2. forms of corrosion; electrochemical
TRANSCRIPT
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Corrosion Engineering Course
Forms of Corrosion
KAFCO, ChittagongBangladesh
Giel [email protected]
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Contents Forms of Corrosion
• Introduction
• Electrochemical- Uniform corrosion- Galvanic corrosion or contact corrosion
(two metal corrosion)- Pitting- Crevice corrosion; corrosion by differential aeration- Intergranular corrosion- Selective corrosion
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Contents Forms of Corrosion (continued)
• Electrochemical – mechanical
- Stress Corrosion Cracking (SCC)
- Corrosion – fatigue
- Erosion – corrosion
• Physical – metallurgical – mechanical
- Hydrogen (H) embrittlement
- Liquid Metal Embrittlement (LME)
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Contents Forms of Corrosion (continued)
• High temperature - Chemical
- Oxidation / Sulphidation
- CO-attack
- Metal dusting
- Hydrogen (Nelson) attack
- Nitriding
- Creep
• Atmospheric Corrosion
• Soil corrosion – Microbiological Induced Corrosion (MIC)
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Corrosion occurs in several, widely differing forms.
Classification is usually based on factors like:
• Nature of the corrosive environment
• Appearance of the corroded material of construction
• Mechanism of corrosion
Classification of corrosion forms
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Schematic illustration of different types of corrosion according to morphology
General attack Crevice corrosion
Deposit corrosionIntercrystalline corrosion
Graphitic corrosion
Load Stress
Cracks
Stress corrosion cracking (SCC) Corrosion fatigue
Exfoliation corrosion
Oxide film or noble metal
Pitting
Brass (Cu+Zn)
Porous Cu
Selective corrosion
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Electrochemical Electrochemical - mechanical
Physical –
metallurgical
(mechanical)
High temperature
Chemical
Uniform corrosion
Galvanic corrosion
Pitting
Crevice corrosion
Intergranular corrosion
Selective attack
Stress Corrosion
Cracking (S.C.C.)
Corrosion-fatigue
Erosion - corrosion
Hydrogen (H) damage
Liquid Metal Embrittlement (LME)
Oxidation - sulphidation
CO attack
Metal dusting
Hydrogen (H2) attack
Nitriding
Creep
Survey of corrosion phenomena classified according mechanism
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1%
2%
3%
5%
4%
6%
10%
33%
11%
19%
6%
33% uniform corrosion
11% corrosion fatigue
19% transgranular S.C.C.
6% intergranular S.C.C.
2% H-Embrittlement
1% H2 - attack
5% pitting
4% intergranular corrosion
6% mechanical (wear, erosion, cavitation)
3% high temperature
10% other corrosion forms
Forms of corrosion: frequency of occurrence (Basf)
H-embrittlement
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Basf Bayer DuPont
Uniform corrosionLocal attack - pitting - crevice corrosion - galvanic corrosion - intergranular corrosion - selective leachingElectrochemical-mechanical - stress corrosion cracking - corrosion - fatigue - erosion - corrosion Physical - metallurgical - H - embrittlement - liquid metal embrittlement
33
5
4
25116
2-
12
12
171
3722
-
28
1421
106
2437
2-
Other corrosion formsPeriodTotal number of failures
14 84.5 year
34 year
685
Frequency of corrosion failure modes in several chemical industries
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Impact on integrity Non-destructive testing-techniqe
Uniform corrosion
Local attack
- pitting
- crevice corrosion
- galvanic corrosion
- intergranular corrosion
- selective leaching
Mechenical-electromechanical
- corrosion-erosion
- stress corrosion
- corrosion-fatigue
Physical-metalurgical
- H-embrittlement
- liquid metal embrittlement
-
O
-
+
+
-
O
++
+
++
++
(V) UT X EC
V UT X
(V) UT X
V UT X
(V UT X EC
V EC
V UT X EC
M UT X EC
M UT EC
M UT X
M UT X EC
Forms of corrosion
Reference:
++ = very serious
+ = serious
O = moderate
- = miner
V = visual
UT = ultrasonic
