Engineering Chemistry

14/15 Fall semester

Instructor: Rama OktavianEmail: rama.oktavian86@gmail.comOffice 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 elektrokimiaAdanya kontak secara kimia dan elektrik

antaraAnode, 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: -- oxidation reaction: -- reduction reaction:

reaction) (anodic2eZnZn 2 reaction) (cathodic (gas)H2e2H 2

• Other reduction reactions:-- in an acid solution -- in a neutral or base solution

O2 4H 4e 2H2O O2 2H2O 4e 4(OH)

Zinc

oxidation reactionZn Zn2+

2e-Acid solution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e- in the metal

Zn + 2HCl → ZnCl2 + H2

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

Plati

num

meta

l, M

Mn+ ions

ne- H2(gas)

25°C 1M Mn+ sol’n 1M H+ sol’n

2e-

e- e-

H+

H+

--Metal is the anode (-) --Metal is the cathode (+)

Vmetalo 0 (relative to Pt) Vmetal

o 0 (relative to Pt)

Standard Electrode Potential

Mn+ ions

ne-

e- e-

25°C 1M Mn+ sol’n 1M H+ sol’n

Plati

num

meta

l, M

H+ H+

2e-

Korosi• EMF series • Metal with smaller

V (i.e., more active) corrodes.Ex: Cd-Ni cell

metalo

-

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

Cd 25°C

more

anodic

more

cath

odic Au

CuPbSnNiCoCdFeCrZnAlMgNaK

+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 Vmetalo

DV = 0.153V

o

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:

The oxidation reaction that will occur at the local

anodes is

Fe → Fe2+ + 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

O2 + 2H2O + 4e- → 4OH-

(cathodic reaction)

Korosi

Korosi

• Ranks the reactivity of metals/alloys in seawater

more

anodic

(a

ctiv

e)

more

cath

odic

(i

nert

)

PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium

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.

a)Which electrode is the anode?b)Which electrode corrodes?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.

Fe

1.0 M Fe2+ solution

1.0 M Mg 2+ solution

Mg 25°C

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 SO4

2− 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) Uniform or general attack corrosion2) Galvanic or two-metal corrosion3) Pitting corrosion4) Crevice corrosion5) Intergranular corrosion6) Stress corrosion7) Erosion corrosion8) 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

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 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

KorosiAlteration 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.

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