MMET 207 Exam III

Factors affecting corrosion

Materials properties and environment

Material properties

The chemistry
Redox potential
Passivity
Metallurgical factors: when an anodic area is formed

Redox Potential

In material props
Is corrosion thermodynamically favorable- how easily with which a molecule accepts electrons

Passivity

In material props
Formation of a protective film

Metallurgical factors in material props- what affects when anodic areas are formed

Chemical segregation
Presence of multiple phases
Inclusions
Cold Work
Nonuniform stresses

Environment

Understanding the environment is a necessary 1st step in corrosion control
Chem nature, operating conditions in environment, polarization

Operating conditions in environment

intended service life
temp
velocity of corrodent
concentration of solution
aeration
impurities in solution

Chem nature in environment

acids
bases
salts
gasses
solvents

Polarization in environment

the change in corrosion potential with the change in corrosion current

Uniform corrosion

all the surface is exposed to corrodent
material choice and removal of electrolyte can be preventive

Pitting

Local corrosion damage
Use the available data on corrosion to avoid using metals in environments susceptible to pitting. For example, stainless steel should not be used in salt water.

Crevice corrosion

local attack in a crevice
prevented through good gasketing

galvanic corrosion

when 2 dissimilar metals are electrically connected
use galvanic series in material selection

Stress corrosion cracking

spontaneous corrosion-induced cracking of material under static stress (environmentally assisted cracking)

Intergranular attack

Localized
Preferential corrosion at the grain boundaries

Dealloying

1 element in the alloy is preferentially removed

Types of Erosion

Liquid Erosion
Liquid impact
slurry erosion
cavitation erosion

Liquid Impact

Protective film removal
Can be combined with mechanical material removal

Liquid Erosion

Similar to impact erosion but fluid is parallel to the surface

Slurry Erosion

Combined action of corrosion and wear leads to material removal
The same mechanisms as impact erosion, but abrasive particles enhance it.
Ceramic or elastomer coatings can be helpful

Cavitation corrosion/erosion

Formed when the operational pressure is dropped below the vapor pressure of the fluid, causing the formation of gas bubbles that collapse at an increased velocity on the surface of the material, inducing initial cavitation

Determination of corrosion characteristic

Corrosion data: use data available (National Association of Corrosion Engineers)
Corrodent Chem, corrodent concentration, temp, aeration, state of stress
Standardized lab experiments

Standarized tests to determine corrosion characteristics

General corrosion; samples are weighed before/after immersion in a particular corrodent
Galvanic attack- corrosion potential and corrosion current can be measured
Crevice- severe attack will occur under band if metal is susceptible to this
Stress corrosion cracking- cracks will occur on tension side of U bend if metal is susceptible to this
Liquid erosion- effect of velocity on the corrosion rate of metals is determined by weight loss

Factors used to control corrosion

Material selection
environment
design
Cathodic/anodic protection

Material selection

Coating(ie painting metallic coating-anodic to substrate); should be thick and pinhole free-anodic metallic coatings are not sensitive to pinholes; good for atmospheric corrosion but not chemicals
Diffusion treatment: chromizing, nitriding
Heat Treatment: stainless steels after welding becomes sensitized, heat treatment and subsequent quenching addresses it
Surface finish: rough surfaces exp faster corrosion rates

Environmental control

Temp
Velocity
pH: dissolved gasses
Cleaning: prevents buildups that lead to crevice or concentration corrosion
Inhibitors: alter environment; removing O2, absorptive inhibitors slow down the anodic/cathodic rxns by stablishing a passive film

Design in corrosion control

a)Prevent base metal dilution
b)Gasket shouldn't stick out in crevice control
Cleanability to avoid residue corrodent
c)Provide proper drainage
d)Prevent corrodent/solutions from drying and concentrating
e)Provide for inspection; corrosion monitoring
f)prevent galvanic couples- use insulating materials to prevent galvanic corrosion
g)Avoid incomplete weld penetration--> aeration, concentration of electrolyte, and crevice corrosion
h)Avoid water accumulation in outdoor construction
i)Use wear plates to minimize corrosive wear- prevent impingement from fluids

Cathodic protection

Use electrochemical reaction in your favor
Used in soil or in vessels
uses the principle of galvanic corrosion by reversing the flow of current between two dissimilar metals, especially since this type of corrosion is when coupling an active metal with a more noble metal resulting in current to flow and corrosion occurring on the active metal

Anodic protection

Use electrochemical reaction in your favor
More complex than cathodic protection
involves the surface being protected as the anode whereas cathodic protection involves making the metal surface the cathode of an electrochemical cell to control corrosion
the potential of the object is controlled by suppressing the reactivity of the metal so that it stays passive.