X = rontgen
EC = eddy-current
M = magnetic
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Uniform Corrosion
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Expressions of corrosion rates with conversion factors (Factors for conversion to:)
Given Unit g/m2h mm/year mils/year
g/m2 hg/m2 24 hg/dm2 24 h mg/dm2 24 h (mdd)mg/cm2 24 h lbs/ft2 24 hlbs/ft2 year mm/yearmm/monthm/48 h inches/year (ipy)inches/month (ipm)mils/year (mpy)mils/month (mpm)
1,00.0424,17
0,0040,417
203
0,564
0,116d1,39d
0,021d
2,95d35,3d
0,003d0,035d
8,64:d0,360:d36,0:d
0,036:d3,60:d
1760:d4,88:d
1,012
0,180
25,4305
0,0250,305
340:d14,2:d1420:d
1,42:d142:d
69200:d
192:d
39,44737,18
1000
12 0001,012
d = density (specific gravity) of the metalExamples: AISI 304(L) stainless steel = 7,9; titanium = 4,5; aluminium = 2,7
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mA.cm-2 cm.year-1 ipy mdd gm-2.day-1
mA.cm-2 1 0.326 m/nd o.129 m/nd 89.2 lm/dl 8.92 lm/nl
cm.year 3.06 nd/m 1 0.394 274 d 27.4 d
ipy 7.75 nd/m 2.54 1 694 d 69.4 d
mdd 0.0112 n/m 0.00365/d 0.00144/d 1 0.1
g.m-2.day-1 0.112 n/m 0.0365/d 0.0144/d 10 1
Conversion factors for corrosion rates
Reference: ipy : inch per year
mdd : mg.dm-2.day
n : valency of metal
m : atom weight
d : specific gravity (g.cm-3)
e.g. 1cm per year = 0.394 ipy
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Uniform corrosion at 304L heat exchanger tube in acidic ammonium bisulphate environment
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Uniform corrosion at overflow plate out of the NOx absorption column of nitric acid plant
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Ruptured tube out of HP stripper Urea plant
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Top and bottom section out of carbon steel reboiler tube anone/anol recovery caprolactam plant
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Baffle plate (X6NiCrMoCu20-18-2-2) hydrolysis vessel caprolactam plant
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Suction line (304L) in the ammonia absorber of low pressure section of urea plant
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Suction line (304L) in the ammonia absorber of low pressure section of urea plant
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NH3- absorber (304L)
Corroded segment elbow 304L
NH3-gas (traces of O2)
HNO3
Desorption water (traces of NH3)
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Local overall corrosion in AISI 347 wall of precipitation reactor NP plant
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Uniform corrosion can be prevented or reduced by:
- Application of proper material
- Application of a corrosion allowance
- Use of coating systems
- Addition of inhibitor systems
- Application of cathodic (or anodic) protection
Preventive measures
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Hastelloy B-type welded with Hastelloy C-276 corroded in carbamate solution
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Increased corrosion at liquid level in 316L UG carbamate pipe line
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Galvanic corrosion
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Carbon-zinc battery (Leclanche cell)
+
MnO2 in moist ammonium chlorideCarbon
center post
(cathode)
NH4+
CL-
H+
OH-
CL-
NH4+
CL-
NH4+
H+
OH-
CurrentFlow
NH4+
CL- OH- H+
ZincCase
(anode)
Anode: Zn Zn++ + 2e
Cathode: 2MnO2 + 2NH4+ + 2e Mn2O3 + 2H2O + NH3
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eq , H2/H+
corr , Fe
eq , Fe/Fe2+
corr , Zn +Fe
corr , Zn
eq , Zn/Zn2+
cathodic additioncurrent densities
anodic additioncurrent densities
A + B
AB
log icorr, Zn
log icorr , Fe log icorr , Zn conn, met Fe
Explanation of the behaviour of the galvanic coupling of iron and zinc in acidic solution by means of schematic polarization curves
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Parameters influencing galvanic corrosion
- Environmental effects
- Potential difference galvanic couple
