DEGRADATION OF MATERIALS Nov 07egr.uri.edu/wp-uploads/che/DEGRADATIONOFMATERIALS.pdf · Degradation of Materials. ... forms from steel –so unstable on ... combination of electrochemical

DEGRADATION OF

MATERIALS

CVE 549

Nov 28th, 2007.

Richard Brown

Chemical Engineering

URI

Degradation of Materials.

Metals – except for noble metals, they

tend to corrode in wet environments.

M > Mz+ + ze- - Anode- dissolves

Fe -> Fe2+ + 2e-

O2 + 2H2O + 4e- = 4(OH-) – Cathode.

Metal hydroxides form which transform

to oxides. Stability of oxides important.

Unstable oxides – corrosion, stable

oxides no corrosion.

Rust - large volume expansion when it

forms from steel – so unstable on

surface. Chromium and aluminum

oxides reasonably stable, so good

in water environment when no

halides (Cl, F, Br, I).

Rust Expansion

The volume expansion when

rust forms has undesirable

effects. The tensile stresses

formed in reinforced concrete

initiate cracks and eventually

spallation of concrete from the

surface. With paints it will

initiate decohesion and lifts the

paint from the steel, often called

“undercutting”. On nuts, it will

initiate seizure of threads and

make removal of nuts very

difficult.

Note the cracking in the rust,

indicating that it cannot protect

the underlying steel from

moisture.

Passive Alloys

Potential (Volts)

Current Density

µA/cmicrit2

ηa - activation control

Epp

Passive region

Transpassive region

101

Passive metals and alloys are

ones that form surface layers

that are stable in the

environment. The vertical

region of the Evans diagram –

potential v log current density

shows the passive region of

low current density and very

low, near zero, corrosion rates.

Pourbaix Diagram for Iron

Potential – pH diagrams indicate the

combination of electrochemical and

environmental conditions where

unstable and stable films form and

where metals are immune to

corrosion by remaining as an

element rather than reacting to form

an ion.

The Fe region is a region of

immunity.

Fe2+ and 3+ regions represent

corrosion.

Fe(OH)2 and Fe2O3 are passive

regions.

The dashed lines represent where

water will try to initiate corrosion.

Concrete at pH 12.5 falls in a

passive region. (0V SCE and pH

12.5)

Uniform CorrosionDamp wet environment

initiates corrosion of poorly

painted steel tank. Looks bad,

bud as long as a large area

corrodes, then rate of

penetration low.

Pitting and Crevice Corrosion

Crevice and Pitting Corrosion.

Localized Corrosion.

Oxides become unstable due to

halides, local attack using same

reactions.

Crevice corrosion on threaded rod, the

problem here is that the retaining nut

comes loose. It is also difficult to detect

as the corroding region is under the

cover of the nut.

Weld Decay

Welding stainless steel can be an

issue is not conducted properly.

The area adjacent to the weld will

corrode and penetration of the

steel will occur rapidly. This can be

avoided by appropriate material

selection and welding techniques.

Filiform CorrosionFiliform corrosion is surface

corrosion and does not

penetrate into the metal. It

often appears below paint.

The figure on the left shows

filiform corrosion on a steel

component with a clearcoat

applied. Note that although

the corrosion generally

follows the machine

markings, it also does 180

turns, as can be seen in

several locations. Usually

poor paint quality with

porosity or high permeation

rates along with poor

adhesion are reasons for

filiform.

“U” turn.

Selective Leaching

A reasonably slow corrosion

process affecting alloys. A

brass screw has de-zincified

in water over a long time.

The zinc is selectively

removed from the bras and

leaves only porous copper. It

was only when to remove

the screw dezincification was

found as the dimensions of

the screw did not change,

but the mechanical

properties were poor.

Better alloy selection would

help.

Erosion Corrosion

Rapid fluid flow of an aggressive

electrolyte removes material, in this

case a 316L stainless steel flange

from a chemical plant. One issue here

is the purity of the fluid flowing in the

pipes as it is now contaminated with

the elements from the stainless steel

in ion form.

Change alloy and slow fluid flow

would help in this case.

Carbon Fiber Damage

Top figure is unexposed

carbon fiber, the bottom figure

is carbon fiber exposed to a -

0.65V (SCE) voltage.

The white area in a periodic

pattern in the lower figure are

delaminatsion which contan

fliud of pH 11-12 which result

in blisters on carbon fiber

composite surface due to

galvanicinteractions.

These also occur in fiberglass

due to osmotic blistering.