SS general props

Resist corrosion, even at high T
More than 11% Cr (more Cr, better corrosion resistance)
not well suited for reducing environments (e.g. sulfuric acid).
Manufacturing __ is challenging because Cr reacts with oxygen and carbon, so special processes are
used:
Adding ferrochromium to low carbon steel scrap
Use electric furnaces
Ladle treatments
Argon-oxygen decarburization (AOD)
All Expensive
oForm a passive layer in oxidizing environment

5 Metallurgic microstructures of SS

Different alloying elements change the range
of stability of phases.
Ni, C, and N extend the austenite region
Ferritic
Martensitic
Austenitic
Precipitated hardened (PH)
Duplex

Ferritic SS

BCC
Low carbon (

Martensitic SS

BCT(Not a cube)
12-18% Cr
Up to 1.2% C
Alloying elements: same as Ferritic Fe, Cr, C
Carbon expands the gamma loop which makes quench
hardening possible
Chromium carbide is present in the structure
Poor weldability and notch sensitivity
Magnetic
Lowest impact strength
NOT resistant to stress corrosion crack
App: Structural, cutting tools

Austenitic SS

More complex in nature and they have at least 4 alloying elements: Fe, Cr (16-26%), C- lowest, Ni- austenizer(at least 8%, up to 24%)
Quench is needed to maintain the FCC(metastable phase)
By cold work, this metastable austenite is transformed to martensite →very work hardenable.
Nonmagnetic
High weldability
Highest impact strength
App: chemical resistance, creep resistance, tanks, piping

PH alloys

Different types:
Martensitic
Semiaustenitic
Austenitic
• Low carbon
• Ni and Cr have specific ratios
• Al, Ti, and Cu form precipitates
• Needs special heat treatment

Duplex alloys

Si, Mo, V, Al, Nb, Ti, and W promote ferrite formation(general knowledge)
Ni, Co, Mg, Cu, C, and N promote austenite formation
Using different compositions we can form austenite and ferrite next to each other
Compared to all austenite SS: Higher yield strength, Improved welding, Improved stress corrosion
Not immune to stress corrosion and in some environments, ferrite is attacked more severely.

Identification of SS

AISI: three-digit system, the first letter shows the composition type:
200: Cr, Ni, Mg(aus)
300: Cr, Ni(aus)
400: Cr(ferritic/martensitic)
UNS: SXXX00
ASTM standard is based on application: ASTM A 313 covers wires

Ex of Aus SS

301 can esp be cold worked
303, 304, 316 less C
304L, 316L extra low C; used when sensitization is important
high creep R--> load must be carried in furnaces/highT
> 9% Ni--> less work harden

Ex of Martensitic SS

410, 420(both turbine parts), 440C(used for tools)
higher strength, wear resistant

Ex of ferritic SS

430
lower in cost, household appliances
good oxidation R, used in furnaces(don't transform to other phases like other SS)

Types of Cast iron

Gray
malleable
white
ductile

Phys props of SS- Density

similar to other iron-based alloyw

Phys props of SS- Structure

affects mechanical properties and magnetism

Phys props of SS- Conductivity

low electrical (one sixth of carbon steels) and thermal conductivity (less than half of carbon
steels)

Phys props of SS- expansion

austenitic alloys can have 50% larger thermal expansion. Can be problematic in bimetal strips.
Other structures are similar to carbon steels.