- Distance effect
- Area effect
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Metal Potential
magnesiumzinc alloy zamak Z400zincaluminium 99.5%mild steelcast iron GG-2213% Cr-steel (active)18% Cr-8% Ni-steel (active)lead 99.9%brass 60-40coppermonel K70-30 cupronickelchromium and chromium-nickel steels (passive)
-1.32-0.94-0.78-0.67-0.40-0.35
appr. -0.30appr. -0.30
-0.26-0.07+0.10+0.12+0.34
appr. +0.40
Practical galvanic series for a number of metals and alloys in air saturated, neutral seawater
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A number of preventive measures, procedures orpractices can be used to combat or minimizegalvanic corrosion:
- Select combinations as close together as possible in the galvanic series
- Avoid unfavourable area effect- Insulate dissimilar metals wherever possible- Apply coatings with caution- Add inhibitors if possible- Design for replacement of anodic parts- Install a third metal which is anodic to both metals in
galvanic contact
Measures to prevent galvanic corrosion
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Galvanic corrosion in carbon steel T-joint next to brass fitting
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Galvanic corrosion at stainless steel blind plate due to carbon depostis
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Galvanic corrosion of aluminium instrument air line at locations of coupling with carbon steel pipeline
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Galvanic corrosion in carbon steel pipeline welded to stainless steel pipeline in sulphuric acid environment
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Pitting Corrosion
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MeCl2 + H2O
Me(OH)2 + HCl
2H2O + O2 + 4e- 4(OH)- 2H2O + O2 + 4e- 4(OH)-
Me2+ Me2+
Me2+
o o oo
2e- +2 H+ H2 H2 2H+ + 2e-
oo o o
e- e-
e-e-
H2O O2 Cl- Cl- O2H2O
Mechanism of pitting
Passive oxyde layer
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Epitt
Epass
2 H2O O2 + 4 H+ + 4e
Anodic polarization curve
In halide free solution
Me + H2O + X- MeOHX+ + H+ + 3e
Anodic polarization curve
In halide containing solution
2 Me + 3 H2O Me2O3 + 6 H+ + 6e
(passivation)
Me Men+ + n e
(active dissolution)
ipass log i
Anodic polarisation curves of material with active/passive behaviour in a halide free and halide containing environment
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Parameters influencing pitting
- Environmental influences: presence of halidesredox potentialpHtemperature
- Velocity effects
- Alloy composition: PREN = %Cr + 3.3.%Mo + 16.%N
- Metallurgical aspects
- Presence of high temperature oxides
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316
304
430
+ 0,8
+ 0,6
+ 0,4
+ 0,2
0
-0,2
-- 0,41 3 5 7 9 11
pH
Pit
tin
g P
ote
nti
al (
V)
Effect of pH on the pitting potential of several stainless steels in 3% NaCI solution (temp. 25C)
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20 40 60 80 100 °C
Temperature
+ 0,4
+ 0,2
0
-0,2
- 0,4
AISI 316
AISI 304
AISI 430
Effect of temperature on the pitting potential of several stainless steels in a 3% NaCI-solution
Pit
tin
g P
ote
nti
al (
V)
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+ 0,7
+0,6
+ 0,5
+ 0,4
+ 0,3
+ 0,2
+ 0,1
0
- 0,1
10 20 30 40 50 60 weight %
Cr - content
Pit
tin
g P
ote
nti
al (
V)
Effect of Cr-content on the pitting potential of Fe/Cr-alloys in an aerated 0.1 N NaCI solution (temp. 25C)
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0,6
0,5
0,4
0,3
0,2
0,1
00 1 2 3 weight %
Mo-content
Pit
tin
g P
ote
nti
al (
V)
Effect of Mo-content on the pitting potential of Fe-15% Cr-13% Ni-alloys in an aerated 0.1 N NaCI solution (temp. 25C).
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+ 0,15
+ 0,10
+ 0,05
0
0,05
- 0,10 10 20 30 40 50 60 weight %
Ni content
Pit
tin
g P
ote
nti
al (
V)
Effect of Ni-content on the pitting potential of Fe-15% Cr-alloys in an aerated 0.1 N NaCI solution (temp. 25C).
