Question

1. Explain Ecorr, Icorr, Epass, Ipass and Epit (on the potentiodynamic curves).

2. Compare the Control Ti and Coated Ti. Explain. Does coating show improvement in corrosion resistance?

Answer 1

Answer:

(1) Ecorr:

Ecorr represents electrochemical corrosion potential (ECP) in equations as a voltage difference between a metal immersed in a given environment and an appropriate standard reference electrode (SRE), or an electrode which has a stable and defined electrode potential. Metals or alloys based on their corrosion resistance is represented by ECP. It is also known as freely corroding potential, rest potential or open circuit potential.

Various metals or alloys when placed in an electrolyte corrode at different corrosion rates. This corrosion rate rely on the corrosion potential of the metal. This potential difference can be measured by voltmeter after connecting the metal with an SRE. For the reporting of corrosion potential the magnitude of the voltage and its sign are necessary.

At the corrosion potential,

Ecorr = Rate of cathodic reduction = rate of anodic react ion (metal corrosion).

Icorr:

Icorr is shorthand of corrosion current density or corrosion rate.

icorr is the current corresponding to Ecorr, the current between the local cathodes and anodes cannot be measured by conventional (such as ammeter) means.

At Ecorr (freely corroding potential),

the rate of forward process (i_f) is equal to the rate of reverse process i_r that is i_f = i_r

In the polarization curve, linear portions of the measured cathodic and anodic are extrapolated to

Ecorr and the point of intersection yields, icorr.

Epass and Ipass:

Here Epass is called passivation potential and Ipass is passivation current density.

Epit is the potential at which the current sharply increases, When pitting occurs on the forward scan, the reverse scan will trace a hysteresis loop. The potential where the hysteresis loop closes on the reverse scan is called repassivation or protection (EPRO). If the loop does not close, EPRO can be estimated by extrapolating the reverse scan to zero current.

(2) I am assuming Ti as titanium.

To deal with corrosion applications titanium is an stable metal. Titanium contains thin film of oxygen on its surface which protects it from corrosion. It reacts with titanium to form titanium dioxide, which acts as a barrier against corrosion.

This corrosion resitant barrier can only be broken if the Ti placed in an chloride rich environment. In the chemical processing industry, titanium and its alloys offer good corrosion resistance in many process solutions and owe this corrosion resistance to the strong oxide film. This is called control Ti.

Coated Ti:

Although Ti have tremendous corrosion resistance still it's corrosion resistance affected by halides or when Ti slides over Ti galling occure. To prevent this film lubricant is coated on Ti. To coat Ti, obviously there are many benefits other than corrosion resistance.

Titanium is well known corrosion resistance material which in the form of TiN etc. coated on stell substances to protect it from corrosion so there is not much improvement in corrosion resistance after coating.