

As increase from __ to __, the following ________:
-The ability to dampen vibration
-Resistance to thermal shock
-Machinability
-Castability

Class # (20-60)

Secant modulus

Need to use ______ _______ to fine Gray CI's modulus of elasticity

Numbering system for Ductile CI

-Grade number and properties
-Minimum Tensile strength in ksi
-Minimum Yield strength in ksi
-% elongation
Ex: Grade 5(60-40-18)

White

Fracture surface for white CI is _____

Numbering system for malleable CI

ASTM 47 with a 5 digit number
-32510:
-Minimum yield strength – 325
-% elongation – 10
-35018

Malleable CI

Gotten from heat treating white CI (800-900C) for up to 30 hours

Modulus of elasticity

White CI has the highest _______ __ __________

Physical properties of Aluminum

-Low density →one third the weight of steel
-Good thermal and electrical conductivity
-High strength to weight ratio
-High reflectivity
-Good corrosion resistance (passive aluminum oxide layer)
-Not magnetic

Aluminum fabrication

-Easy to cast and machine
-Most alloys are weldable
-Can be given a hard surface by anodizing and hard coating
-Ductile

Aluminum

-Most abundant metal in nature (8% of earth’s crust)
-Nontoxic