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Causes of pitting attack
- Local defects in protective layer
- Adsorption and penetration of specific ions in the oxide layer; e.g. chloride and bromide ions (stagnant conditions)
- Presence of ferric and cupric ions or other species which increase the redox potential (e.g. H2O2) in a chloride containing environment (electron acceptors)
- Foreign metal particles in oxide layer; e.g. iron particles of steel brush in stainless steel
- Stray currents
- Microbiological influences (presence of microbiological slime film)
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Measures to prevent pitting attack
- Avoid presence of chlorides, especially in presence of ferric and cupric ions and other species which increase the redox potential
- Application of inhibitors like silicates, chromates, phosphates
- Increase of pH
- Remove porous oxide layers due to welding
- Avoid stagnant conditions
- Change alloy compositions (PREN = %Cr + 3.3%Mo + 16%N)
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Materials with increased pitting resistance
X8Cr17 (AISI 430)X2CrNi19-11 (AISI 304L)X2CrNiMo17-12-2 (AISI 316L)X2CrNiMoN25-22-2X2CrNiMoN22-5-3X2CrNiMoN17-13-5 (ASN5W)X2NiCrMoCu25-20-4-2 (2RK65)X2NiCrMoCu31-27-4-1 (Sanicro 28)Hastelloy B2Hastelloy C-276 Hastelloy C-4 Hastelloy C-22Titanium, Zirconium, Tantalum
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Extensive growth of pit diameter below surface
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Pitting of 17% Cr steel tube in cooling water with chlorides
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Pitting occurrence depending on Mo content in AISI 304, benzene storage tank with water phase, containing ammonium sulphate and SO2
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Pitting in C-steel recirculation line of condensate tank
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Pitting in X3CrNiMoN17-13-5 (ASN5W) heating coil of separator in MVC recovery of a PVC plant
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Pitting in X3CrNiMoN17-13-5 (ASN5W) heating coil of separator in MVC recovery of a PVC plant
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Pitting in AISI 316 pipeline
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Pitting in AISI 304 (0.0% Mo) in SO2 containing benzene
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Pitting in lower section of horizontal pipeline
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Pitting in 316L heat exchanger tube due to stagnant cooling water
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AISI 316L thermosyphon pipeline of benzoic acid column phenol plant
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AISI 316L thermosyphon pipeline of benzoic acid column phenol plant
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Leakage in nozzle cyclohexanone reactor due to pitting in 316L cladding
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Pitting in 316L cladding of nozzle for agitator in cyclohexanone reactor
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Local overall corrosion in AISI 347 wall of precipitation reactor NP plant
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Pitting in 304L test coupon in FeCl3 environment
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Crevice Corrosion
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Crevice corrosion - initial stage
OH-O2
CI-
O2
M+
C1-
O2
OH-
O2
M+
OH+
O2
M+
M+
OH-
Na+
M+
Na+
Na+
C1-
O2
Na+
ee
e e e
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Crevice corrosion - later stage
OH-O2Na+
O2 OH-
C1-
O2
OH-
O2
OH-
O2
OH-
O2 C1-
C1-
C1-
C1-
M+
M+
M+
M+
M+
M+
C1-
C1-
C1-
O2 C1-
e
e
eee
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Differential aeration cell (U.R. Evans)
+
Electron flow
Air or oxygen
Three-way tap
Miliammeter
Porouspartition
Potassiumchloridesolution
Steel (Fe)cathode
Steel (Fe)anode
Anode: Fe Fe++ + 2e
Cathode: O2 + 2H2O + 4e 4OH-
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Water line attack, caused by a differential aeration cell
FeO2
CATHODE
O2
NaOH FORMED HERE
ANODE FeCl2 FORMED HERE
SEA WATER
AIR
O2
O2 + 2H2O + 4e 4OH-
Fe Fe++ + 2e
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Parameters influencing crevice corrosion
- Design
- Environmentrisk of depositpresence of halides; oxygen
- Alloy composition
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Crevice corrosion is usually attributed to one or more of
the following parameters:
- Lack of oxygen in the crevice
- Build-up of detrimental ion species in the crevice
- Changes in acidity (decrease of pH) in the crevice
- Depletion of an inhibitor in the crevice
Causes of crevice corrosion
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- Application of more resistant construction material
like high Ni / Mo alloys
- Make full penetration welds to avoid crevices
- Design vessels for complete drainage
- Weld (internal bore weld) instead of rolling in tubes in
tubesheets
- Inspect vessels and remove deposits frequently
- Remove wet packing materials during long shut
downs
- Use “solid” non absorbent gaskets, such as PTFE
Preventive measures
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Crevice corrosion at a stainless steel orifice
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Crevice corrosion in carbon steel bottom plate condensate tank due to presence of stainless steel disc.
