Proteksi katodik

Korosi

Prinsip

• Menurunkan potensial sampai daerah imun

• dengan menghubungkan logam dengan logam lain yang potensial reduksinya lebih negatif anoda korban (SA)

• Dengan mengalirkan arus arus terpasang (IC)

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Prinsip

• Proteksi katodik adalah suatu metode perlindungan korosi pada permukaan logam dengan membuat katoda dengan logam lain (anoda korban) atau suatu rangkaian listrik

• Proteksi katodik digunakan untuk melindungi pipa dalam tanah, tangki penyimpanan, kapal, pipa minyak di lepas pantai, dan lain-lain.

Aplikasi proteksi katodik • Petroleum & Petrochemical: underground piping and storage tanks,

above ground storage tank bottoms, internal surfaces of water storage tanks, heat exchangers and storage well casings

• Marine: ships, barges, buoys, steel or reinforced concrete dock structures, offshore pipelines, offshore drilling and production platforms

• Pulp & Paper: effluent clarifiers, underground piping, aboveground storage tanks bottoms, foundation pilings, watermains and effluent discharge piping

• Reinforced concrete structures: bridges, parking garages and foundations

• Municipal: foundation pilings, iron and steel watermains, concrete pressure pipes, sewage treatment clarifiers, sewage pump stations

• Electrical Power Industry :cooling water pipelines & intakes, grounding systems, tower footings, penstocks, condensers.

Kriteria Proteksi

• 850 mV terhadap proteksi katodik yang diaplikasikan,

• -850 mV potensial polarisasi terhadap CSE,• polarisasi minimum 100 mV.

Proteksi katodik Anoda Korban

Pk Anoda korban

http://knol.google.com/k/-/-/22z3waum2dn6b/nulnal/cathprotf1.gif

Prinsip Proteksi katodik Anoda korban

• Menghubungkan logam yang diproteksi dengan logam yang dilindungi

• Logam yang dikorbankan mempunyai potensial elektroda lebih negatif

• Tidak memperlukan arus dari luar

Anoda korban E Zn 2+/Zn Zn 2+ + 2e = Zn - 0,763

2H2O + 2e = H2 + 2OH- - 0,828

E Al3+/Al Al +3 +3 e = Al -1,662

E Mg2+/Mg Mg 2+ + 2e = Mg - 2,363

Diagram Polarisasi

Diagram polarisasi seng (Zn)

Persyaratan

• Mampu menurunkan potensial logam yang diproteksi ke daerah imun

• Memerlukan biaya murah• Mampu dibentuk sesuai dengan ukuran yang

diinginkan• Mengalami korosi merata

Tipe anoda • Typical materials for sacrifical anodes are Magnesium (for soil) and

Aluminium/Zinc /Galvalum / Indium alloys (for seawater or seabeds)• Magnesium anodes bervariasi bentuk dan ukuran dengan berat 17 Anoda Mg

untuk resistivitas tanah 1,000 ohm-cm and 5,000 ohm-cm. Short chunky shapes are suitable for low resistivity areas, but long slender shapes should be employed in Higher resistivity areas

• Zinc anodes are also available in many shapes and sizes. They are appropriate in soils with very low resistivities (750 ohm-cm to 1500 ohm-cm). Favorable environments are sea water and salt marshes. Short chunky shapes are suitable for low resistivity areas, but long slender shapes should be employed in higher resistivity areas

• Aluminum anodes are not commonly used in earth burial applications. Some proprietary aluminum alloy anodes work well in a sea water environment

Hub Anoda Korban & Resistivitas

• Jenis anoda Resistivitas (OhmCm)Al < 150 Zn 150-500Mg >500

Sifat Anoda

Jenis anoda Mg Zn Al1.Masa jenis 1,7 7,5 2,72.E (V/SCE) 1-1,7 1,05 1,103.Teg.dorong,V 0.6-0.8 0,25 0,254. Kapasitas (AH/Kg) 1200 780 27005.Efisiensi (%) 50 95 95

Backfill

• Kantung pembungkus anoda yang berisigypsum 75%bentonit 20%natrium sulfat 5%

Resistivitas anoda yang dihasilkan 50 Ohm-Cm

Fungsi Backfill

• Memberikan lingkungan yang merata arus keluaran tetap

• Menurunkan resistivitas dari fasa anoda dengan tanah

• Mencegah kontak langsung antara anoda dengan tanah

Keuntungan

• Self-powered so no external power source is required.

• Easy field installation.• Low maintenance requirement.• Less likely to cause stray current interference

problems on other structures.• When the current requirement is small, a

galvanic system is more economical than an impressed current system.

Kelemahan

• Low driving voltage.• Limited to use in low resistivity soils.• Low maintenance requirement.• Not an economical source of large amounts of

CP current.• Very Little capacity to control stray current

effects on the protected structure.

Contoh

Pemasangan anoda

Pemantauan

Pemantauan

Proteksi katodik arus terpasang

• Untuk struktur (bangunan) yang lebih besar, anoda korban tidak dapat secara ekonomis mengalirkan arus yang cukup untuk melakukan perlindungan yang menyeluruh.

• Sistem Impressed Current Cathodic Protection menggunakan anoda yang dihubungkan dengan sumber arus searah (DC) yang dinamakan cathodic protection rectifier.

