Pasivasi

Asep Ridwan Setiawan

Pendahuluan

• Pada diagram Eh-pH, ketahanan logam thd korosi ditunjukkan oleh daerah

dimana logam imun (stabil secara termodinamika) atau permukaan logam tsb dilindungi oleh lapisan oksida (pasif).

• Pasivasi disebabkan karena pembentukan lapisan film tipis, impermeable dan melekat dipermukaan logam pada kondisi oksidasi, karena polarisasi anodik.

• Hanya logam/paduan tertentu saja yg mengalami perilaku aktif-pasif.

• Fe terkorosi cepat di asam nitrat encer, tapi tidak terserang pada asam nitrat pekat. Lapisan film oksida yg terbentuk pada asam pekat ditemukan tidak stabil pada larutan asam nitrat encer.

Definisi Pasivasi menurut uhlig

1. Logam aktif pada EMF series atau logam tergolong pasif ketika perilaku elektrokimianya menjadi logam mulia atau kurang aktif.

cth : Cr, Ni, Ti, Zr, dan Baja tahan karat

2, logam atau paduan menjadi pasif, kalau tahan thd korosi pada lingkungan apapun.

cth: Pb di H2So4, Mg di H2O, Fe di HNO3.

2 Jenis passivasi

• a) Logam itu pasif kalau tahan thd korosi dalam kondisi terpolarisasi anodik (noble potential, low corrosion rate).

• b) Logam itu pasif kalau tahan thd korosi meskipun kondisi termodinamika nya memungkinkan utk terkorosi (active potential, low corrosion rate).

Istilah yg digunakan

1. Equilibrium potential (Eeq or EM/Mz+). The potential of an electrode in an electrolyte when the forward rate of reaction is balanced by the rate of reverse reaction (Mz+ + ze M). It can be defined only with respect to a specific electrochemical reaction. This is also written as E◦ and must not be confused with Ecorr.

2. Passive potential (Epassive). The potential of an electrode where a change from an active to a passive state occurs.

3. Flade potential (EF). The potential at which a metal changes from a passive state to an active state.

4. Transpassive potential (Etranspassive). The potential corresponding to the end of passive region which corresponds to the initial point of anodic evolution of oxygen. This may correspond either to the breakdown (electrolysis) voltage of water, or, to the pitting potential.

5. Critical current density (icritical). The maximum current density observed in the active region for a metal or alloy that exhibits an active–passive behavior.

6. Passive current density (ip). The minimum current density required to maintain the thickness of the film in the passive range.

7. Pitting potential (Ep). It is the potential at which there is a sudden increase in the current density due to breakdown of passive film on the metal surface in the anodic region.

Electrochemical basis of active-passive behavior

• Dengan naiknya potensial diatas daerah pasif, maka film pasif akan rusak, arus korosi anodik akan naik terus pada daerah transpasif. Reaksi evolusi oksigen di anoda akan terjadi pada potensial yg lebih tinggi.

• Berdasarkan kurva polarisasi anodik tsb, kita bisa menentukan :

a) Passive potential region.

b) Passive corrosion rate and

c) Necessary conditions to achieve and maintain passivity.

• Kenaikan temperatur dan konsentrasi ion hidrogen (kadar asam tinggi) akan meningkatkan icrit untuk pasivasi. Sedangkan keberadaan ion Cl akan merusak lapisan pasif pada logam.

Pengaruh proses katodik (activation controlled) thd kestabilan pasivasi

3 Kasus yg mungkin ditemui : 1) Only one stable potential at M where the mixed potential theory is satisfied. • High Corrosion rate at M. Eg:- Fe in dil H2SO4, Ti in dil

H2SO4/ HCl. 2) Three points of intersection R, P and N where rate of oxidation is equal to rate of reduction. Point P is not in stable state. Only N and R are stable. • N in active region (high corrosion rate) and R in passive

state (lowest corrosion rate). This system may exist in either active or passive state. Eg:- Cr in dil HCl or H2SO4. Stainless steel in H2SO4 (containing oxidizers).

3) The most desirable condition-spontaneous passivation - Only stable potential S in the passive region. • Eg:- Cr – noble metal alloys in H2SO4 or HCl. Ti – noble

metal alloys in dil H2SO4. 18 – 8 stainless steel in acid (containing Fe+++, O2)

• Pencapaian Kondisi 3 sangat di inginkan untuk pengembangan logam paduan yang tahan korosi.

• Posisi arus max (nose) pada kurva anodik sangat penting. Pasivasi spontan hanya terjadi kalau kurva reaksi katodik tidak menyentuh/mengenai hidung (nose) dari kurva anodik.

• Untuk kurva katodik (reaksi reduksi) tsb, nilai Epp dan icrit akan menentukan apakah logam/paduan tsb akan secara spontan menjadi pasif atau tidak.

• Rapat arus katodik total pada Epp harus sama atau lebih besar dari icrit untuk mencapai pasivasi spontan.

• Kriteria ini bisa dituliskan dalam istilah passivity index (PI), yaitu :

• Untuk PI ≥ 1, Pasivasi secara spontan terjadi. Untuk PI < 1, Pasivasi tdk terjadi secara spontan, meskipun pada kondisi (2) ini, daerah pasif yg stabil ada.