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Crevice corrosion in 304L test coupon
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Filiform corrosion at a bonnet of a car
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n
Filiform corrosion at varnished steel light reflector
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Principle of filiform corrosion
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Intergranular Corrosion
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Grain boundary in a polycrystalline metal (two-dimensional representation)
Grain boundery
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Precipitations on grain boundaries;precipitates are inerte.g. Cr23 C6
Mo6C; W6C.
Depleted zone of components necessary for corrosion resistance.
Precipitates with anodic behaviour versus grains;e.g. Mg5Al8 and MgZn2 in Al-base alloys; Fe4N in iron base materials.
Precipitates withcathodic behaviour versus grains;e.g. CuAl2 in Al-base alloys;Fe3C in iron base alloys.
Anodic grain or grain boundary.
Precipitations affecting intergranular corrosion
1
2
3
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Chromium carbides on grain boundaries of austenitic stainless steel
20 m
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Chromium carbide precipitations on grain bounderies of austenitic stainless steel
50 m
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Concentration of Cr as function of distance from the grain boundery
18
16.4
12
10
Precipitate of(Cr.Fe)23C6
1.6%
criticalvalue
% Cr
grain boundery
distance fromgrain boundery
t = 0
t t = t1
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Weld decay zones
1300
850
450
Tem
per
atur
e °C
weld decay zones
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Weld decay zones
time, sec.
10 20 30 40 50 60
850
450
1250
1050
A
B
C
D
Tem
per
atur
e ºC
B
D
C
A
HAZ
HAZ
weld
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Intergranular attack in austenitic microstructure
125 m
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Intergranular attack in austenitic microstructure
100 m
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- Sensitizing temperature- Chemical analysis of stainless steel
- Carbon content- Titanium or niobium content- Chromium content- Nickel content- Molybdenum content- Silicon content- Nitrogen content
- Microstructure- Ferrite or austenite- Grain size
- Environment to which the sensitised steel is exposed
Aspects influencing intergranular corrosion
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TTC diagram according to Rocha; Influence of carbon content on susceptibility to intergranular corrosion of CrNi18-9
900
800
700
600
500
Sen
sitis
atio
n te
mpe
ratu
re in
ºC
10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5
Annealing time t in hrs
0.02
0.03
0.04
0.06
0.09
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Schematic chart showing solution and precipitation reactions in types 304, 321 and 347
Titanium / Niobium carbides dissolve
Chromium carbides dissolve
Titanium / Niobium carbides precipitate
Chromium carbides dissolve
Chromium carbides precipitate
No reactions
Melting point
°C
1250
850
450
50
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Tests to determine susceptibility to intergranularcorrosion of stainless steels
Test Environment Standard TestdurationIn hours
Criterion Affects
carbide -phase
Huey test 65% HNO3
boilingASTM A262practice CStac spec 53961
5 x 48h weight lossmicrosc. exam.
+ +
HNO3/HFtest
10% HNO3
3% HF 70°CASTM A262practice D
2 x 2h weight loss
+ -Strausstest
6% CuSO4
16% H2SO4 boilingCu–chips
ASTM A262practice E
24h 1 electric resistance2 bending3 noise
+ -
Streichertest
19% g/l Fe2(SO4)3
50% H2SO4 boilingASTM A262practice B
120h weight lossmicrosc. exam
+ -Oxalic acid test
10% oxalic acidroom temp.
