Experimental results of

• Corrosion rate by weight loss

• Corrosion rate by potentiodynamic polarization curve

Omar Farooq 2011906 Khalid Usman Muhammad Tahir

Table of ContentsThe Experiment...........................................................................................................................................3

The weight loss method..........................................................................................................................3

The potentiodynamic technique..............................................................................................................4

Results and discussions...............................................................................................................................5

Potentio-dynamic Curve for Cu-Zn alloy in H2SO4 sol.............................................................................5

Potentio-dynamic Curve for Cu-Zn alloy in NaCl sol................................................................................6

Conclusion...................................................................................................................................................7

Prevention...............................................................................................................................................7

Refrences.....................................................................................................................................................8

The Experiment

The weight loss method

• Sample 1(of 18 g , 1M NaCl sol.) = ∆w= 0.1g

• Sample 2(of 16.59 g , 3.5% NaCl sol.) = ∆w= 0.08g

• Corrosion rate of sample 1 = (534 x ∆w)/(DAT)

= (534 x 0.1g)/(8.5 g/cm^3 x5.4in^2 x 168hr)

= 0.00692 mil/yr

• Corrosion rate of sample 2 = (534 x 0.08g)/(8.5 g/cm^3 x4.99in^2 x 168hr)

= 0.00599 mil/yr

The potentiodynamic technique

The potentiodynamic technique is used to examine the overall corrosion behavior of Cu-Zn alloy. The Gamery equipment used operates by performing a potentio-dynamic scan over a very wide potential range. During the sweep the Cu-Zn alloy undergoes various electrochemical reactions, resulting in corrosion currents. These currents are measured and displayed as a graph by the gamery software

Hydrogen evolution occurs at the cathode, resulting in anodic dessolution or dezincification and uniform corrosion.

Tafel equation gives the parameters that enable understanding of the potentiodynamic curves,

Results and discussions

Potentio-dynamic Curve for Cu-Zn alloy in H2SO4 sol.

Active, Passive and Trans-passive regions are visible in the curve above.

Proposed Mechanism:

• Formation of passivation film

• This film is not porous and thus protective. Because of this corrosion of Al-brass continuous diminshes

Icorr: 1.45E-08

Ecorr: -2.55E-01

Beta A: 0.0528

Beta C: 0.048

Potentio-dynamic Curve for Cu-Zn alloy in NaCl sol.

The anodic polarization curve is similar to curve in Figure 1 but in this case the pseudo-passive film was formed. High current density value indicates that this film is porous and pitting corrosion is possible to occur. A very high increase of current density at transition from Epit in trans-passive area (Figure 2) shows that the formed passive film was very weak.

Icorr: 0.000001

Ecorr: -0.3958

Beta A: 0.12

Beta C: 0.12

Conclusion

The obtained electrochemical parameters presented in Table 2 shows that corrosion rate of CuZn alloy was man times higher in 5 M NaCl solution than in 1M H2SO4 solution. For both cases (Figures 1 and 2) the anodic potentiodynamic polarization curves showed a small passive region. This region is termed as "pseudo-passivity". The corrosion film formed in such a way is not a protective one, but contributes to the formation and propagation of pitting corrosion of the CuZn alloy. The difference between values of i is very small for the both of anodic potentiodynamic polarization curves (Figures 1and 2), and that means that the passive protective layer was not formed. For the formation of compact homogenous protective passive film, in these conditions, the value of I must be smaller than obtained.

• Dezincification (selective corrosion)

The mechanism of dezincification , the entire alloy is dissolved and later one of its constituent is re-plated from solution , copper is deposited as a fine copper dust which zinc is leached out of brass leaving behind a highly porous mass ( zinc has a high tendency to dissolution while copper has a high tendency to plate.

E Zinc = -0.76 volt , E Copper = 0.33 volt )

• Uniform corrosion Greenish corrosion product on surface

Prevention• Use copper alloys with copper content above 85%.

• Use brass alloys with tin , arsenic or antimony addition

• Avoid environment where the solution becomes stagnant and deposits accumulate on the metal surface.

Refrences

1. .http:// www.uobabylon.edu.iq/uobColeges/ad_downloads/4_10460_432.pdf 2. 10th International Research/Expert Conference

”Trends in the Development of Machinery and Associated Technology”TMT 2006, Barcelona-Lloret de Mar, Spain, 11-15 September, 2006ELECTROCHEMICAL BEHAVIOUR OF Al-BRASS IN SEAWATERLoreta PomenicFaculty of EngineeringVukovarska 58, 51000 RijekaCroatia