A comparison of the behavior of two active-passive alloys under an activation controlled cathodic system.

Paduan A terkorosi di X, sedangkan paduan B secara spontan mengalami pasivasi di Y.

The above two alloys are exposed to a cathodic process under complete diffusion control.

Paduan A secara spontan mengalami pasivasi di X, sedangkan paduan B mengalami 2 keadaan stabil, yaitu aktif pada Q dan pasif pada Y.

Kesimpulan

Two significant factors emerge out of the above observations.

a) To achieve passive behavior where cathodic reduction is activation controlled, a metal or alloy with an active Epp is superior.

b) If the reduction process is diffusion controlled, a metal or alloy having a small icrit will passivate faster.

Desain paduan tahan korosi

• Untuk mengembangkan logam paduan yg tahan korosi melalui kriteria pasivasi, bisa dilakukan 2 pendekatan dibawah ini:

• a) Meningkatkan kemudahan pasivasi dengan mengurangi icrit atau membuat Epp lebih aktif. Kurva anodik bisa berubah dengan cara alloying (untuk menurunkan icrit). Contoh : Titanium, Chromium – alloying additions, molybdenum, nickel tantalum dan columbium.

• b) Meningkatkan laju reaksi reduksi katodik. Ini dilakukan dengan alloying dengan logam mulia yg memiliki rapat arus pertukaran (io) untuk reaksi reduksi.

If corrosion is controlled by an activation control reduction process IAC , an alloy which exhibits a very active primary potential must be selected. Conversely, if the reduction process is under diffusion control an alloy with a smaller critical current density must be selected.

Elements, like chromium and nickel, which have a lower icritical and Epassive than iron, reduce the icritical (critical current density) of iron. Addition of up to 18% chromium reduces icritical iron.

• Logam dengan Epp aktif seperti titanium dan chromium, atau paduan yg mengandung logam yg memiliki rapat arus pertukaran tinggi untuk reduksi hidrogen akan mudah mengalami pasivasi secara spontan.

• Pengaruh dari unsur paduan pada ketahanan korosi titanium

Kondisi yg hrs diperhatikan untuk menjaga pasivasi dari suatu logam/paduan

• Corrosion rate is proportional to anodic current density in the active state irrespective of whether the alloy is passive type or not.

• Rate of cathodic reduction must exceed icrit to ensure lower corrosion rates.

• Border line passivity to be avoided. • Avoid breakdown of passive films in oxidizing

environments due to transpassivity. • Stable passive state in oxidizing conditions is

essential

Pengaruh ion Cl thd pasivasi

• Chloride ions breakdown passivity or even at times prevent passivation of Fe, Cr, Ni, Co and stainless steels.

• They can penetrate oxide films through pores and influence exchange current density (overvoltage).

• Breakdown of passivity by chloride ions is local and leads to pitting corrosion.

• Chloride ions break down the passivity and increase the rate of anodic dissolution.

Detrimental role of chloride concentrations and temperature on the passive region and critical anodic current density

• An increase in temperature generally decreases the passive range and increases the critical current density (icritical).

• An increase of temperature decreases polarization and enhances the dissolution kinetics.

Proteksi Anodik

• Anodic protection refers to prevention of corrosion through impressed anodic current.

• This method of protection tested and demonstrated by Edeleanu in 1954 however can be applied only to metals and alloys that exhibit active-passive behavior.

• The interface potential of the structure is increased to passive domain

If an active-passive alloy such as stainless steel is maintained in the passive region through an applied potential (or current) from a potentiostat, its initial corrosion rate (icorr) can be shifted to a low value at ipass

As per mixed-potential theory, Applied anodic current density = oxidation current density – reduction current density.

• Anodic protection is more effective in acid solutions than cathodic protection. Current requirements for cathodic protection in acid solutions are several orders of magnitude higher than that necessary for complete anodic protection. Cathodic protection currents in acid solution can also lead to hydrogen liberation and embrittlement of steels.

• Anodic protection unlike cathodic protection is ideally suited for protection of active-passive alloys in aggressive environments such as high acidity and corrosive chemicals. Hence anodic protection is the most preferred choice for protection of chemical process equipment.

Anodic protection parameters

• a) Protection range – range of potentials in which the metal/alloy exhibits stable passivity.

• b) Critical anodic current density. • c) Flade potential. • Potential corresponding to middle of the passive region can

be taken as optimum for anodic protection. While choosing the desirable protection potential, an assessment of the aggressiveness of the environment need be made. Since chloride ions are detrimental to passivity, higher chloride concentrations can decrease the protection range. Metals and alloys having relatively larger pitting and protection potentials can only be chosen for very aggressive chemical environments. Higher temperatures can deleteriously influence the protection potential.

Anodic protection of inner surface of a steel acid storage tank is shown

Katoda utk Proteksi Anodik

• Inert cathode materials having large surface area preferred.

• Recommended cathode materials for acid and corrosive industrial liquids include platinum-clad brass, chromium-nickel steel, silicon cast iron, copper, Hastelloy C and nickel-plated steel.

• Various types of reference electrodes such as Calomel, Ag/AgCl, Hg/HgSO4 and platinum are used depending on the chemical environment.

Perbandingan Proteksi Katodik vs Anodik