Phys props of SS- modulus of Elasticity

slightly lower than carbon and alloy steels for the same section size SS has more elastic deformation

mech props of SS

used in large structures- vessels, valves, pumps
value strength, toughness, high T strength, and formability
Where higher strength or wear resistance is needed, martensitic 420 and 440C are used.
Nickel above 9% decreases work hardenability in aus
Ferritic SS is used in furnaces where the load must be carried(no phase change)

Types of fabrication

Forming
machining
pickling and passivation
welding
heat treatment

Forming

reshaping of metals while still in the solid state; creates structural parts/components out of metal sheets or tubing
Austenitic SS has high ductility--> no fracture in huge deformations
Ferritic SS as a group are not as formable as carbon steels
In spite of that, cold forming is common

Machining

raw material is cut into a desired final shape and size ; subtractive process
If not modified, much lower machinability compared to B1112
Ferritics are gummy
Austenitics tend to cold work
Consider types 430F, 416, and 303 when heavy machining is needed

Pickling and passivation

to achieve the maximum corrosion resistance a uniform passive film is needed. Therefore, contaminations should be removed.
Pickling removes tightly adhering oxides (produced from welding, heat treatment, ...)
Sulfuric or nitric-hydrofluoric acid is used
For passivation nitric acid, phosphoric acid or citric acid is used. The process removes iron contaminations.
Safety is important- hazardous; Environmental effects

Welding

Avoid welding on martensitics: formation of hardened martensite and cracking are the problems
In ferritics 1. grain growth, 2. HAF can have lower impact resistance, and 3. Sigma phase might be formed
Austenitics are weldable, but sensitization is still a problem

Sensitization

Precipitation/formation of chromium carbides at the grain boundaries of austenitic stainless steel after welding leads to depletion of Cr at the grain boundaries--> corrosion at grain boundaries

Heat Treatment

Ferritics do not quench hardening. The only useful heat treatment on them is annealing (remove stress)
For austenitics, when annealing at high temperatures happens, quenching is needed to prevent sensitization
Low-temperature carburizing for austenitics is possible (case of 50 microns). Good when wear resistance is
expected from them.

Sulfur and selenium

lower corrosion resistance (but easier machining)

Cb (Nb), Ta and Ti

prevent sensitization

Mo

reduce pitting

Corrosion limitations

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

atmospheric corrosion

All classes are excellent in this environment

Sulfuric and nitric acids

concentration and T matters. 316 is good for low and high concentrations of __ acid, 430 is good for __ acid

Phosphoric acid

most grades are good at room T in this environment

Organic solvents

all grades are good in this environment

Gasoline

304 and 316 are excellent in this environment

Chloride service

__ lead to pitting, crevice and stress crack corrosion

Neutral H2O

most grades are good in this environment

Bleaches

all types are attacked in this environment

Gray cast iron

general purpose
Alloying elements: Fe, C, Si
graphite flakes in pearlite or ferrite matrix
weak & brittle in tension
stronger in compression
excellent vibrational dampening
wear resistant
Ex: Class 20

All strengths
Quality of finish for machined surfaces
Resistance to wear
Modulus of elasticity

As increase from 20 to 60, the following increase in gray cast iron naming system

The ability to dampen vibration
Resistance to thermal shock
Machinability
Castability

As increase from 20 to 60, the following decrease in gray cast iron naming system

Ductile cast iron

add Mg and/or Ce--> Fe, C, Si, Mg, Ce
graphite nodules in pearlite or ferrite matrix
matrix often pearlite – stronger but less ductile
Numbering system for Ductile Cast Iron:
(Grade number and properties, Minimum Tensile strength in ksi, Minimum Yield strength in ksi, % elongation)
Ex: Grade 5(60-40-18)

White cast iron

0.5 - 2 wt% Si
pearlite + cementite
very hard and brittle due to Fe3C; wear resistant
The fracture surface is white
No naming system

Malleable cast iron

Graphite rosettes in pearlite or ferrite matrix
heat treat white iron at 800-900°C
reasonably strong and ductile
Numbering:
ASTM 47 with a 5-digit number
Ex: 32510: Minimum yield strength – 325, % elongation – 10

Al phys props

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

Fabrication of Al

Easy to cast and machine
Most alloys are weldable
Can be given a hard surface by anodizing and hard Coating
Ductile--> less possible of forming cracks

From bauxite(Al ore), alumina (aluminum oxide,
Al2O3) is extracted
Alumina is dissolved in cryolite
Alumina is reduced at the anode
Aluminum is denser than the molten salt and is collected at the bottom of the cell

