MMET 207 Exam III

Factors affecting corrosion

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 (<0.2%) 16-20% Cr Alloying elements: Fe, Cr, C Cannot be quenched hardened; nonhardenable Nonhardenable Poor weldability because of: Formation of embrittling phases and carbides, Corrosion resistance is reduced, Reduce C and N below 100 ppm improves weldability Not susceptible to stress corrosion; resistant Easier to fabricate compared to austenitic SS Magnetic High impact strength App: Non-structural, high temperature

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