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 i
crituntuk 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 E
ppdan i
critakan menentukan apakah logam/paduan
tsb akan secara spontan menjadi pasif atau tidak.
• Rapat arus katodik total pada E
ppharus sama atau lebih besar dari i
crituntuk 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 E
ppis superior.
b) If the reduction process is diffusion
controlled, a metal or alloy having a small i
critwill 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 i
critatau membuat E
pplebih aktif. Kurva anodik bisa berubah dengan cara alloying (untuk menurunkan i
crit). 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 (i
o) 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 i
criticaland E
passivethan iron, reduce the i
critical(critical current density) of iron. Addition of up to 18%
chromium reduces i
criticaliron.
• Logam dengan E
ppaktif 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 i
critto 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.