How to extract Al

Wrought Al #ing system

4-digit number for aluminum series
1000 series – last two digits indicate purity beyond 99% Ex. 1025 – 99.25% pure Al
Other series – numbers only represent different specific alloys
Suffix indicating heat treatment or degree cold work

1000 series of Al

commercially pure- 99% pure or higher
used in electrical and chemical fields
excellent corrosion resistance
high thermal and electrical conductivity
low mechanical properties and excellent workability
moderate increases in strength from strain hardening
iron and silicon major impurities

2000 series in Al

Copper – principal alloying element
Heat treatable - require solution heat treatment to obtain best properties
In heat treated condition, mechanical properties close to or even exceed mild steel
May require artificial aging
Do not have as good corrosion resistance as most other Al alloys
Often clad with 6000 series or pure Al
2024 best known alloy

6000 series in Al

Silicon and Magnesium – major alloying elements
Forms magnesium silicide
Heat treatable
Less strong than 2000 or 7000 series
Good formability
Good corrosion resistance
Medium strength
6061 – most widely used and versatile Al alloy

7000 series in Al

Zinc – major alloying element
Heat treatable
High strength
Used for highly stressed parts and aircraft applications
Not arc welded well – resistance welded

8000 series in Al

Other than Cu, Mn, Si, Mg, Mg and Si, Sn

Precipitation Hardening

4th way to strengthen metals including cold work, reducing grain size, and alloying
Accelerate aging process by cooling quickly from alpha to alpha + CuAl2 and then raising T to below alpha phase

2000, 6000, 7000

Which Al series are heat-treatable?

1000, 3000, 4000, 5000

Which Al series are nonheat-treatable?

Cold work wrought Al numbering system

-H1X Strain hardened only
-H2X Strain hardened and partially annealed
-H3X Strain Hardened and stabilized (Mg)
-H4X Strain hardened and lacquered or painted
where X represents the degree of coldworking

Heat treat suffixes

T3 Furnace solution heat treated, quenched and cold worked
T4 Furnace solution heat treated, quenched, and naturally aged
T6 Furnace solution heat treated, quenched and furnace aged

Indoors: Very corrosion resistant, low corrosion rate
Outdoors: 1. Corrosion rate depends on humidity and chemicals on the atmosphere (e.g presence of salt). 2. Faster corrosion rate in the first two years, then a uniform oxide layer is formed.

How does Al perform in atmospheric corrosion

Chemistry (heavy ions such as copper, lead, Ni) and pH are important.
sea water causes pitting.
Detergents lead to heavy attack.
Cathodic protection or coatings to prevent or reduce

How does Al perform in water corrosion

resistant to nitric acid, ammonia, organic solvents (alcohol, acetone, ...)

How does Al perform in chemical corrosion

Due to the low corrosion rate and high affinity to O2- quickly develops a film of Al oxide

In general, why does pure Al have the best corrosion resistance

Anodizing Al

• Converting aluminum to alumina
oHas 2 types, clear coat (or conventional) and hardcoating
oEnhances corrosion resistance
oEnhances wear resistance (hard coating)
• Coating grows perpendicular to the surface
• Corners should be radiused.
• Coating has pores. The pores can be sealed by immersing them in hot water. Dye can also be added to the hot water.
anodizing changes the dimensions

Clear coat or conventional in anodizing

Less surface thickness converted, not as much size growth
Can be colorized with dye
Provides some wear resistance and increased corrosion resistance

hardcoating in anodizing Al

More surface thickness converted, more size growth
Turns dark gray, does not color well
Primarily for wear, and corrosion resistance

The galvanic series rank metals and alloys based on their nobility, or how cathodic or anodic they are.
The electrolyte used in ranking metals in galvanic series is sea water.
If 2 metals are connected in a structure, using galvanic series we can determine which one is going to
be sacrificed and which one is going to be saved. This can be used to protect structures using cathodic
protection.

What information does galvanic series provide? How can that be used in selecting metals in a design?

Oxidation(loses e-)

Fe --> Fe+2+2e-

Reduction(gains e-)

Cu+2+2e- --> Cu