Arus terpasang

Peralatan

• Rectifier (current supply)• Counter electrode• Reference electrode

Instalasi

Reaksi

• Carbon steel and stainless steel (depending on the temperature) exposed to seawater will suffer from corrosion.

• The following reactions will occur on the surface :

• Anodic reaction: Fe → Fe 2+ + 2e-

• Cathodic reactions: O2 + 2H2O + 4e- → 4OH-

2H+ + 2e- → H2(g) eq

PK arus terpasang

Polarisasi dalam air laut

Overvoltage diagram for steel in seawater with protection current IP included

Prinsip

Keuntungan

• Flexibility• Applicable to a variety of applications• Current output may be controlled• Not constrained by low driving voltage• Effective in high resistivity soils

Kelemahan

• Increased maintenance• Higher operating costs• May cause interference on other structures

Tipe anoda

• Anodes for ICCP systems may be in cylindrical, rod shaped, wire, tubular or ribbon form. Materials may be Silicon-Cast Iron, Mixed Metal Oxide, Graphite, Platinum or Titanium coated alloys. Silicon-Cast Iron anodes are the most economical, but also crack easily.

Jenis anoda : Stand off, Flush mounted, Bracelet

Type anoda • Scrap iron is sometimes used as an anode simply because it is available. Non-uniform

consumption, high rate of consumption, and discoloration of surrounding structures are distinct disadvantages.

• Graphite anodes are one of the most commonly used anodes for impressed current systems. Most common applications are to protect underground structures. Graphite anodes are suitable for deep, shallow vertical, or horizontal ground beds with carbonaceous backfill.

• High Silicon Cast Iron anodes are widely used in underground applications in both shallow and deep ground beds. Specially formulated high silicon cast iron anodes are also used in seawater. Although the performance is Improved with coke breez , its use is no critical

• Platinized Titanium anodes take advantage of the low consumption rate and high current density. Voltages in excess of 10 Volts will result in severe pitting of the titanium core causing premature failure.

• Platinized Niobium/Tantalum anodes also take advantage of the properties of platinum, but avoid the low driving voltage restriction of platinized titanium anodes. Breakdown of the niobium oxide film occurs at approximately 120 Volts. Thus these anodes are used where high driving voltage is required.

• Magnetite anodes are quite expensive but have an extremely long life. They are therefore an economical choice for some applications.

• Mixed Metal Oxide anodes consist of a high purity titanium substrate with an applied coating consisting of a mixture of oxides. The titanium serves as a support for the oxide coating. The mixed metal oxide is a crystalline, electrically-conductive coating that activates the titanium and enables it to function as an anode. When applied on titanium, the coating has an extremely low consumption rate, measured in terms of milligrams per year. As a result of this low consumption rate, the tubular dimensions remain nearly constant during the design life of the anode - providing a consistently low resistance anode.

Karakteristik anoda

Instalasi

• Tipe sistem ICCP yang umum untuk jalur pipa terdiri dari rectifier bertenaga arus bolak-balok (AC) dengan output arus DC maksimum antara 10 - 50 ampere dan 50 volt. Terminal positif dari output DC tersebut dihubungkan melalui kabel ke anoda-anoda yang ditanam di dalam tanah. Banyak aplikasi menanam anoda hingga kedalaman 60 m (200 kaki) dengan diameter lubang 25 cm (10 inchi) serta ditimbun dengan conductive coke (material yang dapat meningkatkan performa dan umur dari anoda).

Pemasangan anoda (ground Bed)

Karbon yang ditanam, dan dipadatkan di sekitar anoda, mempunyai dua fungsi:

• menjadi sebuah resistivitas yang sangat rendah, dan memiliki efek meningkatkan ukuran anoda dengan mengakibatkan penurunan resistansi terhadap tanah

• sebagian besar akan diteruskan ke backfill dari anoda melalui kontak langsung sehingga sebagian besar konsumsi material akan terjadi di tepi luar kolom backfill.

Kebutuhan arus proteksi

• Kebutuhan arus proteksi pada pipa mengacu pada densitas arus proteksi yang diperlukan, yakni pipa tanpa proteksi 20 mA/m2, pipa dengan cat/coating 2 mA/m2, pipa dengan cat/coating ditambah dengan isolasi 0,5 mA/m2.

IP = AP x i• IP = Arus proteksi untuk melindungi pipa

(ampere)• AP = Luas permukaan yang akan dilindungi (m2)• i = Densitas arus proteksi yang diperlukan

(mA / m2)20 mA / m2 untuk pipa tidak dilapisi2 mA / m2 untuk pipa yang dicoating

0,5 mA / m2 untuk pipa yang dicoating dan diisolasi

Pemantauan

• Elektroda acuan CSE

Multi meter

Tugas

• Hitung efisiensi anoda Mg dan Zn• Berdasarkan praktikum PK 1 dan 2 buatlah analisa dari hasil

percobaan bagamana kondisi pipa, anoda, isolasi , dan jembatan

• Jelaskan prinsip perlindungan korosi metode proteksi katodik • Jelaskan perbedaan PK anoda korban dan arus terpasang• Sebutkan isi backfill dan apa fungsinya• Hitung kebutuhan arus proteksi dan anoda korban untuk pipa

dengan luas 8 km2 jika rapaat arus nya 0,6 mA/m2.• Jelaskan persyaratan logam yang dapat digunakan sebagai

anoda korban.• Gambarkan dengan diagram E Vs log (i) PK anoda korban dan

arus terpasang!