ASTM A262practice A
1.50 min
1 A/cm3
anodic
etching
pattern + -
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Polarization diagrams for austenitic Cr Ni-steel in sulphuric acid
pote
ntia
l
E
a. Quench-annealed
b. Sensitized
Huey
Streicher
Strauss
Current Density logi
b
a
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- Decrease the carbon content to <0.03 or even <0.02%
- Annealing at 1050C and quenching
- Alloying with strong carbide formers; stabilizing with Ti (5 x C-content) or Nb (10 x C-content)
- Homogeneous annealing at 900C
Measures to prevent intergranular attack of stainless steels
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Weld decay in AISI 304 pipe material welded to a flange
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Weld decay in a 316 pipeline
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Intergranular corrosion in fork flange of level switch
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Intergranular corrosion in 316 fork flange of level switch
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Intergranular corrosion in fork flange of Mobrey level switch
150 m
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Pump impeller 304 cast, completely corroded intergranularly (environment HNO3)
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Knife-line attack in 347 plate material
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Influence of cold-deformation on intergranular corrosion in outer filament of AISI 304 plate material
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Test coupons with intergranular attack after Strauss test
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Strain induced intergranular cracking in AISI 316L urea grade liner material in top head reactor
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Strain induced intergranular cracking in 316L urea grade liner material in in top HP scrubber
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Microphoto of strain induced intergranular cracking
250 m
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Microphoto of strain induced intergranular cracking
100 m
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Microphoto of strain induced intergranular cracking
20 m
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Strain induced intergranular cracking in liner top urea reactor
10 mm
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linercracks
strip
vessel wall
liner
cracks cracks
cracks
strip
vessel wall
ground weld area
cracks
vessel wall
Preferential locations of stress induced intergranular cracking
liner
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Strain induced intergranular cracking in 304L material in top head of AN neutra reactor R6501 urea plant
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Intergranular corrosion in a Hasstelloy C spray nozzle in a saturator cooler
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Weld decay and knife-line attack in Hastelloy B plate material out of a SO2 separator
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Exfoliation corrosion in a bottom plate of an AIMg3 storage tank. The attack started from outside
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Microscopic view of exfoliation corrosion in AIMg3
150 m
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Exfoliation corrosion in AlMg3
100 m
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Exfoliation corrosion in AlMg3
200 m
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Exfoliation corrosion in AlMg3
50 m
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Selective corrosion / selective leaching
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Examples of selective attack
- Selective attack of specific phase in microstructure
- Selective attack of weld deposit material
- Dezincification of brass- layer type dezincification- plug type dezincification
- Graphitization of cast iron
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Selective attack of the pearlite phase in the ferrite-pearlite microstructure of carbon steel (HII)
100 m
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Selective attack of ferrite in a weld in austenitic 316L stainless
50 m
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Selective attack of weld deposit material in carbon steel petrol pipeline
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Potential curves at connection weldsP
oten
tial
Pot
entia
l
Pot
entia
l
Pot
entia
l
Pot
entia
l
Pot
entia
l
differentmaterials Weld decay zone High temperature
zone
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SMAW covered steel electrodes
acid basicrutile
Fe3O4 TiO2 CaF2
SiO2 CaCO3 SiO2 CaCO3 SiO2 CaCO3
MgCO3 MgCO3 MgCO3
- increase in sensitivity for moisture
- higher purity
- increase in mechanical properties
- increase in corrosion resistance
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SMAW covered steel electrodes
acid basicrutile
Fe3O4 TiO2 CaF2
SiO2 CaCO3 SiO2 CaCO3 SiO2 CaCO3
MgCO3 MgCO3 MgCO3
concave flush convex
- increase in contamination- improved executive weldability (appearance)
deep penetration
normal penetration
less penetration,increased risk for fatigue
Compromise: root run + filler layers: basic; cap layer: rutile
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Selective attack of weld deposit material in 304L material
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Selective attack of ferrite in ledeburitic cast chromium steel (CrMo30-2) pump material
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Layer type dezincification of a brass heat exchanger tube
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Layer type dezincification
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Plug type dezincification in a brass heat exchanger tube
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Plug type dezincification
350 m
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Dezincification in brass water tap
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Dezincification and SCC in brass
20 m
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Dezincification in brass
20 m
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Measures to prevent dezincification
- Reducing aggressiveness of environment by means of
changing the pH, chloride and oxygen removal
- Increasing velocity to avoid formation of deposits
- Softening the water can have favourable influence by
preventing scale formation
- Cathodic protection
- Changing alloy composition
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Measures to prevent dezincification (continued)
Changing the alloy:
- Decrease of Zn content: red brass (15% Zn) is almost immune to dezincification
- Addition of 1% tin (Sn) to a 70 – 30 brass (Admiralty Brass)
- Further improvement by addition of As, Sb or P as “inhibitors”. E.g.: Arsenic Admiralty Metal contains about 70% Cu, 29% Zn, 1% Sn and 0.04% As.
As is also added to aluminium (2% Al) brasses.
- For severe corrosive environments: cupro-nickels (70-90% Cu; 10-30%Ni)
NTT Consultancy
Graphitization in cast iron spray nozzle
NTT Consultancy
Graphitization in cast iron partition plate of waste water pump