14/15 Fall semester
Engineering Chemistry
Instructor: Rama Oktavian
Email: rama.oktavian86@gmail.com
Office Hr.: M.13-15, Tu. 13-15, W. 13-15, Th. 13-15, F. 09-11
Korosi
Korosi : Peristiwa
perusakan atau
degradasi
material logam
akibat bereaksi
secara kimia
dengan
lingkungan
Korosi
Contoh: Korosi dari logam yang disebabkan oleh
air pada umumnya merupakan hasil dari reaksi
kimia dimana permukaan logam teroksidasi
membentuk iron oxide (rust).
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 elektrolit yang
memberikan elektron.
Cathode, electrode pada sel elektrolit yang
menerima elektron
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 metal
Korosi
Korosi
• Two reactions are necessary:
2

-- oxidation reaction: Zn  Zn  2e (anodic reaction)
-- reduction reaction: 2H   2e   H 2 (gas) (cathodic reaction)
• Other reduction reactions:
-- in an acid solution
O2  4H  4e   2H2O
Zn + 2HCl → ZnCl2 + H2
-- in a neutral or base solution
O2  2H2O  4e  4(OH) 
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.
Korosi
• Two outcomes:
--Metal sample mass
--Metal sample mass
--Metal is the anode (-)
--Metal is the cathode (+)
o
Vmetal
 0 (relative to Pt)
Standard Electrode Potential
o
Vmetal
 0 (relative to Pt)
Korosi
• EMF series
metal
Au
Cu
Pb
Sn
Ni
Co
Cd
Fe
Cr
Zn
Al
Mg
Na
K
• Metal with smaller
o
Vmetal
+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
o
Vmetal
(i.e., more active)
corrodes.Ex: Cd-Ni cell
o
DV =
0.153V
EMF: Electromotive Force
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:
Korosi
Rusting of iron
The oxidation reaction that
will occur at the local
anodes is
Fe → Fe2+ + 2e- (anodic
reaction)
The reduction reaction that will
occur at the local cathodes is
A piece of iron immersed in
oxygenated water, ferric hyroxide
[Fe(OH)3] will form on its surface
O2 + 2H2O + 4e- → 4OH(cathodic reaction)
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 Alloys
Cadmium
Zinc
Magnesium
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.
Mg
25°C
Fe
a)Which electrode is the anode?
1.0 M
1.0 M
b)Which electrode corrodes?
Mg+ 2 solution Fe 2+ solution
c)In which direction will the electrons flow?
d)In which direction will the anions in the solution move?
e)In which direction will the cations in the solution move?
f)Write an equation for the half-cell reaction at the anode.
g)Write an equation for the half-cell reaction at the cathode.
Korosi
a) The magnesium electrode is the anode.
b) The magnesium electrode corrodes since the
anode in a galvanic cell corrodes (oxidizes).
c) The electrons will flow from the anode, Mg, to the
cathode, Fe.
d) The SO42− anions will flow toward the magnesium
anode.
e) The cations will flow toward the iron cathode.
f) The oxidation reaction that occurs at the
magnesium anode is: Mg → Mg2++2e−
g) The reduction reaction that occurs at the iron
anode is: Fe2++2e−→ Fe .
Korosi
Tipe – tipe korosi:
1)
2)
3)
4)
5)
6)
7)
8)
Uniform or general attack corrosion
Galvanic or two-metal corrosion
Pitting corrosion
Crevice corrosion
Intergranular corrosion
Stress corrosion
Erosion corrosion
Selective leaching or dealloying
Korosi
1. Uniform/general attack (korosi umum)
⇒ Korosi yang umum terjadi pada baja.
⇒ Akibat adanya reaksi kimia atau elektro kimia yang merata
pada permukaan logam
Pencegahannya :
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.
Korosi
1. Uniform/general attack (korosi umum)
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 : katodik
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.
Korosi
4. Pitting corrosion (korosi sumur)
Serangan korosi mengakibatkan
terjadinya lubang-lubang pada logam.
Diameter lubang relatif kecil.
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.
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 embrittlement
Korosi
PENCEGAHAN KOROSI
Korosi dapat dikontrol dengan berbagai cara. Pertimbangan
utama adalah masalah ekonomi, terutama akibat yang
ditimbulkannya .
Kontrol terhadap korosi
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 katodik
dan anodik
Kontrol lingkungan.
(temperatur,
konsentrasi oksigen
dll).
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.
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 &
lacquers
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.
Cathodic protection
 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 pipeline by using
sacrificial Mg anode
Cathodic protection of an
underground tank by using impressed
currents