1415 Fall semester Instructor Rama Oktavian Email ramaoktavian86gmailcom Office Hr M1315 Tu 1315 W 1315 Th 1315 F 0911 Korosi Korosi Peristiwa perusakan atau ID: 249849
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Slide1
Engineering Chemistry
14/15 Fall semester
Instructor: Rama
Oktavian
Email: rama.oktavian86@gmail.com
Office Hr.:
M.13-15, Tu. 13-15, W. 13-15, Th. 13-15, F. 09-11Slide2
Korosi
Korosi
:
Peristiwa
perusakan
atau
degradasi
material
logam
akibat
bereaksi
secara
kimia
dengan
lingkunganSlide3
Korosi
Contoh
:
Korosi
dari
logam
yang
disebabkan
oleh
air
pada
umumnya
merupakan
hasil
dari
reaksi
kimia
dimana
permukaan
logam
teroksidasi
membentuk
iron oxide (rust).Slide4
Korosi
Korosi
merupakan
proses
oksidasi
yang
terjadi
pada
suhu
yang
rendah
Korosi
merupakan
proses
elektrokimia
Adanya
kontak
secara
kimia
dan
elektrik
antara
Anode
, electrode
pada
sel
elektroli
t
yang
memberikan
elektron
.
Cathode
, electrode
pada
sel
elektrolit
yang
menerima
elektron
Slide5
Korosi
•
Korosi
dapat
terjadi
karena
:
--
Karakteristik
dari
logam
yang
memberikan elektron. -- Adanya reaksi oksidasi dan reduksi yang disebakan oleh electron tersebut.
•
Laju
korosi
dipengaruhi
oleh
:
--
Suhu
--
Konsentrasi
dari
reaktan
dan
produk
-- mechanical stress & erosion
-- the energy state of the metalSlide6
KorosiSlide7
Korosi
• Two reactions are necessary:
--
oxidation
reaction:
--
reduction
reaction:
• Other
reduction
reactions:
-- in an acid solution
-- in a neutral or base solution
Zn + 2HCl → ZnCl
2
+ H
2Slide8
Korosi
Galvanic couple:
Two metals electrically connected in a liquid electrolyte wherein one metal becomes an
anode and corrodes
, while the other acts as a cathode.Slide9
Korosi
• Two outcomes:
--
Metal
sample mass
--
Metal
sample mass
--Metal is the anode (-)
--Metal is the cathode (+)
(relative to Pt)
(relative to Pt)
Standard Electrode PotentialSlide10
Korosi
•
EMF
series
•
Metal with smaller
V (i.e., more active)
corrodes.
Ex: Cd-Ni cell
metal
o
Au
Cu
Pb
Sn
Ni
Co
Cd
Fe
Cr
Zn
Al
Mg
Na
K
+1.420 V
+0.340
- 0.126
- 0.136
- 0.250
- 0.277
- 0.403
- 0.440
- 0.744
- 0.763- 1.662
- 2.262- 2.714- 2.924
metal
V
metal
o
DV =
0.153V
o
EMF: Electromotive ForceSlide11
Korosi
The EMF of a standard Ni-Cd galvanic cell is -0.153 V. If the standard half-cell EMF for the oxidation of Ni is -0.250 V, what is the standard half-cell EMF of cadmium if cadmium is the anode?
Solution:
The standard half-cell EMF of the cadmium can be calculated by considering the half-cell reactions:Slide12
The oxidation reaction that will occur at the local anodes is
Fe → Fe
2+
+ 2e
-
(anodic reaction)
Rusting of iron
A piece of iron immersed in oxygenated water, ferric hyroxide [Fe(OH)
3
] will form on its surface
The reduction reaction that will occur at the local cathodes is
O
2
+ 2H
2
O + 4e
-
→ 4OH
-
(cathodic reaction)
KorosiSlide13
Korosi
• Ranks the reactivity of metals/alloys in
seawater
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel
Iron/Steel
Aluminum AlloysCadmiumZincMagnesiumSlide14
Korosi
Consider a magnesium-iron galvanic cell consisting of a magnesium electrode in a solution of 1
M
MgSO4 and an iron electrode in a solution of 1
M
FeSO4. Each electrode and its electrolyte are separated by a porous wall, and the whole cell is at 25°C. Both electrodes are connected with a copper wire.
Which electrode is the anode?
Which electrode corrodes?
In which direction will the electrons flow?
In which direction will the anions in the solution move?
In which direction will the
cations
in the solution move?
Write an equation for the half-cell reaction at the anode.
Write an equation for the half-cell reaction at the cathode.
Fe
1.0 M
Fe
2+
solution
1.0 M
Mg
2
+
solution
Mg
25°CSlide15
Korosi
The magnesium electrode is the anode.
The magnesium electrode corrodes since the anode in a galvanic cell corrodes (oxidizes).
The electrons will flow from the anode, Mg, to the cathode, Fe.
The SO
4
2−
anions will flow toward the magnesium anode.
The cations will flow toward the iron cathode.
The oxidation reaction that occurs at the magnesium anode is: Mg → Mg
2+
+2
e
−
The reduction reaction that occurs at the iron anode is: Fe
2+
+2e−→ Fe .Slide16
Korosi
Tipe – tipe
korosi
:
Uniform or general attack corrosion
Galvanic or two-metal corrosion
Pitting corrosion
Crevice corrosion
Intergranular
corrosion
Stress corrosion
Erosion corrosion
Selective leaching or
dealloyingSlide17
Korosi
1.
Uniform/general attack
(
korosi
umum
)
Korosi
yang
umum
terjadi
pada
baja. Akibat adanya reaksi kimia atau elektro kimia yang merata pada permukaan logamPencegahannya
:
a.
penggunaan
material yang
tepat
dan
pelapisan
(
coating
),
b
. inhibitor (suatu zat yang ditambahkan dalam
konsentrasi yang kecil ke lingkungan untuk menurunkan laju korosi
), c. proteksi katodik. Slide18
Korosi
1.
Uniform/general attack
(
korosi
umum
)Slide19
Korosi
2.
Galvanic corrosion
(korosi galvanis)
Terjadi akibat adanya beda potensial antara dua benda yang terhubung secara elektrolit.
Logam yang ketahanan korosinya kurang:
anodik
logam yang ketahanan korosinya tinggi :
katodikSlide20
Korosi
3.
Crevice corrosion
(
korosi
celah
)
Korosi
lokal
yang
sering
terjadi
pada celah atau daerah yang dilindungi.Penyebabnya: adanya larutan yang terjebak pada lubang dari permukaan gasket, lap joint, atau
kotoran
yang
terjebak
dibawah
baut
atau
pada
kepala rivet.Pencegahannya: sangat sulit. Cara lain dengan mengkondisikan
dalam kelembaban yang rendah (low humidity)
Daerah yang
kemungkinan
terkena
korosi.Slide21
Korosi
4.
Pitting corrosion
(
korosi
sumur
)
Serangan
korosi
mengakibatkan
terjadinya
lubang-lubang pada logam. Diameter lubang relatif kecil. Slide22
Korosi
5.
Erosion corrosion
(
korosi
erosi
)
Proses
korosi
yang
dipercepat
dengan
adanya
gesekan
antara
fluida
korosif
dengan
permukaan
logam
.
Pencegahannya
: Pelapisan (coating) dengan permukaan
yang keras. Proteksi
katodik.Slide23
Korosi
6.
Stress corrosion
(
korosi
tegangan
)
Sering
juga
disebut
stress corrosion cracking
,
yaitu retak (crack) yang disebabkan oleh tegangan tarik (tensile stress) dan korosi yang spesifik. Contoh : hydrogen embrittlementSlide24
Korosi
Kontrol terhadap korosi
PENCEGAHAN KOROSI
Korosi dapat dikontrol dengan berbagai cara. Pertimbangan utama adalah masalah ekonomi, terutama akibat yang ditimbulkannya .
Pemilihan material:
Logam
Non logam
Coating (pelapisan):
Logam
Organik
Non organik
Disain:
menghindari konsentrasi tegangan
menghindari kontak dengan logam tidak sejenis
menghindari adanya jebakan air
Proteksi
k
a
todik
dan
anodik
Kontrol lingkungan.
(temperatur, konsentrasi oksigen dll).Slide25
Korosi
Metallic Selection
General rules for metallic selection
For reducing or, nonoxidizing conditions such as air-free acids & aqueous solutions, Ni & Cu alloy are often used.
For oxidizing conditons, Cr containing alloy are used.
For extremely powerful oxidizing conditions, Ti & its alloy are commonly used.Slide26
Korosi
Coatings
Metallic, inorganic, & organic coatings are applied to metals to prevent or reduce corrosion
Metallic coating eg.; Zinc coating on steel (sacrificial anode)
Inorganic coating (ceramics & glass) eg.; Glass-lined steel vessels
Organic coating eg.; paints, varnishes & lacquersSlide27
Korosi
Alteration of Environment
4 general methods of altering an environment to prevent or reduce corrosion are:
Lower the system temperature to lower the reaction rates and thus reduce corrosion. Certain exceptions exist, such as seawater, for which the temperature should be raised rather than reduced.
Decrease the velocity of corrosive fluids such that erosion corrosion is reduced while fluid stagnation is avoided.
Remove oxygen from water solutions.
Reduce the concentration of corrosive ions in a solution which is corroding a metal.Slide28
The protection of a metal by connecting it to a sacrificial anode or by impressing a DC voltage to make it cathode
Eg;
sacrificial anode
Impressed current
Cathodic protection of an underground tank by using impressed currents
Cathodic protection of an underground pipeline by using sacrificial Mg anode
Cathodic
protection