Stress Corrosion Cracking

Brass is liable to stress corrosion crack

when a tensile stress, in this case a

residual stress, and an aggressive

environment are present, in this case

an ammonia based solution.

Stress corrosion cracking requires both

a tensile stress to open up cracks, and

an aggressive environment, which is

material dependent.

Corrosion Fatigue

Initiation Site.

Fatigue lives are decreased when

an aggressive environment is

present. The component shown

failed by corrosion assisted fatigue.

Corrosion Rates

Uniform or Galvanic Only.

• CR = 0.13 (icorr x EW)

d

• for units of Corr Rate in mils per year, icorr

in mA.cm-2

• EW is the equivalent weight and d is the

density of the metal.

• icorr data available from lab testing.

Laboratory Data for icorr

100101.1

-0.55

-0.53

-0.51

-0.49

-0.47

-0.45

Polarization Data

log Current Densitymicroamps per centimeter squared

Potential V (SCE)

Open Circuit

Potential

Current Density for

calculationof Corrosion Rate

Cathode Reaction

Anode Reaction

100101.1-0.55

-0.53

-0.51

-0.49

-0.47

-0.45

Polarization Data

log Current Densitymicroamps per centimeter squared

Potential V (SCE)

Potentiodynamic test run to find

icorr data from extrapolation.

Raw data from test.

Salt Spray Testing ASTM B117

Salt spray unit, 5% NaCl at 95F and

100% humidity.

Untested steel, steel 1 and steel 2 tested

for same time. Steel 2 is best choice.

Salt fogSalt water Deionzed Water

Water tower and heater

Using Corrosion

Weathering steels are a class of steels

that contain copper to stabilize the oxide

layer that forms when steel is exposed

to a wet environment. It modifies the

oxide to avoid the cracking shown

earlier. It can be used but should not be

coated to allow the oxide layer to build

up uniformly.

Corrosion Protection for Steel

Bridges and Decks.

Sacrificial Coatings on A36 Steel

• Hot dip galvanizing –in plant 4mils thick, difficult to topcoat, adhesion problems

• Metallizing or Thermal Spray – becoming more popular as price drops – need to seal porous coating, 8mils thick. In plant or on site, repair damage, paint adhesion OK.

• Paints – zinc rich primers, topcoats.

Coatings R and D

Cycle Polarization Data of coated

and uncoated Al 2024-T3 in 0.5N NaCl

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

-8 -6 -4 -2 0 2 4 6

Log i (uA/cm2)

E (mV)

URI Coating Base Sample Chromate Coating

Laboratory testing to examine coatings

involves potentiodynamic scans. The

lower the current density for the passive

region and the larger voltage range

indicate a better coating and also

indicate the mechanism of protection.

Electrochemical Impedance

Spectroscopy

5.5pH conversion coating

2.0pH conversion coating

Bare sample

Day 1 comparing different surfaces Day 1 compared to day 30 data

Long term data available and passive circuit modeling for quantitative analysis

30 days exposure

Adhesive Joints-Galvanic Design

0

1000

2000

3000

4000

5000

6000

7000

0 1 2 3 4 5 6

N umb e r o f W e e k s

conventional treatmentperoxide-titanate treatmentfit (conventional treatment)fit (peroxide-titanate treatment)

Partial interfacial failure after 2 weeks Complete interfacial failure after 4 weeks

First ply failure after 6 weeks with URI

non chromate treatment

Failure loads for lap joints after marine exposure.

Interfacial failure was between the conventional chromate surface treatment and the Ti-6Al-4V alloy

Voltage between carbon fiber and chromate 0.52V

Voltage between carbon fiber and URI treatment 0.07V

Removal of galvanic component increased durability of

joint

Why Lamposts Fall.

Lamposts – Failed Coupling

Lamposts - Corrosion Evidence

Vertical rust line on threaded rod

indicating cracked coupling

Different location, note telltale vertical

corrosion marking on threaded rod.

Lamposts Wrought Aluminum

Coupling

Changing from cast alloy to wrought alloy with copper additionsinduced pitting corrosion

Cast Iron Coupling

with Resistance Plate

Polymer Degradation

Ultra Violet exposure – embrittles polymer, these days use clear coats which stop the

process.

Water uptake – polymers such as vinyl esters absorb 1.5% by weight of water can cause

imbalance in propellers.

Water uptake can also cause polymer swelling and delamination.

Dissolution – chemical attack, eg imides in alkaline environment.

Ceramic Degradation